Power Quality Improvement of DC Electrified Railway Distribution SystemsUsing Hybrid Filters

Seyed Hossein Hosseinil, Farhad Shahnial, Mitra Sarhangzadeh2, Ebrahim Babaeil'Electrical and Computer Engineering Faculty, University of Tabriz, Tabriz, Iran

2Tabriz Electrical Power Distribution Company, Tabriz, Iranhosseini@tabrizu.ac.ir, farhadshahnia@yahoo.com, msarhangzadeh@gmail.com

Abstract DC Electrified railways play an important rolefor metropolitan public transportation because of highefficiency, heavy ridership and fast transportation.However, the electrified railways cause great problems forthe power quality of the distribution system feeding thetraction system such as injecting harmonics, reactive powercompensation and low power factor issue. These problemscan be solved by using a hybrid filter in the tractionsubstations. In this paper, a modified PQ control system hasbeen applied for the control of shunt active filter, while thepassive filters are used for reducing most of the severeharmonics. The simulation is done with PSCAD/EMTDCproving the efficiency of the proposed hybrid filter structurefor reducing the current harmonics, voltage and currentbalancing, reactive power compensation and power factorimprovement.

I. INTRODUCTION

Application ofDC electrified railways as a significantmetropolitan means of transportation is increasing greatly.DC Electrified railways play an important role for publictransportation because of high efficiency, heavy ridershipand fast transportation. However, they result in greatpower quality problems for the power distribution systemwhich feeds the traction system. In DC electrifiedrailways, the rectifiers of the traction substations are amajor cause of harmonic distortion in the AC supply.High THD of the system current, harmonics andinterharmonics, reactive power consumption, voltageunbalance and flicker and low power factor problems cansuffer the power distribution system feeding the tractionsystem greatly [1-4].

In anticipation of the proliferation of nonlinear loadsand to limit the problems, recommended guidelines likethe IEEE-Std 519-1992 specify the allowable harmonicassociated in the currents drawn from the utility system.Different methods are utilized for improving the powerquality issues of the power distribution system such asdynamic voltage regulators, statcoms and active or hybridfilters.

In this paper, a hybrid filter structure with a new andsimple control technique is used for power qualityimprovement of the distribution system feeding the DCelectrified railway systems. Studying and comparing thesimulation results carried out with PSCAD/EMTDCsoftware prove that the proposed method can effectivelyeliminate harmonic currents, balance source currents,compensate reactive power and correct power factor ofthe power system, in other words, power quality

improvement of the power system is achieved by theproposed hybrid filter structure and control method.

II. POWER SUPPLY SYSTEM DESCRIPTION

The power distribution system of DC electrifiedrailways include traction substations for stepping downand converting the AC voltage to DC and also lightningsubstations for each station. The schematic diagram of thepower distribution system of a DC electrified railway isshown in Fig. 1.

ACPowera altosn.hXlbniolsm

Fig. 1. Schematic of the power distribution system ofDC electrifiedrail'ways.

The traction substation has one or two 20 kV /0.592kV/ 0.592 kV three winding transformers with Y/Y/Aconnections which step downs the 20 kV AC grid voltageto 592 volts AC. Two six-pulse rectifier units areconnected in parallel to the outputs of the transformerwhich convert the 592 volts AC to 750 volts DC.Therefore, a 12 pulse rectifier is utilized in the tractionsubstation for reducing the amount of harmonics in thesystem. The DC voltage is then transmitted along thetrack through overhead contact or third rail systems. Theschematic diagram of the traction substations is shown inFig. 2. The lighting substation that is used to consumptionof lighting, ventilation and lifts in the stations utilize a 20kV/400 V transformer with Y/A connection.

Fig. 2. Schematic of traction substations ofDC electrified railway.

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For studying the load flow analysis of the tractionpower distribution systems, the power consumption of thetrain operation is necessary to be investigated and thedynamical behavior of the traction system loads along theroute to be studied. Fig. 3 shows the typical speed profileof a train set between two stations which consists ofacceleration, constant speed and braking periods [10].When train is in the brake mode, the regenerative energywill be converted back into the DC line that can provide apart of the required power for the other running trainsalong the track.

