![Library of Congress Cataloging in Publication Data:
Morley, Lloyd A.
Mine power systems.
(Information circular: 9258)
Includes bibliographies.
Includes index.
Supt. of Docs. no.: 128.27:9258.
1. Electricity in mining. I. Title. 11. Series: Informalion circular (United States.
Bureau of Mines); 9258.
TN295.U4 [TN343] 622 s [622'.48] 87-600213
--
For sale by the Superintendent of Documents, U.S. Government Printing Office
Washington, DC 20402](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-2-320.jpg)
![arrangement is continuedthroughout most of the haulage
system except for the most inby segment, which is dead-
ended. In some mines, dc face equipment and small dc
motors are powered fromthe trolley systemthrough a fused
connection (or nip) to the trolley conductor and rail. The
dc distributioncan also serve large motorsdirectly through
switchgear;however, this practice is rare in underground
coal.
All power equipmentused undergroundmust be rugged,
portable, self-contained,and specifically designed for in-
stallation and operation in limited spaces. In addition, all
equipment and the cables connecting them must be pro-
tected against any failures that could cause electrical haz-
ard to personnel. This is primarily provided by protective
relaying built into each system part, with redundancy to
maximize safety (4).
Utility company
metering e -
- N
t-;
B 6
i V) U
Surfoce
Moin
substation
To wrfoce !ads
(hoist,fon, pmps.char)a nmb*) !%aer center
Distribution
hnnskrlner
CONVENTIONAL Rectifier
MINING SECTION
Portable
switchhse
t
Mixellonews loads
(shop, pump, etc.
1
Rectifier B>
starter
m r W t
ad drive 1
,
r k r;~
Trolley system
1 I -:... lfion ] LONGWALL
'mer MINING SECTIW
I ?-?
Mine pcwr center
CONTINWS
MINING SECTION
I-I-N-LL
t t l ' T t t t T
control](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-39-320.jpg)
![From utility to
wbtransmission
s~stem
hSubstation
+ - - -
24 to 13.8kV
I r715i
To 480-V preparation plant loads
Flgure 1.26.-Representative expanded radial distribution
for preparation plant.
The designed system must meet certain minimum
criteria. IEEE (12)has defined these basic criteria for in-
dustrial electrical systems that must be applied to mines:
Safety to personnel and property,
Reliability of operation,
Simplicity,
Maintainability,
Adequate interrupting ability,
Current-limiting capacity,
Selective-system operation,
Voltage regulation,
Potential for expansion, and
First cost.
Of these, safety, reliability, and simplicity are closely re-
lated. All are dependent on good preventive maintenance.
In the cramped uncompromisingenvironment of an under-
ground mine, these are of vital concern. Since continuous
operation is the aim of every mine operator, planned main-
tenance should be held to a minimum. Most routine main-
tenance should be capable of being performed by unskilled
personnel, since it will be done by the miners themselves.
Training for these tasks must be provided.
Adequate interrupting capacity,current-limiting capa-
bility, and selective-systemoperation areprojected at safety
through reliability. The first two areas ensure protection
during a disturbance. Current limiting, when applied to
grounding, is perhaps the most significantpersonnel safety
feature of mine electrical systems. Selective-systemopera-
tion is a design concept that minimizes the effect of system
U t i l i t y meterlng
Substation$,
3,750kVA each
open tie circuit breaker
7 I
Auxiliary
/
[
I
] functions
w i t c h i o r
s one-cool
switchgear
center center center
X
control center
X X
4 8 0 - V motors 4,160-V 4 0 0 - V About 2,500-hp, About 2,500-hp. 4 8 0 - V m o t o r s 4 8 0 - V
crusher motor8 4,160-V motors 4,160-V motors motors
C v w
Raw-coal c i r c u i t coorse- P u m p r , conveyors, blowers, Fine-cool Auxiliaries
c o a l f i l t e r s , fans, jigs, etc. c i r c u i t
c i r c u i t
Figure 1.27.- Representative secondary-eelectlvedistrlbutlon for preparationplant.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-43-320.jpg)
![No restriction is placed on the form of v and i. In dc cir-
cuits they are constant with respect to time, and in ac
circuits they are sinusoidal.
For metals and most other conductors, R is constant.
In other words, its value is not dependent on the amount
of current, i. In some materials, especially in crystalline
materials called semiconductors,R is not constant,and this
characteristicis useful in diodes,amplifiers,surge arresters,
and other devices.
Further experimentsby Ohm indicatedthat the resist-
anceof a piece of metal dependson its size and shape. How-
ever, the resistivity, p, of the metal depends only on its
composition and physical state. This is an inherent prop-
erty that opposescurrent through the conductorjust as the
frictional resistance of a pipe opposes the flow of water
through it. Resistivity is defined asthe resistance of a unit
cube of homoeeneous material: hence. resistivitv can be
thought of as iproperty of the materialat a value
remains the same at all points in a homogeneous conduc-
tor, but if the material is not homogeneous, its resistivity
can vary from point to point. The value may also vary
greatly for different conductors. The concept of resistivity
is often used in the grounding and distribution aspects of
mine electrical systems.
Using the definition, practical resistivity units would
be ohm-centimeter(Q-cm)and ohm-inch(Q-in).However.
resistivity is usuallyexpressedin ohm-meter(Q-m)
(SDand
ohm-circular-mill-foot(English).The ohm-meteris the re-
sistance of a material 1 mmz in cross section with 1m
length. Likewise,the ohm-circular-mill-foot
(usuallyabbre-
viated to Qcmil-ft)refers to the conductorresistance for a
volume 0.001 in (1mil) in diameter and 1 ft long. For
calculating the resistance in this latter case, the cross-
sectionalarea of the conductor is measuredin circular-mills,
which can be found from
whereA = cross-sectionalarea of circular conductor,cmil,
and d = conductor diameter, lo-=in.
bsistivity values of somecommonconductorsare given in
table 2.2.
Table 2.2-Resistivity of some common materials at 20° C
Temperature Resistivity (p)
Material
coefficient (a) 10-8 Q~ namil-fl
Aluminum, commercial...........
Copper, annealed.................
Imn, annealed........................
......................................
Lead
Nichmme...............................
Silver ..................................
.............................
Steel, mild
Tin.........................................
lbngsten................................
The resistance of any specific conductor can be calcu-
lated from the material resistivity using the formula
where R = resistance, Q,
O = conductor length,
A = conductor cross-sectional area,
and p = material resistivity.
If P is in meters and A is in meters squared, then p must
be given in units of ohm-meters.
Electrical resistivity does not remain constant if the
temperature is permitted to change. For most materials,
the resistance increases asthe temperature increases;car-
bon is an exception to this rule (negative temperature co-
efficient,0.005). If the temperature coefficient isknown,the
resistance of a given conductor at a given temperature is
R = Ro [1+ a (t - to)], (2.6)
where R = resistance at temperature t, R,
Ro = resistance at referencetemperatureto,usually
20" C, Q,
a = temperature coefficient, Q/'C,
t = given conductor temperature, O C ,
to = reference temuerature. "C.
At very low temperatures (about -200" C for copper)or as
the melting point is reached, the temperature coefficient
is no longer constant and changes with temperature. As
a result, equation 2.6 is not valid for very high or low
temperatures.
The symbol illustrated in figure 2.1 portrays a resistor
in a circuit, and often its resistance is stated. Again by
definition,
Sometimes, the element's conductance, G, is referenced
and is defined as the reciprocal of resistance:
In circuit analysis,it is occasionallymore convenientto use
conductancethan resistance. Later, the explanationof this
symbol will be generalized.
Kirchhoff's Voltage Law
In the simple series circuit shown in figure 2.2, three
resistors are connected in tandem to form one single closed
loop. Kirchhoff has shown that when several elements are
Figum2.1.-Clrcult element Illurtratlngvoltage polarltyand
cumnt flow dlmtlon.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-47-320.jpg)
![Addition or subtraction of imaginary numbers yields
another imaginary number. Yet, when an imaginary num-
ber is added to a real number, a complex number iscreated.
These have the rectangular form, x +jy (for instance,
3 +j4), where x is the real part and y the imaginary part
or if
Z = x +jy,
then Re[Zl = x I d Z ] = y. (2.95)
Complex numbers can be represented graphically by a
pair of perpendicular axes as shown in figure 2.68. The
horizontal axis is for real quantities, the vertical one for
imaginary. Considering x +jy, if y = 0, the complex num-
ber is a pure real number and falls somewhere on the real
axis. Similarly, if x = 0, the complex number (now be-
ing purely imaginary) exists on the vertical axis. Hence,
complex numbers encompass all real and all imaginary
numbers.
In the case of the rectangular forms
Z = x + jy,
W = u +jy,
the followingcommon definitions and mathematical opera-
tions of complex algebra are applied.
1. Two complex numbers are equal if and only if the
real components are equal and the imaginary components
are equal:
Z = W, IFF x = u, y = v.
2. To sum two complex numbers, the real and imaginary
parts are summed separately:
Z + W = (x 5 u) + j(y + v).
3. The product of a real and an imaginary number is
imaginary:
x(iy) = j(xy).
4. The product of two imaginary numbers is a negative
real number:
(iy)(iv) = -yv.
5. The multiplication of two complex numbers followsthe
rules of algebra (note, an easier way to perform the multi-
plication will be shown):
(x+jy)(u+jv) = xu + jxv +juy - yv
= (XU - yv) +j(xv + uy).
6. By definition, the conjugate of a complex number is
formed by changing the sign of the imaginary part. An
asterisk denotes the conjugate:
Z = x + j y becomes Z* = x - jy.
7. For division, the numerator and denominator are
multiplied by the conjugate of the denominator (again, an
easier method exists):
x +jy - x + jy (u +jv) xu -
- - + j
(
*
)
.
u - jv u - jv (u + jv) ((ua +
: ua + 9
ponential. Figure 2.69 illustrates the conversion of rectan-
gular to trigonometric or polar forms where
Z = x +jy. (2.96)
The absolute value of Z is represented by "r," and
x = rcostl,
y = rsin0,
where 9 = tan-' (
l
)
,
X
r = (xP+ yz)l''. (2.97)
Hence, the trigonometrical form of the complex number is
Z = dcostl + jsino), (2.98a)
with the conjugate
Z* = r(cos0 - jsin0). (2.98b)
The polar form iswidely used in circuit analysis and is sim-
ply written as
and the conjugate,
Z* = r i d (2.99b)
Euler's theorem states that
-jl
-j2
-j 3
Flgum2.88. -Qraphlcalmprerentatlonof complex number.
Besides the rectangular, there are three other general Flgum260.-Trlgonometrlc or polar reprerentationof com-
forms of complex numbers: trigonometric, polar, and ex- plex number.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-74-320.jpg)
![1 with the primary current being
Notice that there is no change in current or voltage
phase angle across the transformer.
5. This primary current can now be used to find
the voltage drop resulting from transformer and
overhead-line impedances. Summing this voltage
dmp with the transformer primary voltage givesthe
desired answer to the problem, the substation out-
put voltage, V,:
I
where Z,,, Z, = overhead-line and transformer
impedances, respectively.
Then,
Because the analysis is per phase, the result is
obviously line-to-neutral voltage. If line-to-lineval-
ues are required, the above answer need only be
multipled by &.It is interesting that in this exam-
ple the phase angle of the substation voltage is
practically the same as that at the motor. This may
not be the case in actual mine power systems.
When the transformer is deltadelta conne&ed, problem
solutions are practically the same as in example 4.7. The
one-line diagram of figure 4.22 provides an illustration.
When the systemisrepresentedper phase(fig.4.23),the only
additional concern is delta-wyetransformation of the trans-
former impedance; then the solution proceeds as before.
However,the processmay not be assimplewhendelta-wyeor
wyedelta transformer connectionsare involved.
Consider the one-line diagram in figure 4.24 that
shows a delta-wye transformer supplying the same motor
as that shown in figure 4.19.Figure 4.25 illustrates one
leg of the three-phase transformer. From this, it can be
seen that secondary line currents appear as phase cur-
rents on the primary, and line-to-neutral secondary volt-
ages become line-to-lineprimary voltages. In other words
(fig. 4.25), for primary voltage in terms of the secondary
(the ideal transformer with turns ratio, a, only):
-
where V
,
, = line-to-lineprimary voltage, V,
and Van = line-to-neutral secondary voltage, V,
and for primary and secondary current,
Z=O.6+jO.6
1 ,--(:)
Substation ~2][Li;=0.07+j0.05-
Y
1
5
0 kW total at
B
a
n
k o
f 3-1
9 XFMRS, logging pf
each 2,400/480~ 480V I~ne-to-line
Z =0.3
+10.9
referred to high side
Figure4.22.-One-line diagramwith deltadelta transformer.
Figure 4.23.-Per-phase diagram of figure 4.22.
Lwd
Substation center
Trailing cable
Z=0.6t j0.6 Z=0.07+j0.5
Y
J L
- 50kW at
1,385 / 2 7 7 V
0.8 lagging pf
Z=O.l+iO.3
2 7 7 V line-to-neutral
referred tdhigh skle
Figure 4.24.-One-line diagram with delta-wyetransformer.
d
Primary ~ecwdar;
Figure 4.25.-One leg of three-phase transformer from
figure 4.24.
However, when performing balanced three-phase
analysis, the parameters of interest are line-bneutral
voltages and line currents. Thus, to continue the analyaia
in a fashion similar 5 that q e d in example 4.7 (the
wye-wye transformer), V
, and I
, must be converted to
their respective per-phase equivalents. Recalling that
and applying this concept to equation 4.18, the primary
line-to-neutral voltage, V
,
,
, is
where fAQ = primary phase current, A,
and I, = secondary line current, A.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-117-320.jpg)
![the allowableeffective(rms)current for the total operating
time when curve correction is necessary:
where W = In p2
H ~ ) ' ' ~ ( T , , ~ ~ ) I ~ ]
1
,
H
, = total operation time for motor, h,
T_= rated allowable tem~erature
rise. O C .
, .
T
, = corrected allowable 'temperature rise (due to
elevation or ambient temperature), O C ,
Hl,I, = a point taken from manufacturer's time-to- Figure 6.61.-Stator field of two-pole, single-phase induc-
rise temperature curve, h, A, tion motor.
H2,12= a second point taken from curve, h, A,
and I = allowed rms current for total operation time
I%, A.
If the allowable current from equation 6.19 is less than
I
,
- from equation 6.17, the motor is overstressed for that
duty cycle. Even though the foregoing was applied to dc
motors in traction locomotive, the same concepts can be
adapted to any mine motor application.
SINGLE-PHASE MOTORS
Although the vast majority of mine motors are three-
phase and dc, single-phasemotors do find widespread use
for auxiliary functions asidefrom the mining process. As a
general rule, single-phaseinduction motors have one run-
ning speed and require a separate means for starting
rotation, usually a separate stator or starting winding.
Motors are classified by their starting method. The most
used techniques are split phase and capacitorstart, which
will be discussed briefly in this section.
Rotating Stator Field
When a single-phase ac voltage is applied to one
stator winding, the current flow produces a magneticfield
with a resultant direction that alternates on a line, as in
line OP in figure 6.61. If a squirrel-cagerotorwinding is in
the stator field, a voltage will be induced in the rotor
conductors, but the current produced will create a mag-
netic field that coincideswith the stator field(fig.6.62).As
no magnetic interaction occurs, no torque is developed,
and the rotor remains stationary (9).
If the rotor is moved by some means, the rotor conduc-
tors cut the stator magneticfield, and the inducedvoltages
are in phase with the current through the stator winding.
However, the rotor winding impedance appears as almost
pure inductance, and rotor current will lag the induced
voltage by almost 90° (fig. 6.63). Thus the rotor magnetic
field is now 90° from the stator field and is termed a
cross-magnetizing field (9). The rotor and stator fields
combine to produce a resultant field that rotates at syn-
chronousspeed. The cross field strength is proportional to
the rotor speed, and is about equal to the stator field
strength at synchronous speed. The same operational
principles that have been given for three-phase induction
Figure 6.62.-Rotor field of stationary two-pole, single-
phase induction motor.
Generatedrotor voltage
Stator current and flux
Rotor currentand flux
Figure 6.63.-Phase relationships between stator and turn-
ing rotor.
motors also hold for single phase; slip must always exist
between the rotating field and the rotor. Because of the
crossfield, the slower rotor speed causes the rotating field
to pulsate. Accordingly, vibration and noise are inherent
with single-phase induction motors.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-181-320.jpg)
![Table 8.7.-Ampacities' for portable power cables, amperes per conductor
Single conductor 2.conductor, 3conductor. 3-conductorround
4-
Conductor 0-2,000 V 2,001- 8,001- 15,001- m u d and rOu~~ta"d 5 6-
O- 8,W1- 15foo1- conductor, conductor, conductor.
Size unshielded 8.000 v 15,000v 25,000v flat, ~ 5 , 0 0 0
v 8,000V 15.000 V 25,000V 0-2,000 v 0-2,000 v 0-2,000 v
shielded shielded shielded 0-2!000 V unshielde,j shielded shielded shielded
AWG:
8............... 83 - - - 72 59 - - - 54 50 48
6............... 109 112 - - 95 79 93 - - 72 68 64
4............... 145 148 - - 127 104 122 - - 93 88 83
3............... 167 171 - - 145 120 140 - - 106 100 95
2............... 192 195 195 - 187 138 159 164 178 122 116 110
1............... 223 225 225 222 191 161 184 187 191 143 138 129
110............ 258 280 259 255 217 186 211 215 218 165 - -
2/0............ 298 299 298 293 250 215 243 246 249 192 - -
310............ 345 345 343 337 266 249 279 283 286 221 - -
410............ 400 400 397 389 326 287 321 325 327 255 - -
MCM:
250........... 445 444 440 430 363 320 355 359 360 280 - -
300........... 500 498 491 480 400 357 398 - - 310 - -
350........... 552 549 543 529 436 394 435 - - 335 - -
400........... 600 596 590 572 470 430 470 - - 356 - -
450........... 650 640 633 615 497 460 503 - - 377 - -
500........... 695 688 878 659 524 487 536 - - 395 - -
550........... 737 732 - - - - - - - - - -
600........... 780 779 - - - - - - - - - -
650........... 820 817 - - - - - - - - - -
700........... 855 845 - - - - - - - - - -
750........... 898 889 - - - - - - - - - -
800........... 925 925 - - - - - - - - - -
900........... 1.010 998 - - - - - - - - - -
'Bawd on a copper conductor temperature of 90% and an ambient air temperature of 40%.
These ampacities are based on single isolated cable in air operated with opencircuitedshield.
NOTE.-Dash indicatescable is not made
Table 8.8.-Ampacities' for three-conductormine power
cables
Conductor size 2,001 to 8.000 V 8,001 to 15,000V
Copper Aluminum Copper Aluminum Copper Aluminum
AWG
93 -
122 124
159 165
184 169
211 218
243 251
279 278
321 342
MCM
250 400 355 360 359 367
300 450 398 395 401 393
350 500 435 42.5 438 424
400 - 470 - 473 -
450 - 502 - 504 -
500 - 536 - 536 -
'Based on ICEA values with an ambient temperature of 40% and a
conductor temperatureof 90°C [taken from "Power Cable Ampacities" (20),
v. 1 for copper conductorsand v. 2 for aluminum conductors].
NOTE.-Dash indicatescable is not made.
Table 8.9.-Correction factorsfor ampacities at various
ambient temperatures.
Ambient Ambient Correction
temperature, OC factor
Cable Heating on Reels
A cable that is used in a confined space can become
overheated with continuous-current flow at the ampacity
rating. Perhaps the best example is a cable bound on a
reel, either for storage purposes or to increase mining
machine mobility. Investigations were conducted as early
as 1931 to identify factors responsible for overheating of
rubber-jacketed cables, with emphasis on increased tem-
peratures occurring in reeled cables (16).
A cable manu-
facturer manual published in 1940was the first to contain
a table of derating factors related to the number of layers
wound on a reel to reduce the current-carrying capacity of
the cable (23).
These factors were included in ICEA spec-
ificationsfor 60°C-ratedcables in 1946and have remained
a standard since that time. Table 8.10 presents the ICEA
values presently required by Federal regulations for all
cable insulations (38).
Research has been conducted since the publication of
the ICEA derating factorsto determine their applicability
to the mining industry. McNiff and Shepherd (23-24)
worked with cyclic currents, comparable to those experi-
enced by shuttle cars in sewice, and steady-state loading
at various percentagesof cable ampacity,with both ac and
dcpower. Derating factorsfor 60°C-ratedcablesabstracted
from these results are presented in table 8.10. An impor-
tant contribution of their work, which cannot be shown in
the table, is identification of the dependence of cable
derating factors on the maximum-limit temperature per-
mitted: at this temperature is increased or reduced, the
derating factor changes accordingly. This was later veri-
fied by Woboditsch (41),
and his values for a limit temper-
ature of 60°C are also given in table 8.10.](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-221-320.jpg)
![Conductor
size
AWG:
14 .....................
12 .....................
10.....................
8 .......................
8 .......................
4 .......................
3 .......................
2 .......................
Maximum
C o n d m allowsble
size i n s t e ~ t e n e ~ ~
settlng, A
AWG:
mble 9.4.--Commonly avallable magnetic-trip ranges for
mlning-aewlce molded-caae bmkers
Frame size, Magnetic-trip Range of allowable
A range, A conductor a i m
100........................................ %
I
-
180 14-lOAWG
10-4 AWG
6-3 AWG
4-1 AWG
6-1 AWG
4-1 AWG
2-210 AWG
4-2/0 AWG
1 AWG-500MCM
2-a0 AWG
1310AWG
a0 AWG - 500 MCM
310 AWG - 500 MCM
1.200 ................................. .... 1;5~)3;000 210 AWG - 500 MCM
2,DOQ-4,000 310 AWG -500 MCM
2.500-5.000 410 AWG -500 MCM
bimetal deflection is dependent upon current and time,
the thermal unit provides a long-time delay for light
overloads and a fast response for heavy overloads. A
representative current-time curve for the thermal unit
alone is shown in figure 9.124; later in this chapter,it will
be described aa an inverse-timecharacteristic. In compar-
ison,figure 9.12Bshowsthe circuitbreaker responsewhen
both thermal and magnetic trip elements are incorpo-
rated. The shaded area for each curve represents a toler-
ance between the minimum and maximum total clearing
time.
The thermal-magnetic unit shown in figure 9.11 is
ambient-temperature seasitive. Assuming the circuit
breaker, cable, and equipment being protected are in the
same ambient temperature, the circuit breaker trips at a
lower current aa the ambient temperature rises in corre-
spondence to safe cable and equipment loadings, which
vary inersely with ambient temperature (19).Thermal-
magnetic trip elements are available that automatically
compensatefor ambient-temperature variations. The am-
bient compensation is obtained through an additional
bimetal strip, which counteracts the overload bimetal.
Such trip units are recommended whenever the protected
conductors and the circuit breakers are in different ambi-
ent temperatures (19).
Most mining-servicemolded-casebreakers with 225-A
frame sizesand abovehave interchangeable trip unita. For
straight magnetic elements these allow different
instantaneous-trip ranges per frame size. However,
thermal-magnetic units can be used to establish a lower
continuouscurrent limit for the breaker. The National
ElectricalCode (13)is used as a guideto definethe current
htaqreticelement closes gap ond
opens ConloCtS an shwt circuit
L
m
;
h L k x + s Closed ~ccnmn
open
Latched Trip@
Figure9.9.-Magnetk-trlp relay.
I
0
LOW
n Intermdie
w^ I
0
.
1
High
0
.
0
1
6
I HI'
' L O ] CURRENT
A Adjustment kmb B Charo3eristics
Figure9.10.-Adjustable Instantaneous settlng.
J/lJzw
Load
~irneti 1 CMltactsc.
Latch
Latched Tripped
Figure 9.11.-Thermal-magnetic action of mdded.care clr-
cult breaker.
CURRENT, 96 CURRENT, %
of brwker rating of breaker rating
A Thermal only 8 Thermal magnetic
Figure 9.12.-Time-curnnt characterlrtlcs for thermal.
magnstb molded- clrcult W e n .](https://image.slidesharecdn.com/electricalengineeringinminingindustries-220618045612-f4dd2ae9/85/Electrical-Engineering-in-Mining-Industries-pdf-255-320.jpg)
Electrical Engineering in Mining Industries.pdf
- 1. Information Circular 9258 Mine Power Systems By Lloyd A. Morley UNITED STATES DEPARTMENT OF THE INTERIOR Manuel Lujan, Jr., Secretary BUREAU OF MINES T S Ary, Director
- 2. Library of Congress Cataloging in Publication Data: Morley, Lloyd A. Mine power systems. (Information circular: 9258) Includes bibliographies. Includes index. Supt. of Docs. no.: 128.27:9258. 1. Electricity in mining. I. Title. 11. Series: Informalion circular (United States. Bureau of Mines); 9258. TN295.U4 [TN343] 622 s [622'.48] 87-600213 -- For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, DC 20402
- 3. The author is grateful to the several individuals and companies that supplied noncopyrighted material for use in this publication. This material is noted by the vari- ous courtesies given throughout the text. Its incorporation does not constitute an en- dorsement by the author, The Pennsylvania State University, the University of Ala- bama, or the Bureau of Mines. Reference to specificproducts, equipment, or manufacturers does not imply endorsement by the Bureau of Mines.
- 4. The application of electricity to the mining industry is a distinctive area of both mining engineering and electrical engineering. The diEcult environment, the dynamic power loads, the cyclic and mobile operation and stringent safety requirements that characterize mining, all place unique demands on the mine power system. No other industry makes such extensive use of portable extensible equipment or has such com- plex grounding problems. Mine power systems can range from relatively simple in- stallations for small surface mines to complex underground systems where the harsh environment of dust, humidity, and cramped spaces stretches the ingenuity and crea- tivity of the engineer to provide reliable service. At the present time there is no up-to-date engineering text available that deals specifically with mine power systems. This has created extensive difficulties for edu- cators, industry engineers, and regulatory agency personnel. The need for a suitable reference for students in mining engineering provided the main impetus for this book, since the technician-level material that was in existence proved unsuitable for teach- ing young engineers who have little practical experience. The objective in preparing this manuscript was to assemble a single engineering reference on mine electrical power systems that is as comprehensive as possible. Ear- lier drafts of this material have been used successfullyto instruct university students in courses ranging from basic electrical engineering through power-system design. It is felt, however, that the usefulness of this material extends beyond that of a student text. While not intended to replace other electrical or mining references, this publica- tion is also an indexed, reasonably comprehensive reference handbook for industry engineers and training personnel, and a sourceof material for electrical engineers who wish to expand their education into industrial power-system applications. Obviously, there will be some omissions; to include all aspects of mine electrical systems in one volume would approach an impossibility, but an attempt has been made to collect together the most significant information, thereby providing the tools needed to con- tinue a knowledgeable involvement in mine electricity. This reference work is divided into three general content areas. Chapters 1through 5 contain information considered elementary, chapters 6 through 1 1deal with power- system components, and chapters 12 through 17 contain specifics on mine power sys- tems. A person familiar with electrical principals can use the earlier chapters as re- view material, but all chapters contain material relevant to mining and discuss the necessary combinations of equipment and components that should be contained in the mine power system. Emphasis throughout is placed on coal mining systems, although much of the material pertains to all mining operations. Both surface and underground power systems are discussed, the latter in more detail since these are the more com- plex systems and encounter the most problems. This publication is a thoroughly upgraded and extensively revised edition of Bureau of Mines Open File Reports 178(1)-82and 178(2)-82,prepared under Bureau contract 50155009by The Pennsylvania State University. It contains new chapters,new illustra- tions, and example problems that were not included in the original report. The assembly of this material has been a major undertaking. Many industry, academic, and Government agency personnel helped to review and critique practically every stage of draft preparation. The original report version was made available to students taking the mine power-systems courses at The Pennsylvania State Univer- sity, and their involvement was critical input to manuscript -- preparation. -- The author is grateful to all the companies and individuals who contributed or cooperated in this effort; so much information could not have been gathered without their help. A specialthanks is owed to the late Robert Stefanko. He originally perceived the need for this text and provided guidance and encouragement throughout the proj- ect that produced the original report version. Others deserving special mention are A. M. Christman, R. H. King, J. A. Kohler, G. W. Luxbacher, T. Novak, J. N. Tomlinson, F. C. Trutt and D. J. Tylavsky. Each contributed directly to the text while on the fac- ulty or staff at The Pennsylvania State University; acknowledgements for their con- tributions are made in the individual chapters.
- 5. CONTENTS Page Page Preface ............................. Abstract ............................. Part l : Fundamentals Chapter l.--Electrical power in mining ...... Mine electrical history ................. Underground mine history ............ Surface mine history ................ Mine power equipment ................ ....................... Substations Switchhouses ...................... Power centers ..................... Distribution equipment .............. Basic distribution arrangements .......... Radial system ..................... Primary-selective system ............. Primary-loop system ................ Secondary-selective system ............ Secondary-spot network .............. Utility company power ................ ...................... Surface mining Power systems in surface mines .......... Main substations and subtransmission ... Surface mine distribution ............. Underground coal mining .............. Room-and-pillar mining .............. Longwall mining ................... Power systems in underground mines ...... Regulations ....................... Underground mine distribution ........ Surface facility power requirements ....... Basic design considerations ............. References ......................... Chapter 2.--Electrical fundamentals I . . . . . . . Basic electrical phenomena ............. .................... Coulomb's law Voltage and current . . . . . . . . . . . . . . . . System of units ...................... Experimental laws and paramcters ........ ....................... Ohm's law Kirchhoff's voltage law .............. KirchofPs current law ............... Series circuits ..................... Parallel circuits .................... The magnctic field ................. Inductance ....................... ...................... Capacitance Electric field ...................... Instantaneous power ................ Idealization and concentration ......... ................. Direct current circuits Direct current and circuit elements ..... Series and parallel resistance .......... ............ Wye-delta transformations Circuit and loop equations ............ Node equations .................... Network theorems .................. Time-varying voltages and currents ....... .............. Steady alternating current ........... Effective alternating current Phasors .......................... ........ Phasors and complex quantities Impedance transforms ............... ................ Steady-state analysis ....... Chapter 3.-Electrical fundamentals I1 ......... Average power and power factor ........... Complex and apparent power Resonance ......................... Series resonance ................... Parallel resonance .................. Transformers ....................... Ideal transformer .................... Actual transformers .................. Conductor loss .................... Leakage reactance .................. ........ Core losses and exciting current ........ Power-transformer construction Transformer models ................ Determination of transformer ..................... parameters ... Transformer efficiency and regulation Autotransformers .................... .............. Multivoltage transformers ....... Current and potential transformers Chapter 4.-Power.systcm concepts .... Basic power circuit .............. Three-phase circuits ............. Balanced three-phase circuits ...... Three-phase system voltages ..... Load connections ............. Line and phase currents ........ Equivalent delta and wye loads ... Three-phase power ............ Three-phase transformers ......... Balanced three-phase circuit analysis . One-line and three-line diagrams ..
- 6. Page Page ........ Circuits containing transformers Per-unit system ...................... Transformer impcdancc .............. ........... Three-winding transformers ...... Per-unit method in system analysis ......... Unbalanced three-phase circuits Fault types ....................... Fault analysis ..................... ............... Symmetrical componcnts Sequence components ............... ........ Sequence-quantity combinations .... Symmetrical-componentrelationship Symmetrical-component impedance ..... .................. Fault calculations ................... Power terminology References ......................... Chapter 5.-Basic solid-state devices and instrumentation ...................... 104 Semiconductors ...................... 104 Diodes and rectifiers .................. 104 Diodc equations ................... 105 Rectifier circuits ................... 105 Cooling .......................... 106 Overloads ........................ 107 Three-phase rectification ............... 107 Rectifier circuits ................... 108 Parallel rectifier operation ............ 109 Transistors ......................... 109 Transistor operation ................ 109 Bipolar-transistor amplifiers ........... 110 Field-effect transistors ............... 112 Silicon-controllcdrcctifiers ............. 113 Integrated circuits .................... 114 Basic instrumentation ................. 114 Basic meter movements ............... 115 Meter-movement applications ......... 116 Wattmeters ....................... 117 Varmeters ........................ 118 Power-factor meters ................ 118 Power-system instrumentation ........... 118 Instrument transformers ............. 118 Single-phase connections ............. 119 Three-phase connections ............. 120 Special instruments ................... 122 Watthour meters ................... 122 Demand meters ................... 122 Bridges .......................... 122 Megohmmeters .................... 123 Phase-sequence indicators . . . . . . . . . . . . 124 Recording instruments ................ 124 Electronic instruments ................ 125 Electronic meters .................. 125 ..................... Oscilloswpes 125 .................... Tape recorders 126 Transducers ...................... 126 ................ Instrument installations 127 Part 11: Power-System Components Chapter 6.--Motors and motor control ...... 129 Alternating current generation ........... 129 Principle of generator operation ........ 129 .............. Generator construction 129 Three-phase generation .............. 131 Direct current generators .............. 131 Motor basics ........................ 133 Torque .......................... 133 ............ Speed-torque relationships 133 .................... Standardization 134 Motor type ....................... 135 Three-phase squirrel-cage induction motors .......................... 136 ......... Elementary three-phase motor 136 ................. Motor conslruction 138 Motor behavior .................... 138 Insulation ........................ 139 ............... Design characteristics 139 ............. Induction-motor starting 141 .......... Wound-rotor induction motors 142 Three-phase synchronous motors ......... 143 ........... Synchronous-motor starting 144 ............ Synchronous-motor torque 145 Generated voltage .................. 146 Power factor ...................... 146 Applications ...................... 147 Direct current motors ................. 147 Elementary motor .................. 147 ............ Actual motor construction 148 Torque .......................... 148 .... Motor connections and performance 148 ............... Ward-Leonard system 152 Mine motors ........................ 153 Applications ...................... 153 ........... Actual equipment operation 153 Single-phase motors .................. 156 ................ Rotating stator lield 156 ................. Split-phase starting 157 ............... Capacitor-start motors 157 References ......................... 158 Chapter 7..-Grounding .................. 159 Grounding systems ................... 160 Ungrounded neutral .................. 160
- 7. ............... Solidly grounded neutral ....... Low-resistance grounded neutral High-resistance grounded neutral ....... Electric shock ....................... Characteristics of mine grounding systems .. Ground beds ...................... ...... Grounding in underground mining ........... Grounding in surface mines ............... Ground-bed construction .................. Ground resistance ....... Electrode co~gwation formulas ............ Two-layer earth structures ................. Soil-heating effects ......... Control of potential gradients ...... Ground-bed resistance measurement Measurement method ............... Ground test instruments ............. ................. Ground-bed resistivity Factors affecting resistivity ............ Resistivity measurements ............. Effect of chemical treatment of soils .... ................. Ground-bed corrosion General ground-bed guidelines .......... Grounding equipment ................. ................. Grounding resistor Grounding transformers ............. .......................... Summary References ......................... Chapter 8.--Distribution ................. Nature of cable distribution ............. ................... Cable components ....................... Conductors Insulation ........................ ...................... Cable jacket .................... Cable shielding Cable types ......................... Cable terminations ................... Cable couplers ...................... Coupler contacts ................... .................. Coupler insulation ................... Coupler housing High-voltage couplers ............... ................ Low-voltage couplers ...................... Cable selection Cable length ...................... ................. Conductor selection Cable installation and handling .......... .................... Borehole cables Feeder cable installation ............. Recommended handling practices ...... Cable failures and repairs .............. Cable testing ...................... .................... Failure location Splicing .......................... Trolley systems ...................... Trolley wire ...................... Trolley feeder ..................... ..... Supports. lubrications. and turnouts ................... Rails and bonds Overhead lines ...................... ................ Overhead-line design Overhead-line electrocutions .......... References ......................... Chapter 9.-Protective equipment and relaying ........................... Switching apparatus .................. ............ Arcs and circuit interruption Switches ........................... Circuit breakers ..................... Circuit breakers for low and medium voltage .......................... Molded case circuit breakers .......... ............... Power circuit breakers High-voltage circuit breakers ............ Typical ratings ..................... Oil circuit breakers ................. Minimum-oil circuit breakers .......... Vacuum circuit breakers ............. Fuses ............................. Low-voltage fuses .................... Non-time-delay fuses ................ Time-delay fuses ................... Dual-element fuse .................. Current-limiting fuses ............... Standard fuses ..................... Nonstandard fuses .................. High-voltagefuses .................... Expulsion types .................... Current-limiting high-voltage fuses ...... Load-break switches ................ Relays ............................ Relay terminology and types .......... Thermal relays .................... Electromagnetic-attraction relays ....... Electromagnetic-induction relays ....... Basic relay connections ................ Alternating current direct relaying ...... Alternating current potential relaying .... Alternating current differential relaying .. Direct current connections ............ Kinds of protection ................... ..................... Control wiring Page 206 207 207 211 211 211 211 215 216 217 218 222 224 224 225 226 226 227 228 232 232 232 232 233 234 235 235 236 236 236 236 236 237 237 237 238 239 240 240 240 241 242 244 244 246 247 247 248 248
- 8. Page Page Phase protection ................... Ground overcurrent ................. Ground-check monitoring ............ ......... Advantages and disadvantages Arrangements for mining .............. Zones of protection ................. ...................... Coordination .............. Ground-fault protection Overloads and short circuits ........... Surface mines ..................... Underground mines ................. ......................... References ....... Chapter 10.Suing protective devices ....................... Fault current Fault-current sources ................ ............. Source equivalent circuit Fault calculations for three-phase systems .. Short-circuit calculation procedures ..... Three-phase calculation example ....... Computer fault analysis .............. Ground-fault current calculations ....... Direct current system faults ............. Device settings ...................... Relay pickup settings ................. Short-circuit protection .............. Overload protection ................. Ground-fault protection .............. Current transformer matching ........... Current transformer accuracy ......... Accuracy calculations................ Low-voltage circuit breaker trips ......... Overload protection ................. Short-circuit protection .............. Low-voltage power circuit breakers ..... Fuses ............................. ....................... Coordination ......................... References Chapter ll.--Transients and overvoltages ..... Transient sources .................... Lightning phenomena ................. Switching transients ................... ............... Capacitance switching .................. Current chopping Prestrike ......................... ........... Direct current interruption General switching transients .......... Other transient phenomena ............. Traveling waves ..................... Electromagnetic phenomena ............ Transient-induced failures .............. Winding response .................. ........ Coupling through transformers Transient protection .................. Surge arresters .................... ............ Surge arrester applications Capacitors and system capacitance ...... ............ Other suppression devices Faraday shields .................... ................ Circuit arrangements ........... Protection of overhead lines Impulse performance of ground beds .... References ......................... Part 111: Mine Power Systems .......... Chapter 12.-Mine power centers ............... Equipment specifications Mine power centers .................. High-voltage cable coupler ............. Interlock switches .................... Disconnect switch .................... High-voltage fuses .................... Surge arrestors ...................... Transformers ....................... SpeciF~cations ..................... Transformer construction ............. Faraday shields .................... Grounding resistor ................... Busway ............................ ............... Outgoing circuit breaker Ground-fault protection ............... .............. Single-phase transformers Metering circuits ..................... ............... Outgoing cable couplers ................ Ground-check monitors ................ Power-factor correction ............... Direct current utilization Rectifier transformer ................. ........................... Rectifier Direct current ground-fault protection schemes ......................... .......... Direct current control circuitry ........ Direct current interrupting devices References ......................... Chapter 13.Switchhouses and substations ... Switchhouses ....................... Switchhouse internal components ......... Switchhouse protective relaying .......... Power circuit breakers ................ Switchhouse control circuits ............. Switchhouse design ...................
- 9. Page Page Substations ......................... Basic substation arrangements ........... Single-ended substations ............. Double-ended substations ............ Substation transformers ................ Substation switching apparatus .......... ........................ Reclosers Disconnect switches and fuses ......... Protective relaying in substations ......... Lightning and surge protection in substations ....................... .................. Substation grounding Substation ground mat ............... .............. Ground-fault protection Additional mine substation loads ......... Portable substations .................. ....... Utility voltage as mine distribution Additional substation design ..................... considerations References ......................... Chapter 14.4olid-state control and relaying .. ....................... Motor control ................ Simple motor control Control systems .................... ..... Physical characteristics of thyristors Direct current applications ............. .......... Alternating current applications ............... Static protective relaying Operation of simpwed solid-state and hybrid relays ...................... Static and electromechanical relay ....................... comparison Static relay mining applications .......... Sensitive earth-leakage system ......... Phase-sensitiveshort-circuit protection ... Solid-state relays in the future ........... .......................... Summary References ......................... Chapter IS.--Batteries and battery charging ... ... Basic battery and battery-charging theory Battery maintenance .................. .......................... Chargers Charging stations .................... Battery-box ventilation ................ 1.1. Simple mine electrical system arrangement . 1.2. Simple radial distribution system ........ 1.3. Power-center type of radial distribution ... 332 Battery surface leakage and faults ........ 375 ............... 332 Battery-charginghazards 377 ......................... 333 References 381 Chapter 16.-Permissibility and hazard .......................... reduction ........................ Terminology ............. Hazard-reduction methods Explosion-proof enclosures ............. Explosion transmission .............. ................... Enclosure joints Enclosure mechanical strength and internal pressures ....................... Enclosure hazards .................. ................ Permissible equipment ........ Permissible equipment schedule ... Maintenance of permissible equipment .................... Coal dust hazards ........ Classifications of dust locations ............... Reducing dust hazards Hazardous locations in preparation plants ......................... ......................... References ................ Chapter 17.-Maintenance ............. Mine maintenance program ............... Economicjustification Preventive maintenance program .................. implementation ..... Techniques of preventive maintenance ......... Basic electrical measurements ............. Insulation measurements ................. Megohmmeter tests .............. Mechanical measurements .......... Continuous-monitoring systems ............................ Corona 406 ................... Corona behavior 408 ................... Corona detection 409 .... Partial-discharge problems in mining 410 .................. Intermachiue arcing 411 ........... Ground direct current offsets 412 .......................... Summary 413 ......................... References 414 .......................... Bibliography 415 ...... Appendix.-Abbreviations and symbols 416 ............................... Index 420 ILLUSTRATIONS
- 10. Page .................................................. Primary-selective distribution system ........................................................... Primary-loop distribution .......................................................... Secondary-selectivesystem .................................................... Secondary-spot network technique ........................................ Representative utility transmission and distribution ..................................................... Subtransmission for surface mine Radial strip mine distribution system .................................................. ........................................... Secondary-selectivedistribution in strip mining Primary-loop design for strip mining .................................................. Radial distribution for strip mine with overhead poleline base line ............................. Radial distribution for strip mine with all-cable distribution .................................. Surface mine distribution system using two base lines ...................................... Openpitpowersystem ............................................................ ..................................................... Layout of underground coal mine ...................................................... Plan view of retreating longwall Subtransmission for underground mine ................................................. Radially distributed underground power system .......................................... Secondary-selective distribution in underground mines ..................................... Utilization in continuous mining section ................................................ Power-system segment with longwall equipment .......................................... ........................................ Diagram of electrical-systemsegment for longwall Parallel-feed haulage system ........................................................ Representative expanded radial distribution for preparation plant ............................. Representative secondary-selectivedistribution for preparation plant ........................... Circuit element illustrating voltage polarity and current flow direction .......................... Simpleseriescircuit ............................................................... Ideal and actual voltage sources ...................................................... Circuit for example 2.1 ............................................................. Demonstration of Kirchhoffs current law ............................................... Simple parallel circuits ............................................................. Ideal and actual current sources ...................................................... Parallel circuit for example 2.2 ....................................................... Simple series circuit and equivalent ................................................... Simpleparallelcircuit ............................................................. Series-parallel circuit for example 2.3 .................................................. Series-parallel circuit for example 2.4 .................................................. Magnetic flux in a straight conductor and in a long coil ..................................... Demonstration of induced current .................................................... Two coils demonstrating mutual inductance ............................................. Long-coil inductance and inductor symbols .............................................. Toroidalcoil .................................................................... Charge. voltage. and current relationships of capacitor ..................................... Electric lines of force between two parallel charged plates .................................. Resistor used to demonstrate instantaneous power ........................................ Simple example of idealization and concentration ......................................... Modeling of load center. trailing cable. and shuttle car ..................................... Basic elements of resistance. inductance. and capacitance ................................... Sirnplilicalion of dc circuit .......................................................... 2.25. Simple circuit reduction ............................................................
- 11. Page ............................................................ Cicuitforexample2.5 ............................................................ Circuitforexample2.6 ............................................. Series-parallel conductancesfor example 2.7 Series-parallel circuit for example 2.8 .................................................. Two-terminal and three-terminal networks .............................................. ................................................... Wye and delta circuit configuration "T"and "nucircuit configurations ..................................................... ............................................................ Commonbridgecircuit ..................................................... Circuit reduction of bridge circuit ................................................................... Partsofcircuit Circuit demonstrating two independent loops ............................................ .................................................... Two-loop circuit for example 2.11 Bridge circuit demonstrating loop analysis .............................................. Three-loop circuit for example 2.12 ................................................... Simple two-node circuit ............................................................ Three-junction circuit ............................................................. Three-junction circuit with grounds ................................................... ........................................ Voltage-source circuit demonstrating node analysis Circuit for examples 2.13, 2.15, and 2.16 ................................................ Circuit for example 2.14 ............................................................ Circuit for demonstrating superposition theorem .......................................... Circuit in figure 2.44 with sources turned off ............................................. Demonstration of reciprocity theorem ................................................. Practical voltage-source model ....................................................... Practical current-source model ....................................................... Source transformation ............................................................. ....................... Circuit in figure 2.44 with current sources transformed to voltage sources ............................................................... Thevenin'stheorem ................................................................ Norton'stheorem Comparison of Thevenin's and Norton's circuits .......................................... Circuit for example 2.17 ............................................................ Active circuit for example 2.18 ....................................................... Circuits illustratingsolution steps to example 2.18 ......................................... Some time-varying electrical waves .................................................... Sinusoidal ac waveform ............................................................ Steady ac showing phase shift ........................................................ Steady ac through resistance ........................................................ ........................................................ Steady ac through inductance Steady ac through capacitance ....................................................... Simple series RL circuit ............................................................ Simple series RC circuit ............................................................ Simple series RLC circuit .......................................................... Graphical representation of complex number ............................................ Trigonometric or polar representation of complex number .................................. Sinusoid versus time and as phasor .................................................... ............................................ Phasor representation of current and voltage Other expressions for phasors ....................................................... .................................... Voltage-current phasor relationships for circuit elements ........................................ Steady sinusoid analysis of simple RL series circuit ........................................ Steady sinusoid analysis of simple RC series circuit
- 12. Page ....................................... 2.76. Steady sinusoid analysis of simple RLC series circuit 2.77. Circuit for example 2.21 ............................................................ 2.78. Circuit for example 2.22 ............................................................ .................................................... 2.79. Two-loop circuit for example 2.23 2.80. Activecircuitforexample2.24 ....................................................... ...................................... Power represented as real and imaginary components Illustration of leading and lagging power factors .......................................... ........................................... Circuit demonstrating sum of complex powers ................................................ Simple series RLC circuit for resonance Plot of impedance magnitude versus frequency for series RLC illustratingresonance ............... Circuits that exhibit parallel resonance ................................................. .............................................. Magnetic coupling between two conductors Magneticcouplingbetweentwocoils .................................................. Demonstration of coil winding sense .................................................. Dot convention for mutal inductance sign ............................................... Demonstration of impedance transfer in transformers ...................................... Ideal transformer with winding resistance included ........................................ Accounting for transformer leakage flux ................................................ Transformer magnetizing current ..................................................... Eddy current and magnetic hysteresis creating power loss in core ............................. .............................................. Equivalent circuit of practical transformer Common power-transformer construction techniques ...................................... Movement of exciting components to input .............................................. ........................................... Transferring secondary components to primary Final simplification of pratical circuit model ............................................. Transformer parameter test series .................................................... ............................................................ Circuit for example 3.8 Comparison of two-winding transformer and autotransformer ................................ Two-winding transformer as an autotransformer .......................................... Examples of transformers for multivoltage applications ..................................... TwotypesofCT's ................................................................ Examples of CT and PT placement in circuit ............................................ Basicpowercircuit ............................................................... Applications of basic power circuit .................................................... Elementary three-phase generation .................................................... Three-phase voltage sources ......................................................... Wye-connected source demonstrating line-to-line and line-to-neutral voltages ..................... ................................................. Balanced three-phase load connections Four-wire wye-to-delta system ....................................................... Balanced delta load illustrating phase and line currents ..................................... Comparison of equivalent delta and wye loads ........................................... Three-single-phasetransformers connected for three-phase operation .......................... ................................... Three-phase diagrams for the transformers of figure 4 . 1 0 ........................................... Open-delta three-phase transformer operation Per-phase reduction of wye-to-wye system .............................................. Per-phase reduction of delta-to-delta system ............................................. ............................................................... Three-linediagram One-line diagram of circuit shown in figure 4.15 ..........................................
- 13. Page .................................... Commonly used symbols for one-line electrical diagrams ......................................................... Symbolsforrelayfunctions ..................................................... One-line diagram for example 4.7 Three-phase diagram of figure 4.19 ................................................... ..................................................... Per-phase diagram of figure 4.19 One-line diagram with delta-delta transformer ........................................... ..................................................... Per-phase diagram of figure 4.22 ............................................ One-line diagram with delta-wye transformer ....................................... One leg of three-phase transformer from figure 4.24 Approximate per-phase equivalent circuit for 750-kVA load-center transformer; impedance referred ................................................................... tohighside Transformer of figure 4.26 with impedance referred to low side ............................... Simplified equivalent circuit of transformer expressed in per-unit .............................. Approximate equivalent circuit of three-winding transformer expressed in per-unit ................. ........................................... One-line diagram of small mine power system Impedance diagram of system in figure 4.30, expressed in per-unit on a 1.000.kVA base ............. ............................................................ Basicfaultdescriptions Positive.sequence. negative.sequence. and zero-sequence vector sets ........................... Symmetrical component addition to obtain unbalanced three-phase set ......................... ....................................... Equivalent delta-connected and wye-connected loads ............................................ Three-phase system with line-to-neutral fault ........................................... Symbol and operation of a p-n junction device ............................................. Bias conditions and current flow for a diode Diode or rectifier characteristic curve .................................................. ............................................... Half-wave rectifier circuit and waveforms ................................................ Single-way full-wave rectifier waveforms Bridge rectifier circuit and waveforms ................................................. Example of filtering a rectifier output .................................................. Heatsinkcooling ................................................................ Heat sink thermal relationships ...................................................... Three-phase half-wave rectifier circuit and output voltage waveform ........................... Three-phase full-wave rectifier circuit with input and output voltage waveforms ................... .................................... Parallel operation of rectifiers using paralleling reactors An n-pn junction transistor ......................................................... .......................................................... A p-n-p junction transistor ......................................... Current relationships for p-n-p and n-p-n devices Common-baseamplifiers ........................................................... .......................................................... Common-emitter amplifier ................................................. Common-emitter characteristic curves Bias techniques for common-emitter amplifiers ........................................... ................................................ Common-collector amplifier arrangment ................................................. Model and symbols for junction FET's ................................................. Example of a junction-FET application .............................................. Model and symbols for MOS-FET devices SCRmodelandsymbol ............................................................ .................................................... SCR equivalent model and circuit .................................................. General characteristic curve for SCR ............................................. Sketch of simple monolithic IC cross section TopviewofanactualIC ...........................................................
- 14. Page ................................................ Examples of symbols employed for IC's .............................................. Permanent-magnet moving coil movements .......................................... Shunting d'Arsonval meter for high-current tests .......................................... D'Arsonval meter used to measure dc potentials ....................................... External shunts used for high-current measurements Simpleohmmetercircuit ........................................................... Rectifier ammeter ................................................................ ................................................. Dynamometer connected as wattmeter Power-factor movement ............................................................ .............................................. Simple instrument-transformer connections ........................... Voltmeter. ammeter. and wattmeter arranged as single-phase system .................................... Use of transducers with standard d'Arsonval movements ................................................... Three-phase wattmeter connections Two-wattmetermethod ............................................................ ......................................... Three-phase power measurement with transducer ...................... Balanced three-phase measurement of voltage. current. and average power Line current measurements with two or three CT's ........................................ ................................... Line-to-line voltage measurements with three or two PT's ................................... Simplified sketch of watthour meter induction mechanism Wheatstonebridgecircuits .......................................................... Kelvindoublebridge .............................................................. .............................................. Megohmmeter testing insulation resistance ................................................. Internal components of megohmmeter Phase-sequenceindicator ........................................................... ............................................................... Strip-chart recorder .................................................. Input circuits on electronic voltmeter Digitaldisplay ................................................................... Cathode-raytube ................................................................. .................................................. Semiconductor illustrating Hall effect ............................................... Production of voltage from magnetic field ...................................................... Demonstration of ac generation ................ Cross section of machine with salient poles on stator and nonsalient poles on rotor ............................ Cross section of machine with nonsalient poles on stator and rotor .................. Simplified sketch of electromechanical machine illustrating physical components .......................................... Elementary four.pole. single-phase ac generator ............................................. Elementary two.pole. three-phase generator ............................................ Elementary four.pole. three-phase generator ...................................................... Demonstration of dc generation .................................... Dc generator with two armature windings at right angles ...................................................... Separately excited dc generator Seriesdcgenerator ............................................................... Shuntdcgenerator ............................................................... Compounddcgenerator ........................................................... ........................................... Current-carrying conductor in a magnetic field .............................................. General speed-torque motor characteristic ............................................ Examples of three frame number dimensions ............................................ Demonstration of induction-motor operation ............................................... Elementary three-phase induction motor ......................................................... Squirrel-cage rotor winding
- 15. ..................... Rotating magnetic field in elementary three.phase. two-pole induction motor ................................................ Induced rotor potential by rotating flux ............................................ Lapped windings of three-phase motor stator Characteristic curves of three-phase induction motor ....................................... ........................ Typical torque-speed characteristic for general-purpose induction motor ........................... Phasor diagrams of rotor and stator flux density for induction motor Typical torque-speed characteristics for NEMA-design three-phase squirrel-cage motors ............ Other rotor-conductor designs ....................................................... ...................................................... Across-the-line magnetic starter Starting methods for induction motors ................................................. .................. Schematic of wound-rotor induction motor showing external resistance controller .............. Torque-speed characteristics for wound-rotor motor with stepped-resistancecontroller ............................... Simplified step starter using individually timed magnetic relays ............................... Sketch showing construction of salient-pole synchronous motor ..................... Simplified diagram of synchronous motor using generator for field excitation ................................... External solid-state supply used to provide field excitation ................................. Schematic of low-speed cylindrical-rotor synchronous motor ................... Controller used to demonstrate general starting method for synchronous motor .................. Typical torque-speed characteristic for synchronous motor with damper winding Effect of load on rotor position ...................................................... Equivalent per-phase circuit of a synchronous motor and phasor diagrams for underexcited and overexcited field winding ......................................................... V-curves for synchronous motor ...................................................... ........................................ Plan view of typical mining shovel showing m-g set Elementary two-pole dc motor ....................................................... Elementary four-pole dc motor ...................................................... .................. Cross-sectional sketch of dc motor showing interpole and compensating windings ................. Interaction between armature and main-field flux to produce main-field distortion Four connections for dc motors ...................................................... Typical characteristics for shunt. series. and compound motors of equal horsepower and speed ....................................................................... ratings .................................... Simplified dc motor schematicswith starting resistances Faceplate manual starter ........................................................... Multiple-switch starting ............................................................ Drum-typestarter ................................................................ ............................... Simplified diagram of dynamic braking applied to shunt motor ........................................ Two-step resistance starting of series-wound motor ......................................... Forward-reverse switching of series-wound motor Dynamic braking applied to series-wound motor .......................................... ............................................ One-step starting of compound-wound motor Basic WardLeonard system ......................................................... ............................ Typical characteristic curves for each motor in traction locomotive Stator field of two.pole. single-phase induction motor ...................................... ............................. Rotor field of stationary two.pole. single-phase induction motor Phase relationshipsbetween stator and turning rotor ....................................... Starting and running stator windings ................................................... ............................................ Centrifugal switch to remove starting winding Capacitor-start motor ............................................................. Illustration of electrical shock hazard .................................................. ............................................. Capacitance coupling in ungrounded system
- 16. Page Solidlygroundedsystem ............................................................ 160 Resistance-grounded systcm ......................................................... 160 Effect of frequency on let-go current for men ............................................ 162 ............................................... Simplified one-line diagram of substation 163 ................................................ Step potentials near grounded structure 163 Touch potentials ncar grounded structure ............................................... 163 ~ine-to-earthfault resulting in current flow through safety ground bed ......................... 163 Lightning stroke to equipment causing current flow through safety ground bed ................... 164 Lightning stroke current through system ground bed causing elevation of safety ground bed .......... 164 One-line diagram of simplified mine power system ........................................ 164 Mied ac-dc mine power system; dc load energized from trolley system ......................... 165 System grounding with current-limiting resistors .......................................... 165 Diode grounding of machine frame ................................................... 165 Resistance of earth surrounding electrode ............................................... 166 Decrease in earth resistance as electrode penetrates deeper soil horizons ........................ 167 Calculated values of resistance and conductance for 314-in rod driven to depth of 25 it ............. 167 Calculated values of resistance and conductance for 314411 rod driven to depth of 100 ft ............ 167 Nomogram to provide resistance of driven rod ........................................... 168 ........................................... Resistance of one ground rod. 314411 diameter 168 Resistance of parallel rods when arranged in straight line or circle with spacing equal to rod length ....................................................................... 168 Variation of earth resistance as numbcr of ground rods is increased for various spacings between rods ........................................................................ 168 Values of coefficient kl as function of length-to-width ratio of area ............................ 169 Values of coefficient k2as function of length-to-widthratio of area ............................ 169 Influence of first-layer height of potentials .............................................. 171 Potential on ground surface due to rod 6 ft long and 1-in diameter buried vertically at various depths ....................................................................... 172 Potential on ground surface due to strips. 1in by 0.1 in. of various lengths buried horizontally at depthof2ft .................................................................. 172 Measuring resistance of grounding system .............................................. 173 Concentric earth shells around ground connection being tested and around current electrode ......... 173 Correct spacing of auxiliary electrodes to give true resistance within 2.0% ....................... 173 Resistivity range of some rocks. minerals. and metals ...................................... 174 Variation in soil resistivity with moisture content ......................................... 175 Typical resistivity curves of solutions ................................................... 175 Diagram for four-electrode resistivity survey showing lines of current flow in two-layer earth ......... 176 Connections for Wenner four-terminal resistivity test using megohmmeter ....................... 176 Typical curve of resistivity versus elcctrodc separation ...................................... 176 ~ . ...................................... Reduction in ground mat resistance by soil treatment 177 Seasonal resistance variations attenuated by soil treatment .................................. 177 Trench model of soil treatment ...................................................... 177 Voltage gradients in earth during ground-fault conditions ................................... 178 . . Delta secondary with rig-zag grounding ................................................ 180 ..................................... Delta secondary with wye-delta grounding transformer 180 Cable distribution in underground coal mines ............................................ 182 ................................................ Cable distribution in surface coal mines 183 Shieldtypes ..................................................................... 186 ......................................... Cross sections of round unshicldcd mining cablcs 188
- 17. Page Cross sections of flat unshielded mining cables ........................................... Cross sections of some shielded mining cables ........................................... Round unshielded mining cables ..................................................... Flat unshielded mining cables ........................................................ Round shielded mining cables ....................................................... Cable types for typical distribution systems in underground coal mines ......................... Cable types for typical distribution systems in surface coal mines .............................. Cable terminations for applications up to 15 kV .......................................... .............................................................. Couplercomponents Simplified one-line diagram for situation described in example 8.4 ............................. Allowable short-circuit currents for insulated copper conductors .............................. Representative end-suspension termination for borehole cable ................................ Messenger wire supports for mine power-feeder cable ..................................... Splice layout using template for staggered connections ..................................... Effective method for removing unwanted insulation ....................................... Staggering splice connections ........................................................ Examples of popular connectors and connections used in splices .............................. Reinsulating power conductors with soft rubber tape ....................................... Typical taped splice in high-voltage shielded cable ........................................ Trolley-wire cross sections .......................................................... Typical trolley-wire and feeder-cable supports ............................................ Trolley-wire semicatenary suspension .................................................. Trolley system accessories .......................................................... Theoretical resistance of bonded joint ................................................. Pole strength calculations ........................................................... Guy and log-anchor calculations ...................................................... ............................. Typical arrangements and pin-insulator spacings on wooded poles Typical system fault current ......................................................... Steps in circuit interruption ......................................................... Arc between two contacts .......................................................... Load-breakswitch ................................................................ Extinguishing arc by increasing the length ............................................... Metal-barrier arc chute assists in arc deionization ......................................... ....................................... Insulated-barrier arc chute used with mametic field - Molded-case circuit breaker components ............................................... Magnetic-trip relay ............................................................... Adjustable instantaneous setting ...................................................... Thermal-magnetic action of molded-case circuit brcakcr .................................... ............................. Time-current characteristics for thermal-magnetic circuit breakers Shunt-trip and undervoltage-release accessories .......................................... Construction and operation of dead-tank OCB ........................................... Turboaction are chamber for OCB's ................................................... Cross section of minimum-oil breaker ................................................. ............................................................. CrosssectionofVCB Operating mechanism for vacuum interrupter ............................................ VCB assembly incorporating a load-break switch ......................................... ........................................................... Common cartridge fuses Inside view of dual-element fuse ...................................................... Current-limiting action of fuses ......................................................
- 18. Energy-limitingactionoffuses ....................................................... High-voltage power fuse and support .................................................. Fusible element under spring tension in high-voltage fuse ................................... ............................................. Cross section of boric acid power fuse refill .............................................. Disassembled refill unit for boric acid fuse Load-break switch with interlocked high-voltage fuses ...................................... ............................................................. Relay contact symbols Temperature-monitoring protector .................................................... Electromechanical-thermal relays ..................................................... ......................................................... Solenoid and clapper relays Polarrelay ..................................................................... ........................................................ Common induction-disk relay ...................................... Front view of induction-disk relay removed from case ...................................... Inverse-time curve compared with definite-time curve Various time characteristics of induction units ........................................... Family of inverse-time characteristics .................................................. ........................................................... Cylinder directional relay Directional overcurrent relay using induction-diskrelay and cylinder relay ....................... ......................................................... Direct relaying in ac system ....................................................... Potential-relaying connections Differential-relayingconnections ..................................................... ....................................................... Dc dircct-relaying connections ...................................................... Typical control wiring for UVR Typical control wiriig for shunt-tripping element ......................................... ..................................... Three-phase overcurrent and short-circuit connections TwoCTapproaches .............................................................. Neutral-resistor current-relaying scheme ................................................ ............................................... Neutral-resistor potential-relaying scheme ................................................ Zero-sequence ground relay connections Ground relay in residual connection ................................................... ............................................................ Broken-delta protection .................................................... Series loop ground-check monitor Transmitter loop ground-check monitor ................................................ .................................................... Bridge-type ground-check monitor ....................................................... Pilotless ground-check monitor ........................... Some difficulties associated with ground-check monitoring in mining ...................................... Pilot interlocking circuit using ground-check monitor Simple surface mine power system illustrating protective relaying ............................. Typical schematic for three-phase molded-case circuit breaker with ground-overcurrent and .......................................................... ground-check protection One-line diagram of simple underground mine power system illustrating protective circuitry .......... ....................................... Diode-grounded system with possible fault indicated Basic grounding-conductor system .................................................... Relayed groundig-conductor system .................................................. Neutral-shiftsystem ............................................................... Current-balance dc ground-fault relaying using saturable reactor .............................. Current-balance dc ground-fault relaying using saturable transformer .......................... ........................................... Fault current waveform illustrating asymmetry ... Multiplying factors applied to three-phase faults to obtain momentary ratings for switching apparatus
- 19. Page Multiplying factors applied to three-phase faults to obtain close-and-latch ratings for switching apparatus .................................................................... ................................................. One-line diagram for fault calculations ..................................... Impedance diagram for one-line diagram of figure 10.4 ......................................................... Simplificationoffigure10.5 ......................................................... Simplificationoffigure10.6 ................................................. Further reduction of example network ...................................................... Equivalentcircuitoffigure10.6 ....................................... Example problem with motor contribution neglected Network to calculate momentary or close-and-latchcurrent duties ............................. Fault current in dc system .......................................................... .............. Available fault current versus distance of fault from rectifier on typical trolley systems ............................................ One-line diagram for pickup setting example ........................................................ Model of CT and its burden .......................... Typical set of saturation curves for 600/5 multiratio bushing-type CT ................. Example of one-line diagram for preparing a coordination curve plot for one path ............ Coordination curve plot for figure 10.17 showing various protective-device characteristics ..................................... Schematic representation of lightning stroke discharge Distribution of crest currents in lightning strokes ......................................... .................... Map showing average number of thunderstorm days per year in United States Striking distances for negative and positive strokes ........................................ ............................... Crest voltages induced on transmission lines by nearby strokes ............................. Simple circuit to illustrate capacitance-switching voltage transients .......................... Voltage and current waveforms before and after current interruption Voltage and current transient waveforms occurring with capacitance switching and restrike .......... Per-phase diagram of 4.16O . V pump-motor circuit ........................................ ........ Voltages and current wavesforms resulting from multiple restrikes after capacitance switching Graphic example of current chopping by breaker interruption ................................ Equivalent circuit of power-system segment with lumped components per phase. neglecting resistance .. Graphic example of chopping voltage transients .......................................... Segmentofminepowersystem ...................................................... Circuit to demonstrate voltage transients in dc system ...................................... Transient overvoltage resulting from current interruption on dc system ......................... An undergrounded system. showing capacitive-current flow .................................. An undergrounded system. with fault on phase A ......................................... The distributed inductance and capacitance of two-wire l i e shown as incremental sections .......... Demonstration of traveling wave on overhead line ........................................ ................................. Incident waves being reflected and refracted at discontinuity Electric field between conductors ..................................................... A 1.2 x 50 wave test used for BIL measurement .......................................... Equivalent circuit of multiturn winding showing distribution inductance and capacitance ............. .......................... Initial voltage distribution across uniform winding from step function ....................... Capacitive coupling of transient voltage through two-windiig transformer Basic valve surge arrester ........................................................... ................... Surge arrester with nonlinear resistance grading to equalize each gap structure Surge approaching surge-arrester-protectedequipment ..................................... .................... Typical surge protection of rotating machinery and dry-insulated transformers Simplified sketch of mine power-system segment .........................................
- 20. ............................................... 11.32. Capacitance for 2,300.V induction motors ............................................ 11.33. Capacitance for 2,300.V synchronous motors 11.34. Overhead ground-wire shielding for low and high distribution towers ........................... 11.35. Static-wire-protection designs of wooded support structures using 30 protective angle ............... 11.36. Ratio of impulse to 60-Hz resistance as a function of peak impulse current. for driven rods .......... 11.37. Impulse breakdown of sand for two moisture conditions using spherical electrodes ................. 11.38. Impulse characteristics of spherical electrode, with seven attached pointed protrusions of various lengths ..................................................................... Typical power centers used in underground wal mines .................................... Schematic illustrating major components in power center .................................. Top view of mine power center showing placement of many internal components ................. Interconnections between input and feedthrough receptacles ................................ Graph illustrating transient crest voltage caused by ribbon-element current-limiting fuse operation .... Comparison of transformer withstand characteristic and surge arrester withstand characteristic ....... Typical primary winding taps on power cable transformer .................................. Zig-zag grounding transformer ........................................... ................................. Delta-wye connection for deriving a neutral Technique for measuring transformer impedance ............................. ............................... Typical X/R ratio versus transformer capacity Typical mine power-center transformer undcr construction . . . . . . . . . . . . . . . . . . . . . . ................................. Completed transformer prior to installation Typical bus work in powcr ccntcr undcr construction .......................... Typical conductor connection to molded-case circuit breaker ..................... Zero-sequence relaying on outgoing circuit with control connections to breaker ....... Zero-sequence relaying with jumpcr in relay case ............................. Neutral relaying applied to grounding-resistor current as backup protection .......... Backup protection devices associated with mine power cables .................... ................................ Typical test circuit for zero-sequence relaying Simple control circuit incorporating one ground-fault relay and one ground-check relay . Simple convenience-outlet circuit for 120- or 240-V single phase .................. .................................................................. Fusemountings .......................................... Typical metering circuit for line-to-line voltages ............................................... Typical metering circuit for line currents .................................................... Typical impedance monitor circuit Block diagram of continuity monitor connected in pilotless mode ............................. ................................ Block diagram of continuity monitor wired for pilot operation ................................. Application of power-factor correction in mine power center General arrangement of dc components for combination power center ......................... Full-wavebridgerectifier ........................................................... .................................... Series reactance to reduce available short-circuit current Separate transformer to increase impedance of dc circuit ................................... Typical full-wave bridge rectifier with two diodes in parallel per leg ............................ .................................................... Diode with RC snubber protection Diode-groundedsystem ............................................................ .................................................... Basic grounding-conductor system Relayed grounding-conductor system .................................................. ............................................................... Neutral-shift system ......................................................... Differential current scheme ............................................... Representative control circuit for rectifier ........................................................ Cross section of dc contactor
- 21. Page ................................................. Diagram for typical single switchhouse ................................ Control circuitry for single switchhouse using battery tripping Diagram for typical double switchhouse ................................................ .............................. Control circuitry for double switchhouse using capacitor tripping ............................................ Typical family of curves for inverse-time relay Illustration of fault location for adjusting selectivity ........................................ .......................... Typical control circuit for double switchhouse using capacitor tripping ............................ Typical control circuit for single switchhouse using battery tripping ............................................ Overall view of main substation serving mine Radial distribution applied to underground mine and its surface facilities ........................ .................... One-line diagram for single-ended substation with fuse-protected transformer ............ One-line diagram for single-ended substation with circuit-breaker-protected transformer Simplified one-line diagram for doubled-ended substation ................................... ........................................ Typical liquid-immersed transformer in substation ....................................................... Dead-tank OCB in substation Standard percentage-differential relaying system for transformer protection ...................... ........................... One-line diagram of substation with percentage-differential relaying Insulation characteristic of liquid-immersed transformer compared with the characteristic of valve ................................................................. surgearrester Plan view showing locations of system and safety ground beds ................................ Typical system ground bed for large substation ........................................... ........................................... Typical system ground bed for small substation ................. Substation feeding both surface and underground loads (no pounding conductor) ..................................... Substation feeding both surface and underground loads ......................................... Typical portable substation to service small mine .......................... Providing mine ground and protective relaying from utility substation ........................................ Use of isolation transformer with utility substation ................................................. Model and circuit symbol for thyristor ............................................... Typical characteristics curve for thyristor ......................................................... Thyristor half-wave rectifier ................................................... Alternating current thyristor control Three-phase control with bidirectional thyristor arrangement ................................. .................................................... Full-wave thyristor bridge rectifier ............................................... Three-phase thyristor-controlled rectifier Simplifiedchoppercontrol .......................................................... ................................................... Basic control-system block diagram Simplified block diagram of a motor controller ........................................... ..................................................... Common thyristor configurations .................................................... Heat sinking of disk-type thyristors Block diagram of ac-dc shuttle car .................................................... ............................................... Block diagram of ac-dc continuous miner ..................................................... Simple variable-frequency control .......................................................... Elementary inverter circuit ................................. Use of variable-frequencydrive on production mining shovel Simplified diagram of current-regulated static belt starter ................................... Simplified diagram of linear-acceleration static belt starter .................................. ....................................................... Types of thyristor firing pulses Thyristor protection for static belt starters .............................................. ......................................................... Protective-relay connections ....................................................... Simple electromechanical relay
- 22. ................................................................ Simplestaticrelay Transistor used as relay ............................................................ .......................................................... Optical transistor as relay ............................................................. Thyristor used as relay Triacusedasrelay ............................................................... ............................................................... Hybrid static relays ....................................................... Simple overcurrent static relay ................................................... Simplified sketch of the SEL system Simplified sketch of the multipoint SEL system ........................................... ................................................ Diode-bridge phase-sensitive protection Equivalent model of figure 14.33 ..................................................... Electronic-comparator method of phase-sensitive protection ................................. Digital-controlled continuous static relay used for timed overcurrent ........................... Composition of lead-acid storage battery in various states of charge ........................... Voltage per cell of a typical lead-acid battery with varying continuous rates of discharge ............ Typical charging process of cell from 18.cell. 725-Ah battery ................................. Simplified schematic of saturable-reactor charger ......................................... Simplified schematic of single-phase thyristor charger ...................................... Two-winding transformer model ...................................................... Representation transformer magnetization curve .......................................... ..................................................... Ferroresonant transformer model .......................................................... Ferroresonant transformer ....................................................... Ferroresonant battery charger Plan of underground charging station .................................................. Circuit for detecting faults in batteries ................................................. Curve of relay current for various fault positions on battery .................................. ............................................ One-line diagram of desired charger features Cross-sectionalsketch of typical explosion-proof enclosure .................................. Typical plane-flange joint ........................................................... Typicalstep-flangejoint ............................................................ Threadedjoint ................................................................... Tongue-and-groovejoint ........................................................... Blindscrewhole ................................................................. .............................. Pressure vent limiting pressure buildup during internal explosion Pressure vent assembly using metal-foam material ........................................ Typical slip-fit straight stufting box and packaging-gland lead entrance .......................... ................ Typical slip-fit angle stuffing box and packing-gland lead entrance with hose clamp Typical slip-fit angle stuffing box and packing-gland lead entrance ............................. ............................................... Typical plug for spare lead-entrance hole Typical threaded straight stuffing box and packing-gland lead entrance with provision for hose ...................................................................... conduit Prototype trailing cable entry with polyurethane grommet ................................... ......................................................... Insulated-stud lead entrance Decision flow chart of class 1 1 . division 1and 2 hazardous locations ............................ 17.1. Circuit modeling a dielectric ........................................................ ............................................ 17.2. Current-voltage characteristics in a dielectric ..... 17.3. Graph relating approximate insulation resistance variation with temperature for rotating machines
- 23. Page Insulation resistance versus application time of test voltage .................................. Megohmmeter test connectionsfor checking cable insulation in line A .......................... ............................................ Megohrnmeter test connectionsfor ac motor Megohrnmeter test connectionsfor dc motor ............................................ ................................................ Spot resistance curve for normal motor Spot resistance curve showing effects of dust and moisture .................................. ............................................... Spot resistance c w e for detective motor .......................................... Megohrnmeter test connections for transformer Tie-resistance curve ............................................................. Three time-resistance curves for deteriorating motor ....................................... Time-resistance curves showing polarization for hypothetical motor ............................ Polarization factor curve for deteriorating motor .......................................... ........................................... Multiple voltage curves for deteriorating motor Circuit for harmonic tests .......................................................... Power-factor versus voltage curves showing tie-up ......................................... Mounting techniques for two vibration transducers ........................................ Four typical vibration measurement points .............................................. Typical vibration severity chart ....................................................... Comparison of acoustic-emission techniques for detecting failing roller bearings ................... Conceptual diagram of generalized mine monitoring and control system ........................ Conductioningas ................................................................ ................................................ Discharge sequence in an ionizing field ............................................................. High-stress geometrics Typical dielectric voids in cables ...................................................... Block diagram for corona-detection system .............................................. High-voltage cable terminations ...................................................... Major insulation void sometimes found in high-voltage coupler terminations ..................... ....................................... Possible stress site in high-voltage coupler insulators Power-conductor transposition on three-conductor type G cable .............................. Application of diode-suppressionbridges in power center ................................... Typical saturable-reactor characteristic ................................................. TABLES SIsymbolsandunits .............................................................. ........................................... Resistivity of some common materials at 20 C IEEE device numbers and functions ................................................... ........................................... Device numbers and letters common to mining ............................................... Motor voltage ratings common to mining Motor insulation classes ............................................................ NEMA class A standard starters for three-phase induction motors ............................ ................................................ Common motors for mining equipment .................................. Current range and effect on a typical man weighing 150 Ib Typical resistances for various contact situations .......................................... Approximate resistance formulas for various electrode configurations .......................... Comparison of grounding grids with other types of electrodes ................................ General resistivity classification ...................................................... Variations in resistivity with geologic age ............................................... ............................................... Typical values of resistivity of some soils ......................................... Variation in soil resistivity with moisture content .... Typical potentials of metals in soil measured from a copper and copper sulfate reference electrode
- 24. Page ............................................... Conductor sizes and cross-sectional areas .................................................. Letters used in alphabetic cable code ................................................ Codes for typical cablcs used in mining Typical diameters for round portable power cables ........................................ ............................................... Typical diameters for flat portable cables ........................................ Specifications for trailing cablcs longer than 500 ft .................................................. Ampacities for portable power cables ........................................ Ampacities for three-conductor mine power cables ............................. Correction factors for ampacitics at various ambient temperatures ...................... Ampacity derating factors for 60 C-rated trailing cables operated on drums .......... Australian specifications for ampacity derating factors for trailing cables operated on drums Some estimated power factors and load factors for various underground coal mining equipment in goodopcratingconditions ......................................................... .............................................. intermittent-duty ratings for trailing cables ......................................... Resistance and reactance of portable power cable Resistance and reactance of mine-power-feeder cable ...................................... Solid-wire breaking strength ......................................................... ....................... Recommended minimum bending radius. unshielded or unarmored cables ......................... Recommcndcd minimum bending radius. shielded and armored cables Trolley-wire specifications .......................................................... .......................................... Characteristic data for solid copper feeder cable Characteristic data for stranded copper feeder cable ....................................... ................................................ Trolley-wire support spacings on curves Resistance of steel rail at 20 C ....................................................... Data for rail-bond cable ............................................................ Minimum vertical conductor clearances as specified by the NESC. applicable to mining and mining-related operations ......................................................... ....................... Minimum distances from overhead lines for equipment booms and masts .................................... Ratings for mining-service molded-case circuit breakers Interrupting-current ratings vcrsus system voltage ......................................... Maximum instantaneous-trip settings .................................................. ................ Commonly available magnetic-trip ranges for mining-service molded-case breakers .................................. Some typical ratings for low-voltage power circuit breakers Typical minimum-oil circuit breaker ratings ............................................. ................................................... Ratings of high-voltage power fuses ................................. Common current ratings of induction-disk overcurrent relays ............................................... Standard burden for current transformers Standard ratings for potential transformers .............................................. ................................... Sample reactances for synchronous arid induction motors .................. Three-phase transformer per-unit impcdanccs for liquid-immersed transformers ........... Three-phase transformers impedances for distribution transformers. including load centers Sample applications of fault calculations ................................................ .................................................... Impedance of cables in figure 10.4 .................................. Burdens of relay elements and ammeter connected to CT's Recommended instantaneous trip settings for 480.. 600.. 1 . 0 4 0.V three-phase trailing-cable protection .. ............ Recommcndcd instantaneous trip settings for 300- and GOO-Vdc trailing-cable protection Recommended station and intermediate surge arresters for resistance-grounded mine power systems to protect oil-immersed transformers ...................................................
- 25. Page Recommended distribution.class. RM.type. surge arresters for resistance-grounded mine power systems ................................... to protect rotating machinery and dry-type transformers Commonly used surge capacitors for limiting voltage rate of rise on rotating machinery and dry-insulated transformers ......................................................... Typical capacitances per phase of power-system components. for shielded power cable SHD. SHD.GC. andSHD+GC .................................................................. ................................. Typical capacitances per phase of power-system components Protective angle versus structure height ................................................ ........................................ Typical current ratings of 400-A load-break switch Typical ratings for combination power centers ........................................... ............................ Standard impedance for liquid-immersed three-phase transformers ...................................... Standard BIL's for oil-immersed power transformers Typical electromechanical and static relay characteristics .................................... .......................... Time-margin comparison between electromechanical and static relays ...... Comparison of induction-disk and static time-overcurrent relay burdens to a current transformer ............................................... Formulas to estimate hydrogen evolution ................ Structural gap dimensions for explosion-proof enclosures as specified by 30 CFR 18 ................... Minimum autoignition tcmpcraturcs versus layer thickness for bituminous coals Common causes of vibration ........................................................
- 26. By Lloyd A. Morleyl ABSTRACT This Bureau of Mines publication presents a comprehensive review of mine elec- trical power-systemtheory and practice. It discusses fundamental theory and the vital aspects to be considered in planning and designing mine electrical power systems. The report is divided intothree major sections. The first presents the history of electricity in mining and the fundamentals of electrical phenomena and components.The second focuses on power-system components: motors, grounding systems, cables, and protec- tive equipment and devices. The final section includes mine power-center equipment, switchhouses and substations, batteries, and mine maintenance. p~ ~-~ - ~ p 'Professor of mining engineering, The Pennsylvania StateUniversity,University Park, PA (nowprofessor and department head, mineral enginwring. University of Alabama, Tuscalwsa, AL).
- 27. CHAPTER 1.-ELECTRICAL POWER IN MINING Probably no other mining area has grown so rapidly yet been as little understood by the average mine worker or operator as the mine electricalpower system. Traditionally, the field has held little interest for the mining engineer, who has tended to avoid it, or for the electrical engineer, who has given it scant attention. But today's mine power system is both complex and subject to numerous legal con- straints, and it is no longer possible to treat it with the indifference of the past. Underground mining machines are among the most compact and rugged equipment over designed, and individ- ual units can have up to 1,000 total horsepower. Mining equipment is usually mobile and self-propelled; most is powered electrically through portable cables and, for safety, must be part of an elaborate grounding system. The ma- chines and power-distribution equipment are seldom sta- tionary, must be adapted to continuous cyclic operation, and must resist daunting levels of dust and vibration. Surface mining can involve the largest earth-moving equipment built, where one piece can have 12,000or more connected horsepower-the largest today is over 30,000 hp. The electrical loads created by this machinery are cyclic and extremely dynamic: the largest excavator, for exam- ple, can require electrical loads that range from 200% motoring to 100% generating every 50 to 60 s, under the most exacting physical conditions. In the ever-movingmin- ing operation where distribution of power must be con- stantly extended and relocated, subjected to abuse by machine and worker alike, the potential for safety hazards is always present. Engineering and maintaining such an electrical system is demanding and challenging. It requires a specialist with knowledge of both mining and electrical engineering. Yet conversely, the effective management of a mine requires that anyone responsible for production and safety also be conversant with the mine electrical system. Management should understand the advantages and disadvantagesof one system over another, for if the power system is poorly de- signed, not only will safety be compromised but the mine operator will pay for the resulting conditions with high power bills, high-cost maintenance, and loss of production. Too often, a new mine is designed to use the type of power system employed in the preceding mine, without a comprehensive power study to determine the system needs and examine the alternatives available. Problems arise in existing mines when new mining equipment has been adopted without due regard for its impact on the operating power system; these problems haunt the mine electrical engineer who must frequently cope with a system that is a mongrel, bred from diverse inheritances from the past combined with recent changes and additions. New laws, standards, and safety requirements must frequently be accommodated by power systems not originally designed to meet their specifications;new and unfamiliar equipment must be grafted to the existing network, and the result can be a hybrid of considerable complexity. This text has been produced to assist the power engineer and the student in understanding these complexities and the principles that lie behind them. The material presented here is structured so an indi- vidual unfamiliar with electrical engineering can first developthe necessary fundamentals before embarking into mine electrical design. A basic physics and calculus knowl- edge is necessary to understand the content completely. The goal has been to assemble the most significant information required for comprehension of mine power systems so that the reader may then progress to more specializedtopics. But first, a brief review of the development of electrical usage in mines is given, in order that the reasons for some of the peculiarities of mine power systems can be appreciated. MINE ELECTRICAL HISTORY Electricity was first introduced into coal mines shortly before the beginning of the 20th century in the form of di- rect current (dc)for rail haulage. This form of current was used because at that time most systems were powered by dc generators. It had a number of advantages for haulage; the most outstanding was that the dc series-woundmotor had (and has) excellent traction characteristics. Speed con- trol was a simple matter of placing a resistance in series with the motor armature or field circuits. Batteries served as the first power source, and hence the vehicle was extremely mobile even though constrained on rails. However, keeping the batteries charged was both- ersome, so trolley wires were soon introduced in several mines. Allowingthe trolley wire to act asoneconductorand the rail as the other provided the simplest form of power distribution yet known to the mining industry. Available haulage machinery of that period was low in horsepower and the mines were relatively small sothe increased resis- tance that reduced voltage and power supplied to the motors was still acceptable. Thus, the dc system at a voltage of 250 or 550 V became firmly entrenched in coal mines. Underground Mine History Underground, the first electrically driven coal mining machine, the coal cutter, was installed in the early 1920's. Although dc offered no special advantage, it was readily available; hence, the machine was equipped with a dc motor and added to the system. The cutter was followed almost immediately by the loader, and it too was driven by dc motors.I f there was rail haulage in the mine, trailing cablea supplied power from the trolley wire and the rail to the machines. The next significant increase in power consumption came with the introduction of the shuttle car, almost 20yr after the coal cutter. Actually, when the shuttle car was first invented in 1937, it was battery powered. The addi- tion of an automatic reeling deviceto handle a trailingcable came later, in an attempt to overcomebattery deficiencies. These trailing cables were also connected to the haulage power system, and this equipment, when combined with the cutters and loaders, placed additional stress on the dc distribution system. At that time, the horsepower required to operate each piece of electrical mining equipment was quite small and no individual machine used a large amount of current. However, when all machines were combined, significant power was required, and because all the conductors offered resistance. voltaee d r o ~ s and transmission losses in the distribution system we; extensive. Alternatingcurrent (ac) would have been more practical because it could have been
- 28. distributedeasily at a higher voltage, thereby reducing cur- rent, voltage drops, and transmission losses. But many States had stringent limitations on maximum voltage levels, usually around 300 V, and with this restriction ac had no advantage over dc. Hence, dc continued to he used to operate the successful combination of cutters, loaders, and shuttle cars. Development in ac-to-dcconversion equipment played an important role in underground coal mine power utiliza- tion throughout this period. Motor-generatorsor synchron- ous converters were originally employed for conversion pur- poises,but in addition to being heavy and bulky, they could not be operated effectively in a gassy and dusty atmosphere, and maintenance requirements were substantial. As a re- sult, most conversion installations were placed on the sur- face with borehole connections to the underground mine. This was acceptable as most mines were then relatively shallow. In the 1930's, the same decade that saw the inception of the shuttle car, mercury-arc-ignitionrectifiers began to be employed to provide dc underground. The arc tubes al- lowed more efficient use of electricity in deeper and larger mines than had previously been possible. As the tubes had no moving parts, maintenance was lower, efficiency was higher, and portability was improved. These rectifiers were usually centrally located in the mines because a liquid heat exchanger made them heavy and bulky. In this way, dis- tribution to the mine rectifier was ac, but distribution throughout most of the mine electrical system was still dc. At about the same time, some mines found that haulage of materials by conveyor couldbe more efficient than haul- age by rail. The conveyors were also powered by dc motors, and stress continued to be added to the electrical system. In the late 1940's, when continuous mining machines first began to be used extensively, dc was again expected toprovide the power. However, the continuous miners nor- mally needed more energy input than the sum of the various conventional mining equipment they replaced, and because the required horsepower created high current demand, dc was found to be entirely unsatisfactory in most cases. The attendant current demand created enormous voltage drops in the distribution system. As a possible solution, the dc supply system was separated from the haulage system, but eventhis was unable to improve voltage regulation.During peak operation periods, voltages at the machines were so farbelow the values called for that even moderate efficiency was impossible. The increasingly large cable sizes required to supply the needed power created difficult cable-handling problems. The use of three-phaseac distribution and motors was an obvious answer, but for at least a decade some min- ingcompanies were reluctant to make the change. In many instances this was because the laws in some States limited maximum voltages in the mine. Lawmakers were convinced that high voltages were synonymous with high safety risks. Some State laws were not updated until the mid 1960's. When higher voltages were finally permitted, the de- sirable economics of ac employment could be realized and there was a swift transformation from dc to ac for both distribution and high-horsepowerloads in underground coal mines. Unfortunately, many mine electrical systems were at least partially modified without concern for the compat- iblity of these changes with the remainder of the system, and various safety and production problems arose. As a result of conversions, mine power systems gener- ally had two voltage levels, one for distribution and one for utilization. The simplified mine electrical arrangement shown in figure 1.1illustrates the results. Here, the sub- station transforms the utility voltage down to distribution levels, which are most often at high voltage greater then 1,000V. Power at this voltage is transmitted or distributed through conductorsfrom the substation to the power center; hence, this system is called the distribution system. The power center or load center, actually a portable substation, transforms the voltage to utilization levels, which are typically at low voltage of 660 V or less, or medium vol- tage of 661to 1,000V. At this level, or face voltage, power is normally delivered to the equipment. Despite this ref- erence to voltage levels, it should be noted that distribu- tion and utilization describe functions of a power system segment, not specific voltage ranges. Originally, primary ac distribution was made at 2,300 or 4,160V. In most mines, these levels were later increased to 7,200 V. Someoperationsrecently increasedthe voltage to 12,470 or 13,200 V for both longwall and continuous- mining applications. Each new distribution voltage, it may be noted, is a factor of /3 times the previous value (012,300) = 4,160). The principal reason for increasing the voltage was that, for the same load, current would he cor- respondingly smaller, and lower distribution losses would result even though the same conductor sizes were used. From the beginning, 440 Vac was the most popular voltage for utilization, despite the fact that when the con- tinous miner proved so successful its horsepower was pro- gressively increased, following the sometimes misguided notion that a directly proportional increase in coal produc- tion would follow. As with dc, the additional horsepower resulted in an increase in trailing-cable sizes, until the weight of the cables was almost more than personnel could handle. To compensate, the most commonmove was to raise the rated motor voltages to 550 Vac. More recently, manu- facturers have produced machines with 950-V (550 (3) motors to further overcome the trailing-cable prohlems. While these changes were being made to ac for machine operation and distribution, the use of dc for haulage con- tinued to be advantageous. In the early 1960's,silicon diode rectifiers with large current capabilities became available. Simple, efficient, and small, these rectifiers were ideally suited for use underground and made ac distribution possi- ble for the entire electrical system except rail haulage. Through the use of rectifiers, the benefits of dc for traction and of ac for distribution and utilization on high power loads could be realized. For example, while continuous miners normally used ac, part of the supply at the power center was rectified to dc, primarily for powering the shuttle cars. These underground electrical systems appeared to be simple, and as a result they did not become the focus of at- tention for some time. Systems were frequently designed and maintained by a "seat-of-the-pants" approach, to the point that ac distribution and equipment were installed g$pe Substation Switchhouse center Power -. Utilization voltage voltage Figure 1.1.-Simple mine electrical system arrangement.
- 29. employingdc concepts.However,ac systemsare more com- plicated than dc systems and call for meticulous planning; if wrong decisions are made, the results can be extremely costlyin terms of safety,production, and economics.A great deal of effort is needed to maintain an electricalpower sup- ply within the requirements of the individual pieces of mining equipment, and mixing ac and dc in the samemine has added greatly to the problems. This brief review of the development of electrical sys- tems in underground coal mineshas shownthat the mines went from minor electrical usage with the introduction of rail haulage to almost total dependency on electricity in a period of 50 yr. In the same period, surface coal mining underwent changesthat were as substantial if less numer- ous. They were centered around the enormous growth in equipment size. Surface Mine History The first mechanizationof strip mining occurred in 1877 with the application of an Otis-typesteam shovel in a Pitts- burg, KS, mine ( 5 1 . 'This early attempt was somewhat un- successful,but it served as an important step in the evolu- tion of strip-miningmachinery.Severalsuccessfulattempts to use steam shovels and draglines were made in the next 30yr, and these proved that the surfacemining of coal was completely practical. In time, the advantages of electricity over steam became more apparent, and the first significant introduction of electric-poweredshovels was made in the early 1910's. Whereas dc series motors were universally employed in underground rail haulage, the first large motors used in surface mining were dc shunt wound because of their constant-speedcharacteristics. These motors almost directly replaced the single-speedsteam enginefoundon practically all shovels prior to that time and allowed an immediate reduction in work forcerequirements. Beforelong another important advance in shovel design occurred: the applica- tion of separate steam enginesto power the shovel motions of hoist, crowd, and swing. This change gave increased flexibility through the individualcontrolof eachoperation. In a shorttime, the two major shovel manufacturers of that era, Marion and Bucyrus, began toproduceboth steam and electricmultimotor shovelswith similar characteristics(3. Sinceseries-wounddc motorshad speed-torquerelationships similar to those of steam engines when they were used for this type of duty, they were employedto drive each shovel motion. The initial distribution for electric shovels was dc be- came of the nature of the power generation, but technolog- ical advances soon made ac power systems superior, and ac motorswere tried with somesuccess.However,by 1927, ac-dc motor-generator(m-g)sets and the invention of the Ward-Leonard control concept caused these efforts to be abandoned. The new control system enabled the motor characteristics to be modified as desired within the motor and generator commutator limits, and as a result, separately excited dc motors became more attractive than series-woundmotors. The m-g sets functioned as on-board power-conversionunits, thereby establishing the use of ac distribution in surface mines. Motor-generatorsets driven by synchronousor induc- tion ac motors, Ward-Leonard control, and separately ex- 'Italicized numbers in parentheses refer to items in the liat of references at the end of this chapter. cited dc motors establishedthe standard, and even now the combinationis used on most mining excavators,especially the larger varieties (2).On smaller machines, some single ac electric-motordrives with either mechanical-frictionor eddy-current clutch systems have evolved, but these are often driven by diesel engines. Present excavating equipment is generally classified into three sizegroups,although actual capacityranges are normally not assigned. Small shovels are used primarily in general excavation,while the intermediate types work at bench mining and coalproduction,and large shovelshan- dle overburden stripping.Draglinesof all sizes are used only for stripping. Small and intermediateequipment originally ran on rails, but crawler mountings that give improved mobility made their first appearance in 1925 (5).Today, small and intermediate-sized shovels and draglines are mounted on two crawlers, while large shovels have eight crawlers(2).Large draglines and some intermediate sizes are usually walking types that feature a circular base or tub that provideslow ground-bearingpressure and a walk- ing device for mobility. The design of surface mine drilling equipment paral- leled excavator development. Initially, most drills were pneumatic-percussion types, but because electricity was readily available in mines, some machines were designed with internal motor-driven compressors. By the early 1950'8, large rotary drilling equipment was necessary to satisfy the blasting requirements of thick, hard overbur- den (6). This drilling equipment was again electrically powered and was very successful. The most outstanding change that has taken place in electrically powered surfacemining equipmenthas been in connected horsepower. For example, a 25-ydSdragline or stripping shovelthat had a maximum total load of around 2,000 hp was consideredenormous in the late 1940's(5,9). By 1955,50-to 70-ydsexcavatorswere being manufactured with maximum horsepower at 4,650 hp. Five years later, shovels had reached a 140-ydS capacity wth 12,000hp of main drive motors ( 7 ) .In 1976, the largest excavator in service had 20,000 hp in m-g set drive motors (4). Distribution and utilization voltages also increasedto keep pace with the peak load demands of this machinery. Sometimesthe mine distribution and machine voltages for these excavatorsremainedthe same.Until the mid-1950's, 4,160 and 2,300Vwere the usual mine levels(9).Then,with the advent of larger concentrated loads, 7,200 V was con- sidered advisable(10).However,this level was found to be unsatisfactory for the newly introduced machines with a capacity larger than 100 yda, and so 13,800-V mine and excavatorvoltage became a standard. With machineshav- ing greater then 200-yd8capacity, 23,000-Vutilization was established (4),but even with these substantial increases in distribution, some loads up to 1,000hp continued to be driven at 480 V (10). Production shovels with loads up to 18 yds commonly stayed at 4,160 and 7,200 V, while in general, 4,160V became standardized for machinery with 1,500 hp or less. As a result, more than one voltage level could be required at a mine when excavators of different sizes were employed. MINE POWER EQUIPMENT A few pieces of power equipment have already been mentionedbut onlyto the extent necessary to describe the concepts of distribution and utilization. The evolution of
- 30. mine systems has resulted in major items of power appa- ratus, each servinga specificfunction(1,9).In general,they can be listed as Generation, Main substations, Portable and unit substations, Switchhouses, Distributiontransformers and power (or load)centers, and Distribution (conductors and connectors). The following paragraphs explain these components only in sufficient detail for their inclusion in system ar- rangements to be understood. More detailed descriptions of substations, switchhouses,and power centers are pre- sented in chapters 12 and 13, while chapter 8 is devoted to distribution.Power generationisbeyondthe scopeof this text, but Ehrhorn and Young (13)provide a thoroughdiscus- sion of generation related to mining. Substations It is common mining practice to purchase all or most power from utility companies if it is available. As utility voltagesusually range from 24to 138kV, a main (primary) substation is required to transform the incoming levels down to a primary distribution voltage forthe mine. In ad- dition to having the transformer. substations contain a complex of switches,protection apparatus, and grounding devices, all having a function in safety. Main substations are often installations. l'he nature of the min- ing operation and its power needs dictate how many main substationsare required and where they shouldbe placed. They may be owned by the utility or the mining company; the decision of ownership is commonly dependent on eco- nomics. However, if the total connectedload is greater than 1,000 hp, mine ownership is often more favorable (13). Portable and unit substations are similar in operation to main substations except they serve to transform the primary distribution voltage to a lower distribution level. The term "unit" means that the substation and power equipment are designed and built as a package. In a typi- cal strip-mining deployment, a large dragline may require 24 kV while the productionshovelsand other miningequip ment need 4,160 V. Switchhouses Switchhouses are ort table eaui~ment that ~rotect the distributioncircuits.fheir intekalcomponent&e chiefly protection devices, with circuit deenergization performed by disconnect switches. oil circuit breakers. or vacuum cir- cuit breakers. The swi&hhousemay containmore than one completeset of devices, forinstance, a doubleswitchhouse. which can independentlyprotecttwo outgoingcircuits.This category encompassesdisconnectswitches,which are power equipment containingonly manual devices,with the prime function of allowing mine power to be removed from the main supply. Power Centers At the outermost distribution points there are power centersand distributiontransformers,whichtransform and convert the distribution voltage to utilization levels. In- cluded here are ac to dc conversion equipment or rectifi- ers, which convert the distribution voltage to dc for use on rail trolley and other systems. The power center, also termed a loadcenter, usually impliesan internalbus,which is defined shortly, in the section, "Radial System." In essence, these are all portable substations, and as with switchhouses,each outgoing circuit has its own set of in- ternal protection components.However,an individual unit may supply from 1 to as many as 20 machines. Power centerscan be consideredthe heart of an underground min- ing section power system. In surfacemines, power centers supplypower to low-voltageauxiliary machinery and loads; there may be no need for this equipment with the primary mining machinery. Distribution Equipment Thiscategoryof major power equipmentisoften referred to as the weakest link in the mine power systems. It en- compasses all the overhead powerlines, cables, cable cou- plers, and trolley lines used to carry power and grounding betweenthe power equipment and eventuallyto the loads. Theconductorsareusually calledfeederswhenthey arepart of distribution; at utilization, when connected to portable mining machines, they are called trailing cables. BASIC DISTRIBUTION ARRANGEMENTS The basic distribution arrangements available for in- dustrial applicationsareradial, primary selective,primary loop, secondary selective,and secondary-spotnetwork (6). Radial systemsare the most popular arrangements in min- ing,though otherconfigurationscanbe foundwhere special circumstances call for greater system reliability (3).Sur- face mines have, of course, greater flexibility than underground mines and employ a wider range of distribu- tion arrangements.Secondary-spotnetworks, which are the most popular system for large facilitiesin other industries, areuncommonbut couldbe appliedtopreparationand mill- ing plants. The followingdescriptionsof the main distribu- tion patterns are based on the Institute of Electrical and ElectronicsEngineers(IEEE)definitions(6).Thisinstitute the leading national professional electrical organization, sets standards and recommended practices that are inter- nationally renowned for their correctness. Radial System Figure 1.2showsradial distribution in its simplestform. Here, a singlepower sourceand substationsupply all equip- ment. The single vertical line represents one connection point for all feeders, or all connecting lines, and is termed a bus. Voltage along the bus is considered to be constant. Radial systems are the least expensive to install as there is no duplication of equipment, and they can be expanded easily by extending the primary feeders. A prime disad- vantage is tied to their simplicity;should a primary com- ponent fail or need service, the entire system is down. An expanded radial system, the load-center radial, is illustrated in figure 1.3.As in figure 1.1,two or more volt- age levels are established, but the feeders form a treelike structure spreading out from the source. This system has the advantagesof the simplesystem and severalotherstoo. If the load centers or distribution transformers are placed as close as practical to the actual loads, most distribution
- 31. Utilization Bus -1 Switch- eauioment Cable or pmerline 0 Figure 1.2.-Simple radial distribution system. - Substation Switchhouse8; - 8' w i t c : ; $ Power center center center Circuit beakers Utilization Figure 1.3.-Power-center type of radial distribution. will be at the higher voltage. This allows decreased con- dudor investment, lowerelectricallosses,and better voltage regulation. Primary-Selective System The primary-selectivesystem (fig. 1.4)adds downtime protection through continuity of service. Each substation can receive power by switchingfrom either of two separate primary feeders. Each feeder should have the ability to cany about 80%of the load, so that one feeder can accept a temporary overload(1 0)and provide continuedoperation if one source should fail. During normal service, each feeder should handle one-half of the load. The system is simpleand reliable but costs are somewhathigher than for the radial system because of the duplication of primary equipment. Primary-Loop System Though found in some mines,the primary-loopsystem (fig.1.5)is not considered good practice. It offersthe advan- tages and disadvantages of the primary-selectivesystem andthe costcan be slightly less,but this configurationcan Main source Substation Main Switches -%" Unit substations- 4 Circuit breakers - + Bus - -+fG To the lwds m-+ Figure 1.4.-Prlmary-selective distribution system. Switchhouse Switchhouse n n Sources Substations 7 4substotiom unit 4 mu? To the loads Loop feeder -/* Figure 1.5.-Primary-loop distribution. result in dangerousconditionswhen a primary feeder fails. For instance, a failedportion canbe energizedat either side, creating an extreme hazard to maintenance personnel. Secondary-Selective System In a secondary-selective system, a pair of substation secondariesare connected through a normallyopen tie cir- cuit breaker as shown in figure 1.6. The arrangement al- lows greater reliability and flexibilitythan do the preced- ing techniques. Normally, the distribution is radial from either substation. If a primary feeder or substation fails, the bad circuit can be removed from service and the tie breaker closed either manually cr automatically.Mainten- ance and repair of either primary circuit is possible with- out creatinga power outage,by shedding nonessential loads for the period of reducedcapacityoperation. Other methods that can be used to provide continuity of service include oversizing both substationssothat one can cany the total
- 32. load, providing forced-aircoolingto the substation in serv- ice for the emergency period, or using the temporary over- loadcapacity of the substationand acceptingthe lossof com- ponent life (6). Economics often justify this double-ended arrangement if substation requirements are above 5,000 kVA. Note that the substation capacity or ability to trans- form power is rated in kilovoltamperes. Secondary-Spot Network In the secondary-spotnetwork,two or more distribution transformers are supplied by separate primary-distribution feedersas illustrated in figure 1.7.The secondariesaretied together through special circuit breakers, called network protectors,to a secondary bus. Radial secondaryfeedersare tapped to the bus and feed the loads. This arrangement createsthe most reliable distribution system available for industrial plants. If a failure occurs in one distribution transformer or primary circuit,perhaps by acting as a load to the bus, its network protector can quickly sense the reversepower flow and immediatelyopen the circuit.Total power interruption can occur only with simultaneous mi5 haps in all primary circuits or a secondary-bus failure. However, this type of system is expensive, and the relia- bility gain is not warranted for the mqjority of mining applications. It may be obvious that these basic distribution tech- niques can be combined into hybrid systems.When this is done, there can be confusion about what is primary or secondary. Ordinarily, the subsystems are defined by the specificationof the substations. This will be demonstrated in the next two sections. UTILITY COMPANY POWER As utility companiesare the principal power sourcefor mines, an understanding of utility systempower transmis- sionA d distributionisimportant.-&n this systemgreatly affectsthe power available to the mine, including voltage regulation, system capacity during power failures in the mine, and overvoltage occurrences. Switchhouses tie breaker I I Switchhouses In a nearby substation, power from a generating sta- tion is transformed up to a transmissionvoltage,commonly 69,000V or more (6).Thispower is carried on transmission lines to major load areas, either supplyinglarge industrial users directlv or uowerinn the utilitv's own distribution substations. ~istribution sibstations stepthe voltage down, this time to a primary-distributionlevel ranging from4,160 to 34,500V,but most often at 12,470or 1 3 , 2 0 0 ~ (6).These stages are illustrated in figure 1.8. The utility service, therefore, can be any of the follow- ing standard values, in kilovolts: 138,115,69,46,34.5,23, 13.8, 12.47, 6.9, 4,800, 4.16, and 2.4 (13).Generally, the deliveredvoltagerangesfrom 23to 138kV,but othervalues such as 480, 2,300, and 7,200 V are also found. What is available tothe mine dependson whether the possiblecon- nection is to the power company transmission system, a primary-distributionsystem, or a distributiontransformer. It is the responsibilityof the mining company to select that voltage best suited to its needs. Primarily, the choice dependson the amount of power purchased. It is not safe to assume that the power company has the capability to sewe a large mine complex from existing primary- distributionlinesor evenfrom the transmission system.The problem sterns from the fluctuating nature of mine loads. For example,large excavatorsin surfacemines canrequire high peak power for a short time, followedby regenerative peak power, cycling within the span of 45 s. The fluctuat- ing load may create voltage and frequency variations be- yond the limit set for other utility customers.Accordingly, -totions -9999 Network - protectors a) > 0 ) 4 Bus ' Switchhouse To the loads Figure 1.7.-Secondary-spot network technique. Genemtinq rtotion Transmission Regulated primary line - / distribution system I Seconday distritutim Substotion tmnsf- Utilizatii equipment Flgure 1.8.-Representative utility transmlsslon and distribution. Flgure 1.6.-Secondary-selective system.
- 33. most large draglines and shovels require power from 69- to 138-kVtransmission systems to get adequateoperational capacity, and the construction of several miles of trans- mission equipment can result in a sizable costfor the mine budget. Regardless of where the main mine substation is tied into the utility complex and who owns that equipment, its outgoing circuits will here be termed the mine primary distribution systemorjust distribution. The incomingpower will most often be referred to as a transmission system. The followingsections identify the main types of mines in the United States, classify the major equipment em- ployed, and describe the power-distribution arrangements that are found in them. Of necessity this can be only a very brief overview, but it is designed to indicate the problems and complexities that can arise in mine power distribution and utilization. Individual topics mentioned here are ex- panded in detail in later chapters. SURFACE MINING Surface mining methods are selected over underground methods when the overburden, the earth above the coal seam, can he removed economically to expose the coal. Productivity, safety, and economics usually favor surface mining of seams less than 150ft deep. Surfacecoal mining consists of four basic operations: overburden removal to ex- pose the coal, coal loading, haulage, and reclamation. The mining method is generally classified according to such physical characteristics astopography or land contour, over- burden thickness, coal thickness, number of coal seams,type of overburden. fraementation characteristics of the over- burden, climate, agd hydrology. The mining method is also affected by Federal and State requirements. The mining method se-lected must protect the health and safety of the workers and minimize environmental disturbance and be designed for the specific set of prevailing physical condi- tions. The major surface mining methods for coal are con- tour mining and area mining. Contour mining methods are commonly used in rolling or mountainous terrain; they are called contour mining because overburden removal progresses around the hillside at the coal seam horizon such that the pit resembles a con- tour line. There are many varieties of contour mining, but in all methods overburden is fragmented by drilling and blasting, and removed to expose the coal seam. The over- burden may be removed by small diesel or electric drag- lines, or by diesel-powered front-end loaders and trucks. In soft overburden and for topsoil removal, scrapers and bull- dozers may be used. Area mining is the predominant stripping method in more level terrain. As its name implies, area mining can cover an extensive region, using various box-cutor strip pit and benching techniques. It may be used to mine both thick and thin seams, or multiple seams; where these seams are dipping, area mining is modified to approximate the open pit methods common in metal mining. In all cases, over- burden handling and reclamation are an integral part of the process. Equipment varies, depending on the scale of the operation, from small draglines and dozers to massive equipment that has more than 30,000 connected horsepower. In general, the magnitude of electrical distribution and utilization is greater in area mining than in contour min- ing. Combination of equipment employed in large multi- seam operations may include tandem draglines, dragline and shovel, pan scraperswith attendant dozers, and drag- line and bucket-wheel excavators. Bucket-wheel excavators can be very effective where overburden is soft and does not require drilling and blasting. Frontend loaders, electric and diesel shovels, ripping dozers, and tracked highlifts can all be combined with truck haulage for coal removal. POWER SYSTEMS IN SURFACE MINES Minepower systemscanbe divided intothree categories, depending upon the purpose of a specific portion: 1. Subtransmission, 2. Primary distribution (or distribution), and 3. Secondary distribution (or utilization). Often, if a subtransmission system is needed, it will have the same general arrangement in any mine. At distribu- tion and utilization, power-system installations can vary greatly,but in some mines distribution and utilization can be the same system. Electrical installations in surface coal mines are regulated under 30 CFR 77 (14). Main Substations and Subtransmission Main substations may range from 500-kVA capacity, supplying 480 V for only pumps and conveyors, to 50,000 kVA, servicing a large strip-miningoperation and prepara- tion plant (10). The substation location is usually an eco- nomic compromise between the cost of running transmis- sion lines and power losses in primary distribution. From the main substation, power is distributed to the various centers of load in the operation. However, individual loads or complexes, such aspreparation plants and other surface facilities, may have large power requirements or be so iso- lated that primary-distribution operation is not practical. In these cases, or for safety reasons, incoming utility trans- mission should be extended closeto the load. The extension is designated a subtransmission system,and the conductors are usually suspended as overhead lines (13). As shown in figure 1.9, subtransmission commonly re- quires a primary switchyard of high-voltage switching ap- paratus for power tapping. Branch circuits arefed through Incoming transmission lines I I Primary Iswitchyard I I ! 8 8 Subtronsmission + i i* S~COM Surfacefacilites ~ubtransmission , - - _ _ - - _ - - - I 1 if primory / selective or Preparation , Plant substotlon I selective desired I on major load concentration T o ~reooration t i T o mine distr~bution Figure 1.8.-Subtransmission for surface mine.
