IN RECENT years, growing concerns for the environment

1264 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 Flyback-Type Single-Phase Utility Interactive Inverter With Power Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System Toshihisa Shimizu, Senior Member, IEEE, Keiji Wada, Member, IEEE, and Naoki Nakamura Abstract In recent years, interest in natural energy has grown in response to increased concern for the environment. Many kinds of inverter circuits and their control schemes for photovoltaic (PV) power generation systems have been studied. A conventional system employs a PV array in which many PV modules are connected in series to obtain sufficient dc input voltage for generating ac utility line voltage from an inverter circuit. However, the total power generated from the PV array is sometimes decreased remarkably when only a few modules are partially covered by shadows, thereby decreasing inherent current generation, and preventing the generation current from attaining its maximum value on the array. To overcome this drawback, an ac module strategy has been proposed. In this system, a low-power dc ac utility interactive inverter is individually mounted on each PV module and operates so as to generate the maximum power from its corresponding PV module. Especially in the case of a single-phase utility interactive inverter, an electrolytic capacitor of large capacitance has been connected on the dc input bus in order to decouple the power pulsation caused by single-phase power generation to the utility line. However, especially during the summer season, the ac module inverters have to operate under a very high atmospheric temperature, and hence the lifetime of the inverter is shortened, because the electrolytic capacitor has a drastically shortened life when used in a high-temperature environment. Of course, we may be able to use film capacitors instead of the electrolytic capacitors if we can pay for the extreme large volume of the inverter. However, this is not a realistic solution for ac module systems. This paper proposes a novel flyback-type utility interactive inverter circuit topology suitable for ac module systems when its lifetime under high atmospheric temperature is taken into account. A most distinctive feature of the proposed system is that the decoupling of power pulsation is executed by an additional circuit that enables employment of film capacitors with small capacitance not only for the dc input line but also for the decoupling circuit, and hence the additional circuit is expected to extend the lifetime of the inverter. The proposed inverter circuit also enables realization of small volume, lightweight, and stable ac current injection into the utility line. A control method suitable for the proposed inverter is also proposed. The effectiveness of the proposed inverter is verified thorough P-SIM simulation and experiments on a 100-W prototype. Index Terms AC module, flyback inverter, photovoltaic (PV) power generation, utility interactive inverter. Manuscript received June 2, 2004; revised September 8, 2005. This work was presented at PESC 02, Cairns Australia, June 2002. This work was supported by the New Energy and Industrial Technology Organization of Japan. Recommended by Associate Editor Z. Chen. T. Shimizu is with the Department of Electrical Engineering, Tokyo Metropolitan University, Tokyo, 192-0397, Japan (e-mail: shimizu@eei. metro-u.ac.jp). K. Wada is with the Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Tokyo 152-8550, Japan. N. Nakamura is with Honda R&D Co., Ltd., Saitama 351-0114, Japan. Digital Object Identifier 10.1109/TPEL.2006.880247 Fig. 1. System configuration of the conventional and the proposed ac module photovoltaic generation systems. (a) Conventional photovoltaic generation system. (b) AC module photovoltaic generation system. I. INTRODUCTION IN RECENT years, growing concerns for the environment have led to increased interest in natural energy sources. Many kinds of inverter circuits and corresponding control schemes for photovoltaic (PV) power generation systems have been studied [1]. As shown in Fig. 1(a), a conventional system uses a PV array in which many PV modules are connected in series to obtain sufficient dc input voltage for generating ac utility line voltage from an inverter circuit. However, difficulty is encountered in avoiding shadows created by neighboring buildings, utility poles, trees, and other obstacles that may partially cover some of the PV modules in the array. As a result, the total output power generated from the PV array decreases 0885-8993/$20.00 2006 IEEE