45 -

37 / I AF 22i..Spe c e o t

/ IT |ill IV,1

Fig. 3. Speed characteristics of the train sets with time variations.

III. POWER IMPROVEMENT FILTER STRUCTURE

A. Passive Filters

Passive filters as classic methods for power qualityimprovement of distribution systems consist of series LCtuned for removing a specific harmonic or blocking abandwich of severe harmonics of nonlinear loads current.These filters have low impedances for the tunedfrequencies such as 5th and 7th and for these frequencies,the lower impedance of the filter in comparison withsystem impedance, the better filtering characteristics ofthe passive filter. Low cost is a great benefit of thesefilters but because of their LC constant parameters, theycannot be efficient power quality improvement facilitiesfor dynamic nonlinear loads. Another problem ofinstallation of passive filters is probable resonancesbetween the impedance of passive filter and the systemresulting in increasing the harmonics of system current.

B. Active Filters

Active filters as modern applications of powerelectronic inverters are used to fulfill the necessity ofcompensating the harmonics of power systems fordynamic conditions such as switching on and off loadsand DG units. Shunt active filters are used at main bus ofdistribution systems for reducing the amount of currentharmonics of the system being inserted to HV network.These filters are controlled as current sources forproducing non-sinusoidal currents according to non-sinusoidal current of loads or system for removing theharmonics and making system current sinusoidal. Thesefilters suffer from high ratings that increase the cost ofproject.

C. Hybrid Filters

Hybrid filters constructed of active and passive filterswith different structures are used for removing thedisadvantages of passive filters such as probability ofresonances and non dynamic responses and also high

costs of active filters, while using the advantage of bothof the filters with lower costs. Different structures ofhybrid filters can be utilized in power systems such asshunt passive filter and series active power filter withnonlinear loads, shunt active and passive filter withnonlinear load, series active and passive filter parallelwith nonlinear load, etc. as shown in Fig. 1.

A)

B)

C)

Fig. 4. Different structures of hybrid filters.

D. Hybrid Filter Topology

Active power filter is widely used in distributionpower networks to improve the power quality where thepurpose of using shunt active power filter is to cancel theload current harmonics fed into the power supply. It canalso contribute to reactive power compensation andbalancing of three-phase currents. In an active powerfilter, a controller determines the harmonics that are to beeliminated and also the reactive power that is to becompensated. The output of this controller is the referenceof a three-phase current controlled inverter. Fig. 5illustrates the schematic diagram of the statcom structureand connections used in the DC traction substation.

Fig. 5. Schematic diagram of the statcom for DC electrified railways.

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In this paper, a Modified PQ reference currentcalculation method is used for controlling the active filterwhere the compensating reference currents are as follows:

[ifa] 1 (a Vj8 ][PL(os)] (1)

LIf,8 A vL A a --QLA=v2+V2 (2)

Utilizing a PI controller for keeping the DC voltagebus to a constant value, the output of controller ismultiplied by capacitor current to calculate Pc losses.This power is added to the compensated active power,and then the compensating reference currents arecalculated as:

1faf 1va - VP PL(os) + PCl 3

The hysteresis current control is applied to themodified PQ compensation method for PWM controllingof the switchings of the inverter.

IV. SIMULATION RESULTS

The simulation is done with PSCAD/EMTDCsoftware where a DC electrified railway system issimulated as described in section 2. the simulation is donefor the system without any filters, with usage of just 5thand 7th shunt passive filters, with usage of just a shuntactive filter and with usage of the hybrid filter structureproposed in section 3.

The Current waveforms of the distribution systemfeeding DC electrified railway without any filters areshown in Fig. 6. The low power factor of the system canalso be studied through the difference between the voltageand current waveforms of the system as shown in Fig. 7.The active and high reactive power consumption of thissystem is also shown in Fig. 8. The current harmonicspectrum of the system shown in Fig. 9, proves thesystem of high THD and harmonic contents.

Fig. 6. Current waveforms of the distribution system feeding DCelectrified railway without any filters.

400 -

341

353250200 -v19 X

Fig. 8. Active and reactive power waveforms of the distribution systemfeeding a DC electrified railway without any filters.

Fig. 9. Harmonic spectrum of the current waveform of the distributionsystem feeding DC electrified railway without any filters.

