A control strategy for MMC variable-frequency drive

The submodule (SM) capacitor voltage fluctuation threatens the security of the entire modular multilevel converter (MMC) variable-frequency drive system. This paper focuses on the control strategy for MMC variable-frequency drive based on permanent magnet synchronous motor (PMSM). The MMC control strategy is divided into the high-speed and low-speed stages. The traditional capacitor voltage balance method is employed at high speed, and the high-frequency injection method is used at low speed. Combined with motor control, the MMC variable-frequency drive control scheme is constructed. To verify the feasibility and effectiveness of the proposed control strategy, simulation and experiment are carried on. The results show that the proposed control strategy suppresses the SM capacitor voltage ripples effectively and shows excellent speedregulation and variable-load performances.