8/3/2019 00062380

1/8

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS. VOL. 26. NO . 6. NOVE M BE RI DE CE M BE R 1990 983

A New Approach to Harmonic Compensationin Power Systems-A Combined Systemof Shunt Passive and SeriesActive Filters

Absfract-A new approach to compensate for harmonics in powersystems is proposed. It is a combined system of a shunt passive filter anda small rated series active filter. The compensation principle is descibed,and some interesting filtering characteristics are discussed in detailtheoretically. Excellent practicability and validity to compensate forharmonics in power systems are dem onstrated experimentally.

~NTRODUCTIONBackground

ARMONIC interference problems generated by bulkH hyristor converters become increasingly serious as theyare widely used in industrial applications andtransmission/distribution systems. So far, shunt passive fil-ters have hitherto been used to suppress harmonics in powersystems. However, shunt passive filters have many problemsto discourage their applications. As shown in Fig. 1, a shuntpassive filter exhibits lower impedance at a tuned harmonicfrequency than the source impedance to reduce the harmoniccurrents flowing into the source. In principle, filtering char-acteristics of the shunt passive filter are determined by theimpedance ratio of the source and the shunt passive filter.Therefore the shunt passive filter has the following problems.i) The source impednace, which is not accurately knownand varies with the system configuration, stronglyinfluences filtering characteristics of the shunt passivefilter.

ii) The shunt passive filter acts as a current sink to theharmonic voltage included in the source voltage, U,,,.In the worst case, the shunt passive filter falls in seriesresonance with the source impedance.iii) At a specific fr eque ncy ( fo = 1/2sJ< , + L,)c,), an antiresonance or parallel reso-nance occurs between the source impedance and theshunt passive filter, which is the so-called harmonic-amplifying phenomenon.Paper IPCSD 90-25, approved by the Industrial Power Conv erter Commit-tee of the Industry Applications Society for presentation at the 1988 IndustryApplications Society for presentation at the 1988 Industry ApplicationsSociety Annual Meeting, Pittsburgh, PA, October 2 - 7 . Manuscript releasedfor publication March 22, 1990.The authors are with the Nagaoka University of Technology, Nagaoka,940-21, Japan.IEEE Log Number 9038516.

VF ( = UL*Fig. 1. Basic principle of shunt passive filter

+ I I I I

- IFig. 2 . Basic principle of shunt active filter.

To solve the preceding problems of the shunt passive filter,shunt active filters using PWM inverters have been studiedand developed in recent years. The basic principle of shuntactive filters was originally presented by H. Sasaki and T.Machida in 1971 [l ]. As shown in Fig. 2, a shunt active filteris controlled in such a way as to actively shape the sourcecurrent, is , into sinusoid by injecting the compensating cur-rent, i This is considered the archetype of shunt activefilters. Since a linear amplifier was used to generate thecompensating current, its realization is unreasonable due tolow efficiency. In 1976, L. Gyugyi and E. C. Strycula [2]presented a family of shunt and series active filters, andestablished the concept of the active filters consisting ofPWM inverters using power transistors [2]. However, noattention has been paid to series active filters and no experi-mental result has been shown in any papers, because there isno available way to shape the source current into sinusoid.

0093-9994/90/1100-0983$01OO 0 990 IEEE

8/3/2019 00062380

2/8

98 4 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 26, NO . 6, NOVEMBERIDECEMBER 1990Harmoi i ic-Producing Load

(ZOkVA)

rIIIISeries Active Filter I

(0.45kVA) II

I&,IIIII

_ _ _ _ _ _ _ _ _ _ _ _ _ I

I

II 5th 7th High-Pass III S h u n t Passive F i l t e r ( l 0 k V A ) IL _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 2

V Fu UF u UFw is u s u s wFig. 3. Circuit configuration of combined system.