SHIMIZU et al.: FLYBACK-TYPE SINGLE-PHASE UTILITY INTERACTIVE INVERTER 1265 Fig. 2. Installation of ac module inverter. Fig. 4. Boost dc ac inverter by Caceres and Babri [8] Fig. 5. Buck, boost, and buck-boost inverter by Vazquez et al. [9]. Fig. 3. Parallel connection of multiple ac module inverters. remarkably [2], [3]. To overcome this defect, an ac module strategy, as shown in Fig. 1(b), has been proposed [4] [6]. In this system, a low-power ac utility interactive inverter is mounted on each individual PV module as shown in Fig. 2, and the inverter operates so as to generate the maximum power from the corresponding PV module. Each output terminal of the individual ac module inverter is connected to the utility line as shown in Fig. 3, and the generation current of each inverter is injected into the utility line. Another advantage of this system lies in that the number of the parallel connected inverters, which is equal to the number of PV modules, can be selected in consideration of the dimensions of the roof on which PV modules are installed. This greatly improves the flexibility of the PV generation system. Furthermore, the ac module concept is expected to lower the manufacturing cost of the inverter, because of the effect of mass production. Especially in the case of single-phase utility interactive inverters, an electrolytic capacitor of large capacitance has been connected to the dc input line in order to decouple the power pulsation caused by single-phase power generation to the utility line. However, especially during the summer season, the ac module inverters must operate under very high atmospheric temperature, because each inverter is mounted on the back-side of the corresponding PV module. Hence, the lifetime of the inverter is shortened, because use at high temperature drastically Fig. 6. Single-stage inverter for ac module by Kjaer et al. [10]. shortens the life of the electrolytic capacitor [7]. Of course, we may be able to use film capacitors instead of the electrolytic capacitors if we can pay for the extreme large volume of the inverter. However, this is not a realistic solution for ac module systems. In order to overcome this defect, power decoupling schemes for the inverters, as shown in Figs. 4 6, have been proposed [8] [10]. On these inverters, two output capacitors are used and the bias voltage induced to these capacitors is modulated so as to reduce the power pulsation on the dc input capacitor and to generate sinusoidal voltage at the ac output terminal. However, the authors did not clearly mention the stable current injection into the utility line in the use of ac module inverter [see (1)]. In order to satisfy the above requirements for an ac module inverter, the authors present a novel flyback-type utility inverter based on the flyback operation theory illustrated in Fig. 7 [11]. One of the advantages of the flyback inverter lies in that an ac current injected from the utility inverter into the utility line can be determined individually, owing to individual flyback operations [12]. Moreover, the flyback inverter enables realization

1266 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 Fig. 7. Main circuit configuration of the proposed ac module inverter. of the current sharing operation among the inverters connected in parallel to the utility line without use of a specific current sharing controller. Another advantage of the proposed inverter lies in provision of a novel power decoupling circuit that removes the low frequency power pulsation that appears on the dc input line, and enables use of small capacitance on both the dc input line and the decoupling circuit. Hence, film capacitors or ceramic capacitors can be used instead of electrolytic capacitors. As a result, an extension of the lifetime of the inverter under high-temperature conditions is expected, because these capacitors have lifetimes much longer than those of the electrolytic capacitors. The proposed inverter circuit also enables realization of small volume, lightweight, and low manufacturing cost. The authors propose a simple but useful operation principle of the inverter, along with the power decoupling circuit based on the discontinuous current mode control on the primary winding of the flyback transformer. An optimum capacitance value required for the power decoupling operation is evaluated. The effectiveness of the proposed inverter is verified thorough P-SIM simulation and the experiments on a 100-W prototype. The ac current waveforms on the parallel connected inverters with which the different generation power are also shown. II. CIRCUIT CONFIGURATION AND OPERATION PRINCIPLE OF THE INVERTER Fig. 7 shows the circuit configuration of the proposed inverter. The circuit consists of two primary-side switches, and ;a flyback transformer, ; ac switches, and ; anac filter, and ; a dc input capacitor, ; and a decoupling capacitor,. In order to block the reverse current flowing through,, and, blocking diodes,,, and, respectively, are connected in series with them. The diode,, denotes the body