Publications

Experimental Characterization of a 750-V 100-kW 16-kHz Bidirectional Isolated DC-DC Converter With a Unity-Turns-Ratio Transformer at Different Voltage Ratios

This paper focuses on precise conversion-efficiency measurement of a bidirectional isolated dc-dc converter using the 1.2-kV 400-A SiC-MOSFET/SBD H-bridge modules, along with a unity-turns-ration transformer. A specially-designed test bench for efficiency measurement is characterized by the cascade connection of the two identical 750-V 100-kW 16-kHz bidirectional isolated dc-dc converters. Although the test bench is equipped with two identical unity-turns-ratio transformers, both converters are capable of operating at different voltage ratios ranging from 0.87 to 1.13. Experimental results verify that the whole conversion efficiency from the dc input to the dc output is as high as 98.9% at the rated power of 100 kW with a voltage ratio of 1.13, and 98.6% at 100 kW with another voltage ratio of 0.87, excluding an auxiliary power loss of 82 W.

Harmonic Analysis of the Output Voltage of a Third-Harmonic-Injected Inverter for LSM Drives

The superconducting magnetic levitation railway system (MAGLEV) under development in Japan uses a pulse-width-modulation (PWM) inverter for driving a linear synchronous motor (LSM). The inverter output voltage contains non-negligible harmonics which cause harmonic resonances in the LSM system, and therefore harmonics of the output voltage have been analyzed in order to control such harmonic resonances.This paper applies a third-harmonic injection method to the inverter for the purpose of enhancing the output voltage without changing the circuit configuration. It performs harmonic analysis of the output voltage of the inverter based on the third-harmonic injection. Validity of the harmonic analysis is verified by computer simulation.

Modeling and Analysis of High-Frequency Leakage Currents Caused by Voltage-Source PWM Inverters.

This paper deals with the high-frequency leakage current caused by a voltage-source PWM inverter. A nonnegligible amount of leakage current would flow through stray capacitors between stator windings and a motor frame due to a, large step change of the common-mode voltage produced by the PWM inverter. An equivalent circuit for the leakage current is proposed, which is expressed by a series resonant circuit. The analysis applying the equivalent circuit gives us such a conclusion that the connection of a common-mode choke in series between the inverter and motor terminal is not effective to reduce the rms and average value of the leakage current, but effective to reduce the peak value. Furthermore, this paper shows that the modeling and analysis are valid in designing a previously proposed suppression method of the leakage current.

Discussion on Triangular-Carrier Frequencies of a Modular Multilevel Cascade Converter Based on Double-Star Chopper Cells (MMCC-DSCC) with Phase-Shifted PWM

No abstract is provided for this article.

Power Electronics

No abstract is provided for this article.

A direct drive of multiple multi-pole synchronous generators for wind energy conversion systems

This paper presents a direct drive of multiple multi-pole synchronous generators for wind energy conversion system. The concept of multiple-synchronous-generator drive is demonstrated by a single current-controlled inverter feeding 2 multi-pole synchronous generators connected in parallel. In addition, the torque unbalance of the drive is investigated by taking into account the parameters mismatch among two synchronous generators. Accordingly, the proposed driver is set up with maximum-power-point-tracking scheme and the given simulation results show the technical feasibility of the proposal for the wind energy conversion system.

Compensation Characteristics of Shunt Active and Series Active Power Filters.

Active power filters have been used in practice to suppress the harmonic interference in power systems. To compensate for harmonic currents of loads active power filters are usually connected to power systems in parallel with the loads. These filters which are called shunt active filters here are very effective to loads that can be considered as current sources, such a thyristor rectifiers with large DC reactances. Many papers have covered the shunt active filters applied to these currentsource loads. No paper, however, has discussed characteristics of the shunt active filters when they are applied to voltage-source loads.On the other hand, since more and more diode rectifiers with capacitive DC filters are used recently, harmonics generated by which become an issue.The diode rectifier with capacitive DC filters behaves like a voltage source rather than a current source. When a shunt active filter is applied to such a diode rectifier, the current injected from the shunt active filter may flow into the diode rectifier. As a result, harmonics of the source current cannot be reduced effectively, and harmonic current flowing into the diode rectifier increases largely.This paper reveals the above-mentioned problem of shunt active filters analytically and experimentally. Then a series active filter is proposed to suppress harmonic current of the diode rectifiers. The features, operating conditions, and considerations of shunt active filters and series active filters are described analytically and demonstrated experimentally. Taking a diode rectifier with capacitive DC filter as a typical voltage-source load, compensation characteristics of shunt active filters and series active filters are discussed by experiment and simulation. The validity of the series active filters is illustrated experimentally.

Classification, Terminology, and Application of the Modular Multilevel Cascade Converter (MMCC)

This paper discusses the modular multilevel cascade converter (MMCC) family based on cascade connection of multiple bidirectional chopper cells or single-phase full-bridge cells. The MMCC family is classified from circuit configuration as follows: the single-star bridge cells (SSBC); the single-delta bridge cells (SDBC); the double-star chopper cells (DSCC); and the double-star bridge cells (DSBC). The term MMCC corresponds to a family name in a person while, for example, the term SSBC corresponds to a given name. Therefore, the term "MMCC-SSBC" can identify the circuit configuration without any confusion. Among the four MMCC family members, the SSBC and DSCC are more practical in cost, performance, and market than the others although a distinct difference exists in application between the SSBC and DSCC. This paper presents application examples of the SSBC to a battery energy storage system (BESS), the SDBC to a static synchronous compensator (STATCOM) for negative-sequence reactive-power control, and the DSCC to a motor drive for fans and blowers, along with their experimental results.

Stability Analysis of a Series Active Filter Integrated with a Double-Series Diode Rectifier

Modeling and damping of high-frequency leakage currents in PWM inverter-fed AC motor drive systems

This paper presents an equivalent circuit for high-frequency leakage currents in PWM inverter-fed AC motors, which forms a series resonant circuit. The analysis based on the equivalent circuit leads to such a conclusion that the connection of a conventional common-mode choke or reactor in series between the AC terminals of a PWM inverter and those of an AC motor is not effective to reduce the RMS and average values of the leakage current, but effective to reduce the peak value. Furthermore, this paper proposes a common-mode transformer which is different in damping principle from the conventional common-mode choke. It is shown theoretically and experimentally that the common-mode transformer is able to reduce the RMS value of the leakage current to 25%, where the core used in the common-mode transformer is smaller than that of the conventional common-mode choke.

Control of a transformerless STATCOM based on the MMCC-SDBC (modular multilevel cascade converter — Single-delta bridge-cells)

This paper describes a transformerless STATCOM (Static synchronous compensator) based on the MMCC-SDBC (modular multilevel cascade converter - single-delta bridge-cells) for independent control of positive- and negative-sequence reactive powers. A three-phase 200-V 6-kVA experimental system is designed, constructed and tested to verify the effectiveness of the topology and the control method including dc voltage regulation.

A passive EMI filter with access to the ungrounded motor neutral line—Its effect on attenuating bearing current

This paper describes the effect of a passive EMI filter on preventing bearing current from flowing inside an inverter‐driven motor. Motor‐bearing damage is often caused by bearing currents resulting from the breakdown of grease films in the motor bearing. The high‐frequency common‐mode voltage generated by the PWM inverter induces a shaft voltage between the rotor and the frame. When the shaft voltage exceeds the breakdown voltage of the grease films, a destructive instantaneous discharge current with a peak value of about 1 A flows through the motor bearing. The passive EMI filter, which is unique in access to the motor neutral line, can reduce the shaft voltage as a result of eliminating the high‐frequency common‐mode voltage from the motor terminals. Hence, no breakdown occurs in the grease film, so that no bearing current flows. The viability and effectiveness of the passive EMI filter is verified by experimental results obtained from a 400‐V, 3.7‐kW laboratory system. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 166(2): 78– 87, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20489

Suppression of common-mode voltage in a PWM rectifier/inverter system

This paper proposes a PWM rectifier/inverter system capable of suppressing not only supply harmonic currents but also electromagnetic interference (EMI). An active common-noise canceler (ACC) developed for this system is characterized by sophisticated connection of a common-mode transformer which can compensate for common-mode voltages produced by both PWM rectifier and inverter. As a result, the size of the common-mode transformer can be reduced to 1/3, compared with the previously proposed ACC. A prototype PWM rectifier/inverter system (2.2 kW) has been implemented and tested. Some experimental results show reduction characteristics of the supply harmonic current and EMI.

Current control of a phase-shifted-PWM STATCOM using the modular multilevel cascade converter based on single-star bridge-cells (MMCC-SSBC)

This paper presents and discusses a phase-shifted-PWM STATCOM using the modular multilevel cascade converter based on single-star bridge-cells (MMCC-SSBC). A three-phase 140-V, 10-kVA downscaled experimental system with a cascade count N =6 is designed, constructed and tested for verifying the performance and functionality of the STATCOM on both steady and transient states. The 13-level line-to-neutral (25-level line-to-line) voltage at the ac side yields an almost sinusoidal line current with a THD of 1.7%. An analytical model for the phase-shifted PWM is proposed, leading to the design of current control gains. Two phase-shifted PWM methods named as one-cell and all-cells update methods are compared theoretically and experimentally. The all-cells update method can reduce the inherent delay time, thus resulting in a faster and more stable system response than the one-cell update method.

Experimental Comparisons Between Modular Multilevel DSCC Inverters and TSBC Converters for Medium-Voltage Motor Drives

This paper makes an intensive comparison in operating performance between a modular multilevel double-star chopper-cells (DSCC) inverter and a modular multilevel triple-star bridge-cells (TSBC) converter. Both inverter and converter are intended to drive medium-voltage motors in industrial applications. First, it makes numerical comparisons, thus, resulting in revealing that the torque and frequency of a driven motor produce a significant effect on capacitor-voltage fluctuation and arm or cluster current in the individual DSCC inverter and TSBC converter. Next, a three-phase DSCC inverter and a three-phase TSBC converter with the same rating as 400 V and 15 kW are designed and compared to drive the following two general purposes and specially-designed induction motors; one is rated at the 380-V, 15-kW, 50-Hz four-pole motor, and the other is at the 320-V, 15-kW, 38-Hz six-pole motor. This experimental comparison based on the two downscaled drive systems confirms the validity of the numerical comparison. Finally, this paper concludes that the DSCC inverter is more suitable for driving medium-voltage high-speed motors loaded with quadratic-torque-to-speed profiles like fans, blowers, pumps, and centrifugal compressors. On the other hand, the TSBC converter is more suitable for driving medium-voltage low-speed high-torque motors like mills, kilns, conveyors, and extruders.

A bidirectional isolated DC/DC converter as a core circuit for 3.3‐kV/6.6‐kV power conversion systems in the next generation

This paper describes a bidirectional isolated DC/DC converter considered as a core circuit for next‐generation 3.3‐kV/6.6‐kV high‐power‐density power conversion systems. The DC/DC converter is intended to use power switching devices based on SiC and/or GaN, which will be available on the market in the near future. A 350‐V, 10‐kW, and 20‐kHz DC/DC converter is designed, constructed, and tested in this paper. It consists of two single‐phase full‐bridge converters with the latest trench‐gate Si‐IGBTs and a 20‐kHz transformer with a nano‐crystalline soft‐magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full‐bridge converters. The overall efficiency from the DC‐input to DC‐output terminals is accurately measured to be as high as 97%, excluding gate drive circuit and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC‐based power switching devices. The loss analysis clarifies that the use of SiC‐based power devices may bring a significant reduction in conducting and switching losses to the DC/DC converter. As a result, the overall efficiency may reach 99% or higher. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 163(2): 75–83, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20505

Bi-Directional Isolated DC-DC Converter for Next-Generation Power Distribution-Comparison of Converters Using Si and SiC Devices

In this paper two bi-directional DC-DC converters for a 1MW next-generation BTB system of a distribution system, as it is applied in Japan, are presented and compared with respect to design, efficiency and power density. One DC-DC converter applies commercially available Si-devices and the other one high voltage SiC switch, which consists of a SiC JFET cascode (MOSFET+1 JFET) in series with five SiC JFETs.In the comparison also the high frequency, high voltage transformer, which ensures galvanic isolation and which is a core element of the DC-DC converter, is examined in detail by analytic calculations and FEM simulations.For validating the analytical considerations a 20kW SiC DC-DC converter has been designed in detail. Measurement results for the switching and conduction losses have been acquired from the SiC and also for a Si system for calculating the losses of the scaled 1MW system.

A Speed-Sensorless Start-Up Method for an Induction Motor Driven by a Modular Multilevel Cascade Inverter Based on Double-Star Chopper Cells (MMCI-DSCC)

A modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) is expected as one of the next-generation multilevel PWM inverters for medium-voltage motor drives. This paper presents theoretical and experimental discussions on a practical speed-sensorless start-up method for an induction motor driven by the MMCI-DSCC from a standstill to a middle speed. This motor drive is suitable, especially for a large-capacity fan- or blower-like load, the torque of which is proportional to a square of the motor mechanical speed. Unlike the so-called “volts-per-hertz” control, three-phase stator currents are based on “feedback” control, whereas their amplitude and frequency are based on “feedforward” control. Although the motor drive has no speed sensor attached to the motor shaft, this method makes a slow start-up stable with the help of a stator current-feedback loop. Experimental results obtained from a 400-V, 15-kW downscaled system verify the stable operating performance from a standstill to a middle speed of 590min-1 with a 40% load torque.