comparison of volt and curr source

8/4/2019 Comparison of Volt and Curr Source

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642 IEEE Transactions on Power Systems, Vol. 14, No . 2, May 1999A Comparison of Voltage Source and Current Source Shunt Active Filter by

Simulation and ExperimentationL. Benchaita, S . SaadateGreen-uhp-cnrs ura 143854500 Vandaeuvre-1Bs-Nancy France

Abstract - The utilization of active power filters insuppressing harmonics generated by nonlinear loads in acdistribution power systems will considerably be generalized inthe near future. With regard to the circuit topology, there aretwo basic kinds of active filter, voltage source and currentsource. This paper presents a comparison of these topologiesfrom different points of view such as power circuit design,semiconductor constraints, filtering quality, robustness,adaptability and load transient b ehaviour.Keywords- ctive power filter, voltage source inverter, currentsource inverter, harmonics, total harmonic distortion, nonlinearload,ac distribution system

I. INTRODUCTIONIn recent years, active power filters have attractedconsiderable attention as an efficient way of minimizingvoltage and current disturbances caused by the increasing useof nonlinear loads in AC distribution systems[l]. The activefilter's functions can vary from harmonic current and flickercompensation [2] to voltage sags correction [SI and voltagebalancing [lo]. Further, the active filter will be used toimprove stability in power system as the capacity of active

filters becomes larger [4]. Consequently, differentconfigurations of active filter, shunt and series, have beenproposed and studied [2,10].With regard to their power circuit topology, active filterscan be categorized into two basic structures:1) Voltage source active filter (VSAF) which consists of

2) Current source active filter (CSAF) made of current fedvoltage fed inverter [7].

A.Salem niaPower & Water Institute of TechnologyP.o. box 16765-1719 Tehran Iran

In spite of this preference, both structures of active powerfilter have been widely considered by the researchers.However, a comparative study between the two types ofactive filter is, until now, missed by the literature except abrief comparison based mainly on economical considerationsIn our opinion, though economical aspect is a convincingjustification for industrial partner, a complete comparisonbetween the two active filter structures should also involx.~=technical analysis. Besides, following remarks should betaken into account :a) The role of the active filter is first to improve powerquality of the AC distribution system.b) The economic reasons which are actually in favor of VSAFmay change in the near future [9]. We think that it maybecome in favor of CSAF when the superconducting coilswill be practically used in higher temperatures.The main target of this paper is the comparison between thetopological and operational features of the two typzs ofactive filter. The active filter is considered to be shuntconnected to the AC mains and acts as harmonic currentcompensator. Its performance is evaluated by total harmonicdistortion factor of the AC mains current. Firstly, by means ofsimulation study, the advantages and disadvantages of eachstructure considering the following comparison points arebrought out : power circuit design, semiconductor constraints,filtering quality, robustness and adaptability and loadtransient regime.Then, the experimental results obtained through two smallrate laboratory prototypes of both structures, VSAF andCSAF, with analogical control are presented and discussed.

141.

11. PRINCIPLE OF SHUNT ACTIVE FILTER ANDSTRUCTURE DESCRIPTIONinverter [51.Basically, an active power filter for harmonic compensationcan operate from a DC energy storage element and does notneed any DC power supply. In order to maintain a constantDC voltage or current in the storage element, only a smallfundamental current is drawn to compensate the active filterlosses. The major drawback of the current source active filter

is its high operating losses with respect to the voltage sourceactive filter. Nevertheless, the CSAF presents some importantoperational features [ 6 ] .

Fig.1 illustrates the typical implementation of the shuntactive filter that incorporates the major topologicaldifferences between the two structures VSAF (fig.la) andCSAF (fig.lb). The basic operating principle is described asfollows.The nonlinear load which consists of a six pulse controlledrectifier draws a non-sinusoidal current I,. The objective ofthe shunt active filter is to prevent the harmonic componentsof this current (1,) o circulate bough AC mains impedance(&fLs)? an d hence avoid distortion Of the commonPoint voltage v,.This is accomplished by Controlling theactive filter in order to generate harmonic currents, I whichmatch in magnitude and phase with those existing in the loadcurrent.

PE-326-wRS-0-06-1998 A Paper re cm "n de d and approved bythe IEEE Power System Dynam ic Performance Committee of the IE EEPower Engineering Society for publication in the I EEE Transactions onPower Systems. Manuscript submitted December 29, 19 97; madeavailable for printing June 1 2, 19 98.

