Medium Voltage Power Conversion Research Articles

Control and Experiment of Pulsewidth-Modulated Modular Multilevel Converters

A modular multilevel converter (MMC) is one of the next-generation multilevel converters intended for high- or medium-voltage power conversion without transformers. The MMC is based on cascade connection of multiple bidirectional chopper-cells per leg, thus requiring voltage-balancing control of the multiple floating DC capacitors. However, no paper has made an explicit discussion on voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of pulsewidth-modulated modular multilevel converters (PWM- MMCs) with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the PWM-MMCs to achieve voltage balancing without any external circuit. The viability of the PWM-MMCs, as well as the effectiveness of the voltage-balancing control, is confirmed by simulation and experiment.

A Comparative Study of Medium-Voltage Power Converter Topologies for Plasma Torch Under Dynamic Operating Conditions

This paper compares several medium-voltage power conversion systems for plasma torch application. The thermal plasma torch rated for 3 MW, 2 kA with the physical size of 1 m long is selected. The dynamic characteristics of the dc arc in a plasma torch are investigated using advanced 3-D magnetohydrodynamics simulation. The arc voltage noise of plusmn10% due to the periodic pressure wave is modeled as a sawtooth waveform in arc power loss term. The parameters of the Cassie-Mayr arc model are calculated based on 3-D simulation. A 12-pulse thyristor rectifier and two-phase staggered three-level step-down dc-dc converter having 12-pulse front-end diode rectifier are compared under the same dynamic operating condition. The three-level dc-dc converter has superior dynamic performance over the 12-pulse thyristor rectifier under the existence of arc disturbance noise. This power converter topology provides higher ignition voltage around 5 kV during ignition phase and higher arc stability.

モジュラー・マルチレベル変換器（MMC）のPWM制御法と動作検証

A modular multilevel converter (MMC) is one of the next-generation multilevel PWM converters intended for high- or medium-voltage power conversion without transformers. The MMC consists of cascade connection of multiple bidirectional PWM chopper-cells, thus requiring voltage-balancing control of their chopper-cells. However, no paper or article has been presented or published on the voltage-balancing control with theoretical and experimental verifications. This paper deals with two types of modular multilevel PWM converters with focus on their circuit configurations and voltage-balancing control. Combination of averaging and balancing controls enables the MMCs to achieve voltage balancing without any external circuit. The viability of the MMCs as well as the effectiveness of the PWM control method is confirmed by simulation and experiment.

The next‐generation medium‐voltage power conversion systems

This paper describes the next‐generation mediumvoltage power conversion systems based on transformerless cascade PWM converters and bidirectional isolated dc‐dc converters. A 350‐V, 10‐kW and 20‐kHz dc‐dc converter is designed, constructed and tested as a core circuit for medium‐voltage power conversion systems. It consists of two single‐phase full‐bridge converters using eight 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%. Loss analysis clarifies that the overall efficiency may reach 99% or higher when SiC‐based power devices are used. In addition, this paper presents the 6.6‐kV transformerless STATCOM(STATic synchronous COMpensator) intended for achieving reactive‐power control in industrial and distribution power systems. It is characterized by direct connection to the 6.6‐kV grids, thus bringing significant reductions in cost, weight and size to the 6.6‐kV STATCOM.

Reduced Switching-Frequency-Modulation Algorithm for High-Power Multilevel Inverters

Multilevel inverters have emerged as attractive high-power medium-voltage power-conversion systems. They are mainly controlled with high-frequency pulsewidth-modulation methods. This is not suitable for very high-power application due to significant switching losses. This paper presents an adaptive duty-cycle modulation algorithm that reduces the switching frequency to a minimum necessary to fulfill the dynamic requirements of the system. Switching losses are, therefore, strongly reduced. This is achieved by using the slope of the voltage reference to adapt the modulation period to ensure that only one-step change between two voltage levels. Simulation and experimental results are presented for a nine-level cascaded inverter. Voltage waveforms obtained for variable frequencies and amplitudes show similar switching patterns, with a reduced and near-constant number of commutations per cycle, regardless of the reference frequency and amplitude.

A Bidirectional Isolated DC–DC Converter as a Core Circuit of the Next-Generation Medium-Voltage Power Conversion System

This paper describes a bidirectional isolated dc-dc converter considered as a core circuit of 3.3-kV/6.6-kV high-power-density power conversion systems in the next generation. The dc-dc converter is intended to use power switching devices based on silicon carbide (SiC) and/or gallium nitride, 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. It consists of two single-phase full-bridge converters with the latest trench-gate insulated gate bipolar transistors 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 and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. 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