Figure 1. System topology uses the Y-D11 transformer.

~ 25 180 kV∠ °

~ 0 kV

~ 25 0 kV∠ °

Figure 2. System topology uses the auto transformer [3].

Compensating Procedures for Power Quality Amplification of AC Electrified Railway Systems

Using FACTS

Mohammad Ali Akbari Baseri, Mehdi Niaki Nezhad, Mohammad Ali Sandidzadeh

Iran University of Science and Technology, Tehran, Iran

m_akbari85@rail.iust.ac.ir, m_niakinezhad@rail.iust.ac.ir, sandidzadeh@iust.ac.ir

Abstract— The low power factor and the voltage fluctuation are two stability problems in many ac electrified railway systems. This paper surveys the usage of several types of Flexible AC Transmission Systems (FACTS) for power quality improvements of an electrified railway system which has been consisted of phase-controlled thyristor converters to feed the DC motor drives of their locomotives. The section of a typical single-phase 25-kV, 50Hz, electrified railway system is loaded with ac–dc thyristor-based locomotives. It is equipped with either static VAR compensator (SVC) or static synchronous compensator (STATCOM). The capability of this system as power conditioner is confirmed by using detailed simulations in MATLAB Simulink software. At first, stability problems which are created without using of these compensators are considered. Then stability investigations such as voltage profile of feeder station and analyzing of power factor steady state are conducted on the impact of these compensators on some case studies. It was found that the SVC and STATCOM considerably improve the power quality parameters of the system in below: regulation of the voltage profile and power factor improvement.

Keywords- electrified railway systems; static synchronous compensator; static VAR compensator; power factor improvement; voltage regulation.

I. INTRODUCTION In general, the problem of reactive power compensation is

viewed from two aspects: load compensation and voltage support. In load compensation the objectives are to increase the value of the system power factor, to balance the real power drawn from the ac supply, to compensate voltage regulation, and to eliminate current harmonic components produced by large and fluctuating nonlinear industrial loads. Voltage support is generally required to reduce voltage fluctuation at a given terminal of a transmission line. Reactive power compensation in transmission systems also improves the stability of the ac system by increasing the maximum active power that can be transmitted. It also helps to maintain a substantially flat voltage profile at all levels of power transmission. Compared with STATCOM with the equal capacity, the performance of SVC is less satisfactory but it is cheaper and its controller is simpler [1, 2].

II. BACKGROUNDS

A. The topology of traction system There are several types of traction transformers which are

consist of single-phase transformer, Y-D11 transformer, Scott transformer and impedance-balanced transformer.

1) Y-D11 transformer In this topology (Figure 1), the currents through the two

traction lines connected respectively with phases (a) and (b) of Y-D11 transformer can be considered as two current sources [1].

2) Auto transformer In the auto transformer scheme, one of the power transformer

traction winding ends is earthed and the other end connected to the catenary wire (Figure 2).

2011 2nd Power Electronics, Drive Systems and Technologies Conference

978-1-61284-421-3/11/$26.00 ©2011 IEEE 518

Figure 3. scheme of CTRL case study with its SVC compensator.

B. FACTS 1) Static VAR Compensator (SVC)

Generally SVCs consist of standard reactive power shunt elements (reactors and capacitors) which are controlled to provide rapid and variable reactive power. They can be grouped into two basic categories, the TSC and the TCR. A SVC with thyristor-based and shunt connected systems has been proposed for electrified railway system. They are composed of a capacitor, which is the VAR generator, and a thyristor-Controlled Reactor (TCR), which behaves as a variable VAR absorbing load (depending on the firing angle of the thyristor valve). Thus, the SVC can inject or absorb a variable amount of reactive power to the railway system. The basic parameters of the TCR are calculated using (1) and (2):

2 2U UL= =

ωQ 2πfQ (1) and

ωLRQ

= (2)

Where Q is the reactive power that absorbed by a single-phase reactor and Q is quality factor of the reactor. To reduce the loss of filters and saving cost, there is a passive filter which has the fixed capacitors are often tuned with small reactors to act as passive filters in the characteristic harmonic frequencies of the TCR. These capacitors in an SVC are tuned with reactors to provide low impedance paths to the characteristic harmonic currents of the TCR. Whereas passive filters could resonance in series or parallel status with source impedances, filters should be designed as damped filters to avoid the possible parallel resonance near even-order harmonic frequencies [4,5,6]. The parameters of 3th, 5th, and 7th harmonic filters are obtained by using (3) and (4):

1

2C

n ωU

Q=C n

(3) and nn

nn Cω1

=Lω (4)

Where nC , nL and nCQ are respectively the capacitance,

inductance and compensation quantity of reactive power of each harmonic current, n= 3, 5, 7. There are three main reasons for using SVCs in electrified traction system: [4]

Voltage support in case of loss of one feeder station: SVCs can be used to support the railway Voltage in case of a feeder station trip. In such a case two sections have to be fed from one station for keep the voltage up in order to maintain traction efficiency and performance.

