Energy and Power Engineering, 2013, 5, 522-526
doi:10.4236/epe.2013.54B100 Published Online July 2013 (http://www.scirp.org/journal/epe)
Research on Co-phase Power Supply Test System
Yuanzhe Zhao, Qunzhan Li, Yankun Xia, Zeliang Shu
School of Electrical Engineering, Southwest Jiaotong University, Chengdu, China
Received February, 2013
ABSTRACT
Co-phase power supply system can solve the problems of power quality of heavy unbalanced three phase, large har-
monics and reactive power and cancel neutral section in electric railway power supply system. In order to do further
research, a co-phase power supply test system is proposed. By mean of analyzing on structures and principles of YNvd
transformer, integrated power flow controller (IPFC) and simulation load, establishing control strategy on IPFC and
simulation load, the system is simulated dynamically. The results illustrate that the scheme can well simulate co-phase
system, and the negative sequence is eliminated, harmonic and reactive power are real-timely compensated in system.
Keywords: Co-phase Supply System; IPFC; YNvd Balanced Transformor; Simulation Load
1. Introduction
Reactive power current, harmonics and unbalanced ac-
tive power current are the outstanding problems in tradi-
tional traction supply system [1]. These problems di-
rectly influence the three-phase industrial grid through
traction substations. With the rapid development of high-
speed and heavy-loading railway, these problems are
gradual prominence, and the neutral section also restricts
the speedpromotion of high-speed train, which influence
the safety, reliability and economy of railway operation.
As locomotives based on PWM converter are widely
used, the distortion of reactive power and harmonics is
decreased partially [2], but the unbalance becomes more
significant than before. The co-phase system can solve
unbalance problem, at the same time, compensate reac-
tive power currents and filter harmonics. And the
co-phase power supply system based on passive symmet-
rical compensation is proposed [2, 3]. With the wide ap-
plication of power electronic devices in railway system,
the co-phase scheme based on active power compensator
which is called IPFC is designed [4, 5]. Using real-time
detection, strategy control, and distribution algorithms on
IPFC, this system can compensate harmonics, reactive
power and negative sequence current real-timely and
accurately.
As shown in Figure 1, In co-phase system, the bal-
anced transformer transformers power from three-phase
of public supply grid to two-phase. (YNvd balanced
transformer is proposed and used in system.) One phase
is connected directly with feeders to supply electric lo-
comotives, and another is connected with phase a by
IPFC, which can transfer active power from β-phase to
α-phase, compensate the loads’ reactive power and filter
the harmonic. So there is single-phase power in supply
area of one traction substation (SS), so that the neutral
section in the substation can be cancelled.
2. Co-phase Power Supply Test System
There are many specific details of co-phase system
needing to be studied, but it is impossible to do lots of
project tests in the railway system. In order to further
research co-phase system, a co-phase system test scheme
is proposed which could be operated and tested in the
public grid of laboratory. As illustrated in Figure 2, test
scheme is composition of YNvd balanced transformer,
IPFC and simulated load. YNvd transformer simulates
traction transformer, simulated load simulates the char-
acteristics of the traction load, IPFC realizes power
transmission and compensation dynamically.
i

L
i
A
B
C


i

L
i


i

i

Figure 1. Co-phase pow er supply system.

Y. Z. ZHAO ET AL. 523
2.1. YNvd Balanced Transformer
YNvd balanced transformer is new kind of three-phase to
two-phase balance transformer, which adopts three-phase
three-column iron core and has the characteristics of high
utilization of iron core, simple structure, easy manufac-
turing and maintenance; the primary side has neutral
point and can be grounded directly, so the reliability of
system protection can be improved. YNvd transformer
connection type is shown in Figure 3. The primary side
of YNvd transformer connects the grid utility, the wind-
ings ωA, ωB and ωB are connected in Y-form. The secon-
dary side composes two phases, ωa1 and ωc1 compose
α-phase in V-form connection which supplies the traction
load, and ωa2, ωb2 and ωc2 compose β-phase in Delta-
form connection which only connects IPFC as shown in
Figure 3.
The relationship of the windings of YNvd transformer
is given by
11 2 2
::: : :::
11
::: : :1:1:1
33
ABCa c abc
KKK
2

