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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 54, NO. 1, FEBRUARY 2007 433

Control Methods and Compensation Characteristicsof a Series Active Filter for a Neutral Conductor

Shigenori Inoue, Student Member, IEEE, Toshihisa Shimizu, Senior Member, IEEE, andKeiji Wada, Member, IEEE

AbstractDue to the advance of information technologies, alarge number of electronic products such as personal computershave been connected to power distribution systems in commer-cial buildings. Hence, voltage distortion on utility outlets andexcessive neutral current on distribution lines have arisen andlead to a number of serious problems in the distribution system.Two control methods and the related compensation characteristicsof a series active filter connected to the neutral conductor arepresented in this paper. The distinct functions of the proposedactive filter are the mitigation of the third-harmonic voltage andthe neutral current in a three-phase four-wire distribution system

in a building. The required power of the proposed active filter isless than 10% of that of the harmonic-producing loads. A controlmethod of the dc capacitor voltage on the active filter circuit is alsodescribed. It is clarified through experiments that one of the twofunctions of the active filter can be realized selectively and the dccapacitor voltage of the active filter can be regulated to a desiredvalue.

Index TermsHarmonic voltage distortion, neutral conductor,series active filter, third harmonic.

I. INTRODUCTION

D

UE TO the advancement of information technologies,

a large number of computer products have been connected

to power distribution systems in commercial buildings. The ac-

to-dc front end of these kinds of electronic products uses diode

rectifiers equipped with smoothing dc capacitors, which flow a

large amount of harmonic current into the utility line.

Especially on three-phase four-wire distribution systems,

the third-harmonic currents are increased. The excessive third-

harmonic currents cause overheating of the neutral conductors

[1]. In order to reduce the neutral current, a passive filter

connected in series with the neutral conductor [2], and the

shunt active filters and their control methods [3], [4], have been

proposed. The shunt active filters can mitigate not only third-

harmonic currents but also higher order harmonic currents.

An active filter connected in series with the neutral conductorin order to mitigate the excessive neutral current has been

presented [5], [6].

Manuscript received July 30, 2004; revised June 22, 2005. Abstract pub-lished on the Internet September 15, 2006.

S. Inoue and T. Shimizu are with the Department of Electrical Engineer-ing, Tokyo Metropolitan University, Tokyo 192-0397, Japan (e-mail: [email protected]).

K. Wada was with the Department of Electrical Engineering, Tokyo Instituteof Technology, Tokyo 152-8552, Japan. He is now with the Department ofElectrical Engineering, Tokyo Metropolitan University, Tokyo 192-0397, Japan(e-mail: [email protected]).

Digital Object Identifier 10.1109/TIE.2006.885511

Fig. 1. Small-scaled model of a three-phase four-wire distribution system ina building.

On the other hand, the voltage drop on both the distribution

transformers and the power cables caused by the third-harmonic

current results in the voltage distortion on utility outlets. A

stadium that experienced the problem was reported in [7]. In

this case, lighting equipment cuts out suddenly due to the third-

harmonic voltage in the three-phase four-wire distribution sys-

tem. However, few papers have reported the mitigation methods

of third-harmonic voltages on utility outlets.

The authors have already presented a series active filter in or-

der to mitigate the third-harmonic voltage on utility outlets [8].

The active filter is controlled to operate only for the third-

harmonic frequency, not producing any fundamental voltage.

Therefore, for the fundamental frequency, the active filter can

be regarded as a short circuit. The active filter, which consists

of a single-phase voltage-source inverter, is connected in series

with the neutral conductor. The required power of the active

filter is less than 10% of that of the harmonic-producing loads.

The active filter can mitigate either the third-harmonic voltage

or the neutral current on the three-phase four-wire distribution

system. Thus, there are two operating modes of the proposed

active filter. In this paper, the control methods for the twooperating modes are presented and verified through the ex-

perimental setup. Besides, a control method for regulating the

dc capacitor voltage and its characteristics is presented. Note

that this paper focuses on the three-phase balanced condition,

resulting in no fundamental neutral current flowing through the

active filter.

