Publications

This paper discusses an optimal operating frequency range and optimal switching dead times for a 6-kW, full-bridge, bidirectional isolated dc-dc converter with focus on improving converter efficiency. The optimal frequency of the dc-dc converter is defined by making its frequency-dependent loss equal to its frequency-independent loss. The maximum efficiency of the dc-dc converter operating at 4 kHz is measured at 98.1% during battery charging and at 98.2% during battery discharging. The converter maintains a high efficiency of more than 97% in a wide range of power transfer.

Conventional passive filters have been broadly used to suppress the harmonics in power systems. However, they have the following disadvantages; (1) Source impedance affects the compensation characteristics. (2) Antiresonance between the source and the passive filter amplifies the harmonic current of the source at the specific frequency. (3) No harmonic compensation is performed at the frequency to which the passive filter is not tuned. On the other hand, active filters which have been studied and put into practical use have the ability to overcome the disadvantages mentioned above, but their initial and running costs are high, because they need voltage-or current-source PWM converters having high speed current response.This paper presents a novel harmonic compensator consisting of an active filter and a passive filter which are connected in series with each other. The passive filter removes the harmonics of the load, and the active filter improves the compensation characteristics of the passive filter. So the proposed filter system can eliminate the problems of the passive filter used alone and needs a small rated active filter, compared with a conventional active filter used alone. Control schemes and compensation characteristics are discussed, and experimental results of a laboratory model are shown.

Unlike traditional passive harmonic filters, modern active harmonic filters have the following multiple functions: harmonic filtering, damping,isolation and termination, reactive-power control for power factor correction and voltage regulation, load balancing, voltage-flicker reduction, and/or their combinations. Significant cost reductions in both power semiconductor devices and signal processing devices have inspired manufactures to put active filters on the market. This paper deals with general pure active filters for power conditioning, and specific hybrid active filters for harmonic filtering of three-phase diode rectifiers.

A 21-level (line-to-line) self-commutated BTB (back-to-back) system based on series connection of sixteen converter-cells has attractive features as follows: i) Both active and reactive powers can be controlled independently even in transient states, ii) The BTB system produces almost sinusoidal voltage at the AC side without performing PWM (pulse-width-modulation) control, so that no harmonic filter is required, iii) The BTB system provides an active-power response as high as 3 ms. This work presents experimental verifications of the self-commutated BTB system for the purpose of achieving power flow control in transmission systems. Experimental results obtained by a 200-V 10-kW laboratory prototype confirm effectiveness and validity of both the proposed system configuration and control strategy developed in this paper.

This paper describes a 6.6-kV battery energy storage system based on a multilevel cascade PWM (pulsewidth- modulation) converter with star configuration. It discusses design concepts with and without a linefrequency transformer for grid connection. The control system consists of state-of-charge (SOC)-balancing control and fault-tolerant control. The former is indispensable for effective utilization of battery energy while the latter is required for maintaining continuous operation during a converter-cell or battery-unit fault. A 200-V, 10-kW, 3.6- kWh laboratory system combining a three-phase cascade PWM converter with nine nickel-metal-hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed control system.

This paper deals with a position-sensorless IPM motor drive system in which rotor position is estimated from current ripples generated by a PWM inverter. The new circuit configuration of a proposed digital controller makes it possible to reduce number of sample and hold circuits compared with the conventional system and to acquire current ripples automatically. Some experimental results show accuracy, response, and generality of the digital controller, and demonstrates a quick discrimination of magnetic polarity.

This paper focuses on voltage-balancing control suitable for a three-level diode-clamped PWM converter used in a hybrid active filter. A three-phase downscaled system including a 400-V, 15-kW three-phase diode rectifier with a capacitive load as a harmonic-producing load is designed, constructed, and tested to verify the filtering performance of the hybrid filter and the voltage-balancing control performance of the three-level PWM converter. Experimental results, along with theoretical analysis, verify that the voltage-balancing control proposed in this paper works properly in all the operating regions from full-load to no-load conditions.

This paper describes a 750-Vdc, 100-kW, 20-kHz bidirectional isolated dual-active-bridge (DAB) dc-dc converter using SiC-MOSFET modules without free-wheeling diodes, focusing on the output voltage ripple and power conversion efficiency. Intermittent operation brings higher conversion efficiencies in a low-power range to the dc-dc converter than does continuous operation because it makes a significant contribution to reducing switching (snubber) and iron losses. Experimental results reveal that the efficiency at 10kW (10% of the rated power) is 95.1% in continuous operation, whereas it is improved to 98.2% in intermittent operation. The combination of intermittent operation with continuous operation makes the efficiencies in a power range from 3 to 100kW higher than 98%, excluding power losses produced by the control and gate-drive circuits. However, the intermittent operation may bring additional voltage ripples to the dc output terminals. Theoretical analysis confirms that the voltage ripples in a power range from 1 to 18kW are lower than 7V in peak-to-peak, which is only 0.9% of 750Vdc. Theoretical results are confirmed by comparison to simulated and experimental ones.