0885-8950/99/$10.00 0 1998 IEEE


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a: VoItage source active fdter b: Current souroe active fdter

Fig. 1. Principlescheme of shunt active fiterA . Description o W A F structureFig. la shows the voltage source structure of shunt activefilter which uses a DC capacitor , C , as an energy storageelement. The inductance, L5 through which the inverter isconnected to the power supply network, ensures firstly thecontrollabilityof the active filter currents and secondly actsas a first order passive filter attenuating the bgh frequencyripples generated by the inverter [2].B. Description of CSAF structure

In fig.b, a current source active filter made upof a currentsource inverter using an inductance, Ld5 as a DC storageelement is given. In this case, the inverter is connected to theA C mains through a second order low pass filter formed byC and L5 The harmonics near to the filter's resonancefrequency are amplified. In order to prevent thisphenomenon an additional resistance has to be added or anappropriate current control which takes into account thederivative of the instantaneous active filter current should beadopted [6].C. The control princ iple o the shunt activejl ter

In both structures, the control system of the active filter iscomposed of two parts ; the calculation of the harmoniccurrents and the generation of the control signals for theinverter semiconductor devices.For the first part, harmonic components of the load currentare identified by use of the instantaneous real and imaginary

power method [3]. In this method, the fundamental currentis shifted to DC frequency which can easily be separatedfrom the other components.The harmonic current is thencalculated from the alternating powers and as shown in

Once the harmonic currents are detected, each of them ortheir sum is used as a reference signal in the inverter currentcontrol loop and thus compared with the real active filtercurrent to generate the switches control signals. For a VSAFtype, the inverter is controlled by an ordinary hysteresistechnique. In fact, the hysteresis control provides a betterlow order harmonic suppression than PWM control, which isthe main target of active filter [121. An appropriate pulsemodulation technique, adapted to the current source invertercircuit topology, i s employed for the CSAF [ 5 ] . Its principleis illustrated in fig.3. It is based on the correction of the twoout of three larger errors of the real active filter current withrespect to its reference. For instance, as long as Aifl is thelargest positive error and AIp is the largest negative one, thecurrent in phase 1, ImI is made equal to Idf ,whilst fm2sequal to -Zdf and Zm3 null. Thus only switches T I and T2' arekept in on state when all others are blocked. The active filtercurrent in phase 1, I, increases and that in phase 2, I J ~ ,decreases resulting in decreasing the errors and Arespectively. This state does not change until the error inphase 3, dP,ecomes larger than AZfl or lower than AI f l ,This principle avoid to command more than two switchessimultaneously which is an important requirement in currentsource inverter control.

fig.2 .

111. COMPARISONA. ower circuif design

The power circuit design of the shunt active filter in bothstructures, VSAF and CSAF, involves selecting theparameters of the DC source and the elements of the ACoutput passive filter as well as the choice of the propersemiconductor devices for each structure.1)- DC source element : The selection of the DC voltage V,or current Id f is made in order to permit the active filter tobecapable to follow its reference. The capacitor in VSAF or the


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Fig.3. The principleof the modulationtechnique of CSAFreactor in CSAF is designed in order to provide a DC voltageor current with acceptable ripples.In the case of VSAF structure, in order to assure the filtercurrent at any instant, the DC voltage V must be at leastequal to 3f2 of the peak value of the line AC mains voltage.For the controllability of the active filter current during therectifier commutation, V, should verify relation (1)[11]:

are the rms value and the angular frequency of the mainsvoltage, and a is the thyristor firing angle.Ones the voltage V, is fur&, the DC side capacitor valuecan be calculated considering an acceptable DC voltageripple AV, as follows :(2)c iV, AI ;

where is the harmonic power to be generated by the activefilter.

In the case of CSAF. the DC current, Idfi must be higherthan the peak value of the harmonic current to be generatedby the active filter. A s in the previous case, the inductance,L, is fixed by an allowable current ripple during eachoperation cycle according to the following expression :

c=- p

Equations ( 2 ) and ( 3 ) show that for smoothing DC voltageor current and thus o improve the performance of the activefilter, a large capacitor or reactor should be used whichincreases the active filters cost. However in low and averagepower applications, a capacitive energy storage element ismore efficient and costs less than an inductive energy storageelement. On the contrary, in the case of high powerapplications, superconductor coils are the most reliableenergy storage elements. Further, these elements have a largeinductance resulting in dc current ripple elimination.It can be deduced that in ordinary power ratings, VSAF ismore suitable thanCSAF with regard to DC element.2)- The output passive f i l t e r : An active filter must beconnected to the A C mains through a decoupling passivefilter. The design of this filter should consider two points :a) Providing a sufficient attenuation of the high switchingripples caused by the inverter.b) Preserving high perfomiance for the active filter.The VSAF is generally connected to the A C mains via a

