Publications

This paper presents the application of a modular multilevel cascade converter based on single-delta bridge-cells (MMCC-SDBC) to a STATic synchronous COMpensator (STATCOM), particularly for negative-sequence reactive-power control. The SDBC is characterized by cascade connection of multiple single-phase H-bridge (or full-bridge) converter cells per leg, which facilitates flexible circuit design, low voltage steps, and low electro-magnetic interference (EMI) emissions. However, there is no published report on such a STATCOM with experimental verification or a control strategy. This paper designs, constructs, and tests a 100-V 5-kVA PWM STATCOM based on the SDBC with focus on the operating principle and performance. Experimental results confirm that this converter can control not only the positive-sequence reactive power but also the negative-sequence reactive power and low-frequency active power intended for the flicker compensation of arc furnaces.

This paper presents a 2 MHz 6 kVA voltage-source inverter for low-temperature plasma generators. A new MOSFET module referred to as a "mega pack" is specially designed and fabricated for high-frequency high-power applications. It has a low-profile package equipped with four terminal plates. The main circuit consists of a single-phase full-bridge inverter using the four new modules. The layout of the modules is characterized by two modules, which are placed back-to-back with each other, forming a half bridge. Both device and circuit designs achieve great reduction of stray inductance in the main circuit. A prototype inverter shows stable operation around frequencies as high as 2 MHz.

This paper provides a theoretical and experimental discussion on ac inductors used in a modular multilevel cascade converter (MMCC) based on triple-star bridge cells (TSBC) for motor drives. The TSBC converter is one of the most promising candidates suitable for low-speed, high-torque motor drives. It requires ac inductors for the cluster current control, and the three kinds of inductors exist: non-coupled inductors, zigzag inductors, and three-legged three-winding inductors. This paper proposes a new set of ac inductors consisting of three three-legged cores with special windings. The proposed set of inductors is characterized by canceling out the flux produced by not only the motor currents but also the supply currents, thus resulting in bringing a significant reduction in size and weight to it. A specially-designed downscaled motor drive system rated at 400 V and 15 kW is used to confirm the validity of the proposed set of inductors.

This paper deals with a self-commutated back-to-back (BTB) high-voltage direct-current (HVDC) link system for the purpose of power flow control and/or frequency change in transmission systems. Each BTB unit consists of two sets of 16 three-phase voltage-source converters, and their AC terminals are connected in series each other via 16 three-phase transformers. Hence, the BTB unit uses totally 192 switching devices capable of achieving gate commutation. This results in a great reduction of voltage and current harmonics without performing PWM control. Simulation results verify the validity of the proposed system configuration and control scheme not only under a normal operating condition but also under a single-line-to-ground fault condition.

This paper presents a battery energy storage system with a modular push-pull PWM converter (MPC), which is intended for grid connection to medium-voltage or high-voltage power systems. The converter is equipped with a center-tapped transformer and two arms consisting of a cascade connection of multiple bidirectional PWM chopper-cells with floating dc capacitors. This paper discusses the operating performance and control method of the MPC, to achieve voltage balancing of all the floating capacitors. Moreover, a comparison is made between the MPC and a modular multilevel converter (MMC) as the battery energy storage system. The validity of the concept and control method developed in this paper is confirmed by computer simulations using the “PSCAD/EMTDC” software package.

This paper deals with a 6.6-kV transformerless STATCOM cascading multiple single-phase H-bridge PWM converters in each phase. The ac voltage of the STATCOM is almost sinusoidal, so that it requires no harmonic filter. Each converter is equipped with a capacitor and a voltage sensor on the dc side, which are electrically isolated from each other. The STATCOM has the capability of self-starting and voltage-balancing without any external power supply or equipment. Experiment by a three-phase 200-V, 10-kVA laboratry system, along with computer simulation, is carried out to confirm the viability and effectiveness of the STATCOM.

No abstract is provided for this article.

This paper focuses on the high-frequency leakage current flowing through the grounded heat sink of a voltage-source pulsewidth-modulation (PWM) inverter in an adjustable-speed motor drive system. A passive electromagnetic interference (EMI) filter is proposed, designed, and tested for a 200-V 3.7-kW drive system. This filter does require access to the ungrounded motor neutral point. An equivalent common-mode circuit for the drive system equipped with the EMI filter is derived from experimental waveforms along with theoretical discussions. Experimental results verify that the EMI filter is viable and effective in eliminating both heat-sink and motor-frame leakage currents from the drive system.

This paper provides a theoretical discussion and comparison in energy balancing between a modular multilevel cascade inverter based on single-star bridge cells (SSBC) and that on single-delta bridge cells (SDBC). Attention is paid to applications involving asymmetric active-power generation in utility-scale grid-tied photovoltaic systems. Both qualitative and quantitative evaluation metrics to assess the energy-balancing capability are introduced and applied to both SSBC and SDBC inverters. As for the SSBC inverter, six zero-sequence voltage waveforms with different harmonic content enabling enhanced energy-balancing capability are analyzed and compared regarding their effectiveness. This paper also emphasizes on the SDBC as an alternative to the SSBC and highlights its superior operating characteristics under asymmetric active-power generation.