Renewable energy sources such as wind turbine generators and photovoltaics produce fluctuating electric power. The fluctuating power can be compensated by installing an energy storage system in the vicinity of these sources. This paper describes a 6.6-kV battery energy storage system based on a cascade pulsewidth-modulation (PWM) converter with focus on a control method for state-of-charge (SOC) balancing of the battery units. A 200-V, 10-kW, 3.6-kWh (13-MJ) laboratory system combining a cascade PWM converter with nine nickel metal hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed balancing control.

This paper describes a transformerless energy storage system based on a cascade multilevel pulsewidth modulation converter with star configuration. The system is intended for power leveling of renewable energy sources, as well as for improving power quality and reliability of a power distribution system. This paper pays attention to active-power control and voltage-balancing control that are indispensable for proper operation of the energy storage system. A 200-V 10-kW 8.8-kJ downscaled laboratory system is designed, constructed, and tested, replacing electric double-layer capacitors with large-capacity electrolytic capacitors. Experimental results obtained from the laboratory system verify the viability and effectiveness of the 6.6-kV energy storage system.

A design strategy for the combined power filter for a three-phase twelve-pulse thyristor rectifier is proposed. The shunt passive filter, which can minimize the output voltage of the series active filter, is designed and tested in a prototype model. A specially designed shunt passive filter makes it possible to reduce the required rating of the series active filter to 60% compared with a conventional shunt passive filter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

This paper discusses the control and design of the 6.6-kV back-to-back (BTB) system combining bidirectional isolated dc/dc converters and modular multilevel cascade pulsewidth modulation (PWM) converters. The system consists of multiple converter cells connected in cascade per phase at both front ends. Each converter cell consists of a bidirectional isolated medium-frequency dc/dc converter and two voltage-source H-bridge (single-phase full-bridge) PWM converters. Extremely low-voltage steps bring a significant reduction in harmonics and electromagnetic interference emissions to the BTB system. This paper designs, constructs, and tests a single-phase downscaled BTB system rated at 120 V and 3.3 kW to verify the viability and effectiveness, leading to the actual system.

This paper focuses on a medium-voltage motor drive requiring no regenerative braking. It is operated not only in a high-speed region but also in a low-speed region with the rated torque, intended for an application to drilling rigs. The power conversion system consists of a front-end diode rectifier, a five-level diode-clamped PWM inverter, and a dc-voltage-balancing circuit including a single coupled inductor. The five-level diode-clamped inverter is characterized by injecting a common ninth-harmonic zero sequence voltage on each of the three-phase reference volt ages for mitigating magnetic-flux fluctuation of the coupled inductor of the dc-voltage-balancing circuit. Moreover, the voltage injection can make the power-loss distribution of eight insulated gate bipolar transistors (IGBTs) per leg close to an even profile. Experimental results obtained from a 200-V 5.5-kW downscaled model confirm the viability and effectiveness of the power conversion system, indicating that the ninth-harmonic zero-sequence voltage injection mitigates the magnetic-flux fluctuation.

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.

Over the last five to ten years, significant progress has been made in high-power semiconductor devices and in their practical applications to power systems. This comes not only from sophisticated semiconductor technology but also from the demand for a higher degree of frequency and voltage stability, and for greater reliability in power systems. This paper deals with an adjustable speed rotary condenser capable of not only reactive power control but also active power control based on a flywheel effect of the rotor. The behavior of a power system consisting of the adjustable speed rotary condenser, a synchronous generator and a transmission line is subjected to a set of nonlinear differential equations. The set of nonlinear equations can be linearized by limiting attention to small perturbation around a reference state, thus leading to the so-called Heffron-Phillips model of the power system. The Heffron-Phillips model derived is effective in analyzing effects of the adjustable speed rotary condenser on power system stabilization. The validity of the analysis is confirmed by computer simulation based on EMTDC. Finally, it is discussed how well power system stabilization is achieved by the rotary condenser. As a result, the rotary condenser has the function of decoupling reactive power control from active power control, thus producing a good effect on power system stabilization which would not be achieved by a conventional inverter-based static var compensator.