12. 2002 Denmark Parallel Operation Active and Passive Filters

7/28/2019 12. 2002 Denmark Parallel Operation Active and Passive Filters

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A Study of Parallel Operations of Active and Passive FiltersZ. Chen, F. Blaabjerg and J. K. Pedersen

Aalborg UniversityInstitute of Energy TechnologyPontoppidanstraede 101

DK-9220 Aalborg East, DenmarkEmails:[email protected], [email protected], [email protected]

Abstract-- This paper reports investigations of the paralleloperations of a current controlled active filter and passive filtersin a system with current harmonic sources. The task of reactivepower and harmonic compensation is shared by the active filterand passive filters. The passive filters are used for supplyingreactive power to the connected ac system and eliminating someharmonics, the active filter is used to cancel out the otherharmonics. This can be achieved by taking the suitable referencecurrent of the active filter and signal processing filters in thecontrol loop.I n parallel combination arrangements of the active and passivefilters can o perate relatively independently, also the compensati onflexibility of the active filter can be fully exploited, such as oneactive filter for several harmonic sources.

The simulation studies on various systems have been performedto evaluate t he effectiveness of the systems. The results s how thatthe power factor is corrected by the passive filters, harmonics areminimized by both active and passive filters and overloading ofthe filter s ystem can be avoided.

I. INTRODUCTIONNonlinear loads such as diode and thyristor rectifiers andunintermptible power supplies produce harmonics and theyoften consume reactive power. Without a proper compensationthe power quality of ac supply systems can be deteriorated.Passive filters have been widely used to absorb harmoniccurrent of nonlinear loads due to their low cost and highefficiency. However, passive filters suffer some drawbacks,such as strong dependence on system impedance, heavy,susceptible to source and load resonance and the variation offilter characteristics due to aging. Active power filters havebeen developed to compensate the problems of passive filters.Although providing excellent compensation results, pure seriesand shunt active filters are not yet a cost-effective solution dueto their high construction and operation costs. Consequently,hybrid filters are attracting more and more attentions, varioustopologies, such as the combination of series active andparallel passive filters [2] and active filter in series withparallel passive filters [3, 4, 51 have been developed tocombine the advantages of passive filters and active filters.Other alternatives to reduce harmonics have also beenproposed, such as the integration of a controllable componentin power electronic converters [6] or the combinations ofsingle-phase and three-phase nonlinear loads [7]. The

proposed solution is more applicable in centralizedcompensation systems.

This paper studies the parallel operation of active andpassive filters. Such a connection has the advantage of relativeindependence: passive filters can be in operation whether theactive filter is in operation or not, and within the rating limit,the active filter can also be able to perform some hc t ions ofthe passive filters. However, the connection of passive filtersmodifies the impedance characteristics of the system.Therefore, the co-operation between the active filter andpassive filters is very important for performing effectivecompensations. In the following, this paper will discuss thesystem configuration of the compensation system, analyze thecharacteristics and discuss the possibility of operating theactive filter and passive filters in parallel. Then the controlstrategy will be discussed in Section 111, and a case ofsimulation study will be presented in Section IV .

11. SYSTEM CONFIGURATIONAND IMPEDANCECHARACTERISTICS

A simple configuration of the active filter and passive filtercombinations is shown in Fig. 1, where the filter system,includmg the passive filters and an active filter, is connected inparallel with a current type of harmonic source, such as aninductively loaded thyristor converter.

kJpassi vefilters

Fig. 1. Schematics of a parallel connected hybrid compensator in a simplesystem.

The passive filters,qf,n Fig. 1 are tuned to remove someharmonics and supply reactive power at the fimdamentalfrequency while the active filter is controlled to cancel otherparts of the harmonics. In this system, the active filter and thepassive filters can operate relatively independent, e.g. one of

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the filter system is out of service, the other can still operatewithin the capacity rating of the device and equipment.