- 34. circuit breakers to protect the subtransmissionline and the utility's system. Dual-busconfigurations are employed if primary-selectiveor secondary-selectivedistribution is de- sired on major loadconcentrationsto provide high reliabil- ity. This additional subtransmissioncircuitry is illustrated in figure 1.9 by dashed lines. Subtransmission circuits, primary switchyards, and main substations are almost always permanent installa- tions located in areas unaffected by the mining operation. The main substation is where the grounding systemfor the mine is established. This ground is carried along the powerlines through overhead conductors or in cables and is connected to the frames of all mobile mining equipment. Surface Mine Distribution Minepower distribution,in its simplestradial form,has already been shownto consist of a substation, distribution, and a power center feedingthe mining equipment.The ar- rangement is very common in small surface operations where the distribution voltage is commonly 4,160 V but can be 2,300 V in olderequipment.In the smallest mines, power is purchased at low-voltageutilization (often480 V) and fed to a distribution box to which motors and equip- ment are connected.At times, simpleradial distribution is employed in large surface mines where only one machine must be served or an extensive primary-distribution net- work cannot be established, as in somecontouroperations. The great majority of strip minesemploy radial distribu- tion, but secondary-selectiveand primary-loopdesignscan alsobe found. Simplifiedexamplesofthe three systemsare provided in figure 1.10to 1.12. In all configurations,a por- tion of the primary distributionis established at a base line or bus. The base line is usually located on the highwall, paralleling the pit forthe entire length of the cut. Its loca- tion is typically maintained 1,500ft ahead of the pit, and it is moved as the pit advances(3).Distribution continues from the base line to the mining equipment, with the con- nections maintained at regular intervals. As the machines move along the pit, the base-lineconnectionsare changed to another convenient location. The base line can consist of overhead polelines or a cable-switchhouseconfiguration,figures 1.13and 1.14 (3). It can be seen that cable distribution brings power intothe pit area, where shieldedtrailing cables connect to the ma- chines. The overhead poleline plus cable arrangement is common in older mining operations,especiallywhen utiliza- tion is at 7,200 V or less (3).Typical spacingbetween poles, or line span, is 200 ft. Drop points are noted in figure 1.13 by triangles. These areterminations between the overhead conductors and the cables, mounted about 8 ft above the ground on poles spaced at regular intervals of around 1,000 to 1,500 ft. Cables connected to the drop points deliver power to skid-mountedswitchhouseslocated on the highwall or in the pit. The switchhousesmay contain manual disconnect switches, which are commonly termed switch skids or disconnect skids, automatic circuit-protection devices or breaker skids,or a combinationof both. The skidscan either be boat design with flat bottomsorhave fabricatedrunners, dependingupon the allowablebearing pressure of the mine terrain. Couplers or plug-receptaclepairs are commonly used for both feeder and trailing-cableconnections.Discon- nect and circuit-protectionfunctions are required for each distribution load, and double switchhouses (two-breaker skids)are frequently employed for two loads. Unit substa- tions oftencontaininternal circuit protectiononthe incom- ing side, and thus do not require a breaker skid. Trailing cablesare usually 1,000ft in length, although lengths to 2,000 R can be found. When longer cables are necessaryto reach a breaker skid,in-linecoupling systems can be used, and these are commonly mounted on small skidsfor easy movement. Trailingcable handling for strip- ping equipmentis often assisted by cable reels mounted on skidsor self-propelledcarriers.Largeexcavatorscanrequire the self-propelledvariety. Switchhouses Base line Bose line Dragline Pit hiqhwall substation Low-voltage Production show1 Pump, lighting Figure 1.10.-Radiei strip mine distribution system. ~ o i n h substation line substotion tie1 breaker Unit substatii shovel substation Production Other pit shovels power Figure 1.11.-Secondary-selective distribution in strip min- ing. Main substation substation Switchhouses . Pit hi$r.mIl Unit substation Stripping shovel Loader Figure 1.12.-Primary-loop design for strip mining.
- 35. Power cornpony supply, Utility company 69kV metering Substation (69kV/7.2 kV) 7.2-kV overhead poleline ( base line 1 ~rdduction 1production ill shovel shovel Drill spore KEY DS Disconnect switch TBS 2-breaker skid Flgure 1.13.-Radlal distribution for strip mine with overhead-polelinebase line. Utility company supply, 138 kV Utility company metering 138-kV O v e r b a c l ; r c ; $ breaker, ! Wwerllne Additional rninina method ! Skid-mounted Substation ( 138kV/25 kV) tri:ti',"itCh L 25-kV cable I Single-breaker skid , I I is used ; Drwline 7 - 1 I 11 .- - - - --... I (transformed trailing cable 7.2-kV cable to 7 , 2 0 0 ~ - by incl~neto on machine) pullback machine / T - - Single-breaker skid 2-breaker skid ! Spore 7-r 2 5 kV/440 V 1 , 5 0 0 ~ 25-kV r---.--- 1 ' , .-- - --! Auxiliary t I eaui~ment . . ragl line I 2-breaker skid. Lateral cables( Jfm Orill- i Production spore- -,.-..-a 31NVel 7.2-kV c o b l e s v 25-kV/7.2-kV transformer Drill 4 Trailing cables Production shovel ---- skid Figure 1.14.-Radial distribution for strip mine with all-cable dlstrlbutlon. The layoutfor an allcable mine distribution,figure 1.14, is very similar to that just described. In this case, how- ever, the base line is assembledusing cable-interconnected switchhouses. As noted in the illustration, the common approach is to use disconnect skids with three internal switchesin the base lineand tohave separate breaker skids in line with the cables feedingthe mnihinery.Another ap- proach is to combinethe single-breakerskids intothe base- line switchhouses. - When a secondary-selective configuration is used, as shown in figure 1.11,a normally open tie circuit breaker is placed in the base line in a location approximatelyequi- distant fromthe main substations. In someoperations,the two substations and the tie circuit breaker may be in the same location, with two feeders running from the substa- tion area to the base line. More than two main substations may be established in very large operations. Primary-loopsystems have occasionally been used in strip mining. It can be noted fromfigure 1.12that the sub- stations actually operate in parallel, considering the base line to be a bus. Here the substations can be smaller than those needed for a radial system.Notwithstanding,certain precautions should be taken with this configuration (9). For example,the substations must be identicalif they are to share the load, but as an unbalanced load distribution is probable on any system, it is likely that the two substa- tions will not be equally loaded.Regardless,because of the safety hazards, primary-loopdistribution is consideredun- satisfactory and is not recommended. Distribution voltage for the surface mine may be 7.2, 13, or 23 kV, and to a lesser extent 4.16 kV. Regardless of the level,drills and production shovelsusually operate at 7,200 or 4,160 V.Therefore, when higher distribution levels are needed, portable unit substations are commonly used in the pit. One instance would be when the load cre- ated by a large machineis several times that for auxiliary machines. Another method is to establish two base lines on the highwall for two distribution voltages, as shown in figure 1.15.Here, a large unit substation interconnectsthe two base lines. Even in this situation, as can be seen in the preceding illustrations, low-voltage unit substations Utility Y Moin substation Switchhouse for 23-kV I / base line Unit substation 23 kV/X 200V Switchhause ~ ~ 7 i ~ ~ O - V 10,000-ft pit Figure 1.15.-Surface mine distribution system using two base lines.
- 36. or power centers are often required for 480-Vauxiliary equipment. The primary purpose of any primary-distribution scheme in a surface mine is to provide a flexible, easily moved or modified power source for the highly mobile mining equipment. System designs must also be considered as an integral part of the total mine operation. Those described have these objectives in mind. As will be seen, the distribution system in any surface or underground mine that serves portable equipment is subject to damage from the mining machinery itself, and as a result, the system must be designed with optimum flexibility and considera- tion for personnel safety. Open pit power systems are quite similar to those in stripping mines but with one main exception: primary dis- tribution typically establishes a ring bus or main that partially or completely encloses the pit. Radial ties to the bus complete the circuit to switchhouses located in the pit, and portable equipment again uses shielded trailing cables. An example is shown in figure 1.16.Distribution voltage is normally 4.16kV, but 7.2or 6.9and 13.8kV are some- times used. Unit substations are employed if equipment voltages are lower. Primary distribution is almost invari- ably through overhead lines. UNDERGROUND COAL MINING Figure 1.17 is a plan view of a typical U.S. underground coal mine. A system of main entries, each 16 to 20 ft in width, is developed from the coal seam access point to the production areas, which are called panels or sections. Pil- lars of coal are left duringmining to support the overburden above the entries. Crosscuts are mined between the entries. The main entries may remain standing for several years while coal is being extracted from the panels. Haulage of Utility Substation Dixonnects limit MINE PIT ''4"" i Utility or Overhead line Poles subtransmission rinq main personnel, supvlies, and coal. together with provision of . -- . - ;entilating air and dust-suppressionwater, and electrical distribution are necessary functions of the main entries I throughout the life of the mine. The mining method is defined by the configuration of the open workings and by the classification of equipment used. The important underground coal mining methods are room and pillar, which may be either conventional or con- tinuous, and longwall. To the miner, the type of mining machinery used is implied by each category. Theroom-and- pillar method remains dominant in the United States, although there has been a recent substantial increase in longwall mining. The choice of a specific mining method is frequently dictatedby such natural conditions of the mine asthe characteristics of the overburden, roof, and floor,plus the seam dip, water, methane, and seam height (11).Es- sentially, the method and equipment selected are based on the combination that will provide the safest and most prof- itable extraction within the given set of geologicconditions, while complyingwith State and Federal health, safety, and environmental regulations. Room-and-Pillar Mining Room-and-pillar mining is named for the regular pat- tern of openings made in the coal seamand was the earliest form of underground coal workings. Conventional Mining The conventional minine method reoresents a direct - evolutionary link with the early mining techniques. It is based on the original loading machine, which came into use Entries Previausly mined brqwall panel, rmf,coved (Gob) Figure 1.16.-Open pit power system. Figure 1.17.-Layout of underground coal mine.
- 37. in the early 1920's. Modern conventional mining consists of six distinct operations: 1. Undercutting the coal face, 2. Drilling holes in the face for blasting, 3. Blastine. -, 4. Loading the broken coal onto a face haulage system, 5. Hauling the coal from the face area to a subseauent haulage system, and 6. Providing roof support. In order,these stepscomprise a mining cycle; after roof sup- port is completed, work begins again at step 1.Ventilation, although essential, is not included as a separatestep in the cycle as it must be provided continuously. Other safety pro- cedures include careful examination of the face and roof after blasting and before each job begins at the face. Mobile self-propelled mining equipment performs most of the operations in conventional mining. The cutting ma- chine, basically an oversized chain saw, is employed to cut a slot at floor level. called the kerf. which allows coal ex- pansion during blasting A face drill follows the cutter and makes several holes in the face with its carbide-tipped auger-type drill bits. Blasting is carried out either by chemical explosives approved as permissible by the U.S. Mine Safety and Health Administration (MSHA) or to a lesser extent by high-pressure air. Permissible ex- plosives will not ignite methane and coal dust when used correctly. A crawler-mounted loading machine loads the broken coal onto the face haulage vehicle, typically a shuttle car. The car isequippped with a chain conveyor that moves the coal from the load end to the discharge end and subse- quently unloads it from the vehicle. Shuttle cars almost invariably work in pairs and move the coal to rail cars or a conveyor belt, which makes up the next stage of materi- als handling in the mine. The roof bolter, sometimes called a roof drill, is a rubber-tired vehicle that secures the roof by first drilling vertical holes and then emplacingroof bolts, which secure the roof either by clamping thin roof layers together to form a thick beam, or by hanging weak strata t o a more competent upper layer. Drilling is usually ac- complished by rotary action with auger-type bits. The re- sulting dust is collected through the bit and hollow drill rod by vacuum. With few exceptions, all these machines are electrically driven, powered via trailing cables from the mine power system. Since the mining equipment is continually moved among several faces in a coordinated plan designed for maximum production efficiency, the handling of trailing cables is a significant part of the mining cycle. The result of coal removal is a system of open rooms divided by coal pillars that support the roof asmining advances toward the property boundaries. When the equipment approaches the property limit, the operation is turned around and retreat mining takes place. If surface subsidence is permitted, the pillars are removed in an organized extractionplan and the roof is allowed to cave. The broken material that then fills the mined void is known as gob. Continuous MYning The heart of the continuous coal mining method is the continuous mining machine, which replaces the conven- tional room-and-pillar unit operations of cutting, drilling, blasting, and loading. The mining fundions of haulage and roof support remain, although some continuousminers also perform roof bolting. The term "continuous" is actually a misnomer because of legal constraints that mandate inter- ruptions in the mining process for safety checks and ven- tilation requirements. The most common form of face haulage in continuous mining is again the shuttle car. One of the main problems associated with continuous mining is the intermittent nature of the shuttle car haulage system, which causesfre- quent delays at the face. As a result, various types of con- tinuous haulage systems have been developed to alleviate this problem. Mobile chain and belt conveyors arethe most popular of these systems, and these are applicable to min- ing low coal. Continuous haulage systems have not been without problems, and some designs have been hampered by poor reliability and lack of maneuverability. Hydraulic systemshave shown great promise; they operate by pulver- izing and slurrying the coal immediately behind the miner, then pumping the slarry to the surface. Longwall Mining Longwall mining is the most popular underground coal mining technique in Europe, and it is growing rapidly in the United States. In contrast to room-and-pillar mining, longwall is capital intensive rather than labor intensive. Longwalls are usually 300to 600ft wide, and the direction of mining with respect to the main entries classifies them as either advancing or retreating longwalls. The latter is the most frequent in the United States. A typical retreating longwall is shown in figure 1.18. The section of coal to be mined, the longwall panel, is first delineated by two room-and-pillar entries or headings driven perpendicular to the main entry. These two head- ings, the headgate and the tailgate, handle haulage equip- ment and ventilation. The longwall panel is then mined back and forth,retreatingtoward the main entry. The roof is allowed to cave immediately as the longwall equipment moves, as is shown by the gob area on the diagram. The longwall equipment consists of an interconnected system of cutting machine, roof support equipment, and Tailgote wnel entries Longwall / Main entries Figure 1.18.-Plan view of retreating longwall.
- 38. conveyor haulage. The cutting machine moves along the face on a conveyor that also carries away the mined coal. Behind the face conveyor, and connected to it, is the roof support equipment,which supportsa protective metal can- opy or shield that extends over the face area. These roof support units provide both the protection and the forward mobility of the system. The typical face conveyor is a flexible armored-chain conveyor powered by motors at the headgate and tailgate. Mined material moves toward the headgate, where it dis- charges to the panel belt via an elevated intermediate haulage unit, the stage loader. Shortwall mining is a less common mining method; it is very similar to longwall mining except that the short- wall panel is normally 150to 200 ft wide. From the stand- ~ o i n t of eoui~ment. shortwall can be considered as a com- promise between rdom and pillar and longwallin that the extractive and face haulage systems are identical to those in continuous mining, while the roof support equipment is similar to that used in longwall mining. POWER SYSTEMS IN UNDERGROUND MINES Regulations Underground mine power systemshave differentchar- acteristicsfmm those for surfacemines, and thesetwobasic mining operations are regulated by separate codes and standards. For instance, although 30 CFR 77 covers elec- trical installations of surfacecoal mines and surfacefacili- ties of undergroundcoal mines,Part 75 regulatesthe under- ground installations and Part 18 specifies standards for electrically powered face machinery(14).Part 77 illustrates an overlap between surface and underground legal de- mands, which is logical because the surface electrical counterparts of both mine types are similar; examples in- clude substations and subtransmission. Figure 1.19can be comparedwith figure 1.9to seethe similarity between sur- Incoming tronsmission lines 1 I Primory I Iswitchyard I I I I m iI and switches circuit I breakers Note: DJol-bus confiqumtion can be used if second vwrce desired To surface loads i. To fon power system Subtronsmission Preparation plant substotion Moin s u b % k 2 substation 1 + Borehole2 Bwehole 1 to undergrourd to undergrwnd Figure 1.19.-Subtransmission for underground mine. face mine and underground mine subtransmission. As a general situation, however, the mine distribution system is related to the mining method.Hence, underground mine systems become different from surface mines at the point where the circuitsleave the substationand go underground. Underground Mine Distribution As shown in figures 1.20 and 1.21,underground mine power systems are somewhat more complicatedthan those for surfaceapplications.Because of the nature of the mine and its service requirements, distribution must almost always be radial (fig. 1.20);the freedom in routing distri- bution enjoyed by surface mines is not available under- ground. For increased reliability, secondary-selectivemain substationsare employed(fig.1.21).The secondary-selective operationisdefined by the use of two substations and mine feeders with a normally open tie breaker. Primary-distri- bution voltage is most commonly 7,200 V; however, older 4,160-V systems can still be found, and 12,470 V has in- creased in popularity in recent years, especially for long- wall operations.The groundingsystemfor the underground mine distributionmust be separatedfrom that used for sur- face equipment. Power and mine groundingare fed underground in in- sulated cables, either through a shaft or borehole or a fresh-airentry. The cablesterminate in disconnectswitches within 500Rof the point of power entry intothe coal seam. These switchesallow total removal of underground power in an emergency. From the disconnects, which may be a part of a switchhouse, the power is distributed through cablesto power centers or rectifiers located as close to the machinery as practical. All the cables on high-voltagecir- cuits,usually involving only distribution,must have shield- ing around each power conductor. The prime load concentrationsin underground mining are createdby the miningsections.Distribution terminates at the section power center, which is a transformer com- bined with a utilization bus and protective circuitry. From this, several face machines are powered through couplers and trailing cables.Power-systemsegmentsfortypical con- tinuous and longwall operationsare given in figures 1.22, 1.23,and 1.24.Rated machine voltagefor most installations is 550 Vac, but 250-Vdc and 440-Vac equipment is used extensively, and 950 Vac has become quite popular for high-horsepowercontinuousminers and longwallshearing machines. In the longwall system, power is fed through controls to the various motors. On conventional or con- tinuous equipment, the utilization approximatesthe radial system shown in figure 1.22. If belt haulage is used, distribution transformers are located close to all major conveyor belt drives and are re- ferredto as belt transformers.After transformation, power is supplied through starter circuitry to the drive motors. With rail haulage,distributionterminates at rectifiers that contain a transformer and rectifier combination. The rec- tifiers are located in an entry or crosscutjust off the rail- way. As shown in figure 1.25, dc power is then supplied through circuit breakersto an overhead conductoror trolley wire and the rail, with additional rectifiers located at reg- ular intervals from 2,000to 5,000ft alongthe rail system. For further protection, the trolley wire is divided into elec- trically isolated segments.The typical rectifier suppliesthe ends of two segments of trolley wire and each feeder has its own protective circuitry to detect malfunctions. Each trolley-wiresegmentiscalled a deadblock. Thisloopfeeding
- 39. arrangement is continuedthroughout most of the haulage system except for the most inby segment, which is dead- ended. In some mines, dc face equipment and small dc motors are powered fromthe trolley systemthrough a fused connection (or nip) to the trolley conductor and rail. The dc distributioncan also serve large motorsdirectly through switchgear;however, this practice is rare in underground coal. All power equipmentused undergroundmust be rugged, portable, self-contained,and specifically designed for in- stallation and operation in limited spaces. In addition, all equipment and the cables connecting them must be pro- tected against any failures that could cause electrical haz- ard to personnel. This is primarily provided by protective relaying built into each system part, with redundancy to maximize safety (4). Utility company metering e - - N t-; B 6 i V) U Surfoce Moin substation To wrfoce !ads (hoist,fon, pmps.char)a nmb*) !%aer center Distribution hnnskrlner CONVENTIONAL Rectifier MINING SECTION Portable switchhse t Mixellonews loads (shop, pump, etc. 1 Rectifier B> starter m r W t ad drive 1 , r k r;~ Trolley system 1 I -:... lfion ] LONGWALL 'mer MINING SECTIW I ?-? Mine pcwr center CONTINWS MINING SECTION I-I-N-LL t t l ' T t t t T control
- 40. I I MAIN I SUBSTATION I AREA I ! I I I Norrnolly open I I tie breaker I I I I --- -- ---J Ground level +b-h4 -Shafts or boreholes seam cool -Y8- [ : I[ ' I -Disconnect switchhouses Y V To other portable To other portoble switchhouses and J loads (1/2 totol) i swlrcnnouses ona Lf J LnTJ loads (112total) T Major lood concentrations Figure 1.21.-Secondary-sel.ctlw distribution in unde~round mines. Low-vdtoge couplers Troiling High-voltage couplers Bolter Shuttle car Shuttle cor Mine power center Feeder Figure 1.22. -Utillutlon in continuous mining section.
- 41. f Motor, 125hp 2 1,000-kVA power center 3 125-hp stage-loader starter 4 Dual 125-hp foce-conveyor starter 5 Dual 75-hp pump and 230-hp shear starter 6 Pump, 75-hp 7 Pump, 75-hp 8 Master control Figure 1.23. -Power-systemsegment with longwall equipment. J High-voltoge feeder cable igh-voltage coupler Power center edium-voltage couplers Medium-voltage cables Cables for control of starters . . . 950-V face 950-V shearing- ,950-V stage- 950-V hydraulic- conveyor motors machine motors loader motor pump motors Figure 1.24.- Diagramof electrlcai-systemsegment for iongwsll.
- 42. / High-voltage feeder Switchhouse Switchhouse Power from substation -<w< ' t r < l , - + To other downstream LA^ switchhauses 4 High-voltage oc input 1.Rectifier 1 Rectifier Dead-block segment Dead-ended segment Figure 1.25. -Parallel-feed haulage system. SURFACE FACILITY POWER REQUIREMENTS The surface activities of any mine, which may include shops, changing rooms, offices, ventilation fans, hoisting equipment, preparation plants, and so forth, can have large power requirements. For safety, these facilities should at least have an isolated power source and at times a separate substation. In preparation plants, the distribution arrangements are almost always expanded radial or secondary selective (8).Representative system layouts are shown infigures 1.26 and 1.27. In both, distribution is at 2.4 to 13.8 kV, with 4,160V the most common level. Power is distributed at one of these levels to centers of electric load. This power may be used directly for high-voltage motors, but usually the voltage is stepped down to supply groups of motors or single high-horsepower motors. The power centers must be in an elevated location or totally enclosed. The rooms used for these and other electrical components may also be pressur- ized to exclude coal dust. The most popular voltage for preparation plant utiliza- tion is 480 V. This voltage is used to drive all motors throughout the plant except those with high-horsepower demands, such as centrifugal dryers and large fan drives, where 2,300or 4,160V iscommonly employed.Thesehigher voltages may also be preferred for any motor that requires continuous service or independence from the power-center loads. Note that 240-Vmot~rs are unsatisfactory for typi- cal preparation plant demands. Most modern preparation plants use group motor control instead of individually housed control units, since this method facilitates main- tenance and enables the interlocking of the various motor functions required for semiautomaticfacilities. All manual controls, indicating lights, and so on are grouped in one central operating panel to allow easy access and visual indication of plant operation. The panel is often called a motor control center (MCC), as shown in figure 1.27. BASIC DESIGN CONSIDERATIONS The goal of the power engineer is to provide an effic- ient, reliable electrical system at maximum safety and for the lowest possible cost. The types of information made available to the power engineer include the expected size of the mine, the anticipated potential expansion, the types of equipment to be used, the haulage methods to be em- ployed, and whether or not power is available from a util- ity company. The amount of capital assigned for the elec- trical system will also be designated.
- 43. From utility to wbtransmission s~stem hSubstation + - - - 24 to 13.8kV I r715i To 480-V preparation plant loads Flgure 1.26.-Representative expanded radial distribution for preparation plant. The designed system must meet certain minimum criteria. IEEE (12)has defined these basic criteria for in- dustrial electrical systems that must be applied to mines: Safety to personnel and property, Reliability of operation, Simplicity, Maintainability, Adequate interrupting ability, Current-limiting capacity, Selective-system operation, Voltage regulation, Potential for expansion, and First cost. Of these, safety, reliability, and simplicity are closely re- lated. All are dependent on good preventive maintenance. In the cramped uncompromisingenvironment of an under- ground mine, these are of vital concern. Since continuous operation is the aim of every mine operator, planned main- tenance should be held to a minimum. Most routine main- tenance should be capable of being performed by unskilled personnel, since it will be done by the miners themselves. Training for these tasks must be provided. Adequate interrupting capacity,current-limiting capa- bility, and selective-systemoperation areprojected at safety through reliability. The first two areas ensure protection during a disturbance. Current limiting, when applied to grounding, is perhaps the most significantpersonnel safety feature of mine electrical systems. Selective-systemopera- tion is a design concept that minimizes the effect of system U t i l i t y meterlng Substation$, 3,750kVA each open tie circuit breaker 7 I Auxiliary / [ I ] functions w i t c h i o r s one-cool switchgear center center center X control center X X 4 8 0 - V motors 4,160-V 4 0 0 - V About 2,500-hp, About 2,500-hp. 4 8 0 - V m o t o r s 4 8 0 - V crusher motor8 4,160-V motors 4,160-V motors motors C v w Raw-coal c i r c u i t coorse- P u m p r , conveyors, blowers, Fine-cool Auxiliaries c o a l f i l t e r s , fans, jigs, etc. c i r c u i t c i r c u i t Figure 1.27.- Representative secondary-eelectlvedistrlbutlon for preparationplant.
- 44. disturbances. Voltage regulation is a limiting factor in system design, particularly underground, and is often the main constraint to system expansion. It should be antici- pated that when the size of the mine is increased,this might involve augmenting the power-system supply through ad- ditional power sources. While first cost is important, it should never be the determining factor, since high-cost equipment, projected at maximizing safety and reliability, can easily offset the in- creased first cost through the reduction in operating costs. At times, this fact appears to elude some company pur- chasing agents. Using the data available, the task of the power engi- neer is to select one combination of power equipment over another, provide power or circuit diagrams, estimate the equipment, operating and maintenace costs, set the speci- fications forthe system, and receive and assess the proposals from suppliers. For success, the engineer requires a firm knowledge of mine power systems, but this understanding cannot be based on a "standard mine electrical system" because such a standard does not exist: no two mines are exactly alike. The engineer must resort to the fundamen- tal concepts, an awareness of what has worked i n the past, and a clear understanding of the legal constraints. This in- formation is provided in the subsequent chapters. REFERENCES 1. American Standards Association. American Standard Safety Rules for Installing and Using Electrical Equipment in and About Coal Mines (M2.1). BuMines IC 8227, 1964. 2. Bergmann, R. W. Excavating Machinery. Ch. in Standard Handbook for Electrical Engineers. McGraw-Hill, 10th ed., 1968. 3. Bucyms-Erie Co. (South Milwaukee, WI). Surface Mining SupervisoryTraining Program. 1976. 4. Cranos, J. C., and D. E. Hamilton. Portable Substations for Mine Power Systems. Ind. Power Syst., v. 19, Mar. 1976. 5. Hollingsworth, J. A., Jr. History of Development of Strip Mining Machines. Bucyrus-ErieCo., South Milwaukee, WI, 1967. 6. Institute of Electricaland ElectronicsEngineers (NewYork). Recommended Practice for Electric Power Distribution for In- dustrial Plants. Stand. 141-1986. 7. Jackson, D., Jr. Coal Mines. Ch. in Standard Handbook for Electrical Engineers. MeGraw-Hill, 10th ed., 1968. 8. Lordi, A. C. Electrification of Coal Cleaning Plants. Mechanization, v. 20, Oct. 1956. 9. . Trends in Open-Pit Mine Power Distribution. Coal Age, v. 66, Jan. 1961. 10. Rein, E. C. Electrical Apparatus for Surface MiningOpera- tions. Ch. in Surface Mining. Soc. Min. Eng. AIME, 1968. 11. Robinson, N., 11.UndergroundCoal MiningEquipment. Ch. in SME MiningEngineering Handbook. Soc. Min. Eng. AIME, v. 1, 1971 -. 12. Stefanko, R. Coal MiningTechnology Theory and Practice. Soc. Min. Eng. AIME, 1983. 13. Thuli, A. J. Power. Sec. in SME MiningEngineering Hand- book, ed. by J. M. Ehrhorn and D. T. Young. Soc. Min. Eng. AIME, v. 2. 1973. 14. U.S. Code of Federal Regulations. Title 30-Mineral Resources; Chapter I-Mine Safety and Health Administration, Department of Labor; Subchapter0-Coal Mine Health and Safety; Part 18-Electric Motor-DrivenMine Equipment and Accessories; Part 75-Mandatory Safety Standards, Underground Coal Mines; Part 77-Mandatory Safety Standards, Surface Coal Mines and Surface Work Areas of Underground Coal Mines; 1981.
- 45. CHAPTER 2.-ELECTRICAL FUNDAMENTALS I The technique used to solve problems in complex elec- tronic circuits or mine power systems is called circuit an- alysis. It involves calculating such circuit properties as cur- rents, voltages, resistances, inductances, and impedances. Circuit analysis serves as the knowledge base on which an understanding of mine electrical systems can be built. This chapter will diverge from classical circuit-analysis presentations by not covering transient effects in circuits. From experience, studying currents and voltages existing in a circuit immediately after a change in circuit configur- . ation can be confusingand cloudsunderstanding of the most used seementsof circuit analvsis. Therefore. although some " - necessary statements will be made, the subject of transi- ents is delayed until chapter 11,where it can be combined with practical examples. This chapter commences by introducing electrical phe- nomena and continues through to a presentation of steady- state ac circuit analysis. Chapter 3, "Electrical Fundamen- tals 11," continues the coverage of basic electrical subjects and startswith the basics of electricalpower consumption. Numerous excellent circuit-analysis textbooks have been produced over the years. Many can be employed ef- fectivelyto cover the subject, and someof these are provided in the bibliography at the end of this book. Because practic- ally all fundamental electrical relationships are considered common knowledge,the conceptsintroduced in this chapter will seldom be referenced other than by giving credit to the discoverer. BASIC ELECTRICAL PHENOMENA The nature of electricity is not yet fully understood,but it is well known as a form of energy that can be conven- iently converted into and utilized as light, heat, and me- chanical Dower.Like all science. knowledgeabout electricitv - has been developed from observation and experimentation. The generalization of this experimental evidence combined withinformation about the nature and behavior of electrons and electron flow forms the basis of electron theory. The atoms of each element consist of a dense nucleus around which electrons travel in well-defined orbits or shells. The subatomic particles, the building blocks out of which atoms are constructed, are of three different kinds: the negatively charged electron, the positively charged pro- ton, and the neutral neutron. The negative charge of the electron, e-, is of the same magnitude asthe positive charge of the proton, e'. No charges of smaller magnitude have yet been concretely observed. Thus the charge of a proton or an electron is taken as the ultimate natural unit of charge. It is these two particles that are of principal interest in electricity. Coulomb's Law The force, F, between two charges, q and q', varies directly as the magnitude of each charge and inversely as the square of the distance (r)between them. This relation- ship, known as Coulomb's law, is represented mathemat- ically by where k = proportionality constant that depends on units used for force, charge, and distance. If force is in newtons, charge in coulombs, and distance in meters, then The unit of charge, the coulomb (C), can be defined as the quantity of charge that, when placed 1m from an equal and similar charge, repels it with a force of 9 x newtons (N). The charge carried by an electron or by a proton is e = 1.602 x 10-ls C. Voltape and Current A proton in the nucleus of an atom can hold only one electron in orbit around it. When an atom contains fewer than the normal number of electrons that the protons can attract, the atom has an excess of positive charge and is said to be positively charged Atoms with an excess of elec- trons are said to be negatively charged The net amount of these chargesis termedpotential or ekctromotiveforce (emf) and ismeasured in volts. The separation of oppositecharges of electricity may be forced by physical motion or may be initiated or complemented by thermal, chemical, or mag- netic causes or even by radiation. The potential differenceor voltage existing between two points can be measured by the work necessary to transfer a unit charge from one point to the other. The volt is the potential between two points when 1joule (J) of work is re- quired to transfer 1coulomb (C)of charge. In other words, In some metals or conductors,electrons in the outermost orbit of the atoms are rather loosely bound to their respec- tive nuclei. These are called conductionelectrons, sincethey can leave the atom upon the application of a small force and become free to move from one atom to another within the material. In some materials, however, all the electrons are tightly bound to their respective atoms. These are called insulators, and in these materials it is exceedinglydifficult, if not impossible, to free any electrons. Conductors and in- sulators are the principal materials used in electrical systems. The application of a voltage across a conductor causes the free electrons within the conductor to move. Electrical current is defined as the motion of electrical charge. If the charge in the conductor isbeing moved at the uniform rate of 1coulomb per second (CIS), then the constant current existing in that conductor is 1ampere (A),the unit of elec- trical current. The amount of current in a conductor can also be measured as the rate of change of the charge flow. Such changing current at any point in time is called in- stantaneous current or i = (the rate of change of charge) = 2, (2.2) where i = instantaneous current, A, q = flow of charge, C, t = time, s. When electricity was first discovered, it was erroneously thought that it was the flow of positive charges. Since the laws of attraction and repulsion were known, the movement was assumed to be from positive to negative. This theory
- 46. was accepted until the discovery of the radio tube, when Where needed, double-subscriptnotation is used to de- it was recognized that the flow was movement of electrons scribe current and voltage. VA, represents the voltage of from negative to positive. However, the concept of positive- point A with respect to point B. IcD represents the current charge flowwas firmly entrenched and has remained stand- flowing through a circuit element from C to D. Note that ard in the United States, and so it will be used here. in the circuit shown in figure 2.1, the voltage VAB causes the current IA, to flow. These meet with the standard for SYSTEM OF UNITS electrical current, which is positive-charge flow from posi- tive to negative. Most material contained in this text is given in the International System of Units (SI);exceptions are calcula- tions that are more conveniently expressed in terms of the English or American engineering systems. A listing of the basic symbols, units, and abbreviations that are used is given in table 2.1. The decimal system is used to relate larger and smaller units to basic units, and standard pre- fues are given to signify the various powers of 10; for example: pico- (p-, nano- (n-, 1 0 9 micro- 01-,lo-%) milli- (m-, lo3) kilo- (k-, loS) mega- (M-, 109 giga- (G-, Voltage, current, and power variables are represented by the letter symbols V, I, and P in both uppercase and lowercase letters. Uppercase letters represent voltage, cur- rent, and power when the variable is constant, as in dc circuits. In ac circuit work, uppercase V and I represent effectivevalues and uppercase P represents average power. Lowercase v, i, and p depict voltage, current, and power when these quantities are varying with time. EXPERIMENTAL LAWS AND PARAMETERS It is remarkable that the entire theory of electrical circuits is based on only six fundamental concepts. One is Ohm's law, two are named for Kirchhoff, two relate to in- ductance and capacitance, and one has to do with power. To understand any electrical system, comprehension of these relationships is mandatory. Ohm's Law GeorgSimon Ohm (1789-1854)discovered that the elec- trical current through most conductors is proportional to the voltage (potential) applied across the conductors. This phenomenon is known as Ohm's law and is expressed mathematically as v = Ri, (2.3) where v = applied potential, V, i = current through the conductor, A, R = proportionality constant known as resistance of conductor, R. Table 2.1-SI symbols and units SI unit Unit Identicel svmbol unlt Charge....................................................................... Current........................................................................... Mltage ........................................................................... Electromotiveform...................................................... Patential difference........................................................ Resistance ................................................................... ..................................................................... Energy, work .................................................................. Power (active)............................. .................................... Power--apparent ............................................................ Power-reactive ............................................................ Resistivity....................................................................... Conductivity ................................................................... Electric flux ................................................................. Electric flux denslty, displacement................................. Electric field strength...................................................... W r m i n i i Relative pe Magnetic flux............................................................... + Magnetomoliveforce .................................................. F...F Reluctance..................................................................... R...R Wrmeance.................................................................... R..P Magnetic flux density...................................................... B Magnetic field strength ................................................ H Wrmeabllily (absolute) ................................................... p Relative permeability.................................................... p, 'V,E indicatesalternative symbols:...U indicatesresenre symbols. coulomb ................... . . . .............................................. ........................................................................ ampere volt ............................................................................ ohm.................. . .......................................................... siemens.................................................................... ohm.............................................................................. siemens....................................................................... ohm............................................................................ s~emens ....................................................................... fara hen joule .......................................................................... .............................................................................. watt ............................................................... .... voltampere : .......................................................... var................. . . ................................................................ ohm-meter ....................................................... siemens per meter coulomb .......... coulomb per sq ............................................................. volt per meter ............................................................. farad per meter (numeric) weber ....................................................................... ampere (amp turn) ....................................................... ....................................................... I ........................................................... ampere per weber ..................................................... rec~proml henry weber per ampere henry.......................................................................... 'tesla ........................................................................... ................. .................................. ampere per meter . . .......................................................... henry per meter (numeric) n VIA S A N n VIA s A N R VIA S A N F C N H WnlA J N.rn W J k V A var Q-m Slm C Clm2 Vlm Flm Sb WbIA H T wb/mz Alm Hlm
- 47. No restriction is placed on the form of v and i. In dc cir- cuits they are constant with respect to time, and in ac circuits they are sinusoidal. For metals and most other conductors, R is constant. In other words, its value is not dependent on the amount of current, i. In some materials, especially in crystalline materials called semiconductors,R is not constant,and this characteristicis useful in diodes,amplifiers,surge arresters, and other devices. Further experimentsby Ohm indicatedthat the resist- anceof a piece of metal dependson its size and shape. How- ever, the resistivity, p, of the metal depends only on its composition and physical state. This is an inherent prop- erty that opposescurrent through the conductorjust as the frictional resistance of a pipe opposes the flow of water through it. Resistivity is defined asthe resistance of a unit cube of homoeeneous material: hence. resistivitv can be thought of as iproperty of the materialat a value remains the same at all points in a homogeneous conduc- tor, but if the material is not homogeneous, its resistivity can vary from point to point. The value may also vary greatly for different conductors. The concept of resistivity is often used in the grounding and distribution aspects of mine electrical systems. Using the definition, practical resistivity units would be ohm-centimeter(Q-cm)and ohm-inch(Q-in).However. resistivity is usuallyexpressedin ohm-meter(Q-m) (SDand ohm-circular-mill-foot(English).The ohm-meteris the re- sistance of a material 1 mmz in cross section with 1m length. Likewise,the ohm-circular-mill-foot (usuallyabbre- viated to Qcmil-ft)refers to the conductorresistance for a volume 0.001 in (1mil) in diameter and 1 ft long. For calculating the resistance in this latter case, the cross- sectionalarea of the conductor is measuredin circular-mills, which can be found from whereA = cross-sectionalarea of circular conductor,cmil, and d = conductor diameter, lo-=in. bsistivity values of somecommonconductorsare given in table 2.2. Table 2.2-Resistivity of some common materials at 20° C Temperature Resistivity (p) Material coefficient (a) 10-8 Q~ namil-fl Aluminum, commercial........... Copper, annealed................. Imn, annealed........................ ...................................... Lead Nichmme............................... Silver .................................. ............................. Steel, mild Tin......................................... lbngsten................................ The resistance of any specific conductor can be calcu- lated from the material resistivity using the formula where R = resistance, Q, O = conductor length, A = conductor cross-sectional area, and p = material resistivity. If P is in meters and A is in meters squared, then p must be given in units of ohm-meters. Electrical resistivity does not remain constant if the temperature is permitted to change. For most materials, the resistance increases asthe temperature increases;car- bon is an exception to this rule (negative temperature co- efficient,0.005). If the temperature coefficient isknown,the resistance of a given conductor at a given temperature is R = Ro [1+ a (t - to)], (2.6) where R = resistance at temperature t, R, Ro = resistance at referencetemperatureto,usually 20" C, Q, a = temperature coefficient, Q/'C, t = given conductor temperature, O C , to = reference temuerature. "C. At very low temperatures (about -200" C for copper)or as the melting point is reached, the temperature coefficient is no longer constant and changes with temperature. As a result, equation 2.6 is not valid for very high or low temperatures. The symbol illustrated in figure 2.1 portrays a resistor in a circuit, and often its resistance is stated. Again by definition, Sometimes, the element's conductance, G, is referenced and is defined as the reciprocal of resistance: In circuit analysis,it is occasionallymore convenientto use conductancethan resistance. Later, the explanationof this symbol will be generalized. Kirchhoff's Voltage Law In the simple series circuit shown in figure 2.2, three resistors are connected in tandem to form one single closed loop. Kirchhoff has shown that when several elements are Figum2.1.-Clrcult element Illurtratlngvoltage polarltyand cumnt flow dlmtlon.
- 48. connected in series, the current in the circuit will adjust itself until the sum of voltage drops in the circuit is equal to the sum of voltage sources in the circuit. This can be restated as the "sum of all voltages around any closed cir- cuit is zero," which is called Kirchhoffs voltage law. For the circuit shown in figure 2.2, veb+ vbs+ vsd+ vda= 0 (2.9) V . b + v*, + Vs* - Vnd = 0 (2.10) v, + v, + v, - v. = 0. (2.11) Obviously, some of these potential differences could be negative and some positive. This circuit shows only resist- ances and a voltage source, but the network could contain other kinds of elements and might be as complicated as desired. However, Kirchhoff found that the sum of the volt- ages around any closed loop in a circuit, such as a-b-c-d,is always zero. The symbol shown in figure 2.2 beside v, represents an ideal voltage source. Such a source maintains a given voltage across its output (terminals)regardless of the load, but actual voltage sources cannot supply an infinite cur- rent if the terminals are short-circuited; that is, they are tied together so the resistance approaches zero. Therefore, actual sources are usually considered to be ideal voltage sources with an internal resistance connected in serieswith the source and the output terminals. The assumption is illustrated in figure 2.3. EXAMPLE 2.1 Find the current I flowing in the single-loopcir- cuit in figure 2.4. SOLUTION. Adhering to the assigned clockwise direction for current, Kirchhoff s voltage law produces the following equation: -50 + v, + 100 +v,= 0, where V, and V, are the voltages across the 1-R and 2-R resistances. From Ohm's law, v, = 11, v, = 21. Inserting these expressions into the voltage law equa- tion produces -50 + 1 1+ 100 + 21 = 0 or 31 = -50, I = -16.7 A. The negative sign statesthat the actual currentflow is in the opposite direction from that shown in fig- ure 2.4. It should be noted that when writing the voltage- law equation, voltages that oppose the assigned cur- rent flow are considered positive, otherwise negative. Therefore, the 100-Vsource is positive, and the 50-V source is negative. The positive signs for V, and V, assumed opposition by the convention shown in fig- ure 2.1. Kirchhoff's Current Law The other law attributed to Kirchhoff specifies that "the sum of all electrical currents flowing toward a junc- tion is zero." In figure 2.5, five wires are soldered together at a common terminal and the current in each wire is measured. If current flowing toward thejunction is called positive (thedirection shown in the figure)and the current outwards is negative (against the arrows), then the sum of the five currents is zero: As was the case for equation 2.9, this equation implies that some currents must be positive, some negative. If two or more loads are connected between two com- mon points or junctions, these elements are said to be in parallel, as shownin figure 2.6A. The same is true for figure 2.6B, and moreover, the two circuits illustrated in figure 2.6 are identical,just drawn differently. It is important to I Ideal J source Ideal voltage source Actuol voltage source Flgure 2.3.- Ideal and actual voltage roumr. Flgure2.4. -Circuit for example 21. Junction Flgure25.- Demonstration of Klffihhoff's cumnt law.
- 49. note that the lines in these and all circuit diagrams usu- ally show no resistance. Each line is only a connection be- tween elements or between an element and a junction. The similarity in the diagrams can be shown using Kirchhoff s current law. In both, there are only two independent junc- tions, a and b, and for either point, The circuit symbol next to i, in figure 2.6 represents an ideal current source. and a similar situation exists for For the circuit in figure 2.9, if the voltage, v, produces the same current, i, through the circuit, then v = iR, but v = i (R, + R, + R, + R,), iR = i (R, + & + R, + R 4 ) , or R = R, + R, + Rs + R 4 . (2.14) Here R is said to be the equivalent resistance for the previous series circuit. In other words, R is the series resistance of that circuit. The same loeic a ~ ~ l i e s to all elec- - .. all practical current sourcesas was mentioned for practical trical elements in series. voltage sources. However, the internal resistance is effec- tivelyconnected in parallel across the ideal current source. Both ideal and actual current sources are shown in fig- ' 1 11 ure 2.7. - 0 - 0 EXAMPLE 2.2 Verify that KirchhofYscurrent law holds forjunc- tion x in figure 2.8. -b - il '! SOLUTION. The three resistances in figure 2.8 are in parallel, and the 100 V produced by the voltage source exists across each. Therefore, by Ohm's law, the current through each resistance is I,, = 100 = 2 A, 50 Kirchhoffs current law states that for junction x, I,, + IS0+ 1100= 7 A. Accordingly, 4 + 2 + 1 + = 7 A . Series Circuits To restate the earlier definition of a series circuit, elements are said to be connected in series if the same cur- rent passes through them. Such isthe situation for the four resistors shown in figure 2.9.It would be convenient to find a resistance, R, that could replace all series resistors. This equivalent resistance can be found by returning to the Ohm and Kirchhoff voltage laws. By Kirchoffs law, v = v, + v, + v, + v,, but by Ohm's law, Figure 2.6.-Simple parallel circuits. (italic letten an, cited in text.) current I - - Ideal I source current source Actual current source Figure 2.7.- ideal and actual current sources. Figure 2.8. -Parallel circuit for example 2.2. Therefore, v = iR, + iR, + iR, + iR,. Figure 2.9. -Simple series circuit and equivalent.
- 50. It is often useful to find the voltage drop acrossjust one element in a series circuit. To arrive at an expression, again refer to figure 2.9. For the current through the circuit, it is obvious that . . . " . I = , - 1 - - - l3 = 14, but . v1 v2 v3 v4 Therefore, = - = - = - = - R1 R2 R3 R,' As before, consider R, the equivalent circuit resistance, and v v1 v v2 Therefore, - - - - - - - - . . - - - R R l ' R R,' In other words, the voltage drop across any one element is equal to the total circuit voltage times the ratio of the ele- ment's resistance to the total circuit resistance. Parallel Circuits Following the discussion of series circuits, it would be useful to have similar equivalence, voltage, and current relationships for parallel circuits. For the circuit shown in figure 2.10, the voltage is the same acrosseach resistor and is a corollary to current through series elements. Using the same basic procedures as for seriescircuits, it can be shown that G = Gl + G, + G3+ G, + - - - - - (2.16~) and also Restated, the total conductance of parallel-connected re- sistors is equal to the sum of all individual conductances. Likewise, the reciprocal of the total resistance of parallel- conducted resistors is equal to the sum of the reciprocals of the individual resistances. A special case that is very often found occurs when two resistances are in parallel. If these resistances are R, and R,, then If current distribution through parallel circuits is of in- terest rather than voltage distribution,Kirchhoff s current law and Ohm's law can be employed to show that and so on for the balance of currents. The preceding paragraphs have been used to show the immediate application of Ohm's law plus Kirchhoff s volt- age and current laws to circuits that have more than one element. The results are extremely valuable in circuit anal- ysis and are used extensively to solve circuit problems. It is important to note now that these concepts are also valid when circuits contain components other than resistance. Later, after the balance of fundamental laws and param- eters have been covered, more applications of these laws will be shown. I EXAMPLE 2 . 3 I A series-parallel circuit is shown in figure 2.11. Find the equivalent resistance. SOLUTION. The objective is to find an equivalent resistance between terminals a and b. The process is to combine resistances in series or in parallel until I the equivalent resistance is obtained. l%e 2-Qand 4-Q resistances between point 1and terminal b are in series, and from equation 2.14, If a 6-R resistance replaces these two series resist- ances, it can be seen that two 6-Q resistances are in parallel between point 1 and terminal b. Applying equation 2.17, which means that a 3-R resistance can replace the two 6-R parallel res~stances. Therefore, the 3-R resistance between point 2 and point 1 is in series with the equivalent of 3Qbetween point 1and terminal b, and again Now between point 2 and terminal b, there are the equivalent of two 6-R resistances in parallel and Consequently between terminal a and terminal b, a 7-R resistance is in series with an equivalent 3-R resistance, and the equivalent resistance of the en- tire circuit is R = 7 t - 3 = 1 0 Q . Figure 2.10. -Simple parallel circuit. 7rl 2 3rl , 2 r l a -7m4fl b L - Figure 2.11. -Serles.paralleI clrcult for example 2.3.
- 51. EXAMPLE 2.4 Find the equivalent resistance of the circuit illus- trated in figure 2.12. SOLUTION. Point b and point b' can be seen in the centerof the circuit,but these are electricallyjust one point, because b and b' are only separated by a line that does not contain an electricalelement.Thus,the 15-Q and 30-51resistances between a and b are in parallel, as are the two 40-Qresistances between b and c. From equation 2.17, (15x30)- -- 15 + 30 and (40x40)- 20 , -- 40 + 40 Therefore, the resistance of the circuit between ter- minal a and terminal d canbe reduced to three series resistances, and the equivalent resistance is R = 10 + 20 + 10 = 40 Q. The Magnetic Field A. M. Ampere was the first scientist to establish that the conductor through which electric current is passing is enclosed in a magnetic field. The relationship is depicted in figure2.13A.After Ampere's discovery,many experirnen- ters tried to reverse the process and create electriccurrent from a magnetic field. Finally, in 1831,Michael Faraday discoveredthat as a magnet is inserted into a coil of wire, an impulseof electrical current will flowthrough the wire. When the magnet remains stationary within the coil, no current is produced. When the magnet is withdrawn,a cur- rent impulse is again observed,but this time it flowsin the opposite direction. The process is demonstrated in figure 2.14.Faraday visualized the effect as a result of magnetic flux lines cutting or moving through the conductor.When- ever relative motion occurs, an emf is produced in the conductor. This disclosure laid the foundation for electro- mechanicalconversion, that is, the conversionfrommechan- ical energy to electrical energy and vice versa, as found in generators and motors. The magnetic field mentioned here is a condition of space.The directionof a magneticfieldflux line isthe direc- tion of force on a magnetic pole, and the flux line density is in proportion to the magnitude of forceon the pole. Each line represents a certain quantity of magnetic flux, meas- ured in webers. It is a magnetic field characteristic that every flux line is a closed curve, forming the concentric- circle pattern shown in figure 2.13A. These conditions of the magnetic field are employed to develop relation- ships in magnetic devices,which are covered in upcoming sections. When a wire is wound into a coil, an interesting action occurs: as the magnetic flux builds up around one wire, it tends to cut through adjacent turns of wire. In this way a voltage is induced into the coil windings. The concept is shown as dashed flux lines around one winding of the coil in figure 2.13B. ion C Figure 2.12.-Series-parallel circuit for example 2.4. Lines of magnetic 0 A flux Figure2.13. -Magnetic flux in a straight conductor (A) and in a long coil (8). Cardboard Bor magnet moving Magnet movingout into the coil of the coil Galvanometer Current Figure 2.14.- Demonstration of inducedcurrent. Inductance Joseph Henry found that electricityflowing in a circuit has a property analogousto mechanical momentum;that is, current is difficult to start but once started it tends to continue. This is the case for any element from a simple conductor to the most complex.Faraday explained the phe- nomenonby visualizingthe magneticfield in spacearound the conductor. In terms of the coil in figure 2.14,the volt- age inducedin the other windingsis proportionalto the rate at which the magnetic flux lines are cutting through the coil. Yet the magnetic flux is also proportional to the cur- rent in the coil. The induced voltage is such that at every instant, it opposes any change in the circuit current. For
- 52. this reason, the induced voltage is called a counterelectro- motive force, cemf. This interrelationship is so important that it has the status of a physical law and is known as Lenz's law after the scientist who first defined it. The property that prevents any change of current in the coil is called self-inductance;hence the coil is known as an inductor. The greater the induced voltage, the greater is the opposition to the change in current flow. Therefore, the cemf produced by a specificchange of current is a measure of circuit inductance. Expressed as a formula, v = Urate of change of current) where v = voltage across coil, V, L = proportionalityconstant known as inductance,H, i = current through coil, A. As noted, inductance is given the symbolL and ismeasured in units called henries in honor of Joseph Henry. A circuit has an inductance if 1H when a current change of 1A/S causes a cemf of 1V to be induced in the coil. The expres- sion "dildt" represents the rate of change of current, i, in the coil. When two separate coils are placed near each other, as shown in figure 2.15, the magnetic field from one coil can cut through the windings of the second coil. It followsthat a change in the current in coil 1can produce an induced voltage in coil 2. This current-voltage relationship is ex- pressed as v, = L,, (rate of change in i,) di, v, = L2, (-). dt Similarly, if the current in coil 2 is changing, it induces a voltage in coil 1: v, = L,,(rate of change in i,) di, v, = L12 (-). dt L , , and L , , are called mutual inductances and are again expressed in henries. The mutual inductances increase if the coils are brought closer together and decrease as the coils are moved further apart. Two magnetically coupled coils are usually called a transformer. Although not by all means obvious, the two mutual inductances of a pair of magnetically coupled circuits are equal, or The self-inductanceof an actual coil is a function of both the coil configuration and the total number of turns. Fur- ther, because the magnetic flux may induce currents in ad- jacent conductors, the environment in which the coils are placed may also have an effect. Numerous inductance equa- tions are available in handbooks and other reference books, each valid for a given coil configuration; consequently, only a fewthat give approximate inductance values are provided here to demonstrate the parameters that affect inductance. The two symbols used to indicate inductance are shown in figure 2.16; the symbol on the right is that commonly found in power-circuit diagrams. For a long coil as shown in figure 2.16, the inductance is where L = self-inductance, H, p = permeability, Hlm (for air, 4n.10-' = 12.566.10-I), N = turns of coil, A = coil cross-sectional area, m2, and P = coil length, m. The coil cross section need not be circular. The formula is only approximatebecause it assumes that all flux lines link all turns of the coil, which cannot occur at the coil ends. However, the formula gives good results for long coils and does reveal the following important relationships. Coil inductance is proportional to the square of the number of turns. Inductance is proportional to the core permeability. Inductance is proportional to the cross-sectional area of the core. Inductance decreases as the length increases. For a shorter single-layer circular solenoid (coil),the induc- tance is approximately where d = coil diameter, wire center to center, m. Figure 2.15.-Two coils demonstretlng mutual Inductance. Symbol - Area Symbol N m Figure 2.16. -Long-coilInductanceand inductor symbols.
- 53. For the toroidal coil of rectangular cross section in figure 2.17, where dl, d, = inner and outer diameters as shown, m, and h = thickness, m. Note that In indicates the natural logarithm, that is, the logarithm to the base e. Capacitance When two conducting surfaces are separated by a dielec- tric or insulating material, an effect known as capacitance is observed. If two electrical conductors are at different potential, there is some storage of charge upon them. A capacitor is a device included in a circuit for the purpose of storing or exchanging this electrical charge. Further, when capacitance is present, the charge observed to flow intothe capacitor is proportional to the voltage applied. This can be expressed as: q = Cv, (2.26) where q = stored charge in capacitor, C, v = applied voltage, V, and C = proportionality constant called capacitance, F. To analyze circuits, a relationship between the voltage applied and the current flowing into and from the capaci- tor is more useful. Current is the rate at which chargeflows (i = dqldt). It therefore holds that for a given capacitance, i = Cirate of change of v) where i = current flowing into the capacitor, A, and v = voltage across capacitor, V. This is very similar to equation 2.19 and, using the discus- sion in that section, capacitance can be defined as that elec- trical circuit property which tends to oppose any change in voltage. The capacitance of a capacitor depends on the size of the conductors or plates, their proximity, and the nature of the material between them. For most dielectric materials, C is constant. the capacitor remains constant, as in dc circuits, no current will flow into or out of the capacitor. Electric Field An electric field exists anywhere in the neighborhood of an electrical charge, for example, between the plates of a capacitor. The direction of this field is by definition the direction of the force on a positively charged exploring par- ticle (aparticle free to move within the electric field). The strength of the field, E, is proportional to the magnitude of the force.If the charge of the exploring particle is q, then the force is where F = force on particle, N, and E = strength of electric field, Vlm. Electric-field flux lines are visualized as issuing from positive electric charge and terminating on negative charge as shown in figure 2.19. @ N turns Cross section Figure 2.17. -Toroldal coil. Symbol f a Equations2.26 and 2.27 have algebraic signs consistent with the arrows in figure 2.18. The symbol shown is for Flgure 2.18. -Charge, voltage, and current ralatlonships of capacitance; note that a positive terminal voltage produces capacitor. - - positive current and hence positive charge. If the voltage across a capacitor is desired, equation 2.27 can be integrated, resulting in 1 v = - I ' idt + V,. co This equation represents the change in voltage across the capacitor from some arbitrary reference time, called t = 0, to a later time, t. Vo is the potential across the capacitor at time t = 0.The expression 1 ' 4 F ~ 0 ' idt" is the voltage change across the capacitance from time Flgure 2.19.- Electrlc lines of force between two parallel t = 0to time t = t. From the formula, if the voltage across charged plates.
- 54. Voltage or potential difference is by definition the in- tegral of electric-field strength or A simple application of this concept can be demonstrated fromfigure 2.19. Ifthe electricfieldbetween the twoparallel plates is constant, the voltage between the plates is Assume that a positively charged particle, q, is released from the positive plate in figure 2.19, the particle being within the electric field and free to move. If it moves, work is performed on it by the electric field. The amount of work can be found by employing the mechanical formula where w = work done, J, F = force on particle, N, and s = distance particle moves, m. Since work is but so F = qE, w = qEs, v = E.s, w = qv. Therefore, when electricity moves from one potential to another, the work doneis equal to the product of the amount of electricity and the potential difference. In the next section, this concept is applied to a common electrical component. Instantaneous Power Consider the resistor shown in figure 2.20. A charge, dq, isfree to move in the resistor from the point sto s +ds. It moves the distance, ds, in time, dt, and is impelled by the electric field in the region, E. The electric field exerts a force on the charge, dq, while it moves through ds, or The work done in this section of the resistor during time dt can be expressed as Flgum 220.- Reslstor used to demonstrate Instantaneous power. Power is work per unit time (inother words, the rate of do- ing work), or for this section of the resistor, where p = power, W. However, the current through the resistor is the rate at which charge flows, i = dqldt; therefore, Current is constant throughout the resistor and is not a function of distance, s. The potential difference across the region, ds, is and the power acrossthe whole resistor, from a to b, is then p = Jhi~.ds =i J b ~ . d s = iv. (2.38) Formula 2.38 represents only the instantaneous power con- sumed by the resistor, or the power occurring at only one instant in time. This is an extremely important formula as it forms the basis for most power relationships. Idealization and Concentration The foregoing has established the elementary laws and parameters that can be applied to investigate electrical circuits. Practical circuits found throughout a mine, or in fad anywhere else, are composed of wires, coils, and elec- trical devices of varying complexity. Before these funda- mentals can be employed, it is necessary to translate the practical world into an ideal and simple world. The trans- lation is called idealization and is in essence the construc- tion of a model. Here, electrical effects that create insig- nificant results are eliminated. For instance, two adjacent conductors in a coil always exhibit capacitance but the capacitance might be so small that the stored charge is negligible. Yet for many situations, the resistance and in- ductance must remain. For every conductor or component in a circuit, resist- ance, capacitance, and inductance are distributed through- out the entire length or breadth of the portion. It would be much simpler to apply the preceding relationships if these circuit parameters were combined or concentrated into separate circuit elements. For most circuit analysis needs, fortunately, these can indeed be consolidated. The fundamental aspects of idealization and concentra- tion are illustrated in figure 2.21A,which shows a voltage generator connectedto a coil of wire and a resistor in series. Figure 2.21B gives the translation. The distributed resist- ance and inductance of the coil have been combined into R, and L, and the coil's capacitance has been ignored. The resistance of the resistor and its lead wires has been con- centrated into one value, R. Finally, the voltage generator is represented by an ideal voltage source and an internal series resistance. Note that the lines shown in figure 2.21B serve only to connect components and exhibit no electrical properties or effects. Another example can be expressed from figure 2.22A. Here, a load center is shown connected to a shuttlecar through a trailing cable.Again, figure 2.22B gives the translation. The distributed resistance, induc- tance, and capacitance of the trailing cable have been
- 55. Wire coil connection generator Resistor Inductor uniform values. Beyond this, the term "dc" is also applied to ordinary or practical currents that are approximately steady. The following section explores dc circuit analysis, an important topicbecause of its extensiveuse for mine haul- age and for driving electroniccomponents.The study of dc analysisat this time allowsthe fundamentalelectricallaws and parameters to be applied and extended without hav- ing the effort clouded by complex current relationships. Flgun2.21. -Simple example of Idealizetionandconcentra- tlon. - Trailina Shuttle car 1 Load cable- center Flgure 2.22 -Modeling of load center, tralllng cable, and rhuttle car. represented by the combined R,, L,., and C,.. Suchmodels can be developed for all portions of a mine distribution system. In this case, although the cable capacitance is rather small, it is shown here to emphasizethat it may not always be negligible. The shuttle-carmotor is depictedby the symbol shown.The symbolis obviouslythe same as that for a source, for reasons given later. After constructing the circuit representation, or sche- matic, as it is most often called, the relationships covered previously in this chapter can be used to solvefor currents and voltageswithin the circuit.A differentialequationwill usually result, but if the circuit contains only resistance, the only necessary expression is Ohm's law. This will be the case for most dc circuit analyses. DIRECT CURRENT CIRCUITS Electrical current consists of the motion of electrical charges in a definite direction. The direction and magni- tude of current canvary with time, and accordingly,all cur- rents can be classified into one of three basic types: Direct current (dc), Alternating current (ac)or sinusoidal current, and Time-varyingcurrent. Direct current is a steady,continuous, unidirectional flow of electricity.In otherwords,voltage0and current O have Direct Current and Circuit Elements Figure 2.23 gives the basic elements of resistance, in- ductance,and capacitance,each having a voltage and cur- rent asshown.Apowerful simplificationof complexcircuits can be understood by examining the effect of dc on these elements.As before, the voltagecurrent relationshipfor the resistor is Ohm's law: V, = IR. (2.39) For the inductor, the voltage across the element is but, because I does not change, dl - = o dt and V, = 0. (2.40) Likewise,for the capacitor,the current through the element is but again, Therefore, inductance and capacitancephenomena are not present under pure dc. In otherwords, the capacitorappears as an open circuit, while an inductor resembles a conduc- tor showing only resistance. An example of this simplifi- cation is available in figure 2.24.The circuit on the left shows all circuit elements, but under dc the effective cir- cuit is given on the right. The result is a simple series- resistance arrangement, and the only voltage-currentrela- tionship necessary for the analysis is again Ohm's law. Series and Parallel Reslstance The expressionsused to find the equivalent resistance of parallel or series resistances are as before: for series, R., = R, + R, + R, + .... + R.; 1 - 1 1 1 1 for parallel, -- - + -+ -+ .... + - ; Rsq R1 Rz Ra Rn using Ohm's law, V = IR., = I(RL+ R,) v v orI=-=-. R., RL + R,
- 56. Resistance R ohm's law :V = I R Power:P=VI I V p= 12R Z * R L Inductance v = L ~ = o ,Power=O I v C Capacitance + t - - - - . + C I v Figure2.23. -Basic elementsof resistance, inductance,and capacitance. Figure 2.24. -Simplification of dc circuit. Figure 2.25. -Simple circuit reduction. Hence, the current through or the voltage across the cir- cuit can be found. By employingthe previously given for- mulas, the voltage-currentrelationshipsfor eachcircuitpart can be determined. This concept can be elaborated consideringfigure 2.25, which shows a series-parallel circuit where an element of the circuit may be in parallel or serieswith other elements. Arrangements such as these can be solved by observing which individual elements are in series or parallel, then making the appropriate combinations. The objective is to gradually reduce the circuitto an equivalent seriesarrange- ment, which can then be replaced by a single equivalent resistance. Simple illustrations of this process have been shown in examples2.3 and 2.4.Accordingly,the circuit in figure 2.25A can be changed to figure 2.25B by the paral- lel combination The circuit in figure 2.25B is then reduced to the circuit in 2.25C for the series combination R., = Rl + &' = Rl + qR, 4 + R, v Afterwards, if V is known, I, = . Further, the current and voltage distributionscan be deter- mined by R1 V , = (-)V and R,' v,= (-)V, Re, R., or V , = R,I, and V , = %'Il. In this way, all voltages and currents in figure 2.25 can be found. In summary, the main process used is the substitu- tion of a single resistance for several series-parallel resist- ances. In concept,the sameterminal resistance( R . . ) implies equivalence and results in identical current and voltage delivered from the source. This process of solution is for- mally known as circuit reduction. The power consumedby all or part of the circuit can be found by applying equation 2.38: P = VI. (2.42) Noting Ohm's law,V = IR, two other convenient power ex- pressions are P = (IR)I = I=R (2.43) and Thesethree expressions can be used to find the power loss, expressedasPR loss,due to conductorresistancebefore the current is deliveredto a load, as well as the power used by that load. EXAMPLE 2.5 For the circuit showninfigure2.26,determinethe current I flowing through the 30-Q resistance, the power supplied to the circuit by the voltage source, and the power consumed by the 15-Q resistance. SOLUTION. The 25-Q, 1 5 4 , and 10-Q series resistances are in parallel with the 50-Q resistance, and The equivalent resistance seen by the 50-V ideal source is the sum of three series resistances: R., = 30 + 25 + 45 = 100 Q. The current delivered by the source is then V 50 I1 - - = - = O . j j A - R., 100 and the power supplied to the circuit by the source is P, = VI, = 50(0.5)= 25 W .
- 57. The current through the 15-Q resistancecan be found by using current division: Therefore,the power consumedby the 15-Rresistance is P = I:R = ( 0 . 2 5 ) ' 1 5 = 0.94W. -- EXAMPLE 2.6 Find the current between points a and b, I.,, in the circuit of figure 2.27. SOLUTION. The circuit is very similar to figure 2.12 as used in example2 . 4 .Point a and point b are at the same potential, so the right-hand side of the circuit is essentially two parallel arrangements of 15-52 and 30-Rresistances.The two parallel arrangements are in series. As the equivalent resistance seen by the 30-Vsource is R,, = 10 + 10 + 10 = 30 R, and total circuit current is Because of effective parallelism, I., I,, I,, and b can be found by Even though the potential at point a is the same as the potential at point b, the line connectinga to b has the ability to carry current. By Kirchhoffs current law for point a, 1- = 1-b + I=, and for point b, 1.b + I b = Id. By either relationship, Iob = 0.67- 0.33= 0.33 A. EXAMPLE 2.7 The circuit in figure 2.28 is a series-parallel ar- rangement of conductances. Find the voltage V. SOLUTION. As total conductance of parallel- connected conductances is the sum of the individual conductances, the combination of the two elements between points a and b is Gab= 2 + 2 = 4. This combined conductance is in series with the 1-S conductance so that total conductance of the circuit portion from point a through point b to point c is GSb, is in parallel with the 2-Sconductancebetween points a and c, and the equivalent conductanceseen by the ideal by the ideal current source is G., = 2.0+ 0.8= 2 . 8 S . I Therefore, the voltage shown in figure 2.28is Flgure2.26. -Circuit for example 2.5. Flgure2.27. -Clrcult for example 2.6. Figure2.28. -Series-parollel conductancea for example 2.7.
- 58. EXAMPLE 2.8 For the circuit shownin figure 2.29,findthe total circuit current, I,, with the components as shown, with the 5-R resistor short-circuited, and with the 5-R resistor open-circuited. SOLUTION. For the circuit as shown, the two 10-Q resistors between points c and d are in parallel and Thisresistance is in serieswith the 5-Rresistanceand these three elements are in parallel with both the 15-Q and 10-R resistances between points b and d. Thus, and R,, is in series with the 7-Rresistance and R.,, = 7 + 3.75 = 10.75 R, and R.,, is in parallel with the 10-R resistance be- tween points and a and d; both are across the 120-V source. Therefore, and total circuit current is When the 5-R resistance in figure 2.29 is short- circuitedor replaced with zero resistance,pointsb and c are electricallythe same.Four resistances are now in parallel between pointsb and d; three 10-Qand the 15-Q resistance. Following the same procedure as before, the equivalent resistance becomes and total circuit current is For the case of an open-circuited5-Q resistance, the resistance between b and c is assumedto be infinite, and the two 10-Qresistances between points c and d are disconnected from the circuit. The equivalent resistance is now and the total circuit current is This example illustrates an important concept. When an element in a circuit is short-circuited,the equivalentresistance of the circuitwill decrease,and total circuit current will increase. Conversely, with an open-circuitedelement,the equivalent resistance of the entire circuit will increase while total circuit current decreases. Wye-Delta Transformations Any of the circuitsnow covered canbe reduced to a two- terminal network, as seen in figure 2.30A. The circuit receives power from an external source and can contain resistance,inductance, and capacitance.Suchnetworks are called passive. For dc, only resistance is of interest, and it can be found from the terminal voltage and current by Numerous circuits can be represented by a two-terminal arrangement. Other circuits, including several in mine power systems,cannot be represented inthis way,but many of these can be resolved into the three-terminal network given in figure 2.30B Even though with three terminals there now exist three voltages and three currents,the con- cept of circuit equivalence still holds; that is, voltagesand currents are identical and the circuits are equivalent. Flgum2.29. -Serles.parallel clrcult tor example 2.8. Flgum 2.30.-Twotermlnal (A) and three-terminal (8)net- works.
- 59. For three-terminal networks,there are two basic circuit configurations:the wye and the delta (A) (fig.2.31). The wye is sometimescalled a star, but the term y is standard. It is sometimes advantageous to replace or substitute the three wye-connected resistances with another set that is delta-connected,or-viceversa. By using equivalenceof input currents and voltagesfor wye and delta circuits,delta-wye(ordelta-to-wye)and ye- delta transformations can be derived.Thusfor equivalence of the circuits in figure 2.31, R.R, + RbR, + R.R. R,s = (2.45) R. R.R, + RbR, + RcR. Figure2.31. -Wye (A) and delta (0)circult configurations. and R,. = (2.47) Rb In other words, the delta is equivalent to the wye if the resistances of the delta are related to the wye by equations 2.45, 2.46, and 2.47. Accordingly, with three terminals a, b, c,containingwye-connededR., Rb,Re,the circuitperform- ance is unaffected by replacing them with a delta-connected R.,, R,,, R,.. Likewise, for equivalenceof delta-to-wyesets, and R.,RbC R. = R.b + Rb. + R,.? R.bR*. R, = R., + Rbr + R,.' Thesetransformations are useful in allowingthree-terminal circuit reduction because they allow substitution when a network does not contain either seriesor parallel elements. The circuit or circuit portion may not outwardly appear as three-terminal, and common examples, n and T, are given in figure 2.32. These are actually delta and wye circuits drawn in a slightly different fashion. It will be shown in chapter 4 that delta and wye circuits are the two most im- portant configurations for power systems. These trans- formations will be called upon again at that point. It has already been shown that when circuit elements are neither all in series nor all in parallel, but have some other series-and-parallelarrangement, the elementscan be handled in groups to reduce the circuit to an equivalent resistance.This important kind of circuit analysis has been called circuit reduction.Now that delta-wyeand wye-delta transformations have been introduced, the substitution process can be employed to solve networks that contain elements neither in series nor parallel. A prime instance is the common bridge circuit shown in figure 2.33. The bridge is one of the most used configurationsin electrical instrumentation. The objectivehere is to find all available currents and voltage drops in the network, and an overall solution approach is illustrated in the following example. L Figure2.32. -T and " 2 'circuit configurations. Figure 233. -Common bridge circuit.
- 60. EXAMPLE 2.9 Consider that the resistances shown in figure 2.33 are as follows: R,= 5 R R , = l O R R , = 1 5 R R, = 20 R R, = 25 R R, = 0.4 52 Find the equivalent resistance of the circuit between points a and b. SOLUTION. The original circuit has been redrawn in figure 2.34A, in which a delta configuration is clearly defined by points a, c, d. The first step for cir- cuit reduction is to convert the delta to a wye. From equations 2.48, 2.49, and 2.50, and R, = (5)o = 2.5 Q, 30 This conversion results in the simple series-parallel circuit in figure 2.34B. Combiningthe serieselements and parallel branches in the center of the circuit fur- ther reduces the circuit to that shown in figure 2.34C: The equivalent resistance of the total circuit is then simply R., = B + R, + R, The total circuit current can now be found using Ohm's law; for instance, if then "b v . b 30 1 = - = - = 2 A . R., 15 Finally, Kirchhoffs current and voltage laws and the voltage and current distribution formulas can be employed to find currents through and the voltage drops across each circuit element. For example, if I, is the current through R,, then R d + R, I4= Id- R, + R, + R d + R, 1 It should be noted that the current through R is also I,, but R, doesnot exist inthe ofiginal circuit of figure 2.33. Thus, a problem exists in finding the currents through R,, R, and R,. One solution would be to solve for the three potentials among points a, c, d and use Ohm's law to find the three currents in the assigned delta connection. Figure2.34. -Circuit mductlonof bridge circuit.
- 61. EXAMPLE 2.10 Consider that the resistancesshownin figure2.33 are R, = 15 R R, = 15 Q R, = 15 Q R, = 20 R R, = 25 Q R, = 10 Q Find the equivalent resistance of the circuit between points a and b. SOLUTION.Three identicalresistancesform a delta configuration in the circuit, or R, = R, = R, = 15 Q. Following the same processes as in example 2.9 for figure 2.34B, R. = (15)(15) = ., 15 + 15 +15 R b = 5 R and R. = 5 Q. Now, the center resistance of figure 2.34C is Rr = (5 + 2016 + 25) = 13,64 *, 5 + 2 0 + 5 + 2 5 and the equivalent resistance of the circuit is R., = 5 + 13.64 + 10 = 28.6 ' 2 . It should be noted that an important situation is es- tablished where all resistances in a delta or a wye configuration are equal. If Ra is each resistance in the delta and R, is that in the wye, then from equa- tions 2.48, 2.49, or 2.50, R, = R A R, R, + R, + R, Ra or R , = - . 3 The majority of delta or wye ~ o ~ g u r a t i o n s used in power systems consist of identical elements in each leg. Much of circuit analysis can be handled by circuit re- duction, but as circuits become more complexthis process becomes cumbersome. Nevertheless, circuit reduction should always be used when it produces results more eas- ily than other methods.There are solutionapproachesthat are more systematic,andthe next two sectionsdiscusstwo of these. Circuit and Loop Equations Before more general solution methods can be identified, the meanings of some words need to be clarified. A node is the position or point in a circuit where two or more elements are connected. When three or more elements ex- tend from a node, the node is called a junction. A branch is a circuitportion existingbetweentwojunctions and may contain one element or several in a series. A loop is a sin- gleclosed path for current. Figure 2.35 illustrates all these circuit parts. The followingtechnique, loopanalysis,is based entirely on Ohm's law and Kirchhoffs voltage law. The analysis principle produces n simultaneousequations requiring the solution of n unknowns, and the unknowns are currents. In loop analysis it is only necessary to determine as many differentcurrentsasthere areindependentloops;that is, the equations are constructedby defining independent- loop currents. For example, in figure 2.36, the current I , , flowing out of source V, and through R., will be around loop 1.Thecurrent flowingfrom sourceV, through R, will be around loop 2. Although not essential, these directions follow the general convention of assigning all reference loopsclockwise.It is sometimesmoredesirabletouse other directions,for instancewith currentsflowingout of a source positive terminal, but it is imperative that the use of cur- rents within a specificloopremainsconsistentafter the loop is assigned. Notice in figure 2.36 that both I, and I, flow through Rb.Depending on the loop direction,that is, the direction defined by I, or I,, the total current through Rbis either I,-I, or I,-I,. Thus if I, and I, can be found, the current through each circuit element can be determined. The first task in loopanalysis isto use Kirchhoffs volt- agelaw to write equations about eachcurrent loop, stating that the sum of voltages about each loop equals zero. For loop 1, Notice that R.1, equals the voltage drop across R., and Rs(Il-I,) equalsthat acrossRb.In the latter case,the voltage can be taken as R,I, - %I,, consideringthat the voltage ~roduced by I ,opposes that produced by I,. Likewise, for loop 2, Figure 2.35. -Parts of circuit. Flgun238. -Circuit dernonstratlngtwo independentloops.
- 62. By rearranging equations 2.51 and 2.52, (R. + RdI, - %I, = V, (2.53) and -R& + (Rb+ R)12= V,, (2.54) which are two simultaneousequationswith two unknowns, I, and &. These can be solved easily by simultaneous methods. It should be noted that one additional loop equation could be written, that for the loop containing both V, and V.. However. this will not ~rovide another inde~endent Flgun 2.37. -Two-loop clrcult for example 2.11. equation. ~nformation concerningthe maximumnimber o f independent equations available will follow shortly. EXAMPLE 2.11 Find the current through the 1.5-Qresistor in figure 2.37 using loop analysis. SOLUTION. Two loops are defined in the figure where the current through the 1.54 resistor is I, + I,. Applying Kirchhoffs voltage law to loop 1, 0.51, + 1.5(11+ I,) + 1.01, = 250 and for loop 2, 0.51, + 1.5(11+ I,) + 1.01, = 300. By simplifying these equations, 31, + 1.51, = 250, 1.51, + 31, = 300. Simultaneous solution of these results is I, = 44.4 A, I, = 77.8 A, and the current through the 1.542resistor is I, + I, = 122.2 A. To further enforcethe concept of loop analysis, again consider the common bridge circuit, which is redrawn in figure 2.38 to include current loops. Three loop equations can be written because there are three possible indepen- dent loops. For loop 1, for loop 2, , ( I - I ) + I + R - 3 ) = % (2.56) for loop 3, R,(I, - I,) + R,(I, - 13 + &I3 = 0. (2.57) Figure2.38.- Brldge clrcult demonstrating loop analysis. Again, rearranging, which are three simultaneous equations with three un- knowns, I,, I,, and I,. The proper combinationof these cur- rents will yield the current through each branch of the cir- cuit. The process was again to employ Kirchhoffs voltage law for the purpose of finding the unknown currents. Other loops about the bridge couldbe assigned and will produce the same valid results. Generally, the particular choice of loops can enhance a desired result. For instance, if only the current through R, of figure 2.38 is desired, establishing one loop current through that resistor would create a more direct solution. EXAMPLE 2.12 Using loop equations, solve for each branch cur- rent in the circuit shown in figure 2.39. SOLUTION. Applying Kirchhoffs voltage law to loops 1 and 2 respectively, 2(11- LJ + 5(11- I,) + 21, = 56, 2(I, - I,) + 101, + 1(I, + I,) = 0.
- 63. As the assignedcurrent for loop 3 passesthrough an ideal current source, Therefore, the equations for loops 1and 2 become 2al - I,) + 5(I, + 6) + 21, = 56, 2(1, - I,) + lOI, + 1(12+ 6) = 0, or 91, - 21, = 26, -21, + 131, = -6. Solutionof the last two simultaneousequationsyields Each branch current can now be resolved from the loop currents. For the branch containing the 2-Q resistor and the 56-V source, for the other 2-Qresistor, and the resistors in the other branches, A loop equation could have been written for loop 3,but it can only state that the voltage drops across the 1-Q,5-Q,and 4-9 resistors in that loop are equal to the voltage across the 6-A ideal current source, which is unknown. Such an equation would only complicate the solution to the problem. As circuits become more complex and the number of possible loops increases, a method for determining the number of required equations is useful. By counting the number ofbranches andjunctions in the circuit,the follow- ing expression provides the necessary number of loop currents: number of equations = branches - (junctions - 1). Fbun2 . 3 0 . -Thme-loop circuit for oxamplo212 For figure 2.38, there are six branches and four junc- tions; therefore, the number of equations needed equals 6 - (4 - 1) = 3. Node Equations In the preceding analysis, Kirchhoffs voltage law established the method of loop equations. Kirchhoffs cur- rent law did not receive any attention,yet it was satisfied. This can be demonstrated with figure 2.38 by taking any junction and summing the currents through it. Consider- ing that the currents through R, andR, flow fromjunction a', hence, Il - Il + I* - = 0. Kirchhoffscurrent law is used directlyin node analysis, and the unknowns are voltagesacrossbranches. The tech- nique by which these voltages are referenced or measured provides a simplifyingprocedure for a circuit being ana- lyzed. Eachjunction or principal nodein a circuit is assigned a number or letter. Voltages can then be measured from eachjunction to one specificjunction, called the reference node. In essence, the reference node is dependent on all other nodes in the circuit. Node analysis consists of find- ing the voltages from eachjunction to the reference node. The procedure can be demonstrated easily with the simple two-junctioncircuit shown in figure 2.40,in which I, Rb,and R,are known. The existingjunctions are A and 0 , and 0is taken as the reference. The voltage from A to 0 is then VAo, and Kirchhoff's current law can be used to write an equation for junction A: I., - Ib - Ic = 0. (2.61~) By Ohm's law, Therefore, V A O V A O I - - - - = o . (2.61b) R b R, since 1IR = G, I. - VaoGs - VAoGc = 0. (2.62) Equation 2.62 can be further solved for unknown, Vao. During the process, an equation was written for each junction, excludingthe reference node. The number of re- quired equations for node analysis is therefore always one less than the number of junctions in a circuit. To illustrate node analysis further, consider the three- junction circuit in figure 2.41.If junction 0 is taken as the reference node, VAoand V , are the unknown voltages. The reference node, which establishes a reference potential acrossthe bottomof the circuit, is normallyassumed at zero potential. Accordingly,the double-subscriptedvoltages are unnecessary and unknown values can be simply called VA and VB.Further, as zero potential is often referenced to earth or ground, a most convenient reference, figure 2.41 can be redrawn as shown in figure 2.42. These circuit elementsare still connected to a referencenode through the ground symbols, as shown. Hence, each of the circuit elements is said to be grounded.
- 64. Now, applying Kirchhoff s current law to junctions A and B, I. - I, - I, = 0, (2.63a) By Ohm's law, Ib= VAGb, 1 , = VBGd, and I , = (VA- VB)Gc. (2.64) The last expression is evident because, by Kirchhoffs voltage law, the voltage across G. is the potential at junc- tion A minus that at junction B. Therefore, which are two simultaneous equations with unknowns, V., and V,. The same procedure can be applied tocircuits with more nodes. The foregoing examples have shown only current sources that are known, but node analysis can alsobe used with voltage sources or known voltages. Such is the case with figure 2.43, where the current through G. is In= (VA- VB)Go; likewise, I, = (V, - VB)G,. The analysis procedure can then continue asbefore. Mixed voltage and current sources can be handled in much the same manner, realizing that the current source establishes the current through the branch in which it is contained. With both loop and node analysis available, a decision must be made as to which technique best suitsthe solution of a circuit. Simply, the one to select is that providing the fewest equations to resolve. Since circuit reduction may still lead to the most efficient procedure for some circuits, it should always be considered. I EXAMPLE 2.13 I Use node analysis to find the voltage across the 0.54 resistance in figure 2.44. SOLUTION. The circuit contains three junctions. If the junction at the bottom of the circuit is taken as the reference node, A and B can be considered as the independent junctions. Here, Kirchhoff s current law yields The unknown voltages for node analysis are VAand V,, existing between each independentjunction and the reference node, where It can also be noted that I Figure2.40.-Simple two-node circuit. =c A - B + + ? + + I e 1 , t 0 Figure2.41.-Three-junction circult. Figure 2.42. -Three-junction circuit wlth grounds. Figure 2.43.-Voltage-source circuit demonstratlng node analysis. 1 AB A - B ( f f ~ ~ ~ , , C W A 1,500A t 0 Figure 2.44.-Circuit for examples 2.13, 2.15, and 2.16.
- 65. Substituting these Ohm's law relationships into the current-law equations produces lV, + 2(VA- V,) = 1,500, 0.5VB- 2(Va -VB) = 1,000. Rearranging, 3VA- 2VB= 1,500, -2Va + 2.5VB = 1,000. Solving these two simultaneous equations gives VA= 1,644 V, V, = 1,716 V. The voltage across the 0.5-Qresistance is then V, - V, - -72 V, which means that the actual voltage polarity is the reverse of that used in the solution and shown in the figure. EXAMPLE 2.14 Find the voltage, V,, across the 1-52 resistor in figure 2.45 using node analysis. SOLUTION. The circuit contains a mixture of cur- rent and voltage sources. This presents a difficulty for applying node analysis, asthe currents associated with the voltage sources are not known. However, as the objective of node analysis is to find unknown voltages, the difficulty can be eliminated by avoiding the voltage sources in the solution. This can be done by assigning nodes on both sides of each ideal volt- age source,treating both nodes and the voltage source together, and applyingKirchhoffs current law to both nodes simultaneously. For instance in figure 2.45, nodes 1and 2 are on both sides of the 6-Vsource, and nodes 3 and 4 are associated with the 12-V source. Each voltage source can be considered a short circuit joining its associated nodes, and current flow into the combined source and two nodes equals current leav- ing the combination. The node-source combinations are often termed supernodes and are signified in figure 2.45 by the enclosed dashed lines. Each super- node reduces the number of nonreference nodes by one, thus greatly simplifying the application of node analysis. , I 6 - - 4 . ._____-_-___-_____- ------ - Figure 2.45. -Circuit for example 2.14. Using this concept for the supernode containing the 6-V source, Kirchhoffs current law gives Notice that Kirchhofs current law for the 12-Vsuper- nodes produces the same equation. Assigning junc- tion 4 as the reference node, the voltages of the cir- cuit associated with the nonreference nodes 1,2, and 3 are V,, V,, and V,, where and v, -12 v . Rewriting the current-law equation 1 Since V, - V, = 6 V, the voltage across the 1-Q resistance is which statesthat the voltage is in the opposite direc- tion to that shown in figure 2.45. Network Theorems Practically any circuit can be analyzed using either circuit reduction, loop equations, or node equations. There are also several theorems that allow the simplification of particular circuits so that these three methods can be ap- plied more easily. The most commonly used theorems are Substitution, Superposition Reciprocity Source transformation, Maximum power transfer, Thevinin's, and Norton's. Substitution has already been used extensively and simply states that equivalent circuits produce equivalent results. The remaining theorems are discussed here. Superposition The superposition theorem relates that for a linear, bi- lateral network with two or more electromotive sources (voltage or current), the response in any element of the
- 66. circuit is equal to the sum of responses obtained by each source acting separately, with all other sources set equal to zero. Although the word "bilateral" is new, it does not create problems in dc analysis because passive circuits un- der dc are always bilateral. This concept will be discussed in more detail later. The meaning of superposition can be illustrated using figure 2.46A, a network with two voltage sources. The theorem relates that 1. If one source is set equal to zero (removing it from the circuit)and the currents produced by the other source are found, 2. Then if the second source is set equal to zero and cur- rents caused by the first source are found, 3. By summing both findings, t,he results are the cur- rents with both sources operating. In other words,by letting V, = 0, as in figure 2.46B, through circuit reduction, R,R, R , , = R, + R, + R3 v, I,,,, = -, R e , R 3 I,,,, = - (--)11,11 R, + R, The second part of the double subscripts is used only to signify that the currents are caused by source 1. The neg- ative sign in the last expression is caused by the current direction assumed in the illustration. The next step is let- ting V, = 0, thus restoring V, (fig. 2.460, R,Rs R . , = R, + - R, + R,' Finally, the sums of steps 1and 2 yield 11 = I,,,, + II~ZI, (2.69) which are the currents with both sources in operation as in figure 2.344. The process is adaptable (andperhaps more useful) for circuits having more than two voltage or cur- rent sources. As with current sources in node analysis, the unknowns in each step are voltages. Nevertheless, super- position allows many sources to be considered separately, and it is of great benefit in the analysis of circuits. EXAMPLE 2.15 Use the superposition theorem to find the voltage across the 0.54 resistance in figure 2.44. Note that this is the same circuit used for example 2.13. SOLUTION. Followingthe first step of the superposi- tion theorem, the 1,000-Acurrent source on the right side of the circuit will be turned off. The circuit is now operating as shown in figure 2.47A. Only I,,,,, need be known t,o solve the problem. Using current divi- sion for the parallel branches, Figure 2.47B shows the second step in the problem solution, where the 1,500-Asource is turned off. Now the current through the 0.54 resistor is Summation of these two findings produces the cur- rent from A to B with both sources operating. I,, = 429 - 571 = 143 A. Thus, It is obvious that this technique produces the answer faster than the process given in example 2.13. How- ever, node analysis may give a more efficient solu- tion with other problems. Reciprocity The reciprocity theorem states that in a linear passive circuit, if a single source in one branch produces a given result in a second branch, the identical source in the sec- ond branch will produce the same result in the first branch. Figure 2.46.-Circuit for demonstrating superposition theorem. ,,-. &-yqr-A ~ ~ ( ~ l ( l l l ln 2n source l,2-AF source ~ f i I B ~ ) / 2n t 1 . 0 3 3 . Flgure 2.47.-Circuit in figure 2.44 with sources turned off.
- 67. This reciprocal action is demonstrated in figure 2.48. In figure 2.48A, if V, produces I, in the branch that goes through R,, moving V ,to the R, branch will produce I, in the original location of V, (fig. 2.48B). The currents I , and V, - +flRFI 11' mRr I , will be equal. The dual form of reciprocity has a similar - -- R5 function in relating a current source to the voltage pro- duced.The great advantage of this theorem is that a source A B may be moved to another location that is more convenient to analyze. Source Transformationand Maximum Power Transfer Before definine the theorems associated with source - transformation and maximum power transfer, it is advis- able to expand the topics of ideal and practical sources. An ideal voltage sourcehas been defined as a device whose terminal voltage is independent of the current that passes through it. Although no such device exists in the practical world, it is convenient to assume a resistance in serieswith an ideal source as a datum, against which the performance of an actual voltage source can be measured. This is shown in figure 2.49 where the performance of a 12-V automotive storage battery is plotted against an ideal voltage source. The internal resistance, R., compensates the output volt- age, V,, for varying load currents, I,. These currents are obtained by changingthe load, R,. It will be found that with small current the practical source approximates the ideal one. But under heavy duty where there are high current and low load resistance, the output voltage drops substan- tially. Using the Ohm and Kirchhoff voltage laws, V, equalsthe voltage of the ideal source,which can be found bv measurine the terminal voltage with no load resistance. Figure248. -Demondratlonof reciprocity theorem. Flgure2.49. -Practical voltage-sourcemodel. Procricol > 0 Ri 2 Ri 3Ri LOAD (RL) Flgure 2.50. -Practical cunent-sourcemodel. 'fhe internay resistance, R,, ca; then be determined by applying a known R, and measuring VL. practical current sources, equations 2.72 and 2.73 must Similarly,figure 2.50 nmdels a practical current SOurce equal 2.74 and 2.75, respectively. It is obvious that both where R, is the internal shunt resistance. The graph illus- sets are interrelated, other words, for load current, trates the effect of this resistance: as the load resistance increases, terminal current decreases. Using Kirchhoff s current law, it can be shown that V. - RtI, I, = - - ---. (2.76) R, + Rr Ri + R, RiRz V, = ( - )Is, (2.74) If equation 2.76 is valid for any load, Rr, it must hold that R, + R, and The output of the ideal current source, I,, can be found by short-circuiting the output terminals and measuring the resulting current. Then R;can be calculated by measuring V, and I, with a known load, R,. Actually, shorting the ter- minals of a source is usually unwise because it could dam- age the real-world source, not to mention being an unsafe practice. I, can also be determined through source transforma- tion, which uses the fact that two sources are equivalent if each produces identical terminal voltage and current in any load. Therefore, for equivalence of practical voltage and where R = the internal resistance for either equivalent practical source, V , = output voltage of ideal voltage source, and I. = output current of ideal current source. This relationship is shown in figure 2.51. The two circuits shownwill be named shortly. Source transformation states that if one source is known, it can be replaced with the other. Note however that even if two practical sources are equivalent, the power that the two internal ideal sources supply and the internal resistances absorb may be quite
- 68. different. Notwithstanding, this substitution is helpful in shown in figure 2.53. Here, the internal configuration is writing network equations because constantcurrent sources unimportant, but the elements must be linear. The sources are more convenient for node equations, and constant- can be either ideal voltage or ideal current. voltage sources are best for loop equations. In addition, Thevenin's theorem states that if an active network the exchange of particular sources may permit direct cir- (fig.2.53A) is attached to any external network (fig.2.53B), cuit reduction. it will behave as if it were simply a single ideal voltage source,V . , in series with a single resistance, R .(fig.2.530. EXAMPLE 2.16 Solve the problem in example 2.13 using only source transformation. SOLUTION.Two practical current sources exist in figure 2.44 between junctions A and 0and between junctions B and 0.Applyingequation 2.78for the left- hand source, and for the right-hand source, R, V, and Rd V , describe two practical voltage sources that can replace the current sourcesbetween junctions A and 0andjunctions B and 0,respectively. Figure 2.52 shows the results of this transformation, where the circuit becomes a simple loop. The current from A to B is now and the voltage between is Source transformation also produced results quicker than node analysis, but again, this might not occur with other circuit configurations. In the above solution, practical current sources were replaced by practical voltage sources. By com- paring figure 2.44 with figure 2.52, it can be seen that points A, B, and 0 exist in both. Caution should always be taken to ensure that a desired node is not lost after the transformation. In other words, the active circuit will appear as a practical I voltage source.Values for V . and R can be found as follows. - Sinceload resistance can vary from zeroto infinity, some value of resistance must exist that will receive the maxi- mum power available from a particular source. It can be proven, using the conceptsjust presented, that an indepen- dent voltage source in series with a resistance, R,,or an independent current source in parallel with a resistance, R., delivers maximum power to a load resistance, Rr, when R, = R.. This is called the maximum power transfer theorem. Thevenin'sand Norton's Theorems These theorems are closely related to source transform- ation. They can be illustrated by considering the active net- work (one that delivers power) with two output terminals when all internal sources are operating normally and no loads are connected,the open-circuitvoltage acrossthe out- put terminals equals V . . With all the ideal sources turned off, a resistance, Ro, can be measured at the terminals. This isbecause when an ideal current source is turned off, it ap- pears as an open circuit (an infinite resistance). An ideal voltage source that is not operating acts as a short circuit, thus having zero resistance. This theorem is important because it means that any linear circuit where the internal components are unknown can be consideredas a constant-voltage sourcein serieswith a resistance. Any circuit reduced to this form is called a Thevenin circuit. Norton's theorem is the corollaryto Thevenin's theorem. Norton relates that if such an active network is attached to any external network, it will behave as a single ideal current source, I., in parallel with a single resistance, R . . The values for V . and R, can be determinedby consider- ing the same linear active network, this time as showmin figure 2.54A, with internal sourcesoperating normally. The Flgure 2.51. -Source transformation. Figure 2.52-Circuit in figure 244 wlth current sources transformed to voltage sources. network network network network A B C Figure 2.53. -1hBvenin's theorem.
- 69. output terminals are short-circuited, and a terminal cur- rent is measured to give the value for I.. R, is found in ex- actly the same way asin Thevenin's theorem. The combina- , tion of these elements gives the practical current source shown in figure 2.54C, which is also known as a Norton circuit. The Thevenin and Norton circuits are obviouslyrelated by source transformation so that if one is known, the other can be constructed. The equations relating the two are shown in figure 2.55. These theorems are usually employed when a series of calculations involves changing one part of a network while keeping another part constant. This manipulation helps to simplify complexcomputations such as power-system short-circuit currents. EXAMPLE 2.17 Find the Thevenin and Norton equivalentsfor the circuit shown in figure 2.56. SOLUTION. Applying Thevenin's theorem, the equivalent resistance of the circuit between a and b with the internal source off is R.. When the 50-V source is off, it acts as a short circuit, shorting out the 50-R resistance in parallel with it. Thus, The voltage across a and b with the internal source operating is V,. Using circuit reduction, the equiva- lent resistance as seen by the 50-Vsourcewith no load across the terminals a and b is (Note that this resistance is not Re.) The current delivered by the source is and from current division, As no current is flowing between terminals a and b, V , shown in figure 2.56is equal to Vub, which is equal to V.. Thus, V , and R describethe Thevenin equivalent, and I .and Rerepresent the Norton equivalent where ALTERNATIVE SOLUTION. The definition for R. in Norton's theorem is the same as in Thevenin's, again, I R. = R., = 7 R. I However, Norton states that if the terminals a and b are short-circuited, the current through that short circuit is L.The short circuit is noted by the dashed line in figure 2.56. Using circuit reduction, the 2-R resistance connected to terminal a is in parallel with the 10-51resistance connected to terminal b, or The equivalent resistance as seen by the 50-Vsource is and the current from the source is From current division, and the current through the shorted terminals is R . and I. again describe the Norton equivalent. Figure 2.54. -Norton's theorem. Figure 2.55.-Comparison of Th4venin1s and Norlon's clrcults. Flgure 2.58. -Clrcuit for example 2.17.
- 70. EXAMPLE 2.18 Determine the Thevenin's and Norton's equiva- lents for the circuit in figure 2.57. SOLUTION. In the branch containing the 900-V source, the two 5-R resistances are in series. If these are combined into one 10-Rresistance, it should be quite obvious that two practical voltage sources ex- ist between junction 1and the junction connected to terminal b. Source transformation can be employed to solve the problem. The resistance and magnitude of the ideal current source of the Norton equivalent to the 900-V and 10-R source are For the Norton equivalent of the 2,250-V and 15-9 source, Figure 2.58A shows the voltage sources transformed topractical current sources. Notice thatjunction 1and thejunction associated with terminal b still exist. Be- tween these two terminals, the 90-and 150-Asources are operating in parallel, and the 10- and 15-R resistances are connectedinparallel. Combiningthese ideal current sources and resistance results in the cir- cuit of figure 2.58B. Again, notice that the afore- mentioned junctions are retained. Converting the 60-Aand 6-R current source to its Thevenin equiva- lent produces the circuit in figure 2.58C. The 6- and 4-Q resistances in series with the 360-V are combined in figure 2.580. The 360-Vsource and 10-Rresistance form a practical voltage source between terminals a and b, and this is converted to its Norton equivalent in figure 2.583. Here, simple combination of the two parallel 10-R resistances yields one answer to the original problem and is shown in figure 2.58F. The remaining answer, the Thevenin equivalent, is in figure 2.586, obtained by source transformation of figure 2.581". To summarize the preceding sections, the fundamental laws and parameters were first applied to circuits under the influence of dc. Expanding upon these laws, several circuit-analysis techniques and theorems were covered.Be- cause only dc was considered, resistance was t,heonly cir- cuit element of interest. As will be shown shortly, most of this theory is also valid for circuits acting under current forms other than dc, where inductance and capacitance may also enter into the picture. TIME-VARYING VOLTAGES AND CURRENTS As the name implies, the magnitude of time-varying voltages and currents may not be constant with time. Con- sequently, the instantaneous values of the voltage and cur- Figure 2.57.-Actlve circuit for example 2.18. Figure2.58. -Circuits illustrating solution stepsto example 2.18. rent waveforms, v and i, must be considered. Both v and i are functions of time, as they were when originally intro- duced in this chapter, and they can assume any form from constant to the most complex. Figure 2.59 presents just a minor sampling of time-varying waveforms to illustrate their general characteristics. As with dc, the method for analyzing circuits that have time-varying current and voltage is first to form a model of the circuit, then to apply the fundamental laws and rela- tionships. Unlike dc circuits, a differential equation usu- ally results. To demonstrate the effect of time-varying and current on circuit elements, this section will first consider a special waveform, steady alternating current (ac).
- 71. An example of a steady-stateac waveform is provided in figure 2.60. The repetitive nature of this sinusoidal func- tion can be expressed mathematically as where i = current at any time, t, I, = crest or maximum value of current, a constant w = radian frequency, radls. The term sinusoid or sine wave is used collectively to in- clude cosinusoidal or cosine-wave expressions. The above equation could also use a sine function, but the cosine is employed for convenience when referring to current. It can be seen in figure 2.60 and equation 2.79 that the instantaneous value of current repeats itself every 2n rad or 360"; that is, the waveform goes thrcugh one complete cycleevery 2n rad. The number of cycles per second is w12n which is defined as the frequency, f, of the waveform or The units of frequency are hertz (Hz). One hertz is equal to 1cycle-per-second(cps),an expression whose use is now obsolete. The commonpower frequency in the United States is 60Hz, for which w = 377 radls, or just simply w = 377. A more general form of ac is where 8 = phase angle. Instead of expressing the phase angle in radians, such as nl6,angular degrees, 30°, arecustomarily used. By adjusting 8, the sinusoid can be moved left (increasing 8) or right (decreasing 8.Such movement is illustrated in figure 2.61. Using the earlier technique of developing differential equationsthrough circuit analysis, steady ac can be applied to pure resistance, inductance, and capacitance to observe what happens. Alternating Current Through Resistance Figure 2.624 shows a resistor of resistance R. From equation 2.79, if the current through this element is by Ohm's law, the voltage developed across the resistor is where V, = RI, = maximum or crest value of voltage waveform, V. Figure 2.62B shows both voltage and current as functions of time. At every instant, v is proportional to i, and v and i are said to be in phase. When two sinusoidal waves are comparedfor phase in this manner, both must be sine waves or cosinewaves; both must be expressed with positive amp- litude and have the same frequency. Figure2.59. -Some tlme-varylng electricalwaves. radians degrees Flgure2.80. -Sinusoldal ac waveform. i= I, cos wt ill ~i=~,,,cos(wt-€11 , / Figure 2.61.-Steady ac showing phase shift. A B Figure 2.82.-Steady ac through resistance.
- 72. Alternating Current Through hductance Suppose that current through the pure inductance of figure 2.63A is again as in equation 2.79. The voltage across the element is Differentiating, v = -wLI,sin(wt) where V , = oLI, = maximum or crest voltage. The term oL is used so frequently that it is provided with a special name, inductive reactance, and is designated "X," where and V , = 1,X. (2.86) Figure 2.63B compares equations 2.79 and 2.84, with i and v asfunctions of time. Here, it can be seen that the current crest is reached at a later time than the crest voltage. The current waveform is said to lag the voltage waveform by 9 0 ' . The phase angle is called lagging. Alternating Current Through Capacitance Consider the capacitance shown in figure 2.644, and let the voltage across it be The current through the capacitor is then Differentiating, i = -wCV,sin(ot) where I, = wCV, = maximum or crest current through the capacitor. As with the inductive resistance, wC is also provided a specialname, capacitivesusceptance,and symbol,"B." Thus, and I,= BV,. (2.90) The relationship between the current and voltage wave- forms (fig.2.64B)is the reverse of the inductance situation; the current waveform is now leading the voltage waveform Flgure2.63.-Steady ac through Inductance. . - 900 A B Flgure 2.64. -Steady ac through capacitance. by 90'. The phase angle is also called leading. The impor- tance of current and voltage waveforms being compared for lagging and leading phase angles will be brought out later in this and the next two chapters. rime-Varying Equations The preceding discussion considered voltage and cur- rent to be steady sinusoids. But what if they are allowed to have any form? To illustrate the consequences, the fun- damental laws and parameters can be applied to the sim- ple series RL, RC, and RLC circuits shown in figures 2.65, 2.66, and 2.67, respectively. For the series RL circuit, using Kirchhoff s voltage law Substituting in the relationships for voltages across resist- ance and inductance, Now for the series RC circuits, Applying the elementary laws, 1 v = iR + -(idt + Vo. (2.92) C The differential equations 2.91 and 2.92 are valid for any voltage and current, no matter what form. As before, V, is the initial charge on the capacitance. Considering figure 2.67, which shows the series RLC combination, V = Vn t V r f Kc; thus, di 1 v = iR + L- + - lo' idt + V,. (2.93) dt C
- 73. To arrive at an equation that is easier to handle mathe- matically, equation 2.93 can be differentiated once: This equation again describes or models the circuit for all electricalsituations, as no restrictions have been placed on voltage and current. The preceding has shown that when voltages and cur- rents represent any form, the application of circuit rela- tionships results in a differential equation. Through clas- sical differential-equation methods, such equations can providethe required solution,but these techniqueswill not be shown because it can confuse the understanding of the vital aspects of electrical fundamental methods. Figure 2.85. -Simple series RL circuit. Figure 2.88. -Simple series RC circuit. lkansients and Circuit Response Solutionof these equations for all situationsyields the completeresponseof the circuit.For linear circuits,the solu- tion will have two parts: forced response and natural response. The forced or steady-stateresponse can be attrib- uted directlyto the appliedsourceor forcingfunction.This is the action of voltage and current within the circuit if no changes or disturbances are made. The natural or transi- ent response is a characteristic of the circuit only, not a result of the sources. Such action occurs when a circuit is disturbed by a change in the applied sources or in one of the circuit elements.After the change,the circuit currents and voltages undergo transition from their original state to the point where their action is again steady state. The time period involved is normally very short, and the occur- rence within the transition is called a transient. For simplicity,the forcingfunctionsmentioned earlier in this chapter were dc, and in network analysis the study was devoted only to resistive circuitsand dc sourcesbecause here only the forced response is present. When both induc- tance and capacitance are circuit elements,both forced and transient responses can be encountered.However, knowl- edge of circuit transients is not required when considering steady-state voltages and currents, aswas seen in the case of steady ac. By far the majority of mine power problems only require knowledgeof steady-statecircuit currents and voltages,and it will be shownthat even though inductance and capacitance might be present, as long as only the steady-stateresponseis considered the solution of differen- tial equations is not needed. However, transient circuit responses are an extremely important input in the design of mine power systems, and they will be explained in de- tail in chapter ll. It his been shown in this section that any resistor, in- ductor, or capacitor carrying a sinusoidal current has a sinusoidal voltage developed across it. Furthermore, the sum or difference of two sinusoidal waveforms with the same frequency is another sinusoid.From these concepts, it can be shown that for a steady-state circuit, if voltage or current at any part of a linear circuit is sinusoidal (alternating at a particular frequency), voltages and cur- rents in every part of the circuit are sinusoidal with the same frequency. STEADY ALTERNATING CURRENT The form of steadyachas alreadybeen shown and used in the analysis of simple ac circuits, but here the concepts of steady-state ac circuit analysis are introduced. This necessitatesa review of a familiar but easily forgottensub- ject, complex algebra. Real numberssuch as2,4, and n are easy to understand in terms of physical things. Any mathematical operation on these numbers always results in another real number, except when the square root of a negative real number is taken. The term cannot be satisfied by any real number. Therefore,the square root of any negative num- ber is called an imaginary number. Mathematicians dis- tinguish imaginary numbersby writing "i"in front of them, but to avoid confusionwith the symbolforcurrent, electri- cal engineers use the symbol "j" where Figure 2.67. -Slrnple serles RLC clrcult.