Using 5th and 7th shunt passive filters can greatlyreduce the harmonic contents and compensate the reactivepower but with lower performance at the load changes.The Current waveforms, power factor, active and reactivepower consumption of the system and current harmonicspectrum for the distribution system feeding the DCelectrified railway are shown in Figures 10-13 where thepassive filters are switched on at 0.5 s.

Fig. 10. Current waveforms of the distribution system feeding DCelectrified railway using passive filters.

Fig. 11. Voltage-Current waveform of the distribution system feedingDC electrified railway using passive filters.

Fig. 12. Active and reactive power waveforms of the distribution systemfeeding a DC electrified railway using passive filters.

Ha ,cwectur

1'0 V V' IHHHHHHHHHJHJHHLFig. 7. Voltage-Current waveform of the distribution system feeding DC .Hm c tm h un a o he r t

electrified railway without any filters. Fig. 13. Harmonic spectrum of the current waveform of the distributionsystem feeding DC electrified railway using passive filters.

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Using a shunt active filter also reduces the harmoniccontents and compensates the reactive power but forhigher costs. The Current waveforms, power factor, activeand reactive power consumption of the system andcurrent harmonic spectrum for the distribution systemfeeding the DC electrified railway are shown in Figures14-18 where the active filter is switched on at 0.555 s. Itis obvious that the active filter can start compensation atdynamic situations much faster than passive filters. Fig.17 also shows the active power consumption of the bareshunt active filter structure.

Fig. 14. Current waveforms of the distribution system feeding DCelectrified railway using shunt active filter.

Fig. 15. Voltage-Current waveform of the distribution system feedingDC electrified railway using shunt active filter.

Fig. 16. Active and reactive power waveforms of the distribution systemfeeding a DC electrified railway using shunt active filter.

uCi aonly afshunt active filteBr.Fig17. Aciveandrleactiv powe conumtionofthi ctvtfltrwhl

600 --

Soo -

400

-10D -

G.&O 0.25 0.5Xc.75 1.00 1.25 1.Sso 1.75 2.W6

Fig. 17. Active and reactive power consumption of the active filter whileusing only a shunt active filter.

Fig. 18. Harmonic spectrum of the current waveform of the distributionsystem feeding DC electrified railway using active filter.

compensation with lower active power consumption,lower costs and higher capabilities at dynamic situationsas shown in Figures 19-23.

Fig. 19. Current waveforms of the distribution system feeding DCelectrified railway using hybrid filters.

Fig. 20. Voltage-Current waveform of the distribution system feedingDC electrified railway using hybrid filter.

a@ p

a L

Fig. 21. Active and reactive power waveforms of the distribution systemfeeding a DC electrified railway using hybrid filter.

Fig. 22. Active and reactive power consumption of the active filter whileusing hybrid filter.

HaTM66 b .>ect M

oL0HHHHHHH HHHHHHFig. 23. Harmonic spectrum of the current waveform of the distribution

system feeding DC electrified railway using hybrid filter.

Comparing the results of current THD and harmoniccontents through the simulation for the system withoutany filters, with usage of just 5th and 7th shunt passivefilters, with usage of just a shunt active filter and thehybrid filter structure proposed in this paper, prove theefficiency of the proposed hybrid filter structure incurrent harmonic reduction as shown in Table I.

Using the proposed hybrid filter structure consistingof a shunt active power filter and 5th and 7th shunt passivefilters has the best performance in harmonic reduction,power factor improvement and reactive power

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TABLE ITHD AND HARMONIC CONTENTS OF TH

SYSTEM FEEDING DC RAILWAYS WITH DI-

System and Filter Configurations

Without any filters

With only 5th & 7th Shunt Passive Filter

With only Shunt Active FilterWith Hybrid Filter (AF+PFs)

V. CONCLUS

In this paper a hybrid filteractive filter and passive filtersquality improvement of the distiDC traction systems. The sinPSCAD/EMTDC for the system i

usage ofjust 5th and 7th shunt passjust a shunt active filter and theproposed in this paper. The resultsthe efficiency of the hybrid fillharmonic reduction, reactive popower factor improvement withproblems.

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