Study of power flow and transmission capacity in multi phase transmission lines

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 10, October (2014), pp. 43-55 © IAEME
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 10, October (2014), pp. 43-55
© IAEME: www.iaeme.com/IJEET.asp
Journal Impact Factor (2014): 6.8310 (Calculated by GISI)
www.jifactor.com
IJEET
© I A E M E
STUDY OF POWER FLOW AND TRANSMISSION
CAPACITY IN MULTI PHASE TRANSMISSION LINES
Vinay Kumar Tripathi, Anil Kumar Bharadwaj
Department of Electrical Engineering, SHIATS, Allahabad
43
ABSTRACT
There are many problems regarding power flow and stability is an issue of vital importance at
EHV (Extra High Voltage) and UHV (Ultra High Voltage) level because of its sensitivity with real
and reactive power changes. An investigation of multiphase system has been carried out in this paper
by extending the techniques of three-phase systems to multi-phase lines and construction of
performance characteristic curves relating to power flow and voltage stability performance of such
system. A comparison of multiphase phase systems is presented in the paper that can be used for
planning, development and design of multiphase transmission network
Keywords: Transmission Lines, Rights-of-Way, Multi-Phase, Power Flow.
I. INTRODUCTION
The contemporary power system network has of load and power flow conditions leading to
problems of line capacity. In this regard multiphase power transmission systems have been
investigated [1]- [9] as potential alternative to increase transmission capacity without increasing
system voltages. The system voltage has already reached extremely high level i.e. EHV and UHV
and it has been found that 4-phase and 12-phase are quite promising. The problems concerning
power flow and stability, particularly voltage stability are of particular interest because of its
sensitivity with changes in real and reactive power. Voltage stability is obtained by keeping specified
voltage magnitude within the set of operating limits under steady state conditions. The problem has
been studied to a great extent in case of three-phase systems [10]-[19]. However the multiphase
system has received little attention and needs similar study. This paper deals with investigation of
these aspects by using techniques of three-phase systems to multi-phase lines. A number of
performance characteristic curves have been obtained relating to power flow and voltage stability of
such system. Considering the multi-phase line as longitudinal or radial or the one linking to systems
on either end, several performance characteristics such as, (i) reactive power requirements
characteristics, (ii) reactive power loss characteristics, (iii) voltage power characteristics (under

loading conditions), (iv) load end real and reactive power operating contour maps, (v) shunt
capacitive support at voltage stability limit using characteristic of voltage dependant load, and (vi)
optimal reactive power at voltage stability limit, loadability curves can be constructed as done in
[22] Employing a sample system a quantitative as well as qualitative analysis is carried out to
highlight relative performances of such systems as compared to their three phase counterparts. Such
performance curves would be highly helpful in planning and evaluating performance of multi-phase.
The paper is organized as follows: Various performance characteristics for multiphase transmission
system have been derived and graphically analyzed in section III to compare the advantages and
limitations of multiphase system with conventional three phase system. The analysis of reactive
power requirement for voltage stability of multiphase and three phase system has been carried out in
section IV. Loadabilty analysis of multiphase system with compensator has been reported in
section V.
II. BASIC CIRCUIT MODEL AND SAMPLE SYSTEM
The basic circuit model with lumped parameters adopted for the appropriate characteristics
analysis of un-compensated EHV power transmission system is represented on single phase
equivalent basis as shown in Fig. 1. Let EA and IA represent the voltage and current at sending end
whereas EB and IB are the voltage and currents at receiving end side. YT and YSH in Fig. 1
represent equivalent series and shunt admittance of an equivalent pi-model of the transmission line.
Figure (1): basic circuit model
For performance evolution three transmission alternatives (3, 4 and 12-phase) with same
number of conductors, same right of way and utilization of the same air space for power transmission
are considered. Each system is energized at 460 kV line-to ground (L-G) voltages and has the same
thermal rating. The line-parameters are based on assumptions of complete transposition of
conductors are depicted in Table 1.
TABLE I: TEST SYSTEM SPECIFICATION LINE PARAMETERS
44
III. PERFORMANCE CHARACTERISTICS
A. Load End Real and Reactive Power Operating Contour Maps
With reference to Fig.1, load end real and reactive power operating contour maps [14] can be
described by the following expression.
| | | | | * 2 *
B BB B BB B S −Y E = eY E (1)