diode of the MOSFET switch,. Magnetizing control of the flyback transformer is performed under a discontinuous current condition, but the volume of the flyback transformer and the ac filter can be reduced by setting the switching frequency high, 50 khz in this case. One of the primary-side switches,, is triggered so as to charge the constant magnetizing energy up to the decoupling Fig. 8. Operation waveforms on the proposed ac module inverter. capacitor in every switching cycle, and hence the averaged dc input current,, is maintained constant. Another primary-side switch,, is then controlled so as to generate the required amplitude of the magnetizing current on the primary winding of the flyback transformer and to release the current from the secondary winding. Finally, the output ac current is polarized by the ac switches and. Figs. 8 and 9(a) (d) show the operation waveforms and the switching stages in one switching interval. The switching interval,, is kept constant, and one switching interval has four operation stages. The time durations in the respective switching stages are defined in this paper as follows: where, are the duty ratios on the respective stages. The detailed operation at the kth switching interval is explained as follows. Stage I : The main switch,, is turned on, but the other switches,,, and, are turned off. Then, the magnetizing current,, flowing thorough the transformer is expressed as (1)

SHIMIZU et al.: FLYBACK-TYPE SINGLE-PHASE UTILITY INTERACTIVE INVERTER 1267 Fig. 9. Equivalent circuits of each operation stage on the proposed ac module inverter. where and are self inductance on the primary winding and dc input voltage, respectively.when the current reaches the required value,, at time, is turned off In the case where the amplitude of current,, is controlled to be maintained constant, amplitude of the averaged current,, which flows from the photovoltaic module can be maintained constant as (2) Stage III : A current,, is released through one of the secondary windings of the transformer and the related ac switch, or. The utility voltage during the kth switching interval,, can be assumed to be constant; therefore, the absolute value of current,, is expressed as As the current reaches zero at the time, then time interval of is expressed as (6) Stage II : When the main switch,, is turned off, but the secondary switches remain turned off, the magnetizing current is released as the current,, flows into the decoupling capacitor,, though the diode,, which is the body diode of the MOSFET switch,, and decreases linearly towards zero. As the capacitor voltage,,is assumed to be constant during one switching period, the current, is expressed as A switch and either of or are turned on in the manner of ZVS. Since the direction of the current,, changes so that the current flows through, the current magnetizes the transformer again. When the amplitude of the current reaches the required value,, as shown in Fig. 8, the switch is turned off. Hence, the time duration on stage II is given as (3) (4) (5) where, is the turn ratio of the transformer.stage IV : No current flows through the transformer winding, and the transformer is initialized. The high-frequency component contained in the current is filtered by the ac filter circuit; therefore, the absolute value of the averaged current,, during the th switching period is iven as Here, the utility voltage,, is given by (9), and the peak current of is controlled as shown by (10), and therefore (8) is expressed as (7) (8) (9) (10) (11)

1268 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 converted into the static storage energy on the decoupling capacitor. In order to extend the lifetime of the inverter, a film capacitor of small capacitance,, is used. The smaller the capacitance of, the larger the voltage variation that appears on the decoupling capacitor. Also, as shown in Fig. 8, sufficient voltage on the decoupling capacitor is required in order to generate the required magnetizing current,. Hence, the capacitance and other circuit parameters should be selected in consideration of the above requirements. The required capacitance,, can be evaluated from the instantaneous power transfer from the ac power fluctuation,, to the stored power fluctuation,, on the decoupling capacitor. The voltage variation on the decoupling capacitor,, and other circuit parameters are derived by the following calculations. The instantaneous ac power,, which includes the power pulsation with twice the utility frequency, can be defined as (13) Fig. 10. Input and output waveforms and the resultant power pulsation on the decoupling capacitor. where,. Low-frequency components of the voltage and current, and, and the stored power fluctuation,, on are expressed as The polarity of current,, is determined by triggering one of the ac