- 74. Addition or subtraction of imaginary numbers yields another imaginary number. Yet, when an imaginary num- ber is added to a real number, a complex number iscreated. These have the rectangular form, x +jy (for instance, 3 +j4), where x is the real part and y the imaginary part or if Z = x +jy, then Re[Zl = x I d Z ] = y. (2.95) Complex numbers can be represented graphically by a pair of perpendicular axes as shown in figure 2.68. The horizontal axis is for real quantities, the vertical one for imaginary. Considering x +jy, if y = 0, the complex num- ber is a pure real number and falls somewhere on the real axis. Similarly, if x = 0, the complex number (now be- ing purely imaginary) exists on the vertical axis. Hence, complex numbers encompass all real and all imaginary numbers. In the case of the rectangular forms Z = x + jy, W = u +jy, the followingcommon definitions and mathematical opera- tions of complex algebra are applied. 1. Two complex numbers are equal if and only if the real components are equal and the imaginary components are equal: Z = W, IFF x = u, y = v. 2. To sum two complex numbers, the real and imaginary parts are summed separately: Z + W = (x 5 u) + j(y + v). 3. The product of a real and an imaginary number is imaginary: x(iy) = j(xy). 4. The product of two imaginary numbers is a negative real number: (iy)(iv) = -yv. 5. The multiplication of two complex numbers followsthe rules of algebra (note, an easier way to perform the multi- plication will be shown): (x+jy)(u+jv) = xu + jxv +juy - yv = (XU - yv) +j(xv + uy). 6. By definition, the conjugate of a complex number is formed by changing the sign of the imaginary part. An asterisk denotes the conjugate: Z = x + j y becomes Z* = x - jy. 7. For division, the numerator and denominator are multiplied by the conjugate of the denominator (again, an easier method exists): x +jy - x + jy (u +jv) xu - - - + j ( * ) . u - jv u - jv (u + jv) ((ua + : ua + 9 ponential. Figure 2.69 illustrates the conversion of rectan- gular to trigonometric or polar forms where Z = x +jy. (2.96) The absolute value of Z is represented by "r," and x = rcostl, y = rsin0, where 9 = tan-' ( l ) , X r = (xP+ yz)l''. (2.97) Hence, the trigonometrical form of the complex number is Z = dcostl + jsino), (2.98a) with the conjugate Z* = r(cos0 - jsin0). (2.98b) The polar form iswidely used in circuit analysis and is sim- ply written as and the conjugate, Z* = r i d (2.99b) Euler's theorem states that -jl -j2 -j 3 Flgum2.88. -Qraphlcalmprerentatlonof complex number. Besides the rectangular, there are three other general Flgum260.-Trlgonometrlc or polar reprerentationof com- forms of complex numbers: trigonometric, polar, and ex- plex number.
- 75. This expression allows a complex number to be written as an exponent, the exponential form, Z = r(cos8 + jsin8) = re" (2.100~) and Z* = re -j@, (2.100b) All four complex forms are therefore identical or The form should be selected that gives the easiest mathematical manipulation of complex numbers. For ad- dition or subtraction, the rectangular expression is best, but multiplication and divisionare much more convenient when the number is in exponential or polar form, the latter be- ing the most used. For instance, in polar, and in exponential, It will be shown shortly that circuits containing resist- ance, inductance, and capacitance can be represented by complex numbers, and that the solution of these circuits under steady ac will use complex algebra. This can be done with almost as much ease as the dc circuit analysis pre- sented earlier. EXAMPLE 2.19 Find the answer to the following expression in polar and rectangular form: (2 + j6)(18 121"). (1.63j)(2.6 + jl) SOLUTION. Both the numerator and denominator of the above expression are multiplicationsof complex quantities. For ease of solution, the rectangular term should be converted to polar. This results in (6.32171.6")(18)2l0) - (1.63190") (2.79121") (6.32)(18) or - 171.6" + 21" - 90" - 21" (1.63)(2.79) or 251-18.4". Effective Alternating Current The power available in the outlets of U.S. homes is a very familiar quantity: it is sinusoidal, having a frequency of 60Hz and a voltage of 115V. But what does 115V actu- ally stand for? The voltage waveform, being a sinusoid, is not constant with time. Therefore, the voltage is certainly not instan- taneous. If a measuring device could be connected to an outlet in order to visually observe the waveform, it would be found that "voltage" is not the maximum value, V,, be- cause this waveform crest is 1 1 5 nor 162.6V. "Voltage" does not describe an average value either, because the average of a sinewave is identicallyzero. As another resort, the average throughout one positive or one negative half- cycle of the waveform could be calculated, but the result gives a measurement of 0.637 V , or 103.5 V. To discover the meaning of the term voltage, the reason for measuring the voltage must be considered. In any system, current and voltage are defined in terms of what they will do. Conse- quently, the voltage is the effective value of the sinusoidal waveform. It is a measure of the effectiveness of the volt- age source in delivering power to a resistive load. The effective value is called root-mean-square (rms). In order to understand rms measurements, it is neces- saq to return to the concept of instantaneouspower, where p = vi. If the power was being developed across a resistance, R, it was shown that and These equations have little practical value for ac as they represent the value of power for a particular instant and in ac this is ever changing. A more effective measure for the value of power is based on the fact that power is the rate of doing work. A reasonable measure is then the ave- rage rate or average power. For average power, P,consumed by the resistance, R, P = ave(p) = ave(i2R)= (ave iZ)R and Average power is then an effective way to measure or quantify ac voltage and current. It has already been seen that the units of voltage and current in dc are easy to com- prehend; the magnitudes are constant with time, and their ability to deliver power is constant. Therefore, it is appro- priate to equate ac and dc rates of work, P.. and P,,, re- spectively, in order to determine an effectivemeasurement for alternating voltages and currents: P,, =IaR = P., = (ave i"R or 12= (ave i2) or I = t m = rms current. (2.101~) Employing the same procedure, V = = rms voltage. (2.101b) Current and voltage in ac are therefore expressed as the square root of the mean-square values or rms. They are sometimes written I,,, and V,,. It can be shown from the voltage and current waveforms (that is, substituting
- 76. I,cos(wt + 8)into equation 2.101a and similarly for voltage) that V, and V,,, = - or V, = V,,.. (2.102b) V T Root-mean-square currents and voltages are used so often that they are directly implied when referring to an ac magnitude. They are almost always used in calcula- tions. For simplicity, tho subscripts of V,,, and I,,, are eliminated in practice, and just V and I are written to in- dicate rms voltages and currents. All commonac voltmeters and ammeters are also calibrated to read rms values. The preceding analysis of average power concepts ap- plies only to resistance. Average power in the steady state supplied to either a theoretically pure inductance or pure capacitance is identically zero. This can be proved by inte- grating instantaneous power to these elements to obtain an average. The results show that the energy received dur- ing one-halfcycle is stored and then transferred back to the source through the balance of the cycle. The stored energy in the capacitance is greatest at the maximum of the volt- age wave, while in the inductance it is maximum at the current-wave crest. Phasors A steady-state sinusoidal current or voltage at a given frequency is characterized by only two parameters: ampli- tude and phase angle. This can be seen in figure 2.70A, which shows two voltage waveforms separated by a phase angle. An ac quantity may also be represented graphically by a phasor, illustrated in figure 2.70B. The phasor is a continually rotating line that shows magnitude and direc- tion (time). In this figure, the phasor is assumed to have a length representative of V,, rotation about point 0, and an angle increasing with time according to 8, = wt +0.The figure shows the line as if a snapshot had been taken, freez- ing action. The alternating quantity, V,cos(ot + e), is the projection of the phasor on the horizontal axis. In other words, as the phasor in figure 2.70B rotates, a plot of its projection on the horizontal axis with time reproduces the waveform in figure 2.70A. The phasor length shown here represents crest voltage but does not necessarily need to be equal to it. It is common practice to draw phasors in terms of effective Oms)values. Although voltage has been employed as an example, phasors can also represent sinu- soidal current, among other things. Voltage and current phasors are both illustrated as rotating lines in figure 2.71A, where i = 1,cos ot. To show both current and voltage, two phasors can be drawn, with one of them advanced by the phase angle, 4. Both lines rotate indefinitely about the axes, and one line will always lead the other in the same relative position; therefore, the axes are superfluous and need not be drawn. Since it is necessary to orient the phasors at a specific point in time, a convenient instant is selected as a reference. For example, in figure 2.71B the phasor is shown where the current phasor angle is zero. Here, the current phasor is termed a reference phasor, and all other phasors are drawn relative to it. Either voltage or current can be selected as the reference. A phasor may be expressed in several ways. To illus- trate the most used expressions, consider figure 2.72A, which shows one phasor displaced from the horizontal by an angle, o t + 0. Recalling complex algebra, the horizon- tal axis can be assigned as a real-axis and the vertical as the imaginary axis. The phasor, V, is then the sum of the real and imaginary components, - V,. and Vim, or V = V,. + V;,. (2.103) lie anqle W UU Figure 2.70.-Sinusoid versus time (A) and as phasor (6). Figure 2.71.-Phasor representation of current (A) and voltage (8). Figure 2.72.mOther expresslons for phasors.
- 77. Figure 2.72B clearly illustrates the rectilinear form of quantities. Equations 2.108~ and 2.108b are assumed to equation 2.103. The real and imaginary components of the represent the general current through and voltage across phasor are each element. V . . = Vcos(ot + 8), (2.104~) - Thus V = Vcos(wt + 8) +jVsin (cut + 8) (2.104~) - or V = V[cos(wt + 8) + jsin (ot + 8)). (2.1044 - V is used to signify that the voltage is a phasor,-and once more the imaginary operator, j, signifies that V., exists on the imaginary axis. Accordingly, the phasor may be considered as the vector sum of two phasors at right angles to each other. Applying Euler's theorem (equation 2.100) to equation 2.104d, The factor, e'-', is superfluous, as it contains no unique information about the phasor, and it can be suppressed: This is calledthe exponential form of the phasor. Thusequa- tion 2.105~ can be expressed in polar form, These phasor forms are very useful in solving ac cir- cuit problems. The terms phasor and vector are often interchanged. Phasors and Complex Quantities When introducingthe action of time-varyingsinusoids, certain voltage-current phase-angle relationships were found to exist for pure resistive, inductive, and capacitive circuit elements. In general, if a steady-state sinusoidal current has the time-domain farm EXAMPLE 2.20 A circuit has the following voltage and current waveforms applied across and through its terminals: v = 282.8 cos (377t - 209, i = 42.4 cos (377t + 25"). Write the phasor expression for voltage and current. What is the phase angle between current and voltage? SOLUTION.The two given expressions are in the time domain, where for the voltage, V , = 282.8 V, 4 = -200, and for the current, I, = 42.4 A, 0 = 25". The phasors for voltage and current are then, respectively, - v = -v" I 4 = 200 1-20"v , f i T - 1 , I = - I ~ = ~ O N A . fT - The current waveform is leading the voltage wave- form, and the phase angle between current and voltage is 4 - 8 = -20. - 25" = -45". I = I,cos(ot + €9, (2.108~) If sinusoid current is applied to a resistance, R, the voltage across it is and voltage, v = Ri. v = V,cos(ot + 4), (2.108b) Applying the general time-domain expressions, current is said to be lagging voltage by the phase angle, 4-8 (or conversely, leading voltage by the phase angle, V,cos(ot + 4) = RI,cos(ot + 8) 8-4). Using the exponential and polar phasors, this current and voltage can also be stated or in exponential form, and V = V@-' + *I, V = V@, or V = 114 (2.109b) Suppressing e'", where I and V = rms values of current and voltage, Vd* = RId", respectively. Before steady-statecircuit analysis can be performed, Or in polar form$ pure circuit elements must again be considered, this time to analyze the voltage-current relationships using complex V 1 4 = RI 1s.
- 78. In phasor form, V )$ and I are the phasor polar representations, This is the same relationship that exists for time-varying waveforms and dc. It is apparent that angles 0 and 4 are equal and that voltage and current are in phase (fig.2.73A). Supposethe same general forms of current and voltages were applied to a pure inductance where, as before, then, using the general exponentials, Differentiating (ee is a constant with time), and suppressing e'-', Thus, in phasor form, V = jwLI. (2.111) The imaginary operator,j, denotes a +90° displacement of voltage from current; such as illustrated in figure 2.73B. In general, if the current phasor has an angle, 8, the voltage phasor angle, 0, is 0 + 90" for a pure inductance. For a pure capacitance, Employing the same process to find equation 2.111, In this case, -j indicates a -90" displacement of the voltage phasor from current, as shown in figure 2.73C. Now that the phasor relationships of the fundamental elements have been covered, the stage is set for impedance transforms. Impedance Transforms The current-voltage relationships for the three fun- damental elements have been found using phasors, as These can be rewritten as voltage-phasorto current-phasor ratios: A very important quantity, impedance, signified by Z, is defined asthe ratio of the phasor voltage to the phasor cur- rent for a circuit or - v z = = I ' (2.113) This expression is often called Ohm's law for ac circuits. Impedance is a complex quantity with dimensionsof ohms, but it is not a phasor. Therefore, the impedance of the pure passive circuit elements, resistance, inductance, and capacitance, are respectively These can be applied directly to circuit analysis when a cir- cuit is in steady state. In other words, element impedances are employedto convert or transform a time-domain circuit model into a form in which the circuit can be analyzed us- ing only complex algebra. Hence the expressions of equa- tion 2.114 are called impedance transforms, and the transformed mathematical model is then in the impedance (or jw) domain. As a result, no differential equations are used to solve a steady ac circuit. All previous fundamental theorems, laws, and circuit- analysis techniques also apply to steady ac circuit analysis using impedances. Thus, an ac circuit representation in the impedance domain is analogous to a dc circuit model. On the other hand, the concept of impedance has no meaning inthe time domainwith time-varyingvoltages and currents. To demonstrate these concepts, consider the simple RL circuit in figure 2.74A, now with a complexvoltage source, that is, a steady-state sinusoid defined as a phasor. Here, the currgnt through the resistance and inductance is the phasor, I; therefore, V , = IR, Pure res~stance : Pure inductance: Pure capacitance: - 7 and 7 in phase ' ilags 7 by 90" I leads 7 by 90" Figure 2.73.-Voltage-current phasor relationships for cir- cuit elements. Figure 2.74.-Steady sinusoid analysis of simple RL series circuit.
- 79. By Kirchhoffs voltage law, - v = v , + v , - or V = IR + TjwL = T(R + joL). The impedance (equivalent)of the entire circuit is then - v Z = = = R + joL. I (2.115) Because impedanceis a complexquantity,it alsohas a polar form: where IZI = (R' + (wLy)",magnitude of impedance, R, A phasor diagram for the circuit current and voltages is given in figure 2.74B. Note that as current is common to both elements, it could be used as the reference phasor. Here, voltage across the resistor, V,, is in phase with cur- rent, while that across the i~ductor, Vr, leads current by 90". The total circuit voltage,V, can be resolved notingthat where = + xa)", magnitude of source voltage, V, and 6 = tan-'(&). V R This last angle is identicalto that found for the impedance. It should be noted that the current and voltage relation- ships for the inductor are as those found previously when time-domain voltages and currents were considered. Now consider figure 2.75A, which shows a simple RC series circuit in which R = TR, - - and = TR - A = T(R - i ) . wC wC The impedance becomes Figure2.758,the circuitphasor diagram,showsthe current- voltage phase-anglerelationships with the voltage across the capacitor now lagging that across the resistor. Continuing the process for an RLC series circuit (fig. 2.76A), the voltage across each element is Figure 2.75.-Steady sinusoid analysis of simple RC series clrcult. Figure2.76.-Steady sinusoid analysisof simple RLCseries circuit. and across the entire circuit, - v=e,+v,+v, or 1 v = TR + TjoL + I(-), (2.118) JWC with the circuit impedance, The foregoing gives the essence of impedance transforms. Each impedance shownin equations2.115,2.117, and 2.119 is the equivalent impedance of that circuit and has the general form Z = R +jX = IZI I!, where R = resistance component, Q, X = reactance component, R, IZI = (Ra+ Xa)", magnitude of impedance, Q, and 0 = tan-'(XR). Here, dependingon the pure circuit elements, the reactive component is X = oL = inductive reactance, Q, X = wC = capacitive reactance, Q, 1 X = WL- -= reactance for series LC elements, R. oc
- 80. From this equation, it can be seen that resistance is con- stant while reactance is variable with frequency. The time-domain expression found for a general series RLC circuit can be used to clarify the transformation process: It has been demonstrated in the impedance domain for steady ac that Accordingly, time-domain differential equations can be changed to the impedance domain when the circuit is under steady ac by 1. Replacing v with v (in rms), 2. Replacing i with I (in rms), d . 3. Replacing -wlth jw, dt 1 4. Replacing I.. .dt with -, and J W 5. Letting V, = 0. However, it is a much more efficient approach to ac circuit analysis to assign the impedances directly using equation 2.114, and soon more will be stated regarding this. Admittance Admittance, which is given the symbol Y, is defined as the reciprocal of impedance, Z, and The units are now siemens, replacing the previous designa- tion, mhos. Admittance istherefore a complex quantity, the real part being conductance, G, and the imaginary compo- nent susceptance, B, or It should be noted that conductance is not the reciprocal of resistance unless reactance is zero, likewise for suscep- tance, reactance, and resistance. In general form, through equating Y and Z, R G = - - - -X and B = - . (2.121b) R2+ X2 R2 + Xa Admittance affords basically the same convenience in steady ac circuit analysis that conductance provides for parallel dc circuits. Steady-State Analysis As previously stated, all circuit-analysis techniques that were covered for dc circuit,sstill apply to steady ac circuits in the impedance domain. These include network reduction, Kirchhoffs laws, loop and node analysis, network theorem, plus delta-wye transforms. Impedances simply replace resistances in the concept, and steady ac sources replace dc. Even with dc, the impedance domain can be used; in other words, dc sources can be thought of as steady-state sinusoids with o = 0.Therefore, with dc, reactance has no effect. A summary of circuit relationships follows, this time including impedance. 1. Impedancesin series. A single equivalent impedance, Z, is 2. Impedances in parallel. A single equivalent here is 3. Admittances in parallel, Y = Y , + Y , + Y , + ... Y . . (2.124) 4. Voltage distribution of series impedances, where v is the input voltage, v,is across Z,, and so on. 5. Current distribution through parallel admittances, where T is the total circuit current, f, is through Y,, and so on. Or parallel impedances, The overlines are removed on the above impedances and admittances simply for convenience, but it should be remembered that all are complex numbers. In essence, ac circuits in the steady state can be solved almost as easily as dc circuits employing only resistance. The major addi- tion is that the solution now uses complex algebra.
- 81. EXAMPLE 2.21 Consider the circuit shown in figure 2.77, where v = 5,880 cos (377t + 53.1°), i = 141.4 cos 377 t. The circuit is under steady-state conditions. What are the values of R and L? SOLUTION. The phasor representations for voltage and current are - v = - 880 153.1" = 4,158 1 5 3 . 1 "V, 0- I = - - 141.4 lo0 = 100(0°A. fl -- The total impedance of the circuit is then - Z = - = 41158 153'10 - - - 41.58 1 5 3 . 1 "Q 1 100 loo - or in rectangular form, Z = 25 + j33.25 R. The real part of this impedance must be the circuit resistance and the imaginary part equal to total reac- tance. Thus, R = 25 R, X = 33.25 Q, but X = wL - 0.3. Therefore, as o = 377 radls, L = 33.25 + 0.3 = 0.09 H. 377 EXAMPLE 2.22 Find the voltage, v,across the 2-Qresistance in figure 2.78. SOLUTION. Circuit reduction appears to be the easiest way to solvethe problem. Noting that w = 377 radls, the reactances of the impedance and capacitance are X, = oL = (377)(0.12 x = 0.045 Q, 1 & = - = 1 = 0.75 Q. w C (377)(3,535 x 10-1 The impedance of the branch containing the im- pedance is Z, = R, + j& = 1+ j0.045 R for the branch with the capacitance Z, = R, + j X , = 1 - j0.75 R. Combining these two parallel impedances in polar form, ZlZz - (1.0 ILGB.lX1.25 1-36.87") = 0.59 1-14,90 R, -- 21+ ZZ 2-jO.705 and the equivalent impedance seen by the 1,000-V source is Z,, = Z + 0.59 1-14.9" = Z + 0.57 - j0.15 = 2.57 - j0.5 = 2.57 1-3.4"R. Assigning the source voltage asthe reference phasor, the total circuit current is - 1,000 0" i. = = & % = 388 1~ A. The voltage across the 2-R resistance is then - V = 21 = (2)(3881=) = 77713.4" V. Figure 2.77.-Circuit for example 2.21. Figure 2.78.-Circuit for example 2.22.
- 82. EXAMPLE 2.23 Calculate the current, I,through the branch indi- cated in figure 2.79 using only loop equations. SOLUTION. Two loop currents have been assigned in the figure. Using Kirchhoff s voltage law, I The solution to these simultaneous equations gives I The current through the 5-R resistance is then - thus, I = j360 A I Or I = 360/90°A. EXAMPLE 2.24 What are the Thevenin's and Norton's equivalents for the circuit shown in figure 2.80? SOLUTION. Applying either Thevenin's or Norton's theorem, the equivalent impedance of the circuit be- tween a and b with the internal source off is Z,. When the steady-statevoltage source is off, it acts as a short circuit, and the 4-R and 12-Qresistances are effec- tively in parallel, and This combined resistance is in series with the jlO-Q reactance, and the series combination is in parallel with the -j6-R capacitance, and z, = z . ,-(3 + jlO)(-j6). 3 + i10 - i6 ' thus, Z. = 12.51-69.8" = 4.3 - j11.7 R. If the terminals a and b are shorted out according to Norton's theorem, a short circuit exists across the capacitance, and the jlO-R impedance and 12-Rresist- ance are placed in parallel. The equivalent impedance of the circuit under this shorted condition as seen by the ideal voltage source is then The circuit delivered by the ideal source is and the current through the jlO-R react.ance and the shorted terminals is Z, and 1, define the components of the Norton equivalent for the circuit in figure 2.80. By source transformation, which is identical to Z, and V, define the components of the Thevenin equivalent. It can be noted in figure 2.80 that the ideal volt- age source is in series with the 4-R resistance in one branch. Therefore,source transformation alone could be employed to solve the problem. The use of subscripts in this example did not follow the format_previo~sly used in the chapter. In other words, Z . , I., and Vi described the equivalent circuits rather than Z,, I,, and V,. The reason is that the subscript zero has a special meaning in three-phase ac circuits, which will be discussed in chapter 4. Figure 2.79.-Two-loop circuit for example 2.23. j l O n Figure 2.80.-Active circuit for example 2.24.
- 83. Chapter 2 has introduced the concepts of electrical cepts are fundamental to electrical engineering, regardless circuit analysis. The fundamental laws were covered first, of application. Thus, comprehension of the contents of this followed by numerous circuit analysis techniques, which chapter is vital to understanding the following chapters. were applied to dc circuits. Steady ac was then presented, The next chapter will continue the study of electrical fun- and the chapter concluded with examples of circuit analy- damentals, with emphasis on power consumption in ac sis on ac circuits under steady-stateconditions. These con- circuits.
- 84. CHAPTER 3.-ELECTRICAL FUNDAMENTALS II The measures of instantaneous power, p, and average The first term of equation 3.1 is constant, while the second power, P, were introduced in chapter 2. Instantaneous is a sinusoid. Thus, taking the average to find average power does not have application in steady ac circuit power results in analysis, so the concept of average power has been devel- oped to gauge the rate at which electricitydoeswork. This chapter continues to build the foundationsfor mine power P = adp) = 2 v,l, cos 0. (3.2) fundamentals that will be expanded into full comprehen- sion in chapter 4. There, the discussion will focus on three-phasepower; here, the purpose isto introducesingle- Realizing that V , = h v and I, = 61, average Power phase power and transformers. becomes AVERAGE POWER AND POWER FACTOR ?b find the average power consumed by a circuit, the resistance of each element can be examined and all the individual power consumptions computed. Reactance, ei- ther capacitiveor inductive, does not affect average power. When all the average powers have been determined, their sum yieldsthe total averagepower deliveredto the circuit. Obviously, if the circuit elements are numerous, the pro- cess can be time consuming, but this approach is some- times necessary. If the average power needs to be determined for the total circuit, it would be more desirable to perform only one calculation by computing average power in terms of the terminal current and voltage in the circuit. Yet, when complex or imaginary componentsexist in the circuit, can they be ignored, as this implies? In other words, the voltage and current waveforms might not be in phase, and when a phase angle is involved, the product of effective voltage and current no longer equals average power. However,instantaneousvoltage and current can be used to calculate averagepower and to demonstrate what occurs if a circuit has reactance. Assume that the following current and voltage are monitored at the terminals of a circuit: i = 1,cos ot, Current is taken as reference, and the phase angle by which voltage leads current is 0. The instantaneouspower consumed is then p = vi = V,I,cos(ot + Okos wt. Rom the trigonometric identity for the product of two cosines, P = VI cos 8, (3.3) in which V and I are root-mean-square(rms) voltage and current at the circuit terminals and 0is their phase angle. If the voltage and current had been dc values, the average power would just be the product of voltage and current. However, when voltage and current are sinusoidal, equa- tion 3.3 specifies that the average power entering any circuit is the product of the effective voltage, effective current, and the cosine of the phase angle. The function cos 0is called the power factor (pf).For a purely resistive load,the phase angle iszero and the power factor is unity. Unity power factor may also exist when inductance and capacitance are present, if the effects of reactive elements cancel. If the circuit is totally reactive (either inductive or capacitive), the phase angle is a positive or negative 90°, the power factor is zero, and average power must be zero. COMPLEX AND APPARENT POWER When there is reactance in a circuit, a component of circuit current is used to transfer stored energy. The energy is periodically stored in and discharged from the reactance. This stored energy adds to circuit current but not to average power because average power to reactive elements is zero. In such cases, the power factor is not unity. Thus, as no work is performed by the added current, the power factor can be considered to be a measure of circuit efficiency or its ability to perform work, and aver- age power, defined by equation 3.3, is often called active power or real power. Power calculations can be simplified if power is de- fined by the complex quantity shown in figure 3.1, which is expressed mathematically as where S = complex power, P = real power, as before, and Q = reactive power or imaginary power, Imaginary power accounts for the energy supplied to the reactive elements. If or v I p = -cos e + V,I,cos(2wt+ e). (3.1) 2 2 P = VI coso,
- 85. Figure 3.1.-Power represented as real and imaginary corn. ponents. then the magnitude of complex power, S, called apparent powel; is S = VI, (3.5) and imaginary power is Voltage and current are again nns, and 0 is the phase angle. Therefore, Complex power is then simply the product of terminal rms voltage and current magnitude acting at a phase angle. Applying dc concepts, the product, VI,is the power appar- ently absorbedby the circuit,hencethe term apparentpower Apparent power, real power, and imaginary power are di- mensionally the same, but to avoid confusion with real power (units of watts), apparent power has units of voltam- peres, and reactive power uses wltamperes reactive. When sinusoidal voltage and current have general form, as in B = vie, instead of using equations 3.4 and 3.7, the following expression is more convenient for computing complex power: - S = V f*, (3.8) where-V = complex voltage, V, and I* = conjugate of complex current, A. Accordingly, - s = V I* = vle - 11 - - 4 = v1le-9 - or s v@Iei-+ = VI&(B-@, where 0 -C $ = phase angle between voltage and current. EXAMPLE 3.1 When operating under normal conditions, an induction motor has been foundto draw 100A when 440 V is across its terminals. Current is lagging voltage by 36.87'. Find the average,reactive, appar- ent, and complex powers for this load. SOLUTION. From equation 3.3, the average power is P = (440X100)cos 36.87' = 35,200W. Using equation 3.6, the reactive power is Q = (440X100)sin 36.87' = 26,400 var. Equation 3.5 defines the apparent power as S = (440x100) = 44,000 VA, and equation 3.4 yields the complex power as - S = 35,200 + j26,400 VA. ALTERNATIVE SOLUTION. If voltage is assigned as the reference phasor, then From equation 3.8, the complex power is - S = (4401O0X1O01-36.87")* 8- or B = (440)0°X100136.870)= 44,000136.870VA, - where the magnitude is the apparent power, or S = 44,000 VA. Converting the polar expression for complex power to a rectangular form, - S = 35,200 + j26,400 VA, which yields P = 35,200 W, Q = 26,400 var. It should be noted that the above solutions are only two of the many possible.
- 86. EXAMPLE 3 . 2 A load consumes 1,250k W at 0.6 lagging power factorwhen 4,160 V at 60 Hz is acrossit. The load is connectedin series with a (0.71 +j0.71)Qimpedance to a constant source. Determine the voltage and power factor at the source. SOLUTION. From the stated conditions, the aver- age power is From equation 3.3,the current through the load is where PI, V,, and cos8, relate the conditions for the load, or For convenience, the voltage across the load can be assigned as the reference phasor, then The load current also flows through the series im- pedance. Using polar expressions, the voltage drop across this impedance is The voltage at the source is then The power factor at the source can be found by fmt calculating the phase angle between current and voltage at the source with the current phasor taken as reference. Here, Therefore, the power factor at the source is cos 8, = cos 52.2O = 0.61 lagging. Any circuit in steady state can be reduced to the general impedance where By relating the complex power consumed by a circuit with this impedance, another useful expression can be found: hence, S = 12(R + jX) = 12R + j12X. Because S = P + jQ, In equation 3.10~. the current I is real rms, not complex. The rms voltages in equation 3.10b are those existing acrossthe individual elements, not acrossthe total circuit. It may already be obviousthat the circuit impedance angle is identical to the power-factorangle. The following also apply. 1.If a circuit contains resistance and capacitance (a capacitiveload,Z = R -jX), the current leadsvoltage, the phase angle is negative, and Q is negative. 2. J fthe circuit is an inductive load (Z = R + jX), the current lags voltage, and the phase angle and Q are positive. In either case, the power factor ranges from zero to unity (purely reactive to purely resistive). A capacitive load is said to have a leading power factor, an inductive load a lagging power factor, as illustrated in figure 3.2. ling A Capacitive load S=VI AQ=VIsine Lagging P=VIcos B Inductive load Figure 3.2.-Illustration of leading (A) and lagging(6) power factors.
- 87. The complex power delivered to several loads is the sum of the complex power consumed by each individual load, no matter how they are interconnected. This rela- tionship can be shown using the simple circuit in figure 3.3. The total complex power to the system is - - - but I = Il + I,; - thus, S =qQ + i,*) = Vf,*+ Vfd = s1+ 8 , This has extensive practical significance. For example, if a circuit has a lagging power factor, a capacitance (with a leading power factor) can be selected and then placed in parallel, so as to negate or reduce the total circuit imagi- nary power (with the capacitance). The net result is to reduce total circuit current, while the load consumes the same real power and thus performs the same work. This is the essence of power-factor improvement. EXAMPLE 3.3 Consider that the two loads shown in figure 3.3 are induction motors operating as follows: P , = 50 kW at 0.6 lagging power factor, P, = 25 kW at 0.8 lagging power factor. Find the overall apparent power and power factor when these consumptions are combined. SOLUTION. The average, apparent, and reactive power for each load are Q1 = S, sine, = 83,333(0.8) = 66.67 kvar, P, = 25 kw, 6, = S2sine, = 31,250(0.6) = 18.75 kvar. Complex power is then Apparent power is the magnitude of complex power, or s = (P+ Q2)lf2 = (75' + 85.422)1" = 113.7 kVA. Figure 3.3.-Circuit demonstrating sum of complex powers. I The power-factor angle is and the power factor of the combination is pf = cos 0 = cos 48.72' = 0.66. EXAMPLE 3.4 The maximum capacity of a piece of power equip- ment is rated by apparent power at 500 kVA. The unit is being loaded by 300kW at 0.6 lagging power factor. The power factor must be improved to 0.8 lagging by adding capacitance in parallel with the equipment. Find the required capacitance in kilovoltamperes reactive. With the capacitance in place, find the reserve capacitythat is available from the power equipment. SOLUTION.For the load on the equipmentwithout the capacitance, Ql = S,sinB, = 500(0.8) = 400 kvar. It can be said from S, that the equipment is fully loaded. When pure capacitance is added, average power remains constant, and only reactive power and apparent power change. For the desired power factor, cose,, Q, = S,sin0, = 375(0.6) = 225 kvar.
- 88. Consequently, the added capacitance causes the total reactive power to decrease. The difference between the reactive power without and with the capacitance must be the amount inserted by the capacitance. In other words, Q, = -(Q, - Q 2 X Q = -(400 - 225) = -175kvar. The negative sign is used here to indicate that the capacitance adds negative reactive power to the system. Finally,the differencebetweenthe apparent power without and with the capacitance yields the reserve capacity available from the equipment, or It can be noted that additional average power can now be added to load on the equipment without exceeding its maximum capacity. For instance, con- sider that average power P will load the equipment so that the equipment is again operating at full capacity. Then, the total average power is Reactive remains constant, Q, = Q, = 225 kvar, and apparent power changes to Therefore, S, = (PT2+ QT2)'", Solving for the new average power, RESONANCE Series Resonance Earlier, the impedance for the simple series RLC circuit shown in figure 3.4 was found to be A special circuit phenomenon can now be demonstrated with this equation. There exists one spec& frequency, o , , where total circuit reactance is zero and the circuit imped- ance is purely resistive, or and At o,, the circuit is said to be in resonance, and Since w = 2nf, the resonance frequency, f,, is given by For a series RLC circuit in resonance,it can be shownthat 1.The appliedvoltage, and the resulting current, f, are in phase, 2. The power factor of the circuit is unity, 3. The impedance, Z, is minimum, and 4. The current, I, is maximum. At all other frequencies that are significantly higher or lower than f,, the series RLC circuit appears as a high impedance. With frequenciesbelow resonance, capacitive reactance is greater than inductive reactance, sothe angle of impedance is negative (total reactance is negative). Above resonance, the situation reverses and the imped- ance angle is positive. This can be seen clearly in figure 3.5 where circuit impedance versus frequency is plotted. The energy stored in a resonance circuit is essentially constant, yet the energy level within the circuit may be many times higher than the energy being supplied from an external source during any period. The source itself does not supply any reactive power, only activepower. The reactive power transfers energy back and forth between the resonant-circuit inductance and capacitance. The re- sult of this energy transferral can be very high voltages, several times the terminal voltage, existing across the inductance and capacitance within the resonant circuit. c+++-+t-O R L C Figure 3.4.-Simple series RLC circuit for resonance. KEY : ( , J FREQUENCY(w),rad/s Figure3.5.-Plot of impedance magnitude versus frequency for series RLC illustrating resonance.
- 89. This situationcanbe the cause of somesevereovewoltages in mine power systems, and the concept will be explored further in chapter 11. The amount of energy stored, compared with that dissipated by the resistance, is related to the shape of the curve representing impedance magnitude, as shown in figure 3.5. This curve is an example of a response curve. The quality factor of a circuit is a measure of the sharp- ness of the response curve and is expressed as a ratio: maximum energy stored per period Q, = 2* total energy lost per period , (3.14) where the period is one complete cycle of the resonant frequency. By finding the ratio of the energy stored in either of the circuit's reactive components to the energy dissipated in the resistance, it can be shown that reactance w , L 1 - - Qo = resistance R w,CR ' (3.15) The quality factor normally has greater applicationin the communications aspects of electrical engineering than in the power aspects.For instance, the width of the response curve is also related to Q, and has great relevance to the tuned circuits used in radio and television. Parallel Resonance The resonance of the simple parallel RLC circuit shown in figure 3.6A is very similar to thatjust discussed. This circuit is obviouslyidealized,but itsperformanceisof general interest. The admittance can be written as and the circuit is in resonancewhen susceptanceB is zera Hence, the circuit exhibits low admittance and high impedance at resonance, while the series RLC circuit had low impedance and high admittance: On the other hand, the resonant frequency is again The statements previously given for series circuits also apply, except that current replaces voltage and voltnge replaces current. This is an example of duality. Anything stated about a series resonant circuit applies to its dual, the parallel resonant circuit, if each word in the left column below is replaced by its opposite word shown in the right column: Series Parallel Voltage................ Current. - Impedance .......... Admittance. Resistance........... Conductance. Reactance ........... Susceptance. Inductance.......... Capacitance. Therefore, Q , , of this parallel resonant circuit is the dual of equation 3.15 or susceptance o,C R - Qo conductance - G w , L ' (3.18) The concept also relates to many fundamentals covered in chapter 2. For example, two circuits are called duals if the loop equations for one have the same forms as the node equations for the other. Because figure 3.6A is idealized (as actual inducting elements must have associated resistance), figures 3.6B and 3.6C are presented to show practical circuits that exhibit parallel resonance. TRANSFORMERS Early in chapter 2, the concept of mutual inductance was introduced. To review, Faraday found that a time- varying current in one circuit would induce a voltage in a nearby circuit. If the adjacent circuits are simply conduc- tors and are labeled 1 and 2, as in figure 3.7, this statement means that i, in circuit 1produces v, in circuit 2, v, in turn causes i, to flow (if circuit 2 is part of a complete loop), then i, induces v, in circuit 1. These interrelated phenomena can be thought of as mag- netic coupling between the two circuits, and it has been shown that di, di v, = L , , -and v, = L,, 2, dt where L,, = L , , = M = mutual inductance, H. Figure 3.6.-Circuits that exhibit parallel resonance. Flgure 3.7.-Magnetic coupling between two conductors. Flow o f current 2 causes magnetic n : '1 field that cuts ill i 2 t _- other conductor 1 ' 2 / -- Magnetic flux lines
- 90. Because of the equality, M is used to represent mutual inductance. These equations are true only for straight wires, and magnetic coupling exists only if voltage and current are time varying. The circuits considered previously were loops or meshes composed of passive and active elements, and these were conductively coupled by common branches or nodes. The following paragraphs develop the concept of magnetic coupling further and introduce the fundamen- tals behind one of the more important components of ac mine power systems, the transformer. Transformers are prime examples of magnetic cou- pling. They are often designed to optimize this coupling, and their operation is based inherently on mutual induc- tance. Transformers are employed to increase the magni- tude of voltage for more economical power transmission or, conversely, to decrease the level to provide voltage more suitable for electrical equipment operation. In essence, these changes can be made with either total isolation or direct conduction between circuits. Instead of straight conductors, assume that two coils are situated side by side, and their magnetic action is passing through any environment (fig. 3.8).The current in coil 1 is then partly the result of self-inductance in coil 1 and mutual inductance from coil 2, and vice versa for coil 2. Expressed mathematically: di di, v - a, A M - * ....) - dt dt (3.19a) di di, v, = ( * M 2 + L2- * ....), dt dt (3.19b) where L,, L, = self-inductancesof coil 1 and coil 2,respec- tively, H, and M = mutual inductance, H. The additional terms implied by these equations exist only if more than two coils (or circuits, or windings) are interacting, and they are presented merely to make the expressions more general. The plus and minus terms of the equations deserve special attention. Sign convention has been well defined for inductors, and coil 1 and coil 2 are inductors when taken individually. A current flowing into the coil pro- duces an opposing voltage, hence the positive sign or polarity. The potential created by mutual inductance, M, however, cannot be treated in the same manner. This voltage may have either positive or negative polarity depending on the winding sense, the direction the coils are wound with respect to one another. Consider the two coils wound on a common core in figure 3.9A. They are wound in the same direction and therefore have the same sense. If a current is flowing into the top of the upper coil, the voltage produced by this current adds to that produced by the same current direction in the lower coil. But in figure 3.9B,the winding sense of the lower coil is reversed so that the same current in the top coil now creates a voltage that opposes the current produced in the lower coil. Therefore, the polarity of mutual-inductance voltages can be found by drawing physical sketches. However, this is impractical in circuit diagrams, and so magnetically coupled coils are often marked with dots that represent the direction of polarity. A dot is placed at the terminals of the coils that are instantaneously at the same polarity as a result of mutual inductance. Thus, in figure 3.10A, i, enters the dotted terminal of L,, v, is sensed positively at the dotted terminal of L,, and In analyzing circuits, it may be more desirableto reference v, as positive at the undotted terminal of L,, as in figure 3.10B. In this case, di, di, V, = -M- dt + L,- dt What is important is that, in either instance, the mutual voltage is produced independently from that of self- induction. -Magnetic coupling "I Ll: ',,-- ! , 'L2 Figure 3.8.-Magnetic cokpling between two coils. A B Figure 3.9.-Demonstration of coil winding sense. NI Ll "42 L2 I, V2 polarlty "2 change IS fov equotlon M 3 20 only A Actual w~nd~nq sense B Dot notatton Figure 3.10.-Qol convention for mutual inductance sign.
- 91. The equationsjust presented are valid for any voltage or current waveform. If the currents are sinusoidal and have a radian frequency, w, transforms can be employed so that for equations 3.20~ and 3.20d, These relationships can be used to analyze circuits con- taining magnetically coupledelements. It shouldbe stated that equations 3.20 and 3.21 relate only to the magneti- cally coupled elements; equations for complete circuits containing these devices will follow. IDEAL TRANSFORMER The level of mutual inductance, M, depends upon the spacing and orientation of the coils and the permeability of the medium between them. In other words, M is a function of the magnetic flux linking between the coils. More will be said about this phenomenon later in the section. In figure 3.10A,by comparingthe power entering L, of the circuit with that stored or available in L , , it can be proved from flux-linking concepts that Consequently, M has an upper limit defined by the geo- metrical mean of the two inductances involved. The ratio of M to its theoretical maximum is called the coefficientof coupling. This is by definition where k can range from zero to unity. Coils having a low coefficientof couplingare said to be loosely coupled. Here the coils could be far apart or have an orientation such that little magnetic flux interacts between them. Loosely coupled circuits may have a k that ranges between 0.01 and 0.10. For tightly coupled circuits, such as air-core coils, k can be around 0.5. A power transformer is a device having two or more tightly coupled coils or windings on a common iron core. The coils are wound and oriented to provide maximum common magnetic flux and can have a coefficient of couplingvery close to 1.00.The usual range is 0.90 to 0.98. Resistance and other power losses are small. The winding receiving power is called a primary; that deliveringpower is called a secondary. In the circuit in figure 3.10, L, is the primary and L, is the secondary. An ideal transformer is an idealized form of transformer where k = 1and losses within the device are zero. Hence, an ideal transformer can deliver all the power it receives. Many usefulrelation- ships for real transformers can be obtained by assuming the ideal transformer case. The self-inductance of a coil has been shown to be proportional to the square of the number of turns forming the coil (N), provided that all the flux, created by the current in the coil, links all the turns (see chapter 2, "Inductance"). If a sinusoidal current, I, flows in a coil of N turna, then the voltage produced across an N-turn coil must be N times that caused in a 1-turncoil. Further, for a sinusoidalvoltage, V , which is constant across an N-turn coil, the current allowed through must be 1/N times that caused in a 1-turn coil. Both these statements can be provedby magnetic field concepts,again assuming that all magnetic flux produced in a coil links all turns. It follows that for an ideal transformer with two windings: where N, = number of turns in primary winding, N, = number of turns in secondary winding, L,, I,, V, = primary winding inductance, rms cur- rent, and rms voltage, respectively, and L,, I,, V, = secondary winding inductance, rms cur- rent, and rms voltage, respectively. For this two-winding arrangement, the voltage and cur- rent can be complex sinusoids. The turns ratio of the transformer, a, is defined as the ratio of the number of turns in the secondarywinding to the turns in the primary winding: Hence, for an ideal transformer, In other words, the sinusoidal voltagesacrossthe primary and secondary windings are in direct proportion to the number of turns of the windings, and the currents are related inversely to the turns. In addition, the last equa- tion shows that the apparent power at the primary and secondary windings is indeed equal: The magnitude of this power in voltamperes is specified for the maximum allowable or rated capacity of power transformers. Another useful transformer relationship can be deter- mined through a demonstration of steady ac circuit anal- ysis with magnetically coupled circuits. Consider figure 3.11A, where a sinusoidal voltage source, V,, with an internal impedance, Z, (the combination is the Thhenin equivalent for a source),is connected to the primary of an ideal transformer. The secondary delivers power to a load impedance,Z , . The vertical lines between the transformer windings indicate that the core is made of iron lamina- tions. The turns ratio above the transformer symbol, l:a, relates a convention of N, to N,. A very useful relationship is the ideal-transformer input impedance with the load connected,that is, the load
- 92. 'Ideal transfwmer A Figure 3.11.-Demonstration of impedance transfer in transformers. that the source sees through the transformer. Loop equa- tions can be used to solvethe problem. Two loops,fl and 1 , (both express complex currents), are available in the circuit; the loops are magnetically coupled through the transformer. Employing Kirchhoffs voltage law for loop 1, and for loop 2, M is again the mutual inductance. Notice that current enters the dot of the primary and leaves the dot on the secondary,making the sign of M negative.Rewritingthese into standard loop-equation form gives 0 = -I j o ~ + UZ,+ joL,). Solving for I,, Therefore, the impedance seen by the source, Z , , , is the ratio of the source voltage to terminal current, or but M2 = L1L2, then There must be total coupling between primary and sec- ondary windings for an ideal transformer; thus, the self- inductances, Ll and L,, have no effect in the circuit, and their value can be considered infinite. Notwithstanding, the ratio is still finite, as specified by the turns ratio: L2 = a2L1. For this reason, primary and secondary inductances are conventionallynot specified on ideal transformers. When this is related to the input impedance expression, or rearranging, Now allowingL, to tend toward infinity,the input imped- ance for the voltage source becomes Equation 3.28 is significant as it shows that the source sees the load impedance, Z,, through the trans- former as ZL/a2.This means that an ideal transformer has the capability to change an impedance magnitude. There- fore,to assist in circuit analysis,the circuit in figure 3.11A can be redrawn to its equivalent, shown in figure 3.11B. Here, the impedance connected to the secondary is trans- formed to the primary. Obviously, the reverse process, primary to secondary, also holds, but the impedance is multiplied by a2. The impedance angle remains constant in either situation. EXAMPLE 3.5 A 60-Hz single-phase transformer has a rated capacity of 250 kVA and a turns ratio of 15:l. Assuming that the transformer is ideal, find the primary voltage if the secondary voltage is 480 V . What are the magnitudes of primary and secondary currents with these voltages applied and the trans- former operating at full capacity? SOLUTION. For the turns ratio of 15:1, 1 a = -. 15 As the turns ratio specifiesthe secondaryvoltage to the primary, v2 a = - Vl ' and the primary voltage is v2 Vl = - = 15(480)= 7,200 V . a The primary current for 250 kVA at 7,200 V is 250,000 1 I - 7,200 - 35 A, and the secondary current for 250 kVA at 480 V is I - A - 250 OoO - 521 A. - 480
- 93. EXAMPLE 3 . 6 Consider that the circuit shown in figure 3.11A has the following parameters: Z, = 6 + j3 Q, Z,, = 1 + j0.5 Q, V, = 7,200 V, 60 Hz, Turns ratio = 12:l. Find the value of the load impedance (Z,) referred to the transformer primary, the complex power at the source, the transformer secondary voltage ond cur- rent, and the required transformer capacity. SOLUTION.For the specified turns ratio, Transferring the load impedance to the primary, which is the impedance referred to the transformer primary. The total impedance seen by the source is then ZL z,, = z, + -- a2 = 6 + j3 + 144 + j72 = 150 + j75 0. Assigning the source voltage as the reference pha- sor, the transformer primary current is - I, = Z,, = 167.7126.60 7'200100 = 42.91-26.6O A. The transformer secondary current is - I I, = 2 = 12(42.91-26.6O) = 515j -26.6O A, a and the secondary voltage is v2= 12Z, = (5151-26.6OXl + j0.5) The complex power delivered to the load is then - S = &I,* - or S = (5761O0X5l5/26.60) - = 29612622 kVA. This may also be found from Finally, the apparent power demanded by this load is the required transformer capacity, 296 kVA. ACTUAL TRANSFORMERS In actual transformers, a source must furnish the power dissipated by the secondary load plus the power needed to operate the transformer. The additional power is created from losses within the transformer circuit. The transformer capacity, the amount of power it canhandle, is dependent upon the character of these losses, which are dissipated as heat in the core and the windings. Because excessively high temperatures are destructive to insula- tion, the capacity is limited by this rise in temperature, usually specified as an allowable temperature rise above ambient conditions. The major losses in an ironcore transformer are winding resistance (conductor loss), leakage reactance, eddy-current loss, and hysteresis loss. This section will expand upon the ideal-transformer concept to produce a transformer equivalent circuit that accounts for these losses and is a good approximation for real-world trans- former performance under any condition. Conductor Loss As the conductors used for the transformer windings have resistance, current flowing in the primary and sec- ondary produces an 12Rpower loss that creates heat. The loss is minimized by conductors with larger cross sections, but if the resistance is too large to be neglected, primary resistance, R,, and secondary resistance, R,, can be placed in series with the ideal-transformerwindings as shown in figure 3.12. Leakage Reactance For the ideal transformer, all the flux produced by the primary must link with the secondary winding. In the real world, however, a small percentage of the total flux pro- duced fails to link all the secondary turns; this is called leakage flux. Leakage flux can be reduced by placing the primary and secondary windings very close together, per- haps interleaving them. Further reduction comes from Figure 3.12.-Ideal transformer with winding resistance in- cluded.
- 94. winding the coilstightly on the core and providinga short magnetic path between them, thus creating a low- reluctance path betweenthe coils. Nevertheless, even with the best transformer designs, leakage is significant and cannot be neglected. Inductance is the ratio of flux linkage to the current producing the flux, or where d$~ = magnetic flux, Wb, Nd+ = flux linkage of circuit, Wb, and di = current producing flux, A. For transformers with iron or ferromagneticcores, current and flux do not have a linear relationship, and differen- tials must be used. Consider the time-varying primary current, i,, in figure 3.13A, where the changing current produces the magnetic flux, dl, and Nld41 L, = --- di, The part of 4, that links the secondary is +,,; that which only links the primary (or is lost in terms of magnetic coupling) is b,,, where and 1 - Niddlz N I ~ ~ L I L - ------ - - - (3.30~) di, di, ' Figure 3.13.-Accounting for transformer leakage flux. Similarly, although not shown in the figure, N2d4, L, = - di, ' N2d421 + N , d ? , . L, = - - di, di, Interestingly, the coefficient of coupling is also related to flux by The first term in equations 3.30~ and 3 . 3 1 ~ is the transformer mutual inductance, and the second terms are the primary and secondary leakage inductances, LL, and L,,, respectively, or Nld41, N,d+21 M = ------- - - (3.33~) di, di, ' NldCLl L,, = - (3.336) di, ' N,d4,, L , = - . di, These equations hold for effective sinusoidal current. In steady ac analysis, the leakage inductances become leak- age reactances; hence, flux leakage can be represented as an inductance or reactance. Figure 3.13Bshows the addi- tional elements that bring the transformer model closer to a practical transformer. Core Losses and Exciting Current Even with the addition of winding resistance and leakage reactance, equation 3.276for an ideal transformer still applies and can be rewritten as Examination of this expression suggests that whenever I, is zero, I, must be zero. Yet, if an actual transformer primary is connected to an ac source and the secondary is left unconnected (fig.3.14),the primary current will exist, albeit very small. Even though the secondary is open and I, is zero, V, appears across the secondary winding as a sinusoid. This implies that a changing flux in the trans- former core must be produced by the current in the primary, as no other sources of changing flux are avail- able. The portion of primary current that produces the changing flux, called magnetizing current, i,, can be accounted for by adding an inductor, L,, in parallel with the ideal-transformer primary winding.
- 95. The changing flux also induces small currents, eddy currents, in the transformer core material. These have an almost infinite number of closed paths and encircle prac- tically all the flux. Sincethe transformer core has electri- cal resistance, the result is heat in the core and attendant power loss. These eddy currents flow at right angles to the magnetic field, as illustrated in the core cross section of figure 3.15A. The resistance along the eddy-currentpath is approximately proportional to the path length. Obvi- ously, if the path length is decreased, the power dissipated in the eddy-current loopwill drop. Figure 3.15B showsthe core.split lengthwise with a nonconducting layer between the two halves. The result is a desirable decrease in power loss to about two-thirdsof the original power. In practice, transformers are laminated from several thin sheets of steel. Each sheet is sometimescovered with varnish to act as an insulant, but in most cases the oxide layer on each steel sheet is suflicient to produce the necessary high- resistance layers. This can substantially reduce eddy cur- rents but cannot completely eliminate them. As shown in figure 3.15C, energy is also dissipated in the transformer core each time a hysteresis loop is tra- versed. The energy is proportionalto the area enclosed in the hysteresis loop and is called the hysteresis loss of the transformer core. In simple terms, the effect is related to the fact that the core retains some magnetism, and a coercive force is required to overcome this residual mag- netism eachtime the ac current reverses. The loss is due to retentivity or molecular friction. Both eddycurrent and hysteresis losses are propor- tional to frequency and become a major consideration in high-frequency transformer applications. However, these core losses can be satisfactorily approximated at one frequency and one voltage. Good examples are 60-Hz power transformers where neither frequency nor voltage (actually,magnetic saturation of the core)changes drasti- cally in normal operation. To account for these losses, a resistance, Re,is again placed in parallel with the ideal- transformer primary. The sum of the currents through Re and L , is called exciting current, I,, and the total current drawn by the source when the transformer is supplying power to a load is I, + I,. It should be noted that with sinusoidal input voltage to the primary, the exciting current is not a sinusoid but exhibits many harmonic frequencies because of the greatly varying permeability of the transformer core. However, for most purposes it may be assumed as a sinusoid with the same rms value. The equivalent circuit shown in figure 3.16 now contains all the components necessary for it to be a useful model of a practical transformer. In summary, the impor- tant parameters for an equivalent circuit are R,, primary conductor resistance, %, secondary conductor resistance, h,, primary leakage inductance, k , , secondary leakage inductance, Re,a resistance accounting for eddy-current and hysteresis losses, L,, an inductance accounting for magnetizing cur- rent, and An ideal transformer with turns ratio, a = N2/N, R, L , , i, L L ~ R2 +~nj$ vst - - I - ..-- 2 - u i , = magnetizing current Figure 3.14.-Transformer magnetizing current. Eddy-current Eddy-currem Energy Magnetizing layer fwce Figure 3.15.-Eddy current and magnetic hysteresis creating power loss in core. Figure 3.16.-Equivalent circuit of practical transformer. Notice that mutual inductance, M, does not appear in the model sinceit is represented by the turna ratio of the ideal transformer. Power-Transformer Construction The two most widely used transformer types are the core and the shell. In shell construction,both primary and secondary windings are placed on an inner leg of the core. The windings are constructedin layers with an insulating barrier between them, forminga very low-leakageflux. In core construction, the primary and secondary windings are located on separate legs, thus providing maximum isolation between the coils. Both constructions are sketched in figure 3.17. A copper or aluminum conductor is employed to construct each winding, which can have the form of an
- 96. insulated wire with circular or rectangular cross section, or an uninsulated wide metal sheet. The insulated wire is continuously wound in layers, with each layer separated by a sheet of insulating material. With sheet-metal wind- ings, the conductor is wound simultaneously with a con- tinuous sheet of insulating material so that each adjacent conductor t u n is separated by the insulation. The sheet metal is the same width as the transformer winding, and the insulation sheet is slightly wider. Each winding is given a rated capacity, a rated current, and a rated voltage. These ratings depend upon the number of turns in the winding, the magnetic inter- action with other windings, the current-carrying ability of the conductor, as well as the ability to dissipate heat through the insulation to the environment surrounding the winding. It should be obvious that the rated capacity, current, and voltage are mathematically related. Transformer Models Since voltage regulation, efliciency,and heating are of prime importance in mine power systems, detailed power- transformer analysis requires consideration of the com- plete equivalent circuit as shown in figure 3.16. However, because the exciting current, I,, is normally very small compared with load current, I,, a further approximation can be made by placing Reand L , at the transformer input terminals (fig. 3.18). This modification now allows the secondary winding resistance and leakage inductance to be transferred to the primary circuit (fig. 3.19) and com- bined with the primary elements. For many purposes, the exciting current is so small that Re and L, can be removed from the model. Figure 3.20 provides this last simplifica- tion, where the winding resistance and leakage reactance are said to be referred to the primary, and R2 LL, R = R, + - ; - and L, = L,, + 7 . (3.34) a' The primary is sometimes called the high side if its winding has a greater voltage rating (or more turns) than the secondary. The secondary is then called the low side. The terminology is reversed if the secondary has the higher voltage. In steady ac analysis, the inductance becomes a reactance, X , , and R2 L,2 R = R, + yand X, = G,, + -) (3.35a) a a2 ' with the primary impedance simply If desired, the primary impedance can be moved to the secondary of the ideal transformer (thus, referred to the secondary) by multiplying both terms by a'. Secondary Primary Primary I Seconda core Core Core construction Shell construction Figure 3.17.-Common power.transformer construction techniques. Figure 3.18.-Movement of exciting components to Input. Figure 3.19.-Transferring secondary components to primary. Figure 3.20.-Final simplification of practical circuit model.
- 97. EXAMPLE 3.7 A two-windingtransformer has a rated capacity, primary-winding voltage, secondary-winding volt- age, and frequency of 100 kVA, 2,400 V , 240 V, and 60 Hz, respectively. The primary-winding imped- ance is 0.6 + j0.8 n, while the impedance of the secondary winding is 0.005 + j0.007 Q. The trans- former is being used at the end of a feeder to step down voltage to a load.The feederimpedanceis 0.05 + jO.l 0, and the load is 0.3 + j0.4 Q. Find the magnitude of the voltage across the load if the voltage at the source end of the feeder is held constant at 2,400 V. SOLUTION. As core-loss and magnetizing-current elements are not given for the transformer, they must be assumed to be negligible, with the trans- former model being as shown in figure 3.20. The turns ratio is 1110, and the transformer impedance is (equation 3.35) R, x2 Z , = R l + j X l + - + j - a2 a2 0.005 .0.007 = 0.6 + j0.8 + -+J - (l110Y (1110Y = 1.1 + j1.5 n. The load impedance transferred to the primary is ZL - 0.3 + ~0.4 = 30 + j40 n, aZ = (1/10)2 and the total impedance at the source end of the feeder is ZL z,, = z, + Z, + - a2 = 0.05 + jO.l + 1.1 + j1.5 + 30 + j40 = 31.15 + j41.6 = 51.97153.20 Q. The magnitude of current from the source is then I - x = - 2'400 - 46.18 A, - I Z , I 51.97 which is also the current through the transformer primary. Therefore, the magnitude of voltage across the primary is Vl = I, I- I = 46.18(50) = 2,309 V , and that across the secondary and the load is Vz = Vla = - Determination of Transformer Parameters Two tests can provide the necessary elements for the transformer model in figure 3.19, where the exciting current components are at the primary terminals, and secondary parameters are referred to the primary. The first test, the open-circuit or excitation test, is used to find the exciting-currentcomponents.The second- ary of the transformer is unconnected. Rated voltage at rated frequency is applied to the primary winding, and a wattmeter (see chapter 5) is employed to measure the power, P , ,delivered by the source. An ammeter is used to measure rms exciting current, I, (fig. 3.21A). The power corresponds to the core loss; in other words, P , is dissi- pated by Re.R , can be found by where V = rms applied voltage, V , P, = measured average power, W, and Re = core-lossresistance, n. fiom the rms value of Ie,assuming exciting current to be a sinusoid, the input admittance of Re and L, in parallel can be calculated from Realizing that the component accounting for magnetizing current, L,, can be obtained from where 1%= measured rms value of excitingcurrent, A, w = 2 d = applied frequency of source (must be rated frequency of transformer for exact results), and L , = magnetizing inductance, H. Therefore, both Re and L , can be determined from the open-circuit excitation test. The second test is projected at winding resistance and leakage resistance, with both primary and secondary - u 1 _ 1 A Open clrcuit B Short circuit Figure 3.21.-Transformer parameter test series.
- 98. values combined. This is termed the short-circuit or im- pedance test. Here, the secondary terminals are short- circuited and a source is connected to the primary. Voltage at rated frequency is applied to the transformer but at reduced amplitude, so that it produces only rated current in the primary winding and, thus, rated current in the secondary. Current, I,,, and input average power, P,, are again measured. The applied voltage for the test is typically much smaller than rated voltage. Yet the short-circuit(actually, rated) current is much greater than the exciting current, so I, and the associated components can be neglected. As given by figure 3.21B, the equivalent circuit under these conditions can be simplified to a simple series RL combi- nation. The ideal transformer is not needed because the zero load impedance (short circuit), when transferred to the primary, is still zero. Winding resistance and leakage inductance can thus be found from and often applied to transformer secondary-voltagevariations and is defined as V.R. = v ~ L - v F ~ (loo%), (3.41) VFL where V , , = transformer output voltage at full rated secondary current and rated primary volt- age, V, and V , , = transformer output voltage with no second- ary load but rated primary voltage applied, v. V , , and V , , are also called the full-load and mload voltages, respectively. It should be clear that voltage regulation is a function of transformer losses, impedance, and efficiency.The concept is extremelyimportant in mine power systems as it often limits how far a mine can be safely expanded from one power source. (3.38) I where P, = measured average power, W, I, = measured rms short-circuitcurrent, A, V,, = applied rms short-circuitpotential, V, w = 2n-f = rated frequency, radls, R = primary and secondary winding resistance, fl, and L, = primary and secondary leakage inductance, H. It is important to note that these values are valid only forthe frequencyunder which the tests are made. Further, it is neither possible nor necessary to break the resulting components into primary and secondary elements. lhnsforrner Efficiency and Regulation The transformer is designed to be a highly efficient device. However, the output power of a transformer is always less than its input power because of winding conductor losses and core losses. The term eficienqy is used to measure the ability of a transformer to transfer energy from the primary circuit to the secondary circuit. The efficiency is defined as the average-power ratio: Pi, - losses or = Po, 9 = P o , , + losses (3.40b) Pin EXAMPLE 3.8 For the circuit shown in figure 3.22, find the complex power consumed by the transformer load, Z,. If the figure represents the full-load condition, what is the voltage regulation at the transformer secondary? The transformer is considered ideal. SOLUTION. The impedance seen by the 5,000-V source is ZL Z,, = 1+ jl + - a2 = 1 1+ j l l = 15.56145Ofl. Usingthe sourcevoltage as the referencephasor, the current delivered from the source is and the transformer secondary current is The ratio is always less than 1but normally in the range n = 0.95 to 0.98. Efficiency decreases when the device is 5,00vf " 1 60Hz q 11 fv2 z,=o.l+lo.ln operated above or below its voltampere capacity. Voltageregulation is a characteristic of power systems that describes the voltage fluctuations resulting from varying load or current conditions. Voltage regulation is Figure 3.22.-Clrcult for example 3.8.
- 99. I The voltage across the transformer secondary is I I and the complex power delivered to 2 , is I therefore, S, = 1.46145O -MVA. I If the above situation represents the full-load condi- tion, then I v,, = v, = 454.4v. II Under no-load conditions, the load impedance be- comes such a high impedance that the transformer secondary current approaches zero. With no second- ary current, current to the primary of an ideal transformer is also zero. Therefore, the voltage across the primary is equal to the source voltage or I The secondary voltage becomes I Consequently, V , , = V, = 500 V, l and from equation 3.41, I 500 - 454 V.R.= 454 (100)= 10%. AUTOTRANSFORMERS All the transformers discussed so far have been two- winding transformers and have provided electrical isola- tion between the primary and secondary windings. An- other type of transformer, the autotransformer, uses a single winding and does not provide electrical isolation. It is constructed from a continuous winding with a tap connected at a specific point. The autotransformer is compared with an ideal two-windingtransformer in figure 3.23.The advantages and disadvantages of each type of transformer can be illustrated with reference to figure 3.24,where a normal two-winding transformer is shown on the left and is connected to operate as an autotrans- former. The two-winding transformer has the following spec- ifications: N,,V,, I, = primary turns, rated rms voltage, and rated rms current, N,, V,, I, = secondary turns, rated rms voltage, and rated rms current, and the maximum apparent power that can be delivered to a secondary load is s o , = v21,. Figure 3.23.-Comparison of two-winding transformer (A) and autotransformer (B). Figure 3.24.-Two-winding transformer as an autotransformer. 'Ib help visualize the autotransformer action, figure 3.24A is redrawn in figure 3.24B with both windings placed on the same side of the core symbol. For either figure, the output voltage, V,', is now V,' = v, + v,. Transformer rated output current, I,, is still related to rated primary current, I,, by but input current to the autotransformer is now The maximum power that can be transferred to a load at rated output current, I,, is now s, , , = V,'I, + V,I, + V,I,. This expression indicates that the transformer is now able to deliver an increase of V,12 voltamperes over the two-winding connection, yet the transformer windings are still within rated currents and voltages. The reason for the increase is that some input current is transformed by the transformer while the rest is conducted directly to the load. This is the main advantage of the autotransformer over two-winding arrangements. Because primary current is now only a portion of load current, conductor losses in autotransformers are particularly small, and voltage reg- ulation under varying load conditions is usually good. MULTIVOLTAGETRANSFORMERS The transformers considered so far have had only one secondary, but in practice many have two or more second- ary windings. The transformer with two secondary wind- ings in figure 3.25A is able to serve loads with different
- 100. voltage requirements from one source. In such devices the magnetic interaction increases substantially over the two- winding variety because mutual inductance exists be- tween all winding combinations. Taking this into account, the preceding theory can be expanded to model an equiv- alent circuit. Another method for one transformer to serve several voltage applications is to have winding taps on the pri- mary (fig. 3.25B),the secondary (fig. 3.250,or both. When used on the input winding, a higher tap can be selected to account for voltage drops in the circuit that delivers power to the transformer, thus maintaining a desired output voltage. This is a common practice in mining. A special but very widely used application for secondary taps is in utility distribution transformers suppiying240- and 1207 ac service. Here, the winding is center-tapped with equal turns on either side. The voltage magnitude from either line to the tap is 1.20V, and across the total winding, 240 V is available. CURRENTANDPOTENTIALTRANSFORMERS The prime use of transformers in mine power systems should now be apparent: to supply power at different voltage levels to system portions and equipment. Trans- formers are also used extensively to power control circuits, mainly to provide power for circuit breakers and associ- ated circuitry; to power protection devices, usually relays to trip circuit breakers; and for instrumentation. Trans- formers employed for these applications are often given specific names: potentznl transformer,s (PT's) and current transformers (CT's). FT's are merely high-quality two- winding transformers with or without taps. The name is modified because they are used to sense voltage. The current supplied to relays, instruments, and sim- ilar equipment is normally provided by CT's. Some CT's are like the two-winding devices that have just received so much attention. These have a primary with just a few turns of high-current-capacityconductor and a secondary with numerous turns, as illustrated in figure 3.26A. The turns ratio M,/N,) is normally adjusted so that the secondary supplies 5 A when full-loadcurrent flows in the primary. The primary is placed in series with the circuit that is to be measured, and therefore, CT's can be consid- ered as sensing current. Two-winding CT's for high-voltage or high-current circuits, such as those usually found in mine power sys- tems, are very expensive, and as a result bushing-type or donut CT's are more often used. In figure 3.26B, the conductor to be measured passes through a large-diameter ring-shaped laminated iron core and acts as the trans- former primary. The secondary winding, which consists of several turns about the core, supplies current as before. The leakage reactance of this type of CT is obviously high and, coupled with other parameters, results in a low accuracy for current measurements. A schematic illustrat- ing hypothetical placements of a PT and a CT in a simple circuit is provided in figure 3.27. PT's and CT's are important components in instru- mentation and protective circuitry for mine power sys- tems. Their application for instrumentation is presented ,Extra tops Figure 3.25.-Examples of transformersfor multivoltage ap- plications. Primary:a few turns Power Laminated o f high-current C O ~ U C ~ ~ core conductor --. d To instruments 1n s e r i 21 with circuit and relays Nc N? , .. _ I Secondary'several turns of a Ltrumentr ~1 conductor and relays A 8 2-winding transformer Bushing or donut transformer Figure 3.26.-Two types of CT's. ' A ---PT ~~~~~~ 1 ' 1 current Senses T o instruments -- --am. .- To instruments or rebvs Figure 3.27.-Examples of CT and PT placement in circuit. in chapter 5, while chapters 9 and 10 cover their use in protective relaying. The purpose of the foregoingtwo chapters was to ewer many of the basic theoretical aspects behind mine electrical systems. The content was directed towards dc and single- phase ac, and spanned fundamental electrical phenomena, the experimental laws and parameters, dc and ac circuit analysis, and finally, power transformers. Comprehensionof these laws, parameters, and concepts is essential for the understanding of subsequent chapters. This will be very apparent in the next chapter, which introduces power-system concepts and three-phase circuit basics.
- 101. CHAPTER 4.-POWER-SYSTEM CONCEPTS Pbwer systems can be simply described as systems that transmit power from a source to the loads. For the mine, the source is often the secondary of a substation transformer and the loads are motors on mining machin- ery and ancillary equipment.The transmission of power is commonlyperformedby three-phasesystems,which are by nature more complex than the dc and single-phase ac circuits introduced in the previous two chapters. The following sections are primarily concerned with three- phase power systems plus the basic tools and special mathematics needed to study them. Severalreferences are provided at the end of the chapter. As most informationis consideredcommonelectricalengineering knowledge, spe- cific references are seldom cited but can be found in the bibliography. BASIC POWER CIRCUIT Manypower systemsor systemsegmentscanbe reduced to the simpleseriescircuit shown in figure4.1.Thisfamiliar single-phaseac circuit consistsofa sourceor supply voltage, an impedance, and a load or receiver voltage. Such a repre- sentation is &n called the Thknin's equivalent of the power system. Findmg the series circuit may involve many simplifying assumptions or procedures, some of which are yet to be covered, but the result has numerous applications for analyzing the behavior ofelectrical power systems. One specificexampleis analysis of voltage regulation. Here, the sourcevoltage is kept constant, and variations of the load voltage are observed with a range of load-current conditions that cause a change in voltage drop across the impedance. Applying this example to an undergroundcoal mine, the source could be the secondary of a power-center transformer, the impedance could be that of the trailing cable, and the load might be the motors of a continuous miner. On a larger scale, a substation output voltage, a feeder cable, and powercenter primary voltages could constitute a desired Thhvenin's equivalent for analysis. Both these situations are illustrated in figure 4.2. As the chapter unfolds, more applications will become apparent. Actual analysis of the basic power circuit (fig.4.1)can use any applicable technique already given in chapters 2 and 3. For instance, employingthe impedancedomain and Kirchhoffs voltage law yields or and THREE-PHASE CIRCUITS The term single phase has been applied to ac systemswhere power is delivered from a single sinusoidal source. When power is transmitted to a load by applying two or more sinusoidal sources with fixed phase differ- ences, the power system is called polyphase. The most popular system that delivers large quantities of power, including both single phase and polyphase, is the three- phase system. The analysis of three-phase circuits can be extremely complicated. Special techniques have been developed to assist in general problem solutions, but even so, the work can be cumbersome. However, three-phase systems are purposely designed to be balanced, and if actual differ- ences existing among phases can be neglected, the analy- sis of three-phase circuits can be almost as simple as analysis of single-phase circuits. BALANCED THREE-PHASE CIRCUITS Balanced three-phase power consists of three gener- ated voltages, each of equal magnitude and frequency but separated by 120°. When these voltages are applied to a system of balanced impedances,balanced currents result. In other words, a balanced three-phase power system can be divided into three portions. Any voltage or current in one portion has a counterpart in another portion, which is identical but 120° out of phase. L vL$ ~ o a q or - receiver Figure 4.1.-Basic power circuit. Continuous Any variable or constant in these equations can be a complex expression. Nevertheless, the equations describe the performance of the power system that the circuit represents, that is, the source voltage for a specific load current and load voltage, and so forth. When three-phase systems are involved, the solution or even the finding of the equivalent circuit must also utilize the additional methods that follow. SOURCE IMPEDANCE LOAD (4.lb) Feeder cable Substation Load center Load Trailing cable center SOURCE IMPEDANCE LOAD Figure 4.2.-Applications of basic power circuit. ( 4 . 1 ~ )
- 102. l b illustrate this voltage generation, consider the elementary three-phase generator illustrated in figure 4.3A. The armature consists of three single stationary conductors displaced by 120°, and a magnetic field struc- ture rotates counterclockwise within. As the rotating magnetic flux cuts each winding, a voltage is induced. Thesevoltages are out of phase with one another, as shown in figure 4.3B. A composite of these instantaneous volt- ages is provided in figure 4.3C to exemplify the phase relationships, which alsocan be clarifiedwith phasors(fig. 4.30). It can be noted with either representation that the voltage in winding aa' reaches a maximum first, followed by bb', and then cc'. This defines the positive sequence, abcab. . ., that is evident from the counterclockwise ro- tating phasors of figure 4.30. If the phasors are allowedto rotate in the opposite direction (clockwise),the sequence termed negative (cbacba . . .). An outstanding advantage of balanced three-phase systemsis that they provide a more uniform flow of energy than single-phase or even two-phase systems. The 120° timing means that the individual power waves in each phase never reach zero at the same time, and more important, the total instantaneous power from all three phases remains constant. For three-phase motors, this translatesto convenientstarting,constant torque, and low vibration. It would seem logical that if three phases provide a substantialincreasein operationefficiency,more equally spaced phases would result in even greater im- provement. However, three-phase systems are generally more economical than other polyphase systems because the complications caused by additional phases &set the slight efficiency increase. A source supplying these three-phase voltages is nor- mally connected in either delta or wye. As shownin figure 4.4, either configuration can, in practice, be closely ap- proximated by ideal voltage sources or in some cases by ideal voltage sources in series with small internal imped- ances. Three-phase sources always have three terminals, which are called lineterminals, but may alsohavea fourth terminal, the neutral connection.Theseterminals produce three separate potentials between any two line terminals that are called line-bline voltages. Also generated are three separate voltages between each line terminal and the neutral, be it a direct connection as in figure 4.4 or some imaginary neutral point. These are termed line bneutral potentials. Three-Phase System Voltages Line-to-linevoltage can be considered as a condition existing between two phases, while line-to-neutral is a condition for one phase only. Obviously,interrelationships must exist between these two voltage notations, as well as among the voltages of one notation. The wyeconneded source of figure 4.5A can be employed to demonstrate the correspondence. - If the line-to-neutral voltages, v,, & , . , and V,, are positive sequences and Lhe phasor of V , is taken aa reference, then V , , and V , are related to V , by S I ~ I ~ conductor armoture winding A Generator B Individual waveforms C Combined waveforms 0 Voltage phasors Figure4.3.-Elementary three-phase generation. ,Line terminal [Line conductor I voltage voltage C c c Figure 4.4.-Three-phase voltage sources. A B C 3-phase Line-to-linem d line- Gmphicol w y e source to-neutral volmge coffitructiin phasors Figure 4.5.-Wyetonnected source demonstrating Ilne-to. line and line-to-neutralvoltages.
- 103. Equations 4 . 2 ~ and b relate that if a specific phasor representing one phase voltage is rotated 120°, it is identical to the phasor for another phase. By Kirchhoffs voltage law, the line-to-linevoltage is equal to the sum of the two line-to-neutral voltages; for instance, between phases a and b, V,, = Van + V,, (4.3a) - but Vnb = -V bn - - and V,, = V,,l -120°; hence, v , = van- Van(-120° (4.3b) Equation 4 . 3 ~ is truly significant because it states the relationship between line-to-lineand line-to-neutral volt- ages for balanced three-phase systems. In particular, the following can be extracted: It is important to note that, in addition to the foregoing identities, for a balanced three-phase system, - - - and V,, + V,, + V,, = 0. (4.56) A phasor diagram illustrating all line-to-line and line- to-neutralvoltagesof these systemsis giveninfigure4.5B. Here the correspondenceby equation 4 . 3 ~ is apparent. The reasoning used for voltages can be applied to currents, and this will be handled shortly. Load Connections As with sources, balanced three-phase loads can be connected delta or wye. However, the interest in three- phase circuits comes from how delta or wye sourcessupply power to delta or wye loads. The usual combinations or systems are Four-wire, wye to wye; 9 Three-wire,wye to wye; Three-wire,wye to delta; Delta to delta; and Four-wire, wye to delta. By analyzing each combination, certain advantages and disadvantages can be seen, and some important points about balanced three-phase systems can be gained. For purposes of discussion, the lines connecting sources to loads are assumed to have no impedance, although obvi- ously, in the real world, they must have impedance. Figure 4.6A showsthe first arrangement to be consid- ered, the four-wire wye to wye. .The source here could be either a generator or the secondaries of an ideal three- phase transformer, and the load, Z,, Z,, Z,, could be a motor. These conductorsare connected between the source line terminals and the load; the fourth conductor, the neutral return (orjust simply, the neutral), connects the neutral of the source to the commonjunction of the three load impedances. For perfect conditions, the generation is balanced, distribution impedances per phase (again assumed zero here) are equal, and the load impedance in each phase circuit is-identical.-Hence, the magnitude of the line currents, I,, I,, and I,, must also be equal. By Kirchhoffs A 4-wire wye-to-wye B 3-wire wye-to-wye C Wye-to- delta D Delta- to -delta Figure 4.6.-Balanced three-phase load connections.
- 104. current law and the 120° displacement of the three line known as a four-wire wye-to-delta system and is illus- currents, the neutral-return current must be trated in figure 4.7. It is presently the most popular three-phase power connection arrangement in mining. fa + fb + fe = 0, (4.6) The neutral conductor here is more oftentermed a ground ing conductor A neutral point can also be derived from a delta sourceusing a zig-zagor groundingtransformer (see which means the neutral conductor actually carries no chapter 7). current under this ideal situation.Furthermore, there will be no voltage drop across the neutral, no matter what the neutral impedance is. In other words, the potential at the neutral of the source equals that of the load. If the neutral carries no current under balanced conditions, what purpose does it really serve and can it be removed? Consider figure 4.6B, a three-wire wye-to-wye system, which does not employ the neutral conductor. Although this system is used in some applications, prob- lems can arise, and the role of the neutral conductor is to minimize these problems. In the real world, no balanced three-phasesystem can be perfect, and the sources, the distribution impedances, and the loads can easily become unbalanced, that is, unequal from phase to phase. The result is unbalanced currents and voltages. For example, without the neutral conductor,the neutral of the sourcewill not equal the load neutral, and the resulting load unbalance will produce unequal voltages acrossthe loads, no matter how balanced the source.under this condition, a mining machine motor is likely to deteriorate and the result will be maintenance problems. In addition, safety problems can abound as a result of the unequal neutral potentials alone. Chapter 7 will investigate many of these problems in detail. It is apparent that the neutral conductor does serve a vital role in actual three-phasepower systems. Its sizeand current-carryingability do not need to match those of the phase conductors in order to provide the necessary func- tion. In a properly operating power system, normal condi- tions do cause some neutral current, but this is usually very small compared with the phase current. Hence, neutral conductors could be small if they were based only on the size of the neutral current, but in mining applica- tions, this is not the only criterion. Possible system mal- functions must also be taken into account, and these will be discussed in a later section on unbalanced three-phase circuits. Athree-phase load is more likelyto be delta connected than wye connected. The three-wire wye-to-deltasystem, shown in figure 4.6C, is an example of this arrangement. The prime advantage is that under unbalanced load conditions, the source will deliver power proportionately to each load. Hence the delta-connectedloads need not be preciselybalanced. Flexibility is increasedbecause phase- to-phase loads may be added or removed without signifi- cantly upsetting system operation. With wye-connected loads such changes are difficult or nearly impossible to make. A delta-connected source is shown in figure 4.60. Althoughthis arrangement can be found, it has two major disadvantages. First, a slight unbalance in the sourcecan create large circulating currents around a delta loop (for example,source V , , and load Z,d. This extra current can reduce the available current capacity of the source and also increasepower lossesin the system.Second,it is more difficult for safety purposes to maintain metallic equip- ment frames at the neutral potential of the source. The logical and most economicalpoint to employ as a ground is the neutral of the wye-connected source. This system is Line and Phase Currents Currents in a specificphase conductoror in one leg of a wye-connected source or load are termed line currents. As with line-to-neutralvoltages, they can be considered as a condition of one phase only. Currents flowing between two phases are called phase currents (or line-bline cur rents) and correspond to line-to-linevoltages. An obvious exampleof phase current is that flowingthrough one legof a deltaconnected load. As might be assumed, for the balanced three-phase system, the magnitudes of the three phase currents through the legs of the delta are equal. Figure 4.8A shows a schematic of a balanced delta load with three line currents I,, I,, I, and three phase currents I,,, I,,, I,,. It can be utilized to demonstrate the relation- ship between line and phase currents. Considering only phase a and using Kirchhoffs current law, From the same reasoning that related line-to-neutral to line-to-linevoltages, I (Neutral return or "grounding conductor" Neutral 4 - *or ground Ia+Yb+Tc =O point Figure 4.7.-Four-wire wye-to-delta system. Figure 4.8.-Balanced delta load currents. B illustrating phase and line
- 105. which means that in the balanced case the magnitude of line current is larger than phase current by a factor of J3. The phasors are displaced by 30°. The symmetry of phase and line currents is shown in figure 4.8B. Equivalent Delta and Wye Loads There are many instances where it is desirable to replace a balanced delta--connected load with a wye, or vice versa. The groundwork to perform this change has already been established in chapter 3. From equation 2.48, z,, = - z,, Z", Znb + Z h + ZCR' and so on for Z , , and Z,, in terms of the delta impedances. Equation 4.8 provides equivalence of delta and wye for all situations, including unbalanced loads. However, for bal- anced conditions, the expression reduces to simply Z,, Z = a , , 3 (4.9a) This states that each branch of a balanced delta has three times the impedance of a balanced wye. Now that voltages, current, and equivalent load im- pedances of balanced three-phase systems have been cov- ered, these values can be compared for delta and wye loads. If the load is wye connected, the line current and load current per phase are the same, but theyoltage across each load impedance is he-to-neutral, 1/J3 that of line- to-line. When the load is delta connected, the voltage across one load impedance is line-to-line, while the line current is larger than the hase current through each load impedance by a kctor of &.These concepts are illustrated in figure 4.9 for equivalent delta and wye loads. It is significant to note that the three line-to-linevoltages and three line currents for either connection are identical. Three-Phase Power Because the voltage and current are the same in each impedance of a balanced delta or wye load, the average power consumed by one impedance is one-third of the total power to the load. In a delta load as in figure 4.94 current and voltage are phase and line-to-line, respectively, and If ZU...=L.."n ; zbc s V o ~ ' uC? Vun, If Lon= Zb,=ZCn ,/On= Vob/D, Lo, = cind I , E rob Z , , = 3Z,, and lob =I,/ Figure 4.9.-Comparison o f equivalent delta (A)and wye (6) loads. the angle between them is the angle of impedance. Thus, considering phases a and b, the average power consumed by one element is and total power is or in general, where P,, = average power consumed by each element of a delta load, W, P, = total power consumed by delta load, W, V,, = line-to-line (or system) voltage rms magni- tude, V, $ = magnitude of phase rms current through load, A, and cos0 = power factor of load. When the load is wye connected, line current is through each load, while the voltage is line-to-neutral. Hence, taking phase a (fig. 4.9B),the average power to one element is and total power is where P,, = average power consumed by each element of wye load, W, PT = total power consumed by wye load, W, V,, = magnitude of line-to-neutralrms voltage, V, I, = line rms current magnitude, A, and cos0 = power factor of load. It is important to note that the power-factor angle, 8, is referenced to the sinusoidal voltage across one load and the current through that load. The standard measurement values for three-phase circuits are line-to-linevoltage and line current, which are often the known quantities. Since for balanced systems, and IL = &Ip, both equations 4.116 and 4.136 are also identical to PT = hV,IL (load pf), (4.14) where the power-factorangle is that of a load impedance or an equivalent impedance. It is important to realize that this angle has nothing to do with the angle between VLL and I,, for example, V , , and I,. Of the three three-phase average-power formulas, equation 4.14 is by far the most used.
- 106. Following the single-phase presentation of chapter 3, a balanced three-phase load has reactive power, Q,, and apparent power, S,,in addition to average power, P,. The following expressions apply: and S, = &VL,I, = 3 VLLI, = 3 V,,,I,>. (4.16) Complex power, G,is therefore or for phase a, or for phases a and b, It should be evi&nt that in balanced three-phase systems, complex power, F & . , does not equal -hVabI,*. Basically, all power concepts presented in chapter 3 for single-phase ac also apply to balanced three-phase power. Pbor power factor is worthy of critical attention because it affects the entire system operation by limiting the available power from transformers, hindering voltage regulation, and limiting the currentcarrying ability of conductors and cables. Simply, the result is poor system operation and economy. Thus, power factor can be equated to an indicator of system efficiency. EXAMPLE 4.1 An underground coal mining section contains the following three-phase equipment connected to the secondary of a transformer in the section power center: Continuous miner: 300 kW at 0.6 lagging pf, Twoshuttle cars: each 60 kW at 0.8 lagging pf, Roof bolter: 50 kW at 0.8 lagging pf, Feeder-breaker: 100 kW at 0.6 lagging pf. Find the capacity of the power-center transformer necessary to operate these machines. SOLUTION.The complex power to several loads ia the sum of the complex power consumed by each individual load. Thus, the complex power for each load must be found first. For the continuous miner, and Q, = S, sin6, or Q1 = 500 (0.8) = 400 kvar Accordingly, for both shuttle cars, P, = 120 kW, S, = 150 kVA, Q, = 90 kvar, For the roof bolter, For the feeder-breaker, The total average and reactive power are then respectively P., = PI + P, + P, + P, = 300 + 120 + 50 + 100 = 570kW, The total complex is then sirr~ply The required capacity of the transformer is equal to the apparent power of the load. Therefore, transformer capacity = ST = 873 kVA. It should be noted tliat, as in chapter 3, the solution to the problem cannot be assunied to be the simple summation of the apparent powers for all the loads. The only case where this is possible is where all loads are operating with the same power factor. EXAMPLE 4.2 For the combined consumptions of example 4.1, find the necessary total capacitance in kilovoltam- peres reactive to improve the overall power factor to 0.8 lagging. The capacitance will be connected across the transformer secondary. SOLZITION. The combined complex power for the preceding problem is When pure capacitance is added. average power will remain constant, but reactive and apparent power will decrease. Therefore, cosO,, = 0.8 lagging, I
- 107. Qn, = Sn, sinen,, = (712.5X0.6) = 427.5 kvar. The difference between this new or improved reac- tive power and that without the capacitance is the reactive power less the capacitance, or Qc = (QT - Qn-) = (661 - 427.5) = 233.5 kvar. It can be noted that this example is much like example 3.4. The concept of power-factor improve- ment has been repeated here to show the similarity of most power problems, be they single phase or three phase. THREE-PHASE TRANSFORMERS Considerable background information about trans- formers was presented in chapter 3, and most of that theory is also applicable to three-phase transformers. The prime purpose is the same as with single-phase systems, to provide the different voltages required for distribution and equipment operation. The transformer can be constructed as either a single three-phase unit or a bank of three single-phaseunits. The only difference between the two is that the three-phase unit has all windings placed on a common core. The connections can best be described by considering a bank of three single-phase two-winding transformers. Every coil is insulated from the rest, and there are three primary and three secondary windings, all of which can be interconnected independently. The primary and secondary windings can be delta or wye connected while complete electrical separation is retained between all the primary and secondary windings. The possible connections are wye to wye, delta to delta, delta to wye, or wye to delta. Figure 4.10 illustrates the physical connections of each combina- tion, and figure 4.11 shows the corresponding symbols used in the three-phasecircuit diagram. Any one of these combinations can be found in or about mine installations, but mine power transformers are typically delta to wye. Delta-to-wye connections are popular in mine power systems because of the load advantages of the delta connection of the primary, which is in essence the load for the incoming power. The neutral of the wyeconnected secondary provides a good grounding point for the outgo- ing system from the transformer and does not shift poten- tial under unbalanced load conditions. The delta-wye winding combination does not generate third-harmonic (180 Hz for 60-Hz systems) voltages and currents that hamper delta-delta and wye-wye connections. The second most popular transformer configuration in mines is the delta to delta. Although systems requiring a grounding neutral point create some difficulty for the delta secondaries, the delta-delta connection has one sub- stantial advantage. If one of the single-phasetransformers fails, operation can be continued by removing the defective unit and operating the two remaining transformers as Wye-to-wye Delta-to- delta Delta- to-wye Wye- to-delto Figure 4.10.mThree singlephase transformers connected for three-phase operation. y ) - - ; + , N e u t r a l f%bc' C b' d d Wye-to-wye Delto- to-delta Delto- to- wye Wye-to-delta Figure 4.11.-Threephase dlagrams for the transformers of figure 4.10. open delta. This open-delta or V connection can be illus- trated by the two single-phase transformers shown in figure 4.12. Although it is an unsymmetrical connection, it does provide a symmetrical three-phase power input and output. However, using the two transformers in this man- ner reduces capacity to 57.7% of the three-transformer kilovoltampere rating. Nevertheless, it is an effective emergency measure. The open-delta configuration is some- times used as a temporary circuit; for example, when the completion of delta is postponed until load conditions warrant a third unit.
- 108. Figure4.12.-Open-dalta three-phasetransformeroperation. Calculationswith delta-to-deltaand wye-to-wyetrans- formersare straightforwardand easy to comprehend.With delta to delta, primary line-to-line voltages and phase currents are transformed to secondary line-to-line and phase values, while for wye-to-wye transformers, line- to-neutral voltages and line currents transfer directly. Delta-to-wyeand wye-to-deltacombinations are different. With a delta-to-wye codiguration, primary line-to-line voltages become secondary line-to-neutral, and primary phase currents transform to secondary line currents. Through this, the current and voltage for all three phases shift in phase by 30° across the transformer. EXAMPLE 4.3 The main substation at a mine contains a delta- delta connectedtransformer bank composedof three identical single-phasetransformers. With rated volt- age applied, a 6,000-kW load at 0.8 lagging power factor is causing the transformer bank to be fully loaded.The rated primary and secondaryvoltages of each single-phasetransformer are 36 kV and 7.2 kV, respectively. 1.Find the capacity of each single-phasetrans- former in the bank. 2. What are the magnitudes of the primary and secondary currents in each singlephase transformer? 3. What are the magnitudes of the primary and secondaryline currents to and fromthe transformer bank? SOLUTION: The problem states that the trans- formerbank is fully loaded by an average power, P,. Thus, the capacity or apparent power load, S,,of the bank is available from The capacity of each single-phasetransformer,%,is one-third the total bank capacity, or If the transformer is assumed to be ideal, current and voltage in the primary or secondaryare related to apparent power by Hence, the primary and secondary currents in each single-phasetransformer are It can be noted that these currents also correspond to the transformer turns ratio, a, which is 115or 0.2. Because the transformer bank is delta-delta con- nected, the currents in each transformer are alsothe phase currents in the bank. Therefore, the line currents to and from the bank are, respectively, 1 and I,, = &69) = 120 A, I BALANCED THREE-PHASE CIRCUIT ANALYSIS By definition, any element in one phase of a balanced (or symmetrical) system is duplicated in the other two phases. In other words, currents and voltagesforthe other phases are equal in magnitude but displaced symmetri- cally in phase position. Therefore, the analysis of voltage, current, impedance, and power in one phase can provide complete knowledge about the entire three-phase system. In addition, reactions between phases, such as phase currents, line-to-line voLtsges, or line-to-line connected - .. - - - impedances, may be represented by an equivalent line or line-to-neutral value by using delta-wye transformations. The solution technique is called per-phase or single-phase analysis. The technique has wide application because almost all three-phase power systems that are operating normally are approximatelybalanced. As a simple demonstration of the concept, consider figure 4.13A, which illustrates a wye generator connected through line resistance to a wye-connectedload. In figure 4.13B, one phase of this circuit is extracted, and here V is one leg of the wye-connectedsource, R is the line resistance per phase, Z is a "single-leg" impedance of the wye-connected load, and The unconnected points, n and n', are the neutrals of the source and load, respectively. For the balanced system, the vectorial sum of all three line currents is zero. Hence, the current between n and n' is zero, and the potential at n equals that at n'. Accord- ingly, the two neutrals can be joined as shown in figure 4.13C. This last diagram is the single-phase equivalent
- 109. circuit, single-phase diagram, or per-phase reduction of figure 4.13A. It should be noted that figure 4.136is indeed a basic power circuit, siniilar to figure 4.1. Reduction of circuits containing delta-connected sources and loads is almost as easy, but one additional step is involved: the application of delta-wye transformation. Figure 4.14 demonstrates a simple example. Here, all sources and loads must be wye connected. The aim is to convert delta connections to wye using equation 4.9, and line-to-linevoltages and phase currents to line-to-neutral and line, respectively. Thus, figure 4.14R is the per-phase equivalent of figure 4.14A. The simplified representation of the balanced three- phase circuit can now be analyzed, employing all the single-phase techniques previously discussed. When the solution is found, the three-phase parameters can be determined by reversing the reduction. This need only be performed when line-to-line or phase values are required; no changes are necessary with line-to-neutral and line parameters, as can be seen in figure 4.13. EXAMPLE 4.4 A load has a balanced delta-connected imped- ance of 5 5 145O per leg. This load is connected through three balanced line impedances of 1 + jl 0 to a three-phase source that has a line-to-line voltage of 500 V. What is the magnitude of line current deliv- ered to the load? SOLUTION. This problem is basically the same as that for the circuit in figure 4.14, except here a line impedance exists between the source and the delta- connected load. As a per-phase solution is called for, the delta load must be transformed to an equivalent wye: i The per-phase equivalent impedance as seen by the source is simply the sum of the line impedance and the equivalent wye impedance o f the load, or z,, = z, + z , = 1 + j l + 1.18 + j1.18 = 2.18 + j2.18 = 3.08 145O Q. - The magnitude of this impedance divided into the line-to-neutral voltage across any one phase yields the answer. I Ivon/=Ivbnl=Ivcnl =Iv/ R , = Rb = R , = R z,=Zb=ZC=Z Figure 4.13.-Per-phase reductlon of wye-to-wyesystem. Figure 4.14.-Per-phase reductionof delta-tadelta system. EXAMPLE 4.5 I A three-phase 200-hp induction motor has a full-load efficiency of 90%, power factor of 0.85 lagging, and a rated terminal voltage of 950 V line- to-line. Find an equivalent deltaconnected imped- ance for the motor when it is operating at full load under rated voltage. SOLUTION. Perhaps the best way to start this solution is to find the per-phase average power consumed by the motor under the stated conditions. The total average power input to the motor can be calculated from where hp is the motor horsepower and q is its efficiency. Thus, As single-phase analysis is desirable, the power consumed by each element of the equivalent wye- connected load is needed:
- 110. Equation 4.12 can now be used to find the line current to the motor, or The line-to-neutral voltage divided by this line cur- rent is the magnitude of each leg of the equivalent wye-connected impedance for the motor, and the impedance angle is identical to the power-factor angle. Therefore, as the equivalent delta-connected impedance is re- quested, Z , = 3 2 , = 13.9131.8O -L ' EXAMPLE 4.6 A production shovel, operating at full load, uses 1,200 kW at 0.9 lagging power factor with 3,750V line-to-line at the machine. The shovel is supplied through a trailing cable that has an impedance of 0.04 + j0.030 per phase. If the voltage at the source side of the trailing cable is maintained constant, what is the voltage regulation at the machine? SOLUTION. The per-phase equivalent circuit for this problem is again similar to figure 4.148. The equivalent impedance of the shovel is not necessary, but the line currents and line-to-neutral voltage conditions for full load and no load are. Those for full load are 3,750 V , , = 7= 2,165 V line-to-neutral, PP - I,, = - 400'000 = 205.3 A. pL os8 (2,165X0.9) The line-to-neutral voltage of the constant source, VNL,can be found by computing the voltage drop across the trailing cable, V,, then adding it to the voltage at the machine. The full-load voltage at the machine can be assigned a zero phase angle. Thus, based on the given power factor, - V,, = 2,165/0° -V, The voltage drop across the trailing cable is The voltage at the constant source is - - . v,, = v,, + V,, = 10 + j2 + 2,165 The subscript for this voltage is used to signify that for these conditions it is the no-load voltage at the machine. In other words, under no load, line current through and the voltage drop across the trailing cable will be zero, and the voltage at the machine will be the same as at the source voltage. Conse- quently, the voltage regulation is The so1utio:l may be difEcult when there are delta- wye or wye-delta transformers in the system because of the 30° phase shift of voltage and current between the wind- ings. In other words, a iine-to-neutraltransformer second- ary voltage transfers to a line-to-line primary value and vice versa. Obviously, when the three-phase system is not balanced, the per-phase reduction method cannot be ap- plied, and other more specialized techniques are required. These are discussed at the end of this chapter. One-Line and Three-line Diagrams It is now apparent that practically all three-phase circuits consist of three conductors, three transformers, and so forth. When all these components are shown in a schematic, the drawing is called a three-phase diagram, as given in figure 4.15. S-ach diagrams can be extremely helpful, especially when the circuits are concentrated in a piece of machinery or power equipment, because they allow a complete view of component interconnections. They are imperative in manufacturing or troubleshooting. However, when the circuits are large, as in a complete mine power system, three-line diab~amsare not only
- 111. cumbersome to draw but also exceedingly difficult to read and interpret. Furthermore, if the power system is nor- mally balanced, a three-line diagram is unnecessary since the system is always solved as a single-phase circuit composed of one line conductor and a neutral return. In these cases, the three-line diagram is replaced by a one- line diagram in which the interconnectionsor conductors between components are represented by single lines plus conventional symbols. This is a further simplification of the per-phase diagram because the completed circuit through the neutral is omitted. One-line diagrams are an invaluable tool in analysis, in designing new power sys- tems, or in modernizing existing ones. Furthermore, since circuit diagrams of coal mine power systems are required by Federal law (30 CFR 75, 77), a one-linediagram is the most practical way to comply. Figure 4.16 shows a one-line diagram designed to convey in concise form the significant information about the syatem shown in figure 4.15. In such diagrams it is usually implied that all information is per-phase, unless stated otherwise. Hence it is vital to remember that each device shown is actually installed in triplicate. The con- ventional notations are line or line-to-neutral impedance, line current, and line-to-neutral voltage. If line-to-line values arelisted, they shouldbe stated as such. Every line, symbol, figure, and letter has a definite meaning. Conse- quently, when a one-line diagram is constructed, specifk conventions (1-2)' must be followed to ensure that the result willbe complete,accurate, and correctly interpreted by anyone. The following guidelines are recommended. Relative geographic relationships for the power- system components should be maintained as far as prac- tical. The typical mine map provides an excellent layout guide for mining applications. Because of the shorthand form and definite meaning of every entry, duplication must be avoided. Standard numbers and symbols are mandatory, and those commonly used in mining are shown in figures 4.17 and 4.18 and tables 4.1 and 4.2. Many of the devices listed have not yet been covered but are included here for completeness. All known facts about the power system should be shown on the diagram, including Apparatus ratings (volts, amperes, power, and so on), Ratiosandtapsof current and potential transformers, Power-transformer winding connections, Relay functions, and Size and type of conductors. The amount of information shown depends on the purpose of the diagram. For instance, if the diagram is to assist in studying the power flow to loads throughout the system (a load-flowstudy), the location of circuit breakers and relays is unimportant. However, for the solution of other power problems, complete knowledge of these de- vices can be mandatory. It is important that the one-line diagram contain only known facts; implications and guesses can lead to disastrous results. On many one-line diagrams, knowledge of future electrical plans is very helpful, and this information can be entered either diagrammatically or through explana- tory notes. Finally, the diagram should include correct title data so that the installation is clearly identified and cannot be confused with another or portion thereof. Y -Y Transformer Figure 4.15.-Three-line diagram. referred to high side - If machine, could be circle symbol Italicized numbem in parenthesesrefer to items in the listof references Figure 4.16.-One-line dlagram of circuit shown in figure at the end of this chapter. 4.15.
- 112. Figure 4.17.-Commonly used symbols for one4ine electrical diagrams. Air circuit breaker, removable type DH i Magnetic starter //- D~sconnecting switch, nondrawout - D Pothead ---i1111- Battery Reactor, magnetic core Circuit breaker, nondrawout type (oil or vacuum) +!+ Current-limiting breaker, drawout type 4 + Disconnecting switch, drawout type -11111 Ground Cable terminations +[- Power transformer Air circuit breaker, drawout type 3 - Disconnecting fuse, nondrawout + Current transformer -+ilm Surge arrestor * - Rectifier bank Jb 3-phase power transformer connected delta- wye Air circuit breaker, type, series trip -an-%- Drawout fuse + t Potential transformer + I IIII Surge capacitor Reactor, nonmagneticcore A Delta
- 113. Figure 4.17.-Commonly used symbols for one-llne electrical diagrams-Con. 88 +v!+ Overvoltage w Undercurrent ---+ Directional overcurrent T Pilot wire (differential current + b Directional distance Synchro check - Overfrequency tbL< Undervoltage 4 + + b Differential current 1 1 1 1 w Directional ground overcurrent = Pilot wire (directional comparison) 8l~ll+b Directional ground distance +tf, Auto synchronizing * Overtemperature - Overcurrent # l l l l 4 + b Differential ground current __aBL_) Directional power 4 + + b Distance e Carrier directional comparison (phase and control) eb Phase bolance Note: . I I I / ~ - ~ Ground overcurrent - Balanced current GP Gas pressure . 1 l l H + * Ground distance Carrier phase comparison -3-a-b Phase rotation For directional relays, arrow points toward fault that will cause tripping
- 114. Figure4.18.-Symbols for relay functions. - - - Wye, neutral ground + - - - Zig-zag ground @%,, Current transformer with ammeter; letter indicates instrument type PT 27 Relay G Relays connected to CT's and PT'S. Number indicates relay type function > Induction motor or general source t- 4 Future breaker position. Removable type R E L A Y F U N C T I O N S 1 - - 1 f T Ground Overcurrent Differential -JWV-- - E l - - Resistor - Dummy circuit beaker. Removable type * m Synchronous motor , - E l - Instrument transfer switch. Letter indicates type
- 115. Table 4.1 .-IEEE device8 numbers and functions (1) Device Function II Master element. Time-delay starting or closing relay. Checking or interlocking reiay. Master contactor. Stoppingdevice. Starting circuit breaker. Anode circuit breaker. Control power-disconnecting device. Reversingdevice. Unit sequence switch. ReSe~ed for future application. Overspeeddevice. Synchronous-speeddevice. Underspeeddevice. Speed, or frequency matchingdevice. Resewedfor future application. Shunting or discharge switch. Accelerating or. Starting to running transition contactor. Electricallyoperated valve. Distance relay. Equalizer circuit breaker. Temperature control device. Resewedfor future application. Synchronizingor synchronism check. Apparatus thermal device. Undervoltage relay. Resewedfor future application. Isolating contactor. Annunciator relay. Separate excitation device. Directional power relay. Position swttch. Motoraperatedsequence switch. Brushaperatingor slipring short-circuitingdevice. Polarily device. Undercurrent or underpower relay. Bearing protectivedevice. Resewed for future application. Field relay. Field circuit breaker. Running circuit breaker. Manual transfer or selector device. Unit sequence starting relay. Resewed for future application. Reverse-phasebalancecurrent relay. Phase-sequencevoltage relay. Incompletesequence relay. Machineor transformer thermal relay. lnstantaneous overcurrent or rateof-rise relay. Table 4.2.-Device numbers and letters common to mining (2) Device Function 1.....................Control switch. 3.....................Plus interlock relay. 37................... Ground-continuitycheck undercurrent relay. 49 D ............... Diode thermal relay. 49 GR............. Grounding resistor thermal relay. 49 T................ Transformer thermal relay. 50...................Instantaneous overcurrent relay-ac. 50 G ...............Instantaneous overcurrent relay-dc-connected in ground wire. 50 N ............... Instantaneousovercurrent relay-ac-connected to neutral. 50 Z................ Instantaneouscurrent-balancerelay-ac-zero sequence. 51................... Time delay overcurrent relay-ac. 51 N ............... Timedelay overcurrent relay-ac-connected to neutral. 51 Z................ Time-delay current-balancerelay-ac-zero sequence. 52................... Circuit breaker-ac. 59 GR............. Ground protective relay-dc-unbalance relay. 72................... Circuit breaker--dc. 76................... A .................... D .................... GD.................. PF .................. v .................... V A................... VAR ................ W ................... WH................. Overcurrent relay-dc. Ammeter. Demand meter. Ground detector. Power factor. Voltmeter. Volt-ammeter. Varmeter. Wanmeter. Wanhour meter. Device Function Time overcurrent relay-ac. Circuit breaker-ac. Exciter or dc generator relay. High-speeddc circuit breaker. Power-factor relay. Field applicationrelay. Shortcircuitingor rounding device. Power rectifier misire relay. Ovewoltage relay. Voltage balance relay. Current balance relay. Time delay stopping or opening relay. Liquid or gas pressure level or flow relay. Ground protective relay. Governor. Notching or jogging device. ac directionalovercurrent relay. Blocking relay. Permissivecontml device. Electricallyoperated rheostat. Resewed for future application. Circuit breaker-dc. Load resistor contactor. Alarm relay. Positionchanging mechanism. Overcurrent relay-dc. Pulse transminer. Phaseanglemeasuring or outof-stepprotectiverelay. Reclosing relay-ac. Resewed for future application. Frequency relay. Reclosing relay-dc. Automatic selective control or transfer relay. Operating mechanism. Carrier or pilot-wirereceiver relay. Locking-out relay. Differentialprotective relay. Auxiliary motor or motor generator, Line switch. Regulating device. Voltage directional relay. Voltage and power directional relay. Field-changingcontactor. Tripping or tripfree relay. Resewed for future application. Do. DO. DO. D O . The remainder of the text will employ one-line and per-phasediagrams almost exclusively.The main thing to remember is that practically every item represents three identical or corresponding items in the actual system. Even when the normally balanced systembecomes unbal- anced through component failures, the same diagram is used, the only change being the notation for the specific failure. Circuits Containing Transformers As previously stated, solving a balanced three-phase system problem by per-phaseanalysis is as simple as the single-phase techniques covered in chapters 2 and 3. However, the solution is not so clear when delta-wye or wye-deltatransformers are involved.Perhaps the simplest way to demonstrate the approach is first to illustrate a problem solution where the per-phase reduction is of a straightforward wye-wye transformer, then change to a delta-deltaor delta-wyetransformer and show the compli- cations that might arise.