The above equation is an equation of a circle with | | * 2
45
BB B Y E as centre and | | * *
BB B e Y E as its
radius. Thus all states having the constant amplitudes of e (=EAYBA/YBB) lie on the circles with these
parameters on the S-plane (Fig. 2). Each circle (Fig. 2) represents the locus of |SB|,the receiving end
complex power, for a stable value of EB (receiving end voltage), varying within the range of 0.94 p.
u. to 1.0 p. u. that assuming sending end voltage EA to be constant at 1.0 p.u., the load end real and
reactive power operating contour maps, for 3, 4 and 12-phase are constructed for 460 kV and 200 km
transmission lines, on p.u. basis, are shown in Fig. 2. It is evident from the curves in Fig. 2 that the
power handling capacity of multi-phase (employing more than 3-phase) transmission system is
higher for specified sending end and receiving end voltages while maintaining the similar nature of
variation.
Fig.2: Real reactive power operating contours with same receiving end voltages (EB = 0.94 p. u.)
Furthermore, Figure 3 (a-c) confirm enhancement of power handling capacity for 3, 4 and 12-
phase transmission system with 4% increase of receiving end voltage EB1 = 0.94, EB2 = 0.95 and
EB3 = 1.0 p.u. maintaining sending end voltage (EA) as it is i.e. EA = 1.0 p.u. It is observed that
reactive and real power handling capacity for 3-phase, 4-phase and 12-phase systems are increased
by 10.80 %, 7.30 %, 6.70 % and 4 %, 3.340% %, 3.380 % respectively. As supplement, it can be
seen from Figs. 3(a-c) that increment in real and reactive power capacity for 3-phase, 4-phase and
12-phase takes place with an increase of 9 % in EB (within voltage stable zone) keeping EA constant
at its previous value i.e. EA=1.0 p.u.. It is observed that the corresponding percentage increase of in
real and reactive power capacity of 3-phase, 4-phase and 12-phase are found to be 9, 8.66, 8.63 and
16.31, 12.77 and 14.30 respectively.
Fig. 3 (a): Real reactive power contours of a 3-phase line with EB1 = 0.94, E2 = 0.95 and
EB3 = 1.0 p.u

Fig.3 (b): Real reactive power contour maps of a 4-phase line with EB1 = 0.94, EB2 = 0.95 and
EB3 = 1.0 p.u
Fig.3 (c): Real reactive power contour maps of a 12-phase line with EB1 = 0.94, EB2 = 0.95 and
EB3 = 1.0 p.u
It is clear from the above contour maps that the reactive power transfer capacity increases
46
with the increase of number of phases.
B. Reactive Power Loss Characteristic
The reactive losses (QSL) in an EHV power transmission line are given by
= X
(2)
=
Q E I
SL B B
E
Q I X
B
SL
2
2 0 ( )

If a constant power (QSL= EBIB), is assumed that the rate of change of series reactive power
losses with voltage is obtained from (2) as:
Q X
dQ −
SL SL
3
2
0 2( )
B
E
dV
= (3)

So, the rate of change of reactive power loss with respect to receiving end voltage is inversely
proportional of the cube of receiving end voltage. Further an increment in series reactive power
losses (dQSL) would be possible with a decline in the transmission voltage in order of EB
47
3.
Employing (3) curves of (dQSL /dEB) versus EB (p.u.) are drawn for the sample cases of 3, 4 and 12-
phase lines and depicted in fig. 4 for a length of 400 km.
dQ
SL
B
dE
Fig. 4: Reactive power loss characteristics of three transmission alternatives
From the curves in Fig. 4, it is noticed that the series reactive power losses are very small the
losses are smaller in multi-phase transmission lines than that in its three-phase counterpart. This
implies that the voltage stability limit of multi-phase transmission systems is be increased, making
multi-phase systems more secured from the voltage stability view point. Equation (2) can further be
written as
E I
1

) ( =
Q E I B B
= (4)
SIL
E X
B
SL B A
2
2 ( )
/

where, SIL is the surge impedance loading and is given by EB
2 /X. Based on (4) the results are shown
in the plots of QSL versus SIL vide Fig. 5 for a length of 400 km.
Fig. 5: Profile of series reactive power losses (QSL) n verses surge impedance loading (SIL)
Also from Fig.5 it is seen that the reactive power losses smaller in multi-phase lines. In view
of this, multi-phase systems (employing more phases than three) are more secure from voltage
stability view point.

