Shunt Active Power Filter Based on Fuzzy-Hysteresis Controller for Electric Traction System

Smitha Krishnan M.Tech Scholar

Department of Electrical Engineering College of Engineering, Trivandrum, Kerala, India

haismithakrishnan@yahoo.com

Lathika B.S. Assistant Professor

Department of Electrical Engineering College of Engineering, Trivandrum, Kerala, India

bslathika@yahoo.com

Abstract— With the progress of power electronic devices in electric railway, harmonics in power systems become increasingly of concern. This paper presents a Shunt Active Power Filter (SAPF) with fuzzy-hysterisis controller and a new compensating current detection method. The APF consist of a three phase converter and a Scott transformer. The Scott transformer, taken as an isolation transformer, not only connects the three-phase converter to the traction power system, but also converts the traction power system to a nearly balanced three-phase power system. Therefore a three-phase converter topology can be used in APF. The performance analysis of THD and power factor is done to realize the accuracy of this active filter.

Keywords- Shunt Active Power Filter, Scott transformer, Fuzzy controller, Hysteresis Controller, Electrified Railway, THD.

I. INTRODUCTION Nowadays single phase rectifiers are widely used in the

electrical traction loads in many countries, which caused the issues of harmonics and reactive power. The harmonics and reactive power have negative influences on the power supply system. To compensate reactive power, fixed capacitors have been widely used. One of the main disadvantages of fixed capacitor is that its compensation amount is also fixed and cannot be changed with the variation of load. Another one is that resonance may occur between the fixed capacitor and the impedance of power supply system [1]-[3].

Recently TSF (Thyristor Switched Filter) has been used. There are several group of passive filter in TSF.With the variation of reactive power; an appropriate number of groups of TSF are switched on. Thus the compensation amount of TSF can be adjusted with the variation of load. However, resonance will still possible to occur between TSF and the impedance of power supply system.

To avoid this problems Shunt Active power Filter (SAPF) is used, which utilize the features of modern power electronic technology [1], [4]. Different types of APF’s are used in electric traction system. All of them are composed of two single phase converters that have a common DC bus. The harmonics and negative sequence current can be compensated effectively.Howewever, when the complexity of control is increased the use of power electronics components in the topologies also increased. In addition, these APF use instantaneous power theory to determine the compensating

currents, which is not suitable for single phase traction power systems.

Here proposed a new shunt active power filter composed of a three-phase converter and a Scott transformer. The controller part includes fuzzy controller and FBD detection method of compensating current. Since the voltage source converter is important part in APF, more care is given to design of DC side capacitor. The controlling of DC capacitor voltage along with harmonic current control is adopted here to improve the system behavior. Efficiency of operation can be increased by modifying fuzzy controller with fuzzy-hysteresis controller. The harmonics generated by traction has been analyzed through the modeling of traction load by incorporating different speed conditions. The performance of proposed shunt active power filter has been illustrated through MATLAB/SIMULINK and verified the results.

II. PROPOSED SYSTEM

The traction power system in India is powered by a 50-Hz 25-kV power supply. A typical traction power system is shown in Fig. 1 [5]. Here a three-phase 110-kV power system is transformed into two 25-kV single-phase subsystems. Since the traction substation could be equivalent to three-phase to two-phase transformation equipment, the traction power system can be taken as a two-phase circuit or as a serious unbalanced three-phase circuit where one phase is connected to ground. Therefore, a three-phase converter could be used to improve the power quality in two feeders of a traction substation. The proposed SAPF is shown in Fig. 1.

In the traction power system shown in Fig. 1, the traction substation is illustrated with an impedance matching balance traction transformer. The voltages of the secondary side, U and

U , are orthogonal to each other, where 2/ jeUU .If currents

in the secondary side meet 2/ jeII , then the traction

transformer is balanced and the negative-sequence currents in the primary side is zero. Thus, if SAPF transfers part of the active power between two feeders to keep the balance of the traction transformer, the negative-sequence current could be eliminated. In Fig. 1, Li and Li are the load currents in phase α and β.

pLi and pLi are the active power currents of loads. The

active power currents that SAPF should transfer between two -

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)(21

pLipLipcipci

Fig. 1. Schematic diagram of active power filter for electrified railway phases are

So the power rating of SAPF is determined by the loads in two phases. In an extreme case, one phase is fully loaded and the other phase is non loaded. The SAPF capability is equal to half of the full load. With proper control, the reactive power and harmonic can be compensated together. A three-phase converter topology cannot be used in conventional Active Power Filter directly for traction system so that a Scott transformer, which is composed of two single-phase transformers aT and bT , is used to solve the problem. It has the following advantages.

1) Isolate the converter from traction power system.

2) Reduce the voltage level of three-phase converter.

3) Connect the three-phase converter to the two-phase traction power system.

With the Scott transformer, the traction power system is transformed to a near balanced three phase power system. That means a general three-phase converter can be used in the proposed SAPF. Since the traction substation is acknowledged as a special three-phase power system, the SAPF also can be used in conventional unbalanced three-phase power system.

III. TRACTION LOAD MODELING The traction power supply system being studied as a 25kV

single-phase current collection catenary system with two track-side feeder stations [6]. In many modern electric railways most harmonic currents are generated by thyristor-converter of electric railway vehicle. A simplified circuit

diagram for the thyristor-controlled railway vehicle is shown in Fig. 2. The transformers, thyristor-converters and DC motors are mounted at the bottom of the train. For each motoring unit, a phase-controlled thyristor converter delivers power to the motors. By firing different thyristors four notches are made available to control the speed of the traction motors. For notch 1 and 2 only the upper rectifier is put into operation, while for notch 3 and 4, the lower rectifier is also put into operation [7].

The train driver can use the notch setting to deliver appropriate power to accelerate the train. Both Notch 1 and 2 are used for low speed driving. The operation of Notch 1 setting can only deliver limited power to drive the train at very low speed because only one of the bridge is operating and the firing angle for Notch 1 is limited to 120O. The speed range of

Fig. 2. Block diagram of traction loco with rectifier circuit

(a) (b)

(c) (d)

Fig. 3. Equivalent circuit of rectifier cicuit while (a) Notch 1 (b) Notch 2 (c) Notch 3 (d) Notch 4 is working.

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)sincos(1)( titiU

tpG

Notch 1 is always below 10 km/hr. Notch 2 setting allows the firing angle to adjust according to the train speed. However, Notch 2 again turns on one bridge only. Notch 3 allows the operation of another bridge, therefore delivering more power to drive the electric motors. Notch 4 allows the motor to operate in field weakening region at high speed range. The equivalent circuit corresponds to four notches is shown in Fig. 3. During modeling different speed conditions are achieved by switching the rectifier circuit in above said pattern.

IV. CONTROL STRATEGY

A. Detection Method Instantaneous reactive power theory is widely used in active

power filters, but it cannot be used in traction power systems directly because the theory is based on the three-phase circuit.

In 1932, Fryze proposed a method to analyze relations between voltages, currents, and power quantities. Buch-holz and Depenbrock improved this method and labelled it the “FBD-method” [5]. The main characteristic of the FBD-method is to split a current into its power component and the remaining total non active component. The waveform of the power component is identical with the waveform of the corresponding voltage. Since the voltages in traction power systems contain large amounts of harmonic components, the power currents determined by the FBD method and the voltages have the same harmonic components. A real time detection method based on the FBD method is proposed to solve the problem. It is shown in Fig. 4.

The phase-locked loop (PLL) generates the two reference voltages tcos and tsin .The reference voltages are synchronized with u and u , respectively, and only contain

fundamental components. pi and

pi are the power currents in each load. The total non active currents are

ci andci . Both of

them contain total reactive power, and harmonic currents. They are the compensating currents for the two phases. In the traction power system shown in Fig. 1, assume that the fundamental voltage in each feeder is

Where ωt is the phase angle of voltage. The equivalent

conductance Gp (t) is

Where U is voltage vector and is given by

then,

Fig. 4. schematic diagram of current detection by FBD method

After LPF, the equivalent conductance is

So the power current vector is

Since there is no instantaneous value of voltage in (7), using two reference voltages generated by PLL to replace the fundamental voltages, the power current and the total non active current can be measured accurately. Because the waveforms of power currents in each phase are identical with the reference voltages, respectively, the power currents have no harmonic components and the measured results are not affected by the waveforms of voltages.

The difference between total current vector and power current vector is total non active current vector, in which, the elements of the vector are the compensating currents in each phase. If the SAPF could generate the compensating currents accurately, the traction transformer will only output the power currents. It is easy to prove that if PLL generates reference voltages synchronized with each phase voltage, the detection method can be used for three-phase or multiphase circuits. After the compensating currents have been measured, the reference currents of the three-phase converter can be calculated by the currents transformation of the Scott transformer.

(8)

Where k is the phase voltage ratio between the three-phase side and the two-phase side of Scott transformer.