In the beginning, shunt active filters were proposed tosuppress the harmonics generated by large rated thyristorconverters and inverters used in HVDC transmission sys-tems. However, they could not be realized in real powersystems because high-power high-speed switching deviceswere unavailable in the 1970s. Then N. Mohan et al. [3]presented a practical means for injecting the compensatingcurrent, which was implemented by using naturally commu-tated thyristor inverters with a specially designed passivecircuit to reduce the fundamental voltage rating of the shuntactive filter. However, the thyristor inverters generate unde-sirable high-order harmonics, which thus discourages theirpracticability.With remarkable development and advances in switchingspeed and capacity of power semiconductor devices in the1980s, shunt active filters using PWM inverters have beenstudied, with a focus on their practical applications in realpower systems [4]- [l l] . At the same time, the followingproblems of shunt active filters have been pointed out, delay-ing their practical uses [5], [7].

i)

ii)

iii)

It is difficult to realize a large rated PWM inverterwith rapid current response and low loss for use as amain circuit of shunt active filters.The initial cost is high as compared with that of shuntpassive filters, and shunt active filters are inferior inefficiency to shunt passive filters.Injected currents by shunt active filters may flow intoshunt passive filters and capacitors connected on thepower system [131.

A New A pproach to Harmonic CompensationConventional shunt passive and active filters have the

aforementioned problems, which make their practical appli-cations difficult. Therefore a basic question is asked: Isthere an approach to solve the problems of shunt passivefilters and those of shunt active filters?

As is known, filtering characteristics of a shunt passivefilter partially depend on the source impedance, which is notaccurately known and is predominantly inductive. Theimpedance of the shunt passive filter should be lower than thesource impedance at a turned frequency to provide the atten-tuation required. Hence the higher the source impedance, thebetter the filtering characteristics. However, the sourceimpedance should exhibit a negligible amount of impedanceat the fundamental frequency so that it does not cause anyappreciable fundamental voltage drop. These two require-ments, which contradict each other, can be satisfied only byinserting an active impedance in series with the ac source.Also, series and parallel resonances in the shunt passivefilter, which are partially caused by the inductive sourceimpedance, can be eliminated by inserting an activeimpedance. The active impedance can be implemented by aseries active filter using voltage-source PWM inverters.Hence a new approach, which combines the use of a shuntpassive filter and a small rated series active filter, is theanswer to the question.

SYSTEMONFIGURATIONFig. 3 shows a system configuration of the proposedapproach to harmonic compensation, which is an experimen-tal circuit developed in this paper. Fig. 4 shows a detailedcircuit of a series active filter on a per-phase base. A passivefilter consisting of a 5th- and 7th-tuned LC filter and ahigh-pass filter is shunted with a three-phase six-pulse thyris-tor converter of rating 20 kVA, which is considered a typical

harmonic-producing load. The circuit constants of the shuntpassive filter of rating 10 kVA are shown in Table I. Threesingle-phase voltage-source PWM inverters of rating 0.45kVA are inserted in series with the source impedance throughthree single-phase current transformers (CTs; turns ratio =1:20), thus forming a series active filter. A single-phasediode rectifier of rating 50 VA is connected on the dc side of

8/3/2019 00062380

3/8

PENG et al . : HARMONIC COMPENSATION IN POWER SYSTEMS 9 8 5Source r - - - i Load

I :C TLmJ

LI

I 11Fig. 4. Detailed circuit configuration of series active filter on per-phasebase.

I I IFig. 5. Equivalent circuit on per-phase base for Fig. 3

TABLE ICIRCUITONSTANTSF THE S H U N T A S S I V EILTER5th L = 1.20mH C = 340 pF Q = 47th L = 1.20mH C = 170 p F Q = 4HP F L = 0.26 mH C = 3 0 0 pF R = 3 Q

the inverters, supplying the energy corresponding to theswitching and conducting losses in the inverters. The purposeof the CTs is not only to isolate the PW M inverters from thepower system, but also to match the voltage and currentrating of the PW M inverters with that of the power system.The function of the series active filter is not to directlycompensate for the harmonics of the rectifier, but to improvethe filtering characteristics of the shunt passive filter and tosolve the problems of the shunt passive filter used alone. Inother words, the series active filter acts not as a harmoniccompensator but as a harmonic isolator. Hence the requiredrating of the series active filter is much smaller than that of aconventional shunt active filter.

