Isolator and bushing

TESTSOF INSULATORS
 Type Test To Check The Design Features
 Routine Test To Check The Quality Of The
Individual Test Piece.
 High Voltage Tests Include
(i) Power frequency tests
(ii) Impulse tests
2

TESTSOF INSULATORS
POWER FREQUENCY TESTS
(a) Dry and wet flashover tests:
 a.c voltage of power frequency is applied across the insulator and
increased at a uniform rate of 2% per second of 75%of ther
estimated test voltage.
 If the test is conducted under normal conditions without any rain –
dry flashover test.
 If the test is conducted under normal conditions of rain –wet
flashover test
(b) Dry and wet withstand tests(one minute)
The test piece should withstand the specified voltage which is
applied under dry or wet conditions.
3

IMPULSETESTSONINSULATORS
 Impulse withstand voltage test:
If the test object has withstood the subsequent applications
of standard impulse voltage then it is passed the test
 Impulse flashover test:
The average value between 40% and 60% failure is taken,
then the insulator surface should not be damaged.
 Pollution Testing:
Pollution causes corrosion ,deterioration of the material,
partial discharges and radio interference. Salt fog test is done.
4

TESTINGOF BUSHINGS
Power frequency tests
(a ) Power Factor-Voltage Test:
 Set up as in service or immersed in oil.
 Conductor to HV and tank to earth.
 Voltage is applied up to the line value in increasing steps and then
reduced.
 The capacitance and power factor are recorded in each step.
(b) Internal or Partial discharge Test:
 To find the deterioration or failure due to internal discharges
 Conducted using partial discharge arrangements
 Performance is observed from voltage Vs discharge magnitude.
 It is a routine test.
(c ) Momentary Withstand Test at Power frequency
 Based on IS:2009
 The bushing has to withstand the applied test voltage without
flashover or puncture for 30 sec.
5

TESTINGOF BUSHINGS
(d) One Minute withstand Test at Power Frequency
 Most common & routine test
 Test is carried in dry &wet for one minute.
 In wet test, rain arrangement is mounted as in service.
 Properly designed bushing should withstand without flashover for
one minute.
(e) Visible Discharge Test at Power Frequency
 Conducted based on IS:2009
 Conducted to determine radio interference during service
 Conducted in dark room
 Should not be any visible discharges other than arcing horns/
guard rings.
6

TESTINGOF BUSHINGS
Impulse voltage tests:
 Full wave Withstand Test
 The bushing is tested for either polarity voltages
 Five consecutive full wave is applied
 If two of them flashed over, then 10 additional applications
are done.
 If the test object has withstood the subsequent applications
of standard impulse voltage then it is passed the test.
Chopper Wave withstand Test
 Sometimes done on HV bushings (220kV, 400kV and above)
 Switching surge flashover test is included for HV bushings.
 This is also carried out same as above full wave test.
7

TESTINGOF BUSHINGS
Temperature Rise and Thermal Stability Tests
 To observe the temperature rise and to ensure that it doesn’t
go into ‘thermal runaway’ condition.
 Temperature rise test is done at ambient temperature
(below 400C) at a rated power frequency.
 The steady temperature rise should not exceed 450C.
 Test is carried out for long time & increase in temperature is
less than 10C/hr.
 This test is enough to produce large dielectric loss and
thermal in stability.
 Thermal stability test is done for bushing rated for 132 kV
above.
 Carried out on the bushing immersed in oil at max. service
temperature with 86% of normal system voltage.
 This is a type test for low rating and routine test for high
ratings.
8

TESTINGOFISOLATORSANDCIRCUIT
BREAKERS
Isolator:
 Off-load or minimum current breaking mechanical
switch.
 Explained according to “IS:9921 Part-1, 1981”.
 Interrupting small currents(0.5A): Capacitive currents
of bushings, busbars etc.,
Circuit Breaker:
 Onload or high current breaking switch
Testing of Circuit Breaker:
 To evalute,
 Constructional & operating characteristics
 Electrical characteristics
9