However, the connection of passive filters does modify theimpedance characteristics of the system. There is a possibilitythat the passive filters could take the current injected by theactive filter, then both active and passive filters could beoverloaded.The single-phase equivalent circuit shown in Fig. 2may beused to analyze the system characteristics under a balancedoperation condition.

The passive filters provide a harmonic sink, the lowimpedance path, to divert some current harmonics. For a sixpulse thyristor converter load, the passive filter may bedesigned with branches tuned to 5* and 7* harmonicfrequencies and a high pass branch for the high frequency.Fig.3 shows the impedance characteristics of the ac system andpassive filters (5", 7* and high-pass) without the active filtercircuit, where the parameters are based on the system inreference [2]. The supply ac system is assumed to have aninductive characteristic as shown by the solid line in Fig. 3,while the passive filters tuned to 5* , 7& and higher harmonicspresent the characteristics shown in the dashed line.

It can be clearly seen that the passive filter circuit provideslower impedance than the system impedance at the tunedfrequencies (5* and 7" harmonics) and a range of designedhigher frequencies. The passive filter is a bypass branch to theload harmonic current. In the frequency bands where thepassive filters present lower impedance than that of the acsupply system, the higher harmonic deviation capability isobtained. The lower the harmonic impedance of the passivefilters in comparison with the system impedance the better theharmonic filtering performance. Obviously due to the presenceof fimdamental voltage, the passive filter bank can alsogenerate fimdamental frequency reactive power.A parallel connected and current controlled active filter

injects current harmonics into the ac system to cancel theharmonic current from the harmonic sources. An active filterconnected in parallel with the grid and a passive harmonicfilter will behave like a current source, therefore, a currentcontrolled parallel connected active filter requires a lowsystem impedance at the frequenciesof the injecting current tobe effective.

" Current

Iaf 'ctivefilter+

0 1w 2w Po 4ca 5w Em 7w 800 OW 1m1091 ' " f ' 4 1f re quBYWl

Fig. 3. Impedance characteristics of ac system grid and passive filters.However, the parallel connection of passive filters changes

the impedance characteristics and presents the danger of takingthe current injected by the active filter. If the current generatedby the active filter falls into the passive filter frequency band,most of the current produced by the active filter would betaken by the passive filters instead of compensating theharmonics generated by the non-linear load. Consequently,both active and passive filters can be overloaded.

An ideal operation condition may be achieved by separatingthe compensation frequency bands of the active filter andpassive filters. For example, the operation frequencies of theactive filter should be arranged to avoid the tuned frequenciesof the passive filters. This is the basic idea of the paralleloperations of the combined filter systems.

Fig. 4 shows the single-phase equivalent circuits of thediscussed hybrid filter topology. Fig. 4a is for the frequencybands beyond the working frequency range of the active filter,while Fig. 4b is for the frequency bands within the active filterworking frequency range. As shown in Fig. 4a, for thefrequency range of the active filter working frequencies, thetransfer fimction of load harmonic current to the harmoniccurrent entering the system is given in equation (1).

Fig. 2. Single-phase equivalent circuit of a combined filter system.

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(b)Fig.4. Hybrid filter system. (a) Hannonic frequenciesoutof active filter(h:harmonics number.)

range @) harmonic frequencieswithin active filter range.

The active filter is represented with a current source whichcan be controlled in proportion to the harmonic current I&, andexpressed as G A I&, as illustrated in Fig. 4b. Then there aretwo current sources in the simple circuit, a harmonic currentsource driven by the load and a harmonic compensationcurrent source controlled by the active filter. The harmoniccontents of the source current can be determined by

where G A is the gain of the active filter. For the harmonicswhich the active filter is not to compensate, the gain is zero.Therefore the source current would be distributed based on therelationship between the passive filter impedance and the acsupply system impedance, given in equation (1).