B. Fuzzy Logic Based DC Bus Voltage Controller APF dc bus capacitor voltage is an important parameter to be

controlled. If this control is not done properly, source current will deteriorate and lapse from sinusoidal waveform. In this paper, a fuzzy logic based dc bus voltage controller is used to

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)sin(2

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regulate the dc bus voltage of the SAPF [8]. Since fuzzy control rules are derived from a heuristic knowledge of system behavior, neither precise mathematical modeling nor complex computations are needed. Block diagram of SAPF with fuzzy logic control for regulating the dc bus capacitor voltage is shown in Fig. 5. Capacitor voltage is sensed using a voltage sensor and compared with the set reference voltage (Vdcref). Input variables of the fuzzy controller are capacitor voltage error (e) and change in voltage error (Δe) at the k-the sampling time as given below;

where α and β are input scaling factors. Output of the fuzzy controller is the change of adjustment current (ΔIad) and actual adjustment current is determined as below;

)(.)1()( kadIkadikadI (10)

where γ is output scaling factor. This adjustment current will supply the losses in the converter and will be added to peak value of reference source current. The fuzzy control rule design involves defining rules that relate the input variables to the output model properties. The triangular type membership functions are selected for inputs and output. Arrangement of triangular membership functions ensures that for any combination of (e) and (Δe), maximum four rules are applied. In this way the computation time can be reduced.

Fig. 5. Block diagram of fuzzy control of SAPF for electric traction

TABLE I RULE TABLE

ece NNBB NNMM NNSS ZZEE PPSS PPMM PPBB

NNBB PB PB PM PM PS PS ZE NNMM PB PM PM PS PS ZE NS NNSS PM PM PS PS ZE NS NS ZZEE PM PS PS ZE NS NS NM PPSS PS PS ZE NS NS NM NM PPMM PS ZE NS NS NM NM NB PPBB ZE NS NS NM NM NB NB

Fig. 6. The membership functions for inputs e and Δe

The input membership functions are shown in Fig. 6.To ensure converter output voltage stable near the set point 49 rules are derived as seen in Table I. The dynamic nature of traction load current can be incorporated by these fuzzy rules.

C. Fuzzy-Hysterisis Controller This novel method combines the fuzzy control and hysteresis

control methods [9]. Here fuzzy is used for voltage control and hysterisis for current control. By doing so, width of the hysteresis band can be modulated. It allows a fast current control along with voltage control. The hysteresis band is given by

2916dc f S s

m f c f

diV dt

V L VHB f L L

(11)

where fm is the modulating frequency, Lf is the filter inductance and is is the source current. The reference current is calculated from the instantaneous power drawn from the load.

The active power drawn by load is given by

Therefore if active power of load before and after compensation is equalized, peak value of reference source current can be calculated as

The peak value of reference source current produced by the fuzzy controller is Iad and which is used to find the reference source current Is(t).And finally reference filter current is calculated by subtracting extracted harmonic current from reference source current as in (17) Where Ih is the extracted harmonic current. The block diagram of fuzzy-hysteresis control of SAPF is shown in Fig. 7.

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V. SIMULATION RESULTS

A. Fuzzy Controller In this section the active power filter has been realized

through fuzzy controller. The parameters used for simulation is tabulated in table II [10]. The modeling and simulations has been done using MATLAB/SIMULINK. During simulation the traction loco was assumed to be run in four speeds and speed variation has been applied after every 0.5ms so that the total simulation time is taken as 2ms for simplicity. Also the traction loco load is connected for one side of the traction power system with maximum load capacity.

Before compensation with controller, the source current contains harmonic content due to traction load and it will be appeared along with source current as sown in Fig. 8. The spectral analysis shows that source current having Total Harmonic Distortion (THD) of 54%. A proposed SAPF with fuzzy control is used to overcome this problem. The extraction has been done with FBD method and the fuzzy controller has

Fig. 7. Block diagram of fuzzy-hysteresis control of SAPF for electric traction.

TABLE II SYSTEM PARAMETERS

Parameters Numerical Value System voltage and frequency 25 KV,50 HZ

Feeder impedence 5.0 Ω,10.0 mH

Loco transformer 100 KVA,11V/440V

Snubber circuit of rectifier R= 50 Ω, C=4.7 µF

Traction motor

1 phase DC Type,630 KW 750 V,900 A,895 rpm

Series impedance R= 5.0 Ω, L= 4.9mH

DC link capacitor and voltage 33.720 V,47.5µF

Injecting inductor 0.85 mH

VSI switching frequency 3 KHZ

Fig. 8. The current at traction substion without any APF

been designed along with PWM to mitigate harmonics produced by the traction load. The following figure shows the extracted harmonic current and compensated current produced by the APF. After compensation source current has been found to be reduced harmoniccontent and it is shown in Fig. 10.The simulation results shown here are obtained with the condition is that loco moving with high speed ie, in forth Notch.

B. Fuzzy-Hysteresis Controller The Fuzzy control of SAPF has some limitations. This

method cannot incorporate all non linearities in the traction system. The fuzzy controller is modified to a fuzzy-hysteresis controller for harmonic current control along with voltage control. A higher switching frequency can be used in this method.