BREAKERS
Electrical Characteristics:
 Arcing voltage
 Current chopping characteristics
 Residual currents
 Rate of decrease of conductance of the arc space and the plasma
 Shunting effects in interruption
Physical Characteristics:
 Arc extinguishing medium
 Pressure developed at the point of interruption
 Speed of contact travelling
 Number of breaks
 Size of the arcing chamber
 Material and configuration of the circuit interruption
10

BREAKERS
Circuit Characteristics:
 Degree of electrical loading
 Applied voltage
 Type of fault
 Time of interruption
 Frequency
 Power factor
 Rate of rise of recovery voltage
 Re-stricking voltage
 Decrease in AC component of the short circuit current
 DC component of the short circuit current
11

BREAKERS
 Dielectric tests:
Consists of over voltage withstand tests of power
frequency, lightning and switching impulse voltages
Tested for internal & external insulation with CB in
both the open & closed position.
Voltage in Open position >15% of that of closed
position.
During test, CB is mounted on insulators above
ground to avoid ground flash over.
 Impulse tests:
Impulse test and switching surge tests with
switching over voltage are done.
12

BREAKERS
 Thermal tests:
To check the thermal behaviour of the breakers
Rated current through all three phases of the
switchgear is passed continuously for a period long
enough to achieve steady state conditions
Temperature rise must not exceed 40°C when the
rated normal current is less than 800 amps and 50°C
if it is 800 amps and above
Contact resistances between the isolating contacts
and between the moving and fixed contacts is
important. These points are generally the main
sources of excessive heat generation.
13

BREAKERS
 Mechanical Test:
To ensure the open and closing with out
mechanical failure
It requires 500(some times 20,000) operations
without failure and with no adjustment of the
mechanism.
A resulting change in the material or dimensions of a
particular component may considerably improve the
life and efficiency of the mechanism.
14

BREAKERS
Short circuit tests:
 To check the ability to safely interrupt the fault
currents.
 To determine the making and breaking capacities at
different load currents
 Methods of conducting short circuit tests,
i. Direct tests
i. Using the power utility system as the source.
ii. Using a short circuit generator as the source
ii. Synthetic Tests
15

BREAKERS
Direct tests -Using the power utility system as the source:
 To check the ability to make or break in normal load conditions or
short circuit conditions in the network itself
 Done during limited energy consumption
 Advantages:
1. Tested under actual conditions in a network
2. Special cases (like breaking of charging current of long lines, very short
line faults etc.,) can be tested
3. Thermal & dynamic effects of short circuit currents and applications of
safety devices can be studied
 Disadvantages:
1. Can be tested only in rated voltage and capacity of the network
2. Test is only at light load conditions
3. Inconvenience and expensive installation of control and measuring
equipment is required in the field.
16

BREAKERS
Direct Testing-Short circuit test in laboratories:
 To test the CBs at different voltages & different SC currents
 The setup consists of,
 A SC generator
 Master CB
 Resistors
 Reactors and
 Measuring devices
 The make switch initiates the circuit short circuit & master
switch isolates the test device from the source at the end of
predetermined time.
 If the test device failed to operate, master CB can be tripped.
17
Back up
breaker
Short Circuit
Testing Gen
Reactor
Test device
CT
Making
Switch
Circuit
Capacitance
G

BREAKERS
Synthetic Testing of CBs:
 Heavy current at low voltage is applied
 Recovery voltage is simulated by high voltage, small current
source
 Procedure:
 Auxiliary breaker 3 and test circuit breaker T closed, making
switch 1 is closed.  Current flows through test CB.
 At time t0, the test CB begins to open and the master breaker 1
becomes to clear the gen circuit.
18
1
2 3 4 5 6
Vc Cc C0 Cv
IvIc
T

BREAKERS
 At time t1, just before zero of the gen current, the trigger gap
6 closes and high frequency current from capacitance Cv
flows through the arc of the gap
 At time t2, gen current is zero. Master CB 1 is opened
 The current from Cv will flow through test CB and full
voltage will be available
 At the instant of breaking, the source is disconnected and
high voltage is supplied by auxiliary CB 4
19
1
2 3 4 5 6
Vc Cc C0 Cv
IvIc
T