II. SYSTEM CONFIGURATION

Fig. 1 shows the 3-kVA small-scaled model of a three-

phase four-wire distribution system in a building that is used

in theoretical analysis and experiments in this paper. Table I

0278-0046/$25.00 2007 IEEE

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434 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 54, NO. 1, FEBRUARY 2007

TABLE IPARAMETERS OF THE CIRCUIT IN FIG . 1

shows the circuit parameters of this model. The model consists

of a Y step-down transformer, three nonlinear loads, andan active filter. The nonlinear loads, each of which consists

of a diode rectifier and a dcdc converter (Densei-Lambda

PH300H280-24), represent a number of computers and areconnected to each phase-to-neutral line. AC front-end circuits

in computers consume the required power regardless of the

ac-side voltage distortion. Then, the dcdc converters in the

model are controlled to consume a constant 2-kW active power.

A 0.86-mH (4.5%) inductor LN is added to the experimentalsetup in order to simulate the inductance of a neutral conductor

of an actual distribution system in a building.

The active filter is connected in series with the neutral

conductor via a switching-ripple LC filter Lf and Cf and amatching transformer (MT). A 100-F electrolytic capacitorCAF is connected to the dc bus of the active filter. The imple-

mentation on the control circuit is executed by a DSP (TexasInstruments TMS320VC33150 MHz).

III. METHOD OF THIRD-H ARMONIC

VOLTAGE MITIGATION

A. Principle of Neutral Point Voltage and Third-Harmonic

Voltage Mitigation

Under balanced conditions, the third-harmonic component is

classified as a zero-sequence component. In this section, the

mitigation principle of the third-harmonic voltage is introduced

based on Fig. 2, which shows the zero-sequence equivalent

circuits of Fig. 1. In Fig. 2, the neutral current IN is assumedto be an ideal third-harmonic current source because diode

rectifiers equipped with dc capacitors behave as harmonic

current sources when the proposed series active filter mitigates

the third-harmonic voltage on the neutral point [8]. The active

filter is represented as an ideally controlled voltage source, and

RS and LS are the resistance and the leakage inductance ofthe Y step-down transformer, respectively. VN is a phasorexpression of the neutral point voltage vN given by

vN =vLa + vLb + vLc

3(1)

where vLa, vLb, and vLc are the utility outlet voltages oneach phase. The value ofvN mainly means the third-harmonic

Fig. 2. Zero-sequence equivalent circuits when the neutral current is assumedto be a current source. (a) Without an active filter. (b) With an active filter.

Fig. 3. Phasor diagrams explaining the neutral point voltage mitigation.(a) Without an active filter. (b) With an active filter.

component contained in each phase voltage vLa, vLb, and vLc.

Hence, the mitigation ofvN results in the reduction of the third-harmonic component in the utility outlet voltage.

Fig. 3 shows phasor diagrams of the voltages and currents

in Fig. 2. It should be noted that the angular frequency of those

phasors is the third-harmonic frequency contained in the utility

line voltage. The direction of each real axis in Fig. 3(a) and (b)

is set to the direction of the neutral current IN. The phasedifference between VZ and IN is expressed as

= tan13L0R0

(2)

where R0 = RS/3, L0 = LS/3 + LN, and 3 (= 300[rad/s])

is the third-harmonic angular frequency. Control of the activefilter is performed on a rotating frame synchronized with the

third-harmonic frequency as explained in the next section. Each

value on the complex planes in Fig. 3 corresponds to the one on

the rotating frame, which is implemented in the controller of

the active filter in Fig. 4.

The voltage drop across the zero-sequence impedances VZcan be calculated as

VZ = (R0 + j3L0)IN. (3)

When the active filter is not installed, as shown in Fig. 2(a), the

neutral point voltage VN can then be obtained as

VN = VZ = (R0 + j3L0)IN. (4)

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INOUE et al.: CONTROL METHODS AND COMPENSATION CHARACTERISTICS OF A SERIES ACTIVE FILTER 435

Fig. 4. Block diagram of the controller for the mitigation of third-harmonicvoltages at the utility outlet.