Fig. 9. The extracted harmonic current by FBD method and compensating current generated by the SAPF using fuzzy controller.

Fig. 10. Current at traction substation after active filter compensation.

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The fuzzy controller is designed in such a way that the instantaneous values of fundamental frequencies of the current can be modulated continuously with in a fixed hysteresis band. The compensated current produced by the fuzzy-hysteresis controller is shown in Fig. 11. After compensation, source current is balanced and free from harmonics with THD of 5.5% with exact estimation of fundamental frequency. This is shown in Fig. 12.

VI. PERFORMANCE ANALYSIS The APF with fuzzy controller and fuzzy-hysteresis

controller has been studied. Table III gives performance comparison of fuzzy and fuzzy-hysterisis control of SAPF for traction load harmonic compensation. With the fuzzy controller DC voltage of three phase converter has been regulated and a better compensation was obtained. This is shown in Fig. 13. With fuzzy-hysteresis controller THD has been reduced to 5.5% for traction power system. It can be concluded that the performance of APF with fuzzy-hysteresis control improves THD as compared to other control methods. The power factor and reactive power compensation were also improved with fuzzy-hysteresis controller. The results obtained by these controllers are with in the harmonic limit recommended by the

Fig. 11. The compensting current and generated by the SAPF using fuzzy-hysterisis controller.

.

Fig. 12. Current traction substation with fuzzy-hysterisis control of active power filter.

TABLE III PERFORMANCE COMPARISON OF CONTROLLERS

Method used System Condition

THD (%)

Power Factor

Before Compensation 54 0.72

Fuzzy After

Compensation 10 0.83

Before Compensation 54 0.72 Fuzzy-

Hysteresis After

Compensation 5.5 0.97

Fig. 13. DC voltage of three phase converter with fuzzy-hysterisis controller.

IEEE standard 519-1992.

VII. CONCLUSIONS A new modeling method for electric railway vehicle has

been proposed by incorporating different speed conditions. SAPF has been designed for the traction load based on fuzzy controller and fuzzy-hysteresis controller. Its unique advantage is that the DC voltage can be regulated using fuzzy controller and a better harmonic compensation is possible. The tracking efficiency can be improved with fuzzy-hysteresis controller. With the system model, power factor and harmonics have been studied and found that it follows the IEEE standards.

REFERENCES [1] Pee Chin Tan,Poh Chiang loh and Donald Grahamme Holmes “Optimal

impedence termination of 25 KV electrictrified railway system for improved power quality”, IEEE Trans. Power Del., Vol.20,No.2,Apr 2005,pp.1703-1710.

[2] Xianghenz Xu,Baichao Chen, “Study on systhesis control of power Qulity for Electified railway” IEEE workshop on power electronics and intelligent transportation system ,2008.

[3] Federica Foiadelli,Paolo Pinta, “Stastical method for harmonic propagation studies in electric traction supply system”, IEEE International Conf. on Harmonics and Quality of Power, 2004.

[4] Pee-chin,Robert E. morrison and Donald Grahamme Holmes,”Votage form factor in an 25 KV electrified railway system using a shunt active filter based on voltage detection”,IEEE Trans. Ind. Appl., Vol. 39,No.2,Mar. 2003,pp. 67-72.

[5] Zhuo Sun,Xinjian Jiang, “A novel active power quality compensator topology for electrified railway” IEEE Trans. Power Electr., Vol. 19 No.4,Julay 2004,pp.560-565.

[6] Joachim Holtz,Hienz Jurgen, “The propagation of harmonic current Generated by inverter- fed locomotive in distribution overhead supply”IEEE Trans. Power Electr., Vol. 4, No.2, May 1989, pp. 168-174.

[7] K.H Yuen,Z.M Ye,M.H Pong, “Modeling of traction harmonic current using statistical method” IEEE International conf. on Power Electronics and drive Systems, July 1999, pp. 194-199.

[8] S.K. Jain, P. Agrawal and H.O. Gupta, “Fuzzy Logic controlled shunt active power filter for power quality improvement”, IEEE Proc. Electrical Power Appliances, Vol. 149, No. 5, September 2002, pp. 317-328.

[9] M. Kale and E. Ozdemir, “A novel adaptive hysteresis band current controller for shunt active power filter,” in Proc. of the IEEE Conf. Control Applications, Vol. 2, Mar. 2003, pp. 1118-1123.

[10] Mahesh K. Mishra,K. Karthikeyan, “Design and analysis of voltage source inverter for active compensators to compensate unbalanced and non-linear loads”,The 8th International Power Engineering conf. (IPEC 2007), pp. 649-654.

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