- 116. EXAMPLE 4.7 Consider figure 4.19, which shows a one-line diagram of a substation supplying power through about 1 mile of overhead line (power conductors on poles) to a three-phase wye-wye transformer bank, then through a trailing cable to a three-phase induc- tion motor. The motor consumes an average three- phase power of 150 kW,operating at 0.8 lagging power factor. The problem is to find the rms voltage needed at the substation output to provide the rated motor terminal voltage of 480 V line-to-line. A three-phase diagram of the circuit is shown in figure 4.20 for reference. The first step in the solu- tion is usually to develop a per-phase circuit as shown in figure 4.21.Although the solution could be Substation Load center - - 1.365/ 2 7 7 V 0.8 lagging pf Z=O.l t j 0 . 3 2 7 7 V Ihne-to-neutml referred to high side Figure 4.19.-One-line diagram for example 4.7. ( t ) z1 secondary os a source VLL=480V z1 ZOL=Z1=0.6+j0.6 Z , , = Z2=0.07+j0.05 -=2 Y-Y Transformer Figure 4.20.-Three-phase diagram ol figure 4.19. Figure 4.21.-Per-phase dlagram of figure 4.19. performed directly from the one-line diagram, the per-phase diagram allows direct application of single-phase techniques. The following should be noted in figure 4.21. The line and trailing-cable conductor imped- ances are now illustrated as circuit elements. Only one phase of the transformer bank is shown, represented as an impedance in series with the primary of an ideal transformer. The trans- former turns ratio is computed from where these rated voltages are line-to-neutral rms. The induction mot,or is represented by its single-phase equivalent, P,,, where The solution can now follow a stepwise process. 1. Assume the motor terminal volta e, V,, is the rated 277-V rms line-to-neutral (4801d). 2. Compute motor line-current magnitude, I,, using P, = V, I, (load pfl or I, = Pp - 50'000 = 225.5 A. VLOoadpfl - (277X0.8) If the motor terminal voltage is taken as the refer- ence phasor, this current has a phase angle deter- mined by the load power factor. Therefore the motor current phasor is 3. I, is then employed to find the voltage drop across the per-phase equivalent of the trailing cable, Z,,. When this is added to the motor terminal voltage, the voltage at the ideal transformer second- ary, V , , is 4. For the ideal transformer with a turns ratio, a, of 115, the voltage across the primary is - - v, v,, = - a
- 117. 1 with the primary current being Notice that there is no change in current or voltage phase angle across the transformer. 5. This primary current can now be used to find the voltage drop resulting from transformer and overhead-line impedances. Summing this voltage dmp with the transformer primary voltage givesthe desired answer to the problem, the substation out- put voltage, V,: I where Z,,, Z, = overhead-line and transformer impedances, respectively. Then, Because the analysis is per phase, the result is obviously line-to-neutral voltage. If line-to-lineval- ues are required, the above answer need only be multipled by &.It is interesting that in this exam- ple the phase angle of the substation voltage is practically the same as that at the motor. This may not be the case in actual mine power systems. When the transformer is deltadelta conne&ed, problem solutions are practically the same as in example 4.7. The one-line diagram of figure 4.22 provides an illustration. When the systemisrepresentedper phase(fig.4.23),the only additional concern is delta-wyetransformation of the trans- former impedance; then the solution proceeds as before. However,the processmay not be assimplewhendelta-wyeor wyedelta transformer connectionsare involved. Consider the one-line diagram in figure 4.24 that shows a delta-wye transformer supplying the same motor as that shown in figure 4.19.Figure 4.25 illustrates one leg of the three-phase transformer. From this, it can be seen that secondary line currents appear as phase cur- rents on the primary, and line-to-neutral secondary volt- ages become line-to-lineprimary voltages. In other words (fig. 4.25), for primary voltage in terms of the secondary (the ideal transformer with turns ratio, a, only): - where V , , = line-to-lineprimary voltage, V, and Van = line-to-neutral secondary voltage, V, and for primary and secondary current, Z=O.6+jO.6 1 ,--(:) Substation ~2][Li;=0.07+j0.05- Y 1 5 0 kW total at B a n k o f 3-1 9 XFMRS, logging pf each 2,400/480~ 480V I~ne-to-line Z =0.3 +10.9 referred to high side Figure4.22.-One-line diagramwith deltadelta transformer. Figure 4.23.-Per-phase diagram of figure 4.22. Lwd Substation center Trailing cable Z=0.6t j0.6 Z=0.07+j0.5 Y J L - 50kW at 1,385 / 2 7 7 V 0.8 lagging pf Z=O.l+iO.3 2 7 7 V line-to-neutral referred tdhigh skle Figure 4.24.-One-line diagram with delta-wyetransformer. d Primary ~ecwdar; Figure 4.25.-One leg of three-phase transformer from figure 4.24. However, when performing balanced three-phase analysis, the parameters of interest are line-bneutral voltages and line currents. Thus, to continue the analyaia in a fashion similar 5 that q e d in example 4.7 (the wye-wye transformer), V , and I , must be converted to their respective per-phase equivalents. Recalling that and applying this concept to equation 4.18, the primary line-to-neutral voltage, V , , , is where fAQ = primary phase current, A, and I, = secondary line current, A.
- 118. Employing equation 4.7b to convert equation 4.19, pri- mary line current, I,, is - - I, = a I,. (4.22) Equations 4.21 and 4.22 simply state that the phase shifis that occur across delta-wyeor wye-delta-connectedtrans- formers do not interfere with the analysis when this is performed per phase. Analysis can be enhanced by chang- ing the delta primary or secondary to an equivalent wye connection, thus enabling the construction of a per-phase diagram for the entire system. Concerningthe actual per-phaseanalysis, it has been shown that the three-phase circuit is reduced by a process no more difficult than the single-phase work covered in chapters 2 and 3. The next sectionwill present a technique that further simplifiespower-system analysis. PER-UNIT SYSTEM Problemsrelated to electricalcircuits shouldbe solved in terms of volts, amperes, voltamperes and ohms. The answersto mine power problems,and indeedany electrical problem, almost always require these terms, but in the pmcess of computations it is often more convenient to express these quantities in percent or per-unit (pu), re- ferred to some arbitrarily chosen base. For example, if a base voltage of 100kV is selected, voltages of 90,120, and 125 kV have percent representations of 90%, 120%,and 125%,or per-unit values of 0.9, 1.2, and 1.26 pu, respec- tively. Both percent and per-unit values express a ratio of a specific quantity to the base quantity. Per-unit is given as a decimal,whereas the ratio in percent is 100times the per-unit value. These expressions, especially per-unit, are becoming standard for equipment specifications. There is a definite advantage in using per-unit nota- tion wer percent. Per-unit multiplication or division yields a result in per-unit. However, the product of two percent quantities must be divided by 100 to obtain a percent an8Wer. For example,if two quantities are both 0.9 pu or 9096, then and (90%)(90%)# 8,10096 but Consequently, per-unit notation will be employed almost exclusively, the only exception being where conventions dictate otherwise. Voltage, current, voltamperes, and impedance are obviously interrelated forany specificcircuit or system.As a result, the selection of any two determines the base values for the remaining twa For example, if the current and voltage bases are specific, the base impedance and base voltamperes can be found. Since three-phase circuits are usually solved as a single line with a neutral, base quantities in the per-unit system are line current, line- to-neutral voltage, per-phase impedance, and per-phase voltamperes. The mathematical interrelationsof the bases are as follows: where Vb = base line-to-neutral voltage, V, kVb = base line-to-neutral voltage, kV, kVAb = base per-phasevoltamperes, kVA, MVAb = base per-phasewltamperes, MVA, I, = base line current, A, and Z ,= base per-phase impedance, 0. All these formulas are adaptations of the fundamental Ohm's law and power material; the last three are ex- pressed in kilovolts and kilovoltamperes because of the levels normally found in power systems. It should be remembered that line-to-line voltages and total power Qilovoltamperes or megavoltamperes) are customarily specified; these mu@ be changed to line-to-neutral volt- ages (dividingby J3) and per-phasepower (dividing by 3) before equations 4.23 through 4.26 can be applied. To apply the per-unit system to power problems, base values for kilwoltamperes and kilovolts are normally selectedfirst in order to minimize calculationsas much as possible. Base values for impedance and current are then found.Next, all the actual voltages, currents, impedances, and powers in the power system or system segment are expressed as a ratio to the base quantities; these are the per-unit quantities. Problems are then solved in per-unit, with the answers converted back to actual parameters. The actual values and per-unit quantities are related by where Z , , I,, V , , V A , = actual values of impedance, current, voltage, and power, re- spectively, 0, A, V, VA, and Z , , , $,, V , , V h U= per-unit values of impedance, current, voltage, and power, re- spectively, pun, PUA, PUV, puVA. It is important to note that all impedances in a problem are referenced to the same base im~edance. whether thw are pure resistance or pure reactan'e. The kame holds f& all average, reactive, or apparent powers, which are refer- enced to the base kilovoltamperes.
- 119. Often, per-unit impedancesor percent impedancesof a system component have already been assigned to a base referenced to the component or power-system segment in which that componentis located. These impedancescan be changed to another base impedance by where Z , , = per-unit impedance of specified component, pun, kV,, kVA, = base kV and kVA used to reference Z,,, V, VA. kV,, KVA, = base kV and kVA to which new per-unit impedance is to be referenced, V, VA, and Z,, = new per-unit impedance referenced to kV, and kVAb,pu 0. Transformer Impedance 'hansformers are the most common devices in power systems where the component impedance is referenced to the rated power and voltage of the component.Convention- ally, percent impedance is specified, but this can be converted to per-unit simply by dividing by 100. A major advantage of using per-unit computations is seen when circuits are connected through transformers. Throughthe proper selectionof voltage bases,the per-unit impedance of the transformer is the same no matter which winding is used. Consequently, if exciting and magnetiz- ing currents are ignored, as they often can be in power systems, the transformers become a simple series imped- ance in per-unit calculations. In other words, the ideal transformer is not needed in the equivalent circuit. Exam- ple 4.8 explores this advantage. EXAMPLE 4.8 Consider a 750-kVA power-center transformer, the approximate per-phase equivalent circuit for which is shown in figure 4.26.The per-phaseratings are 250 kVA, 5,000/1,000 V, and 5-0 reactance re- ferred to the high side. Following convention, the base power kVAb, is 250 kVA, and the base voltage for the high side kVbl is 5 kV. One kilovolt is selected as the low-side base voltage, kVb,. With these, the high-side base quantities can be calcu- lated using equations 4.23 through 4.26: kVAb,= 250 kVA, kVbl = 5 kV, Ibl = 50 A, and Zbl = 100 0. The per-unit impedance of the transformer is thus Now consider the actual transformer impedance as it would appear referred to the low side, as in figure 4.27. The base quantities on the low side are I kVAb,= 250 kVA, kVb2 = 1 kV, Ib2 = 250 A, and G2 = 4n. Notice that the base power does not change and the low-side base voltage defines base current and im- pedance. The per-unit transformer impedance as seen from the low side is Therefore, the per-unit impedance of the trans- former is the same,regardless of the side it is viewed from, and the per-unit equivalent circuit is simply the series circuit shown in figure 4.28. Here, the input and output voltages are now expressed in per-unit since the transformer is operating at rated voltage. Primary Secondary Figure 4.26.-Approximate per-phase equivalent clrcuil for 750-kVA load-center transformer; impedance referred to high side. Figure 4.27.-Transformer of figure 4.26 with impedance referred to low side. Figure 4.28.-Simplified equivalent circuit of transformer expressed in per-unit.
- 120. Three-Winding Transformers In chapter 3 and to this point in chapter 4, equivalent circuits have been shown only for two-windingtransform- ers, those having only one primary and one secondary winding. However, many transformers in mine power systems have three windings, with the third winding termed the tertiary or second secondary. These include power-centertransformers supplyingtwo different utiliza- tion voltages, such as 950 and 550 Vac to face equipment or 550 and 250 Vdc to machinery. The latter case not only uses a three-winding transformer but also three-phase rectification, which will be described in chapter 5. Both the primary and secondary windings of the two-winding transformer have the same kilovoltampere capacity or rating, but the three windings of a three- winding transformer may have different kilovoltampere ratings. The impedance of each winding may be given in percent or per-unit, based on each winding rating. The three impedances can also be measured by the following short-circuit test, where rated voltage is applied to the primary for Z , , and Z , , and to the secondary for Z,,: Z , , = leakage impedance measured in primary (or first winding),with secondary (or second wind- ing) short-circuitedand tertiary (or third wind- ing) open, Q, Z , , = leakage impedance measured in primary with tertiary short-circuitedand secondary open, Q, Z,, = leakage impedance measured in secondary with tertiary short-circuitedand primary open, 0. The impedances of the primary, secondary, and tertiary windings are found from z, = ; ( z,, + z, - Z,), 1 where Z,, Z,, Z, = impedancesof primary, secondary, and tertiary, respectively,Q. In equations 4.32 and 4.33, all impedances (Z,,, Z,,, Z,) must be referred to the primary winding voltage. If Z , is obtained from the described measurements, the imped- ance is referred to the secondary-windingvoltage, hence it must be transferred. The approximate per-phase equivalent circuit for a three-winding transformer with the winding impedances of Z , , ,Z , ,and Z, is providedin figure 4.29. Magnetizingand excitingcurrents are ignored. The terminals p, s, and t are the primary, secondary, and tertiary connections; the common point is unrelated to the system neutral. The three impedances must be in the per-unit system, as was the case for the equivalent circuit in figure 4.28. Hence they must have the same kilovoltampere base. Further, voltage bases for the circuits connected through the trans- former must have the same ratios as the turns ratio of the transformer windings; that is, primary to secondary, pri- mary to tertiary, which are actually the same asthe ratios of the related winding voltages. Per-Unit Method in System Analysis As mentioned earlier, the use of per-unit equivalents in the analysis of power-system problems can greatly simplify the work involved, especially when the system contains transformers and different voltage levels. How- ever, as per-unit calculations require the change of famil- iar parameters (ohms, volts, amperes, and so on) into values representing a ratio, this advantage is often difi- cult to comprehend. Example 4.9 will illustrate the per- unit method of analysis using the one-line diagram pro- vided in figure 4.30, which could represent a mine power system in the early stagesof development. All power levels listed are given per-phase; those shown for the mining equipment represent consumption. The voltages are all line-to-neutral. I EXAMPLE 4.9 I The informationrequired could be the voltageor current level at any point. Regardless, solution by the per-unit method first requires translation of the impedance of all componentsto the same base. The base selection is arbitrary, but for convenience, the largest kilovoltampere capacity of a system compo- nent is usually taken. In this case a good base would 1-linesymbol Equivalent circuil Figure 4.29.-Approximate equivalent circuit of three. winding transformer expressed in per-unit. Trailing cable Miner Load 2~0.03 +jO.O1 P=57kW Substation -Feeder cable- center d y I(2-47 kvar V=320V Z=0.13+j0.06 Trailing coMe 1,000 kVA 150kVA z.o.1 + jo.02 Shuttle 40/7.2 kV Z2/350kV car 2.7% 2~4.5% <BUS P=4kW Q=5 kvar Figure 4.30.-One-line diagramof smallminepower system.
- 121. be 1,000 kVA, correspondingto the per-phasecapac- ity of the substation. But two base parameters are needed in order to define the four base quantities; as the nominal voltage for each voltage level can be or can approach a constant, the system voltages are an excellent choice for the second base parameter. For figure 4.30, these would be the line-to-neutral volt- ages of 40 kV at the utility, 7.2 kV at mine distri- bution, and 350 V at mine utilization. Note that the 1 system voltages are usually given as line-to-linein one-line diagrams, so they must be changed to line-to-neutral values to employ the formulas given here. In any event, base voltages must correspond with the turns ratio of any interconnecting trans- former. The ones selected do. With base kilovoltamperes and base kilovolts specified, the base quantities can be calculated using equations 4.23 through 4.26. 1.For the utility: 2. For mine distribution: 3. For mine utilization: The per-unit representations for all components of the mine system can now be found, and the needed formulas are equations 4.27 through 4.31. 1. For the substation: percent reactance is 7% or 0.07 pu based on the transformer rated kilovoltam- peres, referred to the high side, 40 kV. Z , , kVAb kV, , zpu - kVA. kv) where Z P , , = j0.07 pu, thus, Z , , = j0.07 pu. 2. For the feeder cable: actual impedance is given, ZA 0.13 + j0.06 and z = - = PU Zb2 52 = (0.0025 + j0.0012) pu. 3. For the load center: percent reactance is 4.5% or 0.045 pu based on the transformer rated kilovolt- amperes, referred to the high side, 7.20 kV. where Z , , , = j0.045 pu, kVAb = 1,000 kVA, and (jO.045X1,OOO) . zpu= 150 = 30.3 pu. 4. For the trailing cables: actual impedancesare again given. Miner. ZA = 0.03 + j0.01 Q, = (0.25 + j0.083) pu. Shuttle car. ZA = 0.1 + j0.02 n,
- 122. 6. For the machines: consumption is given in terms of average and reactive power. Miner. P = 57 kW, Q = 45 kvar kVAA (57 + j45) thus, kVA,, = -s kVA, 1,000 = (0.057 + j0.045) pu. Shuttle car. kVAA = (4 + j5) kVA, hence, kV4, = (0.004 + j0.005) pu. At this point,the entire systemoffigure 4.30 may be redrawn into the impedance diagram in figure 4.31. Figure 4.31, when compared with a per-phase diagram in the impedance domain such as figure 4.21, illustrates the advantage that simplified per- unit computations lend to power-system analysis. The circuit shown is merely a series-parallel ar- rangement of basic electrical elements, and obvi- ously it may be further simplifiedif desired, say into an equivalent per-unit impedance. This is only one example; an actual appreciation of power-system analysis by per-unit techniques can come only through experience. The impedance diagram can be used for the solution of most power problems. Suppose currents under normal operation are desired. One method is to apply known voltages at system points and calcu- late the resulting currents and voltages. For in- stance, suppose the line-to-neutral at the miner is 320 V (about 555 V line-to-line).The per-unit equiv- alent of this is - kV,=----k V ~ 0.32 - 0.91 pu. kVb, 0.35 The line current through the miner trailing cable is then $u = (0'057 - J0.045) = 0.063 - j0.049 pu. 0.91 The conjugateof power is employed because voltage is implied as the reference phasor following the conventions stated earlier. The process is then con- tinued through the entire system.When the desired per-unit values are obtained, they are simply con- verted to actual values. Considering the current in the miner trailing cable, the actual line current is (0.0025~j0.0012)(0.833+ j0.167)p~ - - . - - Shuttle car kVA *(0.004+ j0.005)p u Figure 4.31.-Impedance diagram of system in figure 4.30, expressed in per-unit on a 1,000-kVA base. UNBALANCED THREE-PHASE CIRCUITS The solution of balanced three-phase circuit problems is usually accomplishedby converting the constants, cur- rents, and voltages to per-phase values. Because symme- try determines the magnitude and phase position of all currents and voltages, actions occurring between phases can be represented by equivalent impedances. Further- more, currents and voltages for the other phases are equal in magnitude to those in the per-phase solution but are simply displaced symmetrically in phase position. This is extremely important because normally operating three- phase power systems can usually be approximated as balanced.However,the solutionof unbalancedthree-phase circuits or balanced circuits with unbalanced loads does not permit the same simplification. Mine power systemsare designedto have a high degree of reliability and therefore to operate in a balanced mode. But at times,equipmentfailuresand unintentionalor inten- tional disturbances from outside sources can result in an unbalanced operation. The most common sites for mine power-systemunbalance are equipmenttrailing cables. The consequence of unbalance is abnormal currents and volt- ages, and if safeguards are not designed into the system to protect against these anomalies, the safety of personnel as well as equipment can be compromised. The protective circuitrywithin the mine power system serves as the safety valve for such hamdous -- -- malfunndtns. Power-system unbalance can occur either from open circuits or from faults. A fault occurs whenever electricity strays from its proper path. Faults can be visualized as the connecting together of two or more conductors that nor- mally operate with a voltage between them. The connec- tion that creates a fault can be from physical contact or an arc caused by current flow through a gaseous medium. A short circuit is one type of fault. Currents in the power system resulting from an open circuit or a fault can be exceedingly large. An overload is not a fault. The term overload merely implies that currents exceed those for which the power system was designed. Such currents are usually much smaller than fault currents. Nevertheless, overloads can create equipment failures by exceedingthe thermal design limits of the system. If not corrected, the overload can result in a hazard to personnel. However, such problems occur only with unattended overload operation for an extendedtime period, whereas faults can create an unsafe condition almost instantly. Both circuit breakers and fuses are used to protect circuits from excessive current flow, be it a result of faulting or overloading.The circuit-interrupting operation consists of parting a pair of contacts, and since an arc is
- 123. drawn between the contacts. the Drocess must also extin- guish the arc. The interruption is handledmechanically in the circuit breaker, and the excessivecurrent is monitored electrically or thermally. Fuse operation is based on sim- ple thermal operation.The fusible element is responsive to the heat of an overload or fault current and melts open. The fuse jacket is employed to extinguish the subsequent arc. A complete discussion of interrupting devices and the associated protective circuitry is presented in chapter 9. Fault Types The fault type often seen in literature is called a bolted fault, which can be described as a zero-resistance short circuit between two or more conductors. In reality most faults are not dead shorts but have somefinite value of resistance. Faults may be classed as permanent or temporary. A permanent fault is one where equipment operation is impossible and repairs are mandatory. A temporary fault is intermittent in nature. For instance, two closely spaced overhead conductors may cause trouble only on windy days, when they can be forced into contact or close prox- imity by the wind. A very sinister fault type is the arcing fault. This condition is now believed to be the most common fault. When two conductors of different potential are in very close proximitv. the intervening mace between them can - . - - be consideredas a spark gap.If the two conductorsare part of an ac vower svstem, the insulating material between the conductors &ay break down wgen the sinusoidal waveform reaches a certain value. Fault current will then flow. The potential drop across the conducting gap, which is actually an impedance, remains at a nearly constant level. It is this energy source, releasing terrific quantities of heat, that causes the devastation that is typical of an arcing fault. Soon after the sinusoid reverses polarity, the arc quenches until the spark-gap breakdown voltage or restrike potential is reattained. This repetitive arcing is almost always self-sustaining at ac voltage levels of 480V and above. Depending on how the fault occurs, it may be de- scribed as three-phase, line-to-line,or line-to-neutral. In mining, the cable shields and the grounding system of the equipment are at the same potential as the system neu- tral, and line-to-neutral faults are the most prevalent. Line-to-line faults and line-to-neutral faults are unbal- anced or unsymmetrical, but the three-phase fault is balanced or symmetrical. These three basic fault descrip- tions are illustrated in figure 4.32.The impedance, al- though very small, is shown to signify its finite value. -- --- -- - -- - - -- Line to neutrol Line to l~ne 3 phase Flgure 4.32.-Basic fault descriptions. Fault Analysis Fault analysis is a desirableand offen mandatory part of any mine power-system analysis. As faulting of some nature can occur at any time, knowledge of how faults affect currents and voltages is necessary to design proper protection and to ensure personnel protection. Although faults usually occur in mine-system trailing cables, the actual fault location and time of occurrence is unpredict- able. Consequently,fault analysis is frequently effectedon a trial-and-errorbasis, searching for a worst case solution. It is necessary to assume a fault location, the configura- tion of power-system components prior to the fault, and sometimes the system loads. Such an effort can result in numerous calculations,to the point where digital comput- ers can be extremely advantageous. Nevertheless, the results provide invaluable information on which to base the design of the mine power system. Though not particularly common,fault analysis using three-phase faults has distinct advantages. Using this method,balanced faults, like balanced loads,can be inves- tigated on an equivalent per-phase basis and therefore become as simple as faults on single-phaselines. In the majority of cases, bolted three-phase faults cause larger fault currents than line-to-lineor line-to-neutralevents. Unsymmetrical faults are often of high interest in mine power systems. Instances include finding a mini- mum fault current or current flowing in the system neutral conductors. When an unsymmetrical fault is placed on the system, the balance is disrupted. It is possible to solve an unbalanced power system by using a three-phase diagram with symbols assigned to the quan- tities in each phase and carrying the phase solutions simultaneously.This complicatesthe problem enormously, but it can be simplified by applying the method of sym- metrical components, which reduces the solution of such problems to a systematic form. The reduction is particu- larly applicable to balanced systems operating under unsymmetrical faults. SYMMETRICAL COMPONENTS The method of symmetrical components provides a means for determining the currents and voltages at any point of an unbalanced three-phase power system. In this method, the unsymmetrical phasors representing the un- balanced voltage or current are expressed as the sum of three symmetrical phasor sets. These phasor sets or sym- metrical components are designated as the positive ae- quence, negative sequence, and zero sequence. The first two consist of three balanced phasors with equal magni- tude, set 120° apart. The zero-sequence set has three phasors equal in magnitude but operating in the same time. The componentsare illustrated in figure4.33,where the instantaneous values may be determinedby projection upon the X-axis. The positive-, negative-, and zero- sequence components are then employed to solve the unbalanced-systemproblem. These sequencesare so com- mon in power-systemterminology that they are oftenused to describe the quality of system operation. It might be asked why the resolution of three phasors into nine phasors simplifies the solution of unbalanced power systems. The answer is straightforward.The resolu- tion results in three symmetrical systems in which each
- 124. Figure 4.33.-Positive-sequence, negative-sequence, and zero-sequence vector sets. balanced phasor set can be treated separately, just as in balanced three-phase systems. In other words, the power system can be reduced to per-phase values, then analyzed separately for each symmetrical component. This analysis hmges on the fact that currents and voltages of different sequences do not read upon each other: currents of the positive sequence produce only positive-sequence voltage drops; the same is true for the negative and zero sequences. In additionto aiding analysis,the method of syrnmetri- cal components separates electrical parameters into parts that canrepresentbetter criteria of the controllingfactorefor certain phenomena. For example, the presence o f negative- sequence current or voltage immediately implies that the system is unbalanced, and this can be utilized to detect malfunctioning power systems. Grounding phenomena are othergoodcriterionexamples;neutral current isvery closely related to zero-sequencecomponents. Sequence Components The positive sequencefor voltage is composed of three symmetrical phasors, Val, V,,, and V,, for phases a, b, and c, respectively (fig. 4.33).The quantities have equal mag- nitude but are displaced by 120° from each other. There- fore, following equation 4.2b, or rewriting in exponential form, The unit vector, ei120,is used so frequently that it is given the symbol "a" (not to be confused with the transformer turns ratio), where and , 2 = &lzo&lzo= $240 (4.36b) Thus the positive-sequence vectors (equations 4.35) are customarily written as Equations 4.35 and 4.37 also relate to the standardpractice of symmetrical-component calculations; equations are al- ways expressed in terms o f the phase a quantities. There are several mathematical properties of the unit vector a that are useful in computations: and for specific calculations: These allow easy conversion to simpler forms when sym- metrical components are being manipulated mathemati- cally. For the negative and zero sequences (fig. 4.33),the symmetrical voltage sequences can be written and VaO= VbO= VeO. (4.39) Rewriting these equations in terms of the unit vector, a, it is found that for the negative sequence, and for the zero sequence, It is important to note that in all three sequence systems, the subscriptsdenotespecificcomponentsin each phase (a, b, or c).Furthermore, the subscripts, 1,2, and 0 signify whether that component is part of the positive-, negative-,or zero-sequenceset. Using the samereasoning, symmetrical-component equations can also be written for current. Sequence-Quantity Combinations The total voltage or current of any phase is equal to the vectorial sum of the correspondingcomponentsin that
- 125. phase. Figure 4.34 illus&ates this concept for three unbal- anced voltage phasors, V,, V,, and V,. Expressed mathe- matically, Substituting in the equivalent values given by equations 4.37, 4.40 and 4.41, equations 4.42 become These equations state that an unbalanced system can be defined in terms of three balanced phasor sets. In other words, positive-,negative-, and zero-sequencecomponents of phase a can be added together to obtain the unbalanced phasors. Following convention, the equations are ex- pressed only in phase a quantities. Similarly,three unbalanced voltages or currents may be resolved into their symmetrical components. Consider the zero sequence first. By adding equations 4.43a, 4.436, and 4.43~ together, Since 1 +a2 + a = 0, If equation 4.436 is multiplied by a and equation 4.43~ by a2 and these results are added to equation 4.42a, - Therefore, Val = (V, + aVb + a2V,), (4.44b) which relates the positive-sequencecomponent of phase a to the unbalanced vectors. Finally, for the negative .se- quence, if equation 4.436 is multiplied by a2and equation 4.43~ by a, Then, 1 - V,, = ,(V, + a2Vb+ av,). (4.44~) Equations 4.44a,4.446,and 4.44~ are thereforethe reverse of equations 4.43a, 4.436, and 4.43~; they allow the sym- metrical components to be written in terms of the unbal- anced phasors. Symmetrical-Component Relationships Currents in equivalent deltaconnected and wye- connected loads or sources form a good basis to illustrate the existence of symmetrical components in three-phase circuits. Consider t h e two loads shown in figure 4.35, where I,,, I,, and I,, are the three phase currents and I,, I, and 1, are the line currents. These may all be assumed to result from an unbalanced condition. At the three terminals of the delta load, the following relationships are satisfied by Kirchhoffs current law: The zero-sequencecurrents of the wyecomected load are (using equation 4.44~): Substituting equations 4.45 into equation 4.46 it is found that This showsthat the zero-sequencecurrent of a three-phase circuit feeding into a delta connection is always zero. In addition,the currents to a three-phasewyetonnected load with a floating neutral (fig. 4.35B) can have no zero- sequence component. Simply, a neutral-return circuit must be available for zero-sequence currents to flow. However, zero-sequence current may circulate in a delta connectionwithout escaping int? a neutral conductor (see figure 4.354, note directions of I,,, I,, and L ) . Figure 4.34.-Symmetrical component addition to obtain unbalanced three-phase set. Figure 4.35.-Equivalent delta-connected (A) and wye connected (13) loads.
- 126. For transforming positive-sequence line currents to phase currents, it can be shown from which applies equation 4.44b to current, that Using a similar process for negative-sequence currents, When the foregoing is applied to line-to-neutral and line- to-line voltages for figure 4.33, the transformation equa- tions are - - - where V,,, V,,, V,, = zero-, positive-, and negative- sequence line-to-line voltages, - - - v, V,,, Val, V, = zero-, positive-, and negative- sequence line-to-neutral volt- ages, V. These equations demonstrate another general relation- ship of zero-sequencecomponents: a line-to-linevoltage, however unbalanced, can have no zero-sequence compo- nent. Line-to-neutral voltages, on the other hand, may have a zero-sequence value. Symmetrical-ComponentImpedance Before the solution of unbalanced system problems can be discussed, the concepts of impedance under the influence of symmetrical components need to be covered. Impedance relates the current in a circuit to the impressed voltage. Symmetricalcomponent impedance behaves in a similar manner, except that it is sometimes affected by additional parameters. There are three likely cases for a power system: an unbalanced static network, a balanced static network, and the balanced nonstatic network. All these impedance values are created by the fact that positive- and negative-sequence currents produce only positive- and negative-sequence voltage drops, respec- tively. The flow of zero-sequence currents in a neutral can result in an impedancethat is apparently greater than the actual impedance. In an unbalanced static network,the sequenceimped- ances in a particular phase are equal, but not necessarily equal to those in another phase: whereZ,,, Zbo,and Z,, are symmetrical-componentimped- ances for the zero sequence; Z,,, Z,,, and Z,, are symmetrical-componentimpedances for the positive se- quence; and Z,,, Z,,, and Z,, are symmetricalcomponent impedancesfor the negative sequence. The balanced nonstatic network is given by Z,, = Zb2 = Ze2. (4.53~) This statesthat in a balanced nonstatic network the imped- ances in a sequence are equal, but not necessarily equal to the other sequencecomponent impedances.Cables and pow- erlines are included in this case, and spefically, z,, = z, + z,. (4.54) The last likely case is the balanced static network, where It should be obvious that this is a situation where sym- metrical components would not normally be applied. As a general rule, positive- and negative-sequence impedances of a power system are on the same order of magnitude, but the system zero-sequenceimpedance may vary through a very wide range. This range is dependent upon the resistance-to-reactance ratio as seen by the zero-sequencecurrent. Fault Calculations One of the most significant uses for the method of symmetrical components is the computation of voltages and currents resulting from unbalanced faults. The three- phase diagram in figure 4.36 represents a simple power system with a four-wirewye-connectedsource. The imped- ance of each phase conductor is Z, while Z, is the neutral- conductor impedance. A bolted line-to-neutral fault is occurring on a phase a (an x signifies the fault). The resulting current in the fault, 4, is of interest, and the following showshow symmetrical componentscan be used to find its value. Figure 4.36.-Three-phase system with line-to-neutralfault.
- 127. It is apparent from figure 4.36 that the line currents under the fault condition are - w h e ~ - & = current in fault, A, and I,, I,, I, = unbalanced line currents, A. Applying equation 4.43, the symmetrical components of these currents are 1 - 1- fa, = 5 (I, + db+ a2rc)= 3 &, (4.57~) I,, = 5(i, + a2ib+ ai,) = $&, (4.576) - - - 1- therefore, I,, = I,, = I,, = 5 If. (4.58) To define the fault completely, it must be known whether the fault between line a and the neutral is a dead short or exhibits an impedance.Although all faults have a finite impedance,the faulting assumption states that it is bolted. Therefore,fault impedance is zero and the voltage across the fault, V,, is also zero. With this, the current through the fault, &, can be described. However,to perform the required computations, it is necessary to know the force driving the fault current and the impedance existing between this driving potential and the fault location. The source, V , is the driving potential, and it can be wumed aspurelypositive sequence.It is alsoassumedfrom figure 4.36 that the source has negligible internal imped- ance (inpractical situations,however, the source impedance is o f great importance).Therefore,the sourceline-to-neutral potentials are equal to the terminal voltages: - - where V - V , , v , = terminal line-to-neutralvoltages and V,, V,, V, = corresponding ideal-source poten- tials. The impedancesinvolved are simply the line impedanceof phase a (Z) and the neutral impedance (Z,). With these parameters known, the process is now to convert the unbalanced system to symmetrical components,solve the problem in terms of these balanced vectors, and then reconstruct the result for the fault current. Following convention, all work is performed in phase a quantities. Notwithstanding, phase a contains the line- to-neutral fault; thus, it is the only phase involved. Since only positive-sequencevoltage is supplied by the source, the symmetrical components of the driving potential are The impedance to positive-sequence or negative- sequence current in any of the three lines is equal; thus, for phase a, z,, = z, z,, = 2. Zero-sequencecurrent follows a different path from posi- tive or negative sequence. From the source to the fault, zero-sequencecurrent, I,, exists in each line, but from the fault back to the source (through the neutral conductor) the current is 31,. Zero-sequence impedance, Z,,, is there- fore greater than Z. As was implied in the preceding section,the quantification of Z , , or just simply Z,, is not an easy matter. However, in order to limit the amount of fault current flowingin mine power-systemneutrals, large resistances are placed in the neutral circuit. With this in mind, the resistance-to-reactanceratio of the neutral im- pedance, Z,, is very large, and in this instance for mine power systemsunder line-to-neutralfaults, the impedance seen by the zero-sequencecurrent can be approximated as Loop equations for each sequence current can now be expressed for figure 4.36. If a voltage is assumed to exist across the fault, for phase a, - V , , = Z, I, + 6= 0, (4.616) where Van,,V , , , v , , = sequencevoltages for source,V, I,,, I,,, I,, = sequence components of fault current, A, Vn, G,Vm = sequence components of volt- age across fault, V, and Z,,, Z,,, Z , = sequence impedance seen by fault current, fl. Equation 4.59 generalizes the fault condition and is prac- tical because of fault impedance. However, a bolted fault has been assumed to exist; thus, All input to the problem is now available, and simulta- neous solution of equations 4.57 through 4.62 shows that but Z, = Z + 3Z,, then Consequently, symmetrical components have been em- ployed to solve this unbalanced faulting problem. This work can easily be expanded to cover other unbalanced faulting problems, and the process can be employed to solve any unbalanced three-phase or even polyphase condition. Because fault analysis is imperative in protective-device sizing, additional discussion can be found in chapter 10. POWER TERMINOLOGY If the sum of the electrical ratings is made for all equipment in a power complex, the result will provide a
- 128. total connected load. The measure could be expressed in horsepower, but the electrical quantities of kilowatts, kilovoltamperes, or amperes are more suitable units. Note that the connected horsepower can be converted to con- nected kilowatt simply by multiplying by 0.746. Many . loadsoperate intermittently,especially mining production equipment, and other equipment operates at lessthan full load. Accordingly, the demand upon the power source is less than the connected load. This fact is important in the design of any mine power system,as the system should be designed for what the connectedload actually uses, rather than the total connected load. Obviously, these consider- ations have great impact on power-system investment or the capital required to build the system. Because of the importance of assessing equipment power demands, the Institute of Electrical and Electronics Engineers (IEEE)has standard definitions for load combi- nations and their ratios. The important ones follow (3). Demand is the electrical load for an entire complex or a single piece of equipment averaged over a specified time interval. The time or demandinterval is generally 15 min, 30 min, or 1.0 h, and demand is generally expressed in kilowatts, kilovoltamperes,and amperes. Peak load is the maximum load consumed or pro- duced by one piece or a group of equipment in a stated time period. It can be the maximum instantaneous load, the maximum average load, or (loosely) the maximum connected load over the time period. Maximum demand is largest demand that has occurred during a specified time period. Demand factor is the ratio of the maximum demand to the total connected load. Diversity factor is the ratio of the sum of the individual maximum demands for each system part of subdivision to the complete system maximum demand. Load factor is the ratio of the average load to the peak load, both occurring in the same designated time period. This can be implied to be also equal to the ratio of actual power consumedto total connected load in the same time period. Coincident demand is any demand that occurs si- multaneously with any other demand. All these definitions may be applied to the units of average power, apparent power, or current. Thus they are invaluable in power-system design.A few examples are in order to illustrate their versatility. Consider a feeder cable supplying several mining sections in an underground mine. The sum of the con- nected loadson the cable, multiplied by the demand factor of these loads, yields the maximum demand that the cable must carry. When applied to current, this demand would be the maximum amperage. Good demandfadors for mine power systems range from 0.8 to 0.7 depending upon the number of operation sections. The lower value is used when there are fewer producing units, that is, from two to four. The demand factor can be extended to include esti- mates of average load.For instance, the sumof the average loads on the cable, multiplied by the demand factor, provides the average load on the cable. A prime applica- tion here is for approximatingthe current that a conductor is expected to carry. If, for example, 10 identical mining sections draw 53 A each; the conductorsfeeding all these sections would be expected to carry (total average loadxdemand factor) = (averageload) The demand factor and the diversityfactor can be applied to many other mine electrical areas, such as estimating transformer capacities, protective-circuitry continuous ratings, and the load that a utility company must supply. The load factor can be used to estimate the actual loads required by equipment. Here, the total connected load multiplied by the load factor is an approximation of the actual power consumed. It should be noted that the averageloadfactor in undergroundcoal miningtends tobe rather low, mainly because of the cyclic nature of equip- ment operation but also because of the employment of high-horsepower motors that are needed to perform spe- cific functions but only operate for a small fraction of the possible running time. For instance, when cutting and loading, a continuous miner will have all motors operat- ing, thus have a total connected load of 385 hp or (0.746)385 = 287 kW.The average load factor might be 0.6; therefore, the actual power consumed is (0.61287 or 172 kW.The load factor can also be applied to equipment combinations. The maximum power demand normally forms one basis that utility companies use to determine power bills; most often, 1month is the specified time period. Demand meters are often installed at the utility company metering point. Chapter 4 has covered a broad range of fundamentals projected towards three-phase power systems in mining. Items have included balancedthree-phasecircuit analysis, the per-unit system, the method of symmetrical compo- nents, and specific terminology to describe power-system operation.Comprehensionof this material is vital in order to understand many chapters that follow. REFERENCES 1. American National Standards Institute (New York). ANSI StandardDevice Numbers C37.2. 1978. 2. . Graphical Symbols for Electrical Diagrams Y32.2. 1971) -. 3. Instituteo f Electricaland ElectronicsEngineers(NewYork). Recommended Practice for Electric Power Distribution for In- dustrial Plants. Stand. 141-1986.
- 129. CHAPTER 5.-BASIC SOLID-STATE DEVICES AND INSTRUMENTATION Through the advancement of technology, the motor- generator(m-g) sets and Ignitron rectifiersfor power conver- sion used in early mining have been all but replaced by semiconductordevices,except for m-g sets and synchronous rotary converters in specific surface mining equipment. Equipment employing semiconductors exclusively is often termed solidstateor static.In mine power systemsthe use of semiconductors has grown from simple rectification (the conversion of power to d i r e c t current (dc))to include such areas as motor and equipment control, protective relaying, and lightingpower supplies,not to mention extensiveuse in communications and instrumentation. Sincethe topics of solid-statedevices and basic instru- mentation are closely related, they are introduced to- gether in this chapter. The discussion will be primarily informative rather than theoretical. SEMICONDUCTORS Semiconductors are nonmetallic elements that are characterizedhv relativelv Door conductivitv.Siliconisthe ". most popular and germanium the second most important semiconductor in electrical or electronic applications. Semiconductors are useful in electrical circuits because they can pass current in two different conduction modes when impurities or imperfections exist in their crystal lattices. The process of carefully adding impurities to a pure or intrinsic semiconductor crystal while it is being grown is called doping. The impurities are selected for their size, so they will fit into the crystal lattice and provide either an excess or a deficiency of electrons. For example, when a few parts per million of arsenic atoms are added to germanium, or antimony atoms are addedto silicon in the crystal structure, an overabundance of free electrons is created. The result is a net negative effect,and the crystal is termed an n-typesemicondu*r. If a potential is placed acrossthe impure crystal, conduction occurs through an apparent drift of these free electrons.On the other hand, if indium or gallium is used to dope germanium or silicon, a deficiency of electrons exists, and an excess positive charge is created in the doped crystal. Thus, it is called a ptype semiconductor.If a potential is applied, the atoms conduct current by an apparent movement of electron sites or holes. These holes are places in the crystal lattice where an electron can be held tem~orarilv. When there is an abundance of holes. n-type. In the actual production, a single semiconductor crystal (or monocrystalline material)is grown SO that part is doped to create a p-type region, with the balance doped for n-type. A solid-state diode or rectifier has one p-n junction; it is a device that readily passes current in one direction but does not permit appreciable current in the opposite direction. The symbol for a diode or rectifier is given in figure 5.1A. Figure 5.1B is a simple model of a diode that can be used to explain p-n junction electrical operation. When the two semiconductor materials are joined, a charge redistribution occurs. Both the p-region and the n-region contain a high concentration of majority carriers. Elec- trons from the n-materialdiffuseacross thejunction to the p-material; similarly, holes migrate from the p-material into the n-material. The net result of this diffusion is a depletion region with negatively charged (acceptor)ions on the p-side and positively charged (donor) ions on the n-side of the junction. The electric field across the deple- tion region is established and opposes further majority- carrier diffusion, but the field creates a minority-carrier flow across the junction in the opposite direction. Current caused by majority-carrierdiffusion is called injection current, I,, and that from minority carriers, saturation current, I , . If no external voltage is applied to the p-n device (fig. 5.2A), the junction is in equilibrium because the net hole and electron flow acrossthe junction is zero. In other words, injection current equals saturation current. However, if an external voltage is applied with a polarity such that the p-region is positive with respect to the n-region (fig. 5.2B), the depletion-region electric field is decreased, and a large number of majority carriers are able to cross the junction and diffuse toward the device terminals. Hence, injection current is substantially in- creased,and because saturation current remains constant, the result is current flow in the external circuit. In this case, the external voltage polarity is called forward bias, A B p-type Junct~on n-type +- + + + + + + Depletion reglon free electrons generated within the crystal can quickly Figure 5.1.-Symbol (A) and operation (B)of a p-n junction recombine with available atoms. device. The free electrons in the n-material and the holes in p-material are known as majority carriers. However, be- cause of thermal and other energies,free electronsare also found in a lesser amount in the p-typeand a few holes exist in n-typesemiconductors.These are called minority carri- ers. Nevertheless,even with the excess charge, both semi- 11 11 conductor types are electrically neutral. - Ic Ic +-= . , + L & + L I - -L ~ + r l DIODES AND RECTIFIERS n Is= IT Is < 11 Is '11 The operation of most semiconductordevices is depen- A N o external voltwe B Forward bias C Reverse bias dent upon a p-n junction, which is the boundary formed when a piece of p-type material is joined with a piece of Figure 5.2.-Bias conditions and current flow for a diode.
- 130. and the current is forward current. Conversely, reverse bias, an applied voltage of reverse polarity (fig. 5.20, opposes majority-carrier diffusion by enforcing the depletion-region electric field, and current is greatly re- duced. As saturation current is still constant, the external reverse current is primarily a result of minority-carrier diffusion and is therefore very small. Because there are many more majority carriers than minority carriers, the injection current, under forward bias, is orders-of-magnitude greater than the constant saturation current. As external circuit current is the algebraic sum of injection and saturation currents, for- ward current is significantlygreater than reverse current. Furthermore, to enhance this one-directionalcurrent phe- nomenon,junctions are manufactured in which one side of thejunction is more heavily doped than the other. Forward current is then mainly a result of majority carriers from the more heavily doped region. The arrow portion of the diode symbol (fig. 5.1A) points in the same direction as forward current. As a carryoverfrom vacuum-tubeterminology,the side symbol- ized by the arrow is also called an anode (the p-region), with the opposite terminal, the cathode(the n-region). the external current is about equal to the saturation cment. Therefore, by placing a reverse bias across the device and measuring the resulting reverse current, the forward current can be predicted. The foregoing equations result in the theoretical curve, termed a characteristic curve, which is given in figure 5.3. This cuwe diverges from that for an actual diode in one main aspect, the breakdown of the p-n junction noted at point c. Here,the external voltage meets the limit capabilities of thejunction, and a greater reverse voltage will create an avalanche current that can destroy the device. As a result, p-n junctions normally require a rating for maximum reverse voltage or peak inverse volt- age (PN). Zener diodes are of special interest as they operate in this avalanche current area to regulate an applied dc voltage. As long as the p-n junction is operated within the limits of its reverse voltage and forward current,the device can be represented by a very low resistance for forward- bias conditions and a high resistance during reverse bias. Ideally, and for the majority of applications,a diode can be assumed to have zero resistance under forward bias and infinite resistance under reverse bias. Diode Equations Rectifier Circuits The number of minority carriers is dependent upon temperature and the difference in energy levels between the p- and n-regions. If the energy difference is constant, the concentration of minority carriers plus the saturation and reverse currents will vary exponentiallywith temper- ature. Therefore temperature is a limiting factor in diode operation, and the maximum rated current of a given device is determined by the heat-dissipating properties of the device mounting system. The formula relating external and saturation current with the energy difference and temperature is where Is = saturation current, A, q = charge of one electron, 16 x C, V = voltage acrossjunction (lessthan external volt- age, but approximatelyequal to it), V, qV = energy difference between p- and n-materials, K = Boltzmann constant, 1.38 x JIK, and T = junction temperature, K. At room temperature (300 K), or at other temperatures, where TI= 300 K, and T, = other temperature, K. The negative sign for the saturation current denotes it as flowing in the opposite direction to forward current. The equations relate that if voltage is 0.1 V or more negative, A rectifier can be considered as a diode specifically designed or applied to convert power to dc. The principal applicationin mining is to use the unilateral propertiesof the rectifier for direct alternating current (ac) power conversion. With single-phase ac, there are three basic rectifier circuits to perform this function: half-wave, full wave, and bridge. Figure 5.4A illustrates the circuit of a simple half- wave rectifier in which a transformer magnetically cou- ples the source to the rectifier. This could also be direct, unisolated source connection. With a sinusoidal voltage input (fig. 5.4B), the rectifier acts as a switch. When forward biased (positive anode with respect to the cath- ode), the load, R, is electrically connected to the source, but during reverse biasing it is disconnected. In other words, low and high resistances to current exist with respect to the bias condition. These resistances create a pulsating dc waveform acmss the load, as shown in figure 5.4C. This variation of voltage is often termed ripple. 5 L / , " Actual Vni T A G F I 0 V ISI ,------ a VOLTAGE, 1 0 - IV , b Figure 5.3.-Diode or rectifier characteristic curve.
- 131. Only the positive portions of the input sinusoid ap- pear in the pulsating dc output, and as a result, the conversion efficiencyof the half-waverectifier leavesmuch to be desired. The single-way full-wave rectifier is a method of rectifying both the positive and negative por- tions of a sinusoidal voltage input, and it can be analyzed as two half-wave rectifiers. The circuit shown in figure 5.5A utilizes a center-tapped transformer secondary. When referenced to ground,the V, and V,' waveforms (fig. 5.5B) are then 180° out of phase. Therefore,one rectifier is conductingcurrent (forwardbiased)while the other is not (reversebiased). The consequence is pulsating dc power to the load during both the negative and positive portions of the ac input (fig. 5.50. Conversion efficiency is greatly improved over half-wave circuits. Full-wave rectification can also be obtained with the bridge rectifier. As shown in figure 5.6A, the circuit employs a transformer with a single secondary and four rectifiers. During either the positive or negative portions of the input waveforms,two of the rectifiers are effectively in series with the load resistance. For instance, when the top secondarytransformer rectifiersD, and D, are forward biased but Dl and D , are reverse biased, current flows from the top secondary terminal through D,, R , , and D, back to thrtransformer. The rectifierbiasing condition reverses with the transformer secondary polarity (figure 5.6B,bottom), but the current through the load has the same direction. Hence, the same full-wave pulsating dc waveform in figure 5.6Cappears acrossthe load with only half of the secondary turns needed for the single-way full-waverectifier. Although the output of these three basic rectifier circuits is effectively dc and the current flow is in only one direction, the voltage fluctuation or ripple is often too great to be useful. Consequently, filtering is required to change the pulsating voltage to a relatively ripple-free potential. This filtering action is provided by inductors in series with the load, or capacitors shunting (in parallel with) the load, or both. Each of these methods will smooth the voltage output. An example of this filtering is shown in figure 5.7. It will be shown later that such filtering is not needed for dc mining equipment. Cooling It was stated earlier that the operation of a p-n junction is highly dependent upon temperature. It follows that there exists a maximum temperature beyond which the device will be destroyed if operated. Such a point is called the maximum junction operating temperature. For silicon semiconductors, this temperature is usually around 175O to ZOO0 C,for germanium, 85O to 110° C, but the maximum varies according to the individual device and manufacturer. The temperature at which the junction operates is dependent upon the power dissipated in the junction, the ambient temperature, and the ability of the device to transfer heat to the surrounding environment. Devices designed and operated for small currents usually do not need cooling assistance. However, adequate external cool- ing is required in p-n junctions dissipating 1 or more watts. The simplest method is to mount the semiconductor case securely on a heat sink, which is commonly metal with a large surface area. Thermally conductive washers, siliconcompounds, and correct bolting pressure allow good heat transfer from the device to the heat sink, and air Figure 5.4.-Half-wave rectifier circuit and waveforms. Figure 5.5.-Single-way full-waverectifier waveforms. Figure 5.6.-Bridge rectifier circuit and waveforms. Series - Figure 5.7.-Example of filtering a rectifier output. convection transfers heat to the surrounding atmosphere. In high-power applications, forced-air cooling of the heat sink is sometimes employed to increase heat dissipation further. Figure 5.8 illustrates a rect3er using a heat sink for this purpose. The diagram in figure 5.9 represents the typical relationships in all solid-state device between the
- 132. Rectifier 'Heat sink Figure 5.8.-Heat sink cooling. Collector ,+ junctlon temperature Case ' temperature, Tc Heat sink temperature, T , Ambient temperature,To L - - Absolute-zero temperature Figure 5.9.-Heat sink thermal relationships. solid-state device, its heat sink, and the surrounding envi- ronment. The following equation relates these parameters: where Tj = junction temperature, OC, T, = ambient temperature, OC, Pa = power dissipated by junction, W , and Bj, = ambient-to-junction"thermal resistance,"OCm. The last item, thermal resistance, is actually composed of three parts, as shown in figure 5.9, where Ojo = junction-to-casethermal resistance, OCN, 8 , = case-to-heat-sinkthermal resistance, OCm, and 0 , = heat-sink-to-ambient thermal resistance, OCN. Thejunction-totase and the heat-sink-to-ambientthermal resistances are almost always available from manufactur- ers. The thermal resistance between the device case and the heat sink can be neglected if the mounting is carried out correctly as described here. Junction power can be found by the relationship where L , = maximum forward current, A, and V, = junction forward voltage drop, V. Thejunction forward voltages normally range from 0 . 5to 0.75V for silicon and from 0 . 2to 0 . 3V for germanium,but typical values for specific devices are also available from manufacturers. When the total thermal resistance, Oj,, is known, the operating junction temperature can be calcu- lated and compared with the maximum allowed. Overloads The thermal relationship of figure 5.9 shows three capacitances,C,, C,, and C,, which are the thermal capac- itances of the p-n junction, the device case, and the heat sink, respectively.Thermal capacitanceresists changes in temperature in the same way that capacitance restricts voltage change. For the p-n junction, Cj is usually very small; hence, its time constant is also small. This means that the semiconductor must not be overloaded (excessive power dissipation)for more than a few milliseconds;other- wise, the device will be destroyed. For this reason, high- speed overload protection must be applied to semiconduc- tor devices. For rectifiers, the protection takes two forms: against excessive overloads and short circuits in load currents, and against failure in the rectifier itself (over- temperature or excessive voltage). THREE-PHASE RECTIFICATION Large amounts of dc power at either 250 or 500 Vdc are required for locomotivesand face equipment in many mining operations.When more than a fractional kilowatt of dc power is needed from an ac source, a polyphase rectifier circuit is employed. The direct voltage is derived from three-phase ac power, most often from distribution voltages. There are specific advantages to using polyphase rectifier circuits for dc power. As the number of ac phases driving the rectifier is increased (say, above single-phase ac), the frequency of output ripple is increased, the inter- val between rectifier conduction is decreased, and the ripple magnitude in the dc voltage and current waves decreases. Transformers are almost always used between the ac source and the rectifiers. The rectifier transformer per- forms one or more of the following functions: * 'Ib transform the available ac supply voltage to a value needed for the desired dc voltage; To provide the number of phases required to obtain the desired waveshapes of dc voltage, dc current, and ac supply current; To isolate the dc circuit from the ac source; and 'Ib limit, through transformer impedance, damag- ing overcurrents that might flow during malfunctions. It is important to note that the decrease in the rectifier- conduction interval also increases the required trans- former rating. The transformer utilization factor can be defined as the ratio of dc power delivered to the required transformer secondary voltampere rating. The utilization factor has been found to have a maximum value of 0.520 when three-phase ac input is used. This impliesthat from a transformer utilization standpoint, the most economic rectifier-conduction angle is 120°. When power rectifiers are mentioned today, the refer- ence is almost invariably to solid-stateunits using silicon
- 133. rectifiers as the rectifying elements. Indeed, the silicon rectifier is virtually the only type considered for mine power installations. While there are many possible recti- fier circuits, only two or three types are found in mining equipment. Circuits for silicon rectifiers are selected to make the most efficient use of the transformer, and the results usually are the single-phasefull-wavebridge or the three-phase full-wavebridge. The next sectionwill discuss three fundamental three-phase rectifier circuits, and it will be apparent why the full-wavebridge is popular. Rectifier Circuits Rectifier circuits can be classified as single way or double way.Thephase currentsof the transformersecondary (also termed the dc winding) are unidirectional in a single- way circuit hut alternating in the double-way circuit. The simplest three-phase rectifier circuit is the three- phase half-waveshown in figure 5.10A,where a delta-wye transformer is used, with each leg connected to a rectifier anode. The three rectifier cathodes are tied together to form the positive dc bus. The neutral point of the trans- former winding serves as the negative connection for the load, in this case resistance, R. Being a single-wayrecti- fier, each leg of the transformer secondary conducts cur- rent unidirectionally. If the load is pure resistance, the relationship of output voltage (that across the load)versus time is as shown in figure 5.10B.Each rectifier conducts for the cycle portion in which its anode has a higher positive value than the anodes of the other rectifiers. Therefore, each rectifier passes current for 120° of the input three-phase cycle. Since the current through the load is directly proportionalto the output voltage, the load current has the same waveform as voltage. Inspection of the three-phasehalf-waveoutput voltage shows that the ripple voltage is much lower than the single-phase full-wave rectifier circuit. Actually, the rms value of the ripple voltage waveform is only 18%of the averageoutput voltage (thisaverage voltage is the average dc load voltage, V,). If rectifier losses are ignored, since they are very small for silicondiodes,the dc output voltage and the transformer secondary voltage are related by V,, = 0.827V, , , , = 1.17V, , , , (5.7) where V,, = average dc output voltage, V, V,,, = peak value of voltage applied to rectifier circuit, V, and V,, = rms value of voltage applied to rectifier cir- cuit, V. Both V,,, and V,,, are line-to-neutralvoltages.Note that the fundamental frequency is three times the ac line frequency. As a result, any filter components required to lower the ripple voltage further can be much smaller than in single-phaserectifiers. The relationships presented here for the three-phase half-wave rectifier apply only to ideal transformers and rectifiers. In actual circuits, the voltage drop caused by dc current and the transformer secondary-windingresistance creates a dc componentthat pushes transformer magnetic operation toward saturation. Consequently, this simple three-phase rectifier circuit is seldom used. Output ripple can be further reduced by a three-phase full-wave rectifier circuit, connected as shown in figure 5.11A.This circuit is also called the three-phase bridge or Phase Phase A C - 0 180" 360° B wt, deg Figure 5.10.-Three-phase half-waverectifier circuit (A)and output voltage waveform (6). Phose Phose f Phase A El 0 180" 360' C wt, deq Figure 5.11.-Three-phase fuii.wave rectifier circuit (A)with input (13) and output (C)voltage waveforms. a six-phase rectifier. Being a two-way rectifier, the mag- netic saturation problem inthe transformer isnot present. Furthermore, this configurationretains the advantage of 120° conduction for transformer economy, plus a funda- mental ripple frequency of six times the ac source fre- quency. These characteristics make this double-wayrecti- fier circuit of great practical value, and it is the most popular configuration for dc power in mining. The trans- former dc winding may be either wye or delta connected. In the full-waverectifier circuit, each terminal of the transformer secondary is connected to two diodes, one at
- 134. the anode and the other at the cathode. The cathodes of three rectifiers are common and form a positive dc voltage bus, while the common anodeconnectionof the other three rectifiers represents the negative dc voltage bus. The load is connected between these two common points. Each rectifier conductsfor 120° of one input cycle, and current alternates in each transformer winding. However, current flows through a specific combination of rectifiers for only 60° of the input cycle. This combination could be Dl and D, with transformer secondaryterminals A and B. Therefore, the peak-to-peakvoltage across the load resis- tance appears as six-phaseripple as shownin figure5.11C. Analysis of figure 5.11C shows that the rms value of the fundamental component of the ripple voltage is now only4.2% of the average dc output voltage. In addition,the average dc output voltage for ideal rectifiers is where Vdc = average dc output voltage, V, V,,, = peak line-to-line voltage applied to rectifi- ers, V, and V , , , = rms value of line-to-linevoltage applied to rectifiers, V. The foregoing circuits are typical of most polyphase rectifier circuits, but many additional configurations are available. Because mining almost always employs full- wave rectifier circuits, coverage of more circuits is beyond the scope of this text, but the bibliography can be con- sulted if desired. Parallel Rectifier Operation The current cequirements of a rectifier circuit are often too large to be handled by one rectifier for each circuit element. Two or more rectifiers must then be connected in parallel. Direct operation of two silicon rectifiers in parallel is very difficult, because unbalance between the parallel paths can be caused readily by unequal rectifier characteristics (mainly the forward volt- age) and by unequal impedance in the bus bars or cables. The result is that the rectifier with the least forward voltage can be destroyed by overcurrent.?b eliminate this problem, the parallel rectifiers must be forced into sharing the current equally. The method used almost exclusively in mining equip- ment to force current-sharing employs paralleling reac- tors, sometimes called current-balancing transformers. Figure 5.12 shows how several rectifiers can be paralleled using these reactors. The combination acts as one rectify- ingelement in a rectifier circuit such as in figure 5.11A.In figure 5.12, each reactor is a laminated magnetic core linked in opposing polarity by the anode currents of two rectifiers, and the coresare designed not to saturate at the highest expected current. If the two rectifier currents become unequal, the current difference excites a magnetic flux that induces an aiding voltage. This voltage is in- duced in the rectifier leads in a directionthat will equalize the currents. TRANSISTORS The principal tool of the electronics industry is the amplifier, a device that can increase the power level of an input waveform or signal. An amplifier is actually an energy converter in which energy from a power supply is converted by the amplifier to signal energy. The most common device used in amplifiers is the transistor. A bipolar transistor is formed in a manner similar to that of the junction diode, but it consists of two junctions in close proximity and parallel to each other in the same crystal. When a p-region is sandwiched between two n-regions, the device is termed an n - p n transistor, the model and symbol of which are given in figure 5.13A. Similarly,if a thin portion of n-material is bounded by two p-regions, the transistor is termed pn-p, as shown in figure 5.14A. As illustrated, each semiconductorregion is given a name: emitter, base, and collector. Transistor Operation The operation of the transistor is dependent upon the bias voltages across the junctions. If voltages are applied to an n-p-n device as shown in figure 5.13B, the emitter- base junction is forward biased, and the collector-base Fuse - 1 ,F?~ltl"e Rectifier - 1output dc Figure 5.12.-Parallel operation of rectifiers using paral- leling reactors. Base Symbol E c A B Figure 5.13.-An mp.n junction transistor. A B Figure 5.14.-A p.mp junction transistor.
- 135. junction is reverse biased. These are the normal bias conditions,Electrons will flow into the base region, caus- ing an excess of majority carriers there. Because the base region is thin and the potential existing across the two n-regions is much higher than the base-to-emitterpoten- tial, most electrons from the emitter region diffuse across the base and are accelerated into the collector region. The electrons d r i f t across the collector and cause current flow in the collector circuit. However, a small percentage (typ- ically, 5% or less) flows out from the base connection because of recombination with holes in the base region. This process can be considered amplification since the small base current controls the much larger collector current. A p-n-p transistor operates on the same princi- ple, but here it is hole flow rather than electrons that causes the amplification. Consequently, the bias condi- tions are reversed from that for an n-p-n (the normal conditions are shown in figure 5.14B). From the preceding discussion, it would appear that either end of the transistor could be called an emitter because either hole flow or electron flow meates the current amplification, but this is generally not the case. Heat dissipation is much larger in the collector-base junction because of the greater difference in potential. Therefore both pn-p and n-p-n transistors are designed so this heat can be diffused through the collector region. As might be expected, a saturation current resulting from thermally-generated minority carriers flows across the reverse-biasedcollector-basejunction. In the diode, the current is designated "I,;" in a transistor, it is termed "LBO" In the same manner as for diodes, the increase of saturation current with temperature sets the maximum operating temperature for a transistor. Heat sinks are commonly used in high-power transistor applications to diffuse collector-basejunction heat and maintain temper- ature below critical levels. The same calculations that were presented in the preceding section on rectifiers can alsobe appliedto transistors to determine a safe operating temperature. The fraction of constant emitter current that reaches the collector is called alpha, a, and the collector circuit itself can be considered to be the output circuit. Since as much emitter current as possible should be collected, alpha should be as close to 1as possible. When combined with EBO,the collector current, i,, can be expressed in terms of emitter current, i,, as i, = ai, + Lao Figure 5.15 shows the relationship of these currents. However, in practical applications, I , , , is often so small that it can be neglected. Since base current controls collector current, an im- portant expression can be obtained from figure 5.15 using Kirchhoffs current law on either transistor: iB = iE - i,, In terms of collector current, it can be shown that The term, d(1- a), is called beta, 6, and also the dc current amplificationfactor, and This last equation shows the significant effect of temper- ature on transistor operation; that is, the temperature- sensitive EBois multiplied by (1 + B). Even though a is less than 1, , ! 3 may range from 20 to 200 for amplifying transistors. Bipolar-Transistor Amplifiers Bipolar transistors can be operated with any one of the terminals common to the input and output, thus there are three basic circuit arrangements: common-base, common-emitter,and commoncollector.The most popular is common-emitter. Illustrated in figure 5.16, the common-base or grounded-baseconfigurationemploysthe emitter and base terminals as input, with the collector and base terminals supplying output. Current gain, which is the ratio of output to input, is usually just less than 1. Because the emitter-basejunction isforward biased, the circuit has low input impedance as viewed from the input terminals. Because the collector-basejunction is reverse biased, the output impedance is high in comparison to the input. Hence, voltage and power amplification can be realized. Two different circuits, signal and bias, are necessary for the operation of either of the two common-baseampli- fiers shown in figure 5.1. The bias voltage sources, often termed the amplifier power supply, fix the dc level for proper operation of the two junctions. If the signal input and output are not separated electrically from the bias source, as seen in figure 5.16A, the circuit is called a dc amplifier. Although it is beneficial in applicationssuch as amplifying dc voltages for instrumentation, a signal with Figure5.15.-Current relationshipsfor p-n-p(A) and n-p-n(8) devices. A B Figure 5.16.-Common-base amplifiers.
- 136. dc content or offset can interfere with correct transistor biasing. Figure 5.16B illustrates a popular method of removing this problem: the use of capacitorsto isolate the amplifier. The capacitorsexhibit high impedanceto dc but low impedance to ac signals, thus they block input and output dc. As the circuit now reacts only to ac signals, it is called an ac amplifier. It can be noted that transformers can perform a similar function. With either the dc amplifier or the ac amplifier, a small change in input voltage causes significant variation in the injection current across the emitter-basejunction. As previously discussed, most majority carriers diffise to the collector, causing collector current, &. If the load resistance, R , , is small with respect to the transistor output impedance, ic is approximately equal to i,. The collectorcurrent creates voltage variations acrossthe load resistance that can be much larger than the input voltage. In the common-emitter transistor arrangement, the source signal only supplies current to the base. Because base current is much smaller than either the emitter or collector current, current amplification or gain, G,, is high. Neglecting I , , , in equation 5.12,the gain is approx- imately equal to which can be from 10 to several hundred. The input impedanceis also higher than in common-baseamplifiers. Figure 5.17 shows a simple common-emitter ampli- fier. The control action of the base current can be demon- strated by assuming that the base-emitter forward bias is increased. This increase creates a corresponding increase in emitter-basejunction current; thus, collectorcurrent is raised substantially. Because the base current is approxi. mately proportional to but usually much less than collec- tor current, base current is the controlling parameter of the amplifier. The concept of characteristic curves has already been introduced in figure 5.3 in the section on diodes and rectifiers. Characteristic curves are an extremely useful tool for the graphical design and analysis of transistor circuits. Four independent transistor parameters control 12 ,/ Safe operation boundory 4 0 1 0 2 0 the number of necessary curves.When figure 5.17 is used, these parameters are as follows: a and 6 increase with Vo, the collector-to-emitter voltage. in is dependent on i, and VcE. i, is not a linear function of &. When V , , is zero, i, is approximatelyzero, regard- less of i,. Consequently, two sets or families of curves are needed: 1.Collectoror output characteristics, ic versusvcEfor varying values of i , , @ 2. Common-emitter input characteristics, VBEversus i, for varying values of V , , . Figures 5.18A and 5.18B show typical output and input characteristics for an n-p-n transistor connected for common-emitter operation. The nonlinear and propor- tional properties of the four independenttransistor param- eters are evident in the graphs. These curves can be employed for design and analysis purposes. The analysis often uses a load line (the straight line in figure 5.18A)to observe dynamic variations of voltage and current. The dashed line in figure 5.18A isvery important as it delineates the safe operation boundary. Manufacturers specify maximum permissible collector voltage, current, and power dissipation, since outside this area damage to the transistor will probably result. As noted earlier, allow- able power dissipation must be reduced as temperature is increased. Figure 5.17.-Common-emitter amplifier. COLLECTOR-TO-EMITTER BASE CURRENT VOLTAGE (VCE). V (1~). PA A Output 6 Input Figure 5.18.-Commonamitter characteristiccurves.
- 137. Figure 5.17 illustrates an amplifier circuit with two batteries supplying dc for transistor bias, but single dc source for all bias voltages is more desirable in practical applications. Three bias techniques are frequently used for common-emitter amplifiers, and these are shown in figure 5.19. Each circuit uses resistors to supply dc bias to the base for a center bias condition about which the transistor operates. The center condition is termed the quiescent point of the amplifier. Of the circuits illus- trated, the stabilizedbias circuit(0givesthe best thermal stability, maintaining the quiescentpoint within a desired or specified range regardless of the normal operating temperature. The bypass capacitor,shown acrossthe emit- ter resistor of the stabilized bias circuit, establishes a constant base bias bypassing or acting as a low impedance to time-varying voltages. The two preceding amplifier configurationsemployed the collector circuit for output. In the common-collector arrangement, the output is obtained across a load resis- tance in the emitter circuit, as illustrated in figure 5.20. Because the source and output voltages are now in series but have opposing polarities, the circuit gives high input impedance and approximately unity voltage gain, yet current gain is about the same as in common-emitter amplifiers. A main advantage of the common-collectoris that the output impedance is about equal to the load resistance, which is lower than the preceding two connec- tions. This allows the circuit to be adjusted to fit the output needs precisely; hence, this circuit can be used for impedance matching the output of a source signal to the input of another amplifier. - - Field-Effect Pansistors The n-p-n and p-n-p junction transistorsjust covered contained two junctions. Field-effect transistors (FET's) have effectively only one junction but still can operate as amplifiers. These devices are voltage controlled, whereas bipolar transistors can be consideredas current-controlled devices. There are two general classifications: junction FET's and metal oxide semiconductor FET's. Both have very high input impedances, much higher than bipolar transistors and approaching the input impedance of vac- uum tubes. To demonstrate the amplifying action available with FET's, consider the cross-sectionalmodel of an n-channel junction FET, illustrated in figure 5.21A. The gate-to- channel junction is reverse biased by placing the voltage V , between the gate and sourceterminals as shown.The level of V , establishes a specific size of depletion region about the gate semiconductor and within the channel. Changing this reverse bias increases or decreases the size of the depletion region and decreases or increases the available conduction area remaining in the channel. Therefore, voltage changes between the gate and source terminals can control the allowable current through the channel from the drain to the sourceterminals. The action can be employed to amplify voltages or currents. The conduction channel in the junction can be either n-type or p-type semiconductor, with the gate being p- or n-material,respectively. - - Figures 5.21B and 5.21C give the symbols for either junction FET type. An important ad- vantage of FET's over junction transistors is that the source-to-drainchannel is resistive without a diode effect. In essence, this allowsFET's to be operated as electrically controlled resistors. A Fixed bias B Self-bias C Stabilized bias Figure 5.19.-Bias techniques for common-emitter amplifiers. I 0 "cc Figure 5.20.-Common-collector amplifier arrangement. o-semiconductor . S- ;-channel semiconductor B n-channel symbol A Simple bar model C p-channel symbol Figure 5.21.-Model and symbols for junction FET's.
- 138. As an application example, figure 5.22 shows a junc- base. This pn-p base current in turn causes collector tion FET used in a typical amplifier circuit. The input current in the p-n-p transistor. The action between the signal is applied across the gate to the source, with output two transistors has a positive feedback effect because an taken from drain to source. R, is employed to set the increasein current in one transistor creates an increasein proper dc quiescent point bias for the gate, and the the other. Therefore, once conduction in the SCR is estab- capacitor in the source circuit bypasses ac, thus maintain- lished, the gate no longer has any controlling effect, and ing the bias level. In metal oxide semiconductorFET's (or MOS-FET's), the depletion region used in the junction FET is replaced by a thick layer of silicon oxide, a good insulator, and the semiconductoremployedfor the gate isreplaced by a metal conductor, thus forming a high-quality capacitor.A model of a MOS-FET, including the symbols, is given in figure 5.23.The operationof these transistors is similar to that of junction FET's but much more complex. The preceding information on transistors is intended as just an introduction to a few important devices. For complete information,the bibliography must be consulted. The coverage here is justified bemuse transistors are an extremely important, but ofken hidden, segment of mine power systems.The next section will cover another device that hasrevolutionized the controlof electricalmachinery. SILICON-CONTROLLEDRECTIFIERS In past few years, the use of solid-state power equip- ment in mining has accelerated.One primary reason has been the introduction and acceptance of static or solid- state starting of conveyor-beltdrive motors. The heart of these starters is the siliconcontrolled rectifier or SCR. SCR's have many other applications; among these, the most common is in dimmers for home lighting. SCR's, also called thyristors, are three-terminal semi- conductor devices having a four-layer pn-p-n material combination. Figure 5.24A shows a model of the SCR construction. The outer two layers act as a p n junction and the inner layers serve as an element to control that junction. The symbol for the SCR is given in figure 5.24B, and figure 5.25 illustrates how the operation of the three- junction combination can be equated to two transistors connected as shown. The equivalent circuit is represented by one n-pn and one p-n-p transistor. When the bias on the gate, the n-pn transistor base, is negative with respect to the cathode, the n-p-n transistor cannot conduct appreciable current. In other words,it is cut off. As no n-pn transistor collectorcurrent can flow, the p-n-p transistor is also cut off. There is high impedance between the anode and cathode for this bias condition, and the SCR operating condition is called OFF. However, if the gate bias is made positivesothat the n-pn transistor conducta,current will flow into the n-p-n collector from the pn-p transistor Input 1%.,fi TCS 1 . - Output T- "OD Figure 5.22.-Example of a junction-FET application. Source Cote(-) ? ? 1SiO? Drain 7 D, drain D, drain Z S X S i S,source S,swrce Model n-channel p-channel symbol symbol A Depletion mode operation D , droin D, drain Q ? S, source S,source n-channel p-channel symbol symbol B Enhancement mode operation Figure 5.23.-Model and symbols for MOSFET devices. -- J Gc Cathode a SCR A 8 Figure 5.24.-SCR model (A) and symbol (8). Figure 5.25.-SCR equivalent model and circuit.
- 139. the SCR is latched ON; that is, anode-to-cathodeimped- ance becomes very low. The gate cannot turn the SCR conduction OFF. Cessation o f current requires a negative gate bias and an essentially zero anode-to-cathodevoltage. This allows the p-n-p transistor to cut off. The OFF and ON characteristics are apparent in the typical curve provided in figure 5.26. The breakover voltage noted here is the anode-to-cathode potential at which the SCR will turn itself ON. There are many applicationsfor the SCR or thyristor. Some of the devices and system components that the thyristor replaces include Thyratrons, Mercury-arcrectifiers, Saturable-corereactors. Relays and contadors, Rheostats and motor starters, Constant-voltagetransformers, Autotransformers, and Mechanical speed changers. Thyristor applications are a major subject in chapter 14. INTEGRATED CIRCUITS The semiconductordevicesdiscussedsofar are termed discrete components if they are manufactured as single units, for example,one diode or one transistor. They must be combined with other electrical and electronic compo- nents to perform any required function. Manufacturing processes have been refined so that several transistors, diodes, and resistors can be made in a single circuit, or in other words on one single semiconductor chip. Such de- vices are termed integrated circuits(IC's), and their study in electrical engineering is known as microelectronics. 'Ibday,many circuits requiring numerous individual tran- sistors, such a complete amplifiers and digital computers, are packaged in a single semiconductor chip or microcir- cuit. When employingonly one semiconductorchip,the IC is called monolithic; when the unit is created by intercon- necting more than one microcircuit, the device is a hybrid IC. The structure illustrated in figure 5.27 represents the cross section of a simple monolithic IC. The device is fabricated on a chip of p-type semiconductor, termed a substrate, by forming a number of junctions. The three sections shown are electrically isolated by reverse-biased p-n junctions, and the silicon surface is protected by a silicon oxide layer. A thin film of metal is depositedon top Avalanche breakdown /ON characteristics Breakover OFF characteristics Figure 5.26.-General characteristiccurve for SCR. of this layer to interconnect the different regions. The top view of an actual IC is provided in figure 5.28. These devices can contain hundreds of transistors but can be small enough to pass through the eye of a needle. Except for high-powerapplications,IC's are preferred over discrete-componentassembliesbecause they add reli- ability to equipment while reducing both size and cost. Consequently, IC's are employed where specific circuits require many transistors, diodes, and resistors. In circuit diagrams, it is accepted practice to show only the symbol for the s~ecific aoolication: some of these are eiven in figure 5.i9.The &e of IC'S is extremely widespread in recently manufactured mining equipment, especially in control, monitoring, and communications applications. BASIC INSTRUMENTATION Much has been said in the preceding chapters about electrical parameters and their quantification: voltage, current, power factor, power, and so on. Instruments that measure these quantities are necessary to monitor and troubleshoot the operation of a power system and can be used to ensure optimum operation and to find malfunc- tions. The devicescan be indicating instruments or record- ing instruments that are permanently installed in mior nPp? PnP Diffused transistor trcnsrstor resistor p-type substrate - - / - - - - - - - - KEY S~l~con d~ox~de loyor, metol B Base f~lm o n top to interconnect E Em~tter components. C Collector nt, "2. "3 Varrous n-type reglons Figure 5.27.-Sketch of simple monolithic IC cross section. A Silicon wafer slice B 30:1 enlargement o f 1 circuit Figure 5.28.-Top vlew of an actual IC.
- 140. Timer 6 2 0 AND 0 NOR 0 NAND Amplifier within circuit Special-purpose circuit Logic circuit symbols Figure 5.29.-Examples of symbols employed for IC's. equipmentor they can be self-containedand portable. It is not unusual for every piece of power equipmentin or about the mine to have some form of enclosed instrumentation. The devices can range from basic meter movements to transducers connected to on-line computers that monitor the status of the entire power-system complex. The word "meter" is often used as a suffix or part of a compound word that describes the function of the instru- ment. Of all the instruments designed to measure electri- cal quantities, the voltmeter and ammeter are the most basic. Voltmeters measure the potential difference or volt- age between two points and must present a very high impedanceto the circuit so as not to interfere with normal circuit operation. Ammeters measure current flow and must have a near-zeroimpedance. The dc voltmeters and ammetera sense average quantities, while their ac coun- terparts usually provide rms voltage and current values. Instrument current inputs are normally at 5 A, with potential inputs at 120V . The following section will explore the various instm- ments available to the mining industry, commencing with a deemiption of the basic instrument or meter types and then showing how the devices are employed to monitor system quantities. BASIC METER MOVEMENTS A meter movement is an electromechanical device that provides the mechanical motion to an indicator in response to an applied electrical signal. Regardless of the type of meter movement, opposing magnetic fields are employed to activate the indicator or pointer. These move- ments can be classified as electrostatic, dynamometer, moving iron vane, and permanenhmagnet mouing coil. An electrostatic movement is the only type that measures voltage directly as opposedto a voltage-produced current. This meter is basically a variable capacitor with a restoring resistor connected between a fixed and a movable plate or vane. When a difference in potential exists between the plates, the opposing charges produce a mutual attraction and the movable vane will move toward the fixed vane with the deflection proportional to the applied voltage. Upon removal or change in potential, the resistor discharges the capacitance. Thus any current through the movement is merely incidental to the opera- tion. Electrostatic instruments can measure either ac or dc potentials; they have true rms response to ac regardless of waveform shape. Full-scale readings (maximum meter deflection) range from 100 V to 10 kV depending on the movement, with a measurement precision of 0.5% to 2%. A dynamometer movement consist of two coils, one fixed and the other movable. The movable coil rotates in the magnetic field produced by current through the sta- tionary coil. If the current being measured flows through both coils,(that is, they are in series),the resulting torque is proportional to tine current, and the displacement is proportional to the square of current. Thus the pointer deflection indicates the rms value of current. The move- ment can be designed to measure dc or ac very precisely to within 0.1%.However, the dynamometer is not commonly employed as an ammeter. Its prime application is as a wattmeter, which will be described shortly. Moving-iron-vane movements are similar to the dyna- mometer, except the moving coil is replaced by a soft iron vane with no permanent magnetization. Here, current throughthe fued coilproduces a magneticfieldthat induces magnetism in the soft ironvane. The magnetic fields oppose each other, producing torque that deflects the vane with a force proportional to the square of the current. The instru- ment can therefore measure dc or the rms value of ac, but with less precision (1% to 2%)than the dynamometer. The last basic type of meter movement is the permanent-magnet moving-coil or d'Arsonval meter, which is a dc ammeter.The movingelement is a coil of fine wire suspended so that it is free 6 rotate in the field of a permanent magnet. Sketches of typical movements are provided in figure 5.30. When dc flows in the coil, a torque is produced that tends to rotate the coil. The rotation is opposed by someform of spring restraint, usually a helical spring, so that coil motion and thus pointer position is proportional to the coil current. If the dc through the coil is varying so fast that the pointer cannot follow the fluctuations, the pointer will assume a position relative to the average torque, and therefore indicate the average value of current. However, if the current is a sinusoid,the average of moving-coiltorque is zero, and the pointer will not be deflected.Nevertheless,d'Arsonval movements can obtain a precision of 0.1%. For measuring current, both dynamometer and moving-iron-vane movements are often restricted to fre- quencies less than 200 Hz. Yet both these yield true rms readings within thew frequency range. Electrostatic in- struments can be extremely precise for observing voltage, but they are often very delicate and are applicableonly for laboratory use. Even though d'Arsonva1 movements mea- sure only dc, they are the most common type in use for both direct dc measurements and ac measurements using rectification.
- 141. External magnet Moving-coil construction Figure 5.30.-Permanent-magnet moving-coilmovements. Meter-Movement Applications When a d ' h n v a l meter is used as an ammeter, it is inserted in series with the circuit being measured. The current range for this direct application is obviously restricted by the maximum scale reading or maximum current of the movement. D'Arsonval meters can have full-scalelimits from 1.0pA to 50 mA, although the basic movement is considered to be 1.0 mA, which allows measurement from zero to 1.0 mA. For higher current requirements, the meter is shunted with a low resistance as shown in figure 5.31. Such shunts can be tapped to provide severalcurrent ranges, or severalshunts might be available, each selected by a switch to provide a specific current range. Commercially available ammetera of this type offer up to a 50-Afull-scalereading. To measure dc voltages, a d'Arsonval movement is simply placed in series with a selected high resistance, and the combination is connected between the two points where a voltage measurement is desired (fig. 5.32). Be- cause meter deflection is still proportionalto current, the meter scale can be calibrated to read the voltage required to produce a specific current. The sensitivity of such voltmeters is stated in ohms per volt. For instance, if a meter has a range of 0 to 200 and if the movement is to be used to measure 0 to 200 V, the total meter resistance must be As movingcoil resistance, R,, is generally on the order of 50 to 100n,it can be neglected in this case. Sensitivity of the combination is therefore A higher value of sensitivity for a specific meter implies higher quality. Presently,the upper limit for the commer- cially available d ' h n v a l voltmeter is 50 knN. The standard d'Arsonval movement of 0 to 1mA has a coil resistance of 100C & hence, it can be employed to read 0 to 100mV directly. External shunts are utilized for a desired maximum current when the current is higher than measurable by Internal magnet Figure 5.31.-Shunting dlArsonvai meter for high-current tests. Permnent A mognet resi! ---/ Line / + Figure 5.32.-D'Arsonvai meter used to measure dc poten. tials. normal instruments with internal shunts. Figure 5.33 providesa coupleof typical constructionswhere terminals are available for circuit as well as meter connections. Theseare simply standard resistance units, designed to be used with either 50-mV (0- to 50-mA) or 100-mV (0- to 100-mA) movements, in which a current through the shunts is indicated by a specific voltage drop across the shunt. For example, if a shunt is designated 100 mV, 600 A, a reading of 50 mV across the shunt signifiesthat 300 A is flowing in the circuit. Any time that metering or instrumentation is part of dc mine power equipment, it can almost be assumed that external shunts are involved.
- 142. To this point, only the measurement of circuit opera- tion has been considered. A d'Arsonval meter can also be used to measureresistance by the addition of a dc sourcein the dc voltmeter circuit. Consider the circuit shown in figure 5.34, which has a dc movement in series with a dc nstrument terminals source(usually a battery) and one or more resistors, one of which is usually variable to be used for calibration. The unknown resistance to be measured completes the loop. Meter deflectionis still proportional to dc through the loop and is therefore a function of the unknown resistance. Using known resistances, the meter scale can be cali- brated to read resistance directly, and different fixed resistors or multipliers can be used to extend the single Figure 5.33.-External shunts used for high.current scale. The combination is easily calibrated before each use measurements. by adjusting the pointer to zero using the variable resis- tance. The resistance desired could be a simple component or a complex circuit, but the ohmmeter should never be used on an energized circuit because of the internal source. m Combining the d'Arsonval movement with a half- wave or full-waverectifier allows the reading of ac values in terms of dc through the coil. The full-waveor rectifier- ~5~pzs) adjust M e t e r 1 ' - 1 ammeter circuit shown in figure 5.35 is the most common. Unknown Here,current through the movement isI,, and thus, meter z 1.5V res~stance deflectionis proportionalto the average of I,. This reading - is the half-cycle average if the ac is symmetrical (that is, d the dc scale of the meter will read the half-cycle average sinusoidal current). As the rms value of current is usually Figure 5.34.-Simple ohmmeter circuit. desired, the scale is calibrated in rms by multiplying the average current by 1.11. This is the rms value for a sinusoidal waveform only; for any other waveshape, rely- ing on the rectifier circuit can produce large errors. Moving-ironand dynamometermovementsrecord rms current automatically,and many permanent meters built into power equipment to measure ac voltage and current are moving-irontypes.However,the d'Arsonval meters are often preferred because of their greater sensitivity.For ac measurements of voltage or high current, the concepts of high series resistance and low parallel resistance also can be applied to the rectifier, moving-iron,and dynamometer movements, but such practices are not common except in small portable test equipment. It can be seen in the foregoing that the d'Arsonva1 meter is used to measure ac or dcvoltage or current as well as resistance. An instrument incorporatingall these func- tions is called a multimeter. The selection of a specific parallel or series resistance combination provides the needed measurement function and parameter range. Wattmeters As mentioned earlier, the main application for dyna- mometer movements is in wattmeters. Figure 5.36 illus- trates the wattmeter connection. Typically, the fixed coil carries circuit current while the moving coil is connected in series with a high resistance and is attached acrossthe terminals of the circuit (the moving coil can itself be of high resistance). Circuit current flows through the fixed (or current) coil, and the current through the moving (or potential) coil is proportionalto circuit voltage. Therefore, the movement torque is proportional to the product of instantaneous voltage and current, with the indication relative to the produce average or average power. The dynamometer connected as such will measure correctly the average power of a dc or ac circuit of any waveform, even when a power factor is involved. Figure 5.35.-Rectifier ammeter. Source 1 +++ Flgure 5.36.-Dynamometer connected as wattmeter.
- 143. Varmeters In addition to being used for measuring watts, the dynamometer movement has wide application in measur- ing reactive power or vars. This is done in single-phase instruments by shifting the phase of the voltage coil by 90°. The voltage coil f l u x is then in phase with the flux produced by the reactive-currentcomponent in the current coil. Varmeters are installed in the same manner as wattmeters are. Flgure 5.37.-Power-factor movement. Power-Factor Meters A power-factor meter showsthe power factor continu- ously and indicates whether the current is leading or lagging the voltage. The movement resembles a single- phase wattmeter but has no control spring and has two moving potential coils mounted on the same shaft 90° apart. One potential coil (Bof figure 5.37)is in series with a noninductive resistor so that it produces torque propor- tional to the line voltage and in phase with the real component of line current. The other coil (coil A) is in series with a higher quality inductance, so its torque is proportional to the line-current reactive component. The fmed coil (coil C) is again the current coil. With unity power factor, the average torque between coils A and C is zero since the currents are 90° apart, but the currents Ammeter through coils B - 4 C are inphase, sothe torque produced Voltage Current aligns their axes, and the pointer indicates unity power factor(1.0 pD. For leading or lagging power factors,the net Figure 5.38.-Simple instrument-transformerconnections. torque created by currents in coils A, B, and C will swing the movingcoilsto the right or left, aligning the pointer in a position relative to the power factor. Meter scales are therefore calibrated so that the center position is unity power factor, and to the left and right of center are lagging and leading power factors from unity to zero. This section has presented some direct applicationsfor basic meter movements. Some concepts shownhere apply to all electrical parameter measurements, but for ac power systems, additional componentsare normally employed. POWER-SYSTEM INSTRUMENTATION Inchapter 3,the subject of current transformers(CT's) and potential transformers CPT's) was introduced. These devices actually fall under the general category of instru- ment transformers and serve two main functions: 'lb isolate instruments, relays,and meters from line voltage, and 'Ib transform line currents and voltages into values suitable for measurement by standard instruments. Thus, the normal ratings of instrument transformer sec- ondaries are 5.0 A for CT's and 120 V for PT's. This measurement implies not only metering or actual visual readings but also sensing for such purposes as protective relaying. The following material will cover specifics of CT's and FT's as they apply to instrumentation of mine power systems. Chapter 9 will discuss the application of these transformers to protective relaying. Instrument Transformers Instrument transformers are connected in the power system in a manner related to the function they monitor. The primary winding of a CT is placed in series with the line conductorto be measured, or may be the line conduc- tor itself, while a PT is placed acrossthe line voltage to be measured (fig.5.38). The transformers can then be used to extend the application of ac instruments in the same way that shunts and series resistors extend dc instrument usage. In this case, the ratio of a CT or PT is the ratio of primary current or voltage to secondarycurrent or voltage under s~ecified conditions. The secondarvwindingparam- eter is Eoordinated with the connected &strume<iation. 'Ib operate reliably,an instrument must receive infor- mation that accurately represents the conditions existing on the power system. When operated outside of the range for which they are intended, instrument transformers are very nonlinear devices; that is, the output from the trans- former secondary can deviate from being an accurate representation of primary-winding conditions. The amount of deviation is a function of the transformer input level, secondary load, and design. To help with current application of instrument transformers so that they oper- ate in their linear range, the American National Stan- dards Institute (ANSI) has standardized transformer de- signs and secondary loads.' The designs are called accuracy classes, and the secondary load is called the transformer burden. The effects of burden changes are typically more pronounced with CT's and PT's. Preferably, CT burden is expressed as a standard load impedance or its resistance and reactance components.In the past the practice was to specify the value as an apparent power (in voltamperes)at a power factor,the angle of which wasreferencedto a rated secondary current (for example, 0.9 pf of current lagging). Consequently, a CT burden of 0.5-0 impedance could be 'Requirementsfor InstrumentTransformers.C57.13 1968 et. 8eq
- 144. expressed as 12.5 V A at 5 A, assuming the usual 5-A current. However, because of the nonlinear nature of transformers, burden impedance decreases as the second- ary current increases, and a specific burden may apply only to one level of secondary current. As a result, the now nonstandard voltampere ratings are confusing. Further- more, CT burden must be applied not only to the external load but also to all elements of that load, including the interconnecting loads. As the total burden needs to be calculated frequently, manufacturer publications usually provide the burdens of individual components. Potential transformer burden is normally stated as the total exter- nal voltampere load on the secondary at rated secondary voltage. For the best accuracy with either PT's or CT's, the impedance of the burden should be identical to that of the instrumentation, and the accuracy limits stated by ANSI will then apply. The general rule for CT's is that if silicon steel is used for the core, the ampere turns should be at least 1,000 for good accuracy under normal conditions. When a PT has acceptableaccuracy at its rated voltage, it can normally be used over a range from zero to 110%of rated voltage. Operationgreater than 10%overvoltagecan produce excessive errors. Some special precautions are in order whenever cur- rent transformers are in use. A CT secondary should alwaysbe shorted or properly connected to the instrumen- tation (meters, relays, etc.), or dangerous potentials can occur at the secondary terminals and the core can become permanently magnetized. The flux density in the core is normally very low and can rise to saturation without a secondarycurrent. The core can also becomemagnetized if dc is passed through the secondary. In either situation,the transformer ratio can be seriously changed. Furthermore, it is possible for a CT to be damaged through insulation breakdown associated with surges, overloads, and other occurrences. Therefore good practice dictates that tests be conductedprior to installation and periodically thereafter to verify transformer operation. If magnetization is sus- pected, the core can be demagnetized by passing rated 60-Hz current through the secondary with the primary open and gradually reducing the current to zero. When a fault occurs on a line downstream from the CT coupling, the transformer primary current may reach severaltimes the rated value for short periods of time. Two different techniques are available to protect against CT damage. One method is to overdesign the primary winding so that the transformer will not be damaged by the mechanical and thermal effects of moderate overload. The other design is perhaps more desirable. Here the CT is selected so that its core is close to the saturation point with normal operating primary current. When a surge current occurs, the secondary current cannot increase in proportion to the primary current and the burden is thus spared much of the shock. (See chapters 9 and 10 for further details as core saturation can seriously affect protective-relay operation.) Single-Phase Connections Figure 5.39 illustrates the measuring device connec- tions needed for a single-phasecircuit in order to observe voltage, current, and average power. This is a simple extension of figure 5.38. Only two instrument transform- ers are required: the PT drives the voltmeter and the wattmeter voltage coil, and the CT suppliescurrent to the ammeter and the wattmeter current coil.For this arrange- ment, the ammeter and voltmeter would probably be moving-iron movements and the wattmeter would be a dynamometer. An alternative instrument arrangement is illustrated in figure 6.40. Here transducers are placed between the instrument transformers and the meter move- ments. Transducers are electronic components that present a standard burden to the transformer and provide an output compatible with the standard d'Arsonval move- ment. This is usually 0 to 1 mA, but 0 to 50 mV and 0 to 100 mV are also available. The transducer output is also adapted to a range of load impedances. With either ar- rangement, three ac power parameters can be measured and the power factor can also be calculated if desired. When any meter movement is employed, the normal reading of the meter should be one-half to three-quartera of the full-scalevalue in order toprovide the best precision. Note that in figures 5.39 and 5.40 the instrument transformer secondaries are grounded. The grounding is needed to prevent a high static potential, which can cause C Power conductors c T ; To loads C s,ia'a current I - J~oil I Wattmeter L ~ ~ Voltmeter Ammeter Figure 5.39.-Voltmeter, ammeter, and wattmeter arranged as single-phase system. I D To loads 0-120V 0-5.OA Voltaae Current I transducer I Figure 5.40.-Use of transducers with standard d'Arsonval movements.
- 145. a higher voltage than normal to appear on the secondar- Current Wattmeters ies. Without grounding, the transformer insulation could connection, J fail. The transformer case should also be grounded for the a x ?- a . - same safety reason. Three-Phase Connections - When the measurement of average power in a three- 1 phase system is required, it seems obvious to place one dvnamometer wattmeter in each ~ h a s e and add the re- A B sklts together. This is shown in fibres 5.41A and 5.41B for a four-wirew e load and a three-wirew e or delta load. The sum of themeter readings is total power for either connection, for any waveform, and whether the system is balanced or not. The common connection of the three wattmeter potential coils may be placed at any potential without affecting the total power readings. If the potential is that of one phase conductor (see figure 5.421,one wattmeter becomes inoperative and thus may be omitted. The result is the two-wattmeter method of three-phase power measurements. Commercially available transduc- ers can be used instead of the two wattmeters. The transducer inputs are two line-to-line voltages and two line currents, and the single output, which is proportional to total power as before, can be used with a standard d'Arsonva1 movement. A circuit arrangement for this method is shown in figure 5.43. Under balanced conditions, the readings from the two-wattmetermethod can be used not only for total power but also to determine the power-factor angle. It can be shown that where Pl,Pz = two power readings, corresponding to ar. rangement in figure 5.42, and 0 = load power-factor angle. If P ,represents a measurement of phase a current, equa- tion 5.14 provides the correct sign for the power-factor angle, thereby specifyingwhether the load is capacitiveor inductive. At times, phase sequence is hard to distinguish in practice, but the equation yields the angle magnitude and this is often sacient information since the reactive characteristics of the load are usually known. If the system is balanced or can be approximated as such, the circuit shown in figure 5.44 can be employed to measure the line-to-linevoltage, line current,power factor, and total average power. The two-wattmeter approach calls for two PT's and two CT's. One PT supplies the voltmeter and one CT provides information to the amme- ter, while the remaining PT and CT supply the power- factormeter sothat the transformer burdens are balanced. It is often useful to observe each line current or line-to-line voltage for major power equipment. Figure 5.45Aprovidesan economical method for the line currents in which only two CT's are needed. If one CT secondary is measured, the current will correspond to the CT phase (that is, phase a or phase c),but if both CT secondaries are in parallel, the current reading is for the phase without the CT (that is, phase b). This metering is theoretically correct only forbalanced voltages,but on most systemsthe voltage is close enough to balance that the two-CT ap- proach gives acceptable precision. If greater accuracy is Figure 5.41.-Three-phase wattmeter connections. Wattmeter,-Current coil * coil Source L Wattmeter '-Current coil Figure 5.42.-Two-wattmeter method. PT CT -,T o - - Load Transducer Figure 5.43.-Three-phase power measurement with transducer. needed, three CT's should be used as shown in figure 5.45B.It is possible to connect the CT secondariesin delta or wye, but the burden impedances should always be wye connected. To observe all three line-to-linevoltages, three potential transformers can be used as in figure 5.46A. The open-delta arrangement shown in figure 5.46B is not as accurate but gives satisfactoryprecision and uses only two PT's. For current or voltage with two or three instrument transformers, power-equipment metering is performed with a voltmeter or ammeter or both. The required phase is switch selectedby connectingthe transformer combina- tion to the meter.
- 146. meter meter rneter output output output Figure 5.44.-Balanced three-phase measurement of voltage, current, and average power. -Load - : Source b C '1 Source b Load C ' I * Meters 6h.S' I , 7 Figure 5.45.-Line current measurements with two or three CT's. Meters - A b J r Load Meters - Figure 5.46.-Line-to-line voltage measurements with three or two PT's.
- 147. SPECIAL INSTRUMENTS Several special, if not very common, instruments are available to perform measurements on specific electrical quantities. These include but are not limited to watthour meters, demand meters, bridges, megohmmeters, and phase-sequence indicators. Each of these is described in the following paragraphs. Watthour Meters The watthour meter is a common power instrument, used in nearly every building to measure consumed elec- trical energy. The typical watthour meter consists of a small induction motor with an aluminum disk that is rotated by a torque proportional to voltage times current at every instant. The principle of operation is similar to that of the dynamometer wattmeter, except the disk is allowed to turn continually with a speed proportional to average power. The number of turns is counted by a train of clocklike gears. The counter thus indicates the product of power and time, or energy, which is measured in kilowatthours. A simplified sketch of the induction mech- anism is shown in figure 5.47. Demand Meters Demand meters are usually of two types (although there are others): integrated demand or lagged demand. The readings may be indicating or recording. Integrated- demand meters consist of an integrating meter element, such as the watthour meter just described, that totals the energy used over the demand interval and drives a maxi- mum indicating device, which can be a passive pointer, display, or chart. The meter can be reset manually, or a timing device can be used to return the drive to zero at the end of the recording period, thus leaving an indication of maximum demand. Lagged-demand meters provide a maximum demand indication that can be subjected to a characteristic time lag by either mechanical or thermal means, but usually the exponential heating curve of electrical equipment is followed. The demand interval is then defined as the time required to indicate 90%of the maximum value of a suddenly applied steady load; thus, maximum demand can be observed. Demand meters, whatever the type, can provide input to the power-system studies. Bridges Bridge circuits yield the most precise measurements of impedance, be it resistance, capacitance,or inductance, for two reasons: the measurements rely on null methods, and comparisons are made directly with standardized impedances that are precisely known. The term null method means that a zero reading or null indicates the correct value. The Wheatstone bridge is the most widely used of these circuits. Shown in figure 5.48, the bridge is dedi- cated to measuring resistance, capacitance,or inductance depending on its internal components. When the Wheatstone bridge is intended to measure resistance (figure 5.484), the circuit consists of two fixed precision resistances, R, and R,, which are known as the ratio arm; a variable precision resistance, &; and the Lin permanent magne< Aluminum disk Simple schematic Permanent rragnet br&e Rotating ent ,-LY-., Curren L ad Eddy currents produced voltage coil by wltoge coil (highly reactive) Disk plan view Figure5.47.-Simplified sketch of watthour meter induction mechanism. A Wheatstone bridge for resistance Audible device or meter Unknown B Impedance measurements with a Wheatstone br~dge Figure 5.48.-Wheatstone bridge circuits. Unknown
- 148. unknown, R,. A dc source suppliescurrent to the arrange- ment, and a galvanometer, G, islocatedat the center of the bridge across points b and d. The galvanometer is simply a very sensitive ammeter with a center-scalezero-reading pointer and the ability to read very small currents in either direction.R, is adjustedto provide a null reading on the galvanometer, which means the potential between b and d must be zero. With this balanced condition, the unknown resistance can be calculated by R2 R, = - R,. R, In commercially available bridges, R,, R,, and R, are all variable and the value of each is readily determined by calibrated dials. Thus, the bridge can measure resistances precisely over a broad range. To measure impedance, R, of the resistance bridge is replaced by Z,, and the unknown is now Z,. An ac source is used, together with some means of measuring the potential between points b and d. This could be a sensitive ac ammeter or an audible device such as a set of head- phones. R, and R, are then adjustedto provide a null, and the balanced condition means that Obviously, the values of Z, and 2 , depend upon the frequency of the ac source. The most typical value used is 1,000Hz. If very low resistances in the order of 10 to 1 . 0mn must be measured, the Kelvin double bridge shown in f i g u r e5.49 can be used. The circuit consistsof ratio arms RAP RB and R,, R,; a connecting link or conductor, R,; a known resistance, Re; the unknown, 4;an adjustable dc source; and a null indicator. The indicator could again be a galvanometer.The resistances r,, r,, r,, and r, are those of the connecting leads between the four-terminal bridge and the resistances to be compared (R,and RJ. Theselead resistances should be in the same ratio as the bridge arms to which they are connected; otherwise, the ratio unbal- ance will cause incorrect measurements. A small adjust- able resistor can be used to balance the lead resistances. The balance equation is thus When R, and R, are so small that R, is comparable, the term in equation 5.17 involving R, can be significant. However, if then the R, term becomes zem The source is aGustable so that current through R,, R,, and R , (the series resistance of which is small in comparison to the bridge) is large enough to allow a measurable milling current through the indicating device, G. An applicationfor the Kelvin double bridge is in the measurement of cable and conductor resistances. Megohmmeters The preceding resistance-measuring devices can be ineffective when resistance is in the many millions of ohms. An important factor here is the resistance of insu- lation, such as that around conductors (fig. 5 . 5 0 ) . One problem in these and other high-resistancemeasurements is to provide ~ ~ c i e n t potential so the resulting current can be detected by an indicating device that provides resistance readings. The instrument designed to perform these tests is called a megohmmeter ( f i g .5 . 5 1 ) ,where the unknown resistance is R,, and R, and R, serve as current- limiting resistors to protect the meter from damage. Galvanometer Figure 5.49.-Kelvin double bridge. Instrument test leads Conductor Indicating scale insulation shows resistance Conductor Megohmmeter Figure 5.50.-Megohmmeter testing insulation resistance. Figure 5.51.-Internal components of megohmmeter.
- 149. The most evident differencebetween the megohmme- ter and the preceding instruments is the hand-driven generator, which supplies the needed dc potential for measurement. The generator applies from 500 to 2,500V depending on the instrument and is tied to the resistance range desired (the higher the measured resistance, the higher the required voltage).Typically, a friction clutch is employedto restrict the generator to rated output voltage. In somemegohmmeters,the potential is frombatteries via an electronicpower supply located within the instrument. As shown in figure 5.51, the meter has two coils mounted over a gapped core. The movement is similar to the d'Arsonva1, but there are no restraining springs,sothe indicator is free to move when there is no output from the generator.If the instrument terminals are open (thatis,R, is infinite) when the generator is operated, current will flowthrough R, and coil A,, and the torque produced will force the pointer counterclockwise to the infinite scale reading. When the terminals are shorted (R,is zero), the torque produced by coil B is greater than that from coil A and this moves the pointer to a zero reading. For measur- ing an unknown resistance, the pointer location is depen- dent upon the opposing torque from the two coils, and the position is a function of R,. Another prime application for megohmmeters is the measurement of ground-bedresistances. These specialized testing procedures are covered in chapter 7. Phase-Sequence Indicators In order to prevent damage or incorrect operation, all condudors in a three-phase distribution system must be properly connected so they will provide the same phase sequenceto all equipment. Correct interconnectionscan at times be difficult to accomplish in mine power systems, especiallywith feeder and trailing cables.At present there is no standard color coding for phase conductors. The phase-sequenceindicator illustrated in figure 5.52can be used to determine the phase relationship of energized three-phase conductors. It falls in the simplest class of testing devices: indicating instruments; other examples include a light bulb with leads to test for the presence of potential, or a battery in series with a light bulb with leads to check continuity by completing the series circuit. The phase-sequence indicator consists of two light bulbs and a capacitor connected in wye, and the lamps are labeled in the two possiblephase combinations. Becauseof this arrangement, one lamp will burn brighter than the other dependingon the connections to the power system. The foregoing has provided information on several devices that are helpful in measuring mine electrical systems. Other instruments that are equally useful for specific applications include the splitcore ac ammeter, a handheld ac ammeter that has its own CT;the synchro- scope, which measures proper phase connections and the correct speed of parallel ac generators; and a frequency meter, which indicates the frequency of an electrical supply in hertz. Often there is also a need to obtain a continuous record of an electrical parameter, and the next section discusses the popular recording devices. RECORDING INSTRUMENTS Many of the direct-reading indicating instruments just presented are also available asrecording instruments. Some of these are very similar to their indicating counter- parts in that they can use the same electrical movements, they differ because the pointer is also used to provide a graphic record on a chart. These are termed chart record- ers; one popular class is strip-chart recorders, so named because the electrical parameter is recorded on a strip of paper. The similarity between the movement of the strip chart recorder and the indicating instruments is illus- trated in figure 5.53.The stripchart recorder movement is actually a d'Arsonva1type. The pen can trace on paper in several ways. Inking.The pen is a capillary tube through which ink flowsfrom a well to the chart. This is perhaps the most used system. ZnkZess. The tip of the pen is a stylus that impacts the paper like a typewriter key with a regular force supplied by a cam, leaving a series of dots. Thermal. The pen tip contains a heating element that leaves a trace by heating specially treated paper. CBA 0 A c Capacitor e ! k Figure 5.52.-Phase-sequence Indicator. Input circuitry M condition input voltage Comparator, compares input with ond establish sensitivity reference,outputs o wltage in of the recorder proportion to the needed position of the servomotor / Amplifier, amplifies comparator output to Circuitrv to condlt~on pen- 2 sensor output Servomotor, ond estobl~sh drives mechanical reference, pen system including to zero settlng w Paper strip /* chart, driven at various icator and constantspeeds - inking pen Figure 5.53.-Strip-chart recorder.
