IEEE ISIE 2006, July 9-12, 2006, Montreal, Quebec, Canada

A Single Phase Multilevel Hybrid Power Filter for Electrified RailwayApplications

Salem Rahmani and Kamal Al-Haddad, Senior Member IEEEEcole de Technologie Superieure

Canada Research Chair in Energy Conversion and Power Electronics CRC-ECPE1100 Notre-Dame West Street, Montreal, Quedbec H3C 1K3, Canada

Phone: (514) 396 8874, Fax: (514) 396 8684, Email: Salem.Rahmani@esstt.rnu.tn, kamal@ele.etsmtl.ca

Abstract- To improve the power quality of tractionpower system, a current control methodimplemented on a multilevel hybrid power filter(MHPF) to compensate harmonics and reactivepower is presented. Regarding the tractionsubstation as a compensating object, the powerquality of a traction substation can be improvedintegrally. The hybrid filter consists of a passivefilter and a low-rated multilevel power converter.The passive filter works not only as a harmonic filtertuned at the 3rd harmonic frequency, but also as aswitching-ripple filters. The multilevel active powerquality compensator uses source voltage reference todetermine the compensating currents for singlephase traction power systems. The rating of theswitching devices for the active filter decreases withthe use a multi-level inverter topology. Simulationresults confirm the validity of the system and showthat the adopted current control method is able tocompensate reactive power and harmonics in thetraction substation single phase 25 kV systems.

Index Terms- Electrified railway, harmonics,reactive power, power quality compensator, hybridmultilevel active power filter.

1. Introduction

In electrified railway systems, electrical locomotives aretreated as single-phase loads; the harmonic currentsinjected by locomotives can result in a range of tractionsystem problems, including trackside over-voltages,increased voltage by a high factor and excessive loworder harmonic currents being fed back into the HVsupply [1]-[4]. Conventional passive power qualitycompensators, such as reactive power compensationcapacitors and passive filters are single-phaseequipment installed at feeder of a traction substation.The speed and load condition of the train changesfrequently. Moreover, passive equipment cannot adjustthe compensating capability to the load needs, whereover- and under-compensation occur frequently due tothe frequently changed dynamic load. A possiblesolution to mitigate for this harmonic complication is toconnect a power quality conditioner based on voltagedetection at the end of the traction feeder [1]-[2]. In atraction system, the locomotive load moves, hence thisis also likely to be the most suitable location for theactive filter considered in a traction application.

The only possible measurement for control is theharmonic voltage measurement approach. Hence theonly realistic control strategy is to measure the feederharmonic voltages at the active filter PCC, and toprocess the voltages to provide a current reference forthe filter. However, the performance of the installedpower quality conditioner depends strongly on theaccuracy and stability of the harmonic extractionalgorithm used to generate the harmonic currentreference to be tracked by the inverter inner currentcontrol loop. In this paper, a new detection method forhybrid multilevel active filter, based on the voltagesource for compensating currents is proposed. Theharmonic currents and reactive power in the feeder of atraction system could be compensated together. Bycontrolling the active power filter to inject out-of-phasecurrent with respect to the feeder voltage, reactivepower can be exchanged to reduce the reactive voltagedrop along the feeder line, therefore achieving voltageregulation along the feeder. The hybrid system consistsof a low-rated multilevel active power filter to provideharmonics mitigation and reactive power compensation,and a passive filter for 3d harmonic frequency and highfrequency oscillation damping. Simulation results provethe detection method validity and show that the MHPFis able to compensate harmonics voltage and reactivepower, in traction feeder section.