 
 (1)
where K is the turn ratio of primary/secondary voltage.
u

i

i

c
i

L
i
A
B
C
u

Figure 2. Co-phase system test scheme Co-phase Power
Supply System.
A
B
C
c1
A

B

C

a1
C
I

B
I

A
I

1a

1c

2a

2c

2b

I

I

I

2
3I

1
3I

a2
c2
b2
1
3I

Figure 3. YNvd transformer connection schemes.
So the voltages of primary and secondary windings
satisfy the formula:
11
0
133
12 1
333
A
B
C
U
UU
K UU






 
 
 

 
 








(2)
So that two output voltages of secondary side are mu-
tual perpendicular and has same amplitude.
Current relations between the primary side and secon-
dary side of YNvd transformer is as follow
11
3
3
12
03
11
3
3
A
B
C
I
I
IK
I
I





 

 

 

 

 








(3)
According to the symmetrical component principle,
the primary side currents can be decomposed as
0
1
2
00
1313
6
313
I
I
Ijj
K
I
Ijj


 

 


 

 









(4)
If
I
jI




, the 2
I

is equal to zero, and the negative
sequence of the primary side is eliminated.
2.2. IPFC
As shown in Figure 4, IPFC is a back-to-back single-
phase converter which is composed of two fully-bridge
converters with one DC-link capacitor, two input AC
inductors. The AC ports are respectively connected with
α phase and β phase of YNvd transformer. In real system,
the output voltage of traction transformer is too high so
that isolated step-down transformers must be placed be-
tween the converter and traction transformer. In test sys-
tem, the step-down transformers are canceled because of
small output voltage.
The voltages of secondary windings of YNvd Trans-
former is assumed as
() sin
( )sin(90 )
ut Ut
ut Ut









 (5)
3
S
8
S
5
S
7
S
1
S
4
S
2
S
6
S
L

L

U


D
C
U
C
I


C
I


I


L
I

U


Figure 4. Structure of IPFC.

Y. Z. ZHAO ET AL.
524
where U represents the amplitude of voltages of secon-
dary windings.
The traction load current is discussed and written as:
111
11
() () ()sin() ()
sincos( )
Lh
pqh
itit itItit
ItItit
h


 
 (6)
where i1 is the fundamental current, ih is harmonic current.
Ip, Iq represent amplitudes of active power, reactive pow-
er, φ1 is the phase difference between i1 and uα.
In co-phase traction power supply system, the α-phase
of IPFC outputs half active power and compensates the
all reactive power and harmonic currents of traction load,
the b-phase of IPFC inputs half active power of traction
load, and active power is transfer between two ports by
DC-link capacitor. The compensated currents of IPFC
are expected as:
11
1
1sin cos
2
1sin(90)
2
p
qh
c
c
p
I
tI ti
i
iIt










 





(7)
The output currents of secondary side of YNvd trans-
former is obtained as
1
1
1sin
2
1
0sin(90 )
2
p
sc
L
sc
p
It
ii
i
ii
It






 

  
 
 

 







(8)
Taking (8) into (3), it yields
11
11
11
11
3
31cos sin
12
2
01
3cossin(90 )
11
2
3
3
sin(30 )
cos sin(90)
3sin(150 )
A
B
C
IIt
IKIt
I
t
It
Kt











 

 

 

  

 

















