A Flexible Reduced Capacitor Voltages Strategy for Variable-Speed Drives with Modular Multilevel Converter

Modular multilevel converter (MMC) has developed into an attractive topology for the variable-speed motor drives. However, the main challenge for the applications is the large low-frequency capacitor voltage fluctuation. This paper proposes a novel control strategy that is able to reduce the average capacitor voltages in a satisfying range when the motor operates below the base speed. Thus, the capacitor voltages can be well controlled under the nominal voltage for much better safety and reliability, and in the meantime the motor is able to operate with lower speed, larger load torque, and lower circulating current in the MMC for the wider range of operation conditions and greater control performances. The effectiveness of the proposed control strategy are demonstrated by the simulation results in PSCAD/EMTDC.