Steady state power factor: SVCs can be used to maintain unity power factor seen from the super grid transformers during normal operation. This ensures that a low tariff for the active power can be used.

Steady state harmonic mitigation: SVCs can be used to suppress the harmonic pollution. The SVC filters are designed not only to accommodate the SVC generation of harmonics but also that of the traction load.

2) Static Compensator (STATCOM)

A typical STATCOM, which consists of a voltage source converter (VSC) and a coupling transformer, connected in shunt with the AC system. In order to the STATCOM operate satisfactorily, its DC voltage is usually controlled to a fixed value. The principle operation of a STATCOM is based on controlling the voltage generated by the converter to control the generated reactive power. The STATCOM control system independent control of active power (DC voltage) and reactive power by controlling the q-axis and d-axis currents respectively. Similar to a SVC, closed-loop AC voltage control can be realized by using an AC voltage controller which generates reactive power order for the STATCOM control system [7].

III. CASE STUDIES

A. UK CTRL As a practical example, Channel Tunnel Rail Link (CTRL) of

UK, has been considered in this case study. Two SVCs are connected to the trackside 25kV bus bar, one to the catenary and other to the feeder. Each one of the three traction feeding points between London and the Channel Tunnel is supported by Static VAR Compensators. Three of these SVCs are mainly for voltage support and are connected on the traction side of the power transformers. The traction load is a section of the typical single-phase 25-kV electrified railway system is loaded with 2.5MW, 50Hz, with two half-controlled series bridges in ac–dc thyristor-based locomotives [8]. The active power of traction load can increase to 120 MW is connected between two phases. The topology of system is shown in figure 3 [4, 5].

B. Compensation of UK CTRL by FACTS As the assumed rail link is designed according to the active

power capacity of power line, SVC and STATCOM are designed to improve the profile of voltage and power factor. This case investigates dynamic performance of model which is similar to UK CTRL and the traction load of up to 30 MW with 12 trains (each train 2.5 MW) is connected between two phases of it. In the following cases, transient response of traction load variation and compensation of them by FACTS has been indicated.

1) System Topology using Auto Transformer a) Eight Trains without any Compansator

This Model consists of the power system, an auto transformer and the load traction with 8 trains. The profiles of voltage and reactive power which need to compensate are shown in figure 4.

519

0 0.05 0.1 0.15 0.2 0.25 0.3-20

-15

-10

-5

0

5

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.30.85

0.9

0.95

1

1.05

Time (Sec)

V rm

s & V

ref (

pu)

V refV rms

Figure 4. Illustration performance of system for eight trains without any compensator.

0 0.05 0.1 0.15 0.2 0.25 0.3-10

-5

0

5

10

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.30.85

0.9

0.95

1

1.05

Time (Sec)

V rm

s & V

ref (

pu)

V refV rms

Figure 5. Illustration performance of system for eight trains with a standalone passive filter compensator.

0 0.05 0.1 0.15 0.2 0.25 0.3-6

-4

-2

0

2

4

6

8

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.30.85

0.9

0.95

1

1.05

Time (Sec)

V rm

s & V

ref (

pu)

V refV rms

Figure 6. Illustration performance of system for eight trains with a TCR-equipped passive filter compensator.

0 0.05 0.1 0.15 0.2 0.25 0.3-40

-30

-20

-10

0

10

20

30

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.30.99

0.995

1

1.005

1.01

1.015

Time (Sec)

V rm

s & V

ref (

pu)

V refV rms

Figure 7. Illustration performance of system for eight trains with a STATCOM compensator.

b) Eight Trains with a Standalone Passive Filter The Model consists of the power system, an auto transformer,

the load traction with 8 trains and a passive filter with designated parameters in table I and II. As the reactive capacity of power line is designed for 12 trains and 8 trains are considered in this case, the injected reactive power of filter capacitors occupies the capacity of power line which has been improved the next cases. The profiles of voltage and reactive power are shown in figure 5.