VN can be separated into two components VNd and VNq as

VN = VNd + jVNq (5)

where

VNd = R0IN VNq = 3L0IN. (6)When the active filter is installed, as shown in Fig. 2(b),

the active filter generates a voltage given by

VAF = KV jVNq (7)

where KV is the control gain of the active filter. The voltagesVN, VZ , and VAF comply with Kirchhoffs voltage law, and thephasor diagram changes from Fig. 3(a) to Fig. 3(b). As a result,

VN is expressed as

VN = VNd + jVNq =

R0 + j3L0

1 + KV IN. (8)

It is clear that the larger the value ofKV is selected, the higherthe performance of reducing VN can be achieved.

The complex power of the active filter PAF is calculated as

PAF = VAF IN = jKV

1 + KV3L0I

2

N. (9)

As can be seen, PAF becomes a pure imaginary number, so theactive filter needs no active power flow for the third-harmonic

voltage mitigation. Note that the active power flowing into

or out of the active filter is used to control the dc capacitor

voltage vC

, as explained in the following sections.

The active filter does not mitigate VNd because mitigationofVNd requires the active filter to supply an active power ofINVNd = I

2

NR0 into the utility line. To avoid the expenses ofadding an external dc power supply to the dc bus on the active

filter inverter, excluding the compensation of VNd is a cost-effective solution from a production point of view.

B. Controller of the Active Filter for Third-Harmonic

Voltage Mitigation

Fig. 4 shows the controller of the active filter for the third-

harmonic voltage mitigation. The part above the dotted line in

Fig. 4 is the controller for the dc capacitor voltage explained inthe next section. Control of the active filter is performed on a

rotating frame synchronized with the third-harmonic frequency

(150 Hz).

The neutral point voltage vN is transformed into orthogonalquantities vN and vN by a time-derivative element

vNvN

= 1 13 ddtvN. (10)The dq components vNd and vNq are transformed from vNand vN as

vNdvNq

=

cos 3i sin 3i sin 3i cos 3i

vNvN

. (11)

The phase angle 3i of the dq coordinates is obtained fromthe third-harmonic component in the neutral current iN, so thedirection of the d-axis is equal to the direction of the neutralcurrent vector iN. The q-axis component of the third-harmonicvoltage vNq is separated through a first-order low-pass filterwith a cutoff frequency of 0.8 Hz. The q-axis output referencevoltage v

AFq is calculated by amplifying vNq by KV as

vAFq = KV vNq . (12)

The d-axis output reference voltage vAFd is given by the con-

trol signal from the controller for the dc capacitor voltage. The

output reference voltage on the stationary frame vAF

is obtained

from vAF

= [vAFd, v

AFq]

T by using the dq inverse trans-form. Pulse width modulation (PWM) signals for the active

filter inverter are generated by comparing vAF

with a 10-kHz

triangular-wave signal.

C. DC Capacitor Voltage Control in the Case of

Third-Harmonic Voltage Mitigation

The voltage across the dc capacitor connected to the dc bus

of the active filter inverter needs to be regulated in order to

ensure proper operation of the active filter. The active filter can

charge the dc capacitor voltage by itself without an external

power supply. The dc capacitor voltage vC is obtained as

vC = 2

CAFpAFdt (13)

where pAF (= vAF iN) is the instantaneous power that theactive filter receives from the distribution system. Conventional

feedback control systems for vC have nonlinear control charac-teristics, as shown in (13). Thus, the control system eases to fall

into an unstable operation. A useful solution to this problem is

the use ofv2C as a control variable instead ofvC [9].Control for the dc capacitor voltage is executed in Fig. 4

above the dotted line. Since the output voltage component to

control the dc capacitor voltage should be in phase with the

neutral current iN, then the output reference voltage on therotating frame should be the d-axis value v

AFd expressed as

vAFd = KCv2C v2C

(14)

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Fig. 5. Block diagram of the dc capacitor voltage control.

