Modular multilevel converter with full-bridge submodules and improved low-frequency ripple suppression for medium-voltage drives

Due to the low-frequency voltage fluctuation problem, application of modular multilevel converters (MMC) in wide-speed-range medium-voltage (MV) drives is still difficult. This paper proposes a new approach to this problem. Firstly, full-bridge submodules (FB-SM) are used in place of the traditional half-bridge submodules (HB-SM). The former can significantly reduce the low-frequency pulsating power in the SM capacitors. Secondly, an improved “square wave plus third-order harmonic injection” with quasi-resonant (QR) control scheme is proposed to transfer the low-frequency ripple power into high-frequency ones. Simulation and experimental results prove the validity of proposed method. A comparison between the FB-SM based and HB-SM based MMCs showed good potential of the former in MV drive applications.