TABLE I. THE DESIGN PARAMETERS OF SEVERAL CASES IN THE SIMULATION.

c) Eight Trains with the Passive Filter and a TCR This Model consists of the power system, an auto

transformer, the load traction with 8 trains and a TCR-equipped passive filter with determined parameters in tables I and II. In this case, the profiles of voltage and reactive power are much better than latter cases (figure 6).

d) Eight Trains with a STATCOM The Model consists of the power system, an auto

transformer, the load traction with 8 trains and a STATCOM. According to transient and steady state responses of the STATCOM in this case, both voltage and reactive power profiles are far better than latter cases (figure 7). However, the structure and controlling procedure of a STATCOM is more complicated than compensators in latter cases.

2) System Topology using Y-D11 transformer a) Eight Trains without any Compansator

This Model consists of the power system, a Y-D11 transformer and the load traction with 8 trains. The topology of system with the Y-D11 transformer and the profiles of voltage and reactive power are indicated in figures 8 and 9, respectively.

System Topology

Reactive Power of a

single-phase TCR

(MVAR)

Reactive Power of a

single-phase TSC

(MVAR)

Reactive Power of a single-phase passive

filter (MVAR)

3th 5th 7th

UK CTRL 3.5 - 26.5 7.5 7.5

Auto Trans. 4.52 - 8.83 2.5 2.5

Y-D11 Trans. 5.52 15.2 - - -

520

0 0.05 0.1 0.15 0.2 0.25 0.-40

-30

-20

-10

0

10

20

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.0.92

0.94

0.96

0.98

1

1.02

Time (Sec)

V rm

s & V

ref (

pu)

V refV rms

Figure 10. Illustration performance of system for eight trains with a SVC compensator.

Figure 8. Topology of system uses the Y-D11 transformer without any

compensator.

0 0.05 0.1 0.15 0.2 0.25 0.3-60

-50

-40

-30

-20

-10

0

10

Q (M

var)

0 0.05 0.1 0.15 0.2 0.25 0.30.92

0.94

0.96

0.98

1

1.02

V rm

s & V

ref (

pu)

Time (Sec)

V refV rms

Figure 9. Illustration performance of system for eight trains without any compensator.

b) Eight Trains with TSC and TCR This Model consists of the power system, a Y-D11

transformer and the load traction with 8 trains and SVC with considered parameters in table I. Since the profiles of voltage and reactive power can be seen in figure 10, the steady state response of SVC with TSC and TCR for Y-D11 topology is much better than the case without any compensator.

TABLE II. THE BASIC PARAMETERS OF ALL FILTERS IN THE SIMULATION

Type of filters C (μF) L (mH)

3th 38.56 29.16

5th 10.91 37.11

7th 10.91 18.94

IV. CONCLUSIONS Whereas the paper proposed to find the best FACTS to

provide reactive power compensation and voltage regulation, the operation of a traction load which is furnished by a SVC has been represented for different case studies and the STATCOM in one case study.

As a traction load model, several 2.5 MW locomotives which are developed in a single-phase 25-kV electrified railway system have been studied in the paper. The results show that:

SVC and STATCOM can amplify the system stability by providing or absorbing required reactive power of the traction load in the railway system.

STATCOM has better capability for providing reactive power compensation in term of low AC voltage than SVC. However, the performance of SVC is less satisfactory, it is cheaper and the control is simpler.

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Negative-sequence Currents of Electric Railway Based on SVC,” 3rd IEEE Conference on Industrial Electronics and Applications, pp. 1958-1963, 2008.

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[3] R. Grunbaum, J. P. Hasler, and B. Thorvaldsson, “FACTS: Powerful Means for Dynamic Load Balancing and Voltage Support of AC Traction Feeders,” Power Tech. Proceedings, Porto, 2001.

[4] R. Grunbaum, “FACTS for power quality improvement in grids feeding high speed rail traction,” Electric Machines & Drives Conference, Antalya, pp. 618-623, 2007.

[5] Davor Vujatovic, and Qingping Zhang, “Harmonics Generated from Railway Operation,” Power Engineering Society General Meeting, Montreal, pp. 1-3, 2006.

[6] Zhihao Ning, Longfu Luo, Yong Li, Jie Zhang, Zhiyu Zhao, and Fusheng Liu, “Analysis and Design of Main Circuit Parameters of New Industry Rectifier System with SVC,” International Conference on Electrical Machines and Systems, pp. 1826-1831, 2008.

[7] Lie Xu, Liangzhong Yao, and Christian Sasse, “Comparison of Using SVC and STATCOM for Wind Farm Integration,” International Conference on Power System Technology, Chongqing, pp. 1-7, 2006.

[8] P. C. Tan, R. E. Morrison, and D. G. Holmes, “Voltage Form Factor Control and Reactive Power Compensation in a 25-kV Electrified Railway System Using a Shunt Active Filter Based on Voltage Detection,” IEEE trans. on industrial applications, vol. 39, no.2, pp. 575-581, 2003.

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