switches, or, in order that the inverter injects ac current,, into the utility line with unity power factor. Therefore, the ac output current,, is given by (12) Note that the waveform of output ac current does not depend on whether other inverters are connected to the same utility line. This means that each parallel connected inverter can inject its inherent ac current into the utility line without the specific ac current sharing control. (14) (15) (16) Hence, the stored energy associated with and, expressed by and, is calculated as (17) (18) III. POWER DECOUPLING FOR REDUCTION OF RIPPLE VOLTAGE ON THE DC-BUS Fig. 10 shows the voltage and current waveforms of the proposed inverter along with the resultant power variation on the decoupling capacitor. In view that the input dc current,, is controlled to be constant as shown in (3), only a pure dc component is contained in the input voltage,, and its value is usually controlled to be a desirable one decided by the maximum power point tracking (MPPT) control which utilizes the characteristics of the PV module or the resultant input power. However, ac output instantaneous power,, contains the power pulsation with twice the utility frequency. Hence, the power pulsation caused by the single-phase power generation is where is the cycle time of the utility voltage. In the case where is satisfied, power pulsation on the dc-bus is transferred into static energy on the decoupling capacitor. Hence, the amplitude of voltage variation,,is calculated as (19) Fig. 11 shows the calculated example of the relationship between the voltage variation,, and the capacitance,. Here, we can see that the voltage variation,, is smaller when the larger dc-bias voltage,, is selected. Also note that the

SHIMIZU et al.: FLYBACK-TYPE SINGLE-PHASE UTILITY INTERACTIVE INVERTER 1269 TABLE I CIRCUIT PARAMETERS FOR THE PROPOSED AC MODULE INVERTER Fig. 11. Capacitance versus voltage variation on the decoupling capacitor. voltage,, must have a positive value,, in order to generate the sufficient magnetizing current,, during stage II; hence, the amplitude of must be smaller than Fig. 12. Control block diagram of the proposed ac module inverter. (20) In view of the fact that the proposed inverter must operate under the discontinuous current mode, the sum of the time duration on stages I III must satisfy the following equation: Substituting (2), (5), and (7) into (21), we obtain (21) When (26) is substituted into (22), required inductance given by (27) In the usual case, the minimum value of, which is denoted by, appears at 3 4 as shown in Fig. 10; therefore, (27) can be expressed as is (22) Meanwhile, the averaged input dc power,, and the averaged output ac effective power,, during one utility cycle are (23) (24) Since, the current is expressed by (25) From (10), (12), and (25), the current is expressed as (26) (28) From the point of view of loss minimization, the inductance,, should be selected close to the maximum limit in (28). Taking (19), (20), and (28) into account, optimum circuit parameters for,, required for performing both the decoupling operation and inverter operation are calculated as shown in Table I. Note that the capacitance,, necessary for reducing the voltage pulsation on the dc-bus is decreased to 1/100 1/200 that required in the conventional method. Fig. 12 shows the control block diagram of the proposed inverter. The control for conveying the pure dc power from a photovoltaic module is executed at the upper part of the control block diagram. The dc voltage command,, is changed depending on the result of maximum power point tracking control for the PV module. In this system, many kinds of MPPT algorithm, such as hill climbing method, dp/dv method, perturbation and observation can be applicable [13] [15]. Meanwhile, the control for injecting the ac current into the utility line

1270 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 Fig. 14. External view of the prototype ac module inverter. Fig. 13. Simulation waveforms. (a) Waveforms on one cycle of the utility waveform. (b) Waveforms on some switching periods. and for keeping the dc bias voltage,, on the power decoupling capacitor is executed at the lower part of the control block diagram. Turn-on action of the main switch,, is triggered by the trigger signal,, which has a switching cycle of. The input dc current command,, which is given by the dc voltage controller,, is compared with actual current,, and both the turn-off action of and the turn-on action of are executed. The current command,, which determines the output ac current, is executed by multiplying the output signals of the decoupling capacitor voltage controller,, and the absolute generator, ABS, of utility ac voltage waveform. Turn-off action of the switch is executed by comparing the current command,, and actual current,. Gate