Publications

This paper deals with "unified power quality conditioners" (UPQCs) which aim to integrate series active and shunt active filters. The main purpose of a UPQC is to compensate for voltage flicker/imbalance, reactive power, negative sequence current and harmonics. In other words, the UPQC has the capability of improving power quality at the point of installation on power distribution systems or industrial power systems. This paper discusses the control strategy of the UPQC, with the focus on the flow of instantaneous active and reactive power inside the UPQC. Some interesting experimental results obtained from a laboratory model of 20 kVA, along with theoretical analyses, are shown to verify the viability and effectiveness of the UPQC.

Attention has been paid to practical applications of series active filters and series active capacitors into power systems. These consist of both voltage-source PWM inverters and matching transformers (MTs) for series-connection to power systems. The purpose of the matching transformer is not only to isolate the PWM inverter from the source, but also to match the voltage and current ratings of the PWM inverter with those of the source. The output terminals on the ac side of the PWM inverter exhibit a high impedance because the impedance on the source side of the matching transformer is multiplied by N2 where the MT's turns ratio is 1: N. The PWM inverter consists of semiconductor switching devices such as MOSFETs or IGBTs. It is therefore essential to connect a small-rated LC passive filter for suppressing switching ripples appearing in the output voltage of the PWM inverter. Oscillation between a small-rated inductor and capacitor may occur because of the high impedance on the ac side of the PWM inverter, causing undesirable high frequency currents to flow into the power system.This paper proposes a new control scheme for the series active filter, aiming at suppressing the voltage oscillation between the small-rated inductor and capacitor. It is shown theoretically that the proposed simple current feedback can suppress the oscillation so that no undesirable current flows into the source. The proposed scheme is actually applied to a hybrid series active/shunt passive filter, and then a design example is shown. Digital simulation confirms that the proposed scheme can damp the oscillation effectively. In addition, experimental results demonstrate the effectiveness and practicability of the proposed control scheme.

No abstract is provided for this article.

This paper deals with a shunt active filter which will be installed by an electric utility, putting much emphasis on the control strategy and the best point of installation of the shunt active filter on a feeder in a power distribution system. The objective of the shunt active filter is to damp harmonic propagation, which results from harmonic resonance between many capacitors for power factor improvement and line inductors in the feeder, rather than to minimize voltage distortion throughout the feeder. Harmonic mitigation is a welcome by-product of the shunt active filter, which comes from damping of harmonic propagation. This paper concludes that the shunt active filter based on detection of voltage at the point of installation is superior in stability to others, and that the best site selection is not the beginning terminal but the end terminal of the primary line in the feeder. Computer simulation is performed to verify the validity and effectiveness of the shunt active filter by means of an analog circuit simulator, which is characterized by installing it on a feeder of a radial distribution system in a residential area.

No abstract is provided for this article.

This paper addresses shaft end-to-end and shaft-to-frame voltages that appear in a 400-V 15-kW induction motor driven by a voltage-source PWM inverter. Although both shaft voltages have been known to be associated to common-mode voltages, this paper demonstrates a shaft end-to-end voltage in the absence of common-mode voltages. Extensive experiments are performed with the motor subjected to differential-mode voltages. Results show that differential-mode voltages applied across motor phases also cause a shaft end-to-end voltage. It is concluded that the combination of differential-mode voltages with high dv/dt and high-frequency motor asymmetry causes a shaft end-to-end voltage. A model of one motor phase is presented to explain the generation of the shaft end-to-end voltage.

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 presents a modular multilevel PWM inverter for medium-voltage motor drives, focusing on its control method and operating performance. It is especially suitable for fans, blowers, pumps, and compressors, the load torque of which is proportional to a square of rotating speed. Particular attention is paid to dc-capacitor voltage fluctuation of each chopper-cell because it may affect the voltage rating of power switching devices used. This paper derives theoretical equations related to an amount of the voltage fluctuation. A laboratory system rated at 400 V and 15 kW is designed and built up to confirm the validity and effectiveness of the nine-level (17-level in line-toline) PWM inverter intended for medium-voltage motor drives.