TESTINGOF CABLES
Different tests on cables are
i. Mechanical tests like bending test,dripping and
drainage test, and fire resistance and corrosion tests
ii. Thermal duty tests
iii. Dielectric power factor tests
iv. Power frequency withstand voltage tests
v. Impulse withstand voltage tests
vi. Partial discharge test
vii. Life expectancy tests
20

TESTINGOF CABLES
Dielectric power factor tests:
 Done using HV Schering Bridge
 The p.f or dissipation factor ‘tan’ is measured at 0.5, 1.0, 1.66 and
2.0 times the rated phase-to-ground voltage of the cable
 Max. value of p.f and difference in p.f b/w rated voltage and 1.66
times of rated voltage is specified.
 The difference between the rated voltage and 2.0 times of rated
voltage is also specified
 A choke is used in series with the cable to form a resonant circuit.
 This improves the power factor and rises the test voltage b/w the
cable core and the sheath to the required value when a HV and
high capacity source is used.
21

TESTINGOF CABLES
High voltage testing on Cables:
 Power frequency HV A.C, DC and impulse voltages are applied to test
the withstanding capability
 Continuity is checked with high voltage at the time of manufacturing
 Routine test:
 Cable should withstand 2.5 times of the rated voltage for 10 mins
without damage in insulation
 Type test:
 Done on samples with HVDC & impulses
 DC Test:1.8 times of the rated voltage (-ve) applied for 30 mins.
 Impulse Test: 1.2/50µS wave applied. Cable should withstand 5
consecutive impulses without any damage
 After impulse test, power frequency & power factor test is
conducted to ensure that no failure occurred during impulse test.
22

TESTINGOF CABLES
Partial Discharge test:
i. Discharge measurement:
 Life time of insulation depends on the internal discharges. So, PD
measurement is important.
 In this test, weakness of insulation or faults can be detected
 Fig(i) and (ii) shows the connection to discharge detector through
coupling capacitor.
23
F
Equivalent Circuit of Cable for
discharges
C HV
D.D
F
C HV
D.D
F
(ii)
(i)

TESTINGOF CABLES
 If the coupling capacitor connected, transient wave will be
received directly from the discharge cavity and second wave
from the wave end i.e., two transient pulses is detected
 In circuit shown in fig (ii), no severe reflection is occurred
except a second order effect of negligible magnitude.
 Two transients arrive at both ends of the cable-super
imposition of the two pulses detected-give serious error in
measurement of discharge
ii. Location of discharges
 Voltage dip caused by discharge or fault is travelled along the length
& determined at the ends
 Time duration b/w the consecutive pulses can be determined
 The shape of the voltage gives information on the nature of
discharges
24

TESTINGOF CABLES
iii. Scanning Method:
 Cable is passed through high electric field and discharge location is
identified.
 Cable core is passed through a tube of insulating material filled with
distilled water
 Four ring electrodes (two @ ends+two @ middle) mounted in contact
with water.
 Middle electrode given to HV. If a discharge occurs in the portion b/w the
middle electrodes, as the cable is passed b/w the middle electrode’s
portion, the discharge is detected and located at the length of cable.
iv. Life Test
 For reliability studies in service.
 Accelerated life tests conducted with increased voltages to determine the
expected life time.
K-Constant depends on Field condition and material
n- Life index depends on material
25






 n
1
-
m KtE

TESTINGOF TRANSFORMERS
 Transformer is one of the most expensive and important
equipment in power system.
 If it is not suitably designed its failure may cause a lengthy
and costly outage.
 Therefore, it is very important to be cautious while
designing its insulation, so that it can withstand transient
over voltage both due to switching and lightning.
 The high voltage testing of transformers is, therefore, very
important and would be discussed here. Other tests like
temperature rise, short circuit, open circuit etc. are not
considered here.
 However,these can be found in the relevant standard
specification.
26