signals for two ac switches, and, are given from the polarity signal of and a gate signal of. IV. SIMULATION AND EXPERIMENTAL RESULTS Fig. 13(a) shows the simulated waveforms of the proposed inverter. The peak amplitude of the current,, is maintained constant, but the envelope of the peak value of current,, has a rectified sinusoidal waveform. Hence, the secondary current,, and the resultant ac output current,, have sinusoidal waveforms and unity power factor. Furthermore, voltage,, on the decoupling capacitor pulsates at twice the utility frequency. Fig. 13(b) shows the detailed operation waveforms during several switching periods. When turns on, the primary current increases linearly. turns off when the current reaches the constant value,, and the primary current is transferred to the capacitor. Following this action, the switch turns on and the primary current increases again, and turns off when the current reaches the peak value,. Finally, the magnetizing current is released from a secondary winding. Fig. 14 shows the external view of the prototype ac module inverter with 100 W output. The layout of the major components is also highlighted. Power MOSFETs, a dc capacitor, a decoupling capacitor, an ac filter, and an EMI filter are located around the flyback transformer. The dimensions of the inverter are 150 mm (W), 140 mm (D), and 35 mm (H). We can see that dc and decoupling capacitors used at both dc input and the power decoupling circuit are sufficiently small. Fig. 15 shows the operation waveforms of the experimental setup. The output current is controlled to produce a sinusoidal waveform with unity power factor, and the total harmonic distortion of the output current is less than 5%. These results satisfy a required condition for interlinking a power converter to a utility line in Japan [16], [17]. The input dc current and voltage are maintained constant, and the resultant power pulsation caused by single-phase power generation appears on the decoupling capacitor voltage,. The maximum conversion efficiency of the proposed inverter is only 70%. This is because that the rated input dc voltage is set to very low, 35 V, and hence the power loss on the power MOSFET devices becomes large. This imperfect result should be improved by using devices exhibiting high performance and low conduction loss. Also, revising the modulation method will give us a useful solution. Fig. 16(a) and (b) shows the circuit configuration of two parallel connected inverters, along with the operation waveforms. Output ac currents on both the inverters have sinusoidal waveforms with unity power factor. Each amplitude of output ac current can be determined individually, depending on the current command of the individual inverter. In this case, the output currents, and, of these inverters are 0.6 A and 0.3 A, re-

SHIMIZU et al.: FLYBACK-TYPE SINGLE-PHASE UTILITY INTERACTIVE INVERTER 1271 This circuit topology and corresponding control circuit are very simple, and hence significant reductions are expected in the production volume and costs of this circuit. V. CONCLUSION A novel PV inverter circuit suitable for an ac module system is presented. Utilization of the high-frequency flyback action of the transformer realizes ac current injection with low harmonic distortion into the utility line. Furthermore, a dc power smoothing circuit that enables a decrease in the low-frequency ripple voltage and a reduction in the total capacitance on the dc input side is presented. As can be shown, the electrolytic capacitors on the dc input side can be exchanged for film capacitors of small capacitance. Hence, the problems of short lifetime and large volume of the ac module inverter system can be solved. The effectiveness of the proposed inverter is confirmed through simulation and experimental results. Improvement of conversion efficiency can be expected by utilizing the low loss switching devices and revising the modulation method, and those will be the important task in this kind of power decoupling technology. Fig. 15. Experimental waveforms. Fig. 16. Experimental results of the parallel connected ac module inverter. (a) Schematic of parallel connection. (b) Experimental waveforms. spectively, and the total output current, 0.9 A, is injected into the utility line. We can see that the proposed inverters can attain stable, parallel operation. REFERENCES [1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, Power inverter topologies for photovoltaic modules a review, in Proc. IEEE IAS Annu. Meeting, 2002, vol. 2, pp. 782 788. [2] L. E. de Graaf and T. C. J. van der Weiden, Characteristics and performance of a PV-system consisting of 20 AC modules, in Proc. IEEE World Conf. Exhibition Photovolt. Solar Energy Conv. Conf., 1994, pp. 921 924. [3] T. Shimizu, M. Hirakata, T. Kamezawa, and H. Watanabe, Generation