111. ACTIVEILTERCONTROLA. Control Strategy of The Active Filter

The control techniques of active filters have been studied bymany researchers. Various methods based on both frequencydomain and time domain have been developed and reported inthe literature. Typically, time domain based methods providefast response, such as the instantaneous power theory [ l ] andsynchronous rotating reference frame methods. In theinstantaneous power theory, the compensation commandsignals are obtained from the instantaneous active power andthe instantaneous reactive power. This method does notrequire the phase synchronization. The use of theinstantaneous power theory based method is reported in thispaper.

Using the instantaneous power theory, the three phasevoltages and currents are transformed into a two axesstationary reference.

2 1 -112 -112[cl={ 0 &I2 4512,Wherei ip are a nd p axis currents,v, vp are a and p axis voltages,i,, ib , i , are a, b and c three phase currents,vu,vb, v, are a, b and c three phase voltages.Then the instantaneous power components can ,e calculatedas

(4)

The fundamental power is represented by the dccomponents of the instantaneous real and reactive power, jjand i j , and the harmonics are corresponding to the accomponents of the instantaneous real and reactive power,and G . herefore, for harmonic compensation, the three phasecompensation currents can be calculated as

The block diagram of the control system is shown in Fig. 5.Three phase currents and three phase voltages are sampled, theinstantaneous real power and reactive power are calculated.The currents, I& , are taken at the point where the effect ofpassive filters has been counted and voltages are sampled atthe terminal .o f the active filter. The high frequencycomponents of the instantaneous power represent theharmonics, which is used to produce the harmonic currentrequired to be compensated. Therefore, in the block diagramof Fig. 5, the high-pass filters are used to block the dccomponents of the instantaneous power and allow the accomponents of the instantaneous power to pass to produce thereference current waveform Iam,*bC y using (5).

B . The Signal Processing FiltersIn order to avoid overload both the active filter and the

passive filters, a signal-processing filter as shown in Fig. 4 isadded into the control loop. This filter removes the undesirableharmonic components in the reference current waveform of thecurrent controlled active filter.

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1 a f h . a . b . c .i,ibi c

calculation calculationT o activefilterva V b V c

-100

-160

-200

Fig.5.Block diagramof control system for the active filter.

--

In the studied system, the passive filter characteristicsshown in Fig. 3, the signal filters should be designed tolimit the frequency signals within the tuned frequency bandsof the passive filters so that the compensation currentgenerated by the active filter would not overload the passivefilters.

Iv. IMULATION EXAMPLESThe simulation studies have been carried out on the

system shown in Fig. 1. The software packagePSCADEMTDC has been used. The hybrid filter systemconsists of an active filter and passive filters, which areconnected in parallel with a harmonic producing load, i.e., athree-phase thyristor bridge converter of the rating of 20kVA.

The passive filters are consisting of the branches tuned to5*, 7* and higher harmonics (HPF). The parameters of thepassive filters and ac supply system are given in Table I [2],which have the corresponding impedance frequencycharacteristics as shown in Fig. 3. The active filter is aPWM voltage source converter, which works in currentcontrol mode.

Fig. 6 shows a group of results. These current waveformsare in the sequence of non-linear load, passive filter, activefilter and ac system downward. The active filter is switchedinto operation at 0.1 seconds. The reactive powercompensation effects of the passive filters can be clearlyseen by comparing the load and the ac system currents. Itcan also be seen that the operation of the active filter has nosignificant effects on the passive filters.

Figs. 7 and 8 present the current harmonic spectra beforeand after the active filter is activated respectively. Theharmonics are presented as the percentage of thefundamental current of the thyristor converter load current.

Simulation results show that the active filter and passivefilters can share their responsibility of harmonic andreactive power compensation. Also, the current carried bythe active filter is reduced in comparison with the case of aconventional pure active compensator. Consequently, therating of active filters is reduced.TABLE

CIRmPARAMETERSPassive Filter

5th L=1.2mH C=340 pF7th L=1.2mH C=170pFHPF L=0.26mH C=300pF R=3R

I O3 "4 -60

3fi o-50

SvstemZs=0.02pu (inductive)

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(c)only)

Fig. 7.Current harmonics before active filter activated (passive filter(a) Thyristor converter harmonic spectra.