 Induced over voltage test:
 Transformer secondary is excited by HFAC(100 to 400Hz) to about
twice the rated voltage
 This reduces the core saturation and also limits the charging current
necessary in large X-mer
 The insulation withstand strength can also be checked
 Partial Discharge test:
 To assess the magnitude of discharges
 Transformer is connected as a test specimen and the discharge
measurements are made
 Location and severity of fault is ascertained using the travelling wave
theory technique
 Measurements are to be made at all the terminals of the transformer
 Insulation should be so designed that the discharge measurement
should be much below the value of 104 pC.
27

Impulse Testing of Transformer:
 To determine the ability of the insulation to withstand
transient voltages
 In short rise time of impulses, the voltage distribution along
the winding will not be uniform
 The equivalent circuit of the transformer winding for
impulses is shown in Fig.1.
Fig.1: Equivalent circuit of a transformer for impulse voltage
28

 Impulse voltage applied to the equivalent sets up uneven
voltage distribution and oscillatory voltage higher than the
applied voltage
 Impulse tests:
Full wave standard impulse
Chopped wave standard impulse (Chopping time: 3 to 6S)
 The winding which is not subjected to test are short
circuited and connected to ground
 Short circuiting reduces the impedance of transformer and
hence create problems in adjusting the standard waveshape
of impulse generators
29

Procedure for Impulse Test:
 The schematic diagram of the transformer connection for
impulse test is shown in Fig.2
Fig.2: Arrangement for Impulse test of transformer
 The voltage and current waveforms are recorded during test.
Sometimes, the transferred voltage in secondary and neutral
current are also recorded.
30

Impulse testing consists of the following steps:
i. Application of impulse of magnitude 75% of the Basic Impulse Level
(BIL) of the transformer under test.
ii. One full wave of 100% of BIL.
iii. Two chopped wave of 115% of BIL.
iv. One full wave of 100% BIL and
v. One full wave of 75% of BIL.
 During impulse testing the fault can be located by general
observation like noise in the tank or smoke or bubble in the
breather.
 If there is a fault, it appears on the Oscilloscope as a partial or
complete collapse of the applied voltage.
 Study of the wave form of the neutral current also indicated
the type of fault.
31

 If an arc occurs between the turns or from turn to the
ground, a train of high frequency pulses are seen on the
oscilloscope and wave shape of impulse changes.
 If it is only a partial discharge, high frequency oscillations
are observed but no change in wave shape occurs.
 Impulse strength of the transformer winding is same for
either polarity of wave whereas the flash over voltage for
bushing is different for different polarity.
32

TESTINGOF SURGEDIVERTERS
(i ) Power frequency spark over test
 It is a routine test.
 The test is conducted using a series resistance to limit the current
in case a spark over occurs.
 It has to withstand 1.5 times the rated value of the voltage for 5
successive applications.
 Test is done under both dry and wet conditions.
(ii ) 100% standard impulse spark over test
 This test is conducted to ensure that the diverter operates
positively when over voltage of impulse nature occur.
 The test is done with both positive and negative polarity
waveforms.
 The magnitude of the voltage at which 100% flashover occurs is
the required spark over voltage.
33

(iii) Residual voltage test:
 This test is conducted on pro-rated diverters of ratings in the range 3 to 12
kV only.
 Standard impulse wave of 1/50µS is applied, voltage across it is recorded.
 Magnitude of the current 2 X Rated current
 A graph is drawn b/w current magnitude and voltage across pro-rated
unit and residual voltage is calculated
 V1=rating of the complete unit
 V2=rating of the prorated unit tested
 VR1=residual voltage of the complete unit
 VR2=residual voltage of the prorated unit
 V1/V2= VR1/ VR2
 V1/= V2 .(VR1/ VR2)
 Let, VRM – Max. permissible residual voltage of the unit
Multiplying factor, r = (VRM /V1)
Diverter is said to be passed when VR2<rV2
34