Fig. 6. Zero-sequence equivalent circuits when the neutral point voltage VNis assumed to be an ideal voltage source. (a) Without an active filter. (b) Withan active filter operated to behave as an inductor.

where KC is the proportional gain of the dc capacitor voltagecontrol.

Fig. 5 shows a block diagram of the feedback control system

ofv2C. In Fig. 5, the 300-Hz (= 2 150 Hz) ripple in the single-phase instantaneous power pAF is ignored, and the rms valueof the neutral current IN is assumed to be constant despitethe operation of the active filter. Under these assumptions, the

control system becomes a linear first-order system expressed as

F(s) = V2C(s)

V2C (s)= 1

1 + CAF2KCIN

s. (15)

IV. METHOD OF NEUTRAL CURRENT REDUCTION

A. Principle of Neutral Current Reduction

Fig. 6 shows the zero-sequence equivalent circuits of Fig. 1

when the active filter reduces the neutral current. In Fig. 6,

the neutral point voltage VN is assumed to be an ideal third-harmonic voltage source because the diode rectifiers equipped

with dc capacitors behave as harmonic voltage sources when

the proposed series active filter operates in a way that increasesthe impedance of the neutral conductor [10].

Fig. 7 shows the phasor diagrams of the voltages and currents

in Fig. 6. The direction of each real axis in Fig. 7(a) and (b) is

set to the direction of the neutral point voltage VN. Control ofthe active filter is performed on a rotating frame synchronized

with the third-harmonic frequency as explained in the next

section. Each value on the complex planes in Fig. 7 corresponds

to the one on the rotating frame, which is implemented in the

controller of the active filter in Fig. 8.

When the active filter is not installed as shown in Fig. 6(a),

the neutral current IN is calculated as

IN = VNR0 + j3L0

= R0 j3LNR20

+ 23L20

VN. (16)

Fig. 7. Phasor diagrams explaining neutral current reduction. (a) Without anactive filter. (b) With an active filter.

Fig. 8. Block diagram of the controller for reduction of neutral current.

IN can be separated into two components, as shown inFig. 7(a), as

IN = INd + jINq (17)

where

INd = R0R20

+ 23L20

VN INq =3LN

R20

+ 23L20

VN. (18)

In order to reduce the neutral current, the impedance of the

neutral conductor needs to be increased. Therefore, the active

filter has to operate as either a resistor or an inductor.

A mitigation method of the neutral current has been pro-

posed, in which a series active filter operates as a resistor forthe third-harmonic frequency [5]. However, this method may

result in excessive dc capacitor voltage because the active filter

receives an active power from the distribution system. Hence,

a method in which the active filter is operated as an inductor is

proposed, as shown in Fig. 7(b), in order to prevent the active

power flow.

To realize such inductive operation of the active filter, the

neutral current IN is measured, and the active filter generatesan output voltage of

VAF = jKI IN = KI(INq + jINd). (19)

As the active filter has an effective inductive reactance of[KI], the phasor diagram changes from Fig. 7(a) to Fig. 7(b),

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and the neutral current IN is then given by

IN = VNR0 + j(3L0 + KI)

. (20)

It is clear that the larger the value ofKI is selected, the higherthe performance of reducing IN can be achieved.

The complex power of the active filter PAF is calculated as

PAF =VAF IN=jKI IN IN=j

KIR20

+ (3L0 + KI)2V2N. (21)

As can be seen, PAF becomes a pure imaginary number likethat in (9), so the active filter needs no active power flow for the

neutral current reduction. Note that the active power flowing

into or out of the active filter is used to control the dc capacitor

voltage vC as explained in the following sections.

B. Controller of the Active Filter for Neutral Current

Reduction

Fig. 8 shows a controller of the active filter for the neutral

current reduction. The part above the dotted line in Fig. 8 is

a controller for the dc capacitor voltage explained in the next

section.

