Reduction of Half-Arm Current Stresses and Flying-Capacitor Voltage Ripples of Flying-Capacitor MMCs

Abstract

In this paper, a new control scheme that can reduce the current stresses on switching devices, flying capacitors, and inductors for flying-capacitor modular multilevel converters (FC-MMCs) is proposed, which is based on redistribution of power difference between upper and lower arms through the flying capacitors. In addition, a way to determine the frequency range of the square-wave voltage to be injected into the FC-MMC is suggested, which is able to improve the current control performance and reduce the flyingcapacitor voltage ripples. The effectiveness of the proposed control scheme has been verified for a 4160-V/1-MW simulation model and a 230-V/3-kW prototype of the FC-MMC. INDEX TERMS Arm current, medium-voltage motor drive, modular multilevel converters (MMC), submodule (SM) capacitor voltage ripple.

A modified submodule of modular multilevel converter using active power decoupling method for reducing capacitor voltage ripple under low‐frequency operation

“…In addition, the voltage of the auxiliary capacitor is designed to allow significant voltage fluctuations that require a much smaller capacitance than the capacitance in the original SM. To reduce the voltage fluctuation of the SM capacitor, the ripple components of the SM are transferred to the auxiliary capacitor C cf by controlling the auxiliary capacitor v cf to follow the voltage reference in (23). As shown in Figure 4, the block diagram on the left side indicates the auxiliary capacitor voltage control used to generate the tracking current reference.…”

“…The responses of the APD capacitor voltages (V cf ) with the corresponding reference calculated in (23) for the first SM in the upper and lower arm are shown in Figure 8. The large voltage fluctuation of the auxiliary capacitors (v cf ) in the SMs is accept- able because the APD capacitors C cf are not directly connected to the system.…”

“…In recent years, various types of research have been conducted on both control strategies and modified topologies to reduce the SM capacitor voltage fluctuation in low-frequency operation [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In terms of control strategies, commonmode injection methods have been proposed to remove the fundamental frequency ripple components of capacitor voltage fluctuation by injecting a high-frequency common-mode voltage (CMV) into the AC output voltage and circulating current with the same frequency into each phase leg [13][14][15][16][17][18].…”

“…For these reasons, flying capacitor modular multilevel converters (FC-MMC) are considered the modified topology methods that have been proposed to reduce the capacitor voltage fluctuation in the entire speed range of medium-voltage drives without imposing the CMV on the motor winding [19][20][21][22][23]. The structure of an FC-MMC is similar to a conventional MMC structure with three auxiliary capacitors that connect to the middle node of each arm to transfer the power difference between the upper and lower arms in low-frequency operation.…”

Common-Mode Voltage Reduction of Modular Multilevel Converter Using Adaptive High-Frequency Injection Method for Medium-Voltage Motor Drives

“…By injecting high-frequency square-wave waveforms of CMV and circulating current instead of sinusoidal waveforms, the peak-to-peak value of the circulating current can be reduced significantly [11]. However, square-waveform CMV may pose significant challenges in terms of dv/dt at the motor terminal, which can jeopardize the longevity of motor bearings and render them unsuitable for practical applications [20,21]. To address these concerns, numerous investigations have been undertaken to guarantee the dependability and optimal functionality of converter systems across the complete spectrum of motor drive speeds [13][14][15].…”