Control strategy for improved dynamic performance of variable-speed drives with the Modular Multilevel Converter

This paper presents a control scheme for the Modular Multilevel Converter (MMC) to drive a variable-speed AC machine, especially focusing on improving dynamic performance. Theoretically, the energy balance in the cell capacitors is prone to be unstable at startup and low frequency operation. Therefore, the MMC topology essentially requires advanced strategies for energy balancing so as to suppress the voltage pulsation of each cell capacitor. The pulsation is getting larger as the output frequency of MMC is getting lower. This paper proposes a strategy for robust dynamic response even at zero output frequency of MMC employing leg offset voltage injection. With this proposed strategy, an AC machine has been driven from standstill to rated speