Energy and Power Engineering, 2013, 5, 483-487
doi:10.4236/epe.2013.54B093 Published Online July 2013 (http://www.scirp.org/journal/epe)
The Analysis of Bilateral Power Supply for
AC Electrified Railway
Zhengqing Han, Zhen Fang, Changqing Li, Dong Li
Southwest Jiaotong University, Chengdu, China
Received March, 2013
ABSTRACT
In China, the unilateral power supply has been used in AC electrified railway, which depends on the management mode
of power system. For better economic and technical indexes, all-parallel AT traction system is widely used in passenger
dedicated line. In this paper, the equivalent circuit model of unilateral power supply and bilateral power supply is inves-
tigated, and based on this model, the difference between unilateral power supply impedance and bilateral power supply
impedance is discussed.
Keywords: All-parallel AT Traction System; Bilateral Power Supply; System Analysis; Unilateral Power Supply
1. Introduction
As the structure of the power supply and the management
mode is different among different countries, the applica-
tion of power supply or bilateral power supply mainly
depends on the conditions of each country. The electri-
fied railway of former Soviet Union adopts the bilateral
power supply, and some European countries also adopt
the bilateral power supply. However, with the collapse of
the Soviet Union, the study on bilateral power supply has
been decreased [1]. In China, the unilateral power supply
is widely used in AC electrified railway due to the re-
strictions of the power supply system’s management
mode.
In this paper, based on the unilateral all-parallel AT
traction system, the connection mode of the section post
is discussed and the model of the bilateral all-parallel AT
traction system is built. Then based on the model, the
short circuit impedance is analyzed. Using the actual data
of the passenger transport line, the short circuit imped-
ance of the unilateral power supply and the bilateral
power supply are calculated. Finally, the impedances of
the two systems are compared.
2. The Principles of Unilateral Power Supply
and Bilateral Power Supply
Unilateral power supply is that the catenaries gain elec-
trical power from one traction substation on one side, an
electrical sectioning at the exit of traction substation and
two adjacent supply arms. When an electrical train is in
operation, it only can get electrical from one traction
substation.
For bilateral power supply, the catenaries gain electri-
cal power from two adjacent traction substations exists
on both sides. When the train passes, it can get electricity
from two traction substations, but the relay protection is
more complicated [2].
The theoretical analysis of the unilateral and bilateral
all-parallel AT traction systems is based on the hypothe-
sis as follows. The auto-transformer is considered as an
ideal transformer, which ignores the leakage reactance
and field current. The steel rail is considered to be insu-
lated to the ground, and the common reactance between
two lines is ignored. The theoretical derivation can be
simplified, and based on the assumption, the influence of
the rail leakage reactance and AT leakage in the electri-
cal calculation can be removed [3].
3. The Theoretical Analysis of the Unilate ral
Power Supply
3.1. The Model of Traction Power Supply System
As shown in Figure 1, the traction power supply system
is composed of traction transformers, traction network
and section post. The traction power supply system
transforms the three-phase voltage of 220 kV into 27.5
kV single-phase voltages. The substation connects the
power system by exchanging phase connections, which
makes the adjacent power supply arm the same voltage
phase and is good for the bilateral power supply.
3.2. The Theoretical Analysis of Unilateral Pow-
er Supply Impedance
From reference [1], the current distribution of unilateral

Z. Q. HAN ET AL.
484
all-parallel AT traction system is shown in Figure 2,
where A represents the distance between the train and the
traction substation; D represents the length of AT section;
and X represents the distance between the train and the
nearest AT traction substation.
For analyzing the impedance of the unilateral power
supply, when T-R short circuit occurs, so the equivalent
circuit diagram is given in Figure 3.
When the T-R short circuit occurs at distance A, and
the equation can be listed as follows:
Figure 1. The struct ur e of the traction power supply sy stem.
1
4
I

1
4I

1
4I

1
4I

I

A
D
X
11T
I

12T
I

11
R
I

12
R
I

1F
I

2T
I

2
R
I

2
F
I

1
T
1
R
1
F
2
T
2
R
2
F
L
U

55kV
'
1
U

'
2
U

'
1
U

'
2
U

1
U

2
U

1
U

2
U

A
Figure 2. The current distribution of the unilateral power
supply.
2
D
Z
3
DZ
3
DZ
2
()
D
XZ 2
X
Z
1
DZ
I

1
X
Z
1
()
D
XZ
D
A
L
U

27.5kV
Figure 3. The equivalent circuit diagram of the unilateral
all-parallel AT traction system.
12
34
11 21
23
3242 4
2
111
23
23
23
(2 )
() 2
1()
2
()
2( )
II I
II I
IXZIDXZ
3
Z
Z
IXZI DXZID
Z
Z
UAXZIXZI
ZZ
XZ IAXI
ZZ





 



 





 
 

 