The neutral current iN is transformed into a dq vectorbased on a time-derivative element and a dq transform. Thephase angle 3v of the dq coordinates is obtained from theneutral point voltage vN. The third-harmonic components inthe neutral current vector i

N= [i

Nd, iNq

]T are separated bytwo first-order low-pass filters. The output reference voltage

for the neutral current reduction on the rotating frame vAFI =

[vAFId , v

AFIq ]

T is given asvAFId

vAFIq

= KI

cos

2 sin

2

sin 2

cos 2

iNdiNq

= KI

iNqiNd

.

(22)

C. DC Capacitor Voltage Control in the Case of Neutral

Current Reduction

Voltage control on CAF can be performed by generating

a voltage component in phase with iN. In order to achievecontrol, the phase angle of iN, in other words, the direction ofthe vector iN on the rotating frame, has to be calculated. Theoutput reference voltage for the dc capacitor voltage control on

the rotating frame vAFC = [v

AFCd , v

AFCq ]

T is given from the

control signal KC(v2C v2C) and the vector iN as

vAFCd

vAFCq

= KC

v2C v2C

1|iN|iNdiNq

(23)

where

|iN

|= i2Nd + i2Nq. (24)

Note that the transfer function V2C/V2C is the same as (15).

Fig. 9. Experimental waveforms when the active filter is not activated.

The total output reference voltage on the rotating frame vAF

is obtained by adding (23) to (22) as

vAF = v

AFI + v

AFC. (25)

V. EXPERIMENTAL RESULTS

A. Third-Harmonic Voltage Mitigation

Fig. 9 shows experimental waveforms when the active filter

is not activated, and Fig. 10 shows the waveforms when the

active filter is operated to mitigate the third-harmonic voltage

at a gain of KV = 5. Note that dc capacitor voltage controlis performed with a reference of vC = 150 V and a gainof KC = 1.0 103 V1 in both Figs. 9 and 10. The loadrectifiers consume a constant 2-kW active power, regardless of

the operating mode of the active filter.

In Fig. 10, the total harmonic distortion (THD) of the utility

outlet voltage vLa is reduced from 13.3% to 5.3% by the third-

harmonic voltage mitigation. The phase current iSa and theneutral current iN are not changed, and the dc capacitor voltagevC is regulated to around 150 V. The required power of theactive filter PAF is calculated from Fig. 10 as

PAF = VAF IN = 11 V 17.5 A = 192 VA. (26)

This value agrees well with the calculation result in (9) (which

is 220 VA) and is less than 7% of the load power (which is

3 kVA). This calculation was carried out by substituting IN =17.5 A and the circuit parameters in Table I to (9).

Fig. 11 shows the relationship among the neutral point

voltage VN, the neutral current IN, and the gain KV . When

KV increases from 0 to 6, VN is reduced from 13.4 to 5.0 V.However, setting KV to a value larger than 4 does not contribute

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Fig. 10. Experimental waveforms for an active filter at a gain ofKV = 5operating to mitigate the third-harmonic voltage.

Fig. 11. Relationship among VN, IN, and gain KV.

to the further mitigation of VN because the active filter canmitigate only the q-axis component VNq , and VNd remains.It is also noted that the neutral current IN remains nearlyconstant at 17.5 A regardless ofKV . This confirms the validityof the assumption that neutral current is a current source, as

mentioned in Section III-A.

The optimal value of KV depends on the line impedanceand system stability. An excessively large value of KV doesnot contribute to the third-harmonic voltage mitigation and may

cause unstable operation of the active filter.

B. Neutral Current Reduction

Fig. 12 shows experimental waveforms when the active filter

is operated to reduce the neutral current at a gain ofKI = 6 ,which is equal to the unit impedance (100%) of the system.

The dc capacitor voltage control is performed under the same

conditions as Figs. 9 and 10. In Fig. 12, the neutral current is

mitigated from 16.9 to 3.2 A by the active filter. Unfortunately,the THD of the utility outlet voltage vLa increases to 20.3%

Fig. 12. Experimental waveforms for an active filter at a gain ofKI = 6 operating to reduce the neutral current.

