Makoto Hagiwara

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 Medium-Voltage Motor Drive With a Modular Multilevel PWM Inverter

This paper describes the control and operating performance of a modular multilevel PWM inverter for a transformerless medium-voltage motor drive. The inverter is prominent in the modular arm structure consisting of a cascaded stack of multiple bidirectional chopper-cells. The dominant ac-voltage fluctuation with the same frequency as the motor (inverter) frequency occurs across the dc capacitor of each chopper-cell. The magnitude of the voltage fluctuation is inversely proportional to the motor frequency. This paper achieves theoretical analysis on the voltage fluctuation, leading to system design. A downscaled model 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-to-line) PWM inverter for a medium-voltage motor drive.

Control and Analysis of the Modular Multilevel Cascade Converter Based on Double-Star Chopper-Cells (MMCC-DSCC)

This paper presents the modular multilevel cascade converter based on double-star chopper-cells, which is intended for grid connection to medium-voltage power systems without using line-frequency transformers. The converter is characterized by a modular arm structure consisting of cascade connection of multiple bidirectional pulsewidth modulation chopper-cells with floating dc capacitors. This arm structure requires voltage-balancing control of all the dc capacitors. However, the voltage control combining an averaging control with an individual-balancing control imposes certain limitations on operating conditions. This paper proposes an arm-balancing control to achieve voltage balancing under all the operating conditions. The validity of the arm-balancing control as well as the theory developed in this paper is confirmed by computer simulation and experiment.

PWM control and experiment of modular multilevel converters

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 and floating dc capacitors per leg, thus requiring voltage-balancing control of their chopper-cells. However, no paper has been discussed explicitly on 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.

Negative-Sequence Reactive-Power Control by a PWM STATCOM Based on a Modular Multilevel Cascade Converter (MMCC-SDBC)

This paper presents the application of a modular multilevel cascade converter based on single-delta bridge cells (SDBCs) 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, thus facilitating flexible circuit design, low-voltage steps, and low-electromagnetic-interference emissions. This paper designs, constructs, and tests a 100-V 5-kVA pulsewidth-modulated STATCOM based on the SDBC, with focus on the operating principle and performance. Experimental results verify that it can control not only positive-sequence reactive power but also negative-sequence reactive power and low-frequency active power intended for flicker compensation of arc furnaces.

Start-Up and Low-Speed Operation of an Electric Motor Driven by a Modular Multilevel Cascade Inverter

This paper describes start-up and low-speed operation of an electric motor driven by a modular multilevel cascade inverter based on double-star chopper cells. This paper proposes a square-wave method to suppress the peak circulating current. The theoretical analysis developed in this paper reveals that the peak circulating current when using the square-wave method gets smaller by 50% than that when using the sinusoidal-wave method proposed in the previous work. The experimental results obtained from a 400-V 15-kW downscaled system verify that stable operation is achieved at an ultralow speed of 17 min-1 with a load torque of τL = 40%, as well as “three-phase” dc-current feeding operation. Moreover, the motor can start up from a standstill without producing any overvoltage or overcurrent.

Experimental Verification of a Modular Multilevel Cascade Inverter Based on Double-Star Bridge Cells

A modular multilevel cascade inverter based on double-star bridge cells (MMCI-DSBC) is expected to be one of the next-generation medium-voltage pulsewidth modulation (PWM) inverters intended for grid connections. This inverter is formed by six modular arms, each of which consists of a cascaded stack of multiple full-bridge converters. The DSBC is different from the traditional two-level PWM inverter in that the ac voltages are independent of the dc link voltage. Hence, the DSBC is suitable for a grid-connected inverter with a time-varying dc link voltage. This paper describes the DSBC with a focus on its operating principles and performance. The validity of the inverter is confirmed by experiments using a three-phase, 200-V, and 10-kW downscaled system.

Modeling and Analysis of Switching-Ripple Voltage on the DC Link Between a Diode Rectifier and a Modular Multilevel Cascade Inverter (MMCI)

This paper focuses on the common dc-link voltage between a three-phase diode rectifier and a modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) for a medium-voltage motor drive. This motor drive can be operated even when no capacitor exists on the dc link. However, a nonnegligible, but predictable, amount of switching-ripple voltage occurs on the dc link. This paper achieves modeling and analysis of the switching-ripple voltage, thus making it possible to design a small-sized dc passive filter consisting of series connection of a film capacitor and a damping resistor. A 400-V, 15-kW down scaled system is used to confirm the effectiveness of the analysis and the dc filter. Experimental results show that the switching-ripple voltage can be attenuated satisfactorily by the dc filter, and that the power loss dissipated in the damping resistor is negligible, compared to the rated power of 15 kW.

Control and Experiment of a Modular Multilevel Cascade Converter Based on Triple-Star Bridge Cells

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.

Performance of a self-commutated BTB HVDC link system under a single-line-to-ground fault condition

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.