- 150. The simplest unit provides a curved recording as the pen swings in an arc, but articulated pen arms are also available that produce linear or rectilinear traces. The paper chart moves past the pen at a predetermined speed driven by an electric motor or a mechanical-springclock- work mechanism. This recorder provides a continuous record of the average or rms value of the electrical param- eter of interest, which is advantageous in obtaining records of equipment operation,for example,the electrical performance of a mining machine. A variation of these recorders uses a round chart, driven like a disk on a record player but at very slow speed. These charts can be built into major equipment to provide permanent records. Sometimes recordings of the actual electrical wave- forms are needed to study power systems. This calls for an instrument that can resolve instantaneous values of elec- trical parameters. Electromechanical instruments that have this resolution are called oscillographs, and the movement in most of these is a sensitive galvanometer o f low mass. Two types of writing systems are normally available: Direct writing. This is similar to either the inking or thermal strip-chart recorder types. The pen has high inertia, and instrument response is about 0.5 to 100 Hz (some to dc). Optical. Instead of a pen, the movement drives a low-massmirror that deflects a light beam that exposes a light-sensitive paper. Developingis required to obtain the record, but the system can have resolution to 10,000Hz. For many applications, magnetic tape recorders and oscil- loscopes,both electronic instruments, find favor over oscil- lographs. However, oscillographs still have some practical use, especially where an extended-time hard copy is needed immediately. An example would be in measuring neutral currents existing on three-phase equipment, which can have dc as well as ac components. ELECTRONIC INSTRUMENTS The employment of complex and sophisticatedcontrol equipment in the mining industry is continuing to in- crease. Instances include solid-state motor starters, elec- tronic protective relaying, computer logic circuits on min- ing machinery, and so on. These types of systems require precise voltage, current, and waveform measurements that are not possible with the preceding instruments. Certain phenomena existing on power systems, such as transients, require precise measurements with frequency response into the megahertz. Electronic measuring equip- ment answers this need. This section will introduce only the more popular instruments. Electronic Meters These instruments use many of the basic circuits that have been described for multimeters; that is, series resis- tances for voltage (fig. 5.544), voltage-dropfor resistance (fig. 5.548),and shuntsfor current. The prime difference is that a scaled-downdc voltage, which is proportional to the actual circuit voltage, current, or resistance, is amplified by electronics. When the parameter is sinusoid, the ac is rectified before amplification. The amplified signal then drives the indicating device. In the past, vacuum tubes Mult~plier resistors Filter,removes Range switch any ac superimposed selection Amplifier, amplifies of sensitivity Calibration I00v Meter V i , , 1 4 - Feedkk, stabilizes am~lifiercharacteristics I 4 Full-scale sensitivity of voltmeter A dc voltmeter Range, value per division on meter scale - - , , Amplifier, amplifies dc voltage L 1 , ~ ~ ~ fl drop across unknown resistance. whvki;qzportional to I ~ S Res~stance being measured Meter, I $ fthrouah measured I B Ohmmeter Figure 5.54.-Input circuits on electronic voltmeter. performed the amplification, termed a uacuum-tube volb meter or VTVM;more recently, solid-statedevices (IC's or FET's) have become the most popular. The indicating device can be of two types: the familiar d'Arsonva1 movement or a digital indicator. The electro- mechanical displays or movements described thus far can be termed analog. The digital display is an indicating output assembly that takes the measurement results (voltage, current, average power, etc.) and through elec- tronics gives a visual indication in a discrete number, as shown in figure 5.55. The actual display can be by Nixie tube (a gas-discharge tube), seven-segment incandescent filament, light-emitting diodes (LED), or liquid-crystal display (LCD).The electronicsin the display assembly use logic or binary mathematics to convert the analog output of the measurements and drive the visual display. These digital displays are replacing their analogcounterpartsin many applications. Electronic meters might appear rather complicated, but an important advantage is gained through the cir- cuitry: the instrument can be made sothat its interference with the circuit being measured is negligible. Typical input impedance of most electronic voltmeters is 11Mil. Oscilloscopes Oscilloscopesare electronic instruments that provide a real-time display of waveforms. They are available with responses from dc to hundreds of megahertz and thus can
- 151. ANALOG -TO -DIGITAL CONVERTER, generates time intervol in proportion to input voltage Counter, measures time intervol and outputs o sequence o f pulses to display the result, at a h ~ g hrate such thot disploy appears continually reference; when the output Of that device changes from zero to a positive voltage; the 2 important comparison points are zero (for start Sampling-rate generator, establishes pulse) and the input Reference (romp) Display (7-segment devices ore shown); voltage magnitude generator, produces the starting point for output pulses from counter activate (stop pulse an increasing voltage each measurement and proper digit and segments o f thot waveform to be the time between digit, then another digit and its proper compared with the consecutive segments, sequentially until the ~nput measurements number is displayed Figure 5.55.-Digital display. be used to observe a large range of electrical phenomena including those of extremely short duration. The reason these instruments have such a broad frequency range is that they are not constrained by mechanical inertia. The heart of the oscilloscopeis a cathode-raytube or CRT (fig. 5.56). A fine beam of electrons is deflected by an electro- static field in relationship to the voltage or current being investigated. The beam then impinges on a fluorescent screen to create a luminous display. The electrostatic field is normally established by two pairs of deflecting plates; one provides deflection vertically, the other horizontally. When a waveformis observed,the horizontalpair is driven electronicallyby a sweepsignaltoprovide a time base, and the vertical pair creates a field in response to the instan- taneous value of the electrical parameter. In some CRT's, additional pairs of vertical plates are availablethat enable more than one trace to be displayed on the screen. This allows direct comparison of two or more waveforms. A camera can be used in conjunctionwith the oscilloscope to provide a permanent record. Tape Recorders The familiar magnetic tape recorder records a signal by magnetizing a thin strip of tape. The nonmetallic tape is coated with a very thin layer of magnetic material such as iron oxide, thereby providing a relatively permanent record of a signal. The signal can be an analog recording, or digital, or in direct relationship to the measuredparam- eter. With the digital recording, the signal is converted electronically to the binary system and the binary coun- terpart is recorded. The recording can be played back numerous times, and the output very closely matches the input that was observed. Analog tapes, which can have frequencyranges from dc to over 20,000Hz, can be used as input to strip-chart recorders to provide hard copies or input to various electronicinstruments that perform anal- ysis of the electrical parameters. Digital recordingscan be made compatible with digital computers for swift and elaborate analysis of the data. Transducers perform the transfer of information from the power system to the electronic instruments. A trans- ducer can be described as a device that provides an electrical signal output in response to a specific measure- ment. Thus, potential transformers and series-dropping resistors can be considered voltage transducers, and cur- rent transformers and shunts, current transducers. Another popular current-sensing device employs the Hall-effect principle. Hall-effectcurrent transducers mea- sure the effect of an electromagnetic field on a semicon- ductor. Basically,such devicesoperate on the interaction of magnetic force and the movement of charge through a semiconductor.Consider figure 5.57 in which a current, I,, is flowing and a magnetic field is acting perpendicular to the current. The magnetic field will deflect the charge carriers in proportion to the field strength. This action produces a Hall-effect voltage, as shown, which is in
- 152. Vert~cal Attenuator, restr~cts Attenuator and Delays vert~caldeflection input the range of the preamplifier may be until sweep generator starts terminals signal to ampl~fy o combined plug-in so full waveform is unit for versatility displayed on CRT screen Deloy network I - - To internally synchronize sweep generator ,u/with the vertical s~anal Generates precise sawtooth voltage waveform with Electron gun emits, controls, accelerotes ond focuses electron Attenuator and preamplifier may be used to drive horizontal a combined plug-in deflection plates from unit for versatility Horizontal -A external source if desired scale, t~me Simplified block diagram Sinusoid displayed with sweep Figure 5.56.-Cathode-ray tube. Input resistance Magnetic Control current KEY x, y, z Dimensions of Hall-effect device Figure 5.57.-Semiconductor Illustrating Hall effect. proportion to the magnetic field. Current flow in a conduc- tor produces an electromagnetic field that can be mea- sured by a Hall-effect device, thus producing a voltage output in proportion to the current. Most times, the magnetic field requires concentration. In some Hall-effect instruments, this is performed by a core of magnetic, low-retentivity material that can be clipped around a conductor. The semiconductor is mounted on the core and oriented at right angles to the induced magnetic field. The combined unit appears much like a splittore CT, and through this method dc as well as ac currents can be measured with high precision. Many instruments used for precise power-system measurements employ Hall-effect devices. The output from a transducer is sometimes incompat- ible with the input of the instrument, or for safety reasons is not isolated from the power system. In these instances, the signal requires conditioning, and electronic circuitry, usually amplifiers, is called upon to perform the task. INSTRUMENT INSTALLATIONS It is common to find several instruments included as part of major power-equipment circuitry. As a summary to this chapter, the following describes the typical locations for measuring instruments within a power system. 1.The termination of utility transmission lines: Voltmeters, Ammeters, Wattmeters, Varmeters or power factor meters, Watthour meters, Demand meters, and Frequency meters.
- 153. 2. Substation secondaries(outgoingdistribution): Voltmeters, Ammeters, Wattmeters, Varmeters or power factor meters, Watthour meters (demand attachment optional), and Test blocks (or connection points) for portable instruments. 3. Switchhouses,load centers, and rectifiers: Voltmeters, Ammeters, and %st blocks for portable instruments. 4. Machinery: Voltmeters (optional), Ammeters, Elapsed-timemeters (optional),and Watthour meters (alsooptional). Two points must be considered when applying the above listing. At the higher transmission voltages, say 69 kV and up, it is sometimes advantageous to have the utility metering point at the substation transformer secondary. This would eliminate item 1 and might add additional instruments to item 2 for reasons of economics.The capital required for high-voltage metering can prohibit its use. The test blocks listed for portable instruments are neces- sary because they provide an easy avenue for maintenance and trouble shooting. In general, the test blocks consist of a series of terminals to which permanent connections are made to important circuit portions, such as major compo- nents. The block is located on the surface of the equip- ment, accessible only to maintenance personnel, and can eliminate the need for some permanent instruments. However, as was mentioned at the beginning, voltmeters and ammeters are considered to be the minimum perma- nent instrumentation within mine power equipment.
- 154. CHAPTER 6.-MOTORS AND MOTOR CONTROL The subject of this chapter is the electromechanical conversion equipment that links electrical and mechani- cal systems and makes it possible to convert from one energy form to the other. The primary electromechanical devicesare generators and motors. In generators, mechan- ical power is used to generate electrical power. Electric motors can be viewed as generators in reverse; they convert electrical power into mechanical power. The word "motor" can be applied to a device that converts energy of any form into mechanical power, but for purposes of this chapter the term is restricted to those machines that receive electrical energy. Generators have limited but important applications in most mining operations. The principal functions are in motor-generator (m-g) sets for surfaceexcavating machin- ery and mine hoists, and for providingemergency power to ventilation fans and hoisting equipment. Motors, on the other hand, are used so extensivelythat they are the most important mechanical source in mining machinery and the most important loads on the mine electrical system. By far the majority of mines, milling plants, preparation plants, and other related mining activities would find it virtually impossible to operate without electric motors. Generators and motow can be studied independently of each other, but comprehension of motor operation is more easily obtained when generation is covered first. Consequently, the chapter will follow this format. In view of their relative importance,motors and their control will be the principal chapter discussion. The content will be elementary, but the objective is to provide sufficientinfor- mation so that the effect of motors on the mine power system and specific motor applications in mining can be appreciated. ALTERNATING CURRENT GENERATION In chapter 2, it was demonstrated that a voltage is induced in a conductor when there is relative motion between the conductor and a magnetic field. This electro- magnetic induction concept, called Faraday's law, is the basic principle behind the generation of voltage in electric I machines. The following paragraphs return to this funda- mental concept but in a slightly different fashion and serve as a transition between induced voltages in induc- I tors or transformers and generators. In figure 6.1, a conductor is under the influence of a magnetic field. If a force is placed on the conductor so that Force on conductor Figure 6.1.-Production of voltage from magnetic field (emf = electromotive force). it moves at right angles to the magnetic-field direction, a voltage will be induced in the conductor. Obviously, when a conductor is part of a closed loop, current will flow. The direction of the current produced depends upon the direc- tions of the magnetic flux and the conductor movement. Fleming's "right-hand ruleJ'is often used to determinethe current direction, and it also helps to illustrate the inter- relationships amongthe three dependentparameters. The thumb, forefinger, and center finger of the right hand are stretched out so they are mutually at right angles to sach other. If the hand is placed such that the forefinger points in the flux direction, with the thumb pointing in the direction of conductor motion, the center finger points in the direction of current flow. When the conductor cuts a magneticfield at a specific relative velocity, it has been found (3)' that the instanta- neous magnitude of the voltage induced, e, can be calcu- lated by where B = magnetic field flux density, T , P = conductor length, M, and v, = conductor velocity at right angles to magnetic flux field, mls. Accordingly, the magnitude of induced potential de- pends upon the flux density, the conductor length, and the conductor velocity relative to the magnetic field. By vary- ing these parameters, a voltage of almost any magnitude can be theoretically produced. It is this electromechanical principle, an adaptation of Faraday's law, that is utilized in the generation of alternating and direct currents (ac and dc). Principle of Generator Operation Figure 6.2 illustrates the basic principle of ac gener- ation. Consider a loop of conductormounted on a mechan- ical drive shaft through an insulating block and rotated in a magnetic field (9).A circular metallic ring, called a slip ring, is connected to each end of the loop, and brushes contact each slip ring surface to allow the connection of stationary conductors. When the loop is rotated by a mechanical drive, a potential is induced in each side of the loop that is proportional to the conductor velocity at right angles to the magnetic field. At the position shownin figure 6.2, the induced voltage is at a maximum (relative right-angle velocity is maximum),but at 90° from this position there will be no induction (relative velocity is zero). The instan- taneous voltages for the entire loop can be added algebra- ically as they are in series. Continued rotation will pro- duce a sinusoidal voltage (fig. 6.2B) and thus sinusoidal current, if the loop is part of a closed circuit. Generator Construction It has been shown that there are basically two re- quirements for generation. The first is a conductor or Italicizednumbers inparenthesesrefertoitems in the list of references at the end o f thia chapter.
- 155. rAxis o f stator field I '-i-'dJ Axis o f rotor field L Vertical axis I Flgure 6.2.-Demonstration of ac generatlon. winding in which a desired voltage is to be induced. This is termed an armature winding,and the structure enclos- ing it is called an armature. The second requirement is a magnetic-field source, and this is normally created by a field winding, although some very small machines use permanent magnets. To classify the rotating and fixed machine portions, the rotating member is referred to as the rotor, while the stationary portion is the stator. The windings are placed on two concentriccylindrical iron cores with a small air gap between so that the flux path in the machine is as efficient as possible. The inner coreusually servesas the rotor. Thin laminations, attimes insulated from each other, are employed to minimize eddy-current loss, as in transformer construction. The structure of either core is one of two types: salient poles or nonsalient poles, which form the center lines of the magnetic field. Salient poles stick out from the cylinder surface (fig. 6.3) and have the windings around them but located near the core surface in the vicinity of the air gap. Nonsalient poles are part of a completely cylindrical surface, with the windings positioned in slots (fig.6.4) (3). The conductors are usually insulated from the core. Sur- rounding the cores and windings is a structure called the frame, with some form of end enclosure. The frame serves to anchor the stationary machine elements to a founda- tion. The end enclosure may contain bearings, of either sleeve, ball, roller, or needle types, that support the rotor shaft and position the rotor properly with respect to the stator. Figure 6.5 is a sketch illustrating these physical components. The function of an electromechanical machine is commonly described in terms of the number of available magnetic poles in the field. Thus, the elementary machine Figure6.3.-Cross section of machinewlth sallent paleaon stator and nonsallent poles on rotor. Axis o f stator field Stotor Axis o f rotor field Rotor ' N A i r gap S I Figure6.4.-Cross sectionof machinewlth nonsallentpoles on stator and rotor. Frame 1 Stotor core Flgure 6.5.-Simpllfled sketch of electromechanical machine illustrating physical components. in figure 6.2 is a two-pole generator. Most generators, however, have more than two poles, usually even numbers of four, six, eight, and so on. Figure 6.6 shows an elemen- tary four-polegenerator (9).Here the armatureneeds only to turn 180° to produce a full sinusoidal cycle in ita winding output.
- 156. The foregoing terminology applies to all electrome- chanical rotating machinery. For practically all ac gener- ators or alternators, the armature is contained in the stator. The field windingis part of the rotor,with a dc field current suppliedthrough slip rings, which is the reverseof the situation discussed previously. Three-Phase Generation A preliminary discussion of three-phase power gener- ation has already been presented in chapter 4 but only in the context of balanced three-phase systems. This section will elaborate on its electromechanicalconversion. Consider figure 6.7, a cross-sectional view of an ele- mentary three-phase, two-pole generator. The machine is termed twepok because of the number of available mag- netic poles in the field winding. Located in the stator, the armature has three single-conductorwindings a, b, and c, whose axes are 120° apart. The rotor containing the field winding is turned at a constant speed by a mechanical power source connected to the rotor shaft, and the field winding is excited by dc. The magneticdux distribution around the air-gap circumference of the machine is de- signed so it forms a sine wave (3).Therefore, the induced voltage in each armature windingvaries sinusoidallywith the familiar 120° displacement among the three gener- ated potentials. For this two-polegenerator, the sinusoidal voltage in each phase winding goes through one full cycle per rotor rotation. The waveformfrequency(hertz)is identicalto the rotor speed (revolutions per second). The sinusoid is thus in time with or synchronized with the mechanical speed, and such ac generators are often termed synchronous genemtors. For 60-Hz output, rotor speed is 60rls or 3,600 rlmin. An example of an elementary four-pole generator is shown in figure 6.8A. Here, the rotor poles alternate between north or south polarity when rotated. Each phase of the armature consists of two windings conneded in aeries, as shown in figure 6.88. The induced voltage per phase thus completestwo cycles for each rotor revolution. DIRECT CURRENT GENERATORS A very elementary two-pole dc generator is shown in figure 6 . 9 .The illustration differs from figure 6 . 2only in that the dc generator has a commutatorin place of the slip rings.The commutator is an annular ring that is split into parts (in this case, two), which are insulated from each Flgure 6.6.-Elementary four-pole, single-phase ac generator. - - . , / Armature winding Armature structure or stator < a Field winding excited by dc thrwgh slip rlngs Field pole produced bv dc in field F~eld structure- w~nd~ng or rotor Flgure 6.7.-Elementary two-pole,three-phasegenerator. @: A 0 -c c' - C -ti -ab 6 C -b -0' -C' b b' 4 Cross sect~on 8 Armature schematic Flgure 6.8.-Elementary four.pole, three-phasegenerator. Generator Voltoge output versus time Flgure 6.9.-Demonstmtlon of dc generation.
- 157. other. Each part is termed a commutator segment. As before, carbon brushes contact the ring surface to allow connection of stationary conductors. During armature rotation, the voltage produced in the loop is a sinusoid,as in the ac generator. The commutator serves to rectify the waveform mechanically since at all times the positive and negative brushes are connectedwith the correct armature- winding polarity. In other words, the connection to the loop reverses or wmmutates every one-half revolution. Thus, the generator output waveform is the same as full-waverectification (fig. 5 . 5 ) .If the rotational direction reverses, so does the brush polarity. Because of the ripple voltage, the two-poledc genera- tor is not realistic. In practical dc generators, the arma- ture consists of many windings, with the commutator having a corresponding number of segments (fig.6.10).In such a case, current from the generator will never drop to zero. When the number of armature windingsis increased, the output ripple voltage decreases,and the average direct voltage will be closer to the peak voltage. Unlike ac generators, dc generators have the arma- ture winding on the rotor and the field winding in the stator. The field must be excitedby dcprovidedby a source, which may be either external or internal. The internal excitation is possible because the armature is a dc source and can supply current to the field as well as the load. However, in order to start generation, the stator core of these machines must have residual magnetism. Gener- ators connected in this way are called self-excited.When the source is external, the generator is termed separately acited This is diagrammed in figure 6.11. Self-excitedgenerators have three configurations,de- pending on the field-winding connection: series, shunt, and compound, as shownrespectively in figures6.12,6.13, and 6.14The terms series and shunt relate directly to the winding connections. The compound generator has two windings, one connected in series and the other shunting the armature. Each of the generator connectionshas a characteristic voltage output versus load current (3).Because the field, armature, and load currents are the same in series gener- ators, the output voltage fluctuates widely with the load. Hence, this connection is rarely used. Although shunt generator voltage output drops slightly as load current is increased, the regulation is satisfactory for many pur- poses. Compound generators are normally connected so the magnetic actions of the shunt and series windings aid each other. The resultant magnetic flux of the field can increase with load current, causing the output voltage to remain nearly constant. The level of output voltagein both the shunt and compound generators can be controlled by the variable resistance in series with the shunt field winding. The resistance in the separately excited genera- tor provides the same function, but precise output-voltage control is obtainedbecausethe field-windingcurrent isnot a function of the load. r Field winding To dc source A& Arrnoture Field rheostat Figure 6.11.-Separately excited dc generator. Field winding Armature To load Figure 6.12.-Series dc generator. , ,,-Field rheostat Brush Cornmutotor Brush Generator O" 90" 180° 270' 360' Voltage output versus time Figure 6.10.-Dc generator with two armature windings al right angles. L ~ i e l d winding Figure 6.13.-Shunt dc generator. {Series field [Field rheostat J# 0 Shunt field Figure 6.14.-Compound dc generator.
- 158. MOTOR BASICS The essential motor parts are similar to those of a generator and include Two concentric cylindrical laminated-iron cores, separated by an air gap, to carry magnetic flux; Two sets of windings,wound or embedded in slotsin the iron cores, either or both excited by dc or ac; and The inactive motor elements, including the frame, end bells, bearings, and so forth. Combinations of these parts are found in practically all motors. Motors employ electrical energy to produce me- chanical force, which is the reverse process from generator operation. The force of interest in motors is that which tends to produce rotation, or torque. Torque Motor torque considerations are based on the funda- mental principle that a mechanical force is exerted on a current-carrying conductor in a magnetic field. A graphic exampleof this situation is shownin figure 6.15.Here, the magnetic field that surrounds the conductor (due to its current) interlinks with the largest magnetic field. This creates a large concentration of magnetic flux at one side of the conductor,which tends to forcethe conductor toward the lesser flux concentration. The result is an instanta- neous force, f, at right angles to the magnetic field. The magnitude of the force depends upon the mag- netic field flux density, the conductorlength, and the level of instantaneous current, and can be calculated for a straight conductor by (3) where f = force, N, B = magnetic field flux density, T, P = conductor length, m, and i = instantaneous current, A. If the conductor is fixed by a radial distance, r, from the center of a rotor shaft, the associated torque, T, is (3) T = Blri, (6.3) where T = torque, N.m, and r = radial distance or moment arm, m. For a winding, the total torque is the summation of the torques for the individual conductors or coil sides. For electromechanicalmachines, this mechanical quantity is termed electromagnetic torque, and when combined with rotation the resultant power quantities follow the rules of mechanics. Another way of visualizing the development of motor torque is the interaction of two magneticfields.A mechan- ical force is exerted on magnetic material, be it a perma- nent magnet or magnetism created by electric current flow. The force tends to align the material with the closest part of a magnetic field, so the north pole of one machine member is directly in line with the south pole of the other member. If the force is acting at a moment arm about a rotor shaft, torque is produced. Even though equation 6 . 3 is expressed in newton meters (adhering to SI units), the quantities normally used are pound-feet, ounce-inches,and gram-centimeters. The common method of relating motor mechanics is by reference to a percent of full-loadtorque. Speed-Torque Relationships Speed-torquecurvesarethe mechanical characteristic curves of a motor; a general example for an induction motor is provided in figure 6.16 for discussion ( 1 5 ) .One application for these curves is to find the most suitable drive for a given machine. As the machineload can alsobe described by a speed-torquecurve (see load torque in the figure), the comparison of the load and motor curves will show if the motor has the necessary characteristics to drive the load and also what the operating point will be. The operating point is the intersection of the two curves. Many other parts of the motor characteristic relate its suitability for a specific application,and someof these are listed below and are shown in the labels of figure 6.16. 1 . Locke&rotor torque. The minimum torque devel- oped by a motor at the instant of power application, sometimescalled breakaway or starting torque. 2. Accelerating torque. The torque developed during the period from zero to full rated speed with rated power applied. The term is ofien used for the net torque between the motor and the load. It is apparent in the figure that this is a nonlinear value with speed. 3 .Breakdown torque. The maximum torque possible from the motor with rated power input, also called maxi- mum torque. 4. Pullup torque. The minimum torque developed during motor acceleration from zero to full rated speed with rated power applied.The minimum can exist in some motors at full rated speed. 5 .FulGload torque. The torque necessary to provide rated output at rated speed with rated power applied. Force on 4conductor Conductor corrying current Flux produced by Resultant distortion in o magnetic field conductor with of magnetic field respect to field Figure 6.15.-Current-carrying conductor in a magnetic fleid.
- 159. 6. Pullout torque. The maximum torque produced by a motor without stalling. This is sometimes incorrectly referred to as the maximum or breakdown torque. If a torque is applied to a motor above this value during operation, it will stall. The other motor terms listed in figure 6.16 are tied to specific motor types or operations yet to be discussed. Tbe National Electrical Manufacturers Association (NEMA) sets standards for the manufacture of electric motors (15),which are used throughout the mining indus- try. NEW standards generally cover seven areas: speed- torque characteristics, frame size, enclosure, horsepower rating, voltage, temperature rise, and application. Al- though machines from different manufacturers should be I Breakdown toraue1 directly interchangeable when they conform to a particu- lar NEMA standard, there may still be some variation between manufacturers. Frame Size Most motors of 250 hp and under are rated according to a frame number that specifies the essential mounting dimensions(fig. 6.17) (15). The same frame number series carers all ac or dc motor types, and a dozen or more different motors might have the same frame. Enclosure Motor enclosures are usually classified as open or totally enclosed Open motors simply have openings, usu- ally in the end plates, to allow air coolingof the windings. Totally enclosed motors prevent passage of air into the enclosure, but these motors are not always sufficiently closed to be air tight. In this general class are the explosion-proofmotors(seechapter 16),dust-ignition-proof motors, dust-tight motors, and waterproof motors. Cooling for these motors can be by air conduction on the outer frame, internal forced air through a pipe,or a liquidcooled (water or oil)outer jacket surrounding the frame. Horsepower Motor horsepower is also standardized, such as 112, 3/4,1,1-1/2,2,3,5,7-1/2,10, 15,20,25,30,40,50,60,75, 100,125,150,200, and 250 at speedsfor 2 to 16poles with 60-Hz operation (15).Abwe 250 hp, the standard powers are related to motor type. When a horsepower is given, it is often combined with a service factor to allow for usual fluctuations in supply voltage or slight overloading. The service factor indicates the permissible overload and is a multiplier applied to the normal horsepower rating, with values ranging from 1.0 to 1.4 depending on the size and type of motor. For instance, a 1.15 service fador would I I Zero speed Synchronous Flgure 6.16.-General speed-torquemotor characteristic. H-diam. ' 4 holes Dimensions in inches Flgure 6.17.-Examples of three frame number dlmendons.
- 160. indicate that a motor can carry 15%more than rated load continuously without overheating (that is, exceeding the rated temperature rise) as long as the frequency, ambient temperature, line voltage, and so on are at rated values. An interesting situation occurs with frame sizes pri- marily intended for motors of less than 1 hp vi.actional horsepower).A fractional-horsepower motor is considered to be any motor built in a fractional-horsepower frame, even if the actual horsepower rating is in excess of 1hp. Most of these motors, however, are only available for single-phaseac. Voltage NEMA voltage designations are specified for most motors, be they three-phase, single-phase, or dc (15). A listing of the voltage ratings (alsocalled the motor termi- nal voltage)common to mining is available in table 6.1. A maximum voltage variation of *10%from rated is per- mitted. With ac motors, the allowable frequency fludua- tion for the power supply is *5%. lkmpemture Rise The allowable temperature rise from an ambient temperature is dependent upon the class of insulation used in the motor (15).The electrical insulation system is one of the most important components of a motor, as its degradation seriously affects the reliability and service life of the motor. Insulation systems are divided into four classes, A, B, F,and H, depending upon their thermal endurance. Ratings for industrial motors are typically based on a 40° C ambient temperature, but some are based on 25O C. Table 6.2 provides the allowable rise for each insulation class; also included are the common insu- lating materials and the maximum "hot-spot" tempera- ture, which is the highest temperature allowable at any part of the motor. Motors built to a specific class and operated so the recommended temperatures are not ex- ceeded may be expected to have a serviceable life of 20 yr with minimal maintenance. However, physical abuse and the electrical stresses discussed in chapter 1 1can seri- ously shorten the motor life regardless of insulation class and operation. ClassA insulated motorsare rare in almost all applications, except for very small horsepowers. Class F and H insulations are most often used in motors for mining applications (17).The maximum surfacetempera- ture of any permissible mining motor must not exceed 150" C (see chapter 16). If the maximum ambient temperature is greater than specified, the allowable temperature rise must be de- creased by the difference in temperature above ambient. Maximum ambient is the highest temperature the motor is normally exposed to. The rise may be increasedby a like amount when the maximum ambient temperature is be- low that specified. These specifications are applicable when operating under typical barometric pressure, aslong as the altitude does not exceed 1,000 m (3,300 ft). Above 1,000 m, the allowable temperature rise must be reduced 1.0%for each 100 m (330ft) above 1,000m. Additional classification standards will be discussed in the following sections, for example, applications as to load-speedand load-torquerequirements. These aretied to the torque-speedcharacteristics, which are related to the motor type. Table 6.1.-Motor voltage ratings common to mining System type ' Nominal system Motor rated voltage, V voltage. V %phase, low voltage.......................... 208 230 3-ohase. medium voltaae................... . . - &phase, high voltage ........................ 2.400 4.160 7.200 12,470 13.200 13.800 Direct current..................................... 300 MX) Sinale ohase ..................................... 120 - . 240 230 ' System voltage designations follow 30 CFR 18, 75, 77. =WYB More suitable for stationary equipment applications. 'Intended use is mobile mining equipment. Table 6.2.-Motor lnsulatlonclasses Temp rise. O C ' ~~i~~~ Insula- tion Open Totally hot-SPot Common insulating motors enclosed temp, materials motors 'C A............... 50 55 105 Cotton, cellulose. paper, organic, enamel-coated wire. B............... 70 75 130 Mica, glass fiber, asbestos. F............... 90 95 155 DO. H .............. 105 115 180 Mica, glass fiber, silicone elastomers, silicone resins, asbestos. 'Allowable rise from ambient temperature. Each class has compatible bonding agents for the materials shown. Motor Type The classification of motor types, which is dependent upon how the stator or rotor windings are excited,results in three general motor classes: induction, synchronous, and dc. The first two are ac machines, and for many applications these are more rugged, require less mainte- nance, and are less expensive than dc motors of equal horsepower and speed ratings. Ac motors can be used effectively for the majority of motor applications except when very high starting torques are required. The most widely used ac type is the squirrel-cage induction motor, so-calledfor its appearance. It has no slip rings, commu- tator, or brushes to wear out and uses the simplest kind of starting equipment. Three-phase squirrel-cage induction motors and series-wounddc motors are the most popular electromechanicalmachines in mining. After the presentation in chapters 2,3, and 4, it could be expected that three-phase motors would be more com- plex than their single-phase and dc counterparts and therefore more difficultto understand. However, although some parts of three-phase motor construction are more complex, their operation is simpler. As just stated, induc- tion and synchronousmachinesarethe two major ac motor types. Synchronous motors correspond to three-phasegen- erators. In typical large machines,dc is applied to the field winding located in the rotor, while three-phase ac (instead
- 161. of being generated) is supplied to armature windings placed in the stator. Inductionmotors alsoreceiveacpower at the stator windings, but ac is delivered to the rotor winding indirectly by induction, in the same manner as in a transformer. THREE-PHASE SQUIRREL-CAGE INDUCTION MOTORS In order to comprehend the operation of induction motors and understand important terminology, it is per- haps best to start with a simple demonstration. Although the motor does not have familiar motor components, its construction and operation do have direct application in induction-diskrelays and watthour meters. Considerfigure 6.18, which depicts an aluminum disk and a horseshoe magnet (8). Both are mounted about the same axis and are free to rotate. When the magnet is rotated, the disk cuts the magnetic lines of force,a voltage is induced, and eddy currents will then flow. Under the magnet's south pole, the eddy currents set up north magnetic poles, and conversely for the north pole of the magnet. Because the pole attracts, the disk rotates, follow- ing the magnet. The disk can never reach the magnet speed, as there would be no relative motion between the two (that is, no induction would exist). The mandatory difference in speed for induction motors is called slip. In conventional induction motors, the action of the disk occ_ursin a rotor winding, and the rotating magnetic field is supplied by the stator winding. For induction-disk relays, the aluminum disk is the same but the induction force is supplied by an ac-driven stator (see chapter 9 and also the description of watthour meters in chapter 5). ElementaryThree-Phase Motor Figure 6.19 illustrates an elementary two-pole,three- phase squirrel-cage induction motor (11). The stator con- sists of three salient poles spaced 120° apart; the stator windings around each pole are connected in wye and energized by a three-phase system. The rotor has three main elements: a shaft (not shown),core, and winding. As with generators, the rotor core is made of iron laminations pressed onto the shaft. The squirrel-cage winding is constructed by embedding heavy copper or aluminum bars in the core slots. The bars are connected to each other by copper or aluminum rings located on both core ends, which complete the closed circuit. In other words, there are no external connections to this rotor winding either by slip rings or a commutator. Figure 6.20 shows the winding construction. When the stator windings are powered by a three- phase system, currents through the coils reach their respective maxima at differentintervals in time. Sincethe three currents are displaced by 120°, the magnetic field generated by each coil is also displacedfrom the other two by 120° (fig. 6.21A). The magnetic field of each winding alternatesfrom north to south;thus, each has the action of two poles. Figure 6.21Bshowsthe instantaneous direction of a stator flux as it passes through the rotor at different time intervals. At zero degrees, for instance, phase A is at maximum north, while phases B and C are weak south poles. At 60°, phase C becomes strongly polarized in the south direction and phases B and A are weak norths. The larger arrows shown in the figure represent the instanta- Permanent aluminum Bearing Pivot Iron plate A Front v i e w B Top view Figure 6.18.-Demonstration of induction-motoroperation. Figure 6.19.-Elementary three.phase induction motor. Figure 6.20.-Squirrel-cage rotor winding. neous direction of the resultant two-pole magnetic field. Consequently, for this example, the magnetic field is rotating counterclockwise. In a transformer, voltages are induced in the second- ary circuit by the primary. The stator of an induction motor ads in the same manner as the primary, with the rotor winding acting as the secondary winding. The rotat- ing magnetic field of the stator cuts the rotor conductors, and motor actionis developed.The relative motion,or slip, between the rotating flux and the rotor generates voltages within the rotor conductors. According to Lenz's law, the voltage induced in each rotor bar will be in a direction opposing the relative
- 162. motion of the rotating flux and the rotor. The induced- voltage direction in the rotor conductors under the intlu- ence of a two-pole rotating magnetic field is shown in figure 6.22, where positive implies that the voltage direc- tion is toward the viewer. The voltage magnitude in each bar depends upon the stator magnetic-fielddensity at that point. These voltages cause currents to flow through the bars, an end ring, adjacent bars, and then back through the other end ring to the origins, in complete loops. The circulating rotor currents produce magnetic fields about each rotor bar. The interaction between the stator field and the fields around the rotor conductors results in a mechanical couple and thus motor torque. Hence, the rotor will rotate in the same direction as the stator field. A simple reversal of any two phase conductors to the stator windings of a three- phase induction motor will reverse the stator phase se- quence and thus reverse the motor rotation. Electrical degrees The speed at which the stator field rotates is termed the synchronous speed of the motor and can be calculated from where n, = synchronous motor speed, rlmin, f = line frequency, Hz, and p = number of magnetic poles presented by stator. As slip is required to produce rotor induction, a squirrel- cage motor may approach but never obtain synchronous speed. Slip can be expressed mathematically as where s = motor slip, expressed as a per-unit decimal or a percent, and n, = actual motor speed, rlmin. Losses will occur in actual motors because of electrical and mechanical inefficiencies. Those prominent in induc- tion motors are Rotor winding loss, related to 12R; Stator winding loss, also an 12Rloss; Stator core loss, caused by eddy currents and hys- teresis in the core iron; and Friction and windage (rotational or mechanical) losses. These are almost pure active powers; therefore they are often expressed in watts. The losses in both windings of induction motors vary as the square of line current, core 60' 120" 180° loss is nearly constant, and unless motor speed varies considerably rotational losses are nearly constant (11). Knowledge of machine losses allows the determination of motor heating and efficiency. The efficien of motor operation is a measure of the ability to convert input power to mechanical power: X)O" 360" output input - total losses Efficiency = input = input , (6.6) Figure 6.21.-Rotating magnetic field in elementary three- phase, two-pole inductionmotor. which may be expressed as a per-unit decimal or a percent. Slip is also related to motor efficiency, being numerically equal to the ratio of winding loss in the rotor to the total rotor power input: Rotor conductor rotor winding loss 8 = rotor power input (6.7~) rotor winding loss or s = motor power input - stator losses ' (6.7b) Stator losses in equation 6.7b include and wind- -- age. Equations 6.4 through 6.7 can be employed to calcu- late the synchronous and actual motor speeds and also the possible power and torque output, realizing that (8) Figure 6.22.-induced rotor potential by rotating flux. power output (watts) = 746 (horsepower output), (6.8~)
- 163. 138 and in which KC% T 3 -, s (6.8b) where hp = horsepower output of motor, n , = actual motor speed, rimin, T = motor torque, ft.lb, K = a torque constant, ft.lbN, 5 = rotor current, A, and R, = rotor resistance, Cl. Motor Construction The elementary salient-pole motor of figure 6.19 is undesirable from the standpoint of the ineffective use of material and space, as well as its overall inefficiency. The main disadvantage is coupledto the distinguishable stator poles. 'lb overcome this problem, actual induction motors have lapped stator coils, as shown in figure 6.23, where several coils make up a stator winding that can be either delta or wye connected.The flux directionsof each coil are illustrated as &,q5B, and each coil contributes to the rotating flux development of the entire stator. The coils and windings are arranged to have the same effect as salient poles, but the poles are not physically distinguish- able. An induction motor is assigned a specificpole num- ber if at any given instant the stator windings set up the same number of magnetic pole fields. The rotor core and squirrel-cage conductors are usu- ally not insulated from each other, because the induced current is effectively contained within the conductors owing to their significantly lower resistance. The rotor core is pulled magnetically toward the stator core across the air gap. If the force is uneven when the rotor turns, the result is vibration. This is detrimental in several ways as it can lead to S t ~ d u r a l insulation failures, premature bearing failures, and misalignments with the motor load. Vibration does not occur if the magnetic effect about the rotor periphery is equal. An additional method for pre- venting vibration is to place rotor conductors in slots skewed to the stator slots so that a rotor slot passes gradually under a stator slot rather than abruptly. This practice also prevents "dead spots," or positions of near- zero or minimum magnetic influence. Another method of eliminating dead spots is to construct the motor so that the number of rotor slots plus the stator slots sums to a prime number. Motor Behavior Figure 6.24 is a graph of the speed, efficiency, power factor, power input, and current load of a typical three- phase induction motor found in mining applications. Fig- ure 6.25 shows a representative torque-speed characteris- tic for a similar machine. These curves can be used to describe the electrical and mechanicaloperation of induc- tion motors under loading. From the typical torque-speed curve, the torque at locked rotor is approximately 150% of rated. The level increases steadily with rotor accelerationto the maximum or breakdown torque. With applied power input, the rotor continuesto accelerate until the slip reductionreduces the Figure6.23.-Lapped windings of three-phasemotor stator. 1 E 80 Z 60 o 30 2 20 a 1 0 W 0 10 2030405060708090100110 2 OUTPUT, hp Figure 6.24.-Characteristic curves of three-phase induc- tion motor. Breakdown o torque 3 0 - 50 1 0 0 SYNCHRONOUS SPEED, % 1 I I 1 0 0 50 0 SLIP. % Figure 6.25.-Typical torque-speed characteristic for general-purposeinduction motor. rotor current to a point where torque is equal to the load torque. Consider the motor running with no load. As the motor is loaded, slip increases, causing an increase of inductionin the rotor. Hence,rotor current rises, resulting in a stronger rotor magnetic field and motor torque. lbrque continues to increase with the increasedshaft load
- 164. until breakdown torque is reached. Any further load results in a slip value that decreases torque. If the high load is sustained, the rotor will stop Because the induction motor operates basically as a transformer, its electrical characteristics, as seen by the power source, will be a reflection of those occurringin the stator winding. Figure 6.26 shows phasor diagrams for rotor current and voltage during three operation points; these are referenced to the flux-density phasor of the stator, B , , , ( 1 1 ). The rotor bars are embedded in the steel core so they have a high reactance (3).At locked-rotor conditions(rotor stationary), the stator magnetic field rotates past the motor at synchronous speed, and the induced voltage in the rotor conductors has the same frequency as the stator (or line frequency). The result is a high ratio of rotor reactance to resistance, and stator current lags stator voltage by a large amount (fig. 6.26A). During rotor acceleration, slip decreases, which also lowers the fre- quency of rotor current and voltage according to the following relationship (8): where f, = frequenq of sinusoidal voltage and current induced in rotor bars, Hz, s = slip, expressed as a decimal, and f = frequency of voltage and current in stator, Hz. Thus, inductive reactance drops, increasing the power factor (fig. 6.26B).Theoretically, if the motor could obtain synchronous speed, the rotor power factor would reach unity (fig. 6.260. However, as this cannot happen in actual squirreltage motors, the maximum power factor is seldomgreater than 0.85 (fig.6.24)and never greater than 0.95. Because the output torque increases with slip, motor speed decreasesslightly as the load increasesfrom no load to full load. Yet efficiencyand power factordroprapidly on low load conditions. Hence, an induction motor should not be operated at much below rated load for any length of time. It is apparent from figure 6.24 that efficiencydimin- ishes when motor load increases above a given value. Consequently, an induction motor should not be over- loadedfor any extendedperiod. Power-factorand eficiency curvesnormally followroughly the samepath; thus, power factor can be considered as an estimate ofmotor operating efficiency. The torque developed by a three-phase induction mo- tor varies as the square of the stator supply voltage, or lbrque or V & , , , . (6.10) Therefore, a 10%reduction from rated stator voltage will cause a 19%reduction in available torque output. Insulation Insulation in motors normally has five forms: strand, turn, lead, crossover, and ground (15). Since the rotor conductors are uninsulated, the insulation of the stator winding conductors is the critical concern. The primary insulating system is that between the windings and the stator core or ground, and the secondary insulation is in strands, turns, leads, and crossovers. Copper magnet wire, and to a much lesser extent aluminum magnet wire, is used to c 0 n S t ~ d the stator winding or coils. Strand insulation is most frequently a resinous coating on the wire. Turn insulation is applied after strands are wound intocoils(orthe actual windings), and this may be a resinous coating, resinous-film taping, paper taping, or a fibrous wrapping. These types of turn insulation are utilized for applicationsof 6,600 V and less; for higher voltages, additional layers of mica or varnished clothtape can be used. Crossoverinsulation is employedto protect wires that cross each other. The crossovers are often the weakest point in winding construction; thus, they require additional protection. Lead insulation is simply insulation about the conductors leading to the windings. Lastly, ground or ground-wall insulation is the major insulation system of the motor and isolates the windings from the core. This insulation is always sub- jected to the highest potential difference and requires the most attention. Design Characteristics Figure 6.27 illustrates the standard NEMA torque- speed characteristics for squirrel-cage induction motors. The shapesof these curvesdependprimarily on the ratio of rotor conductor resistance to reactance. For instance, to obtain a greater locked-rotortorque, as well as a greater slip over the unable load range, rotor conductor resistance may be increased by decreasing the conductor cross- sectional area, or inductivereactance may be decreasedby placing the bars closer to the rotor surface. On the other I cos 8 Vrotor 'stotor - F - I Irotor C A I COS e a !Bstotor VVr I cos e Vrotor Bstotor Irotor Figure 6.26.-Phasor diagrams of rotor and stator flux density for inductionmotor.
- 165. hand, an increase of conductor resistance will decrease overall motor torque and the stator current drawn during locked-rotorconditions. Single-cage rotors, as previously described, are the most rugged and the most used. Double-cage rotors use two conductors, one over the other, per rotor slot (fig. 6.28A) and provide higher starting torques with higher load efficiency and lower running slip than the single cages (14). Here, the higher conductor would have high resistance and low reactance, while the lower set would have low resistance and high reactance. Double-barrotor conductors are often susceptibleto damage on loads with long accelerating times. To overcome this problem, deep- bar rotors (fig. 6.28B) can be used. These have a thermal advantage in that the full conductor area is available for heat dissipation, but the design still approximates the performance of the double bar. Regardless of the design, the torque-speed curves are matched to the squirrel-cage mtor const~dion, which is fixed for a specific motor. In addition to rotor design changes,the actual values of breakdown and locked-rotortorque vary with the horse- 0 50 100 SYNCHRONOUS SPEED. % Flgure 6.27.-Typical torque-speed characteristics for NEMA-designthree-phasesquirrel-cagemotors. power, frequency,and speedratings ofthe motor. Although the operating characteristics are a function of rotor imped- ance, the horsepower rating is mostly dependent upon the power (or kilovoltampere) capacity of the stator and rotor windings. As mtor losses are constrainedto the rotorcage, rotor thermal capacityis limited. Therefore,motor designs that create large rotor currents, such as high-torque high-slip, may have intermittent time ratings or a limited number of allowed successive starts. Unless these con- straints are heeded, improper operation will burn out the rotor winding. The different rotor designs have led to a variety of speed-torque characteristics- To distinguish among-the various types, NEMA uses a code letter system that signifiesspecificrotor constructions(8).Design B servesas the comparison basis for the motor performance of other designsand is oftencalled the general-purposemotor. This design has relatively high efficiency even at light loads and a reasonably high power factor at full load. It has single rotor bars located rather deep in the core but with large-area slots for good heat dissipation. Starting cur- rents range from 4.5 to 5times the rated full-loadcurrent. The design B motor has the broadest industrial applica- tion field. DesignAhas characteristics similar tothose of design B, except that it has a higher breakdowntorque. The rotor conductors are shallower,which decreases rotor reactance but increases the starting current, being five to seven times rated current. As a result, design B motors are often preferred over design A for large motor applications. As shown in figure 6.27, design A motors have the best speed regulation, as evidenced by the steep curve portion be- tween synchronousspeed and breakdown torque (8). Design C motors have a double-cage rotor construc- tion that results in higher locked-rotortorque and lower breakdown torque than those of design B. Starting cur- rents are about 3.5 to 5 times rated current (8).These characteristics are well suited for conveyorbelt drives and other applications that have sudden large load increases, but low or normal starting inertia. The motors are not suited for heavy high-inertia loads because the thermal dissipation is limited and high rotor current tends to concentrate in the upper bars (8). Accordingly, frequent starting of these motors can cause rotor overheating. Very high locked-rotortorque and high slip are found with design D characteristics. Design D's principal appli- cation is for high-inertia loads. The rotor is of high- resistance design with bars located close to the surface (8). A Double-cage rotors B Deep-bar rotor Figure 6.28.-Other rotor-conductordesigns.
- 166. Starting currents range from three to eight times rated load current. The motor is suited for heavy-duty starting, but again, the poor heat dissipation of the rotor design means that starting cannot be frequent. Design F has lower locked-rotor and breakdown torques than does design B. Design F motors also use a double-cagerotor with high resistance in both conductors, which reduces both starting and running current (8). The locked-rotor current is the lowest of all motor designs. Thus, design F motors are applied when starting-current limitations are severe and both starting and maximum torque requirements are low. The design, however, has poor speed regulation, low overload capacity, and usually low full-load efficiency. Induction-Motor Starting From the foregoingit can be seen that if an induction motor is started by directly connecting it to a power system, the momentary starting current can range from three to eight times the full load current. While this will not damage the motor, the high current can cause a significant disturbance on the power system, and, in some cases, activate overcurrent protection devices. However, most induction motors in mining applications are started by directly connecting them to the power system, espe- cially those within mining machines such as continuous miners. The system usually has enough impedance that protective devices can be set above the in-rush current to prevent nuisance tripping. This, however, is a major prob- lem,which isfurther discussedlater in this chapter and in chapter 10.Full-voltagestarting can usually be performed on 440- to 550-Vmotors up to 1,600 hp. NEhIA standard magnetic starters for this range are shown in table 6.3 (3). The jogging service listed in the table refers to frequent stop-start or plugging (reversingunder load)applications. As shown in figure 6.29, the across-the-linestarter is simply three contacts driven by a solenoid, also called a contactor. Pressing the start button energizesthe solenoid, which closes the M contacts. An auxiliary contact set (M,) simultaneously closes and bypasses the start switch. Pressing the stop button deenergizes the solenoid. Above 1,600 hp (but sometimes lower), full-voltage starting becomes impractical even when the load con- nected to the motor can withstand the stress. Common methods for starting these large induction motors are shown in figure 6.30. In basic terminology, all these methods can be called reduced-voltage starting. In figure 6.30A, an autotransformer is used to start the motor at reduced voltage (50% to 80% of rated), thus limiting starting current and torque. When almost at full speed, contactors quickly change the motor from the autotrans- former to the full-voltage supply. Primary resistor or reactor starting (fig. 6.30B) inserts fixed or variable im- pedances in series with the motor; these are shorted out after acceleration.For the wye-deltatechnique (fig.6.300, the motor is started as a wye connection, which places about 58% of the rated delta terminal voltage across the windings, limiting line current to 58%and torque to 35%. After acceleration,motor operation is with a delta connec- tion. Part-winding starting requires that the motor have two identical stator windings (fig. 6.300). Starting uses only onewinding and limits starting current to about 65% of normal, torque to 45%. After acceleration, the second winding is switched in. There are many systemsthat cannot take the shock of full-voltagestarting. One instance is a conveyorbelt drive Table 6.3.-NEMA class A standard starters for three-phase induction motors n horsepower Maximum horsepower rating, A 220 v 5.................. 270 100 200 75 300 .................. 6 540 200 400 150 NAP 7.................. 810 300 600 NAp NAp .................. 8 1.215 450 900 NAp NAp 9.................. 2,250 800 1,600 NAP NAP NAp Not applicable. L1 Start L2 3-phase diagram Control circuit Figure 6.29.-Across-the-line magnetic starter.
- 167. 3-phase supply Ll L2 '-3 m b switch I ~ 3 w A Autotransformer 3-phase supply u B Primary reacter 3-phase supply 3-phase supply D Part winding Figure 6.30.-Starting methods for induction motors. where the horsepower limit for full-voltage starting is perhaps as low as 50hp. The wound-rotor motors described in the next section provide an alternative. WOUND-ROTOR INDUCTION MOTORS As mentioned earlier, the starting and running char- acteristics of an induction motor may be adjusted by varying the resistance-to-reactance(R/X) ratio of the rotor conductors.Instead of rotor bars and end rings, the wound- rotor motor has insulated windings much like the stator, with the same number of poles and windingsplaced in the rotor slots. The windings are usually connected in wye with the ends connectedto three slip rings mounted on the rotor shaft. The brush and slip-ring circuit is completed through a wye-connected set of variable resistances, as shown in figure 6.31.Thus, the external resistance can be used to vary the speed-torquecharacteristics by changing the rotor RIXratio. The stator of the motor is the same as for a squirrel-cage machine. A typical family of wound-rotor motor characteristics is illustrated in figure 6.32 (11).As external resistance is increased, the starting current is decreased and starting torque is increased.For a given shaft load, the reductionin rotor current will result in a speed decrease. Thus when starting a wound-rotor motor, a maximum resistance is inserted in the rotor circuit ( R , curve). As the rotor accelerates, the resistance is reduced until the desired speed is obtained, or if full speed is required, the resis- tance is brought to zero ( R , curve). Therefore the wound- rotor motor can be considered a variable-speed machine. Thermal considerationsdo place a lower speed limit on it, and for self-ventilated motors, continuous rated torque operation below 70% of rated full speed is not recom- mended (15).This lower limit may be reduced to 50% if the motor load is 40% of rated. Applications for wound-rotor motors include loads that require constant-torque, variable-speed drives or for which a sequence of slow-speed steps is needed to limit motor current during acceleration,such as for high-inertia or high-torque loads. Since they are suited to high-torque loads. Since they are suited to high-torque loads, these motors have found extensive used in the mining industry to operate crushers, grinders, ball and roller mills, con- veyor belt drives, and hoists. The automatic starting method for these motors uses definite-timeacceleration where a series of fixed resistances are shorted out one at a time on a predetermined schedule (12). This step starter is shown in a simplified schematic in figure 6.33. When the starting sequence is initiated, all resistors are in serieswith the rotor winding; then the relay
- 168. 3-phase winding on rotor I - 3-phase supply to stator resistor I-' Figure 6.31.-Schematic o f wound-rotor induction motor showing external resistance controller. SYNCHRONOUS SPEED, % Figure 6.32.-Torque-speed characteristics for wound-rotor motor with stepped-resistancecontroller. Stop 0. L.'S L.B TC wntacta lA, 2A, and 3A aresequentiallyclosed,resultingin four speed-torquecharacteristics. The last effectivelyshorts out the rotor winding. Since the sequenceproceeds regard- leas of motor speed, the method requires close coordination with motor characteristics(15).The actual operation o fthe relays is discussed in chapter 9. Whether started automatically or manually, the wound-rotor motor continues to find application for the functions previously mentioned. However, for conveyor belt drives, these motors are now tending to be displaced by squirrel-cage induction motors equipped with solid- state starters (see chapter 14). Reasons for this change involve maintenance problems and a desire to eliminate the failures inherent with brushes, slip rings, and relay contacts. THREE-PHASE SYNCHRONOUS MOTORS The three-phase synchronous motor has a stator and rotor and is similar to the induction motor. The stator and stator winding have the same basic construction and purpose: to receive the power to drive a load (15).However, in this motor, the rotor consistsof field poles connected in series, parallel, or series-parallel combinations and termi- nated at slip rings. The field windings are excited by an external dc source, the exciter. The number of field- winding poles equals the number of magnetic poles present in the stator. A sketch of a typical large synchro- nous motor is shown in figure 6.34 (12). Rotor field excitation is often supplied from a small dc generator mounted on the same rotor shaft, as dia- grammed in figure 6.35. Alternatively, dc supply can be obtained from a three-phase full-wave bridge rectifier, as illustrated in figure 6.36, or by a separate m-g set. Pure synchronous motors are not self-startingand are generally accelerated in the same manner as inductor motors. Salient-polerotors commonly have a squirrelcage winding (fig. 6.34) to produce the necessary induction motor action. Low-speed cylindrical rotors closely resem- ble a wound-rotorinduction motor,but with five slip rings Figure 6.33.-Simplified step starler using individually Figure 6.34.-Sketch showing construction o f saiient-p~lo timed magnetlc relays. synchronous motor.
- 169. (15).As figure 6.37 illustrates, three rings are used for a motor rotor, not specifically to develop induction-motor wound-rotorcircuit, the other two for the dc field (8). These torque to external loads. Some large synchronous motors cylindrical-rotormotors can provide high starting torque are acceleratedby a small inductionmotor mounted onthe to accelerate high-inertia loads. The use of squirrel-cage synchronous-motorshaft (12). The induction motor must windings, however, is intended only to accelerate the have fewer stator poles than the synchronous motor in order to reach the required speed. I armature I I I Synchronous motor armature Figure 6.35.-Simplified diagram of synchronous motor ur. ing generator for field excitation. Synchronous-MotorStalting Figure 6.38 demonstrates the general method of start- ing a synchronous motor (12). Pressing the start button energizes the CR relay, which in turn closes the CR contacts. One set of contacts electrically locks in the start sequence (which can be terminated by pressing the stop button), and the other set energizes the M relay. The M contacts close, and three-phase power is applied to the stator winding. This allows the machine to accelerate as an induction motor. In the simplest procedure for the motor shown in figure 6.34, the motor is allowed to accelerate to the maximum induction speed, where the slip between the stator rotating field and the rotor is very small. A switch is then closed manually to apply dc to the rotor field winding (figs. 6.35-6.37). A steady rotor mag- netic field is thus established that can lock in step with the rotating field of the stator. Thus, the rotor will turn at synchronous speed, which gives the motor its name. How- ever, if dc is applied before maximum induction speed is Figure 6.36.-External solid-state supply used to provlde field excitation. 3-phase damper rotor winding dc field Stator armature 3-phase ~ovable shorting bar Figure 5.37.-Schematic of low-speedcyllndrlcal-rotor synchronous motor.
- 170. achiwed, the rotor may not pull into synchronization and severevibration can occur, caused by repulsion every time a rotating pole passes a stator pole. As a result, most synchronous-motorstarters do not rely on manual control but instead automatically excite the rotor field at the appropriate time. An approach widely used for automatic starting is synchronization based on frequency (12). This technique uses the voltage induced in the field winding during acceleration and before the dc is applied (I).Again refer- ring to figure 6.38,a resistor (R) and inductor (X)are placed across the field winding, with relay FR across the inductor. The inductance of the relay coil is selected to be much lower than that of X. Immediately after starting commences, a high-frequency potential is induced in the field winding, and the majority of current flows through the resistor and the relay coil because the inductance, X, exhibits high reactance. The FR relay opens the FR contacts faster than the interlock contact M , of relay M closes. As the motor accelerates, the frequency of the induced current decreases. When close to synchronous speed, the frequency has decreased to the point where most current flowsthrough the inductor,and the voltage is reduced to the point where the FR relay cannot hold its contacts open. Consequently the FR contacts close and energize relay FS. The FS contacts then close to apply dc excitation to the field winding and remove the resistor from the circuit. Synchronous-Motor Toque Under load,the synchronousmotorbehaves much like a nonslip direct magnetic coupling. The rotor does not develop induction-motor torque; it is magnetically locked to the stator rotating field and is pulled around at basi- cally the same speed. The torque developed is dependent on the hold-inpole strength. Hence,the lock-intorque may be increased by simply increasing the dc supplied to the rotor field winding. If there is a load change, an instanta- 3-phase supply L1 L 2 L3 I I LlI L12 D-C lines naous speed change occurs but only for a few cycles, after which the rotor again attains synchronism(12).The use of a squirrel-cage rotor winding also helps to dampen out speed changes, and it is therefore often called a damper winding. A typical speed-torque characteristic for a synchro- nous motor containing a damper winding is shown in figure 6.39.Because the synchronous-motorportion can- not start itself, the starting torque comes from the damper winding. When the external loading does not exceed the pull-in torque value, the motor can be started and accel- erated to synchronous speed. However, if no significant external load exists and then a load equal to the pull-in torque is applied,the rotor will momentarily dropto about 95% of full speed and then regain synchronism. During the loading and the momentary drop in speed, the rotor assumes a new position and continuesto rotate at synchro- nous speedbut a few degreesbehind the no-loadposition(a in figure 6.40)(9).This sequence can occur for any applied NORMAL FULL-LOAD ARMATURE CURRENT. % NORMAL FULL-LOAD TORQUE, % o O O o o O O O O O o N * a c o " " " ! e s 2 0 1 0 20 30 40 50 60 70 80 90 00 torque Figure 6.39.-Typical torque-speed characteristic for syn. chronous motor with damper winding. Figure 6.38.-Controller used to demonstrate general start- Figure 6.40.-Effect of load on rotor position. Ing method for synchronous motor.
- 171. load up to the synchronous torque level, above which the restoration of synchronous speed is questionable. If the load requirements exceed the pullout torque, the motor loses synchronism, average torque dmps to zero, and the motor stops (12). Generated Voltage ARer excitation has been applied to the field winding, the revolving magnetic field of the rotor cuts the stator conducto~s and induces a voltage in opposition tothe applied voltage. Figure 6.41A shows an equivalent per-phase circuit of a synchronous motor that demonstrates the effect of the generated voltage on the electrical performance of the ma- chine. From KirchhoFs voltage law (ll), where ae = voltage drop due to effective resistance of armature (stator)winding, V, Ix, = voltage drop due to inductive reactance of - armature winding, V, V, = voltage supplied to motor, V, and V, = generated voltage produced by rotor field winding modified by armature reaction, V. Two important phenomena connected with synchronous machines (and some others as well) are immediately evident in the equation. Under dynamic loading condi- tions, if the load delivers a torque to the motor shaft, the rotor produces a generated voltage that is greater than that of the supply, and power is delivered back or regen- erated into the line. Secondly,if the supply voltaxe is then removed, the load acts as a prime mover, and V, will be generated as long as field excitation exists and until the load dissipates its energy. This last phenomenon is espe- cially important when the supply voltage is lost because of a short circuit, since the synchronous motor can deliver significant current to the malfurlction (see chapter 10 for further information). Power Factor Another important effect results from the generated voltage. I n a n ideal synchronous motor under no-load snditions, V, can be equal in magnitude and frequency to V,but 180' out of phase. Hence, with this ideal situation, the motor does not draw current (I). Obviouslv. practical ever, increasingthe dc field strength with the same shaft load can shift V, such that the reactive component of current will change fi-om a lagging phase angle to leading (fig. 6.410. In this condition, the rotor field is termed ouerexcited, and the motor appears as a capacitive load. The leading power factor is one of the most outstanding features of a synchronous motor, as it can be used for power-factor correction. The ability to operate at unity power factor should be obvious; the field winding is referred to as normally excited in this case. Because the phase angle of operation depends upon both field excitation and motor load (angle a),the charac- teristics of synchronous motors are often represented graphically in a form called V-curves. Illustrated in figure 6.42, they allow the selection of a field-excitation current for a load to ~roduce a desired Dower factor 18). The lines drawn to shiw equal power fa&or are termed~om~ounct ing curves (12). Generator 0~ exclter A Field - motors have such losses as windage and friction, which cause a small shift in angular position, a,between the rotor~nd the rotating magnetic field. Here, the phasocs Vc and V, are no longer opposite in position, since V, is shifted clockwise by a as shown in figure 6.41B.-The - e 0 change causes the motor to draw line current (I) to L L maintain the rotor in synchronism with the stator flux. a 0 Under heavier loading, a increases and the motor draws 2 more current. Note that the rotor field actually does no O r a work, and the dc energy supplied to maintain the field is dissipated as a small 12Rheat loss. e 3 - A change in the rotor field strength, say by adjusting o the resistancejn figures 6.35, 6.36, or 6.37, changes the E a magnitude o f x but -not its-angular position. The differ- ence between V, and V,, or V , in figure 6.41, determines the angular position of motor current. When V, is adjusted to produce a motor current that lags applied voltage (fig. 6.41B), the motor is said to be undermited. How- Figure 6.41.-Equivalent per.phase circuit of e synchronous motor (A) and phasor diagrams for (B) underexclted and (C) overexcited field winding. Normal excitation I v C Field current Figure ~ . ~ ~ . - V - C U N ~ S for synchronous motor.
- 172. Applications Elementary Motor In the past, wide use was made of synchronous motors in the mining industry to take advantage of their constant speed and available leading power factors. Applications included ventilation fans, pumps, compressors, grinders, mills, and drive motors on m-g sets to provide power for dc equipment. However, static capacitors have now re- placed motors for power-factor improvement in almost all situations because of their flexibility and ease of installa- tion, and silicon rectifiers have supplanted m-g sets for power conversion. Nevertheless, one very important use of synchronous motors remains today: as the main drive motor in surface excavators. Here, one or more motors directly drive dc generators, which in turn power the dc motors serving the various functions on the machine. Figure 6.43provides a plan view of a typical mining shovel where one synchronous motor drives three dc generators and the motor exciter (10). This subject will be continued at the end of the presentation on dc motors. DIRECT CURRENT MOTORS The dc motor is the most versatile of all electrical machinery. On advantage over all the preceding motors is that its speed may be easily adjusted. The dc motors accounted for the majority of motors within the mine until the 1940's, when ac distribution systemsstarted to replace dc. Induction machines then substituted dc motors, mainly because the ac-to-dcconversion of reasonably large power quantities was cumbersome. However, dc motors continued to hold prominence for some specific loads. In recent years, because ac-to-dcconversion is now very easy, dc motors have replaced some of their induction counter- parts. The reasons behind the extensive use of dc motors in mining will become apparent in the following paragraphs. Ho~st-and-swing joystick , - 800-hp oc motor' arnach~ne generoror Figure 6.43.-Plan view of typical mining shovel showing mg set. Figure 6.44 illustrates an elementary two-pole dc motor with a one-coil armature. With the armature cur- rent flow as shown, the reaction of the armature magnetic field to that of the main field produces forces on conductors A and B, and the torque results in clockwise rotation. The commutator acts as a switch to reverse the armature current each time the conductors pass the neutral plane. To reverse armature rotation, the armature current flow is simply reversed. The two-pole motor is rather impractical. Torque is maximum when the plane of the armature conductor is parallel to the plane of the field, zero when at right angles. Figure 6.45 shows a four-pole armature with a four- segment commutator, but still with a two-polemain field. Here, motor torque does not drop to zero because an armature conductor is always under the magnetic influ- ence of the main field. Actual dc industrial motors have many commutator segments, armature conductors, and Figure 6.44.-Elementary two.pole dc motor. A Elementary motor 6 Torque output versus rotation Figure 6.45.-Elementary four-pole dc motor.
- 173. main field poles. The result is nearly constant torque output. As with the synchronous motor, the dc motor field does not do useful work. It merely provides the necessary medium for the armature windings to push against when developing rotary motion. In all but the very smallest machines, the field is supplied by dc through field wind- ings. The energy expended in these windingsformsan 12R heat loss. Actual Motor Construction The essential motor parts are the armature (rotor), the commutator, and the main field frame and windings (stator). The armature is constructed of steel laminations pressed onto the shaft, with slots parallel to the shaft. The armature windings are placed in the slots and connected to the segmentsof the commutator,which is located at one shaft end. Carbon brushes, mounted on but insulated from the motor frame or one end bell, rise on the commutator segments. The main field windings surround laminated pole pieces that are bolted around the periphery of the motor frame. Interpoles (or commutating poles) are mounted be- tween the field poles (fig. 6.461, and the windings are connected in series with the armature (12).Their purpose is to improvecommutationby opposingarmaturereaction, the distortion of the main magnetic field by the rotating armature field (fig. 6.47). The interpole windings produce a small magnetic field that opposes the main field in the same plane as the brushes. This reduces the magnetic field that is cut by the armature conductors undergoing Cornmutatingfield, Figure 6.46.-Cross-sectional sketch of dc motor showing interpoie and compensatingwindings. Moqnetic neutral (lood) Magnetic ,neutral (load1 +' Armature flux Field flux Resultant distortion of field flux producedby armature flux Figure 6.47.-interaction between armature and main-field flux to produce main-field distortion. commutation (current reversal) and thus reduces brush sparking (3). The armature reaction can he further neutralized through the use of compensating or stablizing windings (12),which are placed in slots on the ends of the main field poles next to the armature (fig. 6.46) and are again connectedin serieswith the armature. Thesewindingsare especially useful in motors intended for variable-speed or reversingoperation. Without the compensation, armature reaction from large loads can neutralize the main field flux (12). Although interpole and compensatingwindings serve valuable functions in dc motor operations, the machines can work without them. As they can obscure the presen- tation of motor operation, these windings will not be included in the following discussion. Toque The torque developed by any electric motor is a measure of its ability to pull against a load. In dc motors, torque is a function of armature current and the magnetic flux density of the main field or (8) T = KOI,, (6.12) where T = motor torque, N.m (times 0.738 = lb.ft), O = magnetic flux per main field pole linking the conductors, Wb, I, = total armature current, A, and K = a proportionality constant, N.m to WbA, and where K = Traked (TA retedx@reted)' where T, , , , IArated,lPrated = rated value for torque, ar- mature current, and flux for motor, respectively. The above equation can be used to find the torque output from a machine, if the rated torque and the changes in armature current and the field flux are known. Motor Connectionsand Performance Exactlylike dc generators,dc motorscanbe connected as separately excited, shunt, series, and compound. These connections are shown in figure 6.48. The performance of a separately excited motor is similar to that of the shunt, and its importance in mining applications is primarily with regard to motor control; thus, this motor will be discussed later. The speed-torquecharacteristics of shunt, series, and compound motors can change drastically de- pending on the connection. Typical curves are illustrated in figure 6.49 (8). Shunt Motors The shunt motor has the main field winding con- nected in parallel with the armature (fig. 6.48). Sincethe field winding is connected acrossthe supply, its resistance must be rather high, but because of space constraints the armature windings have a much smaller resistance. When the motor is energized, armature current, I,, is limited only by its winding resistance and is thus much higher
- 174. Separately excited Series Shunt IL'Ia'If Io'fs ;F-p-+ - Compound Figure 6.48.-Four connections for dc motors. current Lower curves 0 50 loo I50 200 250 ARMATURE CURRENT. % Figure 6.49.-Typical characteristics for shunt, series, and compound motors o f equal horsepower and speed ratings. than field-winding current, I ,However, as soon as the armature startsto rotate, its conductorscut the main field magnetic flux and a counterelectromotive force (cemf)is generated in the windings. This cemf opposes the applied armature voltage and begins to limit armature current. The opposition to current flow can be seen by applying Kirchhoffs voltage law: where V, = supply voltage, V, V , = cemf induced in armature winding, V, and I,, R, = armature current, A, and resistance, 0. The cemf is proportional to the speed of the armature, or where n = armature speed, rlmin, and Q = magnetic flux per main field pole, Wb. As the armature accelerates, the cemf rises, and the armature current drops. Yet, according to equation 6.12, motor torque decreases. A final speed is reached when the cemf is almost equal in magnitude to the supply voltage. If the motor is unloaded, the difference between the terminal voltage and the cemf will allow only enough armature current to overcome friction, winding, and core losses. Under motor loading, the armature slows down, cemf decreases, and more current enters the armature. However as shown in figure 6.49,the speed of the shunt motor remains relatively constant from no-load conditions up to 100% rated and slightly beyond. The speed can be easily adjusted by changing a resistance in series with the field winding. From equation 6.14,weakening the field fluxby decreasingfield current increasesmotor speed. Yet, for a constant field flux, torque varies linearly with armature current (that is, T a I,). If across-the-line starting was attempted with the shunt motor shown in figure 6.48,the cemf would proba- bly not build up fast enough to limit armature current to a safe value, and hence, damage to the commutator, brushes, and the armature winding could result. For this reason, a starting resistance is used in series with the armature (fig. 6.50A)for all dc motors except those o f fractional horsepowers. The resistance is usually selected to limit armature current from 150% to 250% of rated current depending on the starting torque required. The shunt winding is always connected acrossfull line voltage when starting so less armature current is needed to develop the rated torque. Variable starting resistor Variable startingresistor Variable starting resistor rheostat Shunt field A Shunt motor ' ~ d B Series motor ' ~ d C Compound motor Figure 6.50.-Simplified dc motor schematics with starting resistances.
- 175. In mining, manual controllers are found on many dc machines. These are available in three general forms: faceplate, multiple-switch, and drum controllers. Sche- matics for these are shown in figures 6.51,6.52, and 6.53. Lines L 2 L 1 - - - - - - - - - - - 7 .Holding coil Figure 6.51.-Faceplate manual starter. Shunt fields - , . . L--L'-&r~e;-J Figure 6.52.-Multiple-switch starting. Shunt field Figure 6.53.-Drum-type starter. The faceplate starter is often used with small station- ary dc motors. The level is advanced(tothe right) in steps, momentarily stopping at each position to allow the motor to accelerate, until the resistance is removed. A holding coil then maintains the lever in the last position. A spring is used to return the lever to the off position during a power failure or if the lever is left in an intermediate position. Qne method of multiple-switch starting, shown in figure 6.52, uses two double-pole, single-throw switches. The upper switch is closedfirst, energizingthe shunt field through a 100-Q resistor. This allowsthe main field flux to build up to some extent before the armature is connected to the line. Initial inrush current is thus reduced, which helps to prevent brush arcing and the possibility of com- mutator flashover. The lower switch is then closed, ener- gizing the armature through a second resistor, and the motor accelerates. The armature resistor remains con- nected during running. The two line switchesare mechan- ically interlocked so the upper switch must always be closed first. A variation of this technique is to use relay contacts or contactors to supply main field excitation, insert the starting resistance, then bypass the resistance. Drum controllers (fig. 6.53) are frequently used on mine locomotives but are also found on some dc mining machines. A handle-controlledrotary shaft is connected to the switch seements indicated bv dark lines in the figure. - These segments are of various lengths so contact with the stationary contacts can be made at different intervals. When stking, the M1 and M2 contacts engage first, energizing the shunt field and inserting all resistors in series with the armature. The resistors are then removed one at a time by advancing the controller. Although not shown, an additional drum or reversing controller is usually available to reverse armature current and thus motor direction. The use of fixed resistance starting has widespread application in mining. Here the starting resistance re- mains in series with the armature for running. An in- stance would be a small dc motor, such as a pump in a remote location. The resistance gives poor speed regula- tion, but the motor can be started unattended. Dynamic Braking If a shunt motor is running under load and the armature circuit is opened, the inertia of that load will drive the machine as a dc generator. Dynamic braking simply connects a resistance across the armature to dissi- pate the available energy and decelerate the load (fig. 6.54). The braking action is most effective at high arma- ture speeds, becoming negligible at low speeds.The value of resistance, R, is selected from (12) where V, - IaRa = armature voltage at start of braking, v. and I = dbamic braking current, depending upon desired braking level, A. The normal value for I is 150%of rated motor current but I may be as high as 300%for quick stopping.
- 176. SeriesMotor The armature and main field winding are connected in series and both carry load current in a series motor. The magnetic flux, 9, now produced in the main field winding, is proportional to the armature current. Thus, motor torque varies as the square of armature current (To : 1 2 , ) . Furthermore, the main field strength will change with load, causing a speed decrease with increased load. When a series motor is started, cemf builds up as the armature speed increases. During the initial acceleration, the cemf is small,armature and field current are high, and the torque is very high. When the curvesin figure 6.49are compared with the material presented earlier, it can be seen that the starting torque of the series-woundmotor is higher than that of any other motor type. Because of this, it is often said that the series machine has the best traction or starting characteristics. Thus, it is the most used motor for traction purposes in mining; examples include locomotives, shuttle cars, and diesel-electric trucks. Aproblem with this motor, however, is that at light loads, motor speeds may become excessively high; there- fore, series motors must be directly connectedto loadsthat cannot be removed freely. Otherwise, the motor may race to destruction. The method for starting the series motor is similar to that for shunt machines. The arrangement of the starting resistance is shown in fieure 6.50B. A direct a~~lication of contactorcontrolled muitiswitch starting oi a traction motor in a mining machineis illustrated in figure6.55(7). After the power-iource contacts (MI)close, the motor is accelerated with both resistances in series with the arma- ture. The same control that activated the M1 contacts simultaneously energizes a definite-time relay. After a preset time (about 1 . 0s),the relay closes its contacts, and that in turn energizesthe M6 contador, which shunts its starting resistor. The M7 contactscan be used to provide a L1 Shunt field Resistc I - Figure 6.54.-Slmpilfied diagram of dynamic braking ap- plied to shunt motor. second step before full speed is obtained or can be used to enable two-speedoperationof the motor. In the latter case, control circuitry is arranged so that the M7 contacts cannot close before the M6 contacts. Figure 6.56shows a one-step starting arrangement with the addition of forward-reverse control (contacts 1F and 2F close for for- ward, 1R and 2R for reverse)(7). The procedurefor dynamicbraking is identicalto that already described, with the exception of excitation for the main field. The simplified circuit in figure 6.57is one approach and showsthe switchesclosed for motoring(12). Upon dynamic braking, the switches place the armature, series field, and braking resistance in a loop circuit. The series-field connections are reversed to maintain current flow in the same direction. Compound Motors Com~ound motors have both shunt and series field windings-installed on the same poles. The series winding may be differentiallyor cumulativelycompounded,that is, subtracting from or-addingto the magnetizing force of the shunt field. This causes either reduced or increasedarma- ture speed with load. Only the cumulative compound motor characteristics are shown in figure 6.49. Cumulative compoundinggivesgreater torque than is possible with the simple shunt motor, because of the greater amount of main field flux available (8).The increased flux, however, causes the speed to drop off more rapidly than for a shunt motor, but not as much as for a series motor. Therefore, the cumulative compound motor will develop a high torque with any sudden increase in load, but at light loads it will not run away because the shuntfieldprovidesa constant fieldflux. These motors are often applied to loads requiring high starting torque but fairly constant operating speed under normal conditions. Thus, cumulative compoundingcombinesthe characteris- tics of both series and shunt motors. The differential compound motor produces torque that is always lower than that of the shunt motor (8). The amount of series windingcan be adjustedto offset any drop in speed as loading increases, or it may be sufficient to give a slightly higher speed than normal at full load. A motor having constant speed from full load to no load is called flat compounded, while that with slightly higher speed than normal is called over compounded. Again, armature current is traditionally limited by resistance when starting. Figure 6.50Cshows the process in elementary form, and figure 6.58Aillustrates an actual application for a mining machine hydraulic pump motor (7).It can be seen that the two circuits are almost identi- cal. Afixed startingresistor is used for accelerationand as in figure 6.55, the resistor is shunted by the M3 contacts after a definite time period (usually 1.0 s). A semi- automatic variation of this scheme is illustrated in figure Storting Commutator Starting Overload + MI resistor resistor coil MI - 4 A Main line v connector II- I I M6 '"I7 Figure 6.55.-Two-slep resistancestarting of series-woundmotor.
- 177. 6.68B:semi-automatic means that under certain condi- tions the starter requires someattention. The accelerating contacta (A) are arc before, but the contactor coil is placed across the armature circuit. As cemf increases during acceleration, the voltage across the coil causes contad closure at the proper time. Commutotor field Starting M6 Flgure 6.58.-Forward-reverse switching of series-wound motor. Dynamic braking employs a resistance to dissipate energy generated in the armature, involving either the series field (fig. 6.57) or simply the armature itself (fig. 6.54). Ward-Leonard System For large-motor applications, the Ward-Leonard sys- tem provides one of the finest techniques for controlling motor speed over a wide range and in both rotational directions (3).Two specific examples where it is used are mine hoists and surface excavators (2, 4). Figure 6.59 illustrates the basic system.The dc generator is driven at constant speed, typically by a synchronous motor, but some systems employ indudion-motor drives for smaller horsepower applications. The generator and motor field windingsare separately excited (seeexciter in figure 6.43), and the motor is excited with a constant field current. Because the main field of the motor is constant, the speed is directly proportional to its armature cemf ( V , ) . The magnitude of V, is directly dependent upon the generator output voltage (V,) less i,R,, where i, is the Broke 1 Resistonce Figure 6.57.-Dynamic braking applied to series-wound motor. A Starting Commutator Series Overlood 4 resistor field field coil M2 & { I T -@---; A M o ~ n l ~ n e 1 6 I I I I Armature M3 rYYI Shunt field 19 Shunt field Starting resistance Series + - A , '; ; p o ; i n g > 4 . Figure 6.58.-One-step startingo f compound-woundmotor.
- 178. Ro Generotor , - - @ - - , Motor + lVUl I I w =constant Figure 6.59.-Basic Ward-Leonard system. armature current and R , is the combined resistances of the generator and motor armatures. As a result, excellent control of all motor speeds and both acceleration and deceleration is obtained by adjusting the generator field strength. The generator field-winding resistance is high, and so the required level of control power is relatively low. Motor reversing is obtained by changing the current direction in the generator field. Braking is performed by reversing or reducing the generator field current. MINE MOTORS Many mining uses for industrial motors have been covered so far in the chapter; this section servesto clarify some additional applications,but mostly for underground mining equipment. Applications Mine motor functionscan usually be divided into two -~oups:auxiliary and face (17). Auxiliary motors are employed for fans, pumps, conveyors, hoists, compressors, and other vital functions in mines aside from the actual process of mineral extraction. These operationscommonly call for direct use of general-purpose industrial motors, and as their loads are often well defined and continuous, the motor characteristics covered so far are applicable. Face motors are associated with mining equipment, such as continuous miners, shuttle cars, loaders, roof bolters, and locomotives, where they are mounted in the machine. Their duty usually involves cyclic or random loading as well as the possibility of shock loading. The result is higher electrical and mechanical demands than those placed on equipment in other industrial applications. The horsepower rating for a motor is based on the maximum winding temperature for continuous duty or intermittent duty. The temperature rise parameters have already been covered, but the meaning of a duty cycle has not. Continuous duty is quite obvious and refers to a substantially constant load (torque) over an indefinitely long period. Intermittent duty, however, means that load- ing is at alternate intervals of load and no load (motor running idle); load and off; or load, no load, then off (9). Each portion of the cycle is equal and the time interval is specified. In some cases, face motor intermittent duty is given a definite time interval of 15,30,or 60 min, but it is oftenjust listed as "mine duty" (6,17).Tsivitse (17) states that a very successful horsepower rating for face motors has used both the continuous rating and the 60-min rating. The continuous duty is matched to the average or rms requirements of the load, and peak horsepower load- ing is limited to the 60-min value. The rms value for horsepower can be defined as where hpi = mean horsepower during time segment Ati, and CAti = total time interval. The ac motors in mining machines are normally four or six pole with synchronous speeds of 1,800 and 1,200 rtmin, whereas dc motors often have comparable base speeds of 1,750 and 1,175rlmin (6,17). These speeds are high enough to provide adequate horsepower but low enough to have reasonable reliability. Series-wound mo- tors for traction are built to withstand rotation up to 6,000 rpm, such as might occur during maintenance. 'Dable6.4contains a listing of common applicationsfor different motor designs to accommodate the various func- tions found in mining equipment (5,17).Some additional information is warranted. The locked torque of traction motors is set so that the wheel or crawler-tractor treads will lose traction before the motor stalls. Ac motors that are mechanically paralleled, as for coal cutting with a continuous miner, are often sequence started with a delay to limit starting currents. Further, the high-slip charac- teristics mentioned in the table are for load sharing as well as to limit the rate of torque rise during shock loading. The dc motors used in load sharing often have matched speed-torquecharacteristics. Otherwise, the mo- tors are compounded with a differential field that is in series with the armature of the second motor. Table 6.4.-Common motors tor mining equipment Function ac ' dc Traction (direct mupled) ... Design D .............. Serieswound. Hydraulic pump................ Design B or A....... Compound or stabilized series. ' Conveyor.......................... Dosign C .............. Compound. Loading arm..................... ("1 Heavily mmooundnd. . . F - - Cutting............................. (7 Do. 'NEMA desians for induction machines. Speed reguiation from no load to full load from 10% to 15%. Similar to NEMA design A but sometimes with higher locked-rotor and breakdown torque and higher slip per torque value. Speed regulation from no load to full load from 30% to 35%. Actual Equipment Operation Because mining equipment operates at the tail end of the distribution system, voltage drop becomes an impor- tant factor in the selectionand utilization of motors. This is more critical for ac equipment than it is for dc, and this section will discuss some ramifications for two types of machines. Continuous Miners Of all electrical equipment used in U.S. underground coal mines, the continuousminer is the most concentrated simple load. This machine is the heart of present under- ground coal mining systems from both a production and electrical standpoint; hence, determining the load de- mands it makes on the system, or the load factor, is of
- 179. great importance. The machine load factor can be defined as the ratio of actual power consumption to rated motor power. The rated power for the squirrel-cage induction motors used on ac continuous miners is set by the manu- facturer for one motor or a combination of motors. The motors may or may not be built to NEMA standards. Regardless, torque and power are the only commonratings available to judge motor utilization. Horsepower is di- rectly proportional to the product of motor speed and torque, and this power rating can be employed to deter- mine three-phase motor performance. The load factor can be used not only to investigate the effective operation of a particular machine, but also to compare different equip- ment of a specific type, no matter what the rated power. At low load conditions, motor efficiencyand the power factor drop off rapidly. Since the motor functions on the steep portion of the power-factor curve, a small load variation will cause a relatively large motor current fluctuation. This can produce detrimental current peaks and stresses in trailing and feeder cables, particularly where conductors have marginal size. Poor power-factor operation requires correction capacitors or results in util- ity company penalties. Tb analyze the power factor of continuous miners, a recent study (13)investigated the actual operation of 26 different ac continuous miners. These machines had utili- zation voltages of 440, 550, and 950 V, and total rated motor powers from 100 to 535 hp. All were operating in the Appalachian coalfield, with production ranging from 50 to 770 raw tons per shift. Average load factors were deter- mined for each machine and particular attention was paid to the cutting-and-loading machine cycle because here power consumption is the most demanding. The average cutting load factors ranged from 0.26to 1.17 and averaged 0.52for the measured machines. It is significant that this average load factor is much less than the assumed design level of 0.85 that has been popularly used in the industry. When employing all hydraulic, mechanical, and electrical machine components, a 0.60 load factor might be consid- ered satisfactory for the continuous miners studied. Hence, the implication was that many were being used inefficiently. However,drawing conclusions about machine efficiency and utilization based only on the load factor could be misleading because of the numerous factors involved. Many of the low to moderately powered machines in the study had higher-than-average load factors, and some were considered to be totally adequate. During field mea- surements, close attention was paid to the performance of the machine operator, and in almost every case it was found that the operator was pushing his machine as much as possible during sumping (the cutting cycle), because traction approached full slip. From the standpoint of adequate employment, some continuous miners could be termed overpowered, particularly in the case of high- powered machines cutting friable coal. When a high-horsepower machine (500 hp or more) is used on a low-voltagesystem, the demand for large current can create considerable trailing-cable voltage drops and a machine voltage condition that not only hampers opera- tion but reduces the safety levels of the system on which it operates, perhaps by causing a poor power factor or sub- stantiallyreducing motor torque capabilities.In the study, good machine voltage conditions (that is, as closeto *10% as practical)almost invariably resulted in good motor load factors. To obtain the favorable operationrequired not only good voltage but a strong utilization system, that is, using the largest practical trailing-cableconductors and shortest practical trailing-cable lengths. When distribution voltage regulation was bad, poor machine load factors also oc- curred. The situation was most evident on 4,160-Vdistri- bution systems that had been extended beyond their limits. More information on these subjects is presented in chapters 8, 12, and 13. naction Locomotives A specific case study that was associated with the preceding work involved measurements on main-line trac- tion locomotives.The results of the study can be applied to all series-wound motor traction. The company involved was experiencing numerous motor armature failures on their locomotives-up to 47 in 1 yr. Two avenues were explored to determine the problem: inspection of the failed motors and electrical measurements on a typical operat- ing machine. Examination of the motors showed that the commutators were heavily pitted and charred, which was a direct indication of overloading. Subsequent electrical measurements substantiated this suspicion. The locomo- tive apparently experienced severe continuous stress that caused abnormally low motor voltage every time it en- countered a particular curve located on a steep upgrade. When provided with this information, the mining com- pany was able to reduce the locomotive trailing load, and the motor failures diminished. In this case, the very low motor voltage provided the clue to identifying the problem. Unlike ac motors, dc series-wound motors can still operate under low voltage, although their control circuitry might not function prop- erly. Here, the low voltage indicated high current because the trolley system was well maintained and had adequate capacity and properly spaced rectifiers. There is another method that indicates if the motors within a vehicle are being overstressed while performing a specific duty cycle. Every manufacturer supplies charac- teristic curves for its mining equipment. The example in figure 6.60 shows motor characteristics for a small loco- motive using two series-wound motors. The technique consists of finding the current needed by thejob and using the characteristic curvesto compare the needed value with the maximum value allowed per motor. The classical method employs rms current and makes the following assumptions: The vehicle operates under constant velocity while performing a specific function; The motors heat during acceleration and cool for deceleration; and The ampere rating for the motors stabilizes after 8 h of operation. The method requires complete knowledge of the entire duty cycle, which in mining is the repetitive process that places individual demands on the machine. For instance, the track profile for a locomotive under loaded and then unloaded conditions can be divided into segments of equal demand, such as the grade for a specific haulage distance. For each portion of that duty, the torque or tradive effort demand must then be found. Stefanko (16)contains meth- ods to calculate this input information. The characteristic curves are then used to find the current demand and the actual time the machine operated
- 180. a t that current for each portion of the duty cycle. For instance, consider that the locomotive of figure 6.60 is operating on 600 ft of track with a +0.5%grade and has a tractive effort per motor of 1,020 lb. Fkom the tractive effort curve, the current demand is 81 A, and using the speed curve, the machine speed is about 8.1 mih. By simple calculations it can be found that the locomotive would take 0.84 min drawing 81 A to move the 600 ft. The technique is continued until all times and currents are known for each duty cycle portion. A problem occurs in the above procedure when the speed obtained from the curves is greater than that allowed. For example, assume that the next portion of the track profile has a length of 2,400ftat a -0.5%grade, and the tractive effort for each motor has been found to be 340 lb. Fteferring to figure 6.60, current is 42 A, and speed is 12.5 milh. However, say that the maximum allowable speed is 10 mih. In this case, the locomotive would be commonly operated on-off, on-off, and so on, to maintain but not exceed 10 m i h throughout the haulage portion. The time the motors are on and off can be calculated precisely by 1. Finding the time required to travel the distance at maximum allowed speed (2,400 ft at 10 m i h yields 2.73 mid, 2. Determining the time it would take at the speed found from the curve (2,400ft at 12.5mi/%gives 2.18mid, and 3. Subtracting the results of item 2 from 1 (0.55 min). Item 2 provides the time the motors are on (42 A at 2.18 mid, while item 3 gives the off time (zero current for 0.55 mid. With all currents and times known, including those times at zero current, the following equation provides the rms current demanded by the duty cycle: where Ii = current demand for each duty cycle portion, A, ti = time involved for respective current demand, min, and I - , = effective current demand for duty cycle, A. For additional functions performed by haulage locomo- tives, the following factors can be assumed: 1.Switching operations have zero current demand but one-quarter of the actual time is applied in the above summation. 2. If the locomotive is used to load and unload its cars, the maximum tractive effort is employed for the loading process, the minimum for unloading. One-half the actual time involved for each is used for the effective time (ti). 3. Delays are taken as zero current and zero time. Normal delays are assumed to not allow effective motor cooling. Mine motors are presently standardized at a 90° C allowable temperature rise based on a 25O C ambient temperature (17).Older motors, as in figure 6.60, may have a 75O C temperature rise limit but are still based on iroct~ve effort 5 - 4 - 3 - .Time to rise 75" C MOTOR CURRENT. A Figure 6.60.-Typical characteristic curves for each motor in traction locomotive (&ton, 2504 motor, characterlstlc curves on 250 V; pinion, 13 teeth; gear, 69 teeth; wheel diameter, 29 in). the 25O C ambient. The base temperature closely fits the typical conditions found in underground operations. As mentioned earlier in the chapter, the allowable temperature rise is effective to elevations of 3,300 ft (1,000 m); above this, the allowed is reduced 1% for every 330 ft (100 m), or elev - 3,300 % reduction = 330 In addition, for maximum ambient temperatures exceed- ing 25O C, the allowable rise must also be reduced by the difference above the base temperature. For example, a motor with 75O C temperature rise insulation, operating at 6,600 ft elevation in 30° C ambient temperature, has only a 62.5OC allowable temperature rise. Consequently, if the locomotive is operating at 3,300 ft or less in a maximum ambient of 25O C or less, the characteristic curves can be used directly to find if the duty cycle demands exceed that allowed. In other words, the rms current found by equation 6.17 can be compared with the time-to-rise temperature curve. If the resulting time is greater than the actual time involved, the locomo- tive will work under that duty cycle.For example, if I,.,,,, is 80 A, 7.5 h of operation is allowed (fig. 6.60). However, if the allowable temperature rise must be reduced, the manufacturer curves can no longer be used directly and must be corrected. Fortunately, the time-rise curve is very nearly parabolic. Thus, any allowable tem- perature rise curve can be closely approximated by a straight line through two points plotted on log-logpaper, with the two axes representing motor current and opera- tion hours. This process can be time consuming. Using the parabolic relationship, the following formula also gives
- 181. the allowableeffective(rms)current for the total operating time when curve correction is necessary: where W = In p2 H ~ ) ' ' ~ ( T , , ~ ~ ) I ~ ] 1 , H , = total operation time for motor, h, T_= rated allowable tem~erature rise. O C . , . T , = corrected allowable 'temperature rise (due to elevation or ambient temperature), O C , Hl,I, = a point taken from manufacturer's time-to- Figure 6.61.-Stator field of two-pole, single-phase induc- rise temperature curve, h, A, tion motor. H2,12= a second point taken from curve, h, A, and I = allowed rms current for total operation time I%, A. If the allowable current from equation 6.19 is less than I , - from equation 6.17, the motor is overstressed for that duty cycle. Even though the foregoing was applied to dc motors in traction locomotive, the same concepts can be adapted to any mine motor application. SINGLE-PHASE MOTORS Although the vast majority of mine motors are three- phase and dc, single-phasemotors do find widespread use for auxiliary functions asidefrom the mining process. As a general rule, single-phaseinduction motors have one run- ning speed and require a separate means for starting rotation, usually a separate stator or starting winding. Motors are classified by their starting method. The most used techniques are split phase and capacitorstart, which will be discussed briefly in this section. Rotating Stator Field When a single-phase ac voltage is applied to one stator winding, the current flow produces a magneticfield with a resultant direction that alternates on a line, as in line OP in figure 6.61. If a squirrel-cagerotorwinding is in the stator field, a voltage will be induced in the rotor conductors, but the current produced will create a mag- netic field that coincideswith the stator field(fig.6.62).As no magnetic interaction occurs, no torque is developed, and the rotor remains stationary (9). If the rotor is moved by some means, the rotor conduc- tors cut the stator magneticfield, and the inducedvoltages are in phase with the current through the stator winding. However, the rotor winding impedance appears as almost pure inductance, and rotor current will lag the induced voltage by almost 90° (fig. 6.63). Thus the rotor magnetic field is now 90° from the stator field and is termed a cross-magnetizing field (9). The rotor and stator fields combine to produce a resultant field that rotates at syn- chronousspeed. The cross field strength is proportional to the rotor speed, and is about equal to the stator field strength at synchronous speed. The same operational principles that have been given for three-phase induction Figure 6.62.-Rotor field of stationary two-pole, single- phase induction motor. Generatedrotor voltage Stator current and flux Rotor currentand flux Figure 6.63.-Phase relationships between stator and turn- ing rotor. motors also hold for single phase; slip must always exist between the rotating field and the rotor. Because of the crossfield, the slower rotor speed causes the rotating field to pulsate. Accordingly, vibration and noise are inherent with single-phase induction motors.
- 182. Split-Phase Starting Split-phase motors have two stator windings con- nected in parallel, as shown for the two-pole motor in figure 6.64. The impedance of each winding is such that the currents through them are out of phase. One winding, the auxiliary or starting winding, is usually constructed of small-gauge wire and has high resistance and low reac- tance. The running or main winding has a heavier gauge conductor so the winding is of low resistance and high inductance. When energized, the phase angle between the currents through the two windings is only about 30°, but this is enough to produce a rotating magnetic field. The rotating field pulsates, and starting torque is small. Once the motor is started, the rotor cross field is produced. Thus the starting winding is no longer needed, and it is usually disconnected when the rotor speed reaches 70% to 80% of synchronous ( 9 ) .A centrifugal switch mounted on the rotor shaft is almost alwavs used (fig. 6.65). The starting direction determines the final rotating direction. Unlike three-phase motors, single-phase induc- tion motors must be stopped and the starting-winding connections reversed, then reenergized to produce a rotat- ing field in the opposite direction. Capacitor-Start Motors The capacitor-start motor also has two stator wind- ings. The main winding is arranged for direct connection to the power source, and the auxiliary winding is con- nected in series with a capacitor. With this arrangement, the currents through the two windings can be as high as 90° out of phase. Hence, starting torque can approach 100% of rated (9). Typically, the starting winding is disconnected at 70% to 80% of synchronous speed. A centrifugal switch or a relay sensing current through the main stator winding may be used (fig. 6.66). Apart from the high starting torque, the operation of capacitor-start motors is basically the same as split phase. However, popular split-phase motors have an upper power limit of 113 hp, whereas capacitor-start machines can be obtained up to 10 hp. This chapter has introduced the operation and char- acteristics of the motors in common use in the mining industry. Although elementary in nature, the contents of the chapter should not be discounted. The electrical power systems in and about mines have the purpose of ade- quately serving motors. If the characteristics of these loads are not precisely known, it is doubtful that a safe and effective mine power system can be achieved. Running winding ond poles Figure 6.64.-Starting and running dator windings. A Starting 6 Running Centrifugal switch Switchopens at closed M start 75% of speed Figure6.65.-Centrifugal switchto removestartingwinding. Capxitor Centrifugal switch A Running Line I u Figure 6.66.-Capacitor-start motor.
- 183. REFERENCES 1. Allis-Chalmers (Milwaukee, WI). Motor Control-Theory and Practice. 1955. 2. Bergmann, R. W. Excavating Machinery. Ch. in Standard Handbook for Electrical Engineers. McGraw-Hill, 10th ed., 1968. 3. Fitzgerald, .4. E., C. Kingsley, Jr., and A. Kusko. Electric Machinery. McGraw-Hill,3d Ed. 1971. 4. Hardie, R. C. Mine Hoists. Ch. in Standard Handbook for Electrical Engineers. McGraw-Hill, 10th ed., 1968. 5. Hugus, F. R., J. A. Buss, and E. L. Parker. Mining Machine Motor Characteristics. Min. Congr. J., v. 41, May 1955. 6. - Mining Machine Motor Identification. Min. Congr. J., v. 41, Mar. 1955. 7. Joy Manufacturing Co. (Franklin, PA). Direct Current Min- ing Machinery. 5th ed., 1971. 8. Kosow. I. L. ElectricMachinery and Transformers. Prentice- Hall, 1972. 9. Lloyd, T. C. Electric Motors and Their Applications. Wiley- Interscience, 1969. 10. Marion Manufacturing Co. (Marion, OH). 191-M Mining Shovel. Doc. Specification 542-5, 1979. 11. Marklekos, V. E. Electric Machine Theory for Power Engineers. Harper and Row, 1980. 12. Millermaster, R. A. Harwood's Control of Electric Motors. Wiley-Interscience, 4th ed., 1980. 13. Morley, L. A. Utilization and Efficiency in Underground Coal Mine Electrical Systems. Paper in Mine Power Distribution. Proceedings: Bureau of Mines TechnologyTransfer Seminar, Pitts- burgh, Pa., March 19, 1975. BuMines IC 8694, 1975. 14. Oscarson, G. L. The ABC's of Large Induction Motors. E-M Synchronizer. Electrical Machinery Manufacturing Co., Min- neapolis, MN, 1955. 15. Smeaton, R. W. (ed.). Motor Application and Maintenance Handbook. McGraw-Hill, 1969. 16. Stefanko, R. Coal Mining Technology Theory and Practice. Soc. Min. Eng. AIME, 1983. 17. Tsivitse, P. J. Mining Motors. Ch. in Motor Application and Maintenance Handbook, ed. by R. W. Smeaton. McGraw-Hill,1969.
- 184. CHAPTER 7.-GROUNDING' A vital part of any mine power distribution system is the connection to earth or ground, which is referred to as the mine grounding system. It consists of grounded or grounding conductors, extending from ground beds to equipment. A grounded conductor is a power conductor tied to the grounding system; a grounding conductor is separate from the power conductors and is used only to ground exposed metallic parts of the power system. A ground bed, also termed a ground mesh or grounding electrode, as well as other names, is a complex of conduc- tors placed in the earth to provide a low-resistance con- nection to "infinite" earth. The grounding system serves to protect personnel and machinery from the hazards associated with electrical equipment that is operating improperly. The protection afforded can be divided into the following four functions, which are the main purposes behind grounding the system. First, the grounding system must limit potential gradients between conducting materials in a given area (38)."During a ground fault, for instance, a phase conduc- tor comes into contact with a machine frame, and current flows through the equipment; subsequently, the potential of the equipment tends to become elevated above ground potential by an amount equal to the voltage on the conductor. If a person touches the machine, while being simultaneously connected to ground in some manner, the body's potential can become elevated, possibly to a lethal extent. The maximum potential to which a person couldhe exposed when touching a machine frame is equal to the voltage drop along the grounding conductors. Thus, the grounding system must provide a low-resistance path for the fault current to return to the source, and the ground conductors should have low resistance so they can cany the maximum expected fault current without excessive voltage drop. An example of the exposed potential in a surface mining situation is illustrated in figure 7.1 ( 3 8 ) . Second. the eroundina svstem should limit the energy chapter 11). This leads to premature failures, reduced component life, and mysterious "nuisance trips," which can occur without apparent reason. By providing a path between the transformer neutral and ground, most of the sources of transient overvoltages can be reduced or possi- bly eliminated. Last, a grounding system should isolate offending sections by selective relaying of ground faults (44). The sensitivity and time delays of the protective circuitry should be adjusted so a fault in a certain area will cause the local breaker to sense the malfunction and quickly remove power from only the affected section. If the relative tripping levels and speeds are not established correctly, nearby breakers may not trip when they should, and a small problem could escalate into a large calamity. Con- sequently, power to half a mine may go out because of poor relay coordination, and much time could be lost in the effort to trace and locate the trouble spot. Thus, the relaying system must be arranged so, even at the lowest level of the power-distribution chain, sufficient fault cur- rent can flow to enable the protective circuitry to sense it and take remedial action. Chapters 9 and 10 cover the protective circuitry used to provide the function of section isolation, while chapter 1 1 describes the devices employed with the grounding available at thejault location. Heavy arcing or sparking m i - - - can ignite nearby combustible material. The air itself can --- -- --- - - - - - - - - , Groundingconductor become ionized, making it capable of carrying tremendous Neutrol resistor ! amounts of current. A high-energy fault can vaporize breakers, switchgear, and phase conductors, and protec- tive enclosures may be blown apart with explosive force (21). Controlling the maximu& allowable Lult current significantly reduces the danger of fire and holds equip- ment damage to a minimum. Third, the control of overvoltages is essential. An overvoltage condition may occur by accidental contact of equipment with a higher voltage system, or from transient phenomena due to lightning strokes, intermittent ground faults, autotransformer connections, or switching surges (4). The maximum ratings for cable insulation, trans- former windings, relay contactors, and so forth may be temporarily exceeded in these cases. This does not usually result in an immediate breakdown of equipment, but component parts of the electrical system are successively overstressed and weakened by repeated exposure (see 'The author wishes to thank Alan M. Christman, who prepared the original material for many sectionsof this chapterwhile he was a graduate student at The Fbnnsylvania State University. a Italicizednumbersin parentheaesreferto items in the list ofreferenees at the end of this chapter. &%fety ground-bed res~stonce - 3-phase diagram Phose-corductor resistance - Total fault fault on shovel groundirq canductor current Grwnd~ng-conductor whch operator resistance Safety ground-bed resistance - - - - - . . - - - Circuit for line-to-ground fault Figure 7.1.-Illustration of electrlcal shock hazard.
- 185. system for transient overvoltage control. As an introduc- tion, this chapter looks mainly at the first three purposes and presents the common methods of system grounding, the effect of electric shock on human beings, mine ground system characteristics, and ground-bed construction. Ex- tensive information about grounding is contained in prac- tically all subsequent chapters. GROUNDING SYSTEMS Over the past few decades,several differentgrounding philosophies have held sway in the electrical industry, each with its own advantages and disadvantages (20). These methods o f grounding are discussed below. Note that reactance-grounded systems are not presented in the following paragraphs, as they are not normally used in industrial power systems. UngroundedNeutral The ungrounded system was probably the first to be used because of its simplicity. Here there are no inten- tional ground connectionsin the system whatsoever. How- ever, a perfed ungrounded system cannot exist, since any current-carrying conductor may be coupled to ground through numerous paths, including the distributed capac- itance of its wiring, or through motor windings (49).This phenomenon is shown in figure 7.2 (20). The first line- to-groundfault on such a systemwill havevery little effect (27)because there is no way for the fault current to find a complete circuit back to the source, and its magnitude will be very small or nil. Very low fault current means no flash hazard and no equipment damage. Circuit operation con- tinues normally with no interruption of power, an impor- tant consideration in industries where downtime is criti- cal (60). The first fault is often hard to locate because its effects are negligible. Often no repair effort is made until a second fault occurs, with its concomitant hazards of arcing, heavy current flow, and equipment damage. Since the entire system is "floating," there is no control of transient overvoltages. Except for the problem of acciden- tal contact with a higher voltage system, all the other overvoltage sources mentioned previously are enhanced because of distributed capacitance to ground (20). Solidly Grounded Neutral An alternative is the solidly grounded neutral (20). The first ground fault produces a substantial neutral current flow, which may be quickly sensed by protective circuitry,thereby shutting down the bad section. Ovewolt- ages are controlled since the system, as illustrated in Suoolv transformer Ground Figure 7.2.-Capacitance coupling in ungroundedsystem. figure 7.3, now has its neutral solidly referencedto ground (20).The hazards of this system are due to the magnitude of the fault current. Detection equipment must be sensi- tive enough to detect low-level fault currents and fast enough to disconnect bad circuits before heavy faults can disrupt system integrity. Large fault currents, typically several thousand amperes, can explode protective enclo- sures, destroy equipment, and start fires, which is an excellent reason for not using this technique in explosive atmospheres. Low-Resistance Grounded Neutral The low-resistancegrounded-neutral system is estab- lished by inserting a resistor between the system neutral and ground. The resistance is such that ground-fault currents are limited from 50 to 600 A, but are commonly about 400 A (20).Transients are controlled by the ground connection, and ample fault current is available for actu- ating protective relays. The flash hazard is not as serious as in the solidly grounded neutral system, but a current flow of 400 A can still do considerable damage. To limit damage, the least sensitiveground relay shouldrespond to 10% of maximum ground-fault current. A schematic dia- gram of this method is shown in figure 7.4 (20). High-ResistanceGrounded Neutral Perhaps the best technique, and that required by law in coal-mining applications on portable or mobile equip- ment, is the high-resistance grounded system, often re- ferred to as the safety ground system. The neutral ground- ing resistor is sized according to the system voltage level, supply transformer secondarv I I I ~ N / ~ N / w / . / ~ Ground Figure 7.3.-Solidly grounded system. Figure 7.4.-Resistance-grounded system.
- 186. in general to limit ground-fault current at 50 A or less. Where the line-to-neutral potential is 1,000V or less, the groundingresistor must limit fault current to 25 A or less; above 1,000 V, the voltage drop in the grounding circuit external to the resistor must be 100V or less under fault conditions, With this system, sensitive relaying must detect faults on the order of a few amperesto provide fault isolation and facilitate quick location of the trouble spot (60). The level of fault current is also low enough to practically eliminate arcing and flashover dangers. The ground connection also serves to limit the amplitude of overvoltages. However, loads cannot be connected line to neutral, as the grounding conductor must not carry any load current. ELECTRIC SHOCK For a safe grounding system to be efficiently and economically designed, voltage and current levels that are harmful to human beings must be determined. With the trend toward larger and more powerful mining machinery, distribution voltage and current levels have risen propor- tionately. Constant vigilance is required when using elec- tricity if the hazard of electrocution is to be avoided. Even if a shock is nonlethal, involuntary movement caused by the shock may lead to serious injury or death. As an example, a man standing upon a ladder may come into contact with a live wire and fall from his perch (12). Physiologically speaking, the muscles of the body are controlled by electrical impulses transmitted from the brain via the nervous system. These pulses occur at a rate of about 100per second and may be of positive or negative polarity. Fkom this, it can be seen that the human "in- ternal power supply" operates at about 50 Hz, which is exactly the frequency of the electric power generated in Eurove. and is onlv 10 Hz removed from the U.S. Dower gene;ation frequency of 60 Hz. This is an unforthnate coincidence,for tests have shownthat the most dangerous frequencies to which a person can be exposed arepower frequencies in the range of 50 to 60 Hz (12). How sensitive are human beings to the flow of elec- tricity? Tests have indicated that for an average male holding No. 7 AWG (American Wire Gauge) copper-wire electrodes in his hands, 60-Hz ac is first perceived at a level of about 1 mA (12). By intermittently touching or tapping an electric conductor, currents of only 113mA can be felt. In the case of dc, the threshold of perceptionfor the average male is 5.2 mA. Sensitivity levels for women in the cases mentioned above can be found by multiplying the male values by a factor of two-thirds(13). It is gener- ally agreed that the magnitude and duration of the cur- kent are the important shock parameters, rather than the potential differenceor voltage (12),as can be seen in table 7.1 (42). As current magnitude is increased above the level of perception, many test subjects have reported a tingling sensation, the intensity of which increases as the current rises. Generally, muscles in the vicinity of the current path start to contract involuntarily, until finally a point is reached where the subject being tested can no longer release his grip on the conductor (14). The maximum current magnitude that a person can withstand while still able to release the live conductor through the use of muscles stimulated directly by the current, is called the let-go current (fig. 7.5) (14, 16). Tests performed on hun- dreds of volunteers have shown that the maximum let-go current for a healthy adult male is 9.0-mA ac and 60-mA dc. The correspondingvalues for women are 6.0-mAac and 41-mA dc. These safe-limit values apply to 99.5% of the sample population (11). The value of a specific individual's let-go current is virtually constant, even with repeated exposuresto that current level.In addition, these multiple exposures can be tolerated with no ill effects (16). It has been stated that human tissue possesses a negative resistance characteristic. In other words, an increase in current magnitude or contactduration leads to a decrease in the value of skin resistance (17). In any case, if a person has grasped a live conductor and realizes that helshecannot let go,fear-inducedperspiration will cause a lowering of the body's resistance, and more current will flow. For ac, when the current level across the chest reaches more than 18to 22 mA, the chest muscles tighten involuntarily and breathing ceases. Although circulation of blood by the heart is unimpaired, death by asphyxiation can occur within minutes (43). If an individual's initial contact with a live wire causes a current flow ranging from about 50 to 500 mA, ventricular fibrillation may result (48). Under normal conditions, the heart beats with a strong, coordinated rhythm. However, a current passing through the heart when the ventricles (the heart's two large pumping cham- bers) are just starting to relax after a contraction, can cause the various fibers of the heart muscle tobeat weakly in an uncoordinatedmanner (43).In this condition,known as ventricular fibrillation, the heart is almost totally incapacitated and blood circulation decreases practically to nothing. Within 2 min, the brain begins to die because of oxygen deficiency. Once initiated, ventricular fibrilla- tion almost never stops spontaneously, and treatment by trained medical personnel must be secured if the victim is to survive. Obviously, people cannot be ussd as test subjects in ventricular fibrillation experiments because of the high risk involved. Numerous tests have been carried out on several species of animals and the results extrapolated, Table 7.1.-Current range and effect on a typlcal man weighing 150 ib Current Physiologicalphenomena Effect on man Less than 1 mA..........None ......................................................................Imperceptible. 1 mA .......................... Perception threshold Mild sensation. 3 mA .......................... Pain threshold......................................................... Painful sensation. 10 mA ........................ Paralysis threshold of arms and hands...................... Person cannot release hand grip; if no grlp, victim may be thrown clear. TigMer grip because of paralysismay allow more current to flow; may be fatal. 30 mA ........................ Respiratoryparalysis..........................................Stoppage of breathing, frequently fatal. 75 mA ........................ Fibrillation threshold (depends on time) .................... Heart action uncoordinated,probablyfatal. 4 A............................. Heart paralysisthreshold (no fibrillation)...................Heart stops on current passage, normally restartswhen current interrupted. Greater than 5 A.........Tissue burning........................................................ Not fatal unlessvital organs are burned. -
- 187. 1 I I 1 I I I I 0 5 1 0 50 1 0 0 5001,000 5,000 FREQUENCY, Hz Figure 7.5.-Effect o f frequency on letqo current for men. based upon body weight,to coverhuman beings(15).It was Eound that fibrillating current is proportional to body weight and inverselyproportionalto the squareroot of the shock duration. Using 50 kg (110 lb) as a body weight, it has been proposed that the value of current that can be d e l y endured by 99.5%of normal adults without causing ventricular fibrillation is (16) where I = rms ac, mA, and t = shock duration, s. As noted, this equation is valid for values of time between 8.3 ms and 5.0 s (15). It may be seen from the above equation that for a 1-s contact time, theventricular fibrillation threshold current is about 116 mA. Since a normal person has a pulse rate between 60 and 80 beats per minute, the critical phase of the heartbeat (when a person is vulnerable to ventricular fibrillation) occurs about once each second. Therefore during a shock lastingfor 1s or more, the heart must paw through this critical phase (48). As a result, it is thought that ventricular fibrillation is the leading cause of death by electric shock. Higher currents on the order of a few amperes will freeze both the chest and heart muscles, thereby prevent- ing the onset of ventricular fibrillation. Generally, the heart will restart upon the cessation of current flow (48). These current magnitudes are less dangerousstatistically than the lower values where fibrillation is prevalent. Further increases in current level, to 5A and above, may produceserious burns leading to shockand possible death, while current levels that substantially elevate body tem- perature produce immediate death (16). In an electric-shocksituation, the victim's electrical resistance playa an important role in determining how much current will flow, as indicated by Ohm's law: For a human being, at leaat three components of resis- tance have been isolated: contact resistance, skin resis- tance, and internal resistance (43). Contact resistance, as illustrated by table 7.2, depends upon the degree of skin moistness and the area of contact with the live conductor (42). Values of 40,000 to 50,000 Q/cm2are given for dry skin and 1,000 Q/cm2for wet skin (13). Skin resistance depends upon the physical condition of the tissues: A person who does rough, heavy outdoor work may have a skin resistance of 10,000 Q, while a value of 1,000 Q is typical of a sedentary ofice worker (43). Internal resis- tance is the resistance of the body's interior and is gener- ally accepted to be about 500 Q between major extremities (25). Table 7.2.-Typical resistance for varlous contact situatlons, ohms Contact ' Dry skin Wet skin Finger touch.................................................. 500,000 20,000 ................ ........................... Hand on wire . . 50,000 10,000 Finger-thumb grasp....................... . ........... 20,000 5,000 Hand holding pliers..................................... 20,000 2.000 .................................................... Palm much 10,000 1,000 Hand holding 1.5-in pipe............................... 2,000 500 2 hands holding 1.5-in pipe..................... . . . 500 NA Hand, immersed...................................... NAP 200 'Skin surfaceonly: resistance may be lower when skin is cut, blistered,or abraded. Voltage magnitude has some effect upon the body's reaction to electric shock, although current is by far the most important parameter. Potentials greater than about 240 V simply puncture the skin, thereby negating the effects of skin resistance (12). There is also some evidence that overall body resistance varies inversely with the applied voltage, although this is subject to disagreement. The relationship is given by (43). where R = resistance, $2, E = potential, V, and n = 1.5to 1.9. Above about 2,400 V, tissue damage due to burning becomes the major cause of electric-shockinjury (42). Thus it can be seen that the body's response to electricity is extremely complex,and currents on the order of a few milliamperes can be fatal if long continued. CHARACTERISTICS OF MINE GROUNDING SYSTEMS The concept of protecting mine electrical equipment and personnel against the consequencesof ground faults by suitable grounding has existed since electricity was first introduced into coal mines. As early as 1916, the Bureau of Mines recommended equipment frame ground- ing as a means of preventing electrical shock to miners working on or around electricalequipment (6). For the coal mining industry, a suitable grounding system has always been a difEcult problem, more complex and difficultthan in other industries. Ground Beds For mine usage, the electrical distribution cables and overhead transmission circuits carry into the mine one or
- 188. more grounding conductors in addition to the phase con- ductors. Each piece of ac equipment has its frame solidly connected via these grounding conductors to a safety ground bed commonly located near the main surface substation and consisting of buried horizontal conductors or drivenrods, or a combination of both. The neutral of the substation transformer secondary is also connected to the safety ground bed through the neutral groundingresistor, as shown in figure 7.6. It should be noted that many important componentsare missing fromthis diagram, and chapter 13 covers substation circuitry in detail. The substation actually requires two ground beds, maintained some distance apart. Lightning discharges and other transformer primary surging conditions are directed to the system or station ground. The system and safety grounds must be kept separate so current flow intended for one will not enter the other. It is essential for the safe operation of the mine power system that the resistance of the beds be maintained at 5 . 00 or less (3,39, 44). A ground bed with this resistance range is often termed a lowresistance gmund bed. 'lb demonstrate one reason for a low-resistance bed, consider a situation where lightning strikes the substa- tion, and 10,000A is discharged through the surge arrest- ers into the system ground bed. If the groundbed is of 5.0-0 resistance, a potential of 50,000V is developed, and the grounding grid of the ground bed becomes elevated to 50 kV above infinite earth. Depending upon the physical extent of the grid, a person walking through the area underlain by the grid could bridge a lethal potential gradient with his or her feet (2).Metallic objects within the potential gradient field can also be elevated to danger- ous potentials and become lethal to the touch. Typical step and touch potentials are illustrated in figures 7.7 and 7 . 8 (2). These step and touch potential hazards are applicable to both the system and safety ground beds. However, the dangers of a high-resistance safety ground bed are not found close to the bed but at the mining equipment. The most insidious feature of the safety ground system is that the equipment connected to it is maintained not at earth potential, but at the safety ground-bed potential. Unless the bed has low resistance, any safety ground-bedcurrent flow can render every piece of mine equipment potentially lethal. The flow can be createdby faults to earth, coupling from lightning strokes to the system ground, lightning strokes to safety grounded machinery, and stray currents from dc haulage systems.Three such cases are illustrated in figures 7.9,7.10,and 7.11 (9). Consequently, with high-resistance ground beds, an elevated frame potential is a problem not just on the machine where it occurs, but everywhere (10). Substation transformer Surge b G r d i r q 7 resistor ~Mductor system I - safety ground = : ground I Figure 7.6.-Simplified oneline diagram of substation. R3 4 Figure 7.7.-Step potentials near grounded structure. PMential rise above remote earth durinp fault ' . R2 4 Flgure 7.8.-Touch potentials near grounded structure. Power center Lwd Gmurding resistw t Fwlt to Gmnd mmfftim I, f to rmchine fmme ts - I$ Safety groundbed I " - - Flguro 7.9.-Linbtwatih fault multing in current flow through srtety ground bad.
- 189. Lighting // Safety &-- ground F bed Figure7.10.-Lightning stroke to equipmentcausingcurrent flow through safety ground bed. j &+---AA Csurge p current I Mining machine & J * grwrd bed Sub~tOth grwnd Figure 7.11.-Lightning stroke current through system ground bed causing elevation o f safety ground bed. Grounding in Underground Mining Early practice in underground coal mining was to drive a metal rod into the mine floor and use that as a ground. In almost every case this arrangement proved to be totally unacceptable, with test measurements indicat- ing 25-0 or more resistance (28). With the exception of pumps, the contact resistance of mining machinery with the mine floor also proved to be too high for adequate grounding. Rail haulage track systems, even though often poorly bonded, showed much lower resistance to ground than most metallic rods driven specifically for that pur- pose. As a solution, Griffith and Gleim (28)in 1943stated that ". . . consideration should be given to a grounding circuit carried to the outside of the mine." Present coal mine practice does just that. A simple form of the bipower (mixed ac-dc) system in use in underground coal mines today is illustrated in figure 7.12. After transformation, three-phase ac power enters the mine to supplythe variousthree-phaseac loads. Someof the ac power is convertedto dc at rectifier stations to power the locomotive system and, occasionally, dc face equipment. More often, any dc face machinery is powered from rectifiers located in the mine section. Except for the trolley system, all dc as well as the ac equipment frames are connected to a common junction, which is tied to the surface safety ground bed. In order for the system to be effective, grounding conductors must be continuous and Surface substation Neufml 'resistw Borehole - grand Grounding conductor I Combination OC-dc secticm power center Flgure 7.12.-One-llne dlagram of simplified mine power system. this continuity must be verified. Groundcheck monitors ensure this. Trolley locomotives generally utilize the overhead trolley wires as the positive conductor and the tracks as the negative. Neither of these is tied to the rectifier- station frame ground. However, because the track is in contact with the mine floor,the negative conductor for the trolley system is grounded. The dc system that supplies power to face equipment normally employstrailing cables that have neither the negative nor positive conductor grounded. Thus, this subsystem is often ungrounded un- less the supply is obtained from the trolley system. Note that in diode-groundedsystems,the negative conductor is grounded. At each transformation step within the power system, such as in a power center,an additional neutral point must be established on the transformer secondary. The neutral is tied through a grounding resistor to the equipment frame and thus via the groundingconductorsto the safety ground bed (an exception will be discussed later). Even with all these grounding points, the ac ground- ing system must be isolated from separate dc power systems. If it is not, dc may appear in the ac grounding system,thus elevatingit abovetrue ground potential. If an ac ground current is present, it will be offset by the dc level. The principal concern is with trolley installations, where isolation is achieved by having no common points between the ac and dc systems. Various techniques have been tried to maintain separation or to eliminate dc offsets while grounding dc face equipment frames.
- 190. Face Equipment Grounding When a working section utilizes an ac continuous miner energized from a section power center and dc shuttle cars powered from the trolley system, the ground potentials of the dc and ac equipment frames are not necessarilyequal,because of the voltage drop in the track. Jacot (33)suggested that this problem could be solved by isolating the low-voltageac neutral point from the power- center frame and also the high-voltagegroundingsystem, and connecting it via an insulated cable to the track, as shown in figure 7.13. The low-voltage neutral point re- mains connected to the ac face-equipment frames. This techniaue should make the low-voltaee ac and dc eauiu- - - - ~ - . . mcnt frame potentials the same, thus eliminatingdc offset ~roblems. Difficulties can still arise with this method. If any track rail bonds are bad between the ac and dc low-voltageground points, the dc frame potentials might be elevated with respect to the ac frames. Further, the power center must be constantly maintained at a safe distance from the tracks to preserve isolation between the track and high-voltagegrounding systems. Another method is shown in figure 7.14A. Here, a section power center supplies power to ac face equipment and also, through a rectifier, to dc machinery (usually shuttle cars). The rectifier is isolated within the section power center, but the dc output is grounded through a center-tapped current-limitingresistor. All dc equipment frames are then grounded by trailing-cable grounding conductors, which in turn are connected to the center tap of the grounding resistor. The latter point is connected to the high-voltagegrounding system. This has been consid- ered a very safe dc ground protective system because it permits the use of protective circuitry to trip the rectifier breaker in case of a dc ground fault (see chapter 9). However, the use of the center-tapped resistor has been criticized (46).On such a system, any failure to maintain grounding-conductor conductivity or accidental connec- tion of a wrong conductor when splicingcables may lead to a hazard. Nevertheless, an important advantage of the method is that the dc and ac frame potentials can be the same. A more recent method for limiting dc ground-fault Ground conductw from power-center frame to Power center Continuous miner(ac1 resistor Lifted f r m frame -/ conductor Shuttle car (dcl Grounding conductor - Trolley system ROIIb n contact with mine floor Figure 7.13.-Mixed ac-dc mine power system; dc load energized from trolley system. current is similar to high-resistance ac grounding and is illustrated in figure 7.14B. In 1963 the Bureau of Mines accepted the use o f silicon diodes as a means of grounding dc face equipment frames. When a diode is used, the groundingresistors are not needed because the frame is grounded through the diode to the negative conductor, as illustrated in figure 7.15. The diode circuit also includes a ground protective device, which will interrupt the power if a current flows from a positive power conductor to an equipment frame (again, see chapter 9). According to Jones (377, diode groundingshouldensure good ground continuity since the same conductor acts as both a dc negative conductor and the grounding connection. However, a grounding diode only protects the dc system against ground faults within the equipment frame. Current leakage to ground or faults within trailing cables can still present hazards. Power-center Rectifier transfwmer ~romGfety ground bed / Mmhine - conductw frame Paver-center frame ground A Resistors between dc line conductorsand grounding conductor Grwnd~ng resistor Trailing cage r Machhne frame cwductar B Res~storbetween transformer neutral and ground~ng conductor Figure 7.14.-System grounding with current4imiting resistors. Load center Machlne frame rectlfler , , - Connected to frame From safety ground bed diode Grounding diode Figure 7.15.-Diode grounding of machine frame.
- 191. h c k Grounding As previously mentioned for trolley systems, the rec- tifier frame is grounded by the ac system, but the negative conductor is grounded to the mine floor through the track. In order to maintain isolation, there is no internal connec- tion between the rectifier output (or the trolley distribu- tion system) and its frame. However, if the rectifier is sitting on the mine floor, there is a possible common point from the track (dc)to the rectifier frame (ad. Ideally, the common point through the earth is a much higher resis- tance than the rail itself so that all rectifier current returns in the rail. When the rail resistance increases because of poor bonding or crossbonding, some current may flow through the earth to or from the rectifier frame, depending on the rail potential. Thus dc is introduced in the ac ground system. Leakage of trolley-wireinsulator to the roof or rib may have the same effect, although it is less common.This lack of effectiveseparation can cause dc offset currents on any mining machine and electrical system whenever the sum of the mine floor resistance and equipment frame contact resistances is too low and, therefore, dc current flow is permitted through the earth. To help minimize any prob- lems, rectifiers should be located no closer than 25 ft from the track. In severe cases, the rectifier frame can be insulated from the mine floor. The preceding has shown that haulage conversion units are the primary source of dc offset currents. Regard- less of the source, once stray dc currents occur, they can exist on all the ac grounds within the mine. This problem is further complicated since these currents may also travel through water pipes and hoses, or anything conductive. The two most undesirable effects of dc offset currents on the ac ground system are nuisance tripping and inter- machine arcing. Nuisance tripping can occur whenever the offset ac waveform is greater than the relay trip value, and it primarily affects ground-overcurrent relays and ground-check monitors. Intermachine arcing occurs when two machine frame potentials are not the same. While they are touching, a current flow is possible, but when they separate, arcing may occur. These problems are discussed further in chapter 17. Grounding in Surface Mines The typical grounding system for a surface coal mine is similar to that for underground mining. One or more substations with resistance-grounded secondaries are em- ployed to transform the incoming utility voltage to the lower potential used by the mining machines. At this level, pit distribution is carried on overhead lines or cables to supply switchhouses located near the particular piece of equipment. A trailing cable completes the power circuit from the switchhouse to the machine. A switchhouse is sometimes connected via cable to a portable substation, which supplies lower voltage power to production, auxil- iary, or lighting equipment. Substation grounding includes both a system and a safety ground bed, each physically removed and electri- cally isolated from the other. Grounding conductors extend from the safety ground bed to all equipment frames. The neutrals of the transformer secondary of portable substa- tions are resistance grounded to the equipment frame. In contrast with underground coal mines where the entire secondary distribution system is underground, both the primary and secondary lines in a surface mine are out in the open where they are exposed to lightning. In fact, equipment such as draglines and shovels are subject to direct strokes (fig. 7.10).For the best possible protection from lightning, it is essential that the grounding system have as low a surge impedance as possible. The key factor here is to provide many short, direct paths to earth. The specifics of lightning protection for all mines are presented in chapter 11. GROUND-BED CONSTRUCTION Sincethe minesite is determined by the location of the rock or mineral tobe extracted, the conditions required for the installation of an adequate ground bed are not always easily met. If annual rainfall is low or soil resistivity is high, an extensive array of buried metallic conductorsmay be necessary to assure a low-resistance connection to earth. Measurement of soil parameters can be made before the construction of a grounding grid is begun, thereby ascertaining the configuration for the metallic network that will yield the desired values of earth resis- tance and potential gradient. After construction, the re- sistance of the selected ground-bed configuration must be checked. Proper design at the time the ground bed is installed will save much time and expense in later years. Present-day ground beds can be divided into two general categories: meshes and rodbeds. A mesh is a horizontal network of metallic conductors arranged in a grid pattern, which is embedded a short distance below the earth's surface. A rodbed is an interconnected network of vertical metallic rods driven into the earth. The metal- lic components for either ground-bed type are also called electrodes. Ground Resistance Any grounding system exhibits some finite resistance with respect to infinite earth, even though it is completely immersed in the soil. When a fault from a power conductor to earth occurs, current can flow through the ground-bed metallic electrodes, across the soil-metal interface, and into the ground. The greater the surface area of metal in intimate contact with the soil, the lower the resistance. Most of the actual resistance exhibited by each metallic conductor occurs within 6 to 10 ft of the electrode, as illustrated in figure 7.16 (36).If the surrounding soil is viewed as a succession of concentric shells, it is easily seen that the shells adjacent to the electrode have a much smaller cross-sectional area, and hence a higher resistance f Grounding electrode Figure 7.16.-Resistance of earth surroundingelectrode.
- 192. than more distant shells. Consequently,the main factors which determine grounding-gridresistance are the physi- cal dimensions of the system and the innate characteris- tics of the soil, primarily its resistivity (51).Figure 7.17 shows how the total resistance of a driven rod varies as it penetrates soil horizons of different resistivity (36). The electric field around a current-carrying wire is analogousto the electrostatic field surrounding a charged conductor of similar shape. By calculating the capacitance of an electrode immersed in the soil, its resistance can then be determined. For a conductor buried deeply in the earth (29), where R = resistance, Q, p = soil resistivity, 0-m, and C = capacitance, F. If the conductor is relatively near the earth's surface,as is usually the case, the effectsof the conductor image, which is located an equal distance above the surface, must be included in the formula, yielding (29) For multielectrode systems, the capacitance of each conductor plus its mutual capacitance with respect to all other conductors must be calculated. By maximizing the capacitance,the resistance can be minimized,which is the desired goal. The two predominant methods for determining the capacitance of earth-electrodesystemsare Howe's average potential method and Maxwell's method of subareas, each of which has a constant charge density and potential ( 2 9 ) . Howe's technique assumes a uniform charge density on each electrode and then calculates the average surface potential. The capacitance can be found from (21) where C = capacitance, F, Q = charge, C, and V = potential, V. Electrode Configuration Formulas One of the most common ground systems is the rodbed. For a single vertical rod ( 2 2 ) , P 41 R = -(ln - - I), 2571 a where p = soil resistivity (Q-mor D-ft) P = length of rod, m or ft, and a = radius of rod, m or ft. Figure 7.18 shows how the resistance and conductance of a typical driven rod vary as the rod length is increased (23).It can be seen that the resistance curve starts to flatten out, which indicates that a length in excessof 15 ft is ineffective. However the conductance curve is almost linear. Figure 7.19, which is an extended version of the previous graph, shows very clearly that even at depths of 100 ft, the conductance increases in direct proportion to the length (23).If the soil can be easily penetrated, deeper Surface o f earth 350 250 150 50 RESISTANCE. . ! I f Test boring Figure 7.17.-Decrease in earth resistance as electrode penetrates deeper soil horizons. DEPTH OF ROD, ff Figure 7.18.-Calculated values of resistance and conduc- tance for %-In rod driven to depth of 25 It. DEPTH OF ROD, ft Figure 7.10.-Calculated values of resistance and conduct- ance for %.in rod driven to depth of 100 ft.
- 193. rods are always better. The simple nomogram shown in figure 7.20may be used to estimate the resistance of a driven rod without carrying out calculations (55). Figure 7.21shows the effect of soil resistivity on the resistance of a driven rod, as well as the benefits gained by using longer rods (22). It could be pointed out that several shorter ground rods are easier to drive than one long rod of the same total length. However, when using multiple rods, the effects of mutual resistance tend to negate some effective- ness, so the resistance of the group is greater than would be expected unless very large spacings are used between electrodes (23). Figure 7.22 shows this effect for rods spaced at a distance equal to their length, while figure 7.23 shows the advantages that accrue when spacing is increased from 0.5to 100 ft (54). Resistivity, Res~stonce, Rod length, ft 100 Rod 1 0 4 1 0 3 0 ~urning point Figure7.20.-Nomogram to provideresistanceof driven rod. Rod ratio is equal to rod length (feet) divided by rod diameter (inches). Exampleis shown for a rodof U-indiameterand 20-ft length, driven in 500.I3.tt soil resistivity, providing about 354 resistance. ROD LENGTH.ft , 2 0 18 1 6 15 200 400 600 800 RESISTANCE, One of the following two formulas can be applied to systems composed of multiple rods. If the rod spacing- to-length ratio is large (spacing > > length), then (50) where n is the number of rods and the other variables are as previously defined. If the rod spacing-to-lengthratio is small (length >> spacing) (50), where A = (a S,, S,, S,, . . . )l/n and S , , = spacing between electrodes 1and 2,and so forth. I I I l l I , d 1 2 3 4 5 1 0 5 0 NUMBER OF RODS Figure 7.22.-Resistance of parallel rods when arranged In straight line or circle with spacing equal to rod length. A, 0.5 ft B C A, 8, C,0, E ore various rod spaclngs between 0.5 and 1 0 0ft I I I I I 2 4 6 8 1 0 NUMBER OF GROUND RODS Figure 7.21.-Resistance of one ground rod, %-indiameter. Figure 7.23.-Variation of earth resistance as number of ground rods Is Increased for various spacings between rods.
- 194. Aformulafor determining the resistance of grounding meshes is given by (53) where L = total length of buried conduetor, z = burial depth, and B = area enclosed inside mesh perimeter. The constants kland k , depend upon the burial depth and the length-to-widthratio of the mesh and may be deter- mined fromthe graphs shownin figures 7.24 and 7.25 (53). However, for a typical mesh where the length and width are similar and the burial depth is a few feet or less, then k, = 1 . 3and k , = 6. In many cases, combinationsof rods and a mesh are used, especially when driven rods are interconnected by bare conductors that are also buried in the soil. For these situations ( 5 3 ) . where R, = rodbed resistance, and R, = mesh resistance. 1.4 - KEY ; 1.3 Curve A :F o r depth z = 0 F (areaf'z Curve B:Focdepth z = ~ Y W area)'& ~ w v e ~ : ~ w d e p t h z = ~ 1 3 5 7 LENGTH-TO-WIDTH RATIO Figure 7.24.-Values of coefficient k, as function of length- to.width ratio of area. KEY Curve A :For depth z =0 (area)" Curve 8:Fordepth z=- 1 0 u Curve C :Fw depth z = 7 (area)'& 1 3 5 7 LENGTH-TO-WIDTH RATIO Figure 7.25.-Values of coefficient k , as function of length- tewldth ratio of area. The mutual resistance, R,, is (53) P 2L L R,,, =--an- + k,- - I ~ L I k, + I ) , (7.12) where L = total length of mesh conductor, and I = length of one rod. If the soil is of uniform resistivity, adding a mesh to a preexisting rodbed, or vice versa, cannot be justified merely from the viewpoint of reduced resistance, sincethe reduction in resistance will seldom amount to more than 10% to 15%. However, the addition of a mesh to a rodbed will usually smooth out the potential-gradient distribu- tion, and the addition of a rodbed to a horizontal mesh generally attenuates seasonal fluctuations in resistance (23,5 5 ) . Other electrode configurationsare in use but are not as widespread as the two covered above. nble 7 . 3summa- rizes most of the other electrode types and gives formulas for determining their resistance (22). As a fmt approxi- mation, the Laurent formula gives a quick and fairly accurate estimate of the ground resistance of any type of system (56): where L = total length of buried bare conductor, and r = equivalent radius of the system. The equivalent radius of a grounding system varies de- pending upon the exact configuration,but a safe estimate is one-half of the length of the longest diagonal line contained by the system (55). Contact resistance between the surface of the elec- trodes and the soil is not normally a significant factor if the bed has been in existence long enough for the soil to settle and compact,but in new beds it may amount to 20% of the total resistance (57). In summary, the best way to achieve a low-resistance ground is to maximize the periphery or areal extent of the grounding system. Conductor diameter has little effect upon resistance, and mechanical strength requirements should be the primary consideration. Because of wide seasonal variations in the soil resistivity of surfacelayers, deeply buried meshes or deeply driven rods are often preferable. This is also advisableif lower resistivity layers are known to exist at depth. Driven rods are usually preferred over buried meshes for three reasons ( 3 9 ) : The expense of earth removal to bury the mesh is avoided. Rods do not require the packing of earth around the buried electrodesto ensure good earth contact. The use of rods can give a desired resistance more easily than using any other ground-bed form. Note that although formulas are excellent for calculating the theoretical resistance of a grounding bed, the actual resistance should alwaysbe measuredwith an earth tester to ensure system integrity.
- 195. Table 7.3.-Approximate resistanceformulas for various electrode configurations E l e c t r o d e Deserip t i o n c o n f i g u r a t i o n Formula One ground rod: l e n g t h L, r a d i u s a Two ground r o d s ; s p a c i n g 2 R = L 1 ) + L ( 1 - I - + 2 L 4 s>L ~ T L a 7 . . .) 4nS 3s2 5 s Two ground r o d s ; s p a c i n g 2 4 R = L ( l n G L + l n 4 L _ 2 + L _ S + L s<L 4nL a . . .) 5 2L 161.' 512L4 Buried h o r i z o n t a l w i r e ; 2 4 R = D ( I n G L + I n & - 2 + S - S + S . l e n g t h 2L, d e p t h s / 2 47TL a . .) S 2L 1 6 ~ ' 5 1 2 ~ ~ ' Right-angle t u r n of w i r e ; 2 4 l e n g t h of arm L, d e p t h 612 R = A ( I n & + I n * - 0.2373 + 0.2146:+ 0 . 1 0 3 5 % - 0 . 0 4 2 4 5 . . .) 4SL a L* L4 Three-point s t a r ; l e n g t h of 2 4 arm L, d e p t h 612 ~ = & ( l n ~ + 1 n + + 1 . 0 7 1 - o . z o 9 ~ + 0 . 2 3 8 ~ - 0 . 0 5 4 ~ . L L~ L . .) Four-point star; l e n g t h of 2 4 arm L, d e p t h S / Z R = n ( l n a + l n a + 2 . 9 1 2 8TL a - 1 . 0 7 1 ~ + 0 . h 4 5 2 - L~ 0.145%. L . .) S i x - p o i n t s t a r ; l e n g t h o f 2 4 arm L, d e p t h $12 R = & ( I n + In + 6.851 - 3.128 5 + 1.758 - L 0.490 k . . .) L2 L4 Eight-point star; l e n g t h of 2 4 arm L, d e p t h s I 2 R = ~ ( l n ~ + l n & + 1 0 . 9 8 - 5 . 5 1 = + L 9 . 2 6 2 - 1 . 1 7 5 . . .) L2 L King of w i r e ; r i n g d i a m e t e r D, w i r e d i a m e t e r d , d e p t h s / 2 R = 1 ( I n 9+ I n B) 2 n 2 ~ Duried h o r i z o n t a l s t r i p ; 2 R = 9 - ( l n C L + - l e n g t h 2L, s e c t i o n a by b , 4 1 7 ~ a + 1 , C L - 1 2 ( n + 1,)' d e p t h s I 2 , b , a 1 8 Buried h o r i z o n t a l round p l a t e ; r a d i u s a, d e p t h s I 2 .) Buried v e r l i c a l round , = L + L (, +7a:!+d p l a t e ; r a d i u s a , d e p t h s / 2 & . . .) 4ns 24s2 320s Two-Layer Earth Structures In many situations, the soil is not homogeneous but consists of two or more distinct layers that are appmxi- mately horizontal and possess differing resistivity values. The effect of a two-layerstructure upon ground resistance depends upon the top-layer thickness, the relative conduc- tivity of the two layers, and the dimensionsof the ground- ing system with respect to the thickness of the first layer (26).Figure 7.26 showsthe potentials and potential gradi- ents for a mesh system in the first layer of a two-layer configuration where the thickness of the first layer ranges from 0.1 to 1,000 m (18). In this case, the first-layer resistivity (pl)is 2 0 0 O-m,and p2 is 600Q-m.The equiva- lent radius of the grounding grid is 10 m, and the reflec- tion factor (K),as defined by the following equation, is 0.5: P2 - P l K=- p2 + pl' It may be seen that the potential gradient depends almost solely upon the first-layer resistivity if the grounding system is wholly immersed in that layer. The effect of first-layer resistivity upon ground-bed resistance in- creased with the thickness of that layer. Thus, if pl <p2, ground resistance will decrease as top-layer thickness increases. Soil-Heating Effects The manner in which a ground bed responds to the flow of current through it depends upon the magnitude and duration of the loading. Two types of loading have been recognized and will be dealt with separately. Long-term loading of the safety ground bed in a mine power system should consist only of currents due to unbalance, the charging of conductor capacitances, and mutual inductance between conductors. At any rate, in a properly functioning system, the current magnitude
- 196. Mesh des~an KEY Curve A : Potentiol rise o f mesh electrode above remote ground Curve B : Potentiol rtse o f center o f mesh above remote ground Curve C :Potentiol difference between center o f 100 ------ mesh and mesh electrode I 1 I I I I I 0.1 I 10 1 0 0 1,000 FIRST-LAYER H E I G H T , m Figure 7.26.-Influence of first-layer height of potentials. should be on the order of a few amperes. If the bed is very low rate. In this situation, the maximum allowable soil extensive, the dissipation of ground current in the soilmay temperature rise is given by (57) cause only a small rise in soil temperature. Becauseof the negativetemperature coefficientof soil,the actual ground- 0.24i2pT bed resistance will decrease(23).If the temperature rise is g=- So ' (7.17) hiah enough to evaporate some soil moisture, then the resistancewill increase somewhat. Capillary action will tend to restore any moisture, and the soil itself will also conduct away some of the heat. Eventually an equilibrium point will be reached where the system is once again stable, although the soil temperature and ground-bed resistance may be slightly altered. The maximum allow- able ground-bed current is given by (50) where p = soil resistivity, O-m, X = soil thermal conductivity, 1.2 W/(m. OC), and 0 = maximum allowable soil temperature rise, OC. If both sides of the equation are multiplied by R, the maximum permissible applied voltage is found to be Generally,the maximum allowabletemperature is 100°C, at which point total evaporation occurs. Therefore 0 may be replaced by (100 - T)where T is the ambient Celsius temperature. The preceding analysis is subject to two restrictions (57): 1.The thermal conductivity, X, is somewhat tempera- ture dependent, and 2. Soil moisture will start to evaporate at tempera- tures below 100° C. Short-term overloading of the grounding system may occur during certain fault situations, but in a properly functioning system, only the grounding conductors, lo- cated inside cables and with the overheadpowerlines, and the neutral resistor should be subjected to fault current. Should a situation occur in which the ground bed is called upon to handle large currents for a short time, heat conduction through the soil may be ignored because of its where p = soil resistivity, O-m, i = current density at electrode surface, Aim2, T = time, h, 6 = soil density, kg/m3, and a = soil specific heat, kWhI0C-kg. So far only the effects of ac upon soil heating have been discussed. Dc causes completely different phenom- ena. The first of these is polarization. The flow of dc through water causes some of the molecules to dissociate into the constituent gases, hydrogen, and oxygen. The resulting gas bubbles eventually form a film on the electrode surfaces, thereby insulating them from the sur- rounding soil,which leads to a dramatic rise in resistance. In addition, dc causes electr~osmosis (alsoreferred to as endosmosis). Here, moisture present in the soil (which is not electrolyzed)tends to migrate toward the negative electrode of the dc source. Actually, cations present in the soil are attracted to the cathode, and the polar water molecule is normally attached to these positive ions. Again, an increase in resistance is the result. Control of Potential Gradients In addition to providing a low-resistance path to ground, the ground bed should also be designed so that potential gradients in the soil surrounding the bed (step and touch potentials) are held to a minimum for the protection of personnel. As a generalization, it can be stated that meshes are superior to rodbeds as far as potential-gradient control is concerned (18,23).This is illustrated by table 7.4, which compares a variety of grounding systems, each having about the same total length of buried conductor (18).The electrodes are buried to a depth of 0.6 m, and as can be seen, grid C (rodbed) shows significantly higher potential than does grid A (mesh).The potential gradients around a mesh may be decreased by making the meshes smaller. Figures 7.27 and 7.28 showthe improvementwhich can be obtained by burying the grounding system to a greater
- 197. Table 7.4.-Comparison of grounding grids with other types of electrodes Maximum Rod or Total length Length mesh Grid mesh ofburied of rods, : " , % W)'p layout conductor, m m potential depth (24). It is obvious that the deeper a bed can be buried, the better will be the gradient control. A rodbed, where the rods are interconnectedby bare conductorswith the entire system buried to a depth of a few feet, should provide both a low resistance and a safe potential gradi- ent. Building a fence around the perimeter of the ground bed is one way of limiting human exposure to hazardous potential gradients. GROUND-BED RESISTANCE MEASUREMENT Measurement Method The accepted technique for determining the resis- tance to infinite earth of a grounding resistance is called the falLoflpotential method (36).Figure 7.2914 shows a drawing of this arrangement (56).Three terminals are required: the ground under examination, a potential elec- trode, and a current electrode. The current electrode is spaced far from the ground system being tested, and the potential electrode is placed at some point on a straight line between the two. The resistance-measuring equip- ment is operated, and a reading is taken. Here, a known current is passed through the current electrode, the volt- age between the potential electrode and ground is mea- sured and the resistance is the ratio V/I. This process is repeated as the potential electrode is moved farther and farther from the grounding electrode, toward the current 6 . 0 Burial depth, from surface to top of rod, ft 0 5 10 1 5 DISTANCE FROM ROD, ft Figure 7.27.-Potential on ground surface due to rod 6 It long and I-in diameter buried vertically at various depths. KEY Lengthof electrode: A X) ft B 20R 0 4 8 12 1 6 20 DISTANCE FROM 0, ft Figure 7.28.-Potential on ground surface due to strlps, 1 in by 0.1 In, of various lengths buriedhorizontallyat depth of 2 It; values givenare those along line OY perpendicularto lengthof strip. electrode. A graph is then drawn in which the ground resistance is the ordinate and the distance between the ground and potential electrodes is the abscissa. Figure 7.29B shows two typical plots that may result (56).Curve a was taken with the current electrode at a greater distance than in curve b. The flat portion of curve a is an indication that the current electrode is now far enough away from the groundingsystem that the mutual effect no longer exists. This is illustrated in figure 7.30 by the hemispheres of influence surrounding the ground and current electrodes(35). The proper spacing for the measurement probes is based upon hemisphericalelectrodes, soany actual ground system must first be converted to an equivalent hemi- sphere before the needed spacing can be determined (56). This may be approximated by assuming that the equiva- lent radius is equal to one-half the length of the longest
- 198. Current source Ammeter Potential Current Ground p-i Current source Ammeter , L - (J Potential (J current electrode , electrode Ground A FalI-of-potential method DISTANCE ( P ) FROM GROUND TO POTENTIAL ELECTRODE (PE) B Earth resistance curves Distance to Distance to current electrode Figure 7.29.-Measuring resistance of grounding system. 0 20 40 60 80 100 RADIUS OF EQUIVALENT HEMISPHERE, ft Megohmmeter L Potent101 Current electrode electrode electrode Figure 7.30.-Concentric earth shells around ground con- nection being tested and around current electrode. diagonal that can be placed inside the perimeter o f the system (that is, 50% of the maximum bed dimension). Figure 7.31 showsthe proper spacingfor both current and potential electrodes for a given equivalent radius of the grounding system (55).The potential-electrode spacing that yields the true value of ground resistance is equal to about 61.8% of the current-electrode spacing. For large ground systems, it may be impossible to attain the neces- sary spacings for potential and current electrodes result- ing from this technique. In that case, the procedure , Figure7.31.-Correct spaclngof auxiliary eiectrodes to give true resistance within 2.0%. outlined earlier may still be followed,that is, varying the potential electrode spacing while keeping the current electrodeat somefixed spacing as far aspossiblefrom the grounding system. The true resistance may then be de- rived from the resulting graph using one of several avail- able methods (56). Ground Test Instruments Certain precautions should be observed when a ground test instrument is chosen. A machinethat uses dc should be avoided because of problems with polarization and electro-osmosis. Ac is satisfactory, but a frequency slightly removed from the actual power frequency is pref- erable so the effects of stray currents can be avoided. On the other hand, if the frequency used is too far removed from the power frequency, erroneous results may occur since ground resistance (impedance)varies with frequency (45). The leads from the instrument to the electrodes should be spaced as far apart as possible to minimize the effects of mutual inductance and capacitance. In a good
- 199. instrument, the resistance of current and potential probes is not critical, but inferior equipment will give readings that vary widely depending upon the probe resistance. Great accuracy in measuring earth-ground resistance is not critical because the earth resistance measurement techniques themselves can never be precise or accurate. GROUND-BED RESISTIVITY In the discussions on resistance it was pointed out that soil resistivity, p, is an important parameter; specifi- cally, ground-bedresistance is directly proportional to soil resistivity. The resisitivity of a material was defined in chapter 2 as the resistance in ohms between the opposite faces of a unit cube of that material. The value of resistiv- ity varies widely depending upon the substance being measured;for rocks and minerals, it may range from to loL7 Q-cm.A general classificationis shown in table 7.5 (19).Efforts have been made to relate resistivity values to the geologic age of various rocks, as can be seen in table 7.6. As a rule, resistivity increases with rock age 0, but there are exceptions (54). Rock structure enters into resistivity determinations, in addition to geologic age. The resistivity of a newly formed rock depends mainly upon the amount of water it contains. Young rock will generally have a large pore volume and hence a fairly significant quantity of connate water; therefore, it will exhibit a low resistivity. As time passes and the rock is subjected to forces that tend to consolidate, compress, or metamorphose it, the pore MI- ume and water content will decrease, with a subsequent increase in resistivity (5). Hard crystalline rocks are usually bad conductors, but if crushed or badly fractured, their resistivity may decrease because of greater porosity (47).Resistivity values for somecommon soils are given in table 7.7 (55). When completely dry, most rocks and minerals are nonconducting, although some metallic ore bodies will carry current (24).The main soil constituents have very high resistivities, and in fact, the oxides of silicon and aluminum are good insulators (50).Figure 7.32 reviews the resistivities of some common rocks, ores, and metals (47). Factors Affecting Resistivity Several factors can affect resistivity, and these are generally considered to include Moisture content, Dissolved salts, Temperature, Soil type, Grain size and distribution, and Location. The level of influencefor each is describedin the following paragraphs. Soil containing no moisture has a very high resistiv- ity. The addition of water causes a sharp increase in conductivity, but the decrease in resistivity rapidly levels off once the moisture content of the soil reaches about 16 wt %, as shown in figure 7.33 (55).Tests by the Bureau of Standards have indicated that resistivity increases mark- edly when moisture content falls below 2 0 4 ( 3 6 ) . Table 7.5.-General resistivity classification Conductivify characteristicof material Resistivity, fl-cm ..................................................................... GOO^ 10-~-10 .......................................................... Intermediate 10 -10 ........................................................................ poor 10 '"-10 " Table 7.6.-Variations in resistivity with geologic age Creta- Pennsyl- Pre- Approxi- ceous, vanian, Cambrian, mate Quarter- Tertiary, Missis- Ordovician, mmbina reSistiviw' nary Quarter- sippian, Devonian tian with fl-m nary Triassic Cambrian Loam C l a y Chalk }gravel in surface Chalk Trap Diabase Shale LimBstone Shale Sandstone Limestone Sandstone Sandstone Quartzite Dolomite Slate Granite Gneiss Table 7.7.-vpical values of wsistivity of some soils bps of soil Resistivity. n-cm ......................................... Loams, garden soils, etc. 500- 5,000 ciays ...................................................................... 800- 5,000 Clay, sand and ravel mixtures 4.000- 25.000 .................... Sand and graveB ~ : ~ : ~ 6.000- 10.000 ..................................... Slate, shale, sandstone, etc. 1,000- 50,000 ................................................... Crystalline mcks 20,000-1,W0,000 ~ o r b k Rock salt quartz Wet limestonetZZl E m m Wet-to-moist granlte, granulite Mora~ne EZmCiays Hematite we Galena ore Magnetiteore I - Pyritewe Graphite ~ G r a p h i n i c shales Psilanelane, hollandite, pyrolusite RESISTIVITY, SL-m Z E Gold Flgure7.32.-Resistivity rangeof some rocks, minerals, and metals. aPure chalcopyrite 1 :Z , PyrMite Lead - 8 b l , , , l a l l l l l l 10-8 I lo4 108 1012
- 200. KEY Sandy loom Top soil R e d clav I 1 I I I 0 8 1 6 24 M O I S T U R E C O N T E N T , wt % of dry soil Figure 7.33.-Variation in soil resistivity with moisturecon- tent. The conductivityof water is not a constant value, and it has noticeable effectson soil resistivity.Very pure water, such as may be found high in the mountains, has a poor conductivity, and as a result, mountain soil may be very wet and still possess a high resistivity (24). l b a large extent, it is the dissolved salts present in the water that make the solutionconductive. Conductioniselectrolyticin nature; that is, current flowsvia the movement ofpositive and negative ions in solution. Thus, the concentration of dissolved salt, the particular type of salt, and the solution temperature all have an influence upon the degree to which a dissolved salt can lower soil resistivity. Figure 7.34 shows the effect of various salts upon resistivity (55). Water has a large negative temperature coefficient of resistivity, and the transition from liquid to solid state is marked by a dramatic rise in resistivity (31).In addition, most electrolytes have a negative temperature coefficient of resistivity, amounting to about -2.0%I0C (24).lhble 7.8 illustrates this effect (34). Table 7.8.-Effect of temperature on resistivity of water Temperature. Resistiviw, 11 Tmperature, Resistivily. OC' n-cm 'C' Qcm ................... 20......................... 7,200 0 (ice) 30,000 I ....................... ....................... 10 9.900 - 5 79,000 0 (liquid) ............... 13.800 -15 ..................... 330.000 'TOconvert to degreesFahrenheit,multiply by 915 and add 32. When a very high impulsecurrent such asa lightning stroke enters a ground bed, the resulting voltage gradient may be so high that the soil breaks down. These current levels can be extremely damaging to the soil. Lower current levels flowing into a ground system for extended KEY A Copper sulfate B Sodium sulfate C Sodium carbonate D Sodium chloride E Calc~urncarbonate F Sodium hydrate 1001 ' I I I I I 0 004 0.08 0.12 0.16 0.20 SOLUTION, % Figure 7.34.-Typical resistivity curves of solutions. periods may heat the soil to the point where most of its moisture will evaporate. When this condition is reached, soil resistivity increases drastically. Different soils are characterizedby various resistivity levels (table 7.7). ' I b a large extent, this is due to the previously discussed effects of structure as it pertains to conductivity. Loams and clays possess a low resistivity, while shales, sandstones, and crystalline rocks occupy the high end of the scale (50). The nature of the particles making up the soil or rock is another aspect of rock structure, which influences conductivitythrough the rock's ability to trap and retain water. Surface tensions cause water to cling to large soil particles or grains; with small-grained substances, mois- ture simply fills up the multitude of pore spaces between individual particles. The range of particle sizes and their packing determines how much of the volume occupiedby a particular soil will be void space and thus available for filling by water. If most of the grains are the same size, total pore volume may range from 26%to 4696, depending upon the manner in which the grains are packed (19). If a particular rock structure or formation is confined to a small geographicalarea, then it probably has a fairly uniform resistivity, excludingareasof subsequent igneous activity. Should the formation be widespread, however, chances are that variable resistivities will be noted de- pending upon location. This is due to the differences in local conditionsthat may have prevailed over a small area during actual deposition or formation of the rock strata. This may alsobe caused by variations in the ground water properties from place to place within a large region (5). Resistivity Measurements The basic procedure for measuring soil resistivity involvesthe determination of the potential gradient on the earth's surface causedby flow of a known current through the area. l b illustrate the basic technique, assume an earth structure composedof two horizontal layers, the top one of
- 201. high resistivity,p,, and the lower one of low resistivity,p,, as shown by figure 7.35 (19). The thickness of the upper layer is given by h. A power source forces current flow through the ground between the two outer electrodes. At very small electrodespacings,the apparentresistivity will approximate p, since most current flow would be confined to the upper layer. At very wide spacings (much larger than h), the apparent resistivity will be about the same as p ~ , because the majority of the current would flow through the deeper layer. Many methods are available for measuring earth resistivity, such asthe techniques of Gish-Rooney,Lee, and Schlumberger. Most of these procedures are based on the arrangement describedby Wenner (58),which is shown in figure7.36(35).Four uniformly spacedelectrodesareused, and a current source is connected across the two outer terminals while the potential drop is measured acrossthe inner terminals. When the electrode length b is small compared with the spacing a, then the resistivity is (51) where p = resistivity, 0-m or 0-ft, a = spacing between electrodes, m or ft, and R = resistance = V/I, n. Someproblems that may arise from the use of this method are Stray currents due to leakage as from motors, Natural currents due to electrolysis of nearby min- erals, Polarization due to use of a dc source, Inductance between the lead conductors, and Leakage from the conductors and the instrument when in wet areas. The first three problems are circumventedthrough the use of an ac source operating at the nonpower frequency of an instrument that generates the equivalent of a square wave. The use of a well-insulatedinstrument and conduc- tors solves the latter two difficulties. The megohmmeter has all these features and is an excellent apparatus for use in work of this type. lb perform a resistivity survey, the megohmmeter is set up as shown in figure 7.36,the instrument is operated, and a resistance value R is read from the built-in meter. The procedure is then repeated at different electrode spacings.A graph may be made comparing the resistivity, p, with the electrode spacing, a, as shown in figure 7.37 (55).For each value of electrode spacing,there is a corre- sponding value of resistivity, p,, seen by the instrument. This apparent resistivity is equal to the resistivity that a semi-infinite homogeneous earth would display at an equal electrodespacingand an identical value of R. In the example shown, the apparent resistivity decreases as electrodespacing increases. The overall shape of the curve indicates that the soil here is composed of two horizontal layers, with the overlying horizon having a higher resis- tivity then the lower one. As the electrode spacing, a, is increased, more and more of the current flow between the outer electrodesoccurs in the deeper layer of the soil, and this is reflected in the continulousdecrease in the apparent resistivity (5). In a case like the onejust described, a groundinggrid composed of deeply driven vertical rods would be best, since the rods would penetrate into the underlying layer of Figure 7.35.-Diagram for four-electrode resistivity survey showing lines of current flow in two-layer earth. 1 Megahrnmeter Figure 7.36.-Connections for Wenner four-terminal resistivity test using megohmmeter; distance a should be at least 20 times b. ELECTRODE SEPARATION Figure 7.37.-Typical curve of resistivity versus electrode separatlon.
- 202. higher conductivity and thus provide a more effective ground. Additionally, soil horizons near the surface are usually subject to wide seasonal variations in resistivity due to changes in ambient temperature and moisture (40). Tagg (55) presents several methods whereby an accu- rate interface-depth determination may be calculated. Values are read from a standard graph, and multiple calculations are then performed, followed by another graph construction from which the correct depth is read. Core drilling has verified that values derived in this manner agree closely with the actual conditions. Effect of Chemical Treatment of Soils The natural resistivity of some soils is so high that it is virtually impossible to construct a ground bed with a satisfactorily low value of resistance. By injecting into the earth a substance whose resistivity is very low, the local soilresistivity can be effectively reduced, thereby lowering the resistance of a moundine a i d . Such chemical treat- - - - ment acts to increase the apparent dimensions of the metallic electrodes (7). The result of chemical treatment is to reduce ground resistance by a considerable amount, often as much as 15% to 90%. Figure 7.38 shows an example of this effect (36). Generally, the percentage improvement is greater for a very high resistance ground. Substances traditionally used as chemical additives include sodium chloride, calcium chloride, copper sulfate, and magnesium sulfate (36). Newer additives include gels composed of acrylamide, silicic acid, or copper ferrocya- nide. In the past, electrodes were sometimes surrounded by a bed of coke, not a true chemical treatment but rather a partial soil substitute (24). The effectiveness of most treatments in lowering ground-bed resistance is about the same, with the ultimate selection depending upon the criteria of cost, availability, and corrosive properties. A prime disadvantage shared by most chemical treat- ments is the fact that they will corrode most metals ( 7 ) . Magnesium sulfate has little or no corrosive effect, and graphite is also innocuous. Other additives generally speed up the decay of grounding electrodes. Another disadvantage is that chemical treatments are dissipated and carried away by neutral drainage through the soil (36).Acrylamide gel, which is not water soluble, is an exception (34).The rate at which chemical additives are washed away depends upon the soil type and porosity as well as the amount of rainfall. Useful life may range from 6 months to 5 or more years. Tho cost of chemical treatment may be higher than the price of driving longer ground rods to reach deeper, lower resistivity soil layers, but in some instances it is not feasible or desirable to increase penetration depth. As shown in figure 7.39, the seasonal variations in resistance that are exhibited by grounding grids because of temper- ature and moisture fluctuations, are attenuated in those cases where chemical treatment has been applied (36). The best method of application, illustrated in figure 7.40, is to dig a circular trench about 1ft deep and with an inside diameter of 18in around each ground rod (36).The additive is placed into the trench and then covered with earth. The area is then flooded with water to initiate the solution process. In this manner, the solution can perme- ate a greater volume of soil, while any corrosive action is minimized. 1,600 r , / Before treatment July Jan. July Jan. July Jan. c: ..1,200 W u 800 L V) MONTH Flgure 7.38.-Reduction in ground mat resistance by sol1 treatment. - - - ( I -. W C Y Treated 0 k z k i 2 July Jan. July MONTH Figure 7.39.-Seasonal reslstrnce varlations attenuated by soil treatment. After treatment Ground 0 rod Figure 7.40.-Trench model of soil treatment. GROUND-BED CORROSION Corrosion is a phenomenon that must be considered in the design of a ground bed. There are three basic ways by which underground corrosion can occur (52): Dissimilar metals connected together electrically and surrounded by an electrolyte such as soil,
- 203. Dissimilar electrolytes in close proximity to the same piece of buried metal, and Stray electrical current leaving a buried metal structure. In the first mode, variations in electrochemicalpoten- tial provide the key to the dilemma. The standard half- cell, upon which most corrosion work is based, consistsof a copper rod bathed in a saturated copper sulfate solution. When measured with reference to the copper and copper sulfate half-cell,each metal displays a certain character- istic potential, as shown in table 7.9 (61).If two metals are joined and immersed in soil, the one whose potential is more negative will dischargecurrent and be corroded, but the more positive(noble)specieswill collect current and be protected. When only one metal is used, corrosion can still occur because of differences in soil composition. Metal in an oxygen-rich zone will be protected, while metal in a relatively oxygen-poor soil horizon will be attacked. For- eign metallic structures in the grounding-grid vicinity, such as pipes, cable sheaths, and building frames, may also act in conjunction with the ground bed to form an anode-cathodecorrosion situation. Table 7.9.-vplcal potentlals of metals in sol1 measuredfmm a copper and copper sulfate reference electrode Magnesium............................................................... -2.5 Aluminum ..................... . . .................................... -1.3 -1.1 -.7 - .2 The engineer designing a ground-bedsystem is faced with the problem of solving two conflicting sets of de- mands. For safegrounding,a very low resistance is desired between the soil and the buried metallic grid. Toeliminate potential-gradient hazards, all metal structures should be tied together. However, protection requires that under- ground metallic structures be insulated from the corrosive effectsof the soil. Similarly,the soil and metallic structure should be isolated from one another (61). This seeming paradox may be remedied by making the correct choice of ground-bedconductor and by applyingsuitable preventive techniques. Copper makes an ideal ground-bedconductor since it is corrosion resistant, has a high electrical conductivity, and is easy to clamp or weld (61). However, a good all-copper system is often ruined by tying it together with noncopper structures in the same locale, thereby leading to the corrosionof the less noble species(52). If the ground bed must be located in an area where steel or lead are present, two options are available. First, an insulating coatine mav be au~lied to the base metals. If this is not feasibg, anwall-st&igroundingsystem is preferable, or one composed of steel rods connected with insulated copper wire (8). The idea here is to minimize the exposed s&ce area of the more noble metal. Normally, steel electrodes can be improved by applying a heavy zinc coating or by driving zinc electrodes in addition to the steel. Known as sacrificial anodes, the zinc conductors will be preferen- tially attached, thereby protecting the steel members. For extra protection, magnesium may be used instead of zinc. In highly corrosivesoils, it may be necessary to utilize an external power source that supplies dc to the soil in order to nullify the natural corrosioncurrents. This is known as cathodic protection (41).For externally driven anodes,zinc or magnesium may be used; graphite and high-silicacast iron are also suitable. It may be seen that judicious choice of grounding materials and the use of corrosion-preventiontechniques such as cathodic protection can provide a ground bed that is both low in resistance and high in longevity. GENERAL GROUND-BED GUIDELINES The primary objective of a grounding system is "to limit the potential rise above ground that appears on the frame and enclosures of the equipment connected to the power system" (30).Consequently,the station ground and safety ground beds shouldbe spaced at least 50ft apart, even though the law presently permits only a 253, sepa- ration (21). A typical voltage-gradient representation is shown in figure 7.41 (30).The two groundbeds must be far enough apart so current surges through the station ground bed will not cause the safety ground bed to rise to more than 100V above infinite earth. Once the site has been selected, the excellent guide developed by King (39) can be used for the design and constructionof low-resistancedriven-rodgroundbeds. The simplified procedure consists of the followingfour steps. 1.Using the Wenner array, earth resistivity is mea- sured along the two lines at right angles to each other, centered across the proposed ground-bed site. Two mea- surements, with 6-ft and then 18-ft spacings, are taken along each line. 2. Depending on the magnitude and homogeneity of the resistivities measured, the rod length, number, and arrangement are selected fmm tables. These tables (39) are based on the same information presented earlier in this chapter but are too extensive to be reproduced here. I I I I I I System gmnd - DISTANCE, ft I I I Safety ground Figure 7.41.-Voltage gradlents in earth during ground-fault condltlona.
- 204. 3. The selected rod configuration is driven, and the rods are interconnected with flexible, bare copper conduc- tors. Recommended size is 410 AWG, and connection should be clamped or brazed, but never soldered(1). 4. The completed bed is measured by the fall-of- potential method to check that its resistance is below 5.0 Q. If it is more, a new resistivity, p,, is calculated by where p, = old resistivity, and R = measured resistance. Again, using the tables, additional rods are selected and then driven. Afterward,the resistance is again measured. Whatever the procedure used to construct the bed, the resistance should be checked not only when it is installed but periodically thereafter to ensure that it is still func- tioning properly. GROUNDING EQUIPMENT The basic resistance-grounded system consists of a resistance inserted between the power-system neutral point and ground. Specific concerns when selecting the grounding resistor are resistance, time rating, insulation, and connection. A problem also exists if there is no available connection to the power-system neutral. Grounding Resistor The ohmic value of the resistor is determined by the line-to-neutral system voltage and the maximum ground- current limit. As stated earlier, when portable or mobile equipment is involved, the maximum limit on low-voltage and medium-voltage systems is 25 A, and the upper current limit on high voltage is set by the grounding- conductor resistance, because flow through this conductor cannot cause any machine frame potential to be elevated more than 100 V above earth potential. However, high- voltage limits are typically chosen at 25 or 50 A (50 A is the maximum allowed in some States). For instance, if grounding-conductor resistance is 3.3 Q, and maximum allowable ground current is 30 A, then 25 A is normally chosen. When the resistance-groundedsystem is feeding only stationary equipment, there is no specifiedmaximum ground current, but industry practice sometimesspecifies low-resistancegrounding with a 400-Alimit. For all appli- cations, sizingthe ohmic value of the groundingresistor is simply performed by dividing the line-to-neutral voltage by the selected ground-currentlimit; conductor impedance is neglected. The technique is justified by the method of symmetrical components for a line-to-neutralfault. Ground current can be limited at a level less than the restricted maximum, but for high-resistance grounding the smallest value chosen has two concerns: ground-fault relaying and charging current. For maximum safety, ground protective circuitry should sense ground current at a fraction of current limit (see chapter 9). Hence, reliable relay operation with electromechanical devices can be a problem if maximum current is less than 15to 20 A. The other limitation is that ground-fault current should al- waysbe greater than the system-chargingcurrent (59).the current required to charge system capacitance when the system is energized (see chapter 11).When very low ground-relaysettings are used, the charging current may itself cause tripping. The second main concern in selecting a grounding resistor is its time rating, or the ability to dissipate heat. A grounding resistor carries only a very small current under normal system operation, but when a ground fault occurs, the current may approach full value. The high current exists until the circuit breaker removes power from the faulted circuit, which may take from a fractionof a second to several seconds dependingupon the protective circuitry used. With correct fault removal, the physical size of the resistor can be small, as very little heat is produced. However,protection devices have been known to malfunction,and in these instances ground current might continue to flow until the power is removed manually. Thus, the resistor must be able to dissipate the power . produced from full ground current for an extended time when portable or mobile equipment is involved. If not, the resistor can burn open and unground the system. Two ratings that ensure safety are continuous and extended time. These are essentially the same, since the extended- time rating refers to a heat-dissipation ability for 90 days per year (32). lb provide a safety margin, the transformer-neutral side of the resistor (often called the hot side) must be insulated from ground at a level to withstand the line- to-line system voltage. Both resistor ends are at ground potential with normal operationbut under a ground fault, the transformer end can approach line-to-neutral poten- tial. To afford good insulation, it is recommended that the resistor frame be placed on porcelain insulators, not tem- porary supports such as wooden blocks. Furthermore, for wye-connectedsecondaries,the transformer-neutral bush- ing must be insulated to at least line-to-neutral voltage. The last concern is the resistor connection. The grounding resistor is installed between the transformer neutral and the safety ground bed. In substations it is important to use insulated conductors, because bare con- ductors can easily compromise the required separation between the system and safety ground beds. Grounding conductors must extend from the ground-bed side of the resistor. Finally, to minimize resistor conductor lengths, the resistor must be located on the power-source end d distribution, as close as possible to the source power transformer. Distances greater than 100ft are usually too long. Grounding Transformers Delta-wye, wyedelta, and delta-delta power trans- formers are extremely important in mine power distribu- tion because they offer very high impedance to zero- sequencecurrents. As a result, a ground fault existing on the secondary will do no more than raise primary line current. However if the transformer has a delta secondary, there is no neutral point to which the grounding system can be connected.Another case where this occursinvolves mines where the utility company owns the substation and supplies ungrounded delta power. For both these situa- tions, a separate grounding transformer is needed to obtain an artificial neutral. The two types of grounding transformers in general use are the zig-zagand wye-delta, with the former being more popular. As shown in figure 7.42, the zig-zag is a special three-phasetransformer designedfor derivingthe neutral.
- 205. The transformer winding interconnectionsare such that a very high impedance is shown for positive-sequence and negative-sequencecurrents but a very low impedance is exhibited during zero-sequenceflow. A wye-delta grounding-transformer bank uses three identical single-phase transformers (fig. 7.43). The pri- mary windings, rated at line-to-neutral voltage, are con- nected in wye among the power-transformer secondary terminals and the grounding-resistor hot side, and the secondariesare connected in delta. Any secondaryvoltage rating can be used. Normally, no secondary current will flow, but during a ground fault, current will circulate in the secondary. This will cause the ground to be shared by the three transformers such that the neutral point will remain at constant potential. Grounding-transformer capacity only needs to be large enoughto carry the maximum ground-faultcurrent. Groundingtransformers' primaries cannot be fused, as an open fuse will essentially unground the system,creating a dangerous situation. Maintransformer delta secmdary Zig-zag circuit breakers Grounding resistor - I l k Figure 7.42.-Della secondary with zig-zag grounding. Incoming power resistor Grou*i4 iI " - - System ground bed Grounding conductor Safety gmnd bed Equipment Figure 7.43.-Delta secondary with wyedelta grounding transformer. SUMMARY Several basic grounding methodologies exist, and each has its merits. The resistance-groundedneutral sys- tem is superior for mining applicationsinvolving portable or mobile equipment. The design of ground beds is a complex field, and many variables must be examinedin an attempt to derive an optimum configuration. A low value of resistance is of primary importanceso dangerouspoten- tials are not developed on machine frames. High potential gradients in the ground-bed area must also be avoided to prevent injury to personnel. A study of electric shock and its effects on humans is helpful in further delineating this subject.Formulashave been presentedthat may be used to predict the earth resistance of a particular metallic array or to determine how much buried metal is needed to achieve a desired value. In order to verify the ground-bed earth resistance, a description of ground test instrumen- tation, its utilization, and data interpretation was also included. When designing a ground bed, corrosion effects and soil-heatingphenomena, causedby current flow in the ground system, must be considered. The resistivity of the soil in which the ground bed is immersed has a significant effect upon its earth resis- tance. Resistivity in turn is influenced by other factors such as earth composition, temperature, and moisture, and a thorough understanding of these relationships will be of use in metallic grounding-network design. Instru- mentation was again discussed, as well as practical appli- cations such asthe determination of the best locationfor a ground bed. Chemical treatment of soils to increase con- ductivity and attenuate seasonal resistivity variations was reviewed. Correct selection and coordination of protective cir- cuitry is essential to gain the full benefits of a low- resistance ground bed. Protective circuitry must be i~,- stalledto monitor current flow in the ground conductorsor the potential drop across the neutral grounding resistor. When properly coordinated, this protective circuitry will quickly shut down faulty sections of the electrical system. In the event of a fault or short circuit on a piece of mine machinery, its frame may become hot or elevated above ground potential. An unsuspecting miner could be seriously injured or killed if the machine is touched. Fast-acting relays and circuit breakers will minimize the length of time during which this shock hazard exists, and the bad circuit will be isolated from the remainder of the system. These protective devices form the subject of chap- ters 9 and 10. The grounding conductors that tie equip- ment frames to the safety ground bed are discussed in the next chapter, "Distribution." REFERENCES 1. American Institute of Electrical Engineers Committee. Ap- plication Guide on Methods of Substation Grounding. Trans. Am. Inst. Electr. Eng., Part 3, v. 73, Apr. 1954. 2. . Voltage Gradients Through the Ground Under Fault Conditions. Trans. Am. Inst. Electr. Eng., Part 3, v. 77, Oct. 1958. 3. American Standards Association. American Standard Safety Rules for Installing and Using Electrical Equipment in and About Coal Mines (M2.1). BuMines IC 8227, 1964. 4. Brereton, D. S., and H. N. Hickok. System Neutral Ground for Chemical Plant Power Systems. Trans. Am. Inst. Electr. Eng., Part 2, v. 74, Nov. 1955. 5. Card, R. H. Earth Resistivity and Geological Structure. Trans. Am. Inst. Electr. Eng., v. 54, Nov. 1935.
- 206. 6. Clark, H. H., and C. M. Means. Suggested Safety Rules for Installing and Using Electrical Equipment in Bituminous Coal Mines. BuMines TP 138, 1916. 7. Clark, R. J.. and B. 0. Watkins. Some Chemical Treatments To Reduce the Resistance of Ground Connections. Trans. Am. Inst. Electr. Eng., Part 3, v. 79, Dec. 1960. 8. Coleman, W. E., and H. G. Forstick. Electric,al Grounding and Cathodic Protection a t the Fairless Works. Trans. Am. Inst. Electr. Eng., Part 2, v. 74, Mar. 1955. 9. Cooley,W. L. Design Consideration Regarding Separation of Mine Safety Ground and Substation Ground Systems. Paper in Conference Record- IAS 12th Annual Meeting (Los Angeles, CA, Oct. 1977). IEEE, 1977. 10. . Evaluation of In-Mine Grounding System and Codification of Ground Bed Construction and Measurement Techniques (grant G0144138, WV Univ.). BuMines OFR 20-77, 1975; NTIS PB 263 119. 11. Dalziel, C. F. Effect of Wave Form on Let-Go Currents. Trans. Am. Inst. Electr. Eng., v. 62, Dec. 1943. 12. , Electric Shock Hazard. IEEE Spectrum, v. 9, Feb. 1972. 13. .The Threshold of Perception Currents. Trans. Am. Inst. Electr. Eng., v. 62, Dec. 1943. 14. Dalziel, C. F., J. B. Lagen, and J. L. Thurston. Electric Shock. Trans. Am. Inst. Electr. Eng., v. 60, Feb. 1941. 15. Dalziel, C. F., and W. R. Lee. Lethal Electric Currents. IEEE Spectrum, v. 8, Sept. 1971. 16. , Re-evaluation of Lethal Electric Currents. IEEE Trans. Ind. and Gen. Appl., v. 4, Sept.lOct. 1968. 17. Dalziel, C. F., and F. P. Massoglia. Let-Go Currents and Voltages. Trans. Am. Inst. Electr. Eng., Part 2, v. 75, May 1956. 18. Dawalibi, F., and D. Mukhedkar. Optimum Design of Substation Grounding in a Two-Layer Earth Structure; Parts I, 11, and 111. IEEE Trans. Power Appar. and Syst., v. 94, Mar.lApr. 1975. 19. Dobrin, M. B. Introduction to Geophysical Prospecting. McGraw-Hill, 1952. 20. Dornetto, L. D. The Importance of Grounding Systems in the Protection of Personnel and Equipment. Paper in Mine Power Distribution. Proceedings: Bureau of Mines Technology Transfer Seminar, Pittsburgh, Pa., March 19,1975.BuMines IC 8694,1975. 21. Dnnki-Jacobs, J. R. The Effects of Arcing Ground Faults on Low-Voltage System Design. IEEE Trans. Ind. Appl., v. 8, May/- June 1972. 22. Dwight, H. B. Calculations of Resistances to Ground. Trans. Am. Inst. Electr. Eng., v. 55, Dec. 1936. 23. Eaton, J. R. Grounding Electric Circuits Effectively; Parts I, 11, and 111. Gen. Electr. Rev., v. 44, June 1941. 24. Fawssett, E., H. W. Grimmitt, G. F. Shotter, and H. G. Tavlor. Practical Aspects of Earthing. J. Inst. Electr. Eng. (Lon- 33. Jacot, H. D. Grounding Practice in Coal Mines. I'res. at Min. Electro-Mech. Maint. Assoc. and IEEE Min. Ind. Comm. Joint Meet., Greensburg, PA, Apr. 1,1961; available from H. D. Jacot, North American Coal Co., Bismark, NL). 34. James G. Biddle Co. (Plymouth Meeting, PA). Getting Down to Earth. Booklet 25T, 1970. 35. . Megger Ground Tester; Special Instructions. Booklet 25-J-1, undated. 36. Jensen, C. Grounding principles and Practice; I1 Establishing Grounds. Electr. Eng., v. 64, Feb. 1945. 37. Jones, D. R. Frame Grounding D-C Mining Machines With Silicon Diodes. Min. Congr. J., v. 51, May 1965. 38. Kaufmann, R. H. Important Functions Performed by an ef- fective Equi~ment Grounding Svstem. IEEE Trans. Ind. and Gen. - " Appl., v. 6, ~ o v l ~ e c . 1970. 39. King, R. L., H. W. Hill, Jr., R. R. Bafana, and W. L. Cooley. Construction of Driven-Rod Ground Beds. BuMines IC 8767, 1978. 40. Kinyon, A. L. Earth Resistivity Measurements for Ground- ing Grids. Trans. Am. Inst. Electr. Eng., Part 3, v. 80, Dec. 1961. 41. Landry, A. P., and I. N. Howell. Trends in Ground Bed Designs for Cathodic Protection of Underground Structures. Trans. Am. Inst. Electr. Eng., Part 2, 78, Jan. 1960. 42. Lee. R. H. Electrical Safetv in Industrial Plants. IEEE Trans. 1nd.' and Gen. Appl., v. 7, ~;n./Feb 1971. 43. Lee, W. R. Death From Electric Shock. Proc. Inst. Electr. Eng. (London), v. 113, Jan. 1966. 44. Lordi. A. C. How To Safely Ground Mine Power Systems. Coal Age, v. 68, Sept. 1963. 45. McCall, M. C., and L. H. Harrison. Son~e Characteristics of the Ean-th as a Conductor of Electric Current. BuMines RI 4903, 1952. 46. Pace, E. M. Management Attitude on Grounding of Elcc- trical Underground Equipment. KY Min. Inst., Oct. 1963. 47. Parasnis, D. S. Mining Geophysics. Elsevier, 1973. 48. Picken, D. A. The Effects of Electricity on Human Beings. Proc. Inst. Electr. Eng. (London), v. 108, Jan. 1961. 49. Regotti, A. A., and H. S. Robinson. Changing Concepts and Equipment Applied on Grounded Low-Voltage Systems. IEEE Trans. Ind. Appl., v. 8, MayIJune 1972. 50. Rudenberg, R. Grounding Principles and Practice: I Fun- damental Considerations on Ground Currents. Electr. Eng., v. 64, Jan. 1945. 51. Ryder, R. W. Earthing Problems. J. Insl. Electr. Eng. (Lon- don), v. 95, Apr. 1948. 52. Schaefer, L. P. Electrical Grounding Systems and Corro- sion. Trans Am. Inst. Electr. Eng., Part 2, v. 74, May 1955. 53. Schwartz, S. J.Analytical Expressions for the Resistance of Grounding Systems. Trans. Am Inst. Electr. Eng., Part 3, v. 73, Ang. 1954. - - 54. Sunde, E. D. Earth Conduction Effects on Transmission don), v. 87, Oct. 1940. - - Systems. Van Nostrand, 1949. 25. Friedlander, G. D. Electricity in Hospitals: Elimination of 55. Tagg, G. F. Earth Resistances. Pitman Publ. Corp., London, Lethal Hazards. IEEE Spectrum, v. 8, Sept. 1971. 1964. 26. Giao, T. N., and M. P. S a m . Effect of a TwoLayer Earth 56. . Measurement of Earth-Electrode Resistance With on the Electric Field Near HVDC Ground Electrodes. IEEE Trans. Perticular Reference to Earth-Electrode Systems Covering a Power Appar. and Syst., v. 91, Nov.lDec. 1972. Large Area. Proc. Inst. Electr. Eng. (London), v. 111,Dec. 1964. 27. Gieneer. J. A. Fourteen Years of Data on the Overation of 57. 'Tavlor. H. G. The Current-LoadineCa~acitv of Earth Elec- One ~undrevd ungrounded 240 and 480 Volt Industrial D'istribution Systems. IEEE Trans. Ind. and Gen. Appl., v. 2, Mar.lApr. 1966. 28. Griffith, F. E., and E. J. Gleim. Grounding Electrical Equip- ment in and About Coal Mines. BuMines RI 3734, 1943. 29. Gross, E. T. B., B. V. Chitnis, and L. J. Stratton. Grounding Grids for High-Voltage Stations. Trans. Am. Inst. Electr. Eng., Part 3, v. 72, Aug. 1953. 30. Hamilton, D. E. Mine Power Systems: What's Your Ground Practice IQ? Coal Ace. v. 66. Feb. 1961. 31. H & S , P. J. 1 ;~nvekti~ation of Earthing Resistances. J. Inst. Electr. Eng. (London), v. 68, 1930. 32. Institute of Electrical and Electronics Engineers (New York). Requirements, Terminology and Test Procedures for Neutral Grounding Devices. Stand. 32-1972. trodes. ~."lnst. Electr. Eng. (London), v.-77, bet. i935. 58. Wenner, F. A Method of Measuring Earth Resistivity. U.S. Bur. Stand., Sci. Paper 258, Oct. 1915. 59. Westinghouse Electric Corp. Relay-Instrument Div. (Newark, NJ). System Neutral Grounding and Ground Fault Pro- tection. PRSC-4E, Ind. and Commer. Power Syst. Appl. Ser., Feb. 1986. 60. Whitt, R. 0. D. Trends and Practices in Grounding and Ground Fault Protection Using Statis Devices. IEEE Trans. Ind. Appl., v. 12, Mar./Apr. 1973. 61. Zastrow, 0. W. Underground Corrosion and Electrical Grounding. IEEE Trans. Ind. and Gen. Appl., v. 6, MaylJune 1967.
- 207. CHAPTER 8.-DISTRIBUTION' The distribution system within a mine consists of various types of cables that conned equipment to power supply,the conductorsthat formthe trolley system used in many underground mines, and the overhead lines that distribute power in some surface mines. The character of the mining operation imposes constraints on the distribu- tion system unlike those of other industries and magnifies its importance within the overall power system. Mining is by definition constantly mobile; hence, the distribution system must be handled and extended frequently and can be susceptible to damage from mobile equipment. The mobility in turn necessitates efficient methods for joining cables and repairing them in orderto minimize production downtime and operating costs. In all mines there is the potential for electric shock when handling distribution components. In the hazardous environment of an under- ground coal mine, damaged systemscan be a potential fire and gas-ignition source. Proper installation and correct handling practices are essential if these hazards are to be minimized. This chapter's purpose is to introduce the various distribution components used in mine power systems, as well as to discuss their construction, installation, and maintenance. Cable systems are covered first and com- prise the majority of chapter content because of their uniqueness to mining. Typical trolley-system arrange- ments are then presented, and the chapter is concluded with a brief introduction to overhead lines. NATURE OF CABLE DISTRIBUTION It was shown in chapter 1that cables can carry the electricity from the substation, where the power is taken from utility company lines, to the point of utilization by a mining machine, pump, conveyor belt, or other piece of equipment. There are many possible variations in mine distribution, and several types of cables can be put to a similar use. Only the most typical schemes are covered in this chapter, but some notable exceptions are included. Representative systems are depicted for underground coal mines in figure 8.1 and for surface coal mines in figure 8.2. Obviously,the circuits shown in the figures are only simplified examples of actual mine systems. In prac- tice, an underground coal mine would not have one long- wall, one continuousmining section,and one conventional section, but several continuous mining sections or several conventional sections in addition to one or more longwall units. Surface mines would usually have more than one dragline and one stripping shovel, not necessarily all electrically powered. As might be supposed, the kind of cable is tied to the application. Examination of figures 8.1 and 8.2 indicates that somecablesremain in stationary locations for several years, while others are moved frequently. The cables that are connected to mining machines are termed portable by the Insulated Cable Engineers Association (ICEA) stan- 'The author wishes to thank Robert H. King, who prepared original material for many sections of this chapter. Thanks are also extended to JamesN. Tbmlinson,who assembledthe originalsectiononsplicing.andto George Luxbacher, who assembled the original material on conduetor ampacities and cable derating. dards (19-21).2 The Code of Federal Regulations uses the term trailing cables for the specific variety of portable cables used in a mine (38). Trailing cables are flame- resistant flexible cables or cords through which electrical energy is transmitted to a machine or accessory. In underground mines, trailing cables are generally attached to the inby end (toward the face) of the power center or distribution box. The portable cables that feed the power center or are attached to the outby end (toward the portal or shaft) have to be moved when the power center is advanced and retreated (perhaps once every 2 weeks), but they are not moved as often as the trailing cables. The most stationary cables are those that bring power into the mine, for instance down the borehole and from the borehole to the portable switchhouses. These are the feeder cables. A special type, designated mine power feeder, can be used for installationsthat may not be moved for severalyears. However, the use of the word feeder here is to denote a cable type rather than a function in distribution. Both feeder and portable cables can be used for feeder applications, where the cable supplies two or more major loads (38). "Italicizednumbersinparenthesesrefer to itemsin the list d references at the end of this chapter. KEY I Feeder or baretwle coble Entry, shaft, 2 Feeder coble or barehole 3 Pwtable coble 4 Trailing cable CONVENTIONAL UNIT t - - - 4 Water pump t - - -4 Belt feeder 5 s . CONTINUOUS UNIT + 4 Shuttle car + 4 Water pump 2 + 4 Roof bolter D I S T R I B U T E LONGWALL UNlT Figure 8.1.-Cable dlstrlbutlon In undergroundcoal mines.
- 208. KEY ! Feeder cable 2 Portable coble 1 3 Trailing coble Switchhouse 3 ~ r o g l m Switchhouse fluO Main substation I 0 ~ 0 ~ 3 Shovel 1 Sw~tchhwse -3 Water pump 3 L~gMing I . .~ . center Figure 8.2.-Cable distribution in surface coal mines. Similarly, in surface mines, the cables that feed from the switchhousesor unit substations to mobile equipment are trailing cables. Those moved only occasionally, which are not connected directly to a machine, are portable cables. Stationary (or near so) cables can be feeder or portable types. Moving the cable is a constant task both under and above ground. Some trailing cables are placed on reels or spools to facilitate moving. Prime instances of reeled cables are cables associated with the reeling devices on board shuttle cars and with mobile cable reels used in conjunction with many draglines. Trailing cables without reels are usually termed drag cables. Regardless of the application, cables are heavy and cumbersome and must often be manipulated by hand. Although the most fre- quent personnel injuries are strains, bruises, and frac- tures, cable handling is always potentially hazardous,and investigations in mines have indicatedthat exposed "live" conductors are a too-common occurrence. Indeed, most fatalities in cable-handling accidents are a result of rou- tine handling of unshielded cable (25). Constant handling also imposes considerable stress on the cables. While cable life israted by manufacturers at up to 20 yr for other industrial applications, in an under- ground mine the actual cable life does not even approach this. Mine personnel have estimated the life of continuous miner cables at 8 months; roof bolter cables at 7 months; and shuttle car cables at 3 months (25),and within this lifespan the cable usually requires frequent repair. It has been estimated, for example, that 75% of the total ma- chine downtime for shuttle cars is cable related. CABLE COMPONENTS Cables are made up of three basic components: the conductor, the insulation, and the jacket, although there may also be fillers, binding, shielding and armor. In basic cable construction, the conductors are surrounded by in- sulation and thejacket coversthe insulation. The design of these components is heavily dependentupon the physical stresses that the cable must withstand in the mine envi- ronment, including tension, heating, flexure, abrasion, and crushing. Hence, a discussion of typical stresses is helpful prior to describing component specifics, cable types (the various component assembliesinto cables),and cable coding, High cable tensions are characteristic of both drag and reeled cables. When combined with other stresses such as flexure and twisting, tension can be very harmful to cable life. Drag cables are pulled around pillar corners, through mud, and over jagged rocks where the drag resistance is high. Consequently, a considerable force can be required to drag the cable and, thus, high tensions can develop. Machinery that utilizes cable-storagereels also fre- quently causes excessive cable tensions (13).For instance, the stored cable on the shuttle car is either payed out of the reel or spooled up into the reel as the machine is trammed. The tension required is dependent upon mine conditions, machine type, and cable size, but must be sufficientlyhigh to prevent running over or pinching slack cable. However, if tensions become too high as a result of sudden jerks on the cable, cable and splice failures can become excessive. In addition, instantaneously high cable tensions can result in cable whipping. This is common with shuttle cars and also occurs on other machines that utilize cable reel storage devices, such as roof bolters, coal drills, and cutting machines. This whipping action is a hazard to mine personnel,who may be struckby the cables as they handle the cables or work nearby. In addition to excessive cable tensions, high cable temperatures frequently occur on machinery that utilizes cable-storage reels (6). The cable is wound on the reel, layer upon layer. Suchlayering prevents the coolingaction of circulating airflow, and heating occurs. Consequently, the cable jacket and insulation may become softened and more susceptible to damage from cutting, tearing, and abrasion. If excessive temperatures occur, the cablejacket and insulation can actually blister or crack, becoming brittle. Thus, the physicaldamagecausedby heating poses another hazard to mine workers who must handle the cable, especially in a wet mine environment. Another common cable stress prevalent in all mining cables is cable flexure. As with any material that is bent, internal tension and compression occur in flexed cables. These stresses cause relative movement of individual wire strands, abrading one wire against another and gradually deteriorating the conductors. Stresses fatigue the conduc- tors, making them brittle and more susceptibleto further damage. Abrasion is also deleterious to cables and can have severe consequences. Cutting or tearing can occur when the cablebecomes snagged or caught on rock, nails, and so on (6).Rippingor tearing of the cablejacket and insulation often results. Such damage can cause immediate cable failure, but more often than not, the damage goes unno- ticed. In a wet environment, water penetration can create a current path to the outer surface of the cable. An individual could come in contact with the wet cable several feet from a damaged area and still receive a shock that might be fatal. Another important cause of failure is cable crushing (6).This is usually the result of runovers or pinching the
- 209. cable with a machine frame. Here, the conductors are compressed against one another or against the machine, causing the insulation and jacket to split, as well as damaging the conductors. Even if there is no immediate failure, line-to-lineor line-to-neutral faults that result in nuisance tripping of the protective circuit breakers can occur later. Water penetrating into damaged areas of the jacket can eventually work into areas of damaged insula- tion causing short circuits or a safety hazard. Conductors Line and ground currents are carried by either copper or aluminum conductors, depending on the specific char- acteristics required. Copper has high conductivity, is heavier and more flexible, but also more expensive. Be- cause of its greater flexibility, it is used in all portable mining cables. Copper cable conductors are usually composed of many fine wires combined into strands. Varying numbers of strands form the conductor. At the cable manufacturing plant, a cold-drawingprocess is used in which the copper rod passes through successivelysmaller dies to reduce its diameter (5). This process hardens the copper and makes it less flexible, sothat if a soft-tempercopper (strength about 24,000psi) is required, the'wire must be annealed. Con- ductors that require a high tensile strength but are not bent frequently use medium- to hard-temper copper; medium-hard is rated at 40,000psi. Copper conductors can become annealed in service if they are used at high operating temperatures for long periods of time. In fact, copper can lose 58 of its original tensile strength in 10,000h at 70°C (5).Cable manufac- turers should always be consulted about the capability of their products to resist annealing when installed as bore- hole or high-tension overhead cables. lb prevent corrosion by insulation vulcanizing agents, copper strands are usu- ally coated or tinned with lead or tin alloys, though this reduces the surface conductivity. Aluminum conductors are also used in mines. Alumi- num is cheaper, lighter, and less flexible, and has lower conductivity than copper. Aluminum conductivity is 61% that of copper; therefore, an aluminum conductor must have a cross-sectional area 1.59times that of copper to have an equivalent dc resistance. However, copper conduc- tors weigh 3.3times as much as aluminum; soeventhough the cross-sectional area of an aluminum conductor is greater, the total weight of an equivalent-resistance alu- minum conductor is less. Poor flexibility eliminates the use of aluminum in trailing cables. ~luminum is some- times used for feeder cables because of its lower cost, but problems can arise injointing. An improperly constructed joint can allow the formation of aluminum oxides. which increase resistance and cause heating at the connection. Extreme care must alsobe taken to exclude moisturefrom any copper-to-aluminum joints because of the potential for electrolytic corrosion of the aluminum. ConductorSires The cross-sectionalarea of conductorsis important for mechanical strength and is closely related to current- carrying capacity.Sincethe proper capacity isboth a legal requirement and a desirable practicefor safeoperation,an understanding is needed of the methods commonly used to specifycross-sectionalareas and ampacities.In the United States, both the American Wire Gauge (AWG)(or Brown and Sharpe Gauge) and circular-mil designations (MCM) are used (1crnil is the area of a circle that is 1 mil in diameter).The AWGspecifies38steps or sizesbetween No. 36,which is 0.0050in. in diameter, and No. 4 1 0 ,which is 0.4600in. in diameter (5).These sizes closely conform to the steps of the wire-drawingprocess. Table 8.1specifies the cross-sectionalareas and equivalent circular-milsizes for some of the AWG designations. The 38 intermediate sizes are calculated in a geometric progression relating the ratio of any diameter to the next smaller or larger by: Table 8.1.-Conductor sizes and cross-sectionalareas AWG: 22................ 20................ IS................ 18................ 17................ 16................ 15................ 14................ 13................ 12................ 11................ 10................ Cross-sectional Conductor size area nmil in2 MCM: 250.............. 300.............. 350.............. 400.............. 450.............. 500.............. 550.............. 600.............. 650.............. 700.............. 750.............. 800.............. Cross- Conductor size sectional area. in2 Shortcut conductor-sizeapproximationscan be made by applying some simple rules if a table is not available. For example, the diameter will be doubled or halved by moving six sizes up or down the table. The weight, area, and dc resistance is doubled or halved by moving three gauge sizes, and they are changed by a factor of 10over 10 gauge sizes. A convenient reference point from which to apply these rules is the Na 10 wire since its diameter is about 0 . 1 in., its dc resistance is nearly 1fi per 1,000R, and it weighs lor lb per 1,000ft. In applying these rules, it should be remembered that the outer diameter and weight of conductors depends on the stranding configura- tion, which is described below. Federal regulations require grounding conductors to have at least one-half of the cross-sectional area of the power conductors(38).When the power conductor is a Na 8AWG or smaller, the grounding conductor should be the samesize as the power conductor.The ground-check(pilot) conductor must not be smaller than a No. 10AWG (38).
- 210. Conductor Stranding In order to obtain the required flexibility, mining cable conductors are made with numerous small wires rather than a single solid copper rod. These small wires are wound or laid together in strands, which are wound together in a rope in specific patterns. In a shuttle car cable, 37 wires are wound or bunched together, then 7 of these strands are spiraled together to form the conductor. Consequently,the total number ofwires in this case is 259 and equals the number of strands multiplied by the number of wires in each. The cross-sectional area of a stranded conductor is defined as the sum of the area of its component wires. In the simplestterms, conductorflexibilityis greatest when the largest number of small-diameter wires is used. However, a certain amount of tensile strength is also required in mining cable conductors, and the tensile strength is greatest when a small number of larger wires is used. The design of a specific cable must therefore optimizethese opposingfactors,while taking into account the effects of twisting and bunching. Different applica- tions obviously necessitate different configurations. The engineer must examine cable stranding specifications carefullyand selectthe one that best suitsthe application. Where historical information is not available, several types shouldbe tried to find the best performer. Flexibility is also influenced by the method of insulating the power and ground-check conductors and applying the overall jacket. Insulation Insulation of mining cables is required to withstand stress from heat, voltage, and physical abuse. The insula- tion must be specially designed not only to protect mine personnel from electric shock, but also to separate power and grounding circuits effectively. Heating affects insulating materials in different ways, depending on their chemical composition. Heating either sofiens insulation, causing it to lose physical strength, or causes it to age or become brittle. Conse- quently, heat can make insulation lose its original shape, tensile strength, cut resistance, elongation, and effective- ness as an insulator. The main sources of heat are the environment, related to the ambient temperature, and power (12R) loss in the cable conductors. Hence, cable heating is directly connected to the maximum current the conductors can carry safely. Cable manufacturers usually prefer to use a thermo- setting insulation. After being extended over the conduc- tors, this insulation changes chemically by vulcanizing into a material that softens very little within the rated temperature range. The most common insulating com- pounds in this group are neoprene, styrene butadiene (SBR), ethylene propylene (EPR),and crosslinked polyeth- ylene (XLP). These compounds are usually mixed with other materials to achieve improved physical and electri- cal properties. SBR is used in 600-Vtrailing-cable insulation. It has a high modulus of elasticity, good flexibility, and a 75OC temperature rating, and resists damage by crushing from runovers and rock falls. EPR has replaced SBR in many trailing cablesbecause it allows the cable rated voltage to be increased to 2,000 V and the temperature rating to 90°C, while maintaining the same insulation thickness as SBR and neoprene. The EPR emergency-overloadrating is 130°C, and the short-circuitrating is 250°C. XLP is also rated at 90°C for normal operation and is used in high- voltage (>1,000V)mine-feeder and portable strip-mining cables. XLP is a rather stiff material, however, and is not recommended for reeling applications. The cable voltage rating is closely associated with the maximum anticipated operating voltage. The most com- mon ratings for mining cables are 600 V, 2 kV, 5 kV, 8kV, 15kV, and 25 kV. The 5-kV,8-kV,15-kV,and 25-kVratings are used primarily for stationary feeder cables and are generally not connected to mining equipment, except in surfacemines. Usually,4.16-kV distribution requires 5-kV rated cables, 7.2 kV requires 8 kV, and 12.47 kV and 13.2 kV require 15-kVratings. The utilization voltages of 250 Vdc, 440 Vac, and 550 Vdc usually call for 600-Vor 2-kV cables, and medium-voltageapplications (661to 1,000V) need 2-kV insulation. The voltage rating of an insulation is actually based on its ability to withstand a test voltage that is many times the anticipated operating voltage, for a specified period of time. The test procedure and specifications are published in ICEA standards (19-21). Insulating com- pounds have different voltage ratings, which are usually expressed as the amount of voltage they can withstand per mil of thickness. Consequently, higher voltages can be used with any compound by increasing its thickness. Insulation thicknesses are also specified by ICEA. Insulation must resist damage from corona, particu- larly in high-voltageapplications,as discussed in detail in chapter 17. The term partial discharge describesthe type of corona stress imposed on cables. Partial discharges deteriorate insulation by ion bombardment and chemical action from ozone, nitrogen oxides, and nitric acid, which can occur in such voids as found between a stranded conductor surface and the insulation. Hence, insulation voids must be minimized and the insulation must resist the formation of this type of corona. ICEA standards specify corona-extinctionvoltage levels for insulation (19- 21). Ozone resistance is important for high-voltagecable insulation and sometimes for low voltage, and standards are again given by ICEA. Ozone is formed when electrical discharge is present in air, and it attacks compounds containing double carbon bonds, by splitting the carbon chain and deteriorating the material. Radiating cracksare a physical symptom of this occurrence. Insulation must withstand cold temperatures as well as heat, particularly in surface operations: some of the open-pitiron mines in Minnesota and Michigan, for exam- ple, have experienced temperatures as low as -50°C. Cablesstored on the surfaceat underground minesites are also exposed to extremely low temperatures. Most prob- lems occur when a cold cable must withstand mechanical stress, such as bending or impact. Cable Jacket The main purpose of thejacket is to provideprotection for the inner components and hold the assembly in the designed configuration. Jackets are not required to pass ICEA voltage withstand or insulation resistance tests, but tests for tensile strength, elongation, and aging are man- datory. Ozone and discharge-resisting jackets must also pass surface-resistivity and partial-discharge tests. Min- ing cable jackets must withstand an extensive tempera- ture range, maintaining their physical propertiesthrough- out, and furthermore, they must not deteriorate when
- 211. exposed to direct sunlight. Obviously, resistance to abra- sion, crushing, tearing, and impact are extremely impor- tant. Cablejackets must also be resistant to the chemical action of acid or basic mine water and hydraulic fluids, and underground coal mine cable jackets must be flame resistant. Finally, jackets must exclude moisture and be very flexible. One of the most commonly used materials for cable jackets is neoprene, a chloroprene polymer. Nitrile buta- diene and polyvinyl chloride WBWPVC) is also used, particularly where jacket coloring is desired. Chlorosul- fonated polyethylene (CSP)or Hypalon synthetic rubber is alsoused extensively,especially in combinationwith 90% EPR insulation. EPR is used where extreme cold is en- countered and flame resistance is not essential. Armored cables are used in some borehole applications. Here the jacket is a heavy metallic covering that affords extra protection to the conductors. Cable Shielding The ICEA defines the practice of shielding an electri- cal power cable as confiningthe electric field to the inside of the cable insulation or assembly with a grounded conducting medium called a shield (19-21). Two shield types are used in practice: the conductor shield and the insulation shield. Shown in figure 8.3, the conductor shield is placed betweenthe conductor and the insulation, and the insulation shield surrounds the insulation. Two distinct types of materials are employed in con- structing cable shields: nonmetallic and metallic. Nonme- tallic shields may consist of a conductingtape or a layer of extruded conducting compound. The tape may be made from conductingcompound, be a conductingfibrous tape, or be a fibrous tape faced or filled with conducting com- pound. A typical conducting compound is carbon- impregnated rubber, which is commonly referred to as a conductive-rubbel; semiconducting, or semicon shield. Me- tallic shields are nonmagnetic and may consist of a thin metal tape, wire-woven braid, or concentric serving of wires. Copper-braided shields may be made entirely of copper wires or have nylon twine in combination with Insulation, Conductor Insul shield Insulation Figure 8.3.-Shield types. copper wires. Nonmetallic and metallic elements may be juxtaposed to form the shield. Conductor shields are made of nonmetallic materials and are used only in high-voltagecable. The roles of this shieldtype are to eliminate air spacesor voidsbetween the conductor and the insulation and to present a smooth electrodeto the inner insulation surface. To be effective, it must adhere to or remain in intimate contact with the insulation under all conditions. This can substantially reduce the number of sites where partial discharge can form and helpsreduceelectrical stresson the insulation by uniformly distributing the electrical field about the con- ductor. The use of conductor shields becomes critical at higher operating voltages, especially 12.47 kV and above. Insulation shields can perform three principal func- tions. If placed directly over individual conductor insula- tions, along with confining the electric field caused by conductor current within the insulation, the shield helps to maintain a symmetrical radial distribution of voltage stress within the dielectric. The possibility of partial discharges is minimized by precluding tangential and longitudinal stresses, and insulation is utilized to its greatest efficiencyand in the directionof highest strength. This again becomes critical at higher operating voltages. Insulator shields also provide a continuous capacitanceto ground for the conductor along its entire length. The uniformity is important in terms of transients on the power system, and this is discussed in chapter 11. The third function of insulation shields is the most important for mining in view of the extensive handling of cables: reducing the hazard of electric shock. A major cause of electrical fatalities in mining has been workers' cutting into energized unshielded cables, for instance, during repair. Another source has been handling of ener- gized unshielded cables with damagedjacketing and insu- lation or splices(thespot where a cable has been repaired). An insulation shield can be thought of as a safety barrier to penetrating metallic objects. If the percent of coverage of the shield over the insulation is high enough and its impedance is low enough, any metallic object compromis- ing the conductorinsulation will establish a fault between the power conductor and the grounded shield, with s u f l i - cient current to trip the ground-fault protective circuitry. Damage to insulation and jacketing, such as a pinhole, that would cause a handling danger to unshielded cable also creates a probable ground fault in cables with insu- lation shields. An individual touching the penetrating metallic object or handling the damaged shielded cable should be safe from electrocution. Insulation shields are usually metallic.Recently,how- ever, semicon insulation shields for trailing cables have found application in the United Kingdom, Australia, and to a lesser extent, the United States. This is to take advantage of semicon flexibility,especially in reeled-cable situations. CABLE TYPES An identifying code,related to standard specifications designated by ICEA, is embossed on the cable throughout its entire length. The code includes any approval number for flame resistance by the Mine Safety and Health Administration (MSHA) and approval by the Common- wealth of Pennsylvania (indicated by the letter P preced- ing the MSHA approval number). MSHA approval is mandated for cables in underground coal mines, and the
- 212. Pennsylvania approval is necessary for cables used in underground coal mines in that State. The code includesthe term d c where n is the number of power conductorsin the cable, an approved voltage designa- tion, and letters describingthe cable type. Table 8.2 summa- rizes the meaning of the letters used in the code, and table 8.3presents the codesfor typical cabletypesused in mining. Figures 8.4,8.5,and 8.6 correspond to table 8.3 for un- shielded round, unshielded flat, and shielded cable configu- rations,respectively, and detailthe cablecomponentsasseen in cross section. Photographs of actual mining cables are provided in figures 8.7,8.8,and 8.9 and show both side and cross-sectionalviews. Figures 8.10 and 8.11 are similar to figures 8.1.and 8.2 and show common applicationsof cable types in mine power systems. Figure 8.7A is a single-conductorcable insulated for use at 600 V. This specific cable is not widely used. , However, it has found application on twin-reel dc shuttle cars and small locomotiveswith reels; therefore,it must be highly flexible. Single-conductor cable similar to that shown is used extensively for connections inside power equipment, and a typical voltage rating is 15 kV for system voltages less than that level. The most common dc shuttle-car cables are types W and G, figures 8.8A and 8.8B,respectively. The flat con- figuration is used since it allows an increased length on cable reels and is less susceptibleto runover damage than round cables. The type W is used wherediode groundingis allowedin lieu of a separate groundingconductor. Because shuttle car cables are damaged frequently, type W is preferred by some mine operators since it is easier to repair (splice). Flat cable types employed for ac shuttle cars are shown in figure 8.8C and 8.80.The three power conduc- tors are separated by two grounding conductors in figure 8.8Cand by one grounding and one ground-check conduc- tor in figure 8.80.These cables are also used on other equipment with reels, such as cuttingmachines and drills. Table 8.2.-Letters used in alphabetic cable code Code Meaning Comments ..................... G.......................... Contains uninsulatedgrounding conductor(s) Common on low-voltageac systems but used on dc Systems where grounding conductors are needed. ..................... W ........................ Without uninsulatedgroundingconductor(s). Typical on dc dlodegroundedsystems but 1 insulated power conductor may be used as a grounding conductor. ............. GC ....................... Includes insulated ground-check (pilot) conductor. Usedwhere pilot-type ground-continuity monitoringis required, usually replaces 1groundingconductor of type G cable. SH........................ Shielded cab D.......................... Multiple insul Shields surround each individual powerconductor insulation. C.......................... 1 insulation shiel 1 shield surrounds entire cable assembly just inside jacketing. MP ....................... Mine power feed Table 6.3.-Codes for typical cables used in mining. Code Comwnents Comments W ......................... Contains 2 .3, or 4 insulatedpower conductors.......................... G.......................... Contains2 or 3 insulatedpower conductors and 1 to 3 uninsulatedomundino conductors. G-GC ................... Contains 3 in&lated power conductors, 1 or 2 uninsulated grounding conductors, and 1 insulatedground-check conductor. G+GC ................. Contains3 insulatedpower conductors. 3 uninsulatedaroundins conductors, and 1 insulatedground-checr conductor.- SH-D ............. Conlalns 3 sh~elded insulated power conductors. 2 or 3 uninsulated groundingconductors. SH-C ................... Contains 3 insulatedpower conductors, 2 or 3 uninsulated grounding conductors, assembly shielded. SHDGC .............. Contains3 shielded insulatedpower conductors, 1 or 2 uninsulated grounding conductors, and 1 insulated ground-check conductor. SHD+GC ............. Contains3 shielded insulatedpower conductors. 3 uninsulated grounding conductors, and 1 insulatedground-checkconductor. SHC-GC .............. Contains 3 insulatedpower conductors, 1 or 2 uninsulated grounding conductors, 1 groundcheck conductor, assembly shielded. MPF..................... Contains3 shielded insulatedpower conductors, 3 uninsulated groundingconductors. MP-GC ................ Contains 3 shielded insulatedpower conductors, 2 uninsulated grounding conductors, and 1 groundsheck conductor. 'Although not presently available, 2/C cable design for dc systems is possible. See table 8.2. Flat or round cross section. Grounding conductors are placed in the intersticesbetween the power conductors. Flat or mund cross seclion. Groundcheck conductor replaces 1 grounding conductor of type G cable. Flat or round cross section. Presently, for ac syslerns only.' Similar to round 3/C type G cable but has groundcheck conductor in cable center. Insulationshields about each individualconductor, grounding conductors contact shields. Highvoltagecables usually have conductor shields. Round or flat cross sections. Presently for ac svstems onlv.' A flexible wrtable cable. shielding endloses all con'ductors and IS locatedjust under iacketlna. Gmundina conductors should contact shleld. Round br flat cmss sections. Presently for ac systems only.' A flexible ponable cable. Ground-checkconductor replaces 1 grounding conductor of type SH-D cables. Round or flat cmss section. Presently for ac systems only.' A flexible portablecable. Similar to mund 31C type SH-D cable but has groundcheck conductor in cable center. Groundcheckconductor replaces 1 grounding conductor of type SH-C cables. Round or flat cross section. Presentlyfor ac systems only.' A flexible portablecable. Slmilar to round SH-17 cable. Designedfor relativelysfationary high-voltage feeder applications. Similar to round SHD-GC cable. Designed for relatively stationaly high-voltage feeder applications.
- 213. Conductor insulation Uninsulated grounding conductors Insulated Power ground-check conductor conductor Type W Type G Type G-GC Type G+GC -' Figure 8.4.-Cross sections of round unshielded mining cables. Fillers, may rat be neededif ccnduetor insulation f~lls voids C0d"CtW insubticn / 2/C type W Conductor shield, Conductor copper bald if insulaflon SHD-GC, metallic tape if MP-GC Grwnding / cMductOr 2/C type G 3/C type G 3/C type G-GC Figure 8.5.-Cross sections of flat unshielded mining cables. & y p Jacket Grounding conductor may contact shield Conductor shield Gmurdtnq conductors /i contact shield Jacket I Grwndlng conductors nny contact shield Type SHD-GC or MP-GC Type SH-C Flat type SH-D Flat type SHC-GC Figure 8.6.-Cross sections of some shielded mining cables.
- 214. /A/ l/C,SOOV (Bl 2/C type G, 6 0 0 V ( B l 3/C type G-GC, 2,000V (Cl 3/C type G, 6 0 0 V (Cl 3/C type GtGC, 2,000 V (Dl 3/C type G-GC, 6 0 0 V Figure 8.7.-Round unshieided mining cable. (Courtesy Figure 8.8.-Flat unshieldedminingcables. (CourtesyAnacon. Anaconda EricssonCo.) da Ericsson Co.) fAl 3/C type SHD-GC, 2,000V 3/C type SHD-GC, 15kV (BI 3/C type SHDtGC, 2,000V (Dl 3/C type MP-GC, 15kV Figure 8.8.-Round shielded miningcables. (Courtesy Anaconda Ericsson Co.)
- 215. KEY / Borehole cable; 3/Ctype MP-GC; 5,8.15,or 25kV 2 3/C type MP-GC.SHD-GC.or SHDtGC;5,8.15.or25kV 3 3/C type SHD-GC or SHDtGC; 5.8.15.or 25kV 4 3/C type G,G-GC, or GtGC; 2kV 5 3/C type Gor G-GC, flot, 2 kV I 6 2/C type W or G, flot, 2kV I Entry, shaft, k CONVENTIONAL UNIT QSwitchhouse 1 76 6 250-Vdc 250-Vdc Shuttle Shuttle car car R CONTINUOUS UNIT (550 Voc) 1 1 - I r-4- 5 Shuttle cor 1 - - 4 Water pump + 4 Roof bolter control Figure8.10.-Cable types for typical distribution systems in underground coal mines. The ac shuttle cars also utilize round cables of type G or type G-GC. The grounding conductorsare placed in the interstices between the power conductors in the type G, and a ground-checkconductor replaces one of the ground- ing conductorsin the type GGC (fig.8.7B).In additionto limited use on shuttle cars, the majority of longwall shearer, faceconveyor,stage-loader,roof-bolter,feeder, and continuous-miner cables are of this type. In some in- stances, the G-GC configuration can initiate induced voltages in the frame-groundingsystem (see chapter 17). Therefore, the G+GC type shown in figure 8.7C was constructed. Here the three grounding conductorsare laid symmetrically in each interstice, and the ground-check conductor is placed in the center of the cable. There are two basic configurationsfor shieldedcables: the SH-D and the SH-C. As shown in figure 8.6, the shield of the SH-D cable surrounds each insulated conduc- tor; in the SH-C cable, the shielding encloses all power conductors and grounding conductors. The SH-D shield- ing is preferred because the grounding conductor is in intimate contact with the shield, and line-to-lineleakage current is detectable since the shield surrounds each individual power conductor. The SH-C shield, a single braid over the entire assembly, is sometimes found in low-voltageand medium-voltageportable cables. However, special designs are required to assure consistent, low- KEY I 3/C type MP, MP-GC, or M P t GC; 5,415, or 25kV 2 3/C type SHD, SHD-GC. or SHDcGC, 5,8,15,ar 25kV 3 3/C type SHD, SHD-GC,or SHDtGC, 2 kV Main substation t - - - -3 Water pump t - - - - 3 Lighting Power center Figure8.11.-Cable types for typical distributionsystems in surface coal mines. resistance contact between the shield and the grounding conductors. In high-voltagecables, the insulation shield is gener- ally comprised of two parts: an extruded layer or wrap of semiconductingmaterial applied directly over the insula- tion, and a metallic cover applied over the semiconducting layer. The semiconductivematerial is consideredto have a 100%coverage, but an associated high resistivity. If the metallic layer is composed entirely of copper braid, its coverage is 84%while the combination copper-nylonbraid covers 60%. Shielding of unidirectional spirally wound wires, which gives 60% coverage, may also be used. The high-voltageinsulation shield must be in intimate contact with the insulation under all conditions in order to be effective, and the metallic portion serves as a current- carrying medium for charging and leakage currents. Fed- eral regulations require SH-D shielding for high-voltage cables in underground coal mines. Both SH-D and SH-C shielding are permitted for medium-voltage cables. Medium-voltage cables used on reels do not have to be shielded if the insulation is rated at 2 kV (38). Two round shielded-cable configurations, SHD-GC and SHD+GC, are also used extensively for medium- voltage and high-voltage cables. The 2,000-V-ratedSHD- GC cable, shown in figure 8.9A, and the SHD+GC cable in figure 8.9B are common on such equipment as 950-Vac continuous miners and longwall shearers, and on low- voltage surface coal mine equipment. Some high-voltage cables are required to be flexible, for example, surface mine shovel and dragline cables and underground mine distribution cables, which are connected to a portable power center. The SHD-GC cable shown in figure 8.9C is intended for this application.It is rated at 2,5,8,15, or 25 kV depending on insulation thickness.
- 216. Stationary power cables are often mine power feeders of the MP-GC type as shown in figure 8.90(seealsotables 8.2 and 8.3).These cables can also be rated at 5, 8, 15, or 25 kV, but they are less flexible and have higher tensile strength than the SHD-GC type. Shielding is similar but uses different materials. MP-GC cables are also designed to be used in boreholes, aerial installations, ducts, and direct burial. These are the basic power-cable types used currently in the mining industry. Other configurationsare made for specific applications.For example,one double-drumshear- ing machine model requires a six-conductorcablewith two ground-check conductors and a grounding conductor. Ca- ble manufacturers are usually willing to produce these special cables, but they are not a part of normal product lines and the possible variations are too numerous to include here. CABLE TERMINATIONS The termination or end of any cable must encompass a means of sealing and protecting the cable from the weather above ground and contaminants such as dust below ground. It must often provide a means of electrical connection with other conductors. Particularly in the case of high-voltage cables, considerable stress occurs on the dielectric between the terminating point of the cable shield, which is at ground potential, and the end of the condudor, which is at line potential. These electrical stresses are ameliorated through use of a stress cone that forms part of the termination device. The terminating device may take many forms, may be of varied complexity, and may be constructed from differ- ent insulating materials, dependingon the cable type and the application. Taped terminations are very common, particularly at 15 kV and below. A simple sealing lug applied with insulating tape can be used on nonshielded cables, but where the cable is shielded,a stress-reliefcone must be included. This may be preformed of rubber-like synthetic polymer and include an upper insulated cap, or may merely consist of lapped tape built up to the required cone shape. In either case, additional cover tapes are applied over the assembly and a rain hood or other protective housing may be added. An armor terminator provides a watertight grounding for armored cables and may be used in addition to a stress cone and insulation. A potheud is a form of termination housing used frequently in surface mines and above ground at under- ground mines. The pothead is hermatically sealed and thus provides maximum cable protection from the envi- ronment. A typical pothead for a shielded cable is shown in figure 8.12. Note that with shielded cables the termi- nation is taped prior to insertion in the pothead. In 15-kV applications and above, heated liquefied asphaltic or res- inous material is then poured into the pothead cavity. The rate of cooling of this dielectric material must be con- trolled to prevent the formation of voids. The pothead may include a number of aerial and cable connectors. Even though potheads are used, the standard terminationconnector in mines is the coupler. An entire range of complexcouplershas been developed specificallyfor the mining industry to accommodate the unique combina- tion of environmental fadors and operating procedures. CABLE COUPLERS Couplers are the complex sophisticated plugs and sockets used throughout the mine distribution system to connect mobile machinery to trailing cables, to connect cables with one another, and to connect cables to power centers, switchhouses,and substations. All couplers have certain common characteristics: They have either male contacts (plugs) or female contacts (sockets), They are either line mounted (atthe end of a cable) or gear mounted (located on a piece of equipment), 0 They are available in a wide voltage range, from high voltage (feeder cables) to low voltage (equipment related), They are available in a range of sizes to accommo- date different types and ampacities of cable, They all have grounding contacts and may also include ground-checkcontacts, They all have sealing and locking devices and dust covers to protect the contacts when they are not in use. The complexity of couplers is a direct result of the mine environment in which they are used; they must resist damage, be sturdy enough to withstand repeated use, prevent electrical hazards, be watertight, be dust proof, and withstand heat and cold. Somemodels arerated explosion proof. The plugging mechanism must be easy to use yet secure. High-voltage couplers have been used in mine distri- bution for about 40 y r . Most of the initial problems encountered in 4,160-and 7,200-Vsystems have been resolved over the years through constantly improved de- sign. Operating failures are no longer common at these levels. However, some problems are still found in the 15-kV class of couplers, and these have inhibited the switch to higher voltages by many mine operators. No ideal material has yet been developed for insulation; those Covity may be filled h dielectric material Gosk~t - inal Covity may be filled with dielectric material Cable- ! J ' Figure 8.12.-Cable terminations for appllcatlons up to 15 kV (all or a portion o f cable weight is supported by pothead).