fvst order low-pass filter formed by a simple inductance, L, asillustrated by fig.la. In order to provide a sufficientattenuation of the high switching ripples caused by theinverter, an important inductance should be used whichdeteriorates the filtering efficiency if the inverter DC voltageis not high enough. So, n order to improve the attenuation ofthe high frequency harmonics, a third order filter should beemployed [7]. In this case, the filter produces two resonancefrequencies which may complicate the active filter control.The optimized output filter, through which the CSAF isconnected to the AC mains, is a second order low-pass filterrealized by means of a capacitor C, and inductance L, . asshown in figlb. This filter offers a good compromisebetween a sufficient band-pass and a proper attenuation ofswitching hannonics. This can be accomplished by anaccurate selection of the resonance frequency,

, which must be larger than the highestharmonic frequency to be compensated and at the same timemuch lower than the inverter switching fiequency. At a givenresonance frequency, since the capacitor C protects theswitching devices against the overvoltage caused by the highcurrent gradient in the inductance Lp increasing Lf will resultin decreasing Cf and consequently reduces the semiconductorprotection. On the other hand, Lfmust be chosen higher thmthe intemal inductance of the AC mains.Thus, a compromisehas to be made in order to obtain the optinium filter design.It should be noted that harmonic amplification near theresonance fiequency can easily be avoided by using anappropriate current control of the active filter. In fact, in orderto damp the oscillations caused by the resonance indecoupling filter L r C f , a term, kW,, corresponding to theactive filter current derivative should be added to the referencecurrent. One can easily demonstrate that the parameter k actsas a damping factor in active fdter transfer function [ 6 ] .From the forgoing discussions, it can be concluded thatCSAF offers a better compromise than VSAF between controlcomplication and filtering capability of high frequencyharmonics.3)- Semiconductor devices and constraints : Figs.4a and 4bshow the voltage and current waves applied to the switchingdevices during one cycle. In fig. 4a, case of VSAF structure,the voltage which must be supported by one switch i sunipolar and limited by the DC voltage V , The peak value ofthe current which is bi-directional is imposed by the activefilter current. Thus the appropriate semiconductor device maybe an IGBT with an antiparallel diode and must be protectedagainst overcurrent. On the contrary, in CSAF structure asillustrated in fig. 4b, the current is unidirectional and limitedby the DC current source while the voltage is bipolar and itspeak value corresponds to that of line voltage. So the suitablesemiconductor device, in this case, may be a GTQ withreverse voltage blocking Capability or an IGBT with seriesconnected diode and must be protected against overvoltage.Actually IGBT transistor has lower losses and can operate athigher switching Erequencies than GTO thyristor. Hence, fromthis point of view, VSAF which is made of only six IGBTmodules can be preferred to CSAF which needs six seriesdiodes if made of IGBTs or six high losses symmetricalGTOs.

fo = 2 7 r G


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645

10.1- 0.1.0 0.h. a. m- 4 , . . , . * , , , . , . * * , ,0 . b

t (sec.)(a)

-1000 1 . . . . . . . . . . ,' , . . . .

(b 1 t (sec.)Fig. 4. Switches voltage and current constraints : (a) for VSAF, (b) for CSAF

B) Filtering eficiencyThe comparisons are made under the same operatingconditions i.e. both structures of active filter, VSAF andCSAF, operate at the same mean switching fi-equency andhave to generate the same harmonic power. In fig. 5 arecompared the spectrums of the nonlinear load current, I,, thesource current, I,, obtained by use of voltage source andcurrent source active filter when the firing angle of therectifier is ~ 3 0 " .ither VSAF or CSAF improves highly the

THD of the source current. For the first type, the THD isdecreased from 28.2% to 7.6% and for the second fiom28.2% to 6 .3%. The better efficiency of the current sourcewith respect to voltage source can be noticed in fig. 6 by itscapability to eliminate distortion caused during the thyristorscommutation in the rectifier. From fig.6, it appears that theTH D evolution, when a increases, is the same for bothstructuresof the shunt active filter. The two curves coincidefor the low values of the firing angle (a < 20") and thenseparate from each other. The THD deterioration is due to theincrease of the harmonic current gradient which becomeshigher than the maximum gradient of the active filter current.It can be shown from fig. 7, that VSAF is more affected thanCSAF by the variation ofa.