COMPENSATIONRINCIPLEAssuming for simplicitys sake that the voltage-sourcePWM inverter is an ideal controllable voltage source uc , Fig.

3 is represented on a per-phase base by Fig. 5 . The three-phase thyristor converter is also assumed to be a currentsource i, due to. the presence of sufficient inductance on thedc side. Here, Z , is the equivalent impedance of the shuntpassive filter, the constants of which are shown in Table I,and Z, is the source impedance.The series active filter is controlled in such a way as topresent zero impedance to the external circuit at the funda-mental frequency and a high resistance K to source or load

I 1 IFig. 6. Equivalent circuit for fundamental frequency.

I I 1Fig. 7. Equivalent circuit for harmonic frequencies.

harmonics. The control scheme will be discussed later. Forthe fundamental and harmonics, application of the law ofsuperposition to Fig. 5 gives us two equivalent circuits, asshown in Figs. 6and 7, respectively. Here, Vsf s the sourcefundamental voltage and I L f is the load fundamental currentin Fig. 6 , while VSh is the source harmonic voltage and IL his the load harmonic current in Fig. 7.As is seen in Fig. 6 ,the shunt passive filter behaves as a capacitor for powerfactor improvement, but the series active filter does not playany role. Fig. 7 shows that the series active filter acts as aharmonic isolator between the source and load.To demonstrate the compensation principle of the com-bined system, some important equations are derived on thebasis of Fig. 7.Source Harmonic Current: Z Sh

The harmonic current flowing in the source, which isproduced by both the load harmonic current Z L h and thesource harmonic voltage V S h , is given as follows:

Here, Z, is the amplitude of Z,, and Z , is that of Z,.The first term on the right side of ( I ) means that the seriesactive filter acts as a damping resistance, which caneliminate the parallel resonance between the shunt passivefilter and the source impedance, while the second term meansthat the series active filter acts as a blocking resistance,which can prevent the harmonic current produced by thesource harmonic voltage from flowing into the shunt passivefilter. If the resistance K is much larger than the sourceimpedance, variations in the source impedance have no effecton the filtering characteristics of the shunt passive filter, thusreducing the source harmonic current to zero, as shown in(2).

8/3/2019 00062380

4/8

9 8 6 IEEE TRANSACTIONS IOutput Voltage of Series Active Filter: Vc

The output voltage of the series active filter, which is equalto the harmonic voltage appearing across resistance K inFig. 7, is given by

Vc = iFZLhVsh if K 9 ,, z,. (4 )Equation (4 ) implies that the voltage rating of the seriesactive filter is given as a vector sum of the first term on theright side, which is inversely proportional to the qualityfactor of the shunt passive filter, and the second term, whichis equal to the source harmonic voltage.Filter Harmonic Voltage: V F h

The filter harmonic voltage, which is equal to the harmonicvoltage appearing across the shunt passive filter, is given by

( 5 )V F ~ - 2 F Z L h if K % zs, 2 (6)

Equation (6) tells us that the source harmonic voltage doesnot appear on the load side because it applies across theseries active filter.

FILTERINGHARACTERISTICSTo illustrate filtering characteristics of the combined sys-tem, reference is made to the equivalent circuit in Fig. 7.Application of the law of superposition to Fig. 7 makes thefollowing analysis easy. As a result, the absence or presenceof the series active filter produces distinct differences infiltering characteristics.

