Research trends of modular multilevel cascade inverter (MMCI-DSCC)-based medium-voltage motor drives in a low-speed range

A modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) has been expected as one of the next-generation multilevel PWM inverters for medium-voltage high-power motor drives. This inverter consists of cascaded bidirectional chopper cells with dc capacitors. One of concerns for the motor drive is how to achieve stable operation in a low-speed range. Some kind of solution should be taken to achieve stable operation in a low-speed range, because ac-voltage fluctuations in the dc capacitors are inversely proportional to an inverter frequency. This paper describes state-of-the-art research trends in MMCI-DSCC-based motor drives, especially focusing on the mitigation of the ac-voltage fluctuations. Four kinds of mitigation methods including the proposed ones are summarized, compared, and verified by experiments using a 400-V 15-kW downscaled system. The steady-state waveforms show the validity of the theoretical analysis on the peak current in IGBTs, and start-up waveforms show the stable operation from a standstill to 740 min^-1 with 60% load torque.