(9)
Obviously, the two output currents of YNvd trans-
former are same amplitude and in-phase with respective
port voltage in (8), and the three-phase currents of pri-
mary side are also same amplitude in (9). So negative
sequence, harmonics and reactive power can be thor-
oughly compensated by IPFC and YNvd transformer in
co-phase traction power supply system.
2.3. Simulated Load
Simulated load is composition of traction current simula-
tion unit and energy feedback unit, which are back-to-
back connected with dc capacitor. The traction current
simulation unit (TCSU) as traction load absorbs power
from α phase of YNvd transformer. Energy feedback unit
feedbacks active power to public grid (EFU). So that the
active power is transferred from single-phase through dc
link to three-phase.
As shown in Figure 5, the main circuit is composed of
input inductor, dc capacitor, output inductor. Converter
unit and inverter unit exchange energy through the dc
link.
Assuming the port voltage of TCSU is
() sinut Ut



(10)
where uα is also α-phase voltage of YNvd transformer.
By means of reasonable control strategy on traction
current simulation unit, anticipant load current can be
obtained as
1
11
11
() () ()
sin()()
sincos( )
Lh
h
pq
itit it
It it
h
I
tI tit






(11)
So instantaneous power of simulated load is
() ()
LL
putit


(12)
The A-phase voltage of EFU output side (three-phase
side) is:
2
( )sin(30 )
A
ut Ut  
 (13)
where U is the peak value of three-phase voltage.
The three-phase currents of inverter unit are controlled
to output same amplitude currents and feedback power to
grid with unity power factor, the phase A current is as-
sumed as
()sin(30)
AF F
it It  

(14)
where IF is effective value of three-phase current that the
inverter unit feedbacks to public grid.
Then the instantaneous power of phase A is defined as
() ()
AAAF
p
utit (15)
During one periodic, the two ports of simulated load
have same active power ignoring system loss, so we can
obtain:
U


A
L
B
L
C
L
L
I

3
S
1
S
4
S
2
S
5
S
6
S
8
S
7
S
10
S
9
S
2
C
dc
U
Figure 5. Structure of simulated load.

Y. Z. ZHAO ET AL. 525
22
00
2
0
11
(
3
LAB
A
pdtppp dt
TT
pdt
T






)
C
2
(16)
so,
1
cos 3
F
UIU I

 (17)
11
2
cos cos
33
F
UI I
IUK



(18)
The three-phase currents feedback by energy feedback
unit are followed as
11
sin(30)
1cossin(90 )
3sin(150 )
AF
BF
CF
It
II t
Kt
I








 

 


 

  






(19)
According to energy conservation, the two sides have
same active current without considering system loss.
However, compared with (9) and (19), the input three-
phase currents of YNvd transformer are equal to the
output three-phase currents of simulated load, so that the
energy can be recycled.
3. Control Algorithm
3.1. Control Algorithm of IPFC
The detection, control algorithms of IPFC is analyzed to
ensure IPFC real-time achieving comprehensive com-
pensation accurately and real-timely. The dc capacitor
link needs stable dc voltage to make sure IPFC operation
and power transmission. Figure 6 illustrates that the
double-loop control with the inductance current inner
loop and capacitor voltage outer loop is introduced.
By detecting and calculating simulated load current,
the compensation currents are obtained, which are the
parts of command currents of IPFC. The voltage outer
loop is to achieve the constant dc voltage control. The
compensation currents plus the steady-voltage current
which is obtained by voltage outer loop are the compen-
sation command current i*
cα and i* cβ. The current inner
loop controls the IPFC actual compensation currents to
follow the command currents. The PWM signals are
generated by tuning the difference of the actual current
and the command current in PI controller by modulating.
3.2. Control Algorithm of TCSU
TCSU is single-phase rectifier, as shows in Figure 7, the
current control method is used in order to control the
rectifier follow the command current accurately.
Compared with command current iL* and real traction
current iL, the difference is regulated to be modulating
voltage by PI controller and pulse signals for rectifier are
generated by PWM generator.
3.3. EFU
EFU is three-phase inverter, and it is required to output
currents in the unity power factor and control the dc vol-
tage stabilized on setting value. As shown in Figure 8,
the double-loop control with the inductance current inner
loop and capacitor voltage outer loop is used to control
this unit to achieve these functions.
In the α–β reference frame, instantaneous active and
reactive currents are respectively controlled with a PI
regulator and then the true compensating currents can
track the command signals well.
4. Simulation Verification
To verify feasibility and effect of this system, simulation
models of YNvd transformer, IPFC, simulated load were
built by using the Matlab/SIMULINK. The primary side
of YNvd balanced transformer accessed the three-phase
public gird, and voltage of primary side was 220 V, the
voltage of secondary side was 660V. The power factor of
L
i
dc
u
crefd
u
u