Harmonic Current Flowing fro m L oad to SourceNeglecting the harmonic voltage source on the source sidein Fig. 7, the load harmonic current divides between theshunt passive filter and the source in proportion to theadmittance of the parallel branches, as shown in the term firston the right side of (1). The ratio of the source harmoniccurrent to the load current under the assumption of vsh = 0is given by

(7 )The amplitude of (7) is called distribution factor. Figs.8(a) and 8(b) show how the distribution factor against fre-quency varies with resistance K for the source inductancesL s = 2% and 5.6% (200-V 20-kVA 50-Hz base), respec-tively. Here the source resistance is neglected because it ismuch less than the source reactance. Note that harmoniccurrents flowing into the source are amplified for all thefrequencies at which the distribution factor is more than 0dB.

3N INDUSTRY APPLICATIONS, VOL. 26 , NO. 6, NOVEMBERIDECEMBER 1990

10

. o

2ol 1 K=O. o

1

(b )5 . 6% .

- 0Fig. 8 . Distribution factor against frequency. (a) L , = 2 % . b) L , =

Case of the Shunt Passive Filter. used Alone, i.e .,K = 0: It is seen from Fig. 8 that the shunt passive filterfalls in parallel resonance with the source impedance at threefrequencies, because the shunt passive filter consists of twoLC filters and a high pass filter. For example, a parallelresonance appears at the 4th harmonic frequency in Fig. 8,thus amplifying the load harmonic current by 22 dB (= 13times). It is difficult, however, to avoid the condition ofparallel resonance for a cycloconverter, because it producesharmonic currents having frequencies that vary with theoutput frequency.Case of the Combined System, i.e., K = I or 2: Theseries active filter reduces the distribution factor for all thefrequencies as compared with the value obtained without it.No amplification occurs in harmonic current at the threeresonant frequencies, because the series active filter acts as adamping resistance. The sharpest filtering of the 5th-, 7th-,and higher harmonics is obtained in K = 2.Harmonic Current Flowing from Source to Shunt PassiveFilter

Under the assumption of I L h = 0 in Fig. 7, a sourceharmonic current flowing into the shunt passive filter isproduced by the source harmonic voltage. The source har-

8/3/2019 00062380

5/8

PENG er al.: HARMONIC COMPENSATION IN POWER SYSTEMS

2 -

1-

5 1000

I

15 f / f . Z0J , J L(b)Fig. 9. Ratio Z , / Z , against frequency. (a) L , = 2 % . b) L , = 5 .6%.

monic current is expressed by the second term on the rightside of (1):

Here, 2, = 2 + 2 + K and Z , is a rated impedance,i.e., 2 52 on a 200-V 20-kVA 50-Hz base. Figs. 9(a) and 9(b)show how the ratio Z , Z , against frequency varies withresistance K for L , = 2 % and 5. 6%, respectively.Case of K = 0: For example, the shunt passive filter fallsin series resonance with the source impedance at the 4th-harmonic frequency in Fig. 9(b). Even if a small amount ofharmonic voltage at the resonant frequency is included in thesource voltage, the harmonic current flowing in the sourceand the shunt passive filter will be excessive. The conditionof such a series resonance must be avoided, or overheatingand failure will occur in the shunt passive filter.Case of K = I o r 2: The series active filter increases theratio Z , / Z o for all the frequencies, as compared with thevalue obtained without it. No harmonic current flows into thesource or the shunt passive filter at the three resonant fre-quencies, because the series active filter acts as a blockingresistance.CONTROLIRCUIT

To control the series active filter in such a way as topresent zero impedance for the fundamental and pure resis-tance for the harmonics, the reference output voltage of theseries active filter is given byv$ = Ki,, (9 )

where is, is the source harmonic current, which can becalculated by applying the instantaneous real and imaginarypower theory, the so-called p - q theory developed by H.Akagi et al. [12].Transformation of the phase voltages u L u , u L v , and uL w onthe load side and source currents is,,, is, , and is , into a-0

orthogonal coordinates gives the following expressions:

According to [12], the instantaneous real power p and theinstantaneous imaginary power q can be defined as