Fig. 13. Relationship among VN, IN, and gain KI .

as the gain KI increases. In an actual distribution system, ifthe voltage distortion becomes excessive, it may result in lower

dc voltages in diode rectifiers or deterioration of control perfor-

mance of PWM or power-factor correction (PFC) rectifiers. The

dc capacitor voltage vC is regulated to around 125 V despite the

rms value of the neutral current being suppressed. The requiredpower of the active filter PAF is calculated from Fig. 12 as

PAF = VAF IN = 19 V 3.2 A = 61 VA. (27)

This value agrees well with the calculation result in (21) (which

is 62 VA) and is almost 2% of the load power (which is 3 kVA).

The calculation was carried out by substituting VN = 22 V andthe circuit parameters in Table I to (21).

Fig. 13 shows the relationship among VN, IN, and KI. WhenKI increases from 0 to 6 , IN is reduced from 16.9 to 3.2 A.The neutral point voltage VN increases to 22 V at KI 0.8 and is nearly constant when KI > 1 . This confirms the

validity of the assumption that the neutral point voltage is anideal voltage source, as mentioned in Section IV-A.

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Fig. 14. Experimental waveforms when the two operating modes are alter-nated smoothly.

The value of KI in this experiment was chosen to reducethe neutral current as less as possible without making the

active filter unstable, and 6 (100% in per unit expression)is sufficient to do so [11]. Practically, the value ofKI should bechosen to limit the neutral current within a regulation without

causing serious problems due to voltage distortion.

C. Smooth Transition Between the Two Operating Modes

Fig. 14 shows an example of experimental waveforms when

the two operating modes of the active filter are transited

smoothly from the third-harmonic voltage mitigation to the

neutral current reduction. This experiment is conducted undera supposed situation where the state of the distribution system

changes and the operating mode of the active filter has to be

alternated. The dc capacitor of the active filter inverter CAF isnow 3600 F in this experiment.

First, both the gains KV and KI are set to zero. When KVincreases from 0 to 6, and KI remains 0 , the active filterbegins to mitigate the neutral point voltage vN. Then, KVdecreases from 6 to 0, and KI increases from 0 to 6 . At thistime, the operating mode of the active filter is changed, and

the active filter begins to reduce the neutral current iN. Finally,the gain KI decreases to 0 , and the active filter stops theharmonic compensation.

Neither surge voltage nor current occurred in the system in allthe periods in Fig. 14, and it is clarified that the active filter can

change its operating modes smoothly. Note that dc capacitor

voltage control is performed, and vC is maintained between 145and 155 V in all the periods described above.

VI. CONCLUSION

Two operating modes of a series active filter connected inseries with the neutral conductors were presented. Operation

principles for the third-harmonic voltage mitigation and the

neutral current reduction were presented and evaluated through

the 3-kVA experimental setup.

In addition, an effective method of controlling the dc ca-

pacitor voltage of the active filter inverter was presented. The

proposed control system, in which the squared value of the

dc capacitor voltage is used instead of the voltage itself, was

confirmed to operate stably to regulate successfully.

REFERENCES

[1] T. M. Gruz, A survey of neutral currents in three-phase computer powersystems, IEEE Trans. Ind. Appl., vol. 26, no. 4, pp. 719725, Jul./Aug.1990.

[2] T. Key and J. S. Lai, Analysis of harmonic mitigation methods for build-ing wiring systems, IEEE Trans. Power Syst., vol. 13, no. 3, pp. 890897,Aug. 1998.

[3] M. Aredes, J. Hfner, and K. Heumann, Three-phase four-wire shuntactive filter control strategies, IEEE Trans. Power Electron., vol. 12,no. 2, pp. 311318, Mar. 1997.

[4] M. Ishihara, S. Mori, G. Nakagawa, and M. Nishitoba, Developmentof active filter for three-phase four-wire system, presented at the IEEJTechnical Meeting Semiconductor Power Converter, Kobe, Japan, 1996,Paper SPC-96-128. (in Japanese).