sin t


u

sin t


sin t


sin t


*
c
i

*
c
i

c
i

c
i

*
c
u

*
c
u

Figure 6. Control diagram of IPFC.
*
L
i
L
i
Figure 7. Control diagram of TCSU.
d
u
refd
u
u

sin t


L
i
K
A
u
sin t


cos t


32
C
*
q
i
C
p
i
ABC
i
q
i
*
p
i
p
u
1
32
C
﹣
1
C
﹣
q
u
R
ABC
u
Figure 8. Control diagram of EFU.

Y. Z. ZHAO ET AL.
526
occupancy of the third and fifth harmonic currents were
20% and 10% respectively, so the simulated load
command current was followed as
*200sin(30 )40sin(360 )20sin(5150 )
L
it t t  
 
The Figure 9 shows the command current and actual
traction current of TCSU, and he Figure 10 shows the
three phase currents fed back to grid by feedback unit.
The simulation results of simulated load illustrated that
the traction current could follow the command current
precisely, and as shown in Figure 10, the three-phase
currents were all active currents with same amplitude, the
input active power of single-phase was equal to the out-
put.
Figure 9. Command current and actual traction current of
the TCSU.
AF
i
B
F
i
CF
i
The simulated load is to be traction load, and the si-
mulation results of the other units were shown in Figures
11-13.
Figure 10. Three-phase feedback currents of EFU. As shown in Figure 12, the two output currents of the
secondary side of YNvd transformer were totally active
currents with same amplitude and a phase difference of
90° by the compensation of IPFC, so the traction supply
system was resistance load relative to public grid.
c
i

c
i

Figure 13 illustrates that the input currents of primary
side were equal and negative sequence was eliminated by
YNvd transformer and IPFC. The feedback currents of
simulated load were nearly equal to the inputs currents of
primary side of YNvd transformer, which means that
power energy was used circularly and effectively.
Figure 11. Compensation currents of IPFC.
s
i

s
i

REFERENCES
[1] L. Qunzhan, “Technologies and Applications of Shunt
Comprehensive Compensation in Electric Railway Power
Supply System,” C: China:railway press, 1998.
[2] L. Qunzhan, Z. Jinsi and H. Weijun, “Study of a New
Powersupply System for Heavy Haul Electrict Raction.
Journal of the China Railway Society, c 1988, Vol. 10,
No. 4, p. 23231.
Figure 12. Secondary side output currents of YNvd trans-
former.
[3] L. Qunzhan, H. Jianming, and X. Shaofeng, “Power
Quality and Controlof Traction Power Supply System,”
China: Press of Southwest Jiaotong University, 2012.
A
i
B
i
C
i
[4] Z. L. Shu, S. F. and Q. Z. Li, “Single-Phase Back-to-back
Converter for Active Power Balancing, Reactive Power
Compensation, and Harmonic Filtering in Traction Power
System,” Power Electronics, IEEE Transactions on, Vol.
26, No. 2, 2011, pp. 334-343.
doi:10.1109/TPEL.2010.2060360
Figure 13. Primary side input currents of YNvd trans former. [5] W. Guang, L. Qunzhan and H. Jun. “A new cophase trac-
tionpower supply system,” Automation of Electric Power
Systems, Vol. 32, No. 10, 2008, pp. 19-23.
the traction current was assumed as 0.866(lagged), the