- 217. with excellent electrical and chemical properties have been found to have mechanical inadequacies, and vice versa. The combination of dust and dirt with high humid- ity and moisture found in underground mines has posed many problems. In too many instances, these difficulties have been compounded by neglect, impatience, and total disregard for the purpose of a component by those who use them (7). Coupler Contacts The general requirements for coupler contacts are summarized as follows. The coupler contact system should have 1.Adequate current-carrying capacity and low resis- tance, 2. The ability to withstand repeated coupling, 3. Protection from worker abuse, 4. A reliable and easy-to-makeconnectionto the cable conductor, 5. Oxidation and corrosion resistance, 6. Uncoupling feature that allows a pilot or ground check to disengage first and the ground wires to uncouple last, 7. A guidance system to prevent misalignment and bending during coupling, 8. A feature to allow replacement of bent or damaged contacts. It is important that the male and female pins that mate as the coupler is connected are of adequate size and have low contact resistance to prevent excessive heating when car- rying current. Frequent coupling and uncoupling can lead to a poor contact, particularly when a coupler is dropped, not an infrequent occurrence. Contacts can be bent and become dirty. Poor alignment during coupling and attempting to force a connection can also bend the contacts. In either case, the resulting high-resistance connection can lead to problems with overheating. Coupler manufacturers have attempted to reduce damage to contactsby recessing them in the housing and adding guidance systems to facilitate alignment when couplingmust be carried out in restricted spaces. Another possible failure point is the connection be- tween the cable conductor and the contact. Set screws, soldering,thermit welding, and brazing are various meth- ods for securing this connection. Extreme care must be taken when brazing or soldering these connections to remove excess flux, which can destroy coupler insulation. Severe vibration caused by dropping or by bouncing and bumping on a mobile machine such as a battery scoop can loosen a screw or crack a weld. The high-resistancebroken connection then heats, which can cause insulation deteri- oration and a fault. Electrical voids and protrusions caused by an im- proper mating have great significance at voltages greater than 8 kV because these localized nonconformities can become partial-discharge inception points. Hence, the in- sulation should be made of corona-resistantmaterials and the contact designshouldminimize the occurrence of voids and protrusions. Some low-voltage couplers, for example, have a "self-wiping" action to improve the contact; other, high-voltage contacts employ a Multilam band for the same purpose. Coupler Insulation The general requirements for coupler insulation are as follows. The coupler insulation system should have 1 .Adequate dielectric strength, 2. Adequate corona-extinction level, 3. Adequate tracking resistance, 4. Stress-relieffeature, 5. Adequate impulse level, 6. Flame resistance, 7. Resistance to moisture penetration, 8. Insulators that align easily for coupling, 9. Resistance to cracking, chipping, and bending, 10.Resistance to heat deterioration, 11.The ability to withstand repeated coupling, 12. A feature that discourages phase reversal during mounting and coupling. To ensure that coupler insulation does not break down in normal sewice, it should have a dielectricstrength equal to or greater than that of the cable entering the connec- tion. For high-voltage installations, the surface of the insulation should resist arc tracking, a process in which high-current arc dischargescrossthe insulator surfaceand carbonize the material, forming a conductive track. Keep- ing high-voltageinsulators clean and dry will reduce the incidence of arc tracking. A common cause of moisture contamination is droppingthe coupler on a wet mine floor. Insulation, particularly if it has been weakened by partial discharges, is subject to breakdown by high- impulse voltages called transients, which usually occur during switching. Insulation materials must be able to withstand repeated occurrences of these high voltages. Coupler Housing Characteristicsrequired for the housing are as follows. The outer covering should have 1.A reliable easy-to-makeground wire connection, 2. A cable strain-relief mechanism, 3. A guidance system that improves the ease of alignment for coupling, 4. A durable material composition, 5. The ability to withstand repeated coupling, 6. Corrosion resistance, 7. Grommetor packing gland of the correctsizefor the cable used, 8. As little weight as possible, 9. A feature that facilitates ease in coupling and uncoupling. If the coupler is classified as explosion proof, it incorpo- rates a packing gland at the entrance to the housing that usually consists of asbestos fiber packed tightly between the cable and bushing. To be rated explosion proof by MSHA, an explosion that occurs inside the shell should not ignite any methane-air mixture surrounding the cou- pler. Explosion-proof couplers are allowed inby the last open crosscut in underground coal mines by all State and Federal regulations. Connectors without packing glands can be used inby the last open crosscut if they have a pilot or ground-check circuit that interrupts the power before
- 218. the housing is opened. Instead of packing, non-explosion- proof couplershave a rubber grommet that allows cablesof different diameters to fit into the same housing. The cable strain-relief clamp is located on the outside of the cable entrance and prevents cable tension from pulling the conductorsout of their connections.The clamp may be drawn down on the cable jacket by tightening a bolt on either side. If the bolts are not sficiently tight, the clampwill not prevent tensile pullout, and if too tight, the clamp will damage the cable insulation. Both packing glands and strain-relief clamps are made to fit a single cable jacket size or a small range of sizes. Thus knowledge of cable outer dimensions is neces- sary to match the coupler cable entrance to the cable. 'hbles 8.4 and 8.5 contain typical dimensions for round and flat cables, respectively (38). Variations in these values are allowed as long as the packing gland or strain relief is used. High-VoltageCouplers Couplers in the 15-kV, 500-A range are used as connections to switchhouses and power centers, to join high-voltage cables, and for high-voltage machines. A typical high-voltagecoupler is shown in figure 8.13.In the followingparagraphs, the numbers in parentheses refer to this diagram. A high-voltage coupler accommodates the three power conductors (4), one or more grounding condudors (141, and one or more groundcheck conductors (15).lhe contacts(8, 10, 11) are soldered and taped to the prepared conductor cables during installation. The contacts may be of copper, copper berylium, or in some cases, aluminumor brass. Male contacts have a split-pin design or incorporate a Multilam band of torsion-sprunglouvresto improve the power contact. Table 8.4.-vpical diameters for mund portable power cables in inches, 601 to 5,000 V Conductor size G-GC. SHC-GC, SHD-GC, SHD-GC, 2 kV 2 kV <3 kV 3-5 kV AWG: 4.............................. 110................................ 210............................ 3m................................ 410................ . . ......... MCM: 250............................ 350............................. 'Cable not made. Table 8.5.-Typical dlmensionsfor Rat portable cables in inches, 600 V 2conductor Conductor ... A Xonductor, G size, W U AWG Major Minor Major Minor Major Mlnor axis axis axis axis axis axis 8........................... 0.84 0.51 - 6........................... .93 .56 1.02 4........................... 1.05 .61 1.15 3........................... 1.14 .68 1.26 2........................... 1.24 .73 1.35 1........................... 1.40 .81 1.55 110........................ 1.51 .93 1.67 210........................ 1.63 .99 1.85 310........................ 1.77 1.03 2.00 410........................ 1.89 1.10 2.10 NOTE.-Dash indicatescable is not made. KEY / Coble strain-relief clomp 2 Pocking-glond bushing 3 Plugged hdes for pouring potting compound 4 Power conductor (insulation wrapped) with shieMing tope for high voltage 5 Molded stress-relief cone (for high voltoge) 6 Shell engagement mechonisrn 7 Insulation mounting flange Power-conductor contact Power-conductor insulation tube Grounding-conductor contoct Pilot-conductor contoct Metallic-shell grounding point Coupler shell Grounding conductor (from coble) Ground-check conductor Figure 8.13.-Coupler components.
- 219. The insulation materials and configuration vary ac- cording to the manufacturer, but they commonly have three main parts: a molded stress-relief cone(51,insulation tubes (91,and a flange (7).The molded stress-relief cone is now tending to replace hand taping as a method of providing termination stress relief. It combines the func- tions of a stress-relief cone and seal and also serves to position the conductors. The insulating tubes push onto the tapered cylinders of the molding and encase the contacts. Resistant rubber-like polymer tubes are now finding favor over flexible rubber tubes or cups that have a tendency to fold when a misaligned coupling is at- tempted. Both types replace an earlier polyester insulator that could crack and chip under the abuse almost inevita- ble when coupling in the confined spaces of an under- ground mine. The insulation tubes attach to a rigid insulation flange that positions the assembly correctly, seals the contact area from the rest of the coupler, and attaches it to the housing. When the coupler assembly is complete, a potting compound may be poured into the coupler (3) to guard against the formation of moisture. The compound is de- rived from tung oil and sets to a gel-like consistency. Potting compound is not required for 15-kV couplers that use filler moldings, but is frequently used as an added precaution. Asphaltic compounds were originally used as coupler fillers hut these were very difficult to remove if components were to be reused. The coupler housing (13) is metal, usually a high- strength, light-weight, corrosion-resistant cast aluminum that resists physical abuse yet is portable. The coupler housing incorporates a threaded collar or lock ring (6)that secures one coupler to its mate. Some designs have a pin-and-slot mechanism to reduce the number of turns required to lock the connection and simplify alignment. Low-Voltage Couplers The standard sizes for low-voltage and medium- voltage couplers are 225,400,600,800, and 1,200A. Their primary use is to connect mobile equipment to power centers and junction boxes, and to connect cables in the 600-to 1,000-Vrange. Their construction is sturdy but less complex than that of high-voltage couplers. They have either a boxlike shape and are locked by a latch mecha- nism or a cylindrical lock ring similar to those on high- voltage couplers. They do not have stress-relief cones or packing compound. The packing gland is usually replaced by a rubber grommet seal, but these couplers do include a cable strain-relief clamp. Many different contact configu- rations are available to accommodate a wide range of equipment types. Lower powered couplers specialize in quick and easy connection and disconnection for equip- ment that must be changed out frequently. CABLE SELECTION The cable manufacturer can provide a proper cable to a mining company only if the exact operating conditions for the cable are specified. The purchaser has the respon- sibility for writing a purchasing specification that com- pletely describes the operating environment. A revised ICEA listing of the information to be supplied by the purchaser, given below, will be used here to describe the step-by-stepcable selection process. 1.System characteristics: a. Ac or dc. b. Grounding method (i.e., by grounding conductor or diode-grounding circuit). c. Normal operating voltage between lines or con- ductors (line-to-linevoltage). d. Number of pilot or ground-check conductors and type of ground-check monitor. e. Minimum ambient temperature of cable storage and installation. f. Description of cable-installation area (surface mine, borehole, trailing cable, etc.). g. Environment of use (ambient temperature, amount of moisture, amount of sunlight, etc.). h. Maximum and normal operating current. i. Time schedule. j. Delivery point. k. Future changes in the system. 2. Cable characteristics: a. Cable length. b. Cable type, number of conductors, and flat or round configuration. c. Voltage rating. d. Type of conductor (copper or aluminum). e. Conductor size. f. Insulation type. g. Jacket type and color. h. Maximum outside diameter and tolerance. i. Method of conductor identification. j. Special markings (MSHA and P approval num- bers, dating, etc.). k. End attachments (couplers),type of attachment, location of installer, and method of installation. Many of the items in the system characteristics cate- gory are obviously designed to assist the purchaser in identifying a specific cable type. For example, the number of power conductors is determined when ac or dc is specified (la).The need for one or more grounding conduc- tors is noted when the grounding method (lb)is explained. Similarly, the normal operating voltage (lc) leads to the selection of a cable voltage rating that includes the oper- ating voltage and the requirements for shielding. If the ground-continuity monitor requires a groundcheck con- ductor, this should also be noted (Id). Any additional monitoring or remote-control systems may also require pilot conductors. Because cable jackets can crack during installation after being stored outside in extremely cold weather, the ambient temperatures of storage and use (le) should be specified. The installation area, category (10, explains special requirements such as high-tensile- strength conductors or a flame-resistantjacket for a bore- hole cable. Special environmental considerations (lg)that may affect cable life, such as an excessive exposure to sunlight in a surface mine, should be noted. Delivery time schedule (li) and delivery location (lj) are obviously im- portant considerations to be included so that a cable manufacturer can give the proper service. Finally, if changes to the electrical system (lk)are anticipated, they should be considered. Money can be saved by purchasing a
- 220. cable that will accommodate both the present and future systems rather than replacing a cable after a short oper- ating period. Cable Length The second section of specificationsis concerned with the detailed descriptionof a required cable. First the cable length (2a) must be specified.-~any companies prefer to purchase a long length of cable, thereby receiving a price discount, and then cut the required lengths from this stock. For instance, high-voltage feeder cable is usually shipped to a shop where couplers are mounted onto the cable at 1,000-R intervals before the cable, now in the desired lengths, is transported to the mine. However,other factors such as Government regulations and voltage drop must be considered. 'Igble 8.6 gives relevant information for underground trailing cables longer than 500 ft, based on a 60°C-rated insulation (a table for 90°C insulation is not presently available)(38). Table 8.6.-Specifications for trailing cables longer than 500 tt allow- Normal ampacity Resistance at Conductor able length, at 60% copper 60% copper size A tem~erature temoerature. AWG: 6 ................. 550 50 0.512 4 ................. 600 70 ,353 3 ................. 650 80 ,302 2 ................. 700 85 .258 1 ................. 750 110 ,220 110............... 800 130 ,185 410............... 1,000 MCM: 2 5 0 . ............. 1.000 Most of the remaining cable specifications have been discussed earlier in the chapter. Conductor size selection, however,is a complextopicthat requires detailed analysis. Conductor Selection The selectionof the conductor size(2e)is dependenton many parameters, such as ampacity, cable heating, volt- age drop, length, breaking strength, weight, shielding, insulation, and conductor material; the cable application may place emphasis on specific parameters. The correct selection will allow the cable to carry current without overheating or physical damage, to withstand the rugged mine environment, and to limit the voltage drop between the power source and the machines. The ampacity or normal continuous-currentrating of a cable is the current-canying ability of its power conduc- tors. It is dependentupon the ability of the cable assembly to dissipate heat without damaging the insulation. The ampacityrating is usually based on the maximumconduc- tor temperature rise, with the temperature limit chosen on the basis of the specified life expectancy of the cable insulation. The temperature class assigned tothe material used for the conductor insulation describes the maximum allowable sustained conductor temperature in a specified ambient temperature. The popular temperature ratings are 75O and 90°C. Cableinsulation with a 60°C rating can still be found,but this value is no longer used extensively in mining. An ambient temperature of 40°C is used for all ratings. The heat generated in the cable is primarily causedby the 12Rpower loss from current flow through the power- conductor resistance. The dissipation of this heat is a function of (30). The conductor diameter and the number of conduc- tors in the cable; The thickness of the conductor insulation and the cable jacket; The cable configuration and outside dimensions; The heat-transfer properties of the cable compo- nents; and The type of conductor and cable outer jacket, and the ambient temperature. A conductor size (cross-sectional area) within a specific insulation and cable configuration is given a current rating (its ampacity) through calculations using these parameters and the generated heat. Cable ampacities are now designated in the United States by the National Electrical Code (NEC)(2)or by the ICEA for cables manufactured according to its design specifications. Parts 18, 75, and 77, 30 CFR, basically allow compliance with either the NEC or ICEA ratings ( 3 8 ) . However, allowable ampacities for insulated conduc- tors given in the NEC are broad in both scope and application, and the same current value can be specified for one, two, or three conductors in a raceway, cable, or buried directly in earth (2, table 310-16). The broad applicability of the NEC standards implies that a safety factor must be built intoits ratings, and comparison shows that the NEC ampacities are approximately 25% higher than the ICEA ratings. While the NEC values are fine within the scope and objectives of that code, ICEA values are preferred for engineered systems. lbbles 8.7 and 8.8 give the ICEA ampacities for the 90°C-rated cables pre- ferred for mining. 'hble 8.6 includes ampacities for 6O0C- rated cables as specified in 30 CFR 18, and these are similar to the NEC values. The ampacity of a particular cable assumes that all splices,joints, and terminations in the cable are adequate in design and able to operate without restricting the loading on the cable. Considering the large number of splices made in mining cables, this assumption is a very important criterion for the cable rating. The ambient air temperature for the ampacitiesgiven in tables 8.7 and 8.8 is 40°C. If the maximum ambient temperature is differentfrom that specified, the ampacity correction factors shown in table 8.9 should be applied (30).
- 221. Table 8.7.-Ampacities' for portable power cables, amperes per conductor Single conductor 2.conductor, 3conductor. 3-conductorround 4- Conductor 0-2,000 V 2,001- 8,001- 15,001- m u d and rOu~~ta"d 5 6- O- 8,W1- 15foo1- conductor, conductor, conductor. Size unshielded 8.000 v 15,000v 25,000v flat, ~ 5 , 0 0 0 v 8,000V 15.000 V 25,000V 0-2,000 v 0-2,000 v 0-2,000 v shielded shielded shielded 0-2!000 V unshielde,j shielded shielded shielded AWG: 8............... 83 - - - 72 59 - - - 54 50 48 6............... 109 112 - - 95 79 93 - - 72 68 64 4............... 145 148 - - 127 104 122 - - 93 88 83 3............... 167 171 - - 145 120 140 - - 106 100 95 2............... 192 195 195 - 187 138 159 164 178 122 116 110 1............... 223 225 225 222 191 161 184 187 191 143 138 129 110............ 258 280 259 255 217 186 211 215 218 165 - - 2/0............ 298 299 298 293 250 215 243 246 249 192 - - 310............ 345 345 343 337 266 249 279 283 286 221 - - 410............ 400 400 397 389 326 287 321 325 327 255 - - MCM: 250........... 445 444 440 430 363 320 355 359 360 280 - - 300........... 500 498 491 480 400 357 398 - - 310 - - 350........... 552 549 543 529 436 394 435 - - 335 - - 400........... 600 596 590 572 470 430 470 - - 356 - - 450........... 650 640 633 615 497 460 503 - - 377 - - 500........... 695 688 878 659 524 487 536 - - 395 - - 550........... 737 732 - - - - - - - - - - 600........... 780 779 - - - - - - - - - - 650........... 820 817 - - - - - - - - - - 700........... 855 845 - - - - - - - - - - 750........... 898 889 - - - - - - - - - - 800........... 925 925 - - - - - - - - - - 900........... 1.010 998 - - - - - - - - - - 'Bawd on a copper conductor temperature of 90% and an ambient air temperature of 40%. These ampacities are based on single isolated cable in air operated with opencircuitedshield. NOTE.-Dash indicatescable is not made Table 8.8.-Ampacities' for three-conductormine power cables Conductor size 2,001 to 8.000 V 8,001 to 15,000V Copper Aluminum Copper Aluminum Copper Aluminum AWG 93 - 122 124 159 165 184 169 211 218 243 251 279 278 321 342 MCM 250 400 355 360 359 367 300 450 398 395 401 393 350 500 435 42.5 438 424 400 - 470 - 473 - 450 - 502 - 504 - 500 - 536 - 536 - 'Based on ICEA values with an ambient temperature of 40% and a conductor temperatureof 90°C [taken from "Power Cable Ampacities" (20), v. 1 for copper conductorsand v. 2 for aluminum conductors]. NOTE.-Dash indicatescable is not made. Table 8.9.-Correction factorsfor ampacities at various ambient temperatures. Ambient Ambient Correction temperature, OC factor Cable Heating on Reels A cable that is used in a confined space can become overheated with continuous-current flow at the ampacity rating. Perhaps the best example is a cable bound on a reel, either for storage purposes or to increase mining machine mobility. Investigations were conducted as early as 1931 to identify factors responsible for overheating of rubber-jacketed cables, with emphasis on increased tem- peratures occurring in reeled cables (16). A cable manu- facturer manual published in 1940was the first to contain a table of derating factors related to the number of layers wound on a reel to reduce the current-carrying capacity of the cable (23). These factors were included in ICEA spec- ificationsfor 60°C-ratedcables in 1946and have remained a standard since that time. Table 8.10 presents the ICEA values presently required by Federal regulations for all cable insulations (38). Research has been conducted since the publication of the ICEA derating factorsto determine their applicability to the mining industry. McNiff and Shepherd (23-24) worked with cyclic currents, comparable to those experi- enced by shuttle cars in sewice, and steady-state loading at various percentagesof cable ampacity,with both ac and dcpower. Derating factorsfor 60°C-ratedcablesabstracted from these results are presented in table 8.10. An impor- tant contribution of their work, which cannot be shown in the table, is identification of the dependence of cable derating factors on the maximum-limit temperature per- mitted: at this temperature is increased or reduced, the derating factor changes accordingly. This was later veri- fied by Woboditsch (41), and his values for a limit temper- ature of 60°C are also given in table 8.10.
- 222. Table 8.10.-Ampacity derating factors for 60°C-rated trailing cables operated on drums McNiffand Shepherd (23-24) ' woboditsch Number of layers ICEA np ,ip (41)' . . 5........................... NA .32 '34 (4) NA Not available. 'Data for a 2-conductor. No. 4 AWG. woe G cable at a maximum . ,. temperature of 60°C. Data for a 3-conductor, type NTSCE cable at a maximum temperature ot 60%. 'Values from extrapolatedcurves since data did not extend to this range. Cable not made. Cable ampacity must be derated if the cable is used in a confined space. In view of the findings on limit temper- ature change, the ICEA values are probably adequate for 75OC-rated and 90°C-rated cables. It is significant that Australian mining companies have recently accepted the initial derating factors, but with qualification (9), as shown in table 8.11. The ICEA values are specified as pertaining only to round cable, while new values have been generated for flat cable (8).As flat cable usually occupies more volume on a reel than round cable, heat transfer for flat cables should be less, and the lower values appear reasonable. Table 8.11.-Australian specifications for ampacity derating facton for trailing cables operated on drums Number of layers Circular cable Flat cable Current Calculations Current and voltage regulation are the two major concerns in sizing a cable correctly for an intended appli- cation. The effective continuous current through the cable power conductors must be less tban the cable ampacity, with correct derating factors applied. The voltage drop across the distribution and utilization systems must be such that voltage regulation is within the tolerances specified for the loads. For trailing cables serving ma- chines, current is often the determining factor, since these cables are always short enough for voltage regulation not to be a problem. Feeder and portable cables serving many loads, however, are often so longthat voltage drop becomes a principal concern. Even though the cable size may be found adequate in terms of ampacity and voltage drop, other factors may enter into the conductor sizing, such as tensile !oad, weight, and available short-circuit current. There are t,hreebasic methods that can be used to find trailing-cable ampacity: a full-loadcurrent similar to that specified in the NEC, a 30-min effective current demand, and a load-factor approach. Regardless of the method used, the engineer should realize that the typical current re- quirements of mining machinery change continuously over time and may be described as unsteady in nature. The infinite variability of mining conditions makes it difficult to define current levels for any part of a given duty cycle with precision. Calculation of cable ampacity requirements based on a 30-min effective current demand recognizes this vari- ability and also that cable heating varies as the square of current. Here, line current measurements are taken from the machine, and an effective or rms value is found by weighting current with where I , , , , , , = weighted current through cable, A, I = current level for specific increment of time, A, and t = time increment for current level I, s. This method does account for the transient heating and cooling of the cable, which should be considered for match- ing the loading conditions found in mining with the specific limit temperature for the cable; in other words, the ampacity. Through this method, representative machines in typical mining conditions can be measured and a catalog of effective currents canbe assembled for ampacity selection. However, actual measurements are not always possible, and the next two methods do not require them. The full-loadcurrent approach is detailed by MSHA (39) and essentially follows the NEC requirements in sections 430-22.430-23, and 430-24. Here the ampacity of a cable supplying a single motor must be not less tban 125%of the motor full-loadcurrent rating. When two or more motors are supplied through one cable, the ampacity must be at least equal to the sum of the full-load current ratings of all the motors plus 25% of the highest rated motor in the group Provisions are allowed in this approach for adjusting the current requirements of any motor used for intermittent or periodic duty, and for the 60-min-rated motors normally foundin mining (36); that is, the ampacitymay be reducedby 10% or 5%, respectively. The third method uses the machine load factor and appliesthe average power formula (3243).For ac machines, and for dc equipment, where I = machine line current, A, P = (746) (hp) = rated average power of machine, w. ., hp = rated machine horsepower, actual average power consumed LF = rated average power = machine load factor, V = line-to-line machine voltage, V, pf = machine power factor, and TJ = machine efficiency. The formulas may be used for single motors or machines containing a complex of motors. Obviously,the load factor, power factor, and efficiencyof a machine must be known in order to apply this method. With knowledge of typical operating conditions, these can be estimated. Values for
- 223. many underground coal mining machines have been re- searched and may be found in references 28, 32, and 33. A summary of these and values extrapolated from represen- tative underground mining conditions is given in table 8.12; 100% efficiency should be assumed when applying these values to the formulas. However, caution should be taken when using these parameters as they are only representative. If precise currents are necessary, power measurements should be taken to obtain load factors and power factors, and manufacturer specifications consulted for efficiencies. The formulas can alsobe employeddirectly for full-load current calculations by assuming that pf = 0.85, LF = 1and 1 , = 1for ac induction machines, and pf = 1and LF = 1for dc motors. Table 8.12.-Some estimated power factors and load factors for various undergmund coal mlnlng equipment in good operating conditions Machlne Power factor ' Load factor Batrely chargers................................ 1.O 0.8 Belt drives .................... . . . . . ........... .8 .7 Belt feeder......................................... .8 .7 Belt feeder breaker............................ .7 .6 Continuous miners ".......................... . 6 .5 Cutting machines ............................ .7 .6 Drilling machines............................... .8 .7 Lighting...................... . . ................. 1.O 1.O Loading machines ........................... .7 .6 Longwall shearing machines.............. .8 .7 Roof bolters....................................... .6 .3 Section fans ................... . . . ............ .7 .6 Shuttle cars....................................... .6 .4 ' For ac equipment only. For ac or dc equipment. Values are for cutting andlor loadingonly. Values for other machinesare an average over a typical duty cycle. EXAMPLE 8.1 The difference between the last two methods can easily be seen through examples. First consider a 150-hp ac continuous belt-conveyor drive motor rated at 550 V and operating in 20' C ambient temperature. Using the NEC currents (2,table 430- 150) and applying 125% for the full-load current approach, the current used to size the cable would be The ICEA ampacities of Nos. 2 and 1 AWG 31C unshielded round cable from table 8.7, corrected by ; the factors in table 8.9, are (138X1.18) = 163A and (161X1.18) = 190 A, respectively. Hence the No. 1 AWG size would be indicated. Applying a load-factor calculation with table 8.12 data, This relates that a No. 4 AWG 3lC unshielded round cable is adequate with a corrected ampacity of (104X1.18) = 123A. The second method is probably more representative of actual conditions, since the NEC applies a 25% safety factor. EXAMPLE 8.2 A cable size must be found for a 105-hp dc shuttle car. The machine is rated at 250 V, and it is assumed that the maximum ambient temperature is 20° C, and an average of two layers of cable will remain on the reel. The load-factor approach will be used. From the information in table 8.12, a represen- tative load factor for shuttle cars is 0.4. Applying equation 8.4 and assuming 100%eficiency, Ampacities for two-conductor cables from table 8.7 corrected for a 20° C ambient temperature (table 8.9) are for No. 4 AWG, (127X1.18) = 150 A, * for No. 2 AWG, (167X1.18) = 197 A, for No. 1AWG, (191X1.18) = 225 A. This is a reeled application and these ampacities must be derated by the number of layers on the reel. Because of present Federal acceptance, the ICEA derating values from table 8.10 will be used. Thus for two layers, the ampacities must be reduced by 0.65, or for No. 4 AWG, (150X0.65) = 97 A, for No. 2 AWG, (197X0.65) = 128 A, for No. 1AWG, (225X0.65) = 147A. Therefore, No. 4 AWG is too small, and No. 2 AWG would be selected. It can be noted that No 3 AWG was not included in the example. The reason is that this cable is not popular and is not readily available from manufac- turers. EXAMPLE 8.3 Now consider a 550-Vac continuous miner that has five motors (two 50-hp gathering-head motors, two 175-hp cutter motors, and one 135-hp pump motor) for a total connected horsepower of 535 hp. Using the NEC currents (2, table 430-15), applying the intermittent-duty rating for the gathering head and cutter motors, and increasing the highest rated motor in the group by 25%, 50 hp, I = (52X0.9) = 46.8, A 135 hp, I = (133X1.0) = 133 A, 175 hp, I = (168X0.9) = 151.2 A, 175 hp, I = (168X0.9X1.25) = 189 A. Assuming the current phasor angles are such that a direct summation introduces only minor error, total
- 224. current for ampacity selection would be about 520 A. Assuming the machine is operating in good mining condition, and using a load-factor calcula- tion with table 8.12 values, Continuous miners of this size commonly use un- shielded410 trailing cables with 90° C-ratedinsula- tion. If the ambient is 20° C, the ICEA ampacity from table 8.7 corrected with table 8.9 data is (287X1.18) = 339 A. This is considerably below the calculated values of 520 and 431 A. Actual visits to underground mines using continuous miners of the same size (535hp) showed that the 410 cablejackets were not warm to the touch, implying cable- conductor temperatures well below the 90° C limit temperature ( 3 2 ) .Furthermore, the load-factor cal- culation is based on data from machine cutting and loading, and since a continuous miner does not cut and load continuously, the current would be biased toward a worst case situation. Including the other machine operations (tramming, idle, etc.) would lower the load factor and the calculated current, probably below the ICEA ampacity. Regardless, the load-factor approach reflects this utilization envi- ronment more accuratelythan the NEC approach.It should be obviousthat the effectivecurrent demand method would be more precise than either of these approaches. Intennittent Duty Ratings A major problem implied in the preceding example is that intermittent, fluctuating, or cyclic current through a cable has a different effect on cable heating than contin- uous loading. The full-load current or NEC approach for conductor sizing basically assumes continuous loading, but true continuous operation of most mining machinery would be a rare occurrence.Mining is inherently cyclic in nature. The Institute of Electrical and Electronics Engi- neers (IEEE)(17) does publish guidelines for rating elec- trical equipment under various operating conditions, du- rations, and time sequences of duty. Even though these terms have been used previously i6 this text, it is benefi- cial to define them here: Continuous duly Operation at a substantially con- stant load for an indefinitely long time. Sh~rt~time duty. Operation at a substantially con- stant load for a short and definite specified time. Intermittent dub Operation for alternate intervals of load and no-load as definitely specified. Varying duty Operation where the amount of load and the length of time the load is applied are subject to considerablevariation. In an endeavor to overcome the problem of mining duty cycles, the United Kingdom and Australian mining laws permit intermittent-duty ratings for mining trailing cables(9, 37).These ratings for several popular cable sizes are given in table 8.13. It can be noted that in both United Kingdom and Australian practice, the rating criteria are Table 8.13.-Intermittent-duty ratingstor trailing cables Cable Approximate U,S, cable Continuous Intermittent Increase, size. equivalent, current current mm rating, A rating, A % AWG ,...- UNITED KINGDOM ' 70................... 210 205 235 15 95................... 410 247 290 18 AUSTRALIA -- 21................... 4 70 95 36 33................... 2 90 125 39 'Criteria: full-load current for 40 min. no-load current for 10-15 min. 112 full-load currentfor 40 min, no-load current for 10-15 min; ambient at 25%. Criteria:full-load current for 30 min, no-load current for 30 min. independent of the cable size. An attempt to match or classify the duty of mining machineswith the well-defined IEEE categories, however, results in only one conclusion: the typical mining duty is equivalent to a varying-duty classification.Although mining sequences through given events regularly, distances constantly change; hence, equipment utilization changes. In such cases, the IEEE recommends the use of standard application methods to &set the problemsof a nonconstantload, and suggeststhe use of load-factor and rms current calculations. These should be applicable to electrical equipment, such as cables, which are "suffkiently standardized both in per- formance and construction" (17). Voltage Calculations The major concern for voltage calculations is that adequate voltage must be at the machine terminals for proper starting and operation. As stated in chapter 6, the allowable voltage tolerance on all rotating machines is f 10%for normal load conditions. Maintaining adequate voltage is one of the more difficult problems in mining, and is often the main constraint on mine expansionfrom a point of power delivery to the operation. As mentioned earlier, the voltage drop across trailing cables that have been properly selected by current calcu- lations is usually not a problem because of length con- straints in mining. This is especially true in underground wal mining, where the maximum length is restricted by the cable size used (as shown in table 8.6). One problem here, however, is that the maximum practical trailing- cable size that can be used is also constrained by the maximum weight that workers can physically handle. For threetonductor cables, this is considered to be 410 AWG, but use of 410 AWG can cause voltage-regulationrestric- tions on high-horsepoweredmachinery.Trailing-cablevolt- age drop may also be a concern in surface mines where utilization is at distribution voltage levels. Using the allowablevoltage tolerance as a guide, good practice calls for limiting the maximum voltage drop under normal load conditionsto not more than 10%of the nominal system voltage for each voltage level. For surface mines where machinesoperate at the distribution voltage, this would be equivalent to a maximum voltage drop from the substation secondary to the machines. In underground or surface mines containing power centers or a unit substation, this is not so apparent. Again, the maximum voltage drop must be restrained to lo%, but such a drop
- 225. can occur across the trailing cable alone. Consequently, the powercenter or unit-substation primary must be maintained as close to its normal voltage rating as prac- tical. 'Ib obtain this objective in practice can be a very difficult task, because power centers, for example, are usually at the extreme end of the distribution system. However, most mine power-center transformers are de- signed with two 2 . 5 % taps above and below the rated primary voltage. Therefore, when voltage taps are avail- able, the maximum allowable voltage drop under normal load conditionsin the distribution system (from the sub- station to the power centers or unit substations) is 10%. It is interesting to compare the 10% allowance with other electrical applications. For lighting, the NEC recom- mends 1 . 0 % (2).Industriesother than mining consider 2 . 0 % as good-hemellent regulation and 4.0% as satisfactory. For a thoroughvoltage-regulationstudy of a mine, the impedances of the source,the transformers, and all cables must be known. lhbles 8.14 and 8.15 provide typical resistance and 60-Hz reactance values for popular mining cables 0, the missing parameters in these tables imply the cable is not popular or not considered suitable for mining usage. Manufacturer, power-equipment,and util- ity specifications must be consultedfor other information. If cablesizesare not known, an assumptionhas to be made in order to cany out the calculations. Obviously,the loads on the power system must also be known. A circuit diagram must then be prepared and calculations per- formedto seeif there will be adequate voltage levels at the loads. If. calculated voltages are below those tolerated, system impedance must be reduced: the most convenient way is to increase cable qizes. Calculations are again performed to check for the desired result. In other words, the process is basically trial and error. It must be per- formed for normal load conditions; however, it is also recommended that calculations be made to ensure that critical motors can be started under worst case conditions. Even with a small system using the per-unit method, the computations can become so involved that accurate hand calculationsare extremely time consumingor nearly impossible to obtain. Consequently, load-flow computer programs are the only answer; these are discussed further in chapter 10. However, there are some simple hand- calculation procedures that may be used for initial cable sizing, or for quick verification of voltage conditions in an existing system. These methods will be explored in the next example. Table 6.14.-Redstance and reactance of portablepower cable R (ac). 'WMft XL(60HA. IUMR Conductor G-GC size 75OC BO°C G+GC, SHD-GC. SHD-GC, SHD-GC, SHD-GC. SHD-GC, 3 tv 2 kV 5 kV 8 kV 15 kV 25 kV -... AWG: 8............................. 0.838 0.878 0.034 - - - - - 7.......................... .665 .696 ,033 - - - - - 8......................... .... ,528 552 ,032 0.038 0.043 - - - 5............................. .418 ,438 ,031 ,036 ,042 - - - 4 ............................. ,332 ,347 ,031 .035 ,040 0.043 - - 3............................. 263 .275 ,031 .034 ,039 .042 - - 2............................. ,209 ,218 ,029 .033 ,038 ,040 0.044 - 1............................. .I65 ,173 3.030 .033 .036 ,039 ,042 0.046 1M.......................... ,128 ,134 ,029 .032 ,035 .037 ,040 .044 2/0 .......................... .lo2 ,107 ,029 .031 .034 ,036 ,039 ,043 3/0.......................... ,081 .MIS .M8 ,030 .033 .W5 .038 ,041 410.......................... .065 ,088 ,027 ,029 .032 ,034 .036 ,040 MCM: 250 ......................... ,055 ,057 .028 .030 .031 ,033 ,036 .039 300 ......................... .048 .048 ,027 .029 .031 ,032 ,035 ,038 350 ......................... ,039 ,041 ,027 ,029 ,030 ,032 ,034 ,037 400......................... ,035 ,036 .027 ,026 ,030 ,031 ,033 ,036 500......................... ,028 ,029 ,026 .028 ,029 .WO .032 ,035 600......................... ,023 ,024 .026 ,027 ,028 ,030 .032 .034 700 ......................... ,020 ,021 ,026 .027 ,028 ,029 .031 ,033 800 ......................... ,018 ,019 .025 ,026 ,028 ,029 ,030 ,033 800......................... ,016 ,017 ,025 ,026 ,027 ,028 ,030 ,032 1,000 ...................... ,014 ,015 ,025 .026 .M7 ,028 .030 ,032 'Criteria: a. Sizes 8 to 1 based on tinned copper 94.16% conductivity. b. Sizes 1m AWG and larger based on tinned copper 96.16% conductivity. c. Resistanceincreased by increments per ASTM 0-172, Note 7 (3),to compensate for stranding factor. d. Skin effect calculatedaccording to Arnold's Table, NationalBureauof Standards Monograph 125 (29). e. Nominalcross-sectional areas. 'Criteria: a. Basedon conductor dimensions given for class-H ropelayconductorsin table 2.5 of ICEA 5-19-81 (21). b. Extruded-strand shield thickness, 0.015 in. c. Insulationthickness according to nominals given in Interim Standard 8 to ICEA 5-68-518 (19). d. Diameter adder of 0.075 in to allow for semiconductingtape and metal-braid shield. Deviationfrom normal progressiondue to changes in insulation. NOTE.-Dash indicatescabla is not made.
- 226. TbMe 8.15.-Resistance and reactanceof mlne-power-feeder cable 1,000-ft portablecable Substation 10,000-ft feeder Power centers for continuous 5 MVA 7% reactance 69 kV :7.2 kV R (ac). 'UlMA, X, (60 HJ, WMR Conductor size 9O0C MP-GC. MP-GC, MP-GC. 5 kV 8 kV 15kV ~ ~.. -.~. ,,,T mining sections, 7,200-v Bus representing 'primaries double-breaker switchhouse 1,000-ft portable cable Figure 8.14.-Simplified one4ine diagram for situation described in example 8.4. operations. Chapter 4 presented the concept of de- mand factor (DF)whereusing a value from 0.7 to 0.8 is considered reasonable for mining sections: 0.8 correspondingto two sectionsand 0.7 to fouror more sections. Therefore, 4x1............... MCM: I, = DF(1, + I,) (8.5) or I, = (0.8X53 + 53) = 84.8 A. 800.............. .017 ,027 ,028 ,030 900.. ............ .016 ,027 .027 ,029 1.000........... .014 ,026 .027 ,029 'Criteria: a. Based on bare copper 100%conductivily. A 7,200-Vsystem requires the use of 8-kVshielded cables, and the corrected ampacity for No. 6 AWG from table 8.7 or 8.8 and table 8.9 is b. Nominalcmss-sectlonalareas. c. Resistanceincreased by incrementsper ASTM 6-8, Note 3. to compensate for strandlngfactor. d. Skin effect calculated according to Arnold's Table. National Bureau of Standards Monograph 125(29). Criteria: a. Basedon conductor dimensionsgiven for clms B concentric stranded conductors in table 2.2 of ICEA 5-19-81 (21). b. E x t ~ d e d strand shield thickness. 0.015in. c. Insulation thickness according to nomlnals given in Interim Standard 5 to ICEA - 1 6 (19). d. Diameter adder of 0.033in to allow for semiconducting tape and copper-tape shield. ampacity = (93X1.18) = 110 A. I This means that on a current basis the size is ade- quate for all distributioncables.Consideringthe pref- erence of the coal mining industry for using only portablecablesfor flexibility,ground-checkconductors for groundcontinuity monitoring, and 90°C insula- tion, an SHD-GC cable is indicated.'Igble 8.14 canbe consulted for its impedance.It canbe seen in the table that Na 4 AWG is the smallest 8-kV SHD-GC porta- ble cablereadily available.Hence, a N o .4AWG willbe tried. Its impedance per 1,000R is I NOTE.-Dash indicates cable is not made. EXAMPLE 8.4 Distribution cables for a segment of an under- ground coal mine must be sized. A sketch of the situation isprovidedin figure8.14where the loadsare two continuous mining sections. Voltages given are line to line. In-mine measurements and analysis of identical section equipment working in similar condi- tions have shown an effectivecurrent demand of 58A with 0.8 lagging power fador at the powercenter primary, when the continuous miner is cutting and loading.Maximum ambienttemperatureis 20°C.In a detailed study, the substationtransformer impedance must be included. For the sake of demonstration, however,the 7,200-Vline-to-linevoltage at the substa- tion secondary will be assumed constant. The recom- mendationfor allowablevoltagedropis 10%amasthe distribution system. As the impedances of the feeder and portable cables must be known to make the calculation, a good place to start is to estimate line currents and make an initial cable selection by am- pacity. From the given information, Fkferring to figure 8.14, the voltage drop acrossthe distribution line conductorsto either power center is (taking the power-center voltage as the reference phasor): As per-phase analysis is required to compare this drop with that allowed,the line-to-neutralvoltageof the distribution system is used, or 7 200 V,, = J3 = 4,160 V . I The allowable voltage drop is I I, is related to I, and I, but is not necessarilyequal to their sum, because of the diversity of mining I Vd allowable = 0.1(4,160) = 416 V . I
- 227. Therefore, the 315-Vdrop using No. 4 AWG SHD- GC cables is tolerable. If the voltage drop were not acceptable, an increase in cable size would lower the impedance and the drop. This simple example had equal cable lengths to the loads, and currents operating at the same phase angle. It should be noted that typical mining sys- tems have many more loads, varying cable length, varying load power factors, and so forth, and the complexity of hand calculations will increase sub- stantially. Wr-unit techniques are a tremendous help, but computeranalysis is a much more efficient way to solve such problems. Nonetheless, the tech- niques shown here are useful for partial sizing or spot-checkingdistribution cables. Cable Mechanical Strength The tensile load on the cable should be determined from measurements in the mine, bearing in mind the problems discussed at the beginning of this chapter. The power-conductor breaking-strength data in table 8.16 should then be consultedto assure that the conductor size is large enough to carry the tensile load (5).Two things must be considered when using this table. First, ground- ingand ground-checkconductorsshould not supportany of the tensile load, so the overall cable breaking strength should include only the sum of the power-conductor val- ues. Second, the working tension, especially in reeling applications, should not exceed 10% of the breaking Table 8.16.-Solid-wire breaking strength Conductor Hard- Medium- Soft- size, 65.W psi 55,000p i 40,000 p i AWG Ib kg ib kg Ib kg strength because copper begins to elongate at that point. Federal regulations acknowledge the problem of exceeding the cable mechanical strength and mandate a minimum trailing-cable size for underground coal mine face equip- ment: No. 4 AWG for two-conductor dc cables and No. 6 AWG for three-conductorac cables (38). Short-Circuit Currents The emergency-overloadcurrents that copper conduc- tors can withstand without serious insulation damage are shown in the graph in figure 8.15 (5).If the anticipated short-circuitcurrents are greater than those shown in the graph for the initial selection of conductor size, a larger conductor or a better grade of insulation should be chosen. Chapter 10 covers the calculation methods. CABLE INSTALLATIONAND HANDLING Cables must be installed and handled correctly in order to minimize damage from tension, bending, twist- ing, physical wear, cold, heat, and chemical reaction. Cable maintenance costs can be reduced, cable life im- proved, and safety enhanced by proper installation and handling. In other words, the considerable amount of 1 m 8 0 6 0 5 0 4 0 3 0 20 4 m 1 0 0 i 8 5 6 L K u 5 2 u 4 5 3 u n V 2 Conductor :copper i z Curves basedon formula: 1 0.8 .6 I = Short-circuit current, A .5 A = Cwductor area, cmil .4 t = Time of short crrcuit, s .3 I/@ Y O CONDUCTOR SIZE Figure 8.11.-Allowable short.circu1t currents for insulated copper conductors.
- 228. engineering expertise expended in the design, manufac- should be utilized to prevent the clampsfrom loosening.A ture, and selection processes can be wasted if the cable is useful formula for determining the cableclamp spacingis not utilized properly at the mine. 9DL Borehole Cables S 3 - W ' (8.7) The mining or electrical engineer may not have to plan and supervise the installation of a borehole cable frequently; however, since this may be the main power- supply cable for the entire or a large part of an under- ground mine, safety and production are highly dependent on use of the correct techniques. The term borehole cable comes from the common practice of installing a cable in the vertical borehole that has been drilled into an under- ground mine for the purpose of power entry. However, it applies to any cable that is vertically suspended into a mine, regardless of the opening in which it is placed. The typical location other than the borehole is a shaft. Considerable tension is imposed on borehole cables, depending on the weight of the cable and the depth of the mine. Proper conductor selection, installation procedure, and suspension method are necessary to assure that the cable provides trouble-freeservice for the life of the mine. Shaft cables are also subject to damage from moving skips and spillage. An extremely wet environment is often encountered, which may cause corrosion and icing prob- lems. In addition, safety precautions must be taken to keep the cable from breaking loose and falling into the opening during installation. If the power conductors have enough strength to support the weight of the cable during and after installation, messenger wires (wire ropes)with cable-gripping clamps are not necessary. Otherwise, a messenger-wiresuspension method or a metallic-armored cable must be employed. If an unarmored cable such as an MP-GC is used, the followingformulacanbe used to calculatethe safetyfactor for the tensile strength: where, A = total area of power conductors, in 2, T = tensile strength of conductors, psi (24,000 psi for soft drawn copper and 40,000for medium- hard drawn copper), and W = weight of length of cable to be suspended, lb. 1 If the safety factor is greater than 7, an end suspension as shownin figure 8.16 may be used without messengers(18). Equal tensioning of the conductors is imperative. If messengers are needed, the wire ropes must be made from a corrosion-resistantmaterial such as stainless steel. The typical system uses clamps or wire-type cable grips at specified intervals to secure the cable to individ- ual messengers. The cross-sectional area and tensile strength of each messenger must be such that it can support the total weight of itself, the clamp, and at least a cableportion. Proven and tested clampsof the best quality should be used, or they will become the weak link in the installation. The high gripping force necessary on the cable jacket should be spread over a large area so the jacket is not damaged by pinching. The clamps are often vulcanized to the jacket to prevent this. In addition, a jacketing material that is not subject to cold-flowing where S = distance between clamps, ft, D = cable diameter, in, L = clamp length, in, and W = weight of cable, lblft. Generally, clamp spacing is greater than 25 ft and should not be more than 100 ft. An armored borehole cable is used where depth or location precludes the use of messengers. The armor usually consists of a sewing of steel or aluminum alloy wire typically placed over the cablejacket. If this type of cable is chosen,the armor carries the tensile load, and the tension safety factor can be determined by where SF = safety factor, BS = breaking strength of each wire in armor multiplied by number of wires, lb, and W = weight of cable length to be supported, lb. The minimum safety factor for armored cable is 5. Ar- mored cable may be necessary in shaft installations as Figure 8.16.-Representative end-suspension termination for borehole cable.
- 229. protection against jacket damage from skips, cages, and Table 8.17.-Recommended mlnlmum bendlng radius, spillage. unshielded or unarmoredcables, as a multiple of cable Cables can be installed either bv raisine or lowering. diameter - - Messenger-supported cables are usually lowered into posi- tion as each messenger must be clamped at the top. Raising is often preferredfor self-supportedcablesbecause of the need to have a brake on the surface as well as a pulling force at the bottom when a cable is lowered. In either case, the location should be straight and free of obstacles.It is alsoimportant to locate the cable in an area protected from any ground movementthat may result from the mining operation. When a structure is used at the top to support the cable weight, it should not only be strong enough but also be placed on a substantial concrete base. Any sheave wheels utilized during the installation should be larger then the minimum bending radius specified. Rollers should be used to preventjacket damage when the cable is dragged on rough surfaces and to minimize the pulling force by reducing friction. Crews working at the top and bottom should have a good communicationsystem, and the personnel working at the bottom should be ade- quately protected from iqjury shouldthe cablebreak loose and fall. Feeder Cable Installation The power-feeder cable must be located in an area that is protected from damage by mobile equipment. In underground mines, it is supported from the roof in regularly inspected fresh-air courses and haulageways on properly spaced insulated hangers, which may be sup- ported by a messenger wire. Messenger supports are usually installed at 204%intervals, and cable supportclips are placed on a 318-in messenger wire at 4-ft spacings as shownin figure 8.17 (31).Therecommended static load per clip is 100lb. A 1-114-in-diameter hole is drilled in the roof to place a 6-in-longexpansionshell bolt for the messenger- wire support. The cable must not come into contact with any combustible material. In undergroundcoal mines, the cable must be guarded in any location where miners regularly work or pass under it, unless it is 6-112ft above the floor or rail. Extra lengths of cable should be stored in large figure 8 configurationsin a well-ventilatedarea. The bending radii recommended by ICEA,shown in tables 8.17 and 8.18, should be observed for both mine power-feeder and portable cableswhen they are being installed (19-21). During installation, care must be taken not to twist the cable; that is, the reel should be turned so the cable is unrolled rather than pulled from the end of the reel. Finally, damage can be averted if a cable that has been stored on the surfaceduring the winter is brought intothe mine or a workshop to warm before being flexed. Recommended Handling Practices After cables are installed, proper cable-handlingprac- tices can increase personnel safety and cable life. Other- wise, damage can easily occur, especially to trailing ca- bles, such as machine runovers, cutting by sharp edges of machine frames and stress clamps, and abrasion from sharp rocks and mine openings. Research has produced numerous recommendationsto minimize this damage (10, 14).These are presented in the following section and are divided between those directly applicableto underground mining and those for surface mining. It should be noted, however, that some recommendations apply to all mines. Conductor insulation I .O-in diam 1.001-to 2.001-in diam thickness, mils and less 2.000-in diam and over 155and less............... 4 5 6 170 to 310.................. 5 6 7 325 and over .............. NA 7 8 NA Not available. NOTE.-These limits do not apply to bending around curved surfaces in tension during installation. Larger bendsare required for such installations. Table 8.10.-Recommended minimum bending radius, shielded and armored cables, as a multiple of cable diameter Cable type A " " " , & . Minimum bending radius ..... --. Flat tape and wlre ... 12times the overall diameter. interlocked .............. 7 times the overall dameter. except for tape- shielded cables and where a laraer radius is - specified for unshielded cables. Shielded: Tape........................ 12times the overall diameter. Wire........................ Same as for portable cables unless the cable is flat-tape or wire armored. ..................... Portable 6 times the overall diameter for round cables or the minor dimension for flat cables for insulations rated at lessthan 5.001 V. The minimum is 8 times the diameter for cables rated over 5,001V. NOTE.-These limits do not apply to bending around curved surfaces in tension during installation. Larger bends are required for such installations. Cables in Underground Mines For reeled-cableapplications, such as on shuttle cars, the cables must be anchored separately from the power equipment serving as the power source. The cable anchor points should be constructed so as to prevent personnel injury shouldthe tie point pull out of position. When more than one reeled cable is at the tie point location, separate anchor points should be used for each cable. This will ensure that a cable will not whip dangerously should one of the anchor points fail. This precaution will also prevent subsequent cable damage. A shock absorber should be used between the reeled cable and the anchor point to reduce instantaneous cable tensions (jerking).The use of a rubber-tire shock absorber is adequate, provided that a cable clamp is employed rather than tying the cable to the tire. However, other types of shock absorbersmay be more effective. Hydraulic pressure for the machine reel should be checked periodi- cally and set to manufacturer specifications to minimize instantaneous cable tensions. Backspooling is the process of moving a reeled-cable vehicle in a direction opposite from that for which it was primarily designed,for example,where a shuttle car dump point is beyond the tie point in a direction opposite to (outby)the mining face(inby).Researchhas found that the highest cable tensions occur during backspooling, result- ing from the sudden change in reel rotation as the shuttle car passes the tie point (14). Backspooling should be avoided, but if it is necessary, the cable anchor point should be located as far away from the travel entry as practical. This allows more time for the cable reel to change the rotation direction,and thus, cabletension will be less.
- 230. k - 20'-0" maximum * , 20'-0" moximum 20'-0" maximum - 1 j c - -20'-0" maximum 20'- maximum + 20'-0" maximum 20'-0" moximum -- - spacing spacing KEY A Feeder cable E Bulldog clamp 8 Dead-end hook F Expansion shell C Turnbuckle G Messenger wire D Cable clip H Sister hook Figure 8.17.-Messenger wire supports for mine power-feedercable. Minimizing the number of cable friction points be- tween the tie point and the face will ensure the most effective use of a cable shock absorber located at the tie point. Friction points prevent the tensions from being transferred back to the tie point. When slack cable is reeled in, every precaution should be taken to minimize reel momentum to prevent jerking the cable when the slack cable supply is exhausted. Reeling in slack cable slowly and cautiously will help minimize the possibility of whipping the cable. Maintaining a smooth mine bottom, especially in the vicinity of the tie point, will help mini- mize instantaneously high cable tensions resulting from the shuttle car's bouncing over an uneven mine bottom. Minimizing shuttle car speed when rounding pillar cor- ners and passing the tie point will help prevent fast changes in reel momentum. Consequently,instantaneous cable tensions will be less severe. Minimizing the amount of excess cable stored on a reel will prevent heat buildup in the cable.Cable abrasion on the shuttle car can be reduced by assuring that all contact points are smooth and rounded. If possible, install rollers or sheave wheels at contact points between cables and shuttle cars to reduce abrasion and cable flexure. Avoid severely bending and twisting the cable at the tie point and elsewhere. A clamp shouldbe used to limit cable bending at the tie point to 90°. Cabletwisting betweenthe machine and the anchor point can also be minimized by locating the tie point a maximum distance away from the machine travel entry. If possible, locate repairs to the shuttle car cable outby the tie point, where cable stresses are less severe. Recommendationsfor drag-cableinstallations are not as extensive as those for reeled cables but are just as important. First, the length of drag cable that is pulled should be minimized in order to reduce tension. Pillar corner edges should be rounded to prevent cutting or tearing of the cables. Precautions should be taken not to pull the cable over jagged rocks, timber, or other sharp objects that might damage the cable. There are some general practices that should be followed for handling all cables. Insulated gloves should always be worn, particularly when cables are energized. All cablesshouldbe storedin a warm environment during cold winter months. If storage facilities are limited, cold cables should be placed in a warm location for at least 24 h prior to use. Small-gaugeuninsulated wire must not be used to suspend cables from the roof, as it has a tendency to cut the cablejacket. All cable routes should be located in entries where they are safe from runovers. All cables should be checked periodically for damaged areas and electrical deterioration. Cables should be prevented from coming into contact with various oils, greases, or other contaminantsthat may deteriorate the cablejacket. When purchasing cables,make certain that they comply with all Federal and State regulations. In terms of jacket outer dimensions, this precaution will ensure effective use of packing glands and cable-layingdevices.
- 231. Cables in Sugace Mines Various types of equipment are available to assist with cable handling in surface mines, from insulated long-handled hooks to elaborate hydraulic reels and aerial crossover bridges. Despite this, considerable haulage, dragging, and hand-loadingof cables onto sleds and trucks is still required in many surface mines. Superficial cable damage from abrasion is a common problem, as is cable crushing by mobile equipment. The following cable-handling recommendations for surface mines were detailed in a 1981report to the IEEE (10). Systems should be developed for clearly marking cable lines along roadways and in pit areas. Suitable crossovers should be provided; in heavy traffic areas, these should be elevated. Sleds, skids, reels, and so on should be utilized rather than dragging the cable. Nylon rope or any device that can kink the cable should be avoided. Strain relief should be provided where cables are attached to equipment; rope or wire cable should not be used for this purpose. Insulated gloves are in poor condition at many minesites and provide inadequate protection for cable handling. In addition, personnel tend to place the cable across the body, negating any protection afforded by the gloves. Tools designed for cable handling should be clean and in a good state of repair. They should always have insulated handles. Conroy and Mertain (10)have made a very important statement about cable handling that is applicable to all mines: "A training program for all persons engaged in cable handling should be mandatory. This should cover both electrical precautions and procedures-particularly de-energization and lockout-and physical methods. Cable handling tools and devices should be made available to all concerned, and their use should be mandated. Mechanized cable handling equipment should be considered from both a safety and an economic viewpoint; and it may occur that an actual cost saving can be demonstrated for its use." CABLE FAILURES AND REPAIRS Most electrical cables used in mining are designed to have a minimum life expectancy of 20 yr, with a safety factor of about 2. The life expectancy is controlled prima- rily by the service life of the insulating jacketing materi- als, which, as noted earlier, are temperature related. Where specified operating temperatures are exceeded, deterioration of the insulating materials is accelerated and the useful service life is shortened accordingly. Tem- perature is the main factor in the deterioration of nonport- able cables that are fixed in place for extended periods of time, provided that proper installation practices are fol- lowed using good techniques. Portable cables, on the other hand, are frequently exposed to both excess generated temperature and mechanical abuse. As a result, portable cables can experience repeated failures at frequencies directly related to the proximity of the cable to the active mining area, the general mining conditions, and mainte- nance and cable-handling practices. For portable cables, the design life of 20 yr can easily deteriorate to 1or 2 yr of actual in-service use. Cable deterioration due to overheating is a time- dependent function and can go unnoticed in routine min- ing operations. The main indication is that the cable becomes uncomfortably hot to the touch or, in more severe cases, produces smoke or steam in wet conditions. Excess cable on a reel, created, for instance, by not taking into account cable derating factors, is the most probable con- tributing cause of cable overheating. Mechanical wear can also be a timedependent factor in cable failures, as,for example,repeated abrasion on a sheave support or spooling eye on a shuttle car. The most likely causes of failure, however, are those abuses associated with immediate or nearly immediate power interruptions. A prime example is the case where a shuttle car operator exceeds the length of the car umbilical, and the cable is tensioned to the point of failure. Similarly, a shuttle car might run over its owncable, pulling it apart or crushingthe conductors and insulations. One machine running over the cable that powers another machine is also a common abuse that eventually, if not immediately, takes its toll. Obviously, special care and consideration are needed to adapt such a relatively vulnerable item as a power cable to the mining environment. Unfortunately, once a cable has been damaged to the point of requiring a repair, it becomesmore vulnerable than ever, since it is almost impossible to restore its original performance characteristics. Cable Testing Although cables are often not tested routinely at a mine, there are instances where testing is recommended. Manufacturers test the components used in manufactur- ing and do cany out limited testing of completed cables as prescribed by ICEA standards (19-21). When couplers are added to cables at a cable repair shop, further testing can be done and the person responsible should ensure that these tests are performed effectively. The mine should require every cable removed from service to be tested before re-installation. If this were done, many costly in-service failures, production losses, and safety hazards could be prevented. Visual observation of cable condition is an important and simple task that can be carried out even when the cable is in service. It is important to require machine operators to walk to and from their equipment along the cable and visually examine the jacket for damaged areas. Outside diameter and hardness can also be determined on in-service cables. Any significant reduction in the overall diameter is an indication of excessive tension, while increased hardness results from excessive temperature or bending. More extensive evaluations can be made when the cable is out of service and the conductor ends are accessi- ble. Obviously,an ohmmeter can be used to test for broken conductors; however, more sophisticated equipment is nec- essary to locate an open circuit. Insulation damage may be detected by using a megohmmeter or a high-potential tester (hipot), each of which can give an indication of the ability of the insulation to withstand the operating volt- age without allowing excessive line-to-line or line- to-ground leakage currents. Portable megohmmeters and dc hipots can be used in the field, and ac hipots are sometimes available at cable repair shops. In order to test nonshielded cables completely, they must be surrounded by a grounding medium such as a water bath; otherwise, only the insulation directly between power conductors and between power and grounding conductors can be exam- ined. If a shield or armor is present, either can be used as a grounding medium for the test. Two basic types of insulation testing can be accom- plished with these methods: acceptance and maintenance.
- 232. For acceptance testing, the ICEA standard procedures and voltage levels should be followed (19-21). Maintenance testing requires lower voltage levels to avoid damaging the cable during the test. In both tests, the voltage level and duration of test should be adequate to ensure that the cable will perform safely in the intended service. As a general rule-of-thumb, maintenance test voltage is at 50% to 70%of the ICEA acceptancetest values and should be at least as high as the cable rating. Insulation resistance values from megohmmeter testing and leakage currents at specified test voltages, obtained from dc hipot testing, can be used for preventive maintenance scheduling. If records are maintained, these tests can be used to indicatereplace- ment schedules and prevent in-service breakdowns. Failure Location Failure location, often termed fault location, is an- other type of testing that is extremely important because of the susceptibility of mine cables to damage. It is less time consuming to repair or splice a cable in the mine than to replace it, and it is essential to have quick and accurate methods for locating cable failures in order to minimize the loss of production time. The Bureau of Mines has evaluated several methods, some of which follow (11). Some faults are low-resistance short circuits that can be found by visual inspection. Nonvisible short circuits can be blown by applying a high-energy power source to make them visible. However, this practice is not recom- mended within mines because of the potential safety hazards of fire and personnel injury. When there are faults in more than one place or when they cause low-resistance open circuits or high-resistance short circuits, they are extremely difficult to locate. A thumper or capacitance-discharge fault locator has been used successfully in surface mines and in cable repair shops; however, associated safety hazards restrict its use in underground mines. A capacitor is charged until a spark gap breaks down, sending a pulse along the cable. If the resistance is low enough, the pulse will discharge across a short and return. The pulse will not propagate across a high-resistance open circuit at the same intensity as it was transmitted, and an acoustic sensor can be used to locate the area where the signal caused by the pulse became diminished. The time-domain reflectometer (TDR) is another fairly successful method for locating failures. It works on the principle of a reflected pulse that either reinforces or reduces the original signal, depending on whether the discontinuity is an open or a short. The time of arrival of the echo is proportional to the distance to the failure, and the distance is then visually displayed on a meter. An accessory probe is necessary for exact failure location when a TDR is used, since the precise measurement of distance along a cable is difficult in a mine. A tone transmitter can be used in conjunction with an audio probe to locate the failure precisely. An infrared probe can also be used to locate faults where temperature increases are evident. Probes sensitive to lo or 2" F are available; however, a current source must be attached to the cable end. Splicing Once a cable is damaged and made unsafe or inoper- able, the damage must be repaired so that the machine might be put back into service with the least delay. In U.S. mines, repairs of this type can be made on the spot, whereas in some countries, such as the United Kingdom, the cable is replaced in its entirety and transported to a cable repair shop. The Code of Federal Regulations (38) states that "temporary splices in trailing cables or porta- ble cables shall be made in a workmanlike manner and shall be mechanically strong and well insulated." It fur- ther states that "when permanent splices in trailing cables are made, they should be: Mechanically strong with adequate electrical con- ductivity, Entirely insulated and sealed so as to exclude moisture, Vulcanized or otherwise made with suitable mate- rials to provide good bonding to the outer jacket." By Federal regulations, only one temporary splice is permitted in any one cable at any given time, and this must be removed or repaired within 24 h. A permanent splice, as the name suggests, can remain in place indefi- nitely so long as it is safe and effective. The number of permanent splices in a cable is not limited, except by Pennsylvania law where no more than four permanent splices are permitted along with one temporary splice. In other words, a trailing cable may contain five splices but only for a maximum time of 24 h. By law, specially approved splice kits or materials must be used when making a permanent splice repair. These kits and materials are tested and approved by MSHA and given an approval number similar to the approval number for cables. As with cables, a P is added to the MSHA number to signify approval for use in Wnnsyl- vania. Depending on their basic components and outer coverings, splice kits are generally classified as tape splices, cold-sleeve splices or heat-shrink splices. Varia- tions of these three types depend on the manufacturer. Tape splices use tape for the conductor insulation components as well as for the outer jacket replacement materials. In some cases, slit insulation tubes might be used with the insulating tape and in other designs, blanket-type wraps might be applied in combination with moisture sealingtapes to provide an overall covering prior to applying the final layers of tape that form the jacket replacement. Cold-sleeve splices include a variety of conductor insulation materials and usually consist of tapes that are used alone or in combination with slit-tube insulations. The main differences lie in the method of applying the outer jacket replacement. In all cases, a sleeve is slid onto the cable prior to making the conductor connections. In some designs, the splice area is built up using insulating tapes, and then a generous amount of adhesive is applied over the tape. The adhesive also serves as a lubricant and so the tubular covering must be moved into place imme- diately to cover the splice. This covering is designed to be slightly undersized so that it stretches as it is placed over the bulky taped area. The sleeve must be pushed into place or grasped at the end for pulling, otherwise additional drag is produced in grasping the sleeve, and it might be almost impossible to position it properly. Some of the cold-sleeve splice coverings are pre- stretched during the manufacturing packaging process. When it is time to place the sleeve, it is merely moved into place over the splice area and the restraining device is removed, thus allowing the sleeve to shrink (recover)onto the splice area, which has a smaller cross section. One
- 233. such deviceholds the sleeve in an expanded position by use of an inner plastic core, which is progressively collapsed along the length of the sleeve. In another design, the sleeve is held expanded by an adhesive bond between it and a rigid external concentric tube. A solvent is applied between the sleeve and the tube when it is time to allow the sleeve to recover onto the splice area. In still another design, the covering comes with both ends prerolled to- ward the middle in a toroidal fashion. The covering re- mains in this configuration until it is unrolled over the splice area. All of these special cold-splice coverings are designed to facilitate placement of the covering. The adhesive bonding and moisture sealing vary from manu- facturer to manufacturer, with the necessary components included in the kit. The heat-shrink splice coverings are also prestretched but in a different sense. The sleeves are made of special cross-linked polymers, which stretch readily when warm. In the manufacturing process, they are heated to 270°F and expanded radially to a given oversized dimension and then cooled. While at room temperature, they retain this oversized dimension, which easily accommodates place- ment over the spliced cable. When reheated to 250°F, the sleeve will shrink onto the splice area. A factory-applied thermal-melt adhesive on the inside surface of the sleeve softens with the applied heat and forms a moisture seal and adhesive bond between the sleeve and the original cable jacket. Similar smaller sleeves are used for conduc- tor insulation over the individual power conductors. The packaged splicing kits contain all the materials necessary to reinsulate and rejacket the splice area, to- gether with special illustrated instructions. Cleaning ma- terials, a cloth, and a can of solvent might also be included, together with an emery cloth (nonconducting) or scraper, which is used to prepare the surface of the cable jacketing for improving the adhesive bond. The connectors used to rejoin the power, grounding, and ground-check conductors may or may not be included in the kits, depending mainly upon customer specifications. The Bureau of Mines has sponsored research into the causes and prevention of splice failures, with emphasis on shuttle car cables (26, 34-35). Since this research has had a positive influence on the splice kits and insulation procedures used in the mining industry, a brief overview of the results is presented. Deenergizing Procedures In the interest of safety, it is essential to follow strict lockout procedures before cutting into any cable that has been put into use. Improper lockout prior to splicingcables has been a major source of electrocutions in the mining industry. The individual making the repair must go to the power-source end of the cable, disconnect the cable, and tag (danger om and lock out the disconnecting device, which is usually a coupler. This step must never be left for someone else to do. Cable Prepamtion After the cable has been properly disconnected from the power source, the next step is to remove the damaged area and prepare the conductors for splicing. The prepara- tion procedures vary slightly depending on the types of connections used, and a representative procedure is pre- sented in figure8.18. A guide or template is recommended for marking the cable for cutting and removing the insu- lation andjacketing. Such a guide is included in some kits but can be easily fabricated from light-colored material for repeated use. Use of a marking guide can help to standard- ize procedures and increase speed and accuracy. Once the cable pieces are properly marked, the next step is to remove the unwanted insulation. An effective method is presented in figure 8.19. The key here is to use a sharp knife and to take care not to cut all the way into the conductors. Nicking the conductor strands will mini- mize their performance. The conductor connections are usually staggered to help reduce bulkiness (fig. 8.20). The marking guide maintains good positioning, and before the insulation is actually cut into, the guide allows an imme- diate check point to determine that the power conductors are properly registered; that is, black to black, white to white, and so forth. Figure 8.18.-Splice layout using template for staggered connections. Figure 8.19.-Effective method for removing unwanted insulation.
- 234. Flgure 0.20.-Staggering splice connections. Connectors A variety of connectors(fig.8.21)and connector crimp- ing tools are available. It is generally recommended that lapped-joint connections be used where maximum tensile strength is desirable, as in shuttle car cables. Research has shown that the modified crowsfoot connection, when properly installed, can restore 80%to 100%of the original tensile strength for Nos. 6, 4, and 2 A W G conductors,the smaller conductors being easier to restore. The modified crowsfoot connection offers additional advantages of axial symmetry (no mechanical couple) and a small profile (an important consideration with multiconductor cables). The lap joints, being shorter than butt joints, are better for reeling applications since repeated flexure on a long connection might accelerate fatigue failures. The lap joints generally outperform butt connections in tensile strength, and Bureau-of-Mines-sponsoredresearch has shown that restoring tensile strength is probably more important than restoring high flexibility to shuttle car cables. Either way, the lap connections are superior. A major consideration in obtaining high tensile strength is the use of the proper crimping tool for a given connector. Furthermore, tools that reduce or eliminate operator judgment tend to provide the best repeatability, since overcrimping as well as undercrimping can reduce tensile values. The lap connection has also been recommended for restoringthe groundingconductors. In this case, it has been suggestedthat the connectionbe a little forgivingand allow the grounding conductors to slip slightly inside the connec- tor should the cable undergo excess tension. This would causethe power conductorstotake all the tension and would perhaps prevent the grounding conductors from being ten- sioned, so that they would be the last to fail. Although this concept has not been verified, it may have some merit (assuming of course that a good electrical connection is maintained and that otherwise the grounding conductor might not extend ~ ~ c i e n t l y under tension). An important consideration in selecting and install- ing connectorsin reeled cables is awareness of the connec- tor profile after installation. Bulky connectorswith abrupt edges are more difficult to insulate effectively, simply because they tend to cut through the insulation materials with repeated cycling under normal operations. These connectors can also cause excess pressure and fatigue on adjacent grounding conductors, which are uninsulated and somewhat less protected from mechanical abuse. Although generally unsatisfactory for related applica- tions, the butt connection is effective for larger portable Parallel ide connector into Full crowsfoot Figure 8.21.-Examples of popular connectors and connec- tions used in splices. cablessuch asthose used for continuousminers,because it offers the least bulk. Here it does not need to withstand the repetitious flexingso often experiencedby the smaller size cables. Because of the repeated bending stresses, reinsulat- ing procedures require special attention in portable ca- bles. The key is to provide a flexiblejoint and seal where the new insulation contacts the original cable insulation. As shown in figure 8.22, this is best accomplished using soft rubber tape that completely fills the volume and laps over the original insulating material. The lap is important since a tape fill that only butts to the insulation is almost sure to separate after very little flexing. Where it is desirable to use slit tubes as part of the reinsulating procedure, soft tape is recommended underneath and over the tubing. Soft rubber tape alonewill not hold up under repeated cable flexing. Therefore it is further recommended that tougher vinyl tape be applied over the rubber tape. The vinyl tape accomplishestwo objectives:it restrains the soft tape, thus preventing it from squeezing and extruding from its intended area, and it allows the reinsulated connections to slide relative to one another and the
- 235. Cable !nsulat~on Connector , 'Oft rubber' ~ ~ n y i electrfcal tape lnsulOtlnQtape Figure 8.22.-Reinsulating power conductors with soft rub- ber tape. grounding conductors with minimum wear. The vinyl tape can also be used to bind the multiple conductors together for maintaining positioning and limiting excess relative motion. A single-widthwrap of tape near the middle of the splice area is generally sufficient. Care should be taken not to use too much vinyl tape over the splice area, since the final splice covering is generally intended to bond to the inner parts and the vinyl can in some cases make the subsequent adhesive bond less effective. In the case of heat-shrink splices, the conductor insu- lations are also made of heat-shrinkable tubing, and the tubes must be slipped onto the conductors before the connector is applied. When shrinking the tubes with a heat source, care must be taken to avoid overheating or rupturing the insulation on the sharp connector edge, and so forth. After heating, the installer should inspect the work to ensure that the adhesive has sealed the sleeve to the original insulation material. This is especially impor- tant for flat cables where the insulation cross sections are not always smoothly continuous. The heat-shrink insula- tion tubes provide a generous lap over the original insu- lation and are usually tough and resistant to rubbing wear inside the splice. Shielded cables require complete shield replacement over the conductor insulation. This process is similar for all cables but requires more care in high-voltage splices and will be covered later. The outer splice covering provides protection for the more delicate inner splice components and serves basi- cally the same purpose as the rugged cablejacketing. It is important that it be tough and flexible and at the same time maintain an acceptable bond to the original jacket- ing material. Of principal concern is a splice condition generally termed end lipping, the result of the splice- covering ends' pulling away from the cable jacket. When this occurs, contaminants such as fine solids and water can enter the splice and contribute to failure or an unsafe condition. The causes of end lipping are combinations of poor adhesive bonds, discontinuities and dissimilar mate- rials, or simply physical wear as a result of the normal mining process. The amount of end lipping will vary depending on the types of covering used and the conditions to which it is exposed. Various attempts have been made to provide splicing products that resist end lipping, with varying degrees of success. The general recommendation is to prevent occur- rence by making every effort to clean the cable surface where the adhesive bond is to be made. As a minimum, any soiled surfaces should be wiped with a suitable solvent and abraided with nonconductive emery material to reveal a fresh bonding surface. It should be noted that newer cablejackets can be more difficult to bond simply because waxes from the manufacturing process are often on the jacket surface. In general, the heat-shrink sleeves are good abrasion- resistant coverings. However, it should be noted that they are usually stiffer and sometimes require more attention to obtain a good and lasting bond to the cable jacket. Furthermore, a heat-shrink sleeve can take on a thermal set, for example, if it'is allowed to cool in a curved position on a reel and then is later unreeled while still cool. The cold splices are generally quite flexible, but end lipping can result from bending and scuffing on various machine parts. Major cable and splice wear usually occurs during contact with the machine and its spooling mechanism when the relative motion is at a maximum. It is normal practice to tape down the ends of the splice coverings. This can help to reduce end lipping and can also prevent foreign matter from entering an already lipped end. Regular inspection and renewal of the end taping is a must, since abrasive wear and cutting on machine parts can quickly destroy even well-applied end tapes. The use of exposed soft rubber tapes is considered poor cable repair practice. The softer rubber tapes can provide good moisture seals but should be protected with an overcovering of tough vinyl tape. This vinyl tape will help contain the rubber tape, and the lower friction will give better wear characteristics. High-Voltage Cable Splices When splices are required on high-voltage cables in underground mines or in surface mines, problems are introduced by the presence of shields and semiconducting layers. The high voltage means that care must be taken to achieve an excellent splice, that is, one that closely ap- proximates the qualities of the original cable. The splicing procedure is basically the same as that just covered, but the cable insulation and jacket are usually tapered as shown in figure 8.23. 'IBpering is performed to improve the bond, increase the leakage path length, and lessen the chance of a direct vertical path to a ground plane. Extra care and skill are necessary as any damage to the insulation during splicing, such as a small cut, will cause more rapid dielectric failure at higher voltages. In the same context, a small protrusion such as a sharp edged connector or loose wire will be a more noticeable failure initiator as the voltage increases. The presence of semiconductive tape and braid or tape shielding in cables requires extra caution. The shielding system must be separated from the conductor insulation in such a way that residue on the insulation from the semiconducting tape is completely removed before the conductor insulation is reapplied. In addition, the wires from a braid shield must not protrude into the insulation. The shield must be replaced completely,and the grounding conductor must be placed in intimate contact with the shield. Splice Inspection A recommendation for improving splice performance is to inspect splices on a regular basis and use the information to institute new procedures or even new splice kit designs. An opportune time for doing this is before shipping an extensively damaged cable to a repair shop for vulcanized repairs. When the cable is idle and quite
- 236. Outer protective cover topes Inaulat~ng tape bu~ldup Cable shielding / ' 1 Cable jacket Grounding leod Shleldmg tope Stress-control 41 necessary or braid tope Figure 8.23.-Typical taped splice in high-voltage shielded cable. accessible, perhaps stored in a supply yard, old splices can be cut out and scrutinized. Just the simple process of slitting the old splice lengthwise using a sharp linoleum knife can provide good information regarding insulation procedures, wear characteristics, effectiveness of bonds, and soforth. Electrical tests and tensile evaluationscan be made on the insulations. Samplingsof this type can readily provide extensive data on splices with varying amounts of in-service time. TROLLEY SYSTEMS The conductors that provide power for electric track haulage systems form a major part of the power- distribution system in many underground mines. The trolley system is a potential hazard for fires, ignition of methane, and shock since it utilizes uninsulated conduc- tors. The danger in underground coal mines is greater than that in surface mines because of limited space and the presence of methane, However, all mines that utilize trolley conductors can benefit from proper design, selec- tion, and installation of the system components. Several conductors are used in the trolley circuit: trolley wire, feeder cable, rail-bond cable and steel track rails. The trolley wire supplies power directly to a rail- mounted vehicle, such as a mine locomotive, through a collector called a shoe or harp. The trolley wire and collector connection can cause frequent severe arcing, which may damage either part and cause an obvious ignition hazard. Proper positioning of the trolley wire, particularly at curves and switches, correct holding force on the collector, and the required amount of lubrication are necessary to minimize arcing. A feeder cable supplies power to the trolley wire. Consequently, both must be sized properly to provide enough current-carrying capacity yet minimize heating and voltage drop. In addition, rectifiers must be positioned at adequate intervals to supply the proper voltage to the feeder. The current return path utilizes the steel rails, which must have adequate conductivity to minimize the total system resistance. Rails are laid in segments, and the connections between them can loosen or the rails could break; hence, rail-bond cable is installed to maintain continuity. Rail-bond cable is attached at each rail joint, and as a further precaution, between the two rails at specified intervals (cross bonds). Trolley Wire The trolley-wire conductor used in mines is hard- drawn copper, but brass is available for high-speed surface transportation. Round, grooved, figure 8 and figure 9 (deep-section)wire shapes, shown in figure 8.24,are avail- able (31).At one time, round wire was prevalent, but the clamps necessary to support it caused the collectortojump and arc, so it was replaced with the figure 8 shape. Additional problems occurred with the figure 8 because it twisted and kinked when being reeled and unreeled dur- ing installation, and it frequently pulled out of hangers on curves. Consequently, the groovedtype was developed and, together with the figure 9, has almost completely replaced the round and figure 8 shapes. Figure 9 and deep-grooved shapes are almost mandatory with a 350-MCM size and above, because these sizes require large splices and fit- tings and the widths are too large for proper tracking of the collector. The upper section of the wire, to which the support clamp attaches, has the same width dimension whether the wire is grooved, figure 8 or figure 9. Table 8.19provides the necessary specifications for correct wire size selection (31).The most commonwire is 350 or 400 MCM (both often called 610)figure 9. Trolley Feeder In order to reduce voltage drop and supply the neces- sary current, a feeder cable, which is uninsulated and stranded, is hung alongside the trolley wire. Both alumi- num and copper feeders are used, and their size depends on the load drawn by the track vehicles and the voltage regulation desired. Common sizes are 1,000MCM copper or 1,590 MCM aluminum. Tables 8.20 and 8.21 specify copper feeder data (31). As noted in the tables, feeder can be purchased with a weatherproof jacket. Supports, Lubrication, and Turnouts As shown in figure 8.25, the feeder cable and trolley wire can be hung side by side to gain additional support clearance. The feeder can also be used as a messenger to increase the support-bracket spacing, as shown in figure 8.26.In this configuration, a cushioning effect is provided for the trolley wire since the wire is free to flex under pressure. Typical brackets for supporting trolley and feeder are shown in figure 8.25 and 8.26.The amount of deflection or sag between supports can be calculated by where D = sag, in, W = weight, lblft, L = distance between supports, ft, and T = tension, lb. Since the figure 9 350-MCM conductor has a breaking strength of 12,000lb, it can safely be tensioned to 1,200lb, which is 10%of the breaking strength. This will reduce sag and keep the wire straight and level. The dead-end hooks and turnbuckles shown in figure 8.27 are used to install tension in the wire. The maximum spacing recommended for roof- mounted support for a semicatenary installation(fig.8.26) is 20 ft. Direct suspension (fig. 8.25)spacing should be less than 15 ft. Table 8.22 gives support spacings on curves (31). When selecting proper support types and spacings,
- 237. ROUND GROOVED 0.548" s s o 1/0 A W G 2/O A W G 3/0 A W G 4/0 A W G 300MCM d 0 0 0 0.388" 0.429" 0.482" 0.574" 0.620" 2/0 A W G 3/0 A W G 4/0 A W G 300 M C M 350 M C M 0.300" 0.222" 0.250" FIGURE 8. k 8 0.165" 0 2 0.106" 0.312" 0 352" 0.400" 0.450" 0.570" 1/0 A W G 2/0 A W G 3/0 A W G 4/0 A W G 350MCM FIGURE 9 DEEP- SECTION GROOVED I 0 0.4%" 350M C M d 0.552 400 M C M C O P P E R Figure 6.24.-Trolley-wire cross sections. Table 8.19.-Tmlley-wire specifications Cross-sectional dc resistance Nominal Weight (volts drop per amp Minimum Minimum Elongation size. area Type of wire at 20%) tensile breaking within AWG or Actual strength, load, 10 in, MCM Nominal IbIMR Iblmi 4 : ; Or" Psi Ib % MCM MCM in2 /mi Round, harddrawn mpper. 97.16%conductivity ............ 110 105.6 105.6 0.0829 319.5 1,687 2/0 133.1 133.1 .I045 402.8 2,127 310 167.8 167.6 .I318 507.8 2,681 410 211.6 211.6 ,1662 640.5 3,382 300 300.0 300.0 ,2356 908.0 4,794 Grooved. harddrawn coooer. .. . 97.16% mnductivity............. 210 133.1 137.9 ,1083 417.6 2,205 310 167.8 167.3 ,1314 506.4 2.674 410 211.6 212.0 300 300.0 299.8 350 350.0 351.2 Figure 8, harddrawn copper, 97.16% conductivity............. 110 105.6 105.6 2/0 133.1 133.1 310 167.6 167.8 410 211.6 211.6 350 350.0 350.1 Figure 9 deep section, harddrawn copper, 97.16% conductivity 350 350.0 348.9 400 400.0 397.2
- 238. Table 8.20.-Characteristic data for solid copper feeder cable Conductor Section area Overall diameter. in Weight. lWMll Bare wire breaking size. strength. Ib AWG cmil in m Bare Weatherproof Bare Insulated Hard drawn Annealed 0000........................... 211.600 0.1682 107 0.4600 0.6163 641 767 8.143 5.320 000............................. 167;800 .1316 85.0 . 4096 .5659 508 629 6.722 4.220 00 .............................. 133.100 .lo45 67.4 .3648 .5211 403 502 5.519 3.340 0 .............................. 105.500 .08289 53.5 3249 .4812 320 407 4.517 2.650 1 ................................ 83.890 .06573 42.4 .2893 .4456 253 316 3.688 2.100 2 ................................ 66.370 .05213 33.6 .2578 .3826 210 260 3.003 1.670 3 ................................ 52.640 .04134 26.7 .2294 .3544 159 199 2.439 1.325 4 ................................ 41.740 .03276 21.2 .2043 .3293 126 164 1.970 1.050 5 ................................ 33.100 .02600 16.8 .1619 .3069 100 135 1.591 880 6 ................................ 26.250 .02062 13.3 .1620 .2870 79 112 1.280 700 7 ................................ 20.870 .01635 10.6 .1443 .2693 63 NA 1.030 550 6 ................................ 16.510 .01297 8.37 .1285 .2535 50 75 826 440 NA Not available. Table 8.21.-Characteristic data for stranded copper feeder cable Cross-sectional area Number of Overall diameter. Weight. Ib/Mn Rare wire breaking Resistance.MMfl Conductor wires In strenath. Ib at 20%. size in standard cmil in m ~~~~i Bare Weather~rwf Bare Insulated Hard drawn Soft annealed annealed MCM: 2.000 ...................... 1.750 ...................... 1.500 ...................... 1.250 ...................... 1.000 ...................... 900 ......................... 800 ......................... 750 ......................... 700 ......................... 600 ......................... 500 ......................... 450 ......................... 400 ......................... 350 ......................... 300 ......................... 250 ......................... AWG: 0000 ....................... 000 ..................... .... 00 ........................... .......... ......... . . . . NA Not available. 'Sizes AWG 0000 and 000 cable are usually made of 7 strands when bare and 19 strands when insulated.
- 239. KEY A Mine hanger F Bulldog feeder sling 6 Dlrigo spool insulator G Triple yoke C Dual suspension clamp N Mlne hanger D Double yoke J Triple horizontal insulator E Bulldog clamp Figure 8.25.-Typical trolley-wire and feeder.cable supports. END VIEW STRAIGHT LINE SPAN CURVE SPAN KEY A Expansion bolt D Bulldog feeder sling 6 Combinotion clamp without boss E Cornbinotion clamp with boss C Mine hanger Figure 8.26.-Trolly-wire semicatenary suspension.
- 240. KEY A Expansion bolt B Mine honqer C Dirigo spool insulator D Deod-end hook E Insuloted turnbuckle f bod-end cam grip G Feeder-wire stroin clomp H Dead-end clews J Feeder-wire strain clamp K Dead-end eye L Feeder strain clomps Flgure 8.27.-Trolley system accessories. Table 8.22.-Tmlley-wire support spacings on curves Radius of hdaximum spacing, Radius of Maximum spacing, cum, ft, with deflection 1 C U W ~ ft, with defiection angled 5 O angle of 5 O ... 350 and over 150................. 13 300................. 26 120................. 10 250 ................. 22 100................. 9 200 ................. 17 60................... 7 'On straight lines,spacing can be increased to 30 fl where wire is 5 R or more above rail. If wire i s less than 5 R above rail, the limit on inside constructionis 20 R. the weight of the trolley guards must be included. Under- ground coal mine regulations require guards to be posi- tioned wherever personnel normally work or pass under the uninsulated trolley and feeder wires (38).When the roof is uneven or too high for the trolley wire, pole extension brackets can be used from either the roof or rib. For trolley haulage outside the mine, catenary or direct support can be used. Simple catenary support, suspending the trolley wire from a messenger, works best for long haulage distances since 100-ft spans are possible on straight track. Compound catenaries, using two mes- senger wires and subspan catenaries are also employed. Semicatenary or direct suspension can be used on the surface, employing the same components as shown in figures 8.25 and 8.26but mounting the hangers on wooden or metal poles or structures, and the spacing may be increased to 25 or 30 ft. The deflection formula (equation 8.9) may be used for more precise calculations. A properly installed trolley wire looks level and straight, without bends. kinks, or sags. The rubbing surface of the metal should look polished and smooth, not scraped or burned bright. A graphite-based lubricant should be used periodically to form a smooth contact surface and maintain the smooth polished-brown appear- ance. An unlubricated, uneven trolley wire wears out quickly, is unsafe because of arcing, reduces power efi- ciency, and also wears out the collector. I At track switches or turnouts, trolley frogs must be installed at the proper location to assure that the collector will pass on to the correct wire. Normally, a standard lo0 trolley frog is used for any degree of track turnout. The frog must be positioned far enough beyond the track turnout that sufficient side force exists to guide the collector on to the correct wire. ?bo much side force will cause the collector to be pulled off the wire. The location for frog angles is found by determining the position where the collector pole exerts adequate side force on the collec- tor. This point occurs where the pole angle is equal to the track-frog angle plus lo0. Rails and Bonds Another important consideration of the trolley system is the resistance of the current return path: the rails and bonds. The specific resistance of high-carbon steel rails is 118 nlcmil-ft at 20° C, about 12 times that of copper. In table 8.23, this value is used to provide resistance values for mine track per rail. Table 8.23.-Resistance of steel rail at 20°C Rail weight, R e Rail weight, Resistance, Ib/yd IblMft w ~ f i A major concern with rail resistance occurs at switch contacts and bolted rail joints. These points can have very large resistances, sometimes approaching an open circuit. Rail bonds are used to ensure a low-resistance contact or joint. With inadequate bonding, the return current from trolley system loads can stray into the mine floor or earth, and the stray current can cause electrical problems
- 241. throughout the mine, including nuisance tripping of pro- tective circuitry. Rail-bond cable is soft, stranded copper in sizes from 210 to 500 MCM. It is attached with stud terminals or welds across every rail joint. Welded terminals are pre- ferred for permanent application on main-line haulage; however, the amount of heat used must be controlled in order to prevent steel rail recrystallization. Thermite welds have also been used successfully in this operation. Cross bonds are applied at least every 200 ft along the track, so that if one rail or a bond along a rail breaks, the current return path can be completed through the other rail. This practice also halvesthe resistance in the current return circuit by paralleling the two rails. Crossbonds are also recommended at all rail turnouts in conjunction with the switch ~oints. All boids are susceptible to damage by the wheels of derailedmine cars and hence shouldbe located next to ties and secured to the tie side for protection. If possible,joint bonds should be placed under the rail-connectingplates. Bond size is usually determined by voltage drop rather than by currentcanying capacity. It is a general rule that bare conductors will carry 1A per 5,000 cmil of area without excessive heating. Five times this amount can be carried for brief periods, and short bonds on heavy rail can carry 150%of the normal load current. Bondcable resistances are shown in table 8.24 along with the usual sizes accordingto rail weights. The terminal resistance is negligible at 1pQ, and crossbonds are normally equal in length on the track gauge, typically 42 in. Joint bonds are usually 10 in long. The added resistance of joints, ex- pressed in feet of rail, can be found fromthe chart in figure 8.28 (22). Table 8.24.-Data for rail-bond cable Rail-bond Resistanced 1 h Used with rail size at 20°C, lo-% 0 size, lWyd AWG. 410...................................... 49.97 20- 80 MCM: 300.................................... 35.31 80-1M) 500........................ . . ....... 21.16 80-100 OVERHEAD LINES The most common method used for electric power transmission and distribution is overhead conductors. Al- though their size and detailed construction can vary widely, overhead powerlines normally consist of bare me- tallic conductors supported by insulators from some ele- vated structure. The conductors use air space for insula- tion over most of their length, while their elevation protects them from contact with ground objects. Overhead-line installations use numerous types of conductor arrangements and support structures in various combinations. Utility systems range from single wooden poles, carrying conductors at low voltages, to self- supporting steel towers bearing major transmission lines. Wooden polelines with or without crossarms,for example, may be part of a single-phase or three-phase distribution system with voltages of 2.3 to 35 kV. By contrast, steel towers often carry lines transmitting large amounts of power at 115kV and up, connectingmajor load centers of a utility company grid (12, 40). Utility-owned lines are ADDED RESISTANCE PER JOINT (R), ft of roil 20 I 8 1 6 14 1 2 1 0 A 6 4 7 0 0 5 1 0 15 20 25 30 35 40 45 50 ACTUAL LENGTH OF BOND (L), in Figure 8.28.-Theorellcal resistance of bonded joint. The proper moves to make in using this chart have beenindicated by the smalldiagram in the uppercentral portion.Starting wlth the length (L)of the bond, move vertically to the bond capacity (C), then horizontally either right or left to the rail weight (W), then vertically to the equivalent feet of rail. commonly classified by function, which is related to volt- age. There are no utility-wide standards for voltage clas- sification, but the system that is typically used differs f r o mthe classification used in the mining industry. Overhead conductors are arranged in various config- urations to reduce line-to-line contacts due to wind, ice loading, or sudden loss of ice load, and may include different combinations of power, neutral, and static con- ductors. Aluminum conductors with steel reinforcement (ACSR)are commonly used because of their strength and relatively low price, but special applications may call for other materials such as copper (12, 40). The types of overhead-lineinstallations used for mi- ing applications are similar to those in utility distribution systems. Typical are polelines to supply equipment in surface mining and lines feeding surface facilitiesrelated to mining. These lines are normally installed on single wooden poles and may cany only two conductors, as in single-phasesupplies,or have up to six conductors,includ- ing three power, one neutral, one ground-check(pilot),and one static. The polelines may be relatively permanent installations such as those feeding plants, shops, and other surfacefacilities, and long-termpit baselines or ring mains. Chapter 1 includes a discussion of the poleline applicationin strip and open pit mining operations.Some- times, temporary poles are mounted in portable bases (such as concrete-filled tires) for ease of relocation, and these are commonly used in open pit mining operationsto carry power into the pit. Conductors are again usually ACSR, but hard-drawncopper is used where blast damage is a problem (31). If these lines are not installed properly, failures from conductorbreakage, arcing between phases, and structure
- 242. collapse can occur. Obviously, serious safety hazards and costly power outages can result; therefore, proper design and installation are important. In-depth treatment of overhead-line design is provided in such texts as Fink and Carroll (12)and Westinghouse (40),which give excellent detailed summaries of design, installation, and repair practices in overhead distribution. This section is intended to be a brief introduction to overhead-line design combined with some details of the wooden pole structures that are the main type found in mines. Overhead lines, unfortunately, have been a major cause of electrocutions in the mining industry; thus, an extensive discussion of injury prevention is also included. Overhead-Line Design The design of surface overhead lines relies as much on a knowledge of structure and mechanics as it does on electricity. The design is concerned with obtaining the correct size and placement of the structures that support the power and grounding conductors and keep them from damage. Obviously, the vertical weight of several single conductors or one multiconductor cable must be sup- ported. Additional vertical loading can be caused by ice accumulation. The height of the structures must be ade- quate to provide the required ground clearance consider- ing the amount of line sag. At the same time, the struc- tures must be planted firmly enough to counteract the force placed on them by the conductors on a steep slope. Tension is applied in order to install the conductors with the correct amount of sag, and this results in stretch. The stretch causes creep or elongation in the conductors over a period of time, which must be accommodated in the design. Aluminum conductors are particularly susceptible to creep. Another factor that must be considered is the effect of weather. Temperature changes cause expansion and contraction, which affect the amount of deflection. Wind causes the conductors to vibrate vertically and imparts a horizontal force to the structure. Calculation of horizontal forces is particularly important at angle points in the line. Additional large horizontal forces exist when a conductor breaks, and this factor too must be incorporated into the design. The vertical spacing between conductors must be large enough so that arcing does not occur during high winds or when a large accumulation of ice falls off a line. Extremely tedious calculationsfor catenary spanscan result from attempting to take all of these factors into account. Fink and Carroll explain graphic solutions and describe Thomas Charts that assist in the computation process (12).The National Electrical Safety Code (NESC) (2)gives design information for ice loadings, temperature variations, and wind velocities for different areas in the United States. As mentioned earlier, several different types of struc- tures are used to support the conductors. Selection of a specific type depends on the terrain, accessibility require- ments, right-of-way availability, distribution voltage, span length, number of circuits, conductor size, weather, life of installation, availability of material, and economics. The types commonly used are self-supporting and guyed-steel or aluminum towers, steel or aluminum poles, concrete poles, wooden H-frames, and wooden poles. Steel and aluminum structures are used for high-voltage distribu- tion where long service life and long spans are necessary but are only used in some mine power systems within substations. Wooden poles are the most prevalent overhead-line support in the mining industry, so a few details of their design will be presented. Wooden poles are usually constructed from fully treated pine or butt-treated cedar. They are classified by their circumferential dimension measured at a point 6 R from the butt. Consequently, the nominal ultimate strength is the same for all lengths and species of the same class. Wooden poles, classed 1through 7 with this system, have the capability of withstanding the ultimate loads shown in figure 8.29 (12).The correct setting depth for various lengths is also noted in the figure. The setting depth is important to prevent butt "kickout," since the pole is primarily a cantilever column. GUYED POLE UNGUYEO POLE f =Safe bending stress, psi t = Taper, in/ft Moment of lood = P, hi t P2h;+p3h; Safe moment on pole= ' h f dl.dl+t~' 13h(dl+d2) W~nd on pole (W) = Ultimate Taper, Variety ( . ! ; : & Iinches circum 1 bending per ft length Average top diam Pine 7.400 d2- .63h/3.14 Chestnut 6,000 P= Safe load 2in from top PI P2 P3= Wind on wires, lblft M , = Moment of lood Mp=Safe mwnent on pole 2'-0" dl M,=P,h,tP*h2+P3h, Ultimate kmd 4,500 3,700 3,000 I I I I I 2,400 1,900 1,500 7 1,200 Pole length, ft 1 30 1 35 140 145 1 50 1 55 1 60 1 65 Setting depth,% 1 5.51 6.016.01 6.51 7.01 7.01 7.51 8.0 Figure 8.29.-Pole strength calculations.
- 243. Guy wires are used where necessary to assist in supporting the horizontal loads. Usually, 318-ingalvanized steel wire tied to a log-anchored rod is used, as shown in figure 8.30 (12). The ratio of the guy load (L)to the conductor tension (T)is the same as the ratio of guy length (B) to the distance away from the pole base (A). Both wooden and steel crossarms are used on wooden poles. Steel crossarms give better protection from light- ning strokes,but they are more expensivethan equivalent- strength wooden arms. Treated yellow pine or untreated Douglas f i r are commonly used for crossarms. Their length ranges from 10to 25 ft depending on the required conduc- tor spacing. In size they range from 5 by 6 in to 6 by 10in. Two planks, 3 by 8 in, can be mounted on either side of the poles to form a crossarm for heavy conductors. Qpical conductor arrangements and spacing for pin insulators are shown in figure 8.31. The dimension B is often determined by the span length and calculated ac- cording to the method given by Fink and Carroll (12). Because the sleet-jump failure experience, the dimension E should be at least 1 ft. Swing-type insulators require additional spacing to give a clearance for the swing. The clearance between the conductor and any grounded struc- ture must be a t least 0.75 times the dry-flashover distance of the insulator or the "tight-string" distance under an 8-lb wind at 60° F. In addition to the conductors positioned at the insu- lators, overhead ground conductors or static wires are strung above the conductors to give lightning protection. Lightning protection is discussed in detail in chapter 11. Overhead-Line Electrocutions Overhead lines, whether utility transmission and distribution lines or part of the minesite electrical system, present a serious electrical shock hazard to mining per- sonnel. Overhead lines in and near mining operations can be exposed to many types of mobile equipment and even handheld tools. Metallic frames of such equipment, upon contact with energized overhead lines, can become ele- vated above earth potential, and simultaneous contact of the hot frame and ground by an individual can create a path through the body for lethal levels of line-to-ground fault current. Personnel are therefore exposed to a shock hazard through indirect contact with overhead powerlines. Although this mode of electrocution seems (at least out- wardly) straightforward, it has been very difficult to find effective means of prevention. Examination of mining industry statistics since 1970 reveals that one-third of surface coal mine electrical fatal- ities and approximately one-thirdof all electrical fatalities in metal-nonmetal operations are directly attributable to the indirect contact of overhead lines (25). The majority of these accidents involved mobile equipment, and the haz- ard can exist anywhere that high-reaching equipment operates near overhead lines (1). Trucks commonly involved in overhead-line contacts are highway-legal end-dump tandems, triaxles, and trac- tor trailers. They can contact overhead lines, and their frames become subsequently energized, through their beds being raised or driven into lines. Victims normally bridge lethal potentials when stepping from the cab onto the ground or by operating external controls. Mobile cranes, which present a substantial line-contact hazard in other industries, find various uses around a mine site. They range from large, solid-boom, high construction DESIGN DATA F O R GUYS Uitimate Guy strength, Ib 318 in sm. 6,950 3/8inh.s. 10.800 7 / 1 6in s.m. 9,350 %6 in h.s. 1 4 . m h 8-in-d~ameterlag, 5 f t long Weight of cone = 12,000ib Allowable bearing along guy rod 153ton/ft2 Figure 8.30.-Guy and log-anchor calculations. Pole top Two arm Single arm Voltage Spac~ng Figure 8.31.-Typical arrangementsand pin-insulatorspac- ings on wooden poles. cranes to smaller hydraulically powered units with re- tractable booms. Lines can be contacted by the boom or hoisting cable, and in both cases workers around the crane have the greatest shock-hazard exposure. Mobile drilling rigs are susceptible to overhead-line contact because of their masts, which can be raised or driven into the lines. Operators are the most likely victims, bridging potential gradients while operating drill controls.
- 244. In response to the problem, a detailed investigation has been made into these mining accidents, into preven- tion methods used by utility companies and other indus- tries, and into various additional methods that might reduce electrocutionsfrom indirect contact with overhead lines (27). From this effort, typical hazardous mining locations with overhead lines were identified and several recommendations were established to reduce the associ- ated hazards. The listing of areas and situations that pose the greatest overhead-linehazard is important since it shows the target areas for applicationof recommended solutions. These locations can be divided into two groups: mining surface facilities and active excavations in surface mines. Loading and dumping facilities, including stockpiles, loading bins or hoppers, material transfer points, and adjacent areas, yards and roads, are hazardous overhead- line locations for truck operation. Some factors contribut- ing to the risk are operator unfamiliarity with the dump, use of a temporary dump point, and fluctuations in the edge or height of a stockpile. Trucks and cranes can easily be exposed to line hazards near various mine plant areas such as mineral processing, storage, handling installa- tions,refuse dumps, and settling ponds. Constructionsites may or may not be near permanent mining facilities but often present hazards involving construction cranes and preexisting overhead lines. Overhead lines traversing active surface mine work- ings present potentially dangerous situations. The fatali- ties that have occurred in these areas were from lines other than pit power distribution. Hazardsexist primarily over mine benches as well as access and haulage roads. Although not responsible for electrocutions in the past, pit power distribution can create a hazard when overhead lines are used, such as for strip-mine base lines. The recommendations to reduce these hazardous sit- uations include isolating overhead lines from mobile equipment, modification of overhead lines, use of protec- tive devices, and safe work practices, each of which will be discussed in the following paragraphs. Overhead-Line Isolation It is the responsibility of the power engineer in a surface mine to assess the overhead distribution system with regard to the movement of mobile equipment and to ensure that wherever possible overhead lines are isolated from travel routes. This may seem an obvious course of action,but previous accidentshave shownthat correctable hazardous situations are often allowed to exist at mining operations. Where there is frequent dump-bed truck traffic, lines must be restricted from dump sites and approach or exit roads. A safety margin of at least 100ft should be allowed outside normal truck routes. This would allow for limited truck movement beyond the route to account for mechan- ical problems, bed cleaning,backups and temporary dump sites. Roads leading away from dump locations should not be crossedby lines for at least 250 ft beyond the dump site, since beds may not be completelydown as trucks leave the area. This distance would give additional time for the bed to lower or for the driver to recognize the condition. Construction cranes that remain stationary while operating at a project site can be positioned so that line contact cannot occur at any position. Cranes that travel during operation will require barriers around hazardous areas. When a safe distance from overhead lines is being determined, contact by hoist cables and swinging loads should be considered. One situation is which line isolation may not be feasible is where the lines supply power to a surface facility or a nearby installation. In order to eliminate bare overhead conductors in these situations, some alternate method must be used to supply power. One alternative for permanent installations is underground cable. Cables in conduit or directly buried are suitable for lines entering plants, dump facilities, shops, supply yards, and support buildings. Cables similar to those found in mine power distribution, such as MPF and SHD types, are used for buried applications. Cables present a safe alternative to bare overhead conductors in areas where high-reaching equipment must travel. Underground service removes line exposure com- pletely, but overhead cable with pole support may be preferable because of cost, ground conditions, or expected installation life. In either method, the cable should com- pletely span the hazardous area, or its purpose is defeated. These cable runs should continue for a short distance beyond the hazard area to allow for equipment extensions protruding beyond area limits. Overhead lines traversing active surface mine work- ings present a hazard to high-reaching equipment. Whether they are preexisting utility lines or part of mine power distribution, hazards can result for trucks and drills on benches or on haulage and access roads. The removal of these lines from the work area is the most direct solution. This may involve the permanent relocation of a utility line over a proposed open pit or a temporary rerouting of a line about a strip operation.Elimination of overheadlines in a pit power-distributionsystem would probably involve re- placement of cable. Operations such as strip mines can and commonly do use all-cable distribution with good results, provided that proper cable-handling techniques and equipment are used (27).Open pit operations nor- mally use overhead distribution to switchhousesin the pit and shielded trailing cables to mobile equipment. How- ever, none of the fatal accidents examined were due to contact of these overhead distribution lines. In large open pit mines, overhead distribution is the most practical because of the long distances and cable protection require- ments, but where frequent equipment operation poses a contact hazard, cable may be more desirable. When rerouting lines around surface mine work ar- eas, all aspects of the operation should be considered, including surface clearing, reclamation, access roads, and haulage roads, as well as actual mining activities. A safety margin should again be provided beyond normal work areas to account for occasional abnormal truck traffic, excavator booms, and similar situations. Contact with overhead lines can also be avoided by removing the equipment operation from the hazardous area instead of moving the lines. Although this should be a very effective method, sometimes equipment movement is necessary: for instance, access cannot be restricted for cranes in supply yards or trucks in dump areas. However, where lines traverse active surface mine workings, equip- ment could be kept out of any contact-hazardarea. Limit- ing access to lines can be the only economic alternative for a very small strip operation, which may be unable to sustain the cost of relocating even a small overhead distribution line. Nevertheless, any efforts to restrict mo- bile equipment must be carefully planned and imple- mented so as not to hamper normal operations or antago- nize the work force.
- 245. It may be possible to restrict high-reachingequipment from some permanent surface facilities. Where this is possible, it provides an effective and less costly alternative to relocating overhead lines, so long as normal operations are not hindered. Restriction can be accomplished by posting the area, or using barriers such as steel crossbars, which allow only low vehicles (cars and small trucks) into the area. Provisions can easily be included to allow occa- sional entrance of higher equipment. One option for the operation is to avoid the hazard by leaving the overhead-line right-of-way undisturbed. How- ever, this option can result in a loss of resource as well as a disruption in the continuity of mining. The right-of-way may involve forfeiting only a single pass, as in a contour strip operation, but may seriously affect the mine layout if a large-area strip mine is traversed by a major transmis- sion line. In order for a contour mining operation with an overhead powerline across the projected path to continue through the right-of-waybut not mine below the lines, the towers or poles beyond the pit width limits would have to be guyed. The cables could then be removed or lowered into trenches, and all large equipment would be trammed or walked over the right-of-way.The lines would then be replaced, and mining operations would resume on the far side of the overhead lines. Exploration drilling commonly requires operation in unfamiliar surroundings, often under minimal supervi- sion. However, drill sites may usually be relocated to avoid overhead-line hazards. Overhead-Line Modification Solutions discussed prior to this point isolate over- head lines from mobile equipment to reduce the change of contact. There are modificationsto existing overhead lines that can substantially reduce hazards without resorting to the extreme measures stated earlier. Such techniques are important because many cases will arise where an opera- tor cannot eliminate overhead-line hazards nor limit ac- cess to them. Overhead-line heights must never be less than the minimum mandated by Federal regulations (38).These heights are extracted from the NESC for driveways, haul- ageways, and railroads, and 15 ft is stipulated as the minimum height for any high-voltage power line (2).Table 8.25 lists the NESC standards that cover most overhead lines in mining, while table 8.26 provides the required minimum distances for higher voltages. Some hazards can be reduced by raising some over- head lines above the NESC minimums. Where dump-bed truck traffic is a concern, lines over roadways could be raised to clear most dump-bed units without extensive support structure. A line height of 45 ft would place lines above most highway-legal dump-bed trucks, even with their beds fully raised, and would also clear most drills and cranes when they are in transit with their booms and masts lowered. If necessary, it is possible to raise lines to more than 65 ft using single wooden-pole supports. How- ever, the line heights attainable depend upon line spans, cable sag, and surrounding terrain, but in most cases 45 ft is an achievable height. Another line modification that lends itself to road crossings is the guarding of power conductors by effec- tively grounded conductors. If it can be ensured that any accidental contact with power conductors will be simulta- neous contact with grounded conductors, a line-to-ground current will probably be provided. This reduces current flow through an equipment-ground contact and increases the chance of rapid fault clearing by circuit protective devices. However, several grounded conductors will be necessary to ensure simultaneous contact and may make this method impractical because of cost. Under these circumstances, rubber guarding may be used on overhead lines at hazardous crossings. Utilities will often supply electricity to a mining facility substation by running a branch overhead line from their lines. If the branch line creates a contact hazard on or around the mining property, a disconnect switch should be provided external to the utility system and upstream from any contact-hazard area. Should the need arise to work in close proximity to these lines, power could be cut with no disturbance to other utility customers. Discon- nects that are quickly accessible from mine work areas would also encourage deenergization prior to work about lines, but this depends upon ownership of the lines, availability of qualified personnel to cut power, and utility policy. Protective Devices Devices exist that attempt to reduce overhead-line hazards either by insulation from line potentials or warn- ing of overhead-line proximity. Representative of the insu- lation method are insulated boom cages and insulating load hook links;proximity warning devicesare intended to indicate the presence of energized conductors. Most de- vices are directed primarily toward protection of mobile cranes but do have other applications. An insulated boom cage is an enclosure or guard mounted on and electrically isolated from the boom or mast to be protected. If the boom is moved into an energized overhead line, the insulated cage makes initial Table 8.25.-Minimum vertical conductorclearancesas specified by the NESC, applicable to mining and mining-related operations Criteria Nature of surface underneath wires, conductors. or cables Open supply line conductors,ft 750 V to 15 to 16 kv <n kv Locationswhere wires, conductors or cables cross over. ............ Track rails of railroads (except electrified railroads using 28 30 overhead trolley conductors). Roads, streets, alleys, parking lots subject to truck traffic........... 20 22 Other land traversed by vehicles, such as cultivated, grazing, 20 22 forest, orchard, etc. Locationswhere wires, conductors,or cables run along and Roads in rural districts ............................................................... '18 20 within the limits of highways or other road rightof-wasbut do nor overhang the roadway.