Shunt passive filters, which are connected in parallel to harmonic-producing loads, have been widely used for harmonics suppression in power systems because of low costs and high efficiency, although they have inherent problems such as series and parallel resonance. On the other hand, shunt active filters, which are also placed in parallel, have such an inherent problem as initial and running costs are high, compared with those of shunt passive filters. To solve these inherent problems in shunt passive and shunt active filters, the authors have proposed a combined system of a shunt passive filter and a small rated series active filter.The purpose of this paper is to reduce the required rating of the series active filter, considering a practical application of the combined power filter to harmonic compensation for large capacity thyristor converters. The points are summarized as follows:(1) The required rating of the series active filter can be reduced less than one-tenth by connecting a series resonant circuit in parallel to the secondary of current transformer.(2) It is clarified theoretically and verified experimentally that compensation characteristics of the new combined system are equivalent to those of the already proposed system with no series resonant circuit.

A specially designed twelve-pulse thyristor rectifier of 5-8MVA is required as a low-voltage high-current dc power supply for super-conductive material tests. It is, however, too susceptible to low frequency variation of the utility voltage, or to the so-called “flicker” to generate a strong magnetic field with high stability.This paper presents a flicker and harmonic compensator based on the combined system of a small-rated series active filter and a conventional shunt passive filter. The main purpose of the compensator is to remove susceptibility to the flicker from the thyristor rectifier. The series active filter, which is connected in series with the utility, can compensate for flicker components present at the utility grid, the frequency range of which is from 1Hz to 20Hz. In addition, it helps almost all harmonic currents produced by the thyristor rectifier to flow into the shunt passive filter installed in parallel with the harmonic-producing load, i.e. the thyristor rectifier.

This paper provides a chronological overview of the topology for multilevel converters, and discusses their different terminology usages and characteristics. The multilevel converters include three-level neutral-point-clamped (NPC) and neutral-point-piloted (NPP) inverters, three-level and four-level flying-capacitor (FLC) inverters, and a family of modular multilevel cascade converters. Some have already been put into commercial use, some have been on a research and development stage, and others have been on an academic research stage. This paper pays much attention to six family members of the modular multilevel cascade converters, intended for grid-tied applications and medium-voltage high-power motor drives.

A doubly-fed flywheel generator-motor consists of a wound-rotor induction machine and a cycloconverter or a voltage-source PWM rectifier-inverter which is used as an ac excitor. Adjusting the rotor speed makes the generator-motor either release the kinetic energy to the power system or absorb it from the power system. Thus, the generator-motor has the capability of achieving, not only reactive power control, but also active power control based on a flywheel effect of the rotor. A 200-MJ flywheel generator-motor for the purpose of line-frequency regulation on a 132-kV bus has been installed and commissioned by the Okinawa Electric Power Company. However, little literature has been published on control strategy and dynamic performance analysis of doubly-fed flywheel generator-motors. This paper proposes a new control strategy for a doubly-fed flywheel generator-motor, which is characterized by the combination of vector control and decoupling control. The control strategy enables the flywheel generator-motor to perform active power control independent of reactive power control even in transient states. The validity of the theory developed in this paper, along with the effectiveness of the control strategy, is confirmed by computer simulations using EMTDC. In addition, this paper discusses the transient behavior of a magnetizing current of the flywheel generator-motor. The flywheel generator-motor based on leading edge power electronics and electric machine technologies shows promise as a versatile power conditioner, in particular, being capable of repetitively absorbing or releasing electric energy for a short period of time less than a minute.

This paper describes an autonomous-control method for a DC microgrid system having distribution power generators. This system consists of following five generation and control units; a solar-cell generation unit, a wind-turbine generation unit, a battery energy-storage unit, a flywheel power-leveling unit, and an AC grid-connected power control unit. The proposed control method intended for suppression of circulating current detects only the DC grid voltage. Each unit could be controlled autonomously without communicating each other. This method brings high reliability, high-flexibility and maintenance-free operation to the system. Experimental results from a 10 kW-prototype system verify the validity and effectiveness of the proposed control method.