Note that the dimension of q is not watt, volt-ampere, or varbecause vL a * iss and uLB . is, are defined by the product ofthe instantaneous voltage in one phase and the instantaneouscurrent in the other phase.The harmonic components p , and-q, are extracted fromp and q by using a high pass filter. A first-order high passfilter, the cutoff frequency of which is 35 Hz, is used in thefollowing experiment, filtering characteristics in transientstates being taken into account. In the calculation circuit ofi s h , the following calculations are performed:

(13)The reference output voltage given by (9) is compared with atriangle carrier, producing the PWM switching patterns.Here the frequency of the triangle carrier is 15 kHz. There-fore the series active filter operates as a controllable voltagesource, while a conventional active filter operates as a con-trollable current source. Hence a voltage-source PWM in-verter is suitable for the series active filter rather than acurrent-source PWM inverter. The voltage-source PW M in-verter used in the series active filter can be protected againstovervoltage and overcurrent by the following means. All thepower MOSFETs of the upper legs are turned off to releasethe dc capacitor from the secondary of the CTs, while thoseof the lower legs are turned on to short the secondary of theCTs through the on-state power MOSFETs and diodes.

DESIGNF A SMALL ATED ASSIVEILTEROSUPPRESSWITCHINGIPPLESIt is important but difficult to design an effective passivefilter to suppress the switching ripples of voltage or currentgenerated by shunt active filters, because it is necessary toknow the source impedance of the associated power system.In the proposed system, however, the design is simple and

the implementation is easy, because the PWM inverter isinserted to the power system through the CT with large turnsratio.As shown in Fig. 4, the purpose of L , (1.0 mH) andCr(0.33 pF) connected in the secondary of the CT is tosuppress the switching voltage ripples caused by the voltage-

.-

8/3/2019 00062380

6/8

~

98 8L ,

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 26. NO. 6 . NOVEMBERIDECEMBER 1990

i s1

ZF@ +iL

I 1 I(b)Fig. 10. Equivalent circuit for switching ripples o f voltage and current. (a)Equivalent circuit seen from PWM inverter. (b) Equivalent circuit for U,.

source PWM inverter. Power MOSFET's are used in thesystem, and their switching frequency f s is 15 kHz. Fig.10(a) shows the equivalent circuit seen from the PWMinverter, and Fig. 10(b) shows the equivalent circuit for theswitching voltage ripple V,. ZpWMs the amplitude of thesum of Z, and Z,, which are seen from the secondary of theCT and can be represented as

ZPWM = (. , /.I>' 1 % + GI. (14)For the combined system, f, = 15 kHz, n 2 / n , = 20,L, = 2 % , and Z, can be calculated from Table I. Thefollowing relationship can be obtained

Z p w M = 9. 6 kL?9 1 / { 2 ~ ( 2s )C, )= 16 L? 4 ~ ( 2s ) L , = 200 L?. (15)

Note that the frequency of the switching ripples is 2 s inFigs. 3and 4. The harmonic currents caused by the switchingvoltage ripples V , are completely shunted by C,, ardlyflowing into the power system. In addition, almost no switch-ing voltage ripples appear in the primary of the CT's,because the V, are reduced to 1/400.Since the turns ratio of the CT's can be increased to 1:50,the relationship represented in (15) is still valid, even underthe condition of a practical switching frequency of 2 -3 kHz.The small rated passive filter to suppress the switchingripples can simply be designed according to (15).

EXPERIMENTALESULTSTo demonstrate the practicability and validity of such acombined system as shown in Figs. 3 and 4, the followingexperiments were done under the conditions that K = 2 Q ,the dc voltage of the PWM inverters was 120 V, and thecapacitance of the dc capacitor was 1500 pF. The loss of the