%25 1 0 4I,withVSAF

Iv with CSAF1050 5 7 1 1 13 17 19 23 25

t (sec.)Fig. 6 . Load and source current waves from top to bottom ;I, : load current,Is source current(VSAF),2 : source current(CSAF)

% O - HD of i. (wth VSAF).... ..THD of i.(with CS A F )-i

0 I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .d i Q ' O e S b 8 d O ' O e j . ba (degree)

Fig. 7. THD versus firing angle aC) Robustness and ada ptability

The influence of the mains inductance, L,, and the qualityfactor of the output passive filter on the compensationcharacteristics of VSAF and CSAF is discussed in this part.Fig. 8 gives the source current total harmonic distortion as afunction of the AC mains inductance. It demonstrates that inVSAF structure the THD decreases when L, increases. Thisimprovement in the compensation characteristic is theconsequence of the current gradient limitation in the AC sideof the rectifier. However, for CSAF structure as given in fig.8, the inductance L, beyond a certain limit, 75pH in our case,does not favor the harmonic compensation because of thedecrease of the active filter band-pass. This means that CSAFstructure is more sensitive to the variation of the powersupply impedance. In fact, the fi-equencies of some dominantharmonics may be higher than the break frequency of thesecond order passive filter formed by Cfi fi L, and L,.Fig. 9 shows that the quality factor of the additionalinductance Lf does not have a noticeable effect on theharmonic compensation neither in VSAF nor in CSAF.

- ith VSAFWith CSAF

_ - - - -_.---- - -- -4 A -- - -a . B b . B . d - 1 6 0 . do

LS (PH)Fig. 8. THD of the source current versus Lsig. 5. Current spectrums


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646- ith VSAFWith C S A F....

0 j , , , ,0 . h 0 . h o . h o 0 . h 0.630 0.L 0 . h o . 6 a 0 .

Rf(0)Fig. 9. TH D of the source current versus R,

0) oad transient behaviourThe voltage or current variation in the DC side of the activefilter depends on the fluctuation of the fundamental activepower exchanged with the mains. An abrupt decrease of theDC load current of the rectifier results in a brutal increase ofthe DC side voltage or current of the active filter. Themagnitude of this variation depends on the selected values ofCor Ldfand the regulation loop of V, or Idf Figs. 10a and 10bshow the DC source control behaviour when the firing angle,

a, increases fiom 30" to 60" during one half of the mainscycle. Either DC voltage in VSAF or DC current in CSAFreaches a peak value which is about 110% of its referencevalue. Since in load transient regime, VSAF should supporthigher voltages and CSAF higher currents, it could beconcluded that VSAF has a better switch utilization.IV. EXPERIMENTAL DEVICE

Two experimental models of shunt active filter have beenrealized in our laboratory with 20 kVA thyristor rectifier asthe nonlinear load and 220V, 50Hz AC mains with a short-circuit power of 25 kVA. The firing angle a s set at 30".

a00

700

6 B 0 1 , , , I , , , , 10.08 0.10 0.12 0.14 0.16 0.18 0.20t (sec )

400200:4 a40 05 x 4 ,

O.!M O.'lO 0.'12 0.'14 O.Il8 0.'18 0."(h ) t (sec.)

10. DC source control behaviour in case of nonlinear load transient

The VSAF prototype in fig. l l a is made of 6 IGBTmodules. The DC voltage is set at 250V. The results are givenin figs. 1b and 1IC.Each switch of the CSAF prototype (fig.12a) consists of an IGBT in series with a fast diode. For thecurrent waveforms of fig. 12b, the DC current is SA.These experiments (figs. 11 and 12) show that in both cases,VSAF and CSAF, the magnitude of harmonic components areconsiderably reduced in the AC mains. The total harmonicdistortion of this current is decreased fiom 29,3% to 3 3 %with VSAF and to 4 % with CSAF.

V.CONCLUSIONTwo topologies of shunt active filter, voltage source(VSAF) and current source (CSAF) have been compared inthis paper. It can be concluded from the discussions abovethat :1) Both options of active filter VSAF and CSAF achieve themain target of the active filter and hence improve thepower quality of the distribution system.2) The major advantage of the VSAF structure is in its

capacitive energy storage element which is more efficient,smaller and costs less than inductive one used in CSAF.Nevertheless, this advantage may become in favor ofCSAF in near future when superconductor coils arepractically used in higher temperatures.3) A proper sizing of the output passive filter, through whichthe active filter is connected to the AC mains, must takeinto account the inverter switching frequeccy.Theobjectives claimed to this filter may be better satisfiedwith CSAF than VSAF. VSAF is more sensitive to thenonlinear load operating point than CSAF, which meansthat CSAF may be the better effective solution in case ofvariable environmental applications. On the contrary,CSAF is more affected than VSAF by the modification ofAC supply internal inductance.Finally, experimental results confirm the excellentperformance of both realizations of active filter. This shouldchange the actual tendency based mainly on economicalconsiderations in favor of passive filters. Since active filter'scost becomes more interesting in near future, the choicebetween voltage source and current source structure should bemade considering the comparisons proposed in this paper.