2. Multilevel Hybrid Power Filter TopologyMultilevel inverter systems are often proposed for highpower applications, since they have particularadvantages that suit these types of applications [5]-[8].Figure 1 shows the railway traction system where themultilevel hybrid power filter is connected to the pointof common coupling (PCC) of a traction feeder section.The MHPF consists of a multi-level inverter and apassive filter. The multi-level inverter consists of sixswitching devices, four clamping diodes, and twocapacitors, at the inverter input side. An advantage ofmulti-level inverters is that they can reduce the voltageor current ratings of the semiconductor switchingdevices and improve compensation characteristics of theactive filter. The multi-level inverters can offer highquality output voltages since output voltage approachesto sinusoidal waveform as level increases. In addition,the power rating of the switches in the multi-levelinverter is much lower than that of the conventionalvoltage source inverters [5]-[10]. The voltage across

1-4244-0497-5/06/$20.00 C 2006 IEEE I925

each capacitor is VdC/2, and each device voltage stresswill be limited to one capacitor voltage level, VdC/2,through clamping diode. A typical traction powersystem is shown in Figure 1. The traction power systemis powered by a 60-Hz, 25kV power supply. Thetraction system considered consists of a 10 km single-phase contact feeder section with longitudinalimpedance of (0.3 + jO.045) Q/km at 60Hz and a shunt

Feeder substationl-

R

1OKm contactfeeder section

R L-L

LC

capacitance of 0.1LF/km, fed from a substation step-down transformer at 25kV. The substation transformeris represented by its equivalent series inductance andresistance and the contact feeder is modelled as 10 kmpi-section line. While up to locomotive load, nominallyrated at 2.5 MW at 25 kV, is considered. Locomotive ismodelled as a pair of series-connected half-controlledthyristor bridges rectifier.

Four Level Hybrid Power Filter

Z\SAI

Z\SA

i 1)E15S,,(t'l,~~~~~7~ A- Al L

Passive Filter

LA

Train Load A i\S7Ai

Figure 1. Railway traction system model

3. Current control strategy for active filter applied torailway traction system

Low system voltage, line resonance, loss of voltage andharmonic over voltage have been identified as seriousproblems that can limit the performance of 25-kVelectrified railway systems. A shunt active filter acts asa controllable harmonic current source. In principle,harmonic compensation is achieved when the currentsource is controlled to inject harmonic currents of thesame magnitude but opposite phase to the loadharmonic currents. There are basically three methods ofdetermining this current reference for the active filter:[i] by measuring the load harmonic current to becompensated and using this as a reference command;[ii] by measuring source harmonic current andcontrolling the filter to minimize it;[iii] by measuring harmonic voltage at the active filterpoint of common coupling (PCC) and controlling thefilter to minimize the voltage distortion.In a traction system, the locomotive load is physicallymoving. Since the shunt filter is to be added to the farend of a feeder to be most effective, the ac source willbe physically quite distant from the active filter point ofcoupling (PCC). Therefore the harmonic voltagemeasurement approach is the only feasible possibilityfor active filter control. Hence the only realistic controlstrategy is to measure the feeder voltage at the activefilter PCC, and to process the voltage to provide a

current reference for the filter. The proposed controlstrategy is therefore to measure voltage harmonic at theactive filter PCC, by using a band-pass filter. Figure 3shows the active filter control structure that achievesthis mandate. The band-pass filter extracts thefundamental voltage from the measured track voltage,and subtracted this signal from the measured signal tocreate a harmonic currents reference after multiplyingby a constant gain G. Besides harmonic currents, afundamental reactive current reference is also createdfrom the fundamental voltage error, and a real currentreference component is derived from a PI regulator thatcontrols the dc bus voltage of the inverter. Thesecomponents are summed to create the demandedreference for the inner current regulator loop.Mathematically, the band-pass filter transfer functionscan be written as:

F(s) = 2ConS~2 +2 1s+2Where 4 is the damping, which makes possible toregulate the filter band pass width to -3dB, p,, is the cut-off frequency of the filter.The amplitude of transfer function F(jo) is given by:

F(j c0) = 2((o. o

Where Cl the line frequency.

(2)

(1)

926 2

The amplitude of the transfer function F(jnw), for thecut-off frequency and harmonics spectrum offrequencies (nwo) will have the following expression.