No abstract is provided for this article.

This paper discusses two different circuit configurations of the active common-noise canceler (ACC) which has been proposed by the authors.One is characterized by its dc power supply isolated from the dc link of a PWM inverter.The configuration makes it possible to integrate the ACC with a medium-voltage PWM inverter.The other compensates a partial frequency component of the commonmode voltage.The purpose is not to achieve complete cancellation, but to restrict only a slope in a change of the common-mode voltage applied t o an ac motor.As a result, the core size of the common-mode transformer used in the ACC becomes small considerably.Experimental resultsshow good effects of the proposed active circuits on both ground current and conducted EMI.

This paper addresses shaft end-to-end and shaft-to-frame voltages that appear in the 400-V 15-kW induction motor driven by a voltage-source pulsewidth modulation inverter. It has been known that an inverter-fed common-mode voltage causes both shaft voltages. Extensive experiments are performed with an ungrounded motor subjected to a differential-mode voltage. Experimental results show that the differential-mode voltage applied across three-phase, two-phase, or single-phase stator windings also causes a shaft end-to-end voltage. It is concluded that the combination of a steep change in differential-mode voltage and high-frequency motor asymmetry causes a shaft end-to-end voltage. A motor model is presented to explain the generation of shaft end-to-end voltage.

This paper presents a modular multilevel cascade converter based on triple-star bridge cells (MMCC-TSBC), devoting itself to control and experiment. The TSBC is one of the direct ac-to-ac power converters capable of bidirectional power flow with three-phase sinusoidal currents with any power factor at both supply (input) and motor (output) sides. Therefore, it is suitable for medium-voltage high-power motor drives with regenerative braking, intended to replace a conventional line-commutated cycloconverter using thyristors. This paper provides an intensive discussion on how to control the whole TSBC system, how to regulate and balance the dc mean voltages of all the dc capacitors, and how to mitigate their ac voltage fluctuations. The validity and effectiveness of the proposed control strategy and tactics are verified by a three-phase 400-V 15-kW downscaled model.

This paper presents a specially designed third-order resonant circuit intended to achieve fast switching operation for a voltage-source series-resonant inverter using four MOSFETs. The third-order resonant current superimposed on a sinusoidal load current helps to quickly charge or discharge the output capacitance of each MOSFET. This results not only in a reduction of the time required to turn the MOSFET on and off, but also in an improvement of the displacement factor of the inverter. Moreover, the third-order resonant circuit acts as a low-pass filter to eliminate the parasitic oscillation between line inductance and stray capacitance. The viability and effectiveness of the third-order resonant circuit is verified by a 2MHz, 2kW prototype inverter developed for a low-temperature plasma generator.

This paper deals with a self‐commutated BTB (Back‐To‐Back) system for the purpose of power flow control and/or frequency change in transmission systems. Each BTB unit consists of two sets of 16 three‐phase voltage‐source converters, and their AC terminals are connected in series to each other via 16 three‐phase transformers. Hence, the BTB unit uses totally 192 switching devices capable of achieving gate commutation. This results in a great reduction of voltage and current harmonics without performing PWM control. Simulation results verify the validity of the proposed system configuration and control scheme not only under a normal operating condition but also under a single line‐to‐ground fault condition. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 143(3): 68–78, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10124

The purpose of this paper is to perform theoretical analysis of active power flowing into, or out of, a series device in a unified power flow controller (UPFC), and to reveal what amount of electric energy the dc link capacitor absorbs or releases through the series device in a transient state. This paper theoretically derives that the electric energy flowing into, or out of, the series device during a transient state is equal to the difference between the energy stored in the line inductance in a steady state before the start of the transient state and that in another steady state after the end of the transient state. This analysis helps us to determine the required capacity of the dc link capacitor, as well as the required response of dc voltage regulation in the shunt device. Experimental results obtained from a 10-kVA laboratory setup agree well with analytical results, as well as computer-simulated results.

The modular multilevel cascade converter (MMCC) family has been attracting attention from power electronics and power system engineers because it has been showing considerable promise as one of the next-generation high-power high-voltage/medium-voltage power converters. This paper provides an intensive discussion on power-flow analysis of three MMCC members with star configuration, focusing on dc-mean-voltage balance/regulation and ac-voltage mitigation control of all the floating dc capacitors. Most existing analysis methods are based on single-phase circuits, which make it difficult to derive capacitor voltage regulation/mitigation methods relying only on circulating currents and zero-sequence voltage. The power-flow analysis and regulation/mitigation methods developed in this paper are characterized by applying the αβ0 coordinate transformation(s). The validity of the power analysis and the regulation/mitigation method is confirmed by simulation using the PSCAD/EMTDC software package.