I ,: 25OV

fiF:lOOA

L

ih ~LW&J/rA~/nW$n1s ta r tFig 11 . Experimental waveforms in case of L , = 2 %

Fig. 12. Frequency spectra of i s . a) Before started. (b) After started

PWM inverters was supplemented by a single-phase dioderectifier of rating 50 V A .Fig. 11 shows the experimental waveforms on the transientstate at the start of the series active filter in the case ofL , = 2%. Fig. 12 shows the frequency spectra of the sourcecurrent, before and after the start of the series active filter,under the same conditions as Fig. 11. Before the series activefilter was started, not enough filtering effect was obtained, sothat a large amount of harmonic current still remained in thesource current. However, after the series active filter wasstarted, the source current became purely sinusoidal, showingthat the series active filter improves the filtering characteris-tics of the shunt passive filter. In Fig. 11, the rms value of

8/3/2019 00062380

7/8

PENG er a l . : HARMONIC COMPENSATION IN POWER SYSTEMS

_____

989

r V,:250V

vc:mvL.---yMww/M!dstart

Fig. 13. Experimental waveforms In case of L , = 5.6%.

f 1,:lOOA

7 lL:lOOAIvc:20v

L J d b p ? -s t a r t

Fig. 14. Experimental waveforms with no load (i r = 0 ) n case of Ls =2% .the voltage applied to the primary of the CTs was 2.5 V, sothat the VA rating of the series active filter was

2.5V x 60A x 3 = 450VA.This was only 2.3%, s compared with that of the load, i.e.,20 kVA.Fig. 13 shows the experimental waveforms in the case ofL , = 5 .6%. Before the series active filter was started, aparallel resonance at the 4th-harmonic frequency occurred inthe system. A small amount of the 4th-harmonic currentincluded in the load current was largely amplified. As aresult, severe distortion existed in the source current, i s , andthe filter voltage, u F , which was not seen in Fig. 11. How-ever, no parallel resonance appeared after the series activefilter was started.Fig. 14 shows the experimental waveforms with no load inthe case of L , = 2%.Although the source harmonic voltage

was only 1 % , the source harmonic current reached about10%before the series active filter was started. After it wasstarted, no harmonic current flowed into the shunt passivefilter. In addition, no harmonic voltage appeared at theterminals of the shunt passive filter, because the sourceharmonic voltage was applied to the series active filter.The total loss of the series active filter was less than 40 W ,so it is concluded that the combined system proposed in thispaper is far superior in efficiency to conventional shunt activefilters. CONCLUSION

In this paper, the combined system of a shunt passive filterand a small rated series active filter was proposed, givingboth practical and economical points of view. The compensa-tion principle applied to this system was quite different fromconventional shunt and series active power filters, thus suc-ceeding in better filtering characteristics and lower initial andrunning costs. The required rating of the series active filter ismainly determined by the quality factor of the shunt passivefilter. The quality factor of the shunt passive filter used in thisexperiment was 14. Since it is 50-80 for a practical shuntpassive filter in power systems, the required rating of theseries active filter will be reduced to less than 1%, ascompared with the rating of a three-phase six-pulse thyristorconverter or cycloconverter. It is believed that the combinedsystem is the most suitable to harmonic compensation forlarge rated thyristor converters and cycloconverters.

REFERENCES[ I ] H. Sasaki and T. Machida, A new method to eliminate ac harmoniccurrents by magnetic compensation-Conside rations on basic desig n,IEEE Trans. Power Appl . Syst . , vol. 90 , no . 5 , p. 2009, 1971.[2 ] L. Gyugyi and E. C. Strycula, Active ac power filters, is presentedat IEEEIIAS Ann . Meeting, 1976, pp. 529.[3 ] N. Mohan et al . , Active filters for ac harmonic suppression,presented at IEEE/PES Winter Meeting, A77 026-8, 1977.[4] H . Kawahira, T. Nakamura, and S. Nakazawa, Active powerfilters, in Proc. JIEE IPEC-Tokyo, 1983, pp . 981.[5 ] H. Akagi, A. Nabae, and S . Atoh, Control strategy of active powerfilters using multiple voltage-source PWM converters, IEEE Trans.Ind. A p p l . , vol. IA-22, no. 3, pp . 460, May/lune 1986.