HIGH CURRENT IMPULSE TEST ON SURGE DIVERTERS
 Impulse current wave of 4/10µS is applied to pro-rated arrester in
the range of 3 to 12kV.
 Test is repeated for 2 times
 Arrester is allowed to cool to room temperature
The unit is said to pass the test if
i. The power frequency sparkover voltage before and after the test
does not differ by more than 10%
ii. The voltage and current waveforms of the diverter do not differ
in the 2 applications
iii. The non linear resistance elements do not show any puncture or
flashover
35

Other tests are
i. Mechanical tests like porosity test, temperature cycle tests
ii. Pressure relief test
iii. voltage withstand test on the insulator housing
iv. the switching surge flashover test
v. the pollution test
36

INSULATIONCO-ORDINATION
Insulation Coordination:
“The process of bringing the insulation strengths of
electrical equipment and buses into the proper relationship
with expected overvoltages and with the characteristics of the
insulating media and surge protective devices to obtain an
acceptable risk of failure.”
Basic lightning impulse insulation level (BIL):
“The electrical strength of insulation expressed in terms of
the crest value of a standard lightning impulse under standard
atmospheric conditions.”
Basic switching impulse insulation level (BSL)
“The electrical strength of insulation expressed in terms of
the crest value of a standard switching impulse.”
37

Factor of Earthing:
This is the ratio of the highest r.m.s. phase-to-earth power
frequency voltage on a sound phase during an earth fault to
the r.m.s. phase-to-phase power frequency voltage which
would be obtained at the selected location without the fault.
This ratio characterizes, in general terms, the earthing
conditions of a system as viewed from the selected fault
location.
Effectively Earthed System :
A system is said to be effectively earthed if the factor of
earthing does not exceed 80%, and non-effectively earthed
if it does.
38

Statistical Impulse Withstand Voltage:
This is the peak value of a switching or lightning impulse test voltage at
which insulation exhibits, under the specified conditions, a 90%
probability of withstand.
In practice, there is no 100% probability of withstand voltage. Thus the
value chosen is that which has a 10% probability of breakdown.
39

Statistical Impulse Voltage:
This is the switching or lightning overvoltage applied to equipment as
a result of an event of one specific type on the system (line energising,
reclosing, fault occurrence, lightning discharge, etc), the peak value of
which has a 2% probability of being exceeded.
Protective Level of Protective Device:
These are the highest peak voltage value which should not be
exceeded at the terminals of a protective device when switching
impulses and lightning impulses of standard shape and rate values
are applied under specific conditions.
40

Necessity of Insulation Coordination:
i. To ensure the reliability & continuity of service
ii. To minimize the number of failures due to over voltages
iii. To minimize the cost of design, installation and
operation
Requirements of Protective Devices:
 Should not usually flash over for power frequency overvoltages
 Volt-time characteristics of the device must lie below the
withstand voltage of the protected apparatus
 Should be capable of discharging high energies in surges &
recover insulation strength quickly
 Should not allow power frequency follow-on current.
41

Volt-Time Curve
 The breakdown voltage for a particular insulation of flashover voltage
for a gap is a function of both the magnitude of voltage and the time of
application of the voltage.
 Volt-time curve is a graph showing the relation between the crest
flashover voltages and the time to flashover for a series of impulse
applications of a given wave shape.
 Construction of Volt-Time Curve:
 Waves of the same shape but of different peak values are applied to the
insulation whose volt-time curve is required.
 If flashover occurs on the front of the wave, the flashover point gives one point
on the volt-time curve.
 The other possibility is that the flashover occurs just at the peak value of
the wave; this gives another point on the V-T curve.
 The third possibility is that the flashover occurs on the tail side of the
wave.
42

 To find the point on the V-T curve, draw a horizontal line from the peak
value of this wave and also draw a vertical line passing through the point
where the flashover takes place
 The intersection of the horizontal and vertical lines gives the point on the
V-T curve.
43

Steps for Insulation Coordination:
1. Selection of a suitable insulation which is a function of reference
class voltage (i.e., 1.05 X Operating voltage of the system)
2. The design of the various equipments such that the breakdown or
flashover strength of all insulation in the station equals or exceeds
the selected level as in (1)
3. Selection of protective devices that will give the apparatus as
good protection as can be justified economically
44