4) The performance of

REFERENCE[l] L. Guygyi, E.C.Strycula, "Active AC Power Filters",

IAS'76 annual, pp. 529-535.[2] F.Z. Peng, H. Akagi, A. Nabae "A Study of Active PowerFilters Using Quad-Series Voltage-Source PWMConverters for Harmonic Compensation", ZEEE Trans. onpower Elecs., vol. 5 , no. 1, 1990, pp.9-19[3] E.H. Watanabe, R.M. Stephan, M. Aredes, "NewConcepts of Instantaneous Active and Reactive Powers InElectrical System with Generic Loads", IEEE Trans. onPower D elivery, vol. 8, no. 2, 1993, pp. 697-703[4] H. Akagi, "Trends in Active Power Line Conditioners",IEEE Trans. on Power Elecs., vol. 9, no. 3, 1994, pp.

regime : (a) case of VSAF, (b) case of CSAF 263-268


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647.LAC mains380V 50Hz 48 R40m H3.3mF

AC mains380V SoHz

(c )Fig. 1 1. Experimental study : (a) Diagram of VSAF prototype, (b) Currentwaves 10N div and IOms/div and (c ) Current spectrums(from top to bottom : load current, active filter current, sourc e current)

[5] L. Benchaita , S. Saadate, "Current harmonic filtering ofnon-conventional non-linear load by current source activefilter", Proceeding of IEEE-ISIE '96,Warsaw, vol. 2, pp.[6] Y . Hayachi, N. Sato, K. Takahashi "A Novel Control of aCurrent Source Active Filter for AC Power SystemHarmonic Compensation", IEEE Trans. on Inustry App.,vol. 27, no. 2, 1991, pp. 380-384[7] C. Lott,0.Lapierre, H. Poulequen, I' High Power VoltageSource Active filter PWM Active Filter with LowSwitching Effect" Proceeding of EPE '95, Seville[8] V.B. Bhavaraju, P. Enjeti, "A Fast Active Filter to Correct

Line Voltage Sags", IEEE Trans. on Ind. Elecs., vol. 41,no. 3, 1994, pp. 333-338[9] 0. Simon, H. Spaeth, K. P. Juenyst, PKomarek,"Experimental setup of shunt active filter usingsuperconducting magnetic energy storage device",Proceeding of EPE '97,Trandheim, vol. 1, pp.447-452[lo] A. Compos, G. Joos, 0. Ziogas, J. Lindsay, "Analysisand Design of a Series Voltage Compensator for tree-phase Unbalanced Source", IEEE Trans. on Ind. Elecs.,vol. 39, no. 2, 1992, pp. 159-167

636-641

L!JJRectifier0.6m HNy\

I I I I 1 1 1 I I I I

I I I I I f 1 I I I I

( c 1Fig.12. Experimental study : (a) Diagram of CSAF prototype, (b) Currentwaves 10Nd iv and lOms/div and (c ) Current spectrums(from top to bottom : load current, active filter current, source current)

[111 J.H. Xu , "Filtrage actif parallble des harmoniques desreseaux de distribution d'electricite U (in fiench), Ph D thesis,INPL -Nancy, Jan. 1994[12] J.W. Dixon, S. Tepper M., Luis Moran T "Analysis andevaluation of different modulation techniques for activepower filters 'I proceeding of IEEE-APEC '94, Orlando,~01.2, p. 894-900.VI. BIOGRAPHIES

Lahlali Benchaita received the (D.1.E) Eng. Degree from "EcoleNationale Polytechnique", Algeria in 1992. Currently, he is a Ph.D.student in electrical engineering with GREEN-UHP laboratory,Nancy1 university, France.Ahmed SALEM-NI.4 BS c (84) and the MSc(89) from university ofScience an d Technology of T e h r d r a n a nd th e Ph.D degree fromW L- Na nc y, France in 1996. He is actually engaged wi th universityof Power and Water in Tehran.Shahrokh. Saadate BSc (79) from universi ty of Technology ofTehrdran , MSc(86) , Ph .D . (86) and these #&at (95) from INPL-Nancy. is research domain is harmon ic compensation of the mains.

comparison of volt and curr source

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