48
C. Voltage- Power Characteristics
If a power transmission line is assumed to be lossless, then the sending and receiving end
voltages can be expressed as follow [17]-[22]:

A B C cosq sinq (5)

−

E E jZ B B

= +
P JQ
B E
Equation (5) on simplification becomes
( 2 (sin 2 )) 2 0.5
A A C B
q
q
2cos
B
E E Z P
E
± −
= (6)
Where ZC is characteristic impedance and is electrical line length expressed as 0.640 ohms/
km. The expression (6) indicates that if sending end voltage is fixed (EA=1.0 p.u.), the receiving end
voltage varies with load power factor and line length. using (6), the PB-EB curves are constructed in
Figs. 6(a-c) for three alternative transmission systems i.e. 3, 4 and 12-phases assuming line length
400 Km. These figures represent the plots between receiving end voltage and power under specified
operating conditions. It is seen that the maximum receiving end power is found largest for 12-phase
power transmission system.
So, with increased number of phase, maximum power at the receiving end will be
progressively enhanced maintaining the voltage stability at various power factors of the load. The
above statement is also true for specified load power factor and varying the length of transmission
systems, which is evident from Figs. 7 (a -b).
Fig. 6 (a): Receiving end voltage profile for varying power flow at p.f .0.94lag for a 400 km line
Fig. 6 (b): Receiving end voltage profile for varying power flow at p.f .u. for a 400 km line

Fig. 6(c): Receiving end voltage profile for varying power flow at p.f. 0.94 lead for a 400 km line
Fig. 7 (a): Relationship between receiving end voltage and power at u.p.f and line length of 200 km
Fig.7 (b): Relationship between receiving end voltage and power at u.p.f and line length of 400 km
IV. ANALYSES OF REACTIVE POWER REQUIREMENTS
Assuming EA leads EB by an angel , EA can be written as –
49
d d d E E (cos j sin A
j
A = + ) (7)

Substituting (7) in (5) and equating real and imaginary parts of the resulting equation, the
relationship between receiving end reactive power and voltages can be written as
50
d q
sin
( cos cos )
E E E
B B B
q
C
Q
B Z
−
= (8)
Similarly,
( cosd cosq ) / sinq A A B A C Q = −E E − E Z (9)
If EA=EB then
(cosd cosq ) / sinq 2
B A A C Q = −Q = E − Z (10)
For lossless line, cos=1, (10) reduces to
(1 cosd ) / sinq 2 Q E Zc B A = − (11)
where is the transmission angle, ZC is the characteristic impedance and is electrical line
length expressed as 0.0012 rad./ km. Employing (11) the terminal reactive power curves as a
function of power transmitted for different line length are constructed in Figs. 8(a-b)
Fig. 8 (a): Terminal reactive power as a function of Power transmitted for 200 km lines
Fig. 8 (b): Terminal reactive power as a function of power transmitted for 400 km lines.