Conventional method of insulation co-ordination:
 In order to avoid insulation failure, the insulation level of different types
of equipment connected to the system has to be higher than the
magnitude of transient overvoltages that appear on the system.
 The magnitude of transient over-voltages are usually limited to a
protective level by protective devices.
 Thus the insulation level has to be above the protective level by a safe
margin. Normally the impulse insulation level is established at a value 15-
25% above the protective level.
45

Consider the typical co-ordination of a 132 kV transmission line
between the transformer insulation, a line gap (across an insulator
string) and a co-ordinating gap (across the transformer bushing). [Note:
In a rural distribution transformer, a lightning arrester may not be used on account of the high cost
and a co-ordinating gap mounted on the transformer bushing may be the main surge limiting device]
In co-ordinating the system under consideration, we have to
ensure that the equipment used are protected, and that inadvertent
interruptions are kept to a minimum.
The co-ordinating gap must be chosen so as to provide
protection of the transformer under all conditions. However, the line
gaps protecting the line insulation can be set to a higher characteristic
to reduce unnecessary interruptions.
46

For the higher system voltages, the simple approach used
above is inadequate. Also, economic considerations dictate that
insulation coordination be placed on a more scientific basis.
47

Statistical Method of Insulation Co-ordination
At the higher transmission voltages, the length of insulator
strings and the clearances in air do not increase linearly with voltage
but approximately to V1.6 The required number of suspension units
for different overvoltage factors is shown below.
It is seen that the increase in the number of disc units is only
slight for the 220 kV system, with the increase in the overvoltage
factor from 2.0 to 3.5 ,but that there is a rapid increase in the 750kV
system.
48

Thus, while it may be economically feasible to protect the lower voltage
lines up to an overvoltage factor of 3.5 (say), it is definitely not
economically feasible to have an overvoltage factor of more than about 2.0
or 2.5 on the higher voltage lines.
Switching overvoltages is predominant in the higher voltage systems.
However, these may be controlled by proper design of switching devices.
In a statistical study, the statistical distribution of overvoltages has to
be known instead of the possible highest overvoltage.
In statistical method, experimentation and analysis carried to find
probability of occurrence of overvoltages and probability of failure of
insulation.
49

The aim of statistical methods is
to quantify the risk of failure of
insulation through numerical
analysis of the statistical nature of
the overvoltage magnitudes and of
electrical withstand strength of
insulation.
K.Rajkumar50
The risk of failure of the insulation is dependant on the integral of the
product of the overvoltage density function f0(V) and the probability of
insulation failure P(V).
Thus the risk of flashover per switching operation is equal to the area under
the curve
Since we cannot find suitable insulation such that the withstand
distribution does not overlap with the overvoltage distribution, in the
statistical method of analysis, the insulation is selected such that the 2%
overvoltage probability coincides with the 90% withstand probability as
shown.
  dV.P(V)(V)f0

Surge Arresters :Modern Surge arresters are of the gapless Zinc Oxide type.
Previously, Silicon Carbide arresters were used, but their use has been
superceeded by the ZnO arresters, which have a non-linear resistance
characteristic. Thus, it is possible to eliminate the series gaps between the
individual ZnO block making up the arrester.
Selection Procedure for Surge arresters:
1. Determine the continuous arrester voltage. This is usually the system rated
voltage.
2. Select a rated voltage for the arrester.
3. Determine the normal lightning discharge current. Below 36kV, 5kA rated
arresters are chosen. Otherwise, a 10kA rated arrester is used.
4. Determine the required long duration discharge capability.
For rated voltage < 36kV, light duty surge arrester may be specified.
For rated voltage between 36kV and 245kV, heavy duty arresters may be
specified.
For rated voltage >245kV, long duration discharge capabilities may be specified.
51

5. Determine the maximum prospective fault current and protection tripping
times at the location of the surge arrester
and match with the surge arrester duty.
6. Select the surge arrester having porcelain creepage distance in accordance
with the environmental conditions.
7. Determine the surge arrester protection level and match with standard IEC
99 recommendations.
52

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