8/3/2019 00062380

8/8

99 0 IEEE TRANSACTIO NS ON INDUSTRY APPLICATIONS. VOL. 26, NO. 6, NOVEMBERIDECEMBER 1990

r91

1101

K. Komatsugi and T . Imura, Harmonic current compensator com-posed of static power conv erter, presented at IEEE Power Elec-tronics Specialist Conf., 986, pp. 283.M. Takeda et al., Harmonic Current Compensation with ActiveFilter, presented at IEEEIIAS Ann . Meet ing, pp . 808, 1987.A. Nakajima et a l . , Development of Active Filter with SeriesResonant Circuit presented at IEEE Power Electronics SpecialistConf., p. 1168, 1988.M. Kohata et al. , Compensator for harmonics and reactive powerusing static induction thyristors, presented at European Conf. onPower Electronics and Applications, 1987.F. Z . Peng, H. Akagi, and A. Nabae, A study of active power filterusing quad-series voltage-source PWM converters for harmonic com-pensation, presented at IEEE Power Electronics Specialist Conf.,Y . Tanoue, F. Aoyama, and K. Terashima, 5-MVA flicker suppres-sion equipment-A hybrid system of a passive filter and an activefilter using GTO thyristors, Toshiba Rev. , vol. 43, no. 4, 1988.H. Akagi, Y. Kanazawa, and A. Nabae, Instantaneous reactivepower compensators comprising switching devices without energystorage components, IEEE Trans. Ind. A p p l . , vol. IA-20, pp, 625,1984.F. Z. Peng, H . Akagi, and A. Nabae, Some considerations on anactive power filter using quad-series voltage sou rce PWM converters,JIEE PE-86.154, 1986, (in Japanese).

1987, pp. 204-212.

Fang Zheng Peng was born in Hubei Province,China. le received the B.S. degree from WuhanUniversity of Hydraulic and Electric Engineering,China, in 1983. He received the M.E. and Ph.D.degrees in electrical engineering from NagaokaUniversity of Technology, Japan, in 1987 and1990, respectively.He is now working for Toyo Electric Manufac-turing Company, Ltd. His research interests areactive power filters and hybrid systems of activeand passive filters.Dr. Peng is a member of the Institute of Electrical Engineers of Japan.

Hirofumi Akagi (M87) was born in OkayamaPrefecture, Japan, on August 19, 1951. He re-ceived the B. S degree from the Nagoya Instituteof Technology, Nagoya, Japan, in 1974, and theM S. nd Ph.D. degrees from the Tokyo Instituteof Technology, Tokyo, Japan, in 1976 and 1979,respectively, all in electrical engineerin g.Since 1984 he has been an Associate Professorat Nagaoka University of T echnology, Japan. He isengaged in research on ac motor drives, activepower filters, and high-frequency inverters.Dr Akagi is a member of the Institute of Electrical Engineers of Japan.

Akira Nabae (M79-SM 89-F90) was born inEhime Prefecture, Japan, on September 13, 1924.He received the B. S degree from Tokyo Univer-sity, Tokyo, Japan, in 1947, and the Dr . Eng.degree from Wasada University, Japan, in 1987.He joined Toshiba C orporation in 1951 . From1951 to 1970 he was engaged in the research anddevelopment of rectifier and inverter technology atTsurumi Works Engineering Department. From1970 to 1978, he was involved in research anddevelopment of power electronics, especially acdrive systems, at the Heavy Apparatus Engineering Laboratory. Also, from1972 to 1978 he was a nonoccupied Lecturer of Wasada University, Japan.From 1978 to 1990 he was a Professor at Nagaoka University of Technol-ogy, Japan. Since 1990, he has been a Professor in the Electronics Engineer-ing Department at Tokyo Institute of Polytechnics, Japan. He is nowinterested in energy conversion and control systems.Dr . Nabae is a member of the Institute of Electrical Engineers of Japan.