Figs. 8 (a-b) represent the curves between per unit reactive power and per unit active power
for several lengths of 3- phase, 4-phase and 12-phase transmission systems respectively. From
examination of these curves, the following observations and conclusions can be derived. When PB
P0 (SIL), there is obviously an excess of line charging found. As a result, QA is found negative and
QB is obtained positive. This clearly indicates that the reactive power is being absorbed by the
system at both ends. Therefore a reactive inductive) compensation is essential for maintaining the
voltage stability. While PB P0, reactive power is being generated at both ends. Obviously lagging
power factor operation is inevitable. Hence, capacitive compensation is needed for maintaining the
voltage stability of the system. Now from the above comparative analysis of the Fig. 8 it is clearly
evident that the reactive power requirement to maintain stable voltage varying load is least for
highest multi-phase system (12-phase). As the phase order increases, the requirement of reactive
power is reduced gradually for different transmission line lengths. This also implies that the rating of
the compensating devices for multiphase system will be lower.
A. Optimal Reactive Power at Voltage Stability Limit case.
Following [22], for lossless line, the power flow equations may be written as-sin
51
P = A B
d
(12a)
B E E
X
E
Q A B B
B = cosd − (12b)
X
E E
X
The determinant of the Jacobian of load flow equations in a loss less network is given by [19]
1 2 d EAEB EB EA
X
[ ] ( 2 cos )
D J = − + (13)
with [J ] = 0 at voltage stability limit, the receiving end voltage can be written as
2 cosd A B E = E (14)
Using (14) in (12b), the value of reactive power at the limiting stage of voltage stability is given by
Q = B cos 2d
(15)
2
lim
E
X
where, Qlim represents the limiting value of reactive power transfer in a transmission system and it is
critical value for voltage breakdown.
Using (15), the plot of Qlim with respect to EB has been illustrated through Fig. 9. This figure
(Fig. 9) indicates that the reactive power limit for maintaining the voltage stability at different
receiving end voltages for transmission line of length of 400 Km with the range of variation from 1.0
pu. to 0.90 p.u. is further observed that the reactive power limit is gradually reducing as the phase-order
is increasing from 3-phase to 12-phase at each and every point of per unit receiving end
voltage within the range from 1.0 p.u. to 9.0 p.u. Under these circumstances, with increase of number
of phases the rating of the devices required from the point of view of compensation is gradually
diminished. This also implies that the even in uncompensated mode, the higher phase order system

performs well from the voltage stability view point since the requirement of reactive power is
gradually reduced.
Fig. 9: Reactive power limit profile for various systems to have voltage stability for different
receiving end voltages
52
B. Shunt Compensation for Voltage Stability
Shunt capacity support at voltage stability limit using properties of voltage dependent load
[14] is illustrated by the following equation
| |
| E || Y
|
Y −
−
| | sec 2 | |
| |
1
0
T
B
m
B
A BA
SH Y
mP E E
=
− f
(16)
where PO is the power at rated voltage EB and m is the load characteristics for the voltage dependant
loads. Equation (16) provides a relationship for the minimum of SH at load end for getting a stable
voltage state under set of specified operating conditions. Figs. 10 (a) and (b) exhibit the magnitude of
the minimum receiving end bus admittance at different per unit voltage for different power factors in
case of multi-phase systems. The above cases are shown for m = 0.4 and m =1.0 It is observed that
enhancement of shunt capacitive support is required to improve the receiving end voltage magnitude
from the point of view of the stability of the system voltage in case of voltage dependent load (i. e. m
1). Also the requirement of shunt capacitive support rating for voltage dependent load is gradually
reduced as the phase-order of the system is increased from 3 to 12.
Fig.10 (a): Profile of minimum magnitude of load end bus admittance at different voltages with
0.94 p.f. lagging at voltage stable states

Fig.10 (b): profile of minimum magnitude of load end admittance at different voltage with u.p.f. at
voltage stable state
The same analysis has been carried out for variation of length of transmission line and it has
been given in Fig. 11. Figs. 11 (a) and (b) represent the curves - shunt admittance (YSH= =Y'BB)
versus length (L) for different power factors (0.94 lagging unity) at m = 0.5 and m=1 It can be
seen concluded that with increase of number of phases the value of the shunt capacitive rating is
reduced for voltage dependent load at voltage stability limit
Fig. 11(a): Profile of load end admittance for varying line length with p.f.0.94 lag at voltage stability
limit
Fig. 11 (b): Profile of load end admittance for varying line length with u.p.f. at voltage stability limit
53

54
VI. CONCLUSION
The multi-phase lines (4-phase and 12-phase) show progressively increased power handling
capacity, reduced reactive power losses, increased power at the receiving end, reduced reactive
power requirement for maintaining stable load voltage, reduced rating of compensating devices,
better voltage stability in case of voltage dependent load and increased line loadability in
uncompensated as well as compensated condition the phase order is increased the multi-phase lines
may be very attractive and beneficial to electric transmission utility.
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