IF(jno)l =I (3)

From where the attenuation in percent is computed as:

1(4)

The dephasing introduced by the filter is given by:

4, arctg ~2 (5)2 (O2 )O

Indeed, dephasing is theoretically null if one does nothold account of the variation of the frequency of thenetwork, hold for a variation of Awo = Cl--on = XCln Sdephasing in degree introduced by the filter will have asan expression:

18071 (K- arctg 2(x+2 )) (6)

The setting in cascade of two band-pass filters permitthe reduction of the pass band resulting in betterselectivity characteristics filter as seen in figure 2. Thetransfer function amplitude and attenuation are givenrespectively by the following expressions (7) and (8).

|F(jnco)l =

And,

|A(jnco)l =

1

K- n I1+ (1

2n

n2) 2 12

0.8_i 0.6_* 0.4 _

# 0.2_

o~010

Cly mne filterT\Ao filter cascaies

1 2 3 410 10 10 10

1lasCly mne filterT\Ao filter cascaies

-la00 1 2 3 4

10 10 10 10 10Frequency (Hz)

Figure 2. Magnitude and phase of the band-pass filtersused for the extraction of the fundamental voltage.

4. PWM Generation

Two current control schemes can be used to track theline current command: multi-level PWM methodscurrent control and hysteresis current control. Thevariable switching frequency is the main drawback ofhysteresis current control. The multi-level PWMmethods typically uses multi-level carrier-based PWMor space vector PWM. The space vector PWM has adisadvantage that it is difficult to sector distinction dueto increase of vector space with increase of level. Toavoid such a problem, a multi carrier-based PWMmethod is used in the inner control loop to track thereference filter current. Figure 4 illustrates the switchingsignals for active switches SAI, SA2 and SA3 and theAC terminal voltage vc. For an m-level converter, itneeds carrier signal of m-i number. The reference iscontinuously compared with each of the carrier signals.If the reference is greater than a carrier signal, then theactive device is switched on, other wise it is switchedoff. In the multi-level converter, the amplitude ofmodulation index, ma, and the frequency ratio, mfi, aredefined as [6]:

Am(8) a (m-1)AC

mf = fcfm

(1 1)

(12)

In this case, the dephasing is introduced by this filterhas the expression:

4= 360(2-arctg2 (x+i) (9)

The reference current harmonics can be written as:i*ar = K(vB -F(s))

Where all carrier waveforms have carrier frequency f,and peak-to-peak amplitude A,. The reference waveformhas a peak-to-peak amplitude equal to Am, and afrequency equal to fmn.

(10)

927

w

3

I- - - - - -F L l b P FilterFour Lcvcl Hybrid Powcr Filtcr

Figure 3. Control system block diagram

RPference sigiE Caier 1 tier 2 er 3

0 W lTV C-,1T" A

1.5 0.035 0.04 0.045 0.05 0.055

.5

-0.50.035 0.04 0.045 0.05 0.0550.5

0.50.5 0.035 0.04 0.045 0.06 0.0550.5

3 0.50. __ _ __

0.06 0.065

0.06 0.065

0.c6 0.05

0.035 0.04 0.045 0.05 0.055 0.X6 0.65

° [ 01

0.035 0.04 0.045 0.05 0.065 0.06 0.065Time (S)

Figure 4. The multi carrier switching method

5. Filtering characteristics

In the hybrid power filter, APF may be controlled as acurrent controlled voltage source. This is a basic controlstrategy:V-= KIsh (13)Where Ih, is the harmonic component needed to thecompensation of source current i,.The load is assumed as an ideal current source iL,because iL is almost not influenced by the sourcevoltage. Thus, the equivalent circuit of the system whenthe filter control method is applied to the active filter isrepresented by the diagram shown in Figure 5.

When APF is not used (i. e. K=O), the supply voltage isbalanced and supply current Ih gives sinusoidalwaveform as follows:

zsh = p'Lh (14)

sLS +Zpwhere Ih, Zp, ILh, indicates supply current, the passivefilter impedance and the load current, respectively.