[5] P.-T. Cheng, Y.-F. Huang, and C.-C. Hou, Design of neutral har-monic mitigator for three-phase four-wire distribution systems, in Proc.

IEEE/IAS Annu. Meeting, 2001, vol. 1, pp. 164171.[6] P.-T. Cheng, C.-C. Hou, and Y.-F. Huang, Overload prevention, IEEE

Ind. Appl. Mag., vol. 10, no. 6, pp. 2634, Nov./Dec. 2004.[7] K. Sakai, K. Furuya, K. Mimura, and K. Mizuno, Development of third-

harmonic active filter for three phases four line, in Proc. IEIE Nat. Conf.,2001, pp. 203204. (in Japanese).

[8] K. Wada and T. Shimizu, Mitigation method of third-harmonic voltagefor a three-phase four-wire distribution system based on a series activefilter for the neutral conductor, in Proc. IEEE/IAS Annu. Meeting, 2002,vol. 1, pp. 6469.

[9] M. Saitou and T. Shimizu, A single-phase PWM rectifier with dc poweractive filter for fast dc output voltage control, presented at the IEEJTechnical Meeting Semiconductor Power Converter, Kobe, Japan, 2001,Paper SPC-01-106. (in Japanese).

[10] F. Z. Peng, Harmonic sources and filtering approaches, IEEE Ind. Appl.Mag., vol. 7, no. 4, pp. 1825, Jul./Aug. 2001.

[11] H. Fujita and H. Akagi, An approach to harmonic current-free ac/dcpower conversion for large industrial loads: The integration of a series

active filter with a double-series diode rectifier, IEEE Trans. Ind. Appl.,vol. 33, no. 5, pp. 12331240, Sep./Oct. 1997.

Shigenori Inoue (S02) was born in Saitama, Japan,in 1979. He received the B.S. and M.S. degreesfrom Tokyo Metropolitan University, Tokyo, Japan,in 2002 and 2004, respectively, both in electrical en-gineering. He is currently working toward the Ph.D.degree at the Tokyo Institute of Technology, Tokyo.

His research interests include medium-voltagepower conversion systems and next-generation pow-

er switching devices based on SiC and/or GaN.

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Toshihisa Shimizu (M93SM02) was born inTokyo, Japan, in 1955. He received the B.E., M.E.,and Dr.Eng. degrees from Tokyo Metropolitan Uni-versity, Tokyo, Japan, in 1978, 1980, and 1991,respectively, all in electrical engineering.

In 1998, he was a Visiting Professor with theVirginia Polytechnic Institute and State University(Virginia Tech), Blacksburg. In 1980, he was with

Fuji Electric Corporate Research and Development,Ltd. He joined the Department of Electrical Engi-neering, Tokyo Metropolitan University, as an Asso-

ciate Professor in 1993 and has been a Professor since 2005. He has publishedmore than 40 journal papers, 60 international conference proceedings, and fourtechnical books. He holds five patents and has more than ten patents pend-ing. His research interests include power converters, high-frequency inverters,photovoltaic power generations, UPSs, EMI problems, etc.

Dr. Shimizu is a member of the Institute of Electrical Engineers of Japan(IEEJ) and the Japan Society of Power Electronics. He is also an At-LargeMember of the Administrative Committee of the IEEE Power ElectronicsSociety. He received the Transactions Paper Award from the Institute ofElectrical Engineers of Japan in 1999.

Keiji Wada (S98A00M02) was born inHokkaido, Japan, in 1973. He received the B.S. andM.S. degrees from Polytechnic University, Kana-gawa, Japan, in 1995 and 1997, respectively, and thePh.D. degree from Okayama University, Okayama,Japan, in 2000, all in electrical engineering.

From 2000 to 2006, he was a Research Asso-ciate with Tokyo Metropolitan University, Tokyo,

Japan, and Tokyo Institute of Technology, Tokyo.Since 2006, he has been an Associate Professorwith Tokyo Metropolitan University. His researchinterests include active power filters.

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