control circuit for photovoltaic modules, IEEE Trans. Power Electron., vol. 16, no. 3, pp. 293 300, May 2001. [4] S. W. H. de Haan, H. Oldenkamp, and E. J. Wildenbeest, Test results of A 130 W AC module; a modular solar ac power station, in Proc. IEEE WCPEC Conf., 1994, pp. 925 928. [5] R. H. Wills, F. E. Hall, S. J. Strong, and J. H. Wholgewuth, The AC photovoltaic modules, in Proc. IEEE WCPEC Conf., 1996, pp. 1231 1234. [6] S. Yatsuki, K. Wada, and T. Shimizu, A novel AC photovoltaic module system based on the impedance-admittance conversion theory, in Proc. IEEE Power Electron. Spec. Conf. (PESC 01), 2001, pp. 2191 2196. [7] H. Oldenkamp, I. J. De Jong, C. W. A. Baltus, S. A. M. Verhoven, and S. Elstgeest, Reliability and accelerated life tests of the AC module mounted OKE4 Inverter, in Proc. IEEE WCPEC Conf., 1996, pp. 1339 1342. [8] R. O. Caceres and I. Babri, A boost dc-ac converter; analysis, design, and experimentation, IEEE Trans. Power Electron., vol. 14, no. 1, pp. 134 141, Jan. 1999. [9] N. Vazquez, J. Almazan, J. Alvarez, C. Aguliar, and J. Arau, Analysis and experimantel study of buck, boost, and buck-boost inverters, in Proc. IEEE Power Electron. Spec. Conf. (PESC 99), 1999, vol. 2, pp. 801 806. [10] S. B. Kjaer and F. Blaabjerg, A novel single-stage inverter for AC-module with reduced low-frequency ripple penetration, in Proc. 10th Eur. Conf. Power Electron. Appl., 2003, pp. 2 4. [11] T. Shimizu, K. Wada, and N. Nakamura, A flyback-type single phase utility interactive inverter with low-frequency ripple current reduction on the DC input for an AC photovoltaic module system, in Proc. IEEE 34th Annu. Power Electron. Spec. Conf. (PESC 02), 2002, vol. 3, pp. 1483 1488. [12] M. Nagao and K. Harada, Power flow of photovoltaic system using buck-boost PWM power inverter, in Proc. Int. Conf. Power Electron. Drive Syst. (PEDS 07), 1997, vol. 1, pp. 144 149. [13] F. Harashima, H. Inaba, S. Kondo, and N. Takashima, Microprocessor-controlled SIT inverter for solar energy system, IEEE Trans. Ind. Electron., vol. 34, no. 1, pp. 50 55, Feb. 1987.

1272 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 21, NO. 5, SEPTEMBER 2006 [14] S. J. Chiang, K. T. Chang, and C. Y. Yen, Residential photovoltaic energy storage system, IEEE Trans. Ind. Electron., vol. 45, no. 3, pp. 385 394, Jun. 1998. [15] Y. C. Kuo, T. J. Liang, and J. F. Chen, Novel maximum-powertracking controller for photovoltaic energy conversion system, IEEE Trans. Ind. Electron., vol. 48, no. 3, pp. 594 601, Jun. 2001. [16] Japan Electric Association, A Guideline for Harmonic Current Suppression, Tech. Rep. JEAG9702-1995, 1995. [17] International Standard, Edition 1.2, Part3.2: Limits. Limits for Harmonic Current Emissions, IEC-61000-3-2, IEC, 1998. Toshihisa Shimizu (M 93 SM 02) was born in Tokyo, Japan, in 1955. He received the B.E., M.E., and Dr.Eng. degrees in electrical engineering from Tokyo Metropolitan University, in 1978, 1980, and 1991, respectively. In 1998, he was a visiting professor at VPEC, Virginia Polytechnic Institute and State University, Virginia. He joined Fuji Electric Corporate Research and Development, Ltd. in 1980. Since 1993, he has been a member of the Department of Electrical Engineering, Tokyo Metropolitan University, where he is currently a Professor and serves as Department Dhair. Since 2001, he has been an Adjunct Member of the Power Electronics Center, National Institute of Advanced Industrial Science and Technology (AIST), Japan. His research interests include high power-density converters, high frequency inverters, photovoltaic power generations, UPSs, and EMI in power electronics. He published more than 50 journal papers, 60 international conference proceedings, and four technical books. He also holds eight patents and more than ten patents pending. Dr. Shimizu received the Transactions Paper Award from the Institute of Electrical Engineers of Japan in 1999. He has been an At-Large member of the IEEE PELS Adcom since 2004. He is a senior member of the Institute of Electrical Engineers of Japan(IEEJ), and the Japan Society of Power Electronics. Keiji Wada (S 99 M 01) was born in Hokkaido, Japan. He received the B.S. and M.S. degrees from the Tokyo Institute of Polytechnics, Tokyo, Japan, in 1995 and 1997, respectively, and the Ph. D. degree from Okayama University, Okayama, Japan, in 2000, all in electrical engineering. From 2000 to 2003, he was a Research Associate in Tokyo Metropolitan University. Since 2003, he has been with the Tokyo Institute of Technology as a Research Associate. His research interests are power electronics and power quality. Naoki Nakamura was born in Kanagawa, Japan, in 1978. He received the B.S. and M.S. degrees from Tokyo Metropolitan University, Tokyo, Japan, in 2001 and 2003, respectively. In 2003, he joined Honda R&D Co., Ltd., Saitama, Japan.