L',,

) Vif

VAh

c

z

Lp Vt/ )ILf

V =KI1

Figure 5. Equivalent circuit of the system

If the supply impedance o\Ls is much smaller than thefilter impedance Zp, desirable filtering characteristicscould not be obtained.Suppose source voltage is sinusoidal. The supplycurrent Ih, the filter terminal voltage VAh, and the outputvoltage generated by the active filter V, are given asfollows:

928 4

zp'sh 'Lh

K+sL +ZPK+sLs +ZpVAh= 'LhK+sLs +ZP

VC= KZP LVh=K+sL~+Z 'L

the system maintains unity power factor operation.(15) These results confirm that the control strategy

substantial improvements on harmonic content of

(16)

(17)

supply current as well as on dynamic response areconfirmed.

BeFbrecope6ationXl4

x0><-2

IfK >> Zp, all the harmonic injected from the load are

absorbed by the LC filter. When K is infinite, the idealfiltering characteristics can be obtained as follows:ish = 0 (18)VAh = Vsh (19)

VCh = ZpILh + Vsh (20)From these equations, we know that the hybrid filtermay have very good performance. The voltage rating ofthe active switches is reduced to half of that could beobtained with multi-level active power filters.

6. Simulation results

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.145 0.15

500 T I50

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.145 0.15

O-200S 9; _ AOA 10 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.145 0.15

<202 X < t X X<v /

Aft r conpensabon

-500 L XL 40.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.1,45 0. is

Tmne (s)

Figure 6. Steady state simulated waveforms of theSAPF

Numerical simulations using Matlab/Simulink havebeen carried out. A voltage source of 25 KV (rms) at afrequency fs= 60 Hz is considered. It feeds anuncontrolled diode bridge rectifier with a resistive load20 Q, and an inductor L=20mH which represents theAC/DC conversion scheme of the traction system. Thisload produces a distorted current containing all oddharmonics of the fundamental frequency (60 Hz). Thesubstation transformer impedance is L,= 20 mH, R,=100 Q. The parameters PI-section are L= 4.5 mH, R=0.3 Q, C=0.1gF. The active filter has been designedwith parameters L = 50 mH, C = 8000 fF, R = 10 kQ.The DC bus voltages are Vdcl= Vdc2=55KV. Theswitching frequency for the switching devices is fixed to10 kHz. The passive filter tuned at 180Hz. The supplyvoltage (VA) before compensation, load current (iL),MHPF current (ic), supply voltage (VA) aftercompensation, and supply current (is) aftercompensation are depicted in Figure 6. This figureshows that the compensated source current i,(t) is analmost sinusoidal signal in phase with the sourcevoltage vL(t), despite of the highly distorted load currentlL(t). Moreover, the harmonic spectrum of the supplyvoltage and the supply current before and aftercompensation are shown in Figures 7, 8, 9 and 10. Thetotal harmonic distortion (THD) of the supply voltage isreduced from 31.08% before compensation to 0.12%after compensation and the THD of the supply current isreduced from 33.73% before compensation to 0.51%.The speed and load condition of the train alwayschanges frequently. Following to this, it is necessary toexamine the performance of the system when variationsin the nonlinear loads are considered. Figure 11 showsthe response of the system during a load step change.For this, the resistor in the diode rectifier is changedfrom 20Q to 1OQ. During the change of load condition,

II....I .I I

Figure 7. Supply voltage spectrum before compensation

Ii... .Figure 8. Supply current spectrum before compensation

Figure9. Splvta ser ater cme

Figure 9. Supply voltage spectrum after compensation

I-DC T

O ____________________ ____0 C10 100 2:00 20

1 ~ ~ ~ ~ ~ ~ r q c soz)

Figure 10. Supply current spectrum after compensation

929

--

_ T r T r

O_ I_ - -m

=______

_ T r r n

-*0_

II

0.1 0.105 0.11 0.115 0.12 0.125 0.13 0.135 0.14 0.145 0. is

-.

5

4x10

> 2

5000

*- 500

500z 0~

-500

'-'0>2

5000

-500

0.05 0.1 0.15

0.05 0.1 0.15

X10 0.05 0.1 0.15

0.05

0.05

0.1

0.1

0.15

0.15Tine (s)

Figure 11. System response to a 100% step increase ofload current

7. Conclusion

The use of an active filter in traction systems to mitigatethe level of harmonics that are created by the tractionvehicles has promising potential. The performance ofthe installed active power filter depends strongly on theaccuracy and stability of the harmonic extractionalgorithm used to generate the command harmoniccurrent reference to be tracked by the inverter innercurrent control loop. This paper presents a controlalgorithm for the multilevel hybrid power filter tocompensate harmonics and reactive power. Unlike mostother extraction algorithms, the implementation of thealgorithm presented does not require complexmathematical transformations, and therefore has asignificantly faster computation speed, response and issuitable for single-phase traction applications. If themains voltages are distorted which is the case, thealgorithm is capable of maintaining equal distortion inthe compensated current. The accuracy and robustnessof the proposed algorithm have been verified bysimulation.

AcknowledgementThe authors are grateful to Canada Research chair inElectric Energy Conversion and Power Electronics forsupporting this work.

References

[1] K.Wada, H. Fujita, and H. Akagi,"Considerations of a shunt active filter basedon voltage detection for installation on a longdistribution feeder," IEEE Trans. Ind.

Applicat., vol. 38, pp. 1123-1130, July/Aug.2002.

[2] Tan, P.C., Morrison, R.E., and Holmes, D.G.:"Voltage form factor control and reactivepower compensation in a 25kV electrifiedrailway system using a shunt active filter basedon voltage detection", IEEE Trans. Ind. Appl.,2003, 39, pp. 575-581.

[3] Z. Sun, X. Jiang, D. Zhu, and G. Zhang, "ANovel Active Power Quality CompensatorTopology for Electrified Railway," IEEETransactions on Power Electronics, VOL. 19,NO. 4, JULY 2004.

[4] R. E. Morrison, "Power quality issues on actraction systems," in Conf: Rec. 9thInternational Conf: Harmonics and Quality ofPower, 2000, pp. 709-714.

[5] P.C. Loh, D.G. Holmes, Y. Fukuta and T.A.Lipo, "Reduced Common Mode Carrier-BasedModulation Strategies for Cascaded MultilevelInverters", in press IEEE Industry ApplicationsSociety Annual Meeting, 2002.

[6] M. Marchesoni, "High-performance currentcontrol techniques for application to multilevelhigh-power voltage source inverters," IEEETrans. Power Electron., vol. 7, pp. 189-204,Jan. 1992.

[7] D. G. Holmes and B. P. McGrath,"Opportunities for Harmonic Cancellation withCarrier-Based PWM for Two-Level andMultilevel Cascaded Inverters", IEEETransaction on Industry Applications, Vol. 37,No. 2, 2001.

[8] M. D. Manjrekar, P. K. Steimer, and T. A.Lipo, "Hybrid multilevel power conversionsystem: A competitive solution for high-powerapplications," IEEE Trans. Ind. Applicat., vol.36, pp. 834-841, May/June 2000.

[9] G. Carrara, S. Gardella, M. Marchesoni, R.Salutari and G. Sciutto, "A New MultilevelPWM Method: A Theoretical Analysis", IEEETransactions on Power Electronics, Vol. 7,No. 3, pp. 497-505, July 1992.

[10] B.R. Lin and D.J. Chen, "Single-Phase NeutralPoint Clamped AC/DC Converter with theFunction of Power Factor Corrector and ActiveFilter", IEE Proceedings - Electric PowerApplications, Vol. 149, No. 1, pp. 19-30,January 2002.

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