- 246. Table 8.26.-Minimum distances fmm overhead lines for equipment booms and masts (38) Nominal Minimum Nominal Minimum powerline distance, powerline distance. voltage, kv n R 69 to 114.............. 345 to 499 ............ 25 115 to 229 ............ 500 and up........... 35 230 to 344 ............ 20 contact and prevents the boom from becoming energized. The device only protects covered areas and cannot easily guard hoisting ropes, and its effectiveness also depends on the integrity and surface condition of the insulators used ( I ) . During crane hoisting operations, workers steadying or directing a load from the ground are in an extremely hazardous situation should an overhead line be contacted, as they are commonly in contact with both the ground and load. Insulating links can be used to isolate loads from the crane hoisting rope and are placed between the load hook and the hoist rope. The links are constructed of a dielectric such as resin-impregnated fiberglass. A proximity warning device indicates by a visual or audible alarm the proximity of equipment extensions to energized overhead powerlines (15). Unlike cages and links, these devices attempt to prevent equipment-line contact, and the operation is theoretically independent of human judgment, at least so far as indication of powerline presence is concerned. Ideally, such devices alert an oper- ator if the protected equipment extension enters a prede- termined zone about a power conductor. Several types of proximity warning devices are available in the United States, all operating on the principle of electrostatic-field detection (15). The electrostatic field abqut a group of overhead conductors is primarily a function of their volt- age and geometry. The units generally operate by moni- toring 60-Hz electrostatic fields, amplifying, rectifying, and measuring the signal, and then activating an alarm at some preset signal level. The sensor used may be short and effectively a point sensor, which will create a spherical detection area, or a distributed sensor spanning the length of a protected extension. The type, number, and location of these sensors greatly affect the operation of a proximity warning device. Proximity warning devices operate as intended under many circumstances, but their reliability can be compromised by a complex array of factors. These limitations can be grouped into those arising from opera- tional principles of electrostatic-field detection and those which are due to the design of individual devices. The concept of a device to alert equipment operators to possible overhead-line contacts has great merit, but given the inconsistent operation of currently available devices, they should only be applied with full recognition of their limitations. Dangerous conditions can exist where work- ers place too much faith in a warning device or ignore it due to previous unreliable operation. Proximity warning devices are best applied only as a supplement to other overhead-line contact safety measures. Boom cages and insulating load links also have sound theories of operation but problems in implementation, and major drawbacks stem from flashover due to insulator surface conditions. The use of any warning or insulating technique does not relieve the operator from the responsibility of maintaining the minimum line-equipment clearances stated earlier. Safe Work Practices Any attempt to reduce overhead contact hazards at a mining operation must also involve the development of safety awareness within the work force. Training of per- sonnel in safe operation of mobile equipment near over- head lines will complement any other safety method and, in some cases, may be the only effort necessary for preventing indirect-contact electrocutions. The following recommendations include guidelines for work near over- head lines, some passive-warning techniques, and safety training of personnel. Before work is done near high-voltage overhead lines, the areas in question should be thoroughly examined by supervisory personnel and workers to determine the pres- ence of any overhead-line hazards. All overhead lines should be considered energized unless an authorized rep- resentative of the line owner indicates otherwise. If the lines are utility owned, the utility should be contacted for assistance with planning safe operating procedures for the project. Equipment should be operated only by a compe- tent, experienced, qualified operator, and the operations should be observed by a reliable worker, watching for maintenance of minimum clearances and unsafe condi- tions. This observation should be the worker's designated and only task. Another competent worker should be des- ignated to direct the equipment operator, and only this worker should give directions. Standard signals should be agreed upon and used. Booms, masts, beds, and so forth should be in a lowered position when equipment is in transit, and minimum legal clearances should be main- tained. If minimum clearance cannot be provided, the overhead lines in question should be deenergized and visibly grounded. The following procedures should be followed if an energized overhead line is contacted. If contact was mo- mentary and no lines are down, a calm and experienced crew member should be certain that the equipment is no longer in contact and should then assign members of the crew to check for injuries among the work party, to administer first aid if necessary, such as basic life support and cardiopulmonary resuscitation, and to send for an ambulance immediately, to notify supervisory personnel, to check for dangerous equipment damage, and to secure the area for possible accident investigation. If contact is made and maintained, a calm and experienced crew mem- ber should instruct personnel aboard the equipment to remain in place and not to contact the ground, then have the operator move equipment out of contact if possible. Crew members should be assigned to keep all other personnel clear of the area, including equipment, hoisted loads, and fallen lines, to notify appropriate mine super- visory personnel or utility to have lines deenergized, and to send for an ambulance if needed. The crew should not contact any victims still in contact with energized equip- ment. When victims can be rescued safely, the crew should administer first aid, move equipment to a safe position, check for damage, and secure the area for possible acci- dent investigation. Investigation of past fatalities shows clearly how essential it is for workers to be familiar with these procedures, and the importance of regular training in cardiopulmonary resuscitation (CPR). Passive-warning techniques, including signs, stickers, posters, and line indicators, should be highly visible and in color to draw worker attention. They should be to the
- 247. point and simple t o understand. Signs in hazardous areas 16. Ilsley, L. C., and A. B. Hooker. The Overheating of Rubber- should be large enough t o be easily read from approaching Sheathed Trailing Cables. BuMines RI 3104, 1931. equipment and should warn operators well i n advance of 17. Institute of Electrical and Electronics Engineers (New t h e danger. York). General Principles for Rating Electrical Apparatus for paragraphs48.25through 48.28, and 48.31, 30 CFR, Short-Time, Intermittent, or Varying Duty. Stand. 96-1969. 18. . Recommended Practice for Electric Power Distribu- mandate t h e initial training and periodic retraining of tion for Industrial Plants, Stand, 141-1986, mine personnel with respect t o t h e occupational hazards of 19. Insulated Power Cable Engineers Association. Ethylene- mining. ~ i g h - v o l t a g eoverhead-line safety should be in- propylene-Rubber-InsulatedWire and Cable for the Transmission cluded in this training. New employees at surface opera- and Distribution of Electrical Energy. Publ. S-68-516, rev. Jan. tions are often laborers assisting o n or about mobile 1978. - equipment, and i n their initial training they must be 20. . Power Cable Ampacities. Publ. P-46-426, v. 1-2, alerted to the danger presented by overhead lines. Hazards 1962. (IEEE Publ. S-135.) specific to the mining facility i n question should be 21. . Rubber-Insulated Wire and Cable for the Transmis- brought out in initial training and retraining, as well as in sion and Distribution of Electrical Energy. Publ. S-19-81, 5th ed., rev. June 1976. the hazard training required for workers assigned to new 22, Jones, D, C,, M, E, Altimus, and F, W, Myers, Mechanized jobs, particularly new equipment operators. Frequent re- Mining Electrical ~ ~ ~ l i ~ ~ ~ i ~ ~ ~ PA state univ., university park, views of safe practices regarding overhead lines would be continuing ~ d ~ ~ ~ ~ i ~ ~ , 3d ed., 1971. advisable for all operators of high-reaching equipment, 23. McNiff. J. J.. and A. H. She~herd. Current CarrvineCaoaci- regardless of the minimum legally required training. Particularly important is t h e review of safety guidelines with crews about to begin operations with exposure t o overhead lines. Familiarizing supervisory personnel with safety guidelines a n d company policies is also essential if they a r e t o be competent i n directing the work force under hazardous conditions. REFERENCES 1. Allin, G. S., J. T. Wilson, and R. E. Zibolski. A Practical Review of High Voltage Safety Devices for Mobile Cranes(Pres. at Off-Highway Vehicle Meet. and Exhibition, Milwaukee, WI, Sept. 1977.) Soc. Automot. Eng., Paper 770778. 2. American National Standards Institute (NewYork). National Electrical Safety Code. Stand. C2-1977, et seq. 3. American Society for Testing and Materials. Manual on the Use of Thermocouples in Temperature Measurement. ASTM Spec. Tech. Publ. (STP) 470A, 1974. 4. . The Theory and Properties of Thermocouple Elements. ASTM Spec. Tech. Publ. (STP) 492, 1971. 5. Anaconda Co., Wire and Cable Div. (New York). The Mining Cable Engineering Handbook. 1977. 6. Anaconda Wire and Cable Co. (New York). How To Cut Downtime and Extend Cable Life. V. 2, 1975. 7. Bise, C. J. An Evaluation of High-Voltage Cable-Coupler Per- formance for Mine Power Systems. Ph.D. Thesis, PA State Univ., University Park, PA, 1980. 8. Cable Makers Australia Pty. Ltd. (Liverpool, NSW, Australia). The Little Yellow Book of Cable Data. 1979. 9. Cairns, R. A. The Development and Application of Flexible Trailing Cables for Use on Cable Reel Shuttle Cars. Min. Technol., v. 57, NO. 660, Oct. 1957. 10. Conroy, G. G., and C. J. Mertain. Cable Handling in Surface Mines. P a ~ e r in Conference Record-IAS 1981 Minine Industrv - - ~ ~ ~ ~ -~ Technical Eoderince. IEEE, 1981. . , 11. Conroy, G. J. Cable Fault Locating by Electronic Means. Paper in Mine Power Distribution. Proceedings: Bureau of Mines Technolorn Transfer Seminar, Pittsburgh. Pa., March 19. 1975. ~ u M i n e s 7 ~ 8694, 1975. - 12. Fink, D. G., and J. M. Carroll (eds.). Standard Handbook for Electrical Engineers. McGraw-Hill, 10th ed., 1968. 13. FMC Corp. Protection and Troubleshooting of Coal Mine Electrical Cables (contract H0122011). 1. Shuttle Car Reel Test Unit. BuMines OFR 42(1)-74, 1972; NTIS PB 235 639. 14. Hanslovan. J. J. Trailine Cable Stresses in Undermound Mines. M.S. ~hesis, PA State fniv., University Park, PA, i978. 15. Hipp, J. E., F. D. Henson, P. E. Martin, and G. N. Phillips. Evaluation of Proximity Warning Devices (contract J0188082, Southwest Res. Inst.). BnMines OFR 22-80; 1980; NTIS PB 80-144413. , , tyof Portable Power Cables on ~ e e l s . Pres. At AIEE Fall ken. Meet., Chicago, IL., 1957. AIEE Paper 57-1136. 24. . Current Carrying Capacity, Portable Power Cables on Reels. Coal Age, v. 63, Jan. 1958. 25. Morley, L. A., T. Novak, and F. C. Trutt. Electrical-Shock Prevention (contract J0113009, PA State Univ.). Volume I-Protection of Maintenance Personnel. BuMines OFR 177(1)-83, 1982; NTIS PB 84-102946. 26. Morley, L. A,, J. N. Tomlinson, G. Moore, and D. E. Kline. Portable Trailing Cables, Splices, and Couplers Design and In- stallation Considerations (cantract 50199106, PA State Univ.). BuMines OFR 11-83, 1982; NTIS PB 83-170852. 27. Morley, L. A,, F. C. T ~ t t , and G. T. Homce. Electrical- Shock Prevention (contract J0113009, PA State Univ.). Volume IV-Overhead-Line Contact Fatalities. BuMines OFR 177(4)-83. . . . 1982; NTIS PB 84-102979. 28. Morley, L. A,, F. C. Trutt, and R. A. Rivell. Coal Mine Elec- trical System Evaluation (grant G0155003, PA State Univ.). BuMines OFR 61(4)-78, 1976; NTIS PB 283 493. 29. National Bureau of Standards. Thermocouple Reference Tables Based on the IPTS-68. NBS Monogr. 125, 1974. 30. Neher, J. H., and M. H. McGrath. The Calculation of the Temuerature Rise and Load Ca~abilitvof Cable Svstems. Trans. ~ m . n s t .Electr. Eng., Part 3, ; . 76, bct. 1957. " 31. Ohio Brass Co. (Mansfield, OH). Haulage Product Informa- tion and Design Drawings. 1978. 32. Stefanko, K., and L. A. Morley. Mine Electrical Systems Evaluation (grant G0133077, PA State Univ.). Mine Power System Performance. BuMines OFR 76(4)-75, 1974; NTIS PB 245 930. 33. Stefanko, R., L. A. Morley, and A. K. Sinha. Evaluation of Mine Electrical Systems With Respect to Safety, Technology, Economics, and Legal Considerations (grant G0101729, PA State Univ.). Volume 1. Text, Tables, and Analyses. BuMines OFR 70(1)-73,1973; NTIS PB 225 476. 34. Tomlinson, J., T. Rusnak, R. H. King, and L. A. Morley. Splice Testing Using a Figure-S Machine and a New Shuttle Car Simulation (grant G0188036, PA State Univ.). BuMines OFR 80-80, 1979, NTIS PB 80-210222. 35. Trutt, F. C., J. W. Robinson, L. A. Morley, and P. M. Zahn. Electrical Materials Analysis-Arcing (grant G0155197, PA State Univ.). BuMines OFR 90-78, 1977; NTIS PB 284 946. 36. Tsivitse, P. J. Mining Motors. Ch. in Motor Application and Maintenance Handbook, ed. by R. W. Smeaton. McGraw-Hill,1969. 37. U.K. National CoalBoard (London).Flexible Trailing Cables for Use With Coalcutters and for Similar Purposes. N.C.B. Spec. 18811971. 38. U.S. Code of Federal Regulations. Title 30-Mineral Resources; Chapter I-Mine Safety and Health Administration, Department of Labor; Subchapter 0-Coal Mine Health and Safe ty; Part 18-Electric Motor-Driven Mine Equipment and Ac- cessories; Part 75-Mandatory Safety Standards, Underground Coal Mines; Part 77-Mandatory Safety Standards, Surface Coal Mines and Surface Work Areas of Underground Coal Mines; 1981.
- 248. 39. U.S. Mine Safety and Health Administration. Coal Mine 41. Woboditsch, W. Belastbarkeit von AufgetrommeltenBzw. Safety Electrical Inspection Manual, Underground Coal Mines. Sonnenbeschienenen Leitungstmssen(Current Carrying Capacity Apr. 1979. of Reel-Wound and Sun-Exposed Cables). Elektrie, v. 18, Dec. 40. Westinghouse Electric Corp. (EastPittsburgh, PA). Elec- 1974. tricalTransmissionandDistributionReferenceBook.4th ed., 1964.
- 249. CHAPTER 9.-PROTECTIVE EQUIPMENT AND RELAYING Even the best designed electricalsystemsoccasionally experience faults and overloads, or disturbances that cause abnormally high currents. These currents can exist in the ground systemor in the phase conductors. Wherever the occurrence, the situation is likely to precipitate a hazard to either equipment or personnel. Of the basic design criteria that underlie all mine power systems, three are of critical importance in protec- tive equipment and relaying: adequate interrupting capac- ity, current-limitingcapacity, and selective system opera- tion. The first two provide protection to the system during a disturbance, while the third is designed to locate the problem, then minimize its effect. In chapter 7, current limiting and selective relaying were designated as two prime purposes of grounding. It was shown that ground- fault currents can be limited by inserting a resistance in series with the neutral conductor. However, not much has been presented about selective system operation, other than its need. Protective circuitry and protective relaying are the toolsbehind selectivesystem operationand arethe main topics of this chapter. The protective circuitry associated with the power system consists of transducers, relays, and switching ap- paratus. Its role of safeguarding personnel and equipment can be effected manually or automatically.An instance of manual utilization would be removing power from a sys- tem portion for maintenance. An example of automatic operation would be a situation in which protective cir- cuitry first senses then clears each hazardous current resulting from a disturbance. As might be expected, the process of clearing is disconnecting the affected circuit from the power source safely and as quickly as possible, with minimum interference to the system balance. In other words, protective circuitry must isolate a malfunc- tion at a given location with minimum damage to circuits and equipment and minimum operation downtime. The function of protective circuitry to provide detection and isolation is termed selective relaying. All the devices that comprise the protective circuitry in the mine power system thus play a vital role in safety. In fact,protective circuitry is probablythe most important component of the power system and forms a major portion of all power equipment. For example, a switchhouse, which has the principal function of protection, is simply a complex of protective devices. The basic concepts of overloads and faults are intro- duced in chapter 4. Although the removal of destructive overloadsis important, the main concernis the clearing of faults, since their occurrence can be catastrophic. Because of the preponderance of cables, cable shield- ing, and grounded equipment in mine power systems, line-to-neutral faults are the most common, and most of these are arcing with relatively short length and con- trolled distance. Ground-fault current is predominantly limited by neutral grounding resistors, whereas in other industrial applications, ground-fault currents are offen limited by fault impedance. Line-to-lineand three-phase faults can also occur, as when a mobile machine severs a cable during a runover. Extremely large line currents can result, which can be limited in the niine system only by transformer and line-conductor impedances. System components, such as couplers, cables, transformers, bus bars, and disconnect switches,must be capable of withstandingthe momentary mechanical and thermal stresses created by the flow o f fault current through them. Interrupting devices, such as circuit breakers, must be able not only to withstand these momentary faulPthrough stresses, but to interrupt or terminate these anomalous currents. The maximum magnitude of possible fault currents existing in line conductors must be known in order to select adequate ratings of protective equipment. Indeed, this knowledge is required to coordinate protective- circuitry operation for the entire complex. It may also be necessary to know the minimum sustained fault current that is available in the system in order to determine the sensitivity requirements of the current-responsiveprotec- tive devices. These fault magnitudes, both maximum and minimum, are usually estimated by calculation, and the equipment is selected using the calculated results. Becauseof the many hazardsthat can occur, the system must be capable of detecting overloads, short circuits (line faults), undervoltage, and ground faults, as well as any compromisein groundingeonductorcontinuity.Withthe use of resistance grounding in mine power systems, the protec- tive relaying or sensingdevice associatedwith ground faults orzemsequencecurrents isusually handled separatelyfrom thatfor linefaultscausingonly anomalouspositive-sequence or negative-sequencecurrents. In addition, the relaying for overloads may be separate from that for faults.Except for fuse applications,the sensing devices for each function will normally cause the activation, or tripping, of the same circuit-interrupting device no matter what the protection requirements are for an individual location. The sensing devices may be an integralpart of the interrupting appara- tus or be separated from it and connected only through control wiring. This chapter builds upon the material covered in chapter 4, beginning with the main protection compo- nents, switching apparatus and sensing devices. Basic relay connections, relay terminology, and different kinds of protection follow. Finally, typical assemblies and com- binations of protective circuitry are discussed.Essentially, this chapter sets the stage for chapter 10, where fault calculations, device sizing, and coordination are outlined. SWITCHING APPARATUS A switching apparatus is defined as a device for making (closing),breaking (opening),or changingconnec- tions (6).l There are three basic types of apparatus in this classification:switches, circuit breakers, and fuses. All switchingdevicesaregivencertain design ratings, which are a measure of the electrical stresses they can withstand (6). Obviously, the ratings must be correlated with the intended use or duty. A listing and definition of these ratings follows but is restricted to those terms having direct applicationto the development of the topicin this and the subsequent chapter. Further concepts will be added in the discussion of transients and ovewoltages in chapter 11. 1.Voltage. The maximum nominal systemvoltage at a specified frequency (usually line-to-linefor ac devices) on which the device may be installed. 'Italicizednumberain parenthesesreferto items in thelist ofreferences at the end of Wlie chapter.
- 250. 2. Continuous current. The maximum continuous cur- rent that the apparatus may carry. 3. Short-circuit current. Usually, the maximum cur- rent the device is capable of interrupting. This may be further qualified by an interrupting-current or interrupting-capacity rating. 4. Close-and-latch or momentary current. The maxi- mum short-circuit current that the device can withstand during the few cycles after the fault occurs without expe- riencing severe mechanical damage. The ratings of switching apparatus are based on the maximum possible values of fault currents. To help visu- alize the importance of these ratings, consider that a three-phase fault has occurred on a power system. Figure 9.1 illustrates the resulting line current versus time, created by the flow of energy from the source or sources to the fault (7). This asymmetrical waveform is made up of two components: dc and a symmetrical ac. At any instant after the fault occurs, the total fault current equals the sum of the two. The dc component decays to zero in a short time, with the total current gradually changing from asymmetrical to symmetrical. Switching-apparatus ratings, as a measure of the stresses involved during faulting, are based on the sym- metrical rms value. Asymmetry is accounted for by taking the basic symmetrical value and applying multiplying factors. These concepts are presented in detail in chapter 10.However, figure 9.1does provide a useful visualization of rating magnitudes, and these will be discussed in following sections, along with each switching device. ARCS AND CIRCUIT INTERRUPTION After a switching apparatus receives a message that circuit current is to be interrupted, the device proceeds through definite steps to terminate the current (4). These are illustrated in figure 9.2. Under normal operation, the contacts of the apparatus are closed, current flowsthrough the interface, and the outgoing circuit is thus energized. To terminate current, the contacts begin to separate and an electric arc is drawn. The arc is composed of free electron and free positive- ion flow, as shown in figure 9.3. To initiate this arc, free electrons andlor free positive ions must exist between the contacts. Their availability depends upon the following environmental conditions: In air or gm, the conductiveelements aregenerated prior to the initiation of the arcby radiation and cosmicrays, which knock off eledrons &om neutral gas molecules. In a liquid such as oil, the conductive elements exist as impurities. In a vacuum, they can be emitted from the cathode by a high-strength electricfield with the process known as high field emission. The last case can add free electrons to any environment. Even though the voltage between the cathode and anode is low immediately after separation, the free electrons are attracted to the anode, and the positive ions toward the cathode. The electron flow accounts for about 90%of arc current (4). If the voltage across the arc remains large enough, the movement of charge between the contads initiates the mechanisms that can increase and sustain the arc. This I ::I Total asymmetrical current I Ci. , dc component Figure 9.1.-Typical system fault current. ,Contacts closed, circuit energized - 0Load or short circuit / Contacts porting, arc drawn , between contacts QLoad or short circuit J Contacts open, arc extinguished : r "" 5 Load or short circuit deenergized Figure 9.2.-Steps In circuit interruption. Region 6 positive column Cathode Anode - Positive + - Electrons emitted in great number therrnionically w from cathode spot Figure 9.3.-Arc between two contacts.
- 251. again depends upon the environment. In a gas, the free electrons can collide with neutral gas molecules, producing additional freeelectrons and positiveions,termed ionization by collision. In any atmosphere, the collision of heavy posi- tive ions on the cathode produces heat. which can aument - field emission in low-melting-pointmaterials such as copper, creating intense electron discharge from a small area, called a cathode spot, and can cause thermionic emission in high- melting-point substances, such as carbon, where electrons are boiled out by high temperature. Once the arc is established, processes must be brought into play to extinguish it. In general, the greater the arc current and the higher the voltage of the circuit, the more difficult is the problem of arc extinction. The situation is easier in ac systems than in dc systems because the current waveform passes through zero in ac systems. However, the arc can restrike when the voltage rises again if the ionic conditions across the contacts permit. For dc, the arc is readily maintained because a normal cur- rentzero does not exist. Whatever the extinction process, the switching device can open the circuit successfully, provided that the current to be interrupted is within the rated value. However, if the switching device is required to terminate a current well above the design value, the arc between the parting contact may not extinguish or may continue to restrike, and the apparatus could be destroyed by the gas pressure built up within it (5). When a device is designed to interrupt fault current, it is often called an interrupting device; otherwise, it is commonly called a switch and is designed only to open and close a circuit. While some switching apparatus are in- tended to serve only one of these functions, others can do both. SWITCHES A switch has exactly the same definition as switching apparatus, with the qualification that it is a manual device (6); in other words, its operation is a normal or intended occurrence on the power system. The switch types common in mine systems are the disconnect and the load interrupter. Both have the prime function of isolating outgoing circuits from the power source. A disconnect switch is not intended to interrupt circuit current and can be operated only after the circuit power has been removed. Interlocks must be provided to prevent manual operation under load, and latches may be needed to prevent opening from the stresses resulting from fault circuits. Consequently, disconnect switches do not have an interrupting rating; but beyond a continuous- current rating, they may need a momentary-duty or close- and-latch rating for handling fault-through currents. An interrupter or load-break switch differs from a disconnect in that it has an interrupting rating. The device has the capability of terminating currents that do not exceed the continuous-current rating, although this is not its normal operation. Interrupter switches usually have a quick-make, quick-break mechanism, which pro- vides a fast witch-operation speed independent of the handle speed. The illustration in figure 9.4 shows a three-pole device; the mechanism on the right side of the connecting sh& provides the fast operation. Some units can be motor driven, thus allowing remote or automatic operation. In most mining applications, load-break switches need a close-and-latch rating. Where interlocks Figure 9.4.-Load-break switch. (Courtesy Line Power Manufac. turing Corp.) are not employed, this rating indicates the margin of safety when the switch is closed into a faulted circuit. Switches are normally used as disconnects in mining systems regardless of their ratings; in fact, some States require load-break switches with interlocks for all discon- necting applications. These interlocks cause interruption of source power prior to contact separation, and the operation is usually performed through the ground- monitoring circuitry. Load-break switches, used in con- junction with fuses, are employed as interrupters in cer- tain circumstances. CIRCUIT BREAKERS A circuit breaker is primarily an interrupting device, but in some cases it is also used as a switch (6).A circuit breaker can be defined as a device designed to open and close a circuit manually and to open the circuit automat- ically at a specific current level without injury to itself when properly applied. It is available as a single pole, double pole, or triple pole. Manual operation, be it me- chanically or electrically actuated, is again intended where the circuit current is not in excess of rated contin- uous current. Automatic operation is dictated by a system abnormality, such as a fault or an overload. In this case, the device may be called upon to interrupt current in excess of the rated continuous current. Circuit breakers in the mle of interrupting devices must be used with sensing devices to perform their intended function. In medium-voltage and low-voltagemining appli- cations, the operation may be internally controlled by self- contained current-responsive elements, external protective relays. or a combination of both. In high-voltage situations. - the -sensing devices are always separate, with intemnneci tions only through control wiring. Circuit breakers can generally be broken into two classifications: those intended for systems over 1,000 V, and those for 1,000 V and below. Devices in the first class are called power circuit breakers, while the second class is divided into power circuit breakers and molded-case cir- cuit breakers. Following mining standards, circuit break- ers for systems below 661 V are called low voltage; for 661 to 1,000 V, medium voltage; and above 1,000 V, high voltage. It should be noted that IEEE Standards define above 1,000 V to 72,500 V as medium voltage anc! below 1,000 V as low voltage. Low-voltage and medium-voltage circuit breakers are usually considered together and can
- 252. find ac and dc service. High-voltage breakers involve only ac circuits. The next paragraphs look at typical apparatus and operation. Arc rises on horns /" CIRCUIT BREAKERS FOR LOW AND MEDIUM VOLTAGE The term air circuit breaker is often used when referring to molded-case and power circuit breakers de- signed for low-voltage and medium-voltage systems ( 7 ) . Air circuit breakers employ the simplest method of inter- rupting current: extinguishing the arc in normal atmo- sphere by increasing its length (4). Several different pro- cesses can be used to force the arc to lengthen. To illustrate one arc-lengthening technique, consider figure 9.5, where two circuit breaker contacts, a and b, have just separated. The horn-like arrangement of the contacts shown in the figure can be considered an arc chute, which is a barrier that confines, cools, and extin- guishes the arc (5).By the ionization of the air between the contacts, an arc is drawn and heat is generated. The arc extinction action then commences; this is also called deionization because it serves to reduce the free electrons and positive ions in the gas (4).Air currents, created by the heat and confined by the arc chute, force the arc upward to form a loop. Electromagnetic forceswithin the loopfurther encourage the lengthening. As a result of cooling by radiation or convection, the longer arc requires a higher arc voltage to sustain current flow, and thus, the arc is extinguished. As noted earlier, arc interruption in an ac circuit occurs much more easily than in a dc circuit. All voltages and currents in an ac system go through cyclic changes, and consequently, ion-producing effects for the arc are variable too: falling as current becomes smaller, ceasing at current zero (4).Deionizing effects in the arc chute remain steady. To take advantage of this situation, circuit break- ers for ac systems are often designed around the minimum voltage required to establish a cathode spot. Because there is no natural current zero in dc systems, the circuit breaker must force the current to zero. For this to happen, the arc voltage must be greater than the system voltage (14).An enormous amount of heat can be generated in all circuit breakers while the arc exists, and an important function of the circuit breaker assembly is to dissipate this heat safely. The foregoing simple arc-lengthening technique works well for 240-Vacapplications. Conventional practice is to use a single-pole breaker for 120 Vac and a double- pole breaker for 240-Vac single-phase circuits. The latter employs one pole of the circuit breaker in series with each power conductor. For circuits 250 V and above, the direct arc-lengthen- ing approach is not enough; special arc chutes, quenchers, or deionizing chambers are needed to assist in arc termi- nation (5). Figure 9.6 illustrates one approach, where the arc is forced into metallic barriers by magnetic attraction and broken into a series of smaller arcs. Each of these arcs is subjected to lengthening, cooling, and the problem of reestablishing a cathode spot if low-melting-pointmateri- als are used (4).Another approach is depicted in figure 9.7. Because the arc establishes its own electromagnetic field, an external magnetic field can enhance arc lengthening. The process is termed magnetic blowout, and breakers using this principle are called air magnetic. Coils carrying the circuit current in series with the arc can provide the a ( b Arc drawn here Figure 9.5.-Extinguishing arc by increasingthe length. barriers &Arcing contacts 'Main contacts Figure 9.6.-Metabbarrier arc chute assists in arc deioniza. tion. Barriers of insulatina material ~rcing contacts Figure 9.7.-Insulated-barrler arc chute used with magnetic field.
- 253. magnetic field. As shown in the figure, the magneticfield forces the arc into insulated barriers or fins. creatine ,~ ~ further lengthening; recombination and cooling at th; barrier surfaces accelerates deionization(4). In dc mine power circuits below 660 V, air-magnetic breakers are used extensively, especially on trolley sys- tems. With very few exceptions,molded-casebreakers are employed for ac circuits below 1,000 V. In addition, molded-case units are often used to protect low-voltagedc face equipment. Molded-Case Circuit Breakers The molded-case circuit breaker is the most explicit example of interrupting apparatus with self-contained current-responsive elements. It is defined as a breaker that is assembled as an integral unit in a supporting and enclosing housing of insulating material (5).Depending upon the amount of protection desired, these devices can sense internally and then clear undervoltage,overcurrent, and short-circuitconditions. Some tripping elements, that is, the actual components that cause the contacts to start separating, are also externally accessiblethrough control wiring. Hence, other circuit protection can be added. Except for some power circuit breakers of low-voltageand medium-voltage design, all the circuit breakers that will be discussed in this chapter rely solely on outside infor- mation to perform their prime function. Molded-case ap- paratus will be presented first so that many important terms can be introduced. The application of molded-case circuit breakers in mining began in the 1950's with the conversion from low-voltagedc power distribution to ac power distribution and face rectification, expanding further with the trend toward ac face equipment. In fact, Wood and Smith (21) have attributed the introduction of low-height, solid-state rectifier units in underground mines (whichpermitted the use of ac distribution) to molded-case circuit breakers, citing the lack of high-speed dc circuit breakers of the proper height as the previous limitation. Molded-case breakers placed between the transformer and the rectify- ing bridge lowered the height limitation to that of the transformer, allowing a unit design complementary with the mining environment. The largest mining application is trailing-cable pro- tection in underground face areas. The breakers are lo- cated in power centers and provide cable protection on each outgoing circuit, as required by 30 CFR 75.900, in addition to functioning as switching devices. The typical molded-casebreakers, however, are not designed for repet- itive switching. Mining use subjects them to many more operations than found in other industries, and regular or standard circuit breakers generally cannot hold up to the stress. Several manufacturers, recognizing this problem, have produced a special line of mine-duty molded-case breakers, which have stronger construction to withstand the punishment of mine use. Except for external adjustments, molded-casedevices sometimes do not allow field maintenance; many are sealed to prevent tampering. Although some manufactur- ers offer a complete line of replacement components, repairs other than an exchange of easily removable parts, such as arc chutes or trip units, should be made only by qualified repair facilities. This is critical, given the impor- tance of the molded-case circuit breaker in personnel protection. All component parts of these circuit breakers are built into one insulated housing, the molded case. These parts are the operating mechanism, arc extinguishers (arc chutes), contacts, trip elements, and terminal connectors, as shown in figure 9.8 (19).Additional accessoriesmay be included. The molded case is made of a glass polyester or similar synthetic material that combines ruggedness and high dielectricstrength with a compact design. Each type and size of molded case is assigned a frame size or designation for easy identification. This coding, loosely based on an old Underwriters' Laboratories standard, refers to a number of breaker characteristics, including maximum allowable system voltage, maximum allowable continuous current, interrupting capacity, and the physi- cal dimensions of the molded case. Several trip units may be available for a particular frame size, so a specific assembled breaker may have a lower continuous-current rating than the current designationof the frame.Table 9.1 lists the continuous ratings considered to be standard for mining service. The currents in parentheses are the lower current settings available in that frame size from certain manufacturers. Unfortunately, manufacturers have vary- ing design criteria and hence size their units to dissimilar Table 9.1.-Ratings for mining-servicemolded-casecircuit bmakers Frame size,'A Continuous-current ratings,' A ' Regularduty breakers also available in 1,600-, 2.000-, and 2.500-8 frames. Currents in parenthesesare lower settings available in the frame size. Wroided case (frame) Trip eic.rnrj?fs Figure 9.8.-Molded-case circuit breaker components. (Courtesy Westinghouse Electric Corp.)
- 254. specifications. For example, a 225-A, 600-V breaker sup- plied from two separate manufacturers may have different physical dimensions so that direct interchanging is difi- cult, if not impossible. The circuit breakers rated in table 9.1 are generally available as two-poleor three-poleunits at 600 Vac or 300 Vdc, but only as three-pole devices at 1,000 Vac. The two-pole breakers are intended for dc face equipment or single phase ac applications. By convention, one pole is used for each ungrounded conductor in a circuit ( 5 ) . The arc chutes define the interrupting-currentcapac- ity of the assembly in conjunctionwith the insulating and heat-dissipation properties of the molded case. The chutes assist arc deionization by the principle discussedfor figure 9.6. They are also termed arc extinguishers or arc quench- ers by some manufacturers. The breaker case must be mounted vertically with the arc chutes at the top for correct arc-extinctionoperation. Circuit breakers designed for 1,000 V and below are capable of clearing a fault faster than those constructed for high voltage (6). The contacts often begin to part during the first cycle of fault current, and consequently, the breaker must be capableof interrupting the maximum allowablefirst-cycleasymmetricalcurrent. Thus,for lower voltage breakers, the close-and-latch and interrupting ratings are usually the same, a characteristic not found with high-voltage breakers. The rating of these units is carried out on a symmetricalbasis, so multipliers account- ing for the dc offset need not be applied as long as the system XiR ratio does not exceed 6.6 (6)(see chapter 10). nble 9.2 lists typical interrupting ratings versus the system voltages for mine-duty circuit breakers; the ac system values are based on the symmetrical rating. Some manufacturers offer both standard-duty and high- interrupting-capacity breakers for mining service. The table values presented parenthetically indicate the supe- rior construction, which incorporates sturdier contacts and mechanism plus a special high-impactmolded casing. Table 9.2 shows that typical molded-case circuit breakers constructedfor 1,000-Vacmine systemshave only a 10,000-Asymmetricalinterrupting rating. This presents a concern, as available short-circuit currents on high- power 1,000-Vacsystemscan be greater. Instances include longwall mining equipment, which needs a power-center capacity of 1,500kVA or more. To overcome the problem, a manufacturer has introduced molded-casebreakers with a 24,000-A asymmetrical interrupting rating at 1,000 Vac and continuous-currentratings of 600,800,1,000, or 1,200 A. The asymmetrical rating is used to provide more flexibility for designing the breaker into power systems. The function of the operating mechanism of a typical molded-case circuit breaker is to provide a means of opening and closing. It is a toggle mechanism of the quick-make,quick-breaktype, meaning that the contacts snap open or closed independent of the speed of handle movement. The breaker is also trip-free;that is, it cannot be prevented from tripping by holding the breaker handle in the ON position during a fault condition. In additionto indicating whether the breaker is ON or OFF, the operating-mechanismhandle indicates when the breaker is tripped by moving midway between these positions. To reactivate the tripped breaker, the handle must first be moved from the central position to OFF, which resets the mechanism, and then to ON.This distinct trip point is particularly advantageous where molded-case breakers are grouped, as in a power center, because it clearly indicates any faulty circuits. The function of the trip elements is to trip the operating mechanism in the event of prolonged overloador short-circuit current. Two common types of trip elements are used in mining, magnetic and thermal magnetic. When the circuit being protected involves portable or trailing cables, the thermal-magnetic combination is strongly recommended and is mandated by some States. The magnetic trip protects against short circuits, and an electromagnet in series with the load current provides the trip action (19).This type of short circuit is actually a line-to-lineor three-phasefault on ac, or a line-to-linefault on dc systems.When a short occurs,the high fault current causes the electromagnet in the breaker to attract the armature, initiating an unlatching action, which in turn causes the circuit to open (fig. 9.9).The action takes place within 112 s (usually within 1cycle or 16 ms), instanta- neously tripping the breaker. Since tripping takes place with no intentional delay, the magnetic trip is oftencalled the instantaneous-tripelement. Screwdriver slots, located on the front of the trip unit, are used in adjusting the sensitivity (fig. 9.10A). By law, the maximum setting is established by the protection of the minimum conductor size in the circuit (16-17). Table 9.3 lists these maximum settings appliedto trailing cables. Figure 9.10B illustrates a family of time-current curves resulting from the adjust- able range; to the left or below each curve,the breaker will not be tripped magnetically. Typical instantaneous-trip ranges versus frame sizes for mining-servicebreakers are given in table 9.4. Note that this is not a rigorous listing, since some manufacturers will provide any desired trip range with most frame sizes upon request. The other common molded-case breaker type is the thermal-magnetic variety. In addition to providing short- circuit protection, the thermal-magnetic breaker also guards against long-term current overloads existing longer than roughly 10 s, by incorporating thermal trip elements (fig. 9.11). The thermal action is accomplished through use of a bimetal strip heated by load current (19). The strip consists of two pieces of metal bonded together, each with a different coefficient of thermal expansion. A sustained overload causes excessive heating of the strip, resulting in deflectionof the bimetal, which in turn causes the operating mechanism to trip the breaker. Because the Table 9.2.-lnterruptlng-current ratings1versus system voltage, amperes Frame slze, A 240 Vac 460Vac 600Vac 1 ,000Vac 300Vdc2 - - 100.. ............................................ 18,000(65,000) 14,000(25,000) 14,000 (18.000) 10,000 10,000(20,000) 225.............................................. 25.000(65.000) 22.000(35.0001 22.000125.000) 10,000 10,000(20.0001 'Parenthetical ratings are for typical prernium-duty circuit breakers. 'Actual dc interrupting current dependent upon system inductance.
- 255. Conductor size AWG: 14 ..................... 12 ..................... 10..................... 8 ....................... 8 ....................... 4 ....................... 3 ....................... 2 ....................... Maximum C o n d m allowsble size i n s t e ~ t e n e ~ ~ settlng, A AWG: mble 9.4.--Commonly avallable magnetic-trip ranges for mlning-aewlce molded-caae bmkers Frame size, Magnetic-trip Range of allowable A range, A conductor a i m 100........................................ % I - 180 14-lOAWG 10-4 AWG 6-3 AWG 4-1 AWG 6-1 AWG 4-1 AWG 2-210 AWG 4-2/0 AWG 1 AWG-500MCM 2-a0 AWG 1310AWG a0 AWG - 500 MCM 310 AWG - 500 MCM 1.200 ................................. .... 1;5~)3;000 210 AWG - 500 MCM 2,DOQ-4,000 310 AWG -500 MCM 2.500-5.000 410 AWG -500 MCM bimetal deflection is dependent upon current and time, the thermal unit provides a long-time delay for light overloads and a fast response for heavy overloads. A representative current-time curve for the thermal unit alone is shown in figure 9.124; later in this chapter,it will be described aa an inverse-timecharacteristic. In compar- ison,figure 9.12Bshowsthe circuitbreaker responsewhen both thermal and magnetic trip elements are incorpo- rated. The shaded area for each curve represents a toler- ance between the minimum and maximum total clearing time. The thermal-magnetic unit shown in figure 9.11 is ambient-temperature seasitive. Assuming the circuit breaker, cable, and equipment being protected are in the same ambient temperature, the circuit breaker trips at a lower current aa the ambient temperature rises in corre- spondence to safe cable and equipment loadings, which vary inersely with ambient temperature (19).Thermal- magnetic trip elements are available that automatically compensatefor ambient-temperature variations. The am- bient compensation is obtained through an additional bimetal strip, which counteracts the overload bimetal. Such trip units are recommended whenever the protected conductors and the circuit breakers are in different ambi- ent temperatures (19). Most mining-servicemolded-casebreakers with 225-A frame sizesand abovehave interchangeable trip unita. For straight magnetic elements these allow different instantaneous-trip ranges per frame size. However, thermal-magnetic units can be used to establish a lower continuouscurrent limit for the breaker. The National ElectricalCode (13)is used as a guideto definethe current htaqreticelement closes gap ond opens ConloCtS an shwt circuit L m ; h L k x + s Closed ~ccnmn open Latched Trip@ Figure9.9.-Magnetk-trlp relay. I 0 LOW n Intermdie w^ I 0 . 1 High 0 . 0 1 6 I HI' ' L O ] CURRENT A Adjustment kmb B Charo3eristics Figure9.10.-Adjustable Instantaneous settlng. J/lJzw Load ~irneti 1 CMltactsc. Latch Latched Tripped Figure 9.11.-Thermal-magnetic action of mdded.care clr- cult breaker. CURRENT, 96 CURRENT, % of brwker rating of breaker rating A Thermal only 8 Thermal magnetic Figure 9.12.-Time-curnnt characterlrtlcs for thermal. magnstb molded- clrcult W e n .
- 256. at which the long-time-delaythermal element must ini- tiate the circuit-clearing operation and specifies a point that is 126% of the rated equipment or conductor ampac- ity. As seen in figure 9.12A,the circuit breaker will take no action below this current. Hence, the thermal portion defines the continuouscurrent rating of the breaker, speci- fied as 100% at 40" C for conventional (noncompensating) thermal-magneticelements.Obviously,the thermal element current rating cannot exceed the frame rating. Because of the connection, some manufacturers recommend that the continuouscurrent through the breaker be limited to 80% of the frame size. This topic will be continued in chapter 10. Electromechanical magnetic and thermal-magnetic trip elements have been replaced by solid-state compo- nents in some molded-case breakers. Although the solid- state counterparts may become popular in the future,they have not yet achieved wide acceptance in the mining industry. Nevertheless, these breakers are discussed in chapter 14. The last basic breaker components are the terminal connectors. Their function isto connect the circuitbreaker to a desired power source and load. They are usually made of copper and must be constructed so that each conductor can be tightened without removing another. The terminal connectors shown in figure 9.8are for direct connectionof one cable connector per terminal. Many molded-case breakers also have provisionsfor threaded-stud terminals. These studs can be used not only for connection of more than one conductor per terminal, but also for breaker mounting. It should be noted that the type of terminal used on a breaker may change its heat dissipation proper- ties and thus lower its interrupting rating. In addition to the basic components, several accesso- ries are available, of which the most common are the terminal shield, shunt trip, and undervoltage release (UVR).Terminalshieldsprotect personnel from accidental contact with energized terminal connections and are sim- ply plates that shield (guard)the terminals. The other two accessories are used to trip the operating mechanism. A shunt trip is employed to trip a circuit breaker electrically from a remote location. It consists of a momentary-rated solenoid tripping device mounted inside the molded case that activates when control power is applied acrossthe solenoid coil.The magneticfieldcreated by the solenoid moves a plunger, which in turn activates a trip bar. At the same time, a series cutoff switch removes power to the solenoid coil, preventing it from burning up under continuous load. A typical shunt-trip assembly is shown in figure 9.13. The shunt trip can remotely trip the breaker hut cannot remotely operate it. To reclose the breaker, the handle must first be moved to the reset position, then to the ON position. The purpose of the UVR is to trip the breaker when- ever control voltage to the UVR falls below a predeter- mined level, usually 35% to 70%. This device is also mounted inside the breaker frame and consistsof a spring and a solenoid. The spring is cocked or precharged by the operating mechanism when the breaker is closed and is held in the cocked position by the solenoid after closure. If the voltage drops below the required level, the solenoid releases the spring, causing the circuit breaker to trip. The breaker cannot be turned on again until the voltage returns to 80% of normal. The importance of the shunt trip and UVR is far ranging, as they allow the protection capabilities of circuit breakers to be extended. The molded-case breaker alone can provide overload and short-circuit protection in an outgoing circuit. The UVR adds undervoltage protection; in fact, undervoltage protection is normally required at most breaker locations. Note that undervoltageprotection is required for all equipment, hut it is not required on all circuit breakers as long as all equipment downstream from the breaker has undervoltageprotection. The under- voltage protection provided by a UVR is actually "loss- of-voltage" protection since the dropout level is well out- sidethe recommended operating range of most motors (see chapter 6). Through a specific combination of relays and sensing devices, additional types of protection can be appliedthrough shunt or UVRtripping. With a shunt trip, the relay completes the circuit between the control-power source and the solenoid coil. When a UVR is used, the relay removes the control voltage across the solenoid coil. This circuitry will be discussed in detail later in the chapter. The molded-case circuit breaker is the most widely used breaker in mining, even though its employment is restricted to low-voltageand medium-voltagesystems.The principal application is on ac, where it provides high Figure 9.13.-Shunt-trip (A) and undervoltage-release(6)accessories. (Courtesy General Electric Co.)
- 257. interrupting capacityfor short circuits in minimum space. On ac or dc systems, it is often the first protection device called upon to handle electrical problems existing on trailing cablesand mining machinery.Aclear understand- ing of the construction and rating of these breakers is required to assure adequate protection. The operating characteristics must be closely matched with those of the trailing cable to minimize hazards to personnel. Power Circuit Breakers Some mining-industry engineers have found that molded-case circuit breakers cannot handle the available short-circuit currents in certain low-voltage applications, such as the outgoing dc circuits of trolley rectifiers and dc face equipment. The low-voltage power circuit breaker provides an alternative in these cases. Power circuit breakers for applicationsof 1,000V and below are of open construction assembly with metal frames. They are designed to be field maintained under planned periodic inspection, and all parts are accessible for ease of maintenance, repair, and replacement (6-7). The design enables higher endurance ratings and greater repetitive-duty capabilities than are available from molded-case devices. However, power circuit breakers are intended only for service inside enclosures with "dead- front" construction,that is, not accessibleto unauthorized personnel. Electromechanical units are available for long-time tripping, but mechanical-displacementdashpot types are normally used for this function and provide the same overcurrent protection as does the bimetal thermal trip- ping in molded-casebreakers. Although long-timecharac- teristics are not adjustable with bimetal strips, the dash- pots allow the long-time-delay "pickup" current and operation time to be changed. This extends the capabili- ties of the power circuit breaker over the molded case by providing not only short-circuitbut also overloadtripping adjustments, thereby allowing a broader range of applica- tions (7). Low-voltagepower circuit breakers are available with or without direct-acting instantaneous units and with or without long-time-delayunits. Furthermore, most manufacturers offer three different separately adjustable long-time-delayoperation bands as well as three different short-time-delay operation bands. As with molded-case breakers, power breakers are available with either shunt- tripping or UVR units or both. Solid-statedevices are also manufactured for all tripping arrangements. Some typical ratings for low-voltage power circuit breakers are provided in table 9.5 (7).In addition to these Table 9.5.-Some typical ratingsfor low-voltage power circuit breakers Ac system Rated Frame 3-phase short-circuit Range of nominal maximum . . current ratlna. trlDdevlce voltage, v voltage, v S'Ze, ~~mmetrical,"~ current ratings, A listed values, frame sizes are available up to 6,000-A continuous ac current and 12,000-Acontinuousdc current ( 5 ) .These frame sizes are rated to carry 100% of the continuous-current rating inside enclosures at 406C. In power breakers with low current ratings, arc interruption can utilize arc-chute arrangements similar to those used in molded-case breakers. The full air-magnetic arrange- ments described for figure 9.7 are employed for high- current-interruption power breakers. HIGH-VOLTAGE CIRCUIT BREAKERS The power circuit breakers used in high-voltagemin- ing applications include air-magnetic, oil, minimum-oil, and vacuum types. Vacuum circuit breakers or VCB's are by far the most popular because of their small size and high efficiency. Oil circuitbreakers or OCB's oncewere the most common, but their use has dropped substantially in recent years, since the interrupting sizes needed for min- ing are not available. Air-magnetic types are normally limited to surface installations. The next few paragraphs will examine typical apparatus ratings, and then the operation of oil, minimum-oil, and vacuum types will be described; air-magnetic breakers are excluded as their operation is the same as that presented previously for lower voltage breakers. Typical Ratings The typical nominal voltage ratings correspondingto nominal system voltages are 4,160, 7,200, and 13,800V, with 23,000 V used in some strip mines. The system portions of interest are obviously ac. Commoncontinuous- current ratings are 400,600,800, 1,200,and 2,000 A. The majority of mine systems do not call for current greater than 600-A continuous, which has become the most used rating. Interrupting and close-and-latchratings are very im- portant high-voltageparameters (6).For low-voltage and medium-voltagecircuit breakers, the two ratings are usu- ally the same. As high-voltage circuit breakers rarely terminate current flow until a few cycles after the first- cycle peak, the close-and-latchrating must be higher than the interrupting rating. A typical interrupting rating for high-voltagecircuit breakers found in mining is 12,000-A rms symmetrical, while the typical close-and-latchrating is 20,000-A rms asymmetrical. The asymmetrical close- and-latch rating is often found by multiplying the sym- metrical interrupting rating by 1.6 (seechapter 10)(6). High-voltage circuit breakers can also be given an interrupting-capacity class,which is an identifyinggroup- ing rather than a rating. It is expressed in megavoltam- peres, such as 250,350,500, and 750 MVA. The interrupt- ing capacity is related to the interrupting-current rating by (5) where MVA = interrupting capacity, MVA, kV,,,,, = rated system voltage, kV, and k q , , , ,= rated rms interrupting current, kA. Oil Circuit Breakers Even though their popularity has been dropping, OCB's are still used extensively in surface installations,
- 258. especially substations. The commontype of construction is the dead tank, shown in figure 9.14A. This steel tank is partly filled with oil and has a cover with porcelain or other composition bushings or insulators through which the conductors are carried (4-5). The breaker contacts are located below the bushings and are bridged by a conduct- ing crosshead supported by a lift rod. In most designs, two contacts and the crosshead provide two interruptions per pole. The majority of OCB's in mining have three such poles in one tank. The tank has an insulated liner to prevent the arc from striking the tank walls. The entire assembly is oiltight; a vent with oil-separatingproperties permits the escape of any gases generated but prevents the escape of entrained oil. Arc interruption in high-voltage circuit breakers em- ploys the cathode-spot phenomenon combined with arc lengthening and deionization of the arcpath. In the case of the OCB, oil is vaporized as an arc is established between the parting contacts, and this produces a bubble around the arc. The gases within the bubble are generally not conducive to ionization, but in most modern OCB's, an oil-filled insulating chamber surrounds the parting con- tacts (fig. 9.14B).When the moving contact is lowered, the gas generated by the arc portion within the chamber forces oil out through the chamber throat (4). The blast of oil comes into intimate contact with the arc, accelerates the cooling and ion recombination process (fig. 9.140, and carries away available ions. A different arc-chamber ap- proach is shown in figure 9.15. Here the chamber throat is made of laminations so that during interruption, the oil can move radially into the arc path. This is sometimes termed a turbo action. In high-interruptingcapacities, the gases developed within the chamber can be used to blast oil horizontally across the arc path. Whatever the specific design, the chambers are intended to contain the devel- oped high gas pressures and reduce any pressure on the main oil tank (5). After being effectively cooled, the generated gases are allowed to pass through the vent into open air. The result of OCB construction and operation is a very effective arc interrupter. However, beyond availability, there are inherent disadvantages that discourage use of OCB's (4-5). The oil presents a fire hazard, particularly if the tank is ruptured because of unexpected pressure; this has led some Statesto prohibit OCB application in under- ground coal systems above 10,000 V. The oil is bothersome to handle and creates maintenance problems including cleanliness problems. Finally, the inertia of the heavy operating mechanism severely limits operational speed, causing a time delay in opening the arc. Despite these problems, other advantages, which are discussed in chap- ter 11, still make the OCB desirable to many industry engineers. When used underground, the physical size of three- pole units usually limits the interrupting capacity to 100 MVA or less, with continuous-current ratings of 400 A. The operating mechanism on these small OCB's is typi- cally spring-gravity and manual-reset; a handle-driven mechanism (quick break, quick make) is used to close the breaker manually while at the same time automatically tensioning an opening spring. With the breaker engaged, the spring becomes armed, allowing a shunt-trip or UVR device to trigger the breaker opening by releasing the spring. A motor-drivensystem is also availableto closethe breaker, but the tripping method is the same. The motor- driven OCB's can thus be electrically engaged as well as Movlng contact -Stationary contact 011 Moving contact -Insulating chamber oil iet . Arc I ' ! ! B C Figure 9.14.-Construction and operationof dead-tankOCB. Moving contact Oil flows into throat between laminations Figure 9.15.-Turboaction arc chamber for OCB's. tripped. Larger OCB's such as those used in substations are typically motor driven. Minimum-Oil Circuit Breakers Minimum-oil circuit breakers, also termed low volume oil or live tank, enclose each pole in its own small-diameter tank (5).In modern versions, the tank is made of insulated high-strength, high-resistancematerial, and the top and bottom covers are high-dielectric-strength insulators(fig. 9.16). Contacts consist ofa movable vertical rod and a stationary contact in the tank bottom. Oil volume is about 1L, and the top surface of the oil is at atmospheric pressure. Arc extinguishing is assisted by oil blast, and resulting gases are vented to outside air. The operating mechanism can be either manual-reset and spring-trip or motor-reset and spring-trip. Some typical ratings of these breakers are listed in table 9.6. The arrangement of a three-pole minimum-oil unit with moving contacts mechanically interconnected results in a smaller overall package than comparable dead-tank breakers. The smaller mass of moving parts (operating
- 259. Insulation Figure 9.16.-Cross section o f minimum.oil breaker. Table 9.6.-vplcal minimum-oilcircuit breaker ratings Rated voltage, Interrupting capacity, Continuous current. V MVA A mechanism and rods) enables higher operating speeds, while the advantages of oil interruption are maintained. However,the low volume of oil is such that after about five operations, the oil level must be checked. Even though oil-level indicators are available, this can create a main- tenance problem in mining. Vacuum Circuit Breakers With all the circuit breaker types covered so far, a gaseous atmosphere exists between the parting contacts. The gas is ionized by many processes and thus provides free electrons,which move to the anode, and positive ions, which are attracted to the cathode(4).As the positive ions arrive at the cathode, they can cause thermionic or high- field emission of electrons,which has a negative effect on arc interruption. Almost all these phenomena cease to exist if the gas between the breaker contactsisremoved; in other words, if the arc is drawn in a vacuum. For this reason, vacuum is considered an extremely good medium for switching, and circuit breakers have been developed to take advantage of this feature. Figure 9.17 shows a sketch of a VCB, again with one pole. The assembly is sometimes called a bottle. The main advantages of VCB's are Interruption usually occursat the first zero current; There are no blind spots in their interrupting range; They have extraordinarily long life; They are relatively maintenance free; and Recovery of dielectricstrength (betweenthe parting contacts) following interrupting is extremely fast. Figure 9.17.-Cross section o f VCB. / These all result from the fact that the vacuum totally discouragesionization. An important aspect of VCB's is in the long sewice. For instance, if a unit fails to clear a short circuit beyond its interrupting range, but another unit down the line does, the exceeded VCB can be employed again up to the full rating without difficulty. Because of their efficient ratio of size to capacity,they are extremely well suited to underground mining use. Their interrupting capacity for large currents is such that they can be utilized anywhere on high-voltagedistribution, usually without reservation. This flexibility has made the VCB the most popular high-voltage interrupter for distribution systems in min- ing today. Added to these advantages isthe fact that the VCB does not have any physical orientationproblems. This is a consid- erableconstraint with OCB's, where the tanks must always be vertical. Vertical placement o f VCB bottles is sometimes necessary, however, to minimize dust accumulation. Ironically, the high efficiency of vacuum interrupters, which has favored their wide application, is the same property that can lead to severe transients. If care is not taken with VCB installation, switching transient-related problems can occur throughout the mine electrical com- plex. A detailed discussion of this important problem is deferred until chapter 1 1because of related phenomena. The operating mechanism,which includesthe mount- ing structure for the vacuum bottles, is an important factor in proper VCB operation. As a result of the small contact travel distance, usually on the order of 1 1 4in, four criteria are mandatory: Ild ' , . 1.Rugged construction to withstand the shock and stress of equipment movement; 2. A firm, smooth closure motion to prevent contact bounce; 3. Forceful opening of contacts in the case of contact welding; and 4. Clean, smooth opening motion to prevent contact bounce and subsequent arc restriking. Stationary contact In most cases, manufacturers rely on a spring-reset and spring-trip mechanism to meet items 2 through 4, and figure 9.18 illustrates one approach. The closing opera- tion, also termed resetting or reclosing, may be manually or motor driven. The trip solenoid can be a shunt-tripor UVR device, and in some cases, both are used. In VCB applications,the compact sizeof the operating mechanism and mounting structure has made possible a substantial reduction in overall power-equipmentdimen- sions. Manufacturers have even incorporated a disconnect High vacuum
- 260. Contact opening spring (extended) Bounce latch (disengaged) Contact *7 pressure spring - (compressed) $7 Main contacts closed Contact opening . Bounce latch (engaged) (disengaged) Contact travel just completed after tripping Contact oDenlna ce latch (disengaged) otor turns lever Reset latch (d~sengaged ) Main contacts open, ready for reclosing Figure 9.18.-Operating mechanism for vacuum interrupter. (Courtesy McGrew Edison) switch in their designs (fig. 9.19). The operating mecha- nism for the switch is mechanically interlocked with the circuit breaker mechanism. If the switch is opened when the breaker is closed, the interlock trips the circuit breaker prior to switch-contactparting. FUSES The fuse is the simplest and oldest device for inter- rupting an electrical circuit under short-circuit or excessive-overload current (5, 7). Fuses are installed in series with the protected circuit and operate by melting a fusible link. The response is such that the greater the current, the shorter the time to circuit opening,that is, an inverse-timecharacteristic. Fuses may be used in ac or dc circuits. and there is such variation in their timecurrent Figure 9.19.-VCB assembly incorporating a load-break switch. (Courtesy Ensign Electric) characteristics that they are suitable for many special purposes. While circuit breaker contacts rely on external sensing, the fuse acts as both the sensing device and the interrupting device. Unlike circuit breakers, fuses are "one-shot," as their fusible element is destroyed in the circuit-protection process. Fuses are available with interrupting-current ratings up to 200,000-A symmetrical rms, much higher than the capacity of circuit breakers. Fuses are also available with current-limiting abilities to provide maximum protection for all circuit components. Fuses are normally classified as low voltage or high voltuge:The low-voltagetypes are intended for service in systems 600V and below, while the high-voltagevarieties are suitable for installations 2.3 to 161 kV ( 7 ) . LOW-VOLTAGE FUSES Plug fuses and cartridge fuses are the two principal categories of standard low-voltage fuses, and they are classified as non-time-delay, timedelay, duabelement, or currenblimiting (13). There are also miscellaneous and nonstandard fuse classes. As with circuit breakers, there are three general fuse ratings (7); 1 .Current. The maximum dc or rms ac, in amperes, which the fuses will carry without exceeding a specified temperature rise limit (available range: milliamperes to 6,000A). 2. Voltage. The maximum ac or dc voltage at which the fuse is designed to operate (usual low-voltageratings are 600,300,250,or 125 V ac or dc or both). 3 .Interrupting. The assigned maximum short-circuit current that the fuse will safely interrupt (typical ratings are 10,000-, 50,000-, 100,000-, or 200,000-Asymmetrical rms). Specialratings are also given to current-limiting fuses to specify the maximum current and energy the device will let through to the protected circuit when clearing a fault (7).
- 261. Plug fuses are rated at 125V and are available with current ratings up to 30 A. Their use is thus limited to circuits with this voltage rating or less, except that they may be employed on systems having a grounded neutral where the maximum potential to ground of any conductor does not exceed 150 V (7). As a result, plug fuses have limited application in mine power systems (although an extensivepopularity still exists for homes).Cartridge fuse applications, on the contrary,are widespread, to the point where mention of a fuse implies a cartridge. Figure 9.20 shows the three standard low-voltagecartridge-typefuses (7). Non-Time-Delay Fuses As the name implies,these have no intentional built- in delay. They have a very simple construction,consisting of two end terminals joined together by a copper or zinc fusible element. The link is more current sensitive to melting than to time. Non-time-delayfuses are available as one-shot(or nonrenewable)and renewable;the former is the oldest cartridge fuse type in use today (7). With the one-shot, the link is in a sealed enclosure and the entire cartridge must be replaced after interruption. The renew- able fuse can be disassembled,and the link replaced. The lack of intentional time delay and a limited interrupting rating of around 10,000 A have substantially reduced the popularity of these fuses in recent years. Time-Delay Fuses The metal alloy used in time-delay fusible links is not only sensitive to current but also to the time period involved. In other words, a specific current existing for a specified time period is necessary to cause the heat- melting energy of the alloy. Such an arrangement permits harmless high-magnitude, short-duration currents to ex- ist, which are oftentimes necessary for proper system operation, as in motor starting. Dual-Element Fuse Originally designed primarily for motor-circuit pro- tection, the dual-element fuse (fig. 9.21) combines the features of non-time-delayand time-delayunits. The time- delay or thermal cutout is providedfor overloadprotection, while two fuse link elements give short-circuitprotection, blowing in a fraction of a cycle on heavy currents. The thermal cutout will allow the passage of currents as high as five times its continuous rating for up to 10 s. Hence, these fuses may be matched closely to protect the actual motor running current and at the same time be sized to protect wiring and other equipment, and provide both these functions without nuisance blowing. In fact, prop- erly sized dual-element fuses are required on all fuse- protected trailing cables. They are available with up to a 200,000-A symmetrical rms interrupting-current rating, and for further protection, most dual-element fuses also have a current-limitingfeature. Current-Llmiting Fuses Short~ircuit protection requires that a fuse limit the energy delivered by the short circuit to a faulted compo- nent. Obviously, the energy any interrupting device lets through under fault conditions cannot exceed the pro- Ferrule type 0-60 A Knife-blade type Bolt type 70-600A 601-6,000 A Figure 9.20.-Common cartridge fuses. Multiple-bridge sand short-circu~tlink Fiber tube Quartz sand Alloy time-delay filler element Flgure 9.21.-Inside view of dual-elementfuse. tected components withstand rating. Current-limiting fuses provide this protection by restriding or cutting off fault currents before damaging peaks are reached. With very high fault currents, they are extremely fast, limiting current in less than one-quarter cycle, with current inter- ruption occurring within the first one-half cycle. Only a portion of the destruct.ive short-circuit energy that is available is let through. By this, the current-limitingfuse allows the use of lower momentary and interrupting ratings by cutting off current within equipment ratings (7). Figure 9.22 illustrates how the fuse operates:the large waveform represents the available short-circuitcurrent on a faulted system, and the performance of the fuse is superimposed. Restricting energy is a means of limiting the mechan- ical and thermal stress imposed on equipment that is carrying fault current. lb illustrate this energy, consider figures 9.22 and 9.23 and the peak let-throughcurrent, . 'r It has been found that the magnetic forces during a fau t vary as the square of fault current, $ ( 7 ) .These forces translate to mechanicalstress, which could damagetrans- former frames, bus structures, or cable supports. The let-through energy, 12t,represents a measure of the heat- ing effect or thermal energy of the fault with or without the fuse (with the fuse, the value is 9. IZtactually equals ji2dt, the time integral of the current squared for the time under consideration (8).Both $ and 't can be consider- ably reduced when current-limiting uses are used (7). Furthermore, equipment with an 1% withstand rating can be matched with the energy let-through limit of the fuse. Standard Fuses As implied by the foregoing,cartridge fuses come in a wide range of types, sizes,and ratings. Various classes for
- 262. I ' Peak available current - 1 - f T' 1 I I 0 1 F ,A , Peak let-through #clearing Time -----L current II time Melting JL~rcing time time Figure 9.22.-Current-limiting action o f fuses. Time 4 Figure 9.23.-Energy-limiting action o f fuses. low-voltage units have been standardized (15), and a listing of general-purpose fuses follows (the first value listed is the range of continuous currents): Class G: 0 to 60 A, 300 V to ground maximum, 100,000-A symmetrical rms interrupting, current limit- ing, fit only class G fuse holders. Class H: 0 to 600 A, 250 and 600 V, interrupting capacity up to 10,000 A, either one-time or renewable construction, commonly termed the "old NEC fuse." ClassJ:0 to 600 A, 600V, 200,000-Asymmetricalrms interrupting, current limiting, fit only a class J fuse holders. Class K: 0 to 600 A, 250 and 600 V, 50,000-,100,000-, or 200,000-A symmetrical rms interrupting, have the greatest current-limiting effect of all low-voltage fuses (available as straight current limiting, dual-element cur- rent limiting, and dual-element time-delaycurrent limit- ing), fit class H fuse holders. Class L: 601 to 6,000A, 600 V,200,000-Asymmetrical rms interrupting, current limiting, bolt-in mounting. Class R: 0 to 600 A, 250 and 600 V, 200,000-A symmetricalrms interrupting, current limiting similar to class K level 5 fuse, fit only class R fuse holders. Class T: 0 to 600 A, 250 and 600 V, 200,000-A symmetrical rms interrupting, current limiting but effect less than class J fuses, fit only class T fuse holders. Nonstandard Fuses Nonstandard fusesreceive their name because of their special dimensionsor use in special applications;they are not general-purposefuses ( 7 ) .Of the many available, four have important applications in mining: Cable Limiters. These fuses are for use in multicable circuits (paralleled cables) and are placed in series with each cable in parallel. They are designed to provide short-circuit protection to each cable, removing it from power in caseof f a i l h . Cablelimiters arerated according to cable size (AWG 410 and so forth). Semiconductor Fuses. These devices are available in two types: semiconductor-protectionfuses or semiconductor- isolation fuses. Both are used in series with the applica- tion. Protection fuses are employed where solid-state de- vices are to be protected rather than isolated after a failure; they have lower let-through characteristics than other current-limiting fuses. A specific application is pro- tecting a rectifier or thyristor in case of an overload current. Isolation types are high-speed fuses, used to isolate a defective solid-state device in case of its failure. These are mandatory fuses for individual power diodes paralleled in large rectifier banks. Capacitor Fuses. Capacitor fuses are applied in series with power-factor(pf)correction (or other type)capacitors and are used to isolate a failed component by clearing short-circuit current before excessive gas is generated in the capacitor. Welding Fuses. These are current-limiting fuses for use in welder circuits only. The time-current characteris- tics are such that these fuses allow a longer intermittent overload than general-purpose fuses, but still provide short-circuit protection. HIGH-VOLTAGE FUSES High-voltagefuses provide usable protection for 2.3- to 161-kV systems and fall into two general categories: distribution fuse cutouts and power fuses(7). Distribution fuse cutouts were designed for overhead distribution cir- cuits, such as the protection of residential distribution transformers. Even though their employment in utility- type systems is extensive, their use in mining is limited and in some cases restricted. Power fuses are another matter, as certain types offer extremely practical protec- tion in mine power systems. They can be applied to substation, distribution, and potential transformers (in series with the primary) and occasionally to distribution circuit conductors.For surfacemine systems,the fuses are often equipped with contacts, arranged so that the fuse and its mounting act as a disconnect switch (fig. 9.24). There are two basic power fuses, expulsion and current- limiting types, and the next few paragraphs will discuss their operation, ratings, and application. Expulsion Types As with low-voltagefuses,high-voltagetypesstartthe current-interruption process by the melting of a fusible link, but as might be expected, deionization of the atten- dant arc becomes the most substantial part of current termination. To help the process, as shown in figure 9.25, the link is held under tension by a coil spring; upon melting, the spring pulls the contacts apart, lengthening the arc (4).In expulsionfuses,gases are liberated fromthe lining of the current-interrupting chamber by the heat generated from the arc. Both the earliest formof expulsion
- 263. Fiber tube, rStrain element Figure9.24.-High-voltage powerfuse andsupport. (Courtesy S&C Electric Co.) Fusible link Spring Glass tube ~leiible lead Figure 9.25.-Fusible element under spring tension in high- voltage fuse. fuse and distribution fuse cutouts use a liner of organic material to deionize the generated gases by expelling them fromthe fuse holder tube to the surrounding air. The problem with this operation is the attendant flame expul- sion and loud noise. Hence, expulsion fuses are suitable only for outdoor usage, generally in substations remotely located from human habitation (7). The limited interrupting capacity (table 9.7)and unsuitability for indoor use of early expulsion fuses led to the development of the boric acid or solid-materialfuse(7). Here, the interrupting chamber is made of solidboric acid. When exposed to arc heat, the material liberates steam, which can be readily condensed to liquid by venting the gas into a cooling device. The result is an operation with negligible or harmless flame and gas emissions and noise levels. The range of voltage, continuous current, and interrupting ratings is also greatly expanded. High-voltageboric acid fusesare manufactured in two styles (7): the fuse unit (nonrenewable),where the fusible unit, interrupting element, and operating element are all combined in an insulated tube; and the refill unit or fuseholder (renewable), where only the refill unit is re- placed after interruption. Figure 9.26shows the internal components of a refill unit, while figure 9.27illustrates the constructionof the entire fuse. Thble 9.7provides a list of typical ratings for both styles. The fuse-unit style is intended for outdoor use at system voltages of 34.5to 138 kV, while the refill unit can be used indoorsor outdoorson the surface at 2 . 4 to 34.5kV. Boric acid 1 1 IMainfuse Plunger 1 Gap ,Disk, Figure 9.26.-Cross section of boric acid power fuse refill. KEY A Fuseholder B Sprin -and-cable assembly (copper cable carries 1w8 current C,D Fusehoider upper contacts and latch E Fuseholder lower contacts and latch f Refillunit Figure 9.27.-Disassembled refill unit for boric acid fuse. (CourtesyS&C Electric Co.) Current-Limiting High-Voltage Fuses High-voltage current-limiting or silver-sand fuses have the same advantages as previously discussed for low-voltagefusesand are of twodifferent forms(7):those to be used with high-voltage motor starters for high-capacity distribution circuits at 2,400and 4,160V and those for use with potential, distribution, and smallpower transformers from 2.4to 34.5 kV. The operation of either form is such that the arc established by the melting of the fusible element is subjectedto mechanicalrestriction by a powder or sand filler surrounding the fusible element. The tech- nique provides three important features: Current is interrupted quickly without arc-product or gas expulsion. This allows use indoors or in small-size enclosures on the surface or underground. There is no noise fromthe operation,and sincethere is no gas or flame discharge, only normal electrical clearances need by met.
- 264. Table 9.7.-Ratings of high-voltage power fuses Nominal rating, kV Expulsion-type fuse Maximum continuous current, A Maximum interrupting rating, MVA' Boric acid fuse. 1-shot type Maximum Maximum continuous interrupting current. A rating, MVA' - - - - - - - - - - Boric acid fuse, renewable Maximum Maximum continuous interrupting current,A rating. MVA' 200.400.720 155 200,400,720 270 - - 200,400,720 325 200,400,720 620 200,300 750 200,300 1.000 - - - - - - Current-limiting fuse Maximum Maximum continuous interrupting current, A rating, MVA1 100,200,450 155.210.380 450 380 100,200,300,400 310 100.200 820 161................. 100, 200 3,480 - - - - - - '3-phase symmetrical rating. NOTE.-Dashes indicatethat standard fuses are not available in the specific vonage rating. Very high interrupting ratings are available so these fuses can be applied on systems with very high short-circuitcapacity (within their voltage rating). All of the advantages of current-limiting action are available for high voltage. a b l e 9.7 provides a listing of typical ratings for current-limitingfuses. Instead of being rated by current, these fuses can also be "E-rated" (for instance, 100 E instead of 100 A), "C-rated," or "Rrated." The specifica- tions for E and C ratings are as follows: E-ratedfuses: 100E and below, open in 300 s at an rms current within the range of 200% to 240% of the continuous rating of the fuse element; above 100 E, open in 600 s at an rms current within the range of 220% to 264%of the continuous (or E)rating; Grated b e s : open in 1,000s at an rms current within the range of 170% and 240% of the C ratings. E-ratedfuses are consideredas general-purposeor backup fuses. while R-rated devices are intended for use with high-voltagemotor starters ( 7 ) . Load-Break Switches Fused load-break switch It is possible that after the occurrence of a short circuit on a fuse-protectedthree-phase system, only one of the three fuses could open. Here current through the remaining two fuses might be reduced so that they do not d Large fuse open. The system then becomes single phased, which can / cause serious damage to equipment. In a low-voltage ::-I:i : : : Actuator fuse circuit, dual-elementfuses that are closely matchedto the ---1 ---I --1 overcurrent point can usually handle the situation. On I I high-voltage systems, the problem is much more difficult I I Remote when protection is by fuses alone.However, to take advan- tage of the lower cost of fuses and load-break switches 1-2 versus the cost of a high-voltage circuit breaker, some I signaling +J -circuit or switch- manufacturers produce load-break switches with incorpo- o~enlng rated high-voltage fuseholders. An example is shown in Switch circuit figure 9.28where the fuses are interlocked to trip the operating mechanism of the switch if one or more of the Schematic showing interlocks fuses fail. Interlocking is usually accomplished with spe- cial high-voltage fuses that contain a spring-loaded Figure 9.28.-Load-break switch with interlocked hlgh- plunger. Fuse activation releases the plunger, which trips voltage fuses. (Courtesy Line Power ManufscturingGorp.)
- 265. of time and is determined by the heater rating. The trip setting is commonly based on a 40° C ambient tempera- ture, but the relay may be ambient or nonambient com- pensating. Most relays of this type must be manually reset after tripping. An electromechanical-thermal device not using bime- tallics is the melting-alloy or eutectic-alloy relay, figure 9.31B.Being shock resistant and having high contact force, this is considered one of the most reliable thermal relays available, but because of its cost, it is not nearly as popular as the bimetallic type. The alloy melting point is extremely precise and is again related to a specific current-time characteristic. The relay can be reset after tripping and alloy resolidification. Two other thermal devices, resistance or thermistor types and thermocouples, operate with associated elec- tronic equipment to provide very precise temperature sensing and relaying. Here, for example, a probe can be inserted or embedded in a transformer or a motor winding to provide a spot temperature response. This type of device is very popular especially where large horsepower or capacity is involved. Electromagnetic-AttractionRelays There are three electromagnetic-attraction relays in common use: the solenoid, the clapper, and the polar (20). Although their operational speed might vary, all are considered instantaneous relays, since there is no built-in delay for pickup or reset. The solenoid and clapper types are available for ac or dc and are voltage or current actuated. Coil impedance is high for voltage and low for current. Polar units are dc sensing only, but may be used on ac circuits through rectification. All electromagnetic relays are available with NO contacts, NC contacts, or both. In solenoid units, the relay contact movement is initiated by a plunger being drawn into a cylindrical solenoid coil. Typical operating times are 5 to 50 ms, with the longer times associated with operation near the min- imum pickup value (20). A cross-sectional sketch of a solenoid relay is given in figure 9.32A. Four different clapper relays are shown in figure 9.32B. These have a magnetic frame with a movable armature and operate by the attraction of the armature to Adjusting core screw Coil area Magnetic frame -Helical spring A Solenoid-type relay Normallyclosed(break) contact ~ ~ ~ ~ f v l a ~ n e t i c frame Normally open(make)contact Mains~ringfl'~,i 1 , c ~ o ~ C o r e ~ b b , , k&,T- - - Res~dual pln Coil fit- Armature Movingcontact Indicating contact switch (ICS) Contact multiplier contact Residual plating Indicating instantaneous trip ( I I T ) High speed 6 Clapper-type relay Figure 9.32.-Solenoid and clapper relays. (Courtesy Westinghouse Electric Corp.)
- 266. an electromagnetic pole (20). The armature controls the pickup or reset of contacts. As illustrated in figure 9.33, polar relays have a hinged armature in the center of the magnetic structure, which is here shown as an electromagnet but may be a permanent magnet. The relays operate when dc is applied to the actuating coil, and the polarity of the actuating source determines armature action, be it stationary or movement in either direction (10). In some units there is no retaining spring, and through a combination of con- tacts, the relays can sense actuating current through the coil in either direction (20). The pickup and reset values of clapper units are less precise than those of solenoid and polar relays; thus, clapper relays are used often as auxiliary or gq nego devices (20).A common use for polar relays is in dc circuit protection where the actuating source is obtained from a shunt or directly from the circuit (10). A characteristic that should be considered when ap- plying any electromagnetic-attraction relay is the large difference that can exist between pickup and reset values. When an attraction relay picks up, the air gap is short- ened, and a smaller coil current is needed to retain pickup Thus, the reset current may be much lower than the pickup current. The disparity is usually expressed as a percent ratio of reset current to pickup current, and is less pronounced in ac than dc relays. The ac relays can have a reset up to 90%or 95%of pickup, but dc ratios range from 60%to 90%(10).This is no problem in overcurrent appli- cations where relay coil current dropsto zero after pickup, but it is a concern where reset values are important. Electromagnetic-Induction Relays Electromagnetic-inductionrelays are of two general types: induction disk and cylinder (20).Depending on the design, the induction-disk unit can be either a single- quantity or directional relay, whereas cylinder relays are intended to be directional. A single-quantity relay, as might be supposed, is actuated by and compares two sources (10). The most commonly used time-delay relays for system protection employ the induction-diskprinciple (7). Single Quantity Single-quantity timedelay relays of the induction- disk type use the same principle of operation that was described for induction motors in chapter 6, but the physical construction is quite different (20).A sketch of an elementary induction-typedevice is shown in figure 9.34, and most time-delayrelays in use today have this arrange- ment. The disk, made of aluminum, is mounted on a rotating shaft restrained by a spring, and a moving contact is attached to the shaft (fig. 9.35). On one side of the disk is a three-poleelectromagnet;the other side has a common permanent magnet or keeper. The operating torque on the disk is produced by the electromagnet, and the keeper providesa dampingactionor restraint after the disk starts to rotate. The retarding effect of the keeper createsthe time delay or desired time characteristic of the relay. Figure 9.35 is a front-viewillustration of an actual induction-disk relay removed from its drawout case; all important components are indicated. The unit pictured is for overcurrent, but ovenroltage and undervoltage relays are also available and are identical in constructionexcept for the electromagnet coil rating. MovaMe Control 5 Polar!zing spring c Stop rnagnet 1 + To actuating quant~ty Figure 9.33.-Polar relay. Coil terminals J L P n ? a Lag coil / Keeper r a 9 rair gap Figure 9.34.-Common induction-disk relay. Figure 9.35.-Front view of induction-disk relay removed from case. (Courtesy General Electric Co.) The control spring carries current for the moving con- tact. If the actuating quantity driving the electromagnetis of sufficientmagnitude and is sustainedfor enoughtime,the disk will rotate until the movingcontact touchesthe station- ary contact. (Somerelaysuse a lever on the moving diskthat forcesa pair of stationarycontactstoclose,sothat no current
- 267. flows through the control spring and disk.) Pickup of these main contacts triggers the seal-in or time-delay element, which is an electromagnetic-attraction relay with its coil in series and contacts in parallel with the main contacts.When activated, this relay picks up and seals in, thus lightening the currentcanying duty of the main contacts as well as operating a target indicator.ARer pickup, it usually must be reset manually. The tap block at the top of figure 9.35 is to allow different tap sett~ngs on the electromagnet coil. Table 9.8 lists the tap settings generally available in overcurrent relays (n, but some relays have wider ranges than those shown. Each range represents a different operating coil. Voltage relays have a narrower range of adjustment, because they are usually expected to operate within a limited change from the normal magnitude of the actuat- ing quantity (10).Be it a voltage or current relay, the coil and its tap settings are normally selected with respect to the ratios of the potential or current transformer used. Table 9.8.-Common current ratings of Induction-disk overcurrent relays Time-delay elements Typical instantaneous element^,^ Coil range, A Tap settings,' A adiustment ranae. A 'Tap sen ngs w~ll vary slightly according to manulacturer 'Add~tlonalunlts are available for each tame delay range 'Not adjustable As shown in figure 9.35,overcurrent disk relays often have a second (auxiliary) ac-operated instantaneous ele- ment, which is a clapper-type relay (7).The unit is contin- uously adjustable over a calibrated range, and table 9.8 lists some of these representative values. This relay oper- ates in series with the time-delay operating coil and is usually set to operate instantaneously at a current pickup value higher than that of the time-delay element. How- ever, since the same actuating source drives both ele- ments, the instantaneous-relay setting must be coordi- nated not only with the same source but also with the timed element. The instantaneous contacts can be in parallel with the time-delay contacts or can be connected to separate terminals. The unit also has a target indicator, which normally requires manual reset after tripping. The operational characteristic produced by the induction-disk principle is termed Inverse time. Although mentioned earlier in this chapter, the inverse response is illustrated again in figure 9.36 to emphasize that the operating time becomes less as the magnitude of the actuating quantity is increased (10).The more pronounced this effect becomes, the more inverse the curve is said to be. All relay time curves are actually inverse, with the exception of a theoretical definite-time response. By defi- nition, definite-time characteristics imply that the operat- ing time of the relay is unaffected by the magnitude of actuating quantity. In reality, an actual definite-time curve is very slightly inverse (fig. 9.36).Regardless, the term definite time is normally applied to all fixed-time relays that approach this response. The control-spring tension, the damping magnet, and the magnetic plugs (A and B of figure 9.34)provide separate and relatively independent adjustment of the relay inverse-time characteristics. They are preset by the manufacturer, and the common responses are "inverse," "very inverse," "extremely inverse," "short time," and "long time," the first three being the most popular in mining. A comparison of these responses is given in figure 9.37.The need for a specific response depends upon the application, and a few thoughts in terms of overcurrent relays follow (6). When the available fault-current magnitudes vary considerably, faster overall protection is usually gained with an inverse-time response. Very inverse curves provide the best overall protection where fault current remains MAGNITUDE OF ACTUATING QUANTITY A I Figure 9.36.-inverse-time curve compared with definite time curve. W z t- w Z - k a n W a 0 MULTIPLES OF TAP VALUE CURRENT ifl P~ckup value - Figure 9.37.-Various time characteristics of induction units.
- 268. constant (detection of the fault, as seen by the relay, is mainly a function of fault location). Extremely inverse relays are designedto coordinate rather closelywith power fusesand distributioncutouts and are alsoused in systems that have large inrush currents. The actual applicationof these characteristics in the mine is given in chapter 13. The operating time of an induction relay can usually be adjusted by selecting the distance of rotor travel from the reset to the pickup position (10).This is accomplished by adjusting the rest position of the moving-contactstop. The time dial, with evenly divided markings, facilitates positioning. When the response of the relay for different time dial settings is plotted, the result is a family of curves, an example of which is shown in figure 9.38. Current is plotted in terms of multiples of pickup, which enables the curves for a specific relay to be used with any tap setting. Directional The basic ac directional electromagnetic-induction relay or cylinder unit in common use is sketched in figure 9.39. Its operation is similar to that of an induction motor that has salient poles for the stator, except that here the rotor iron is stationary and only the rotor conductor is free to rotate (10, 20). The rotor conductor is a thin-walled aluminum cylinder, and the two actuating quantities, causing I, and I,, independently produce torque on the cylinder. The cylinder drives a moving contact whose travel is restricted to a few degrees by the stationary contact and stops. Reset torque is established by a spiral spring. The ac directional relays are used to distinguish between current supplied in one direction or the other in an ac circuit, by recognizing phase-angle differences be- tween the two actuating quantities (10).(Conversely,a dc directional relay, or polar unit, recognizes differences in polarity.) perform the ac comparison, one actuating value is used as a reference or polarizing quantity. There- fore, the polarizing quantity phase angle must remain fixed while the phase angle of the other fluctuates widely. One application of this technique is in power relays where the unit is polarizedby circuit voltage, with circuit current being the other actuating value. Through this, the cylin- der detects power flow in one direction or the other. Another important application is an ac directional relay combined with an overcurrent relay, as shown in figure 9.40. Here, tripping occurs only when the current has a specific relationship to the voltage, and power flow is in the tripping direction. BASIC RELAY CONNECTIONS In order to sense a malfunction and then supply tripping energy to the appropriate circuit breaker, a relay must be attached in some manner to the power system. Circuit connections for protective relaying are basically not too different from those discussed for instrumentation in chapter 5. Here, however, the relay coil receives the input information, and its contacts pick up or reset, thus affecting the control power to the circuit breaker. Direct relay connections to the monitored circuit are often re- stricted to low-voltage, low-power circuits because most relay current or voltage coils are designed to operate in the vicinity of 5 A or 120 V (4). Obviously, if power-system values exceed these levels, some interface is needed be- M U L T I P L E OF PICKUP Figure 9.38.-Family o f inverse-tlmecharacteristics. Figure 9.39.-Cylinder dlrectlonal relay. tween the monitored circuit and the relays. Again, instru- ment transformers for ac and resistors for dc are used, a subject also introduced in chapter 5. There are five basic relay connectionsused for protec- tive relaying in the mining industry. For ac systems,these are direct, potential, and differential; and for dc work, direct and potential are used. Differentialrelaying is also available for dc, but the circuitry is not considered basic. Although someof the techniques are employed much more frequently than others, this section servesto introduceall these connections. Alternating Current Direct Relaying Direct relaying is used to sense the magnitude of current flow. As shown in figure 9.41A, its simplest form consistsof a current transformer (CT)secondaryconnected to a relay operating coil. Relay pickup current is thus a
- 269. function of line current. For instance, consider that the transformer ampere-turns ratio or current rating is 5015A or 1011and the relay pickup setting is at 0.5 A. This relay would theoretically pick up its contacts when line current is (10X0.5) or 5 A. The purpose of this connection is therefore to provide protective relaying for current in any conductor. The important items to consider in directrelaying are concerned with matching the performance of the CT with that of the relay. IEEEstandards provide most of these ( 7 ) . 1. Ratios. As an obvious starting point after the foregoing example, standard ratios are listed below: Single-ratioCT, amperes: Double-ratioCTwith centered-tappedsecondary,amperes: Multiratio CT with multitapped secondary, amperes (cur- rent ratings higher than those shown are also available): Rating ~ P S 60015 ...................... 5015 10015 150/5 20015 25015 30015 40015 45015 50015 60015 Coil terminal 3 I-----0 co T-_--, B ? A Plug Figure 9.40.-Dlrectlonal overcurrent relay using induction- disk relay and cyllnder relay. Line to be - CT monitored i, HI T r --*- Reby operating coil A Circuit connections B Instantaneous current Figure 0.41.-Direct relaylng In ac systems. The double-ratio and multiratio types provide flexibility through secondarytaps. Thesevalues are for bushing-type or window-type CT's, which are the most popular in the industry. All these have the standard 5-A-ratedsecondary current. 2. Secondary Current. The continuous-currentrating of the secondary should be at least equal to the actual drain, but a full-load secondary current of 3 to 4 A is normal practice. An oversized CT is bad practice, as the percent error is much greater than with a correctly rated CT 3. Sho&Time Ratings. Both thermal and mechanical ratings should be considered. The thermal short-time value relates to the maximum symmetrical rms primary
- 270. current that the CT can cany for 1.0 s without exceeding itsmaximum specifiedwinding temperature. The mechan- ical rating refers to the maximum asymmetrical rms current the CT can withstand without damage. In both cases, the rating is made with the secondary short- circuited. 4. Voltage Rating. Standard voltage ratings are 600, 2,500, 5,000, 8,700, and 15,000 V, and are the same as insulation classes found in mine systems. The CT will operate continuously at 10% above rated voltage without insulation failure. 5. Burden. As defined in chapter 5,burden is the load connected to the CT secondary; expressions used are wlt-amperes at a given power factor or an impedancewith a power factor. The power factor is that of the burden. 'lhble 9.9 lists standard values for CT's at 60 Hz. Relay burdens are sovaried they cannot be listed, but chapter 10 shows how CTburden and relay burden can be compared. 6. Accuracy. Accuracy of a CT relates to its transfor- mation ability. In protective-relaying applications, accu- racy is not only important at normal circuit currents but also at faultcurrent levels. The problem in CT's is that core saturation leads to poor accuracy or ratio errors. Accuracy class designations use a C or T identifyingletter followed by a classification number. C states that percent ratio error can be calculated, whereas T means that the value hasbeen found by testing. The classification number relates to a standard secondary voltage of 10, 20, 50, 100, 200, 400, or 800 V.At this voltage, the CT will deliver to a standard burden, 20 times normal secondary current with 10% ratio error or less, and it will not exceed 10% with any current from 1 to 20 times rated current with a lesser burden. (For example, C200 relates that for a 2.0-0 burden, (20x5) or 100 A can be delivered from the CT without exceeding 10% error. This error can also be - calculated.) 7. filarity. hlarity relates to the correct phasing of primary and secondary currents, and figure 9.41B shows the relative instantaneous directions of current as per standard markings. This allows correct connectionswhen more than one transformer is used, which is imperative in three-phase systems. As can be seen in the foregoing listings, actual man- ufacturer specificationsshould always be consulted before attempting to match CT's with relays for direct-relaying applications. Alternating Current Potential Relaying htential relaying is as simple as direct relaying and enables circuit voltage to be monitored. Figures 9.42A and 9.42B show two applications: sensing voltage across a resistance and between two conductors. There is a poten- tial transformer (PT) between the circuit and the relay. Figure 9.42C gives the polarity correspondenceof instan- taneous voltages between the primary and secondary windings as well as conventional transformer markings. Standard FT's are single-phase, two-winding units con- structed so that the primary and secondary voltages always have a fixed relationship (7). ?b visualize the operation, consider figure 9.42A.The transformer is rated 2,4001120 V or has a 2011 ratio, an ovewoltagerelay is used, and the relay coil is rated at 120 V with the contacts set to pick up at 80% of rated. The contacts will therefore pick up when 1,920 V exists across the resistor. IEEE standards also provide guidelines for FT utili- zation, and a summary of these follows(7). In general. thev - , " are less rigorous than those for CT's. 1. Voltage. Standard voltage ratios are available in table 9.10. When applied to sense voltage between two conductors, the nominal system voltage should be within & 10% of the transformer nameplate rating. When used in three-phase mining systems supplying portable or mobile equipment, primary connections must be line to line. Special ratings, providing other than the standard 120-V secondary, are usually available. Table 9.9.-Standard burdenfor current tmsformers standard General characteristics C h ~ ~ C t , " ~ , " ~ ~ ~ , " , " r ~ ~ t : " d ........ burden , - designation? Resistance Inductance Impedance Apparent power pf (R). n (L). m~ O, n (s), VA 8-0.1........... 0.09 0.116 0.1 2.5 0.9 64.2........... .18 ,232 .2 5.0 .9 8-0.5........... .45 .580 .5 12.5 .9 8-1 .............. .5 2.3 1.O 25 .5 8-2 .............. 1.0 4.6 2.0 50 .5 6-3 .............. 2.0 9.2 4.0 100 .5 6-4 .............. 4.0 18.4 8.0 200 .5 '8-0.1,6-0.2, and 8-0.5ere usually applied for metering purposes;B-1 through B-4 are usually applied for relaying. 2At 5 A, S=1 5 ; for example, for 8-2, =5*2=50 VA. Table 9.10.-Standard ratingsfor potential transformers (Secondary, 120V) Primary. V Ratb 11 Plimary. V Ratio 4Line monitored r--+ --------. T l Ljnes 4-1 (T& Relay coil mon~tored rrrvvy coil G3 A Resistance B Between conductors Instontoneous C Polarity Figure 9.42.-Potential-relaying connections.
- 271. 2. Accuracy. Ratio and phase-angle errors of standard PT's are usually so small they can be neglected, and any standard transformer is satisfactory as long as it is used within its thermal and voltage limits. If the transformer load is within rated burden, the transformer is suitable over the range from zero to 110%of rated voltage. Regard- less, standard accuracy classes do exist for PT's, ranging from 0.3 to 1.2. These values represent the percent ratio corrections to obtain true ratio. 3. Burden. The burden of a PT, or thermal burden limit, is expressed in voltamperes. It is usually s&~cient to add the voltampere ratings of parallel loads arithmeti- cally to obtain a total voltampere burden. Accuracy is usually satisfactory at burdens well below rated, but the transformer voltampere rating should not be exceeded. 4. Fusing. In some instances, fusing the primary of a PT is not advisable, especially when the protective- circuitry's function is to sense a critical overvoltage con- dition (for instance, monitoring the voltage across a grounding resistor). Yet when the PT is connected line to line, it must be protectedin case of PTfailure or secondary conditions that will lead to failure. General practice is to use current-limiting fuses, sized t,othe transformer full- load rating and installed in the primary circuit between each ungrounded conductor of the system. Fuses are preferred over circuit breaker primary protection because the latter is accessible for manual tripping. A major use for PT's in mine systems is to supply control power to protective circuitry; secondary protection in this case is unnecessary. For other loads such as branch circuits for 120-Vconvenienceoutlets, the additional branches should he fused or protected by molded-casecircuit breakers, the latter being general practice. Alternating Current Differential Relaying In differential relaying, a relay is operated by the vector difference of two or more actuating quantities, and relay pickup is determined by a difference threshold (10). Most applications of this scheme are of the current differ- ential type. A basic circuit is shownin figure 9.43A, where the dashed portion represents the area to be protected. Two matched CT's are interconnected,and an wercurrent relay is inserted between them. Under normal conditions, or even when a fault occurs outsidethe protected zone, the CT secondarycurrents will circulate and not flow through the relay coil. However, if the current in both CTprimaries becomes unbalanced, current will flow through the relay in proportion to the vector difference of the current enter- ing and leaving the protected circuit (fig 9.43B). A problem with this basic circuit is that CT's are very difficult to match; on identical units, the same primary current will not always give the same secondary current (7).Thus, the relay must be set so that it does not pick up on maximum error current between the CT's. An approach that usually overcomes the mismatch problem is the per- centage differential connection. As illustrated in figure 9.43C, the main change is that the relay is now an over- current current-balancetype (10). The differential current required to operate the relay is a variable quantity because of the relay restraining coil, and it offsetserrors in the actuating sources. Direct Current Connections In addition to their popularity with ac systems,direct relaying and potential relaying are also the two most used protective relaying connections for dc systems. Direct relaying (fig. 9.44) consists of a dc wercurrent relay connected to a resistance shunt. The relay operatingcoil is matched to the shunt voltage at the desired pickup level (shunt full-loadcurrent rating usually gives 50 or 100mV across the shunt). For low-current applications such as sensing dc current in a groundingconductor,current-relay operating coils are sometimes inserted in-line with the monitored conductor. Potential relays are also directly connected with the coils between the conductors of inter- est. Resistivedividers are at times employed to dropthe dc system voltage down to the coil rating (as discussed in chapter 5). - - Relay - 4 - - - X System portion t o b e protected External lood o r faull 4 Basic circuit under normal conditions + Internal fault B Bosic circuit under abnormal conditions Restroining coil C Percentage different101 relay connections Figure 9.43.-Differential-relaying connections. Monitored + - ltne + Voltage-drop Potential resistor relay coil @overcurrent relay coil Direct relaying Potential relaying Figure 9.44.-Dc direct-relayingconnections.
- 272. the switch mechanism. Precautions must be obsewed when using or considering these devices, and these are discussed in chapters 12 and 13. RELAYS Fklays perform a major role in power-system protec- tion, where their purpose is to detect voltage and current anomalies. They normally receive information about sys- tem conditions through transformers or resistors, which reduce system parameters down to levels that the relays can handle. Upon detection of a problem, a relay operates to supply or remove control power to the shunt or UVR tripping elements of the switching apparatus. Becauseof their function,relays are sometimescalled sensing devices. While transformers might alsobe consid- ered sensing devices, their function in protective relaying is solely as transducers. There are four basic relay types: thermal, electromag- netic attraction, electromagnetic induction, and static. (D'Arsonval movements are actually considered another relay type, but their operation is completely covered in chapter 5).The first three are electromechanicaldevices, and the following paragraphs will present their operation. Static or solid-state relays are discussed in chapter 12, because of related content. Relay Terminology and Types When a relay operates, it is said to close or open its contacts (9). Most relays are restrained by spring control and assume a specificposition, either open or closed,when deenergized: hence there is a normally closed or NC contact and a normally open or NO contact. Symbols for both situations are shown in figure 9.29. When a relay operates to open NC contacts or close NO contacts, it is said to pick up the contacts, and the smallest actuating quantity to cause contact operation is referred to as the pickup value. When a relay operates to close NC contactsor open NO contild, it is said to reset or drop out, and similarly, the largest actuating quantity to cause reset is the reset value of the relay. Whenthe relay is deenergized to reset, the reset value is almost always greater than zero and is often specified as a percentage of normal operation. Most relays have adjustments or tap settings to adapt them to as wide an operating range as possible. The word describing relay operation has a formal meaning; for example, overvoltage relays, overcurrent re- lays, overtemperaturerelays, and so forth. Here the suffix refers to the actuating source (voltage, current, etc.), and the prefix "over" means that the relay picks up to close a set of NO contacts (or open NC contacts)when the actuat- ing quantity exceeds the magnitude at which the relay is adjusted to operate. Similarly, undewoltage, undercur- rent, and undertemperature relays reset to close NC contacts (or open NO contacts)when the actuating quan- tity decreases below a predetermined level. Some relays have both "over" and "under" functions (7, 10). Even with these definite meanings, common usage of relay terminology is rather straightforward. Pickup is used to refer to the point where the relay changes from its normal state to indicate a malfunction,while reset implies that the relay returns to its normal position. The normal position may occur when the relay is energized or deener- gized and depends on the application. Fklays designed for protective circuits are usually provided with some means of visual indication that a specific relay has operated to trip a circuit breaker. These operation indicators or targets are often brightly colored and are operated mechanically or electrically. Specific relay types have been developed to meet special or general system-protectionneeds. Thermalrelays serve directly or indirectly to measure power-systemtem- peratures. Electromagnetic-attraction relays are used to instantaneously detect voltage and current changes. Electromagnetic-inductionrelays allow a time delay be- tween relay detection and contact action. Directional re- lays can sense the direction of current flow. Thermal Relays Thermal relays most commonly employ bimetallic- drivencontactswith an operationsimilarto that described for the molded-case circuit breakers. Another approach is to use ambient temperature, as in the temperature- monitoringprotectorshown in figure9.30.This is a sealed bimetallic thermostat that opens or closes at a specific temperature; it can be used, for example, to sense motor overtemperature if mounted against the end turns of a motor winding. Yet another bimetallic approach is to employ a heater element within the relay enclosures, connected in series with the circuit under consideration, as illustrated in figure 9.31A. The relay trip point for opening or closing the contacts is expressed in amperes,but is also a function 1 T Normally open j; Normally closed Figure 9.29.-Relay contact symbols. Device Contacts closed Flgure 9.30.-Temperature.monltorlng protector. Heoter Thermal reby unit Blmefal / 4 Motor --TO motor clrcurt Control T O clrcult --+ mqnet Cod Ccmtocts closed Normal porition A Bimetallic relay B Melting-alloy relay Flgure 9.31.-Electromechanlcal.thermal relays.
- 273. KINDS OF PROTECTION Several relaying terms describe the protection re- quired in many mine power systems: 1.Undervoltage, 2.Overload (sometimes called overcurrent), 3. Short circuit, 4.Ground overcurrent (or ground fault), 5. Ground continuity, and 6.Overtemperature. Classificationssuch as these are known formally as kinds of protection. The first five are necessary protection on all portable and mobile mining equipment, although excep- tions are provided within Federal regulations (27). This section expands the basic relaying material by describing how each kind of protection is used in the mine power system. The content is mainly pointed at high-voltage, three-phase ac mining systemsand in general is restricted to relaying external from circuit breakers. Accordingly, these kinds of protection imply the following parameters: Line-to-linevoltages for undervoltage, Line overcurrent for overload, Three-phase or line-to-linefaults for short circuit, Faults causing zero-sequence current for ground overcurrent, and Groundingconductorresistanceforgroundcontinuity. Even though overtemperature is listed as item 6,it is usually applied to protect a specific component; thus, it will be discussed in chapters 12 and 13. Control Wiring Figures 9.45 and 9.46 show simplified diagrams of typical control wiring interconnections among the power source, relay contacts, and circuit breaker tripping ele- ments. In both diagrams, a potential transformer supplies 120 Vac with its fused primary connected line to line. Figure 9.45 illustrates cases where the tripping ele- ment is a UVR. The contacts can either reset to remove power from the coil (contactsin series with coil)or close to short it out (contactsparallel the coil).In the latter case, it can be seen that a resistance is placed in series with the contacts. In fact, the UVR itself will trip the breaker if control voltage is decreased in the range of 40% to 60%. Basic shunt-tripping connections are given in figure 9.46,where power is supplied to the element to cause tripping. Here the contacts for the various protective relays are paralleled, and the combination is in serieswith the trip coil; closure of any contact trips the breaker. It should be obvious in figure 9.46Athat the power causing tripping is ac, while in figure 9.46B,it is dc. The capacitor in the dc circuit is employed for energy storage to augment tripping if there is a drop in the PTprimary voltage when a relay contact closes. Phase Protection Phase protection by protective relaying can be over- load, short circuit, or both, depending upon the relays used. (Molded-casecircuit breakers afford this same flexi- bility dependingupon the internaltripping element used.) Time-delayrelays are employed for overload, with instan- taneous units for short circuit. Figure 9.47 illustrates the combined protection for three line conductors, using an Relay contacts open to trip Removing power to UVR Power to load L dp~ resistor Relay contacts closeto trip Shorting out UVR Figure 9.45.-Typical control wiring for UVR. Line-to- Line-to- line M a n u a l PTtuj trip 1 . T G r W 0 - h . , l P Manual trip Shunt trip coil - I L I A Simple B Capactor tripping Figure 9.48.-Typical control wiring for shunt-tripping ele- ment. Time-trip contact T o trip circuit W f L Instantaneous-elementcurrent coil Time-element current coil uProtected equipment Flgure9.47.-Three-phase overcurrent and short-circuitcon- nections.
- 274. induction-disk relay (7). The three current transformers are placed in wye, driving the wye-connected operating coils. The time-delayelement is set on as low a tap setting as practical, enabling protection for sustained moderate overloads. The instantaneous units, however, are set to pick up on a current value slightly higher than the maximum peak load, thereby affordingprotection against short circuits or enormous overloads. The device numbers that were presented in chapter 4 are used extensively to describe the relay function. The number 51 signifies time-delay relays for ac wercurrent, and 50 is used for instantaneous devices. A combination instantaneous and time-delayac overcurrentrelay is often noted by 50151. If the connections are as shown in figure 9.47 and transformer phase-angleerrors are ignored, the secondary currents of each CT are in phase with the primary cur- rents, and each relay responds to abnormal conditions for its respective line (10). This also applies to figure 9.484 where the 50 elements are omitted. If line currents are approximately balanced, short-circuit protection for all three lines can also be provided with an open-delta con- nection, as in figure 9.48B(10).As might be expected,this approach is not as precise as the straight wye connection, but a third overcurrent relay may be inserted in the CT common connection for backup protection (see chapter 5 for a similar discussion on instrumentation). An advan- tage of the wye connectionthat is lost when the open-delta approach is use is the ability to sense zero-sequence currents through residual relaying. Thus, two current transformers are rarely applied as the only means of circuit protection. Ground Overcurrent To this point, the chapter has basically considered power-conductor protective relaying, and the extremely important subject of ground-faultprotection has received only terse reference. Various relay configurationsmay be utilized to provide ground-overcurrent protection, some of which are quite elaborate. However, nearly all these techniques fall into one of five broad classifications (7): direct relaying, potential relaying, residual connection, zero sequence,and broken delta. Directrelaying, potential relaying and zero sequence are frequently used in resistance-grounded mine power systems, with zero- sequence relaying being the most popular. Unless other- wise noted, the following discussion will assume that the system is resistance grounded. The point of application for direct and potential ground-fault relaying is usually restricted to the system neutral point or grounding resistor, whereas the other three techniques can provide protection anywhere in the system. Usually a combination is needed for complete assurance of clearing all ground faults. Direct Relaying The simplest form of ac ground-fault protection is direct or neutral relaying. A current transformer is placed about the grounding condudor and located between the neutral point of the sourcetransformer and the grounding resistor, as shown in figure 9.49. The grounding conductor actsas the primary windingof the CT, while the secondary winding is connected to the ground-overcurrent relay (51N). If the current through the grounding conductor Line Line t L~ i Line Llne A Line I r i A Wye connected 8 Open-delta connected Figure 9.48.-Two CT approaches. Phase relays Grcund wercurrent Neutral aroundina relay (51N) *Pounding - conductor Figure 9.49.-Neutral-resistor current-relayingscheme. exceeds a predetermined value, the relay acts to trip the circuit breaker. In many situations, some ground-currentflow is nor- mal, due to system unbalance, capacitive-chargingcur- rents, or inductive-coupling effects, and so the circuitry must be adjusted to pick up only when the normal level is exceeded. As will be seen, the pickup point should always be less than the system current level. The major disadvantage with this direct relaying method is that, should the grounding resistor or the grounding conductors become open, it will never detect any ground-current flow. The system will continue to operate with no abnormal indication, and then the system can become essentially ungrounded, posing a personnel hazard especially where resistance grounding is manda- tory (12, 14, 18).Accordingly, although the technique does find application on some portions of the ground system, some States do not allow its use on substation grounding resistors, even for a second line of defense. htential Relaying Potential relaying, as shown in figure 9.50, is often used as a sole means of ground-fault protection at the surface substation and can also be used as a backup to other protection schemes at a unit substation or power center. With this method, the primary winding of a PTis connected across the neutral groundingresistor, while the
- 275. secondary winding is connected to a voltage-sensing ground-trip relay (59G). If current flows through the grounding conductor, a voltage is developed across the grounding resistor. When the voltage rises above a preset level, the ground-trip relay causes the circuit breaker to trip. Unlike direct relaying, potential relaying has the advantage of being able to detect a ground fault with the neutral grounding resistor in an open mode of failure. However, if the groundingresistor fails in a shorted mode, potential relaying is rendered inoperable. Zero-Sequence Relay Zero-sequence relaying, also termed balance-flu re- laying, is the most reliable first defense against ground faultsin mine power systems.As shownin figure 9.51,the circuitry consists of a single window-type CT; the three line conductors are passed through the transformer core, forming the CT prima