(1)
As the traction electric network is transformed to the
27.5 kV side, the voltage drop of the traction network is
the voltage difference between the power source and the
train.
1
2
j
UEU


 (2)
where is the 55 kV supply voltage, E

j
U is the volt-
age at the train.
The impedance of traction electric network is equal to
the ratio between the voltage and the line current. On the
occasion of the T-R short circuit,, so the im-
pedance of traction electric network can be deduced.
UU
1223 13
23
2
1223 132
23
()
2( )
()( 2
2( )
AZZZZ ZZ U
ZZZ
I
)
X
DXZZZZZZZ
DZ Z

 





(3)
The T-R short-circuit impedance at the exit of the trac-
tion substation A can be expressed as follows:
1223 13
23
2
1223 132
23
() 24[ 2( )
2
()( 2
]
2( )
A
AZZ ZZZZ U
ZZZI
XDXZZZZZZZ
ZZD

 




)
(4)
1223 13
23
(
2( )
AA
)
Z
ZZZZZ
ZZZ

 (5)
2
1223 132
23
(2
2( )
BB
)
Z
ZZZZZ Z
ZZZ

 (6)
Under the assumptions (5) and (6), the traction electric
network impedance of 27.5 kV side can be simplified.
(1 )
AA BB
X
Z
AZ XZ
D
 (7)
4. The Analysis of the Bilateral All-parallel
at Traction System
4.1. The Bilateral Power Supply Mode
There are three supply modes in multiline sections of

Z. Q. HAN ET AL. 485
bilateral power supply mode: separate power supply,
parallel power supply, and knot power supply.
The analysis shows that the parallel power supply
shown in Figure 4, can effectively reduce traction net-
work impedance and voltage loss, which helps balance
the traction network load. In addition, the parallel power
supply is beneficial to the realization of the relay protec-
tion. However, because of the transverse connection, the
traction network becomes complicated, which increases
the difficulty of fault isolation.
Nowadays, AT power supply mode and direct power
supply are used worldwide in high-speed railways with
backflow. In China, AT power supply mode is com-
monly applied in high speed railway. AT power supply
mode has great technical advantages. Therefore, AT sup-
ply system and all parallel AT supply system are ana-
lyzed and discussed in this paper.
4.2. The Main Electrical Connection of Bilateral
Power Traction Transformer
The realization of bilateral power supply needs some
conditions, for example, two adjacent power supply arms
of two adjacent substations must have same phase. In
order to reduce the influence of the negative-sequence
current on three-phase power system, the substations
adopt phase conversion connection in turn. Therefore, the
traction transformer connection mode must satisfy that
the secondary side has two voltages of different se-
quences and phase. At present, China’s high speed rail-
way traction power supply system widely adopted V/x
transformer, which makes bilateral power supply mode
feasible.
4.3. Bilateral Power Supply Mode
Based on the analysis above, the bilateral power supply
mode is built as in Figure 5, where the main transformer
adopts V/x wiring connection, and the traction network
adopts the AT power supply system, while the bilateral
all-parallel AT traction system is applied in multiple tracks.
4.4. The Short Circuit Impedance of Bilateral
Power Supply
From the bilateral power supply mode, the equivalent
circuit diagram of the bilateral all-parallel AT traction
system can be gained as in Figure 6.
Figure 4. Parallel power supply mode.
U

27.5kV
A
U

27.5kV
B
2
D
Z
3
D
Z
3
D
Z
2
()
D
XZ 2
XZ
1
D
Z
1
XZ
1
()
D
XZ
D
A
L
I

Figure 5. Bilateral power supply mode .
U

27.5kV
A
U

27.5k
V
B
2
D
Z
3
D
Z
3
D
Z
2
()
D
XZ

2
XZ
1
D
Z
1
XZ
1
()
D
XZ

D
A
L
I

Figure 6. The equivalent circuit diagram of bilateral all-
parallel AT traction system.
When the T-R short circuit occurs at A, the equation
can be listed as follows.
23 23 11
23 23
223 1
23
11
23 23
23 23
223 1
() ()
[]
222()2(
(2)( ) ()
[]
22()
()()
[22
() ()
]
2( )2( )
(2 )( ()
[2
a
b
ZZ A XXZZ ZA XXZ
UI )
Z
ZZZ
ZX ZZD X XZ DX
IDDZZ
ZLA DXZDX
UI
ZZ DXZZ LADX
ZZ ZZ
ZXZZD X XZ DX
ID








 











23
)]
2( )
ab
DZ Z
II I
















 
(8)
The T-R short impedance at the exit of substation B
can be expressed as follows:
1223 13
23
2
1223 132
23
()( )
4[ 2( )
()( 2
]
2( )
b
LA ZZZZZZ
ZZZ
LXDXZZZZZZZ
AD ZZ




)
(9)
The traction electric network impedance of 27.5 kV
side can be simplified:

Z. Q. HAN ET AL.
486
the unilateral and bilateral impedances of traction net-
work can be obtained as below:
1223 13
23
2
1223 132
23
(-)( )
2( )
(-)(2 )
2( )
ALA ZZZZZZ U
ZLZ Z
I
X
D XZZZZZZZ
DZ Z

 




(10) Figure 7 shows that the figure of the unilateral and bi-
lateral traction network impedance appears in the shape
of a saddle, and the impedances of the unilateral and bi-
lateral traction network change with distance at the same
time. However, the bilateral traction network impedance
is always smaller than unilateral traction network im-
pedance. Traction power supply reach can be increased
when the lowest voltage requirement is met, which can
decrease substations and offer better conditions for ad-
dress of 220 kV power system incoming lines, mean-
while, power loss is reduced.
Using the equation (5) and (6), the traction electric
network impedance of 27.5kV side can be simplified as
below:
(1 -)(1 -)
AA bb
AX
Z
AZX Z
LD
 (11)
The impedance of the two kinds of power supply
modes has been deduced in part 2.1 and 2.2. It is obvious
that the impedance of unilateral power supply is bigger
than that of bilateral power supply, and the difference
between them is
Z
.
2
AA
A
Z
Z
L
 (12)
5. The Analysis and Comparison of Traction
Network Impedance Based on the
Unilateral or Bilateral Full Parallel at
Power
The parameters of suspension type of the traction net-
work and wires are shown below, which can be found in
a passenger special line.
According to Table 1 and Tabl e 2, and the relation of
the conductor configuration, the traction network im-
pedance calculation procedure in reference [4] is used,
and the parameters are shown in Table 3.
Figure 7. The comparison of the unilateral and bilateral
traction network impedanc e.
Table 1. The suspension type parameters of traction network.
Assuming that the distance between the two adjacent
traction substations is 60 kM (L = 60 kM) and the length
of each AT section is 15 kM (D =15 kM). By transform-
ing the impedance of traction network to the locomotive
side,
NameCatenaryMessenger wire Positive feeder Rail
LineCTMH-150JTMH-120 LGJ-240 P60
Table 2. The basic parameters of wires.
Name Resistance (Ω/km) Section calculation (mm2) Radius calculation (mm) Equivalent radius (mm)
CTMH-150 0.1852 151 7.2 5.616
JTMH-120 0.242 116.99 7 5.306
LGJ-240 0.1136 275.96 9.372 7.104
P60 0.135 77.03 109.1 12.79
Table 3. IMPEDANCE.
111222
1
T R F T R F
T 0.1562 0.5870i 0.0500 0.3110i 0.0500 
1
1
0.3438i 0.0500 0.3331i 0.0500 0.2988i 0.0500 0.2955i
R 0.0500 0.3110i 0.1175 0.5563i 0.0500 0.3032i 0.0500 0.2988i 0.0500 0.3359i 0.0500 0.2814i
F 0.0500 0.3438i 0.0500



2
2
0.3032i 0.1636 0.7420i 0.0500 0.2955i 0.0500 0.2814i 0.0500 0.2719i
T 0.0500 0.3331i 0.0500 0.2988i 0.0500 0.2955i 0.1562 0.5870i 0.0500 0.3110i 0.0500 0.3438i
R 0.0500


2
0.2988i 0.0500 0.3359i 0.0500 0.2814i 0.0500 0.3110i 0.1175 0.5563i 0.0500 0.3032i
F 0.0500 0.2955i 0.0500 0.2814i 0.0500 0.2719i 0.0500 0.3438i 0.0500 0.3032i 0.1636

 0.7420i
 
 
 
 
 
 
 
 
 
 



Z. Q. HAN ET AL. 487
6. Conclusions
Based on the analysis of the unilateral and bilateral trac-
tion power supply model, the bilateral traction network
impedance is smaller than the unilateral traction network
impedance, so the loss of voltage and the power loss
caused by the train will be reduced. With the bilateral
traction power supply, the investment of the auto-passion
phase separations can be reduced, and the capacity and
the reliability of the power supply system can be im-
proved. As the bilateral power supply has such advan-
tages, it will be widely applied in passenger dedicated
lines in China.
REFERENCES
[1] D. Li, “Study on Protective Schemes of Bilateral Traction
Power Supply Systems for High Speed Railways,” 2011,
pp. 34-38.
[2] Z. L. Li, W. Chen and P. Dang, “The Principle Analysis
of Auto-transformer in Electrified Railway,” Journal of
East China Jiaotong University, 1993, pp. 48-53.
[3] X. B. Tan, “AC Electrified Railway Tractive Power Sup-
ply System,” Southwest Jiongtong University Press, 2006,
pp. 44-45.
[4] Ministry of Railways Survey and Design Institute, “A
Handbook of Electrified Railway Design,” China Railway
Publishing House, 1988, pp. 19-25.