@) Passive filter harmonic spectra.(c) Supply system harmonic spectra.

sh.",-*lOn FI.."N ( ,e -,d P-C- .,b-.-)

mmu,.*,on FI un. (-M .m-r -d P u r * - n(b-I

o * -a e-

E o , -io.-!""-&=-S O L -

0 2 -

0 1

-hn , A A A .00 2m 400 am eo0 t o m 1 a l O ,400 ,e00 *a00 -

The total voltage harmonic distortion (THD,) can beTHD, =H , L = H , lWhere I1 and SI are the fundamental current and power

taken by the converter from the grid, I,, and S ,, are the shortcircuit current and power at the point of common coupling(PCC) and H, is the harmonic coefficient and may becalculatedas

calculated as(6)

z s c s c

H c = h h ~ 1 0 0 %dp,( 1 (7)For the above studied case, the I& of the converter

without any filters is 173%, the passive filters reduce the &to 144%, then the active filter reduces the H, further to68%.

Considering a harmonic distortion limit of 5% , theallowed ratio of the converter rating to the system shortcircuit capacityis

For the studied system, the passive filter allows theconverter rating to increase to about 1.2 times of theconverter system without any filters, the combined hybridsystem would further increase the rating of the convertersystem to 2.56 times.

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V. CONCLUSIONSThe proposed paper presents the study of the parallel

operation of a current controlled active filter and passivefilters. The effects of the impedance characteristics havebeen discussed in terms of the effectiveness of filters.

The idea of sharing the compensation tasks of reactivepower and harmonics between the two types of filters hasbeen discussed. The implementation method of using thedigital filter system in the instantaneous power theory basedcontrol method has been presented.

The simulation studies on the presented circuits andcontrol system has demonstrated the effectiveness of theproposed compensation techniques in improving the systemtechnical and economic performance.

Simulation results show that the active filter and passivefilters can share their responsibility of harmonic andreactive power compensation. Consequently, the rating ofactive filters is reduced. Also the controllability of theactive filters can be fully exploited.

VI. REFERENCESH. Akagi, Y. Kanazawa, A. Nabae, : Instantaneous Reactive PowerCompensators Comprising Switching Devices without EnergyStorage Components, IEEE Trans. on Industry Applications, Vol.20, May/June 1984, pp 625-630.F.Z. Peng, H. Akagi, and A. Nabae. A new approach to harmoniccompensation n power systems-A combined system of shunt passiveand series active filters. IEEE Trans. on Industry Applications, Vol.26, No. 6, November/December 1990, pp 983-990.H. Fujita and H. Akagi. A practical approach to harmoniccompensation in power systems - series connection of passive andactive fillers. IEEE Trans. on Industry Applications, Vol. 27, No. 6,November/December 1991, 1020-1025.S. Bhattacharya, P. T. Cheng, and D. Divan. Hybrid solutions forimproving passive filter performance in high power applications.IEEE Trans. on Industry Applications, Vol. 33, No. 3, May/JuneP. T. Cheng, S. Bhattacharya, and D. Divan. Control of square-wave inverters in high power hybrid active filter systems. JEEETrans. on Industry Applications, Vol. 34, No. 3, May/June 1998, pp458472.S. Hansen, P. N. Enjeti, J. Hong Hahn, F Blaabjerg. An ntegratedSingle-Switch Approach to Improve Harmonic Performance ogStandard PWM Adjustable Speed Drives. IEEE Trans.on IndustryApplication, Vol. 36, No. 4, July/August 2000, pp. 1189-1196.S. Hansen, P. Nielsen, F. Blaabjerg. Harmonic Cancellation byMixing Non-linear Single-phase and Three-phase Loads. IEEE/IASTrans. on Industry Applications,Vol. 36, No. 1,2000, pp.152-159.

1997, pp 732-747.

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12. 2002 Denmark Parallel Operation Active and Passive Filters

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