Back-to-back hybrid modular multilevel converters for ac motor drive

Recently, modular multilevel converters (MMC) have attracted extensive research interest in the field of HVDC and motor drives. However in motor drives, it appears that the inherent problem of the fluctuation of capacitor voltage particularly at low frequencies limits its further application. This paper proposes a new back-to-back hybrid MMCs topology, of which one MMC is directly connected to the grid and another MMC that controlled to drive an AC motor is connected to the previous one with a DC bus. This paper proposes the corresponding control strategies to drive the motor in wide speed range with basically constant fluctuation of capacitor voltage under constant-load operation at the same time. The grid-side MMC is controlled to generate constant DC current to the DC bus while the motor-side MMC is regulated to generate a constant AC output current to maintain the load torque. The effectiveness of the topology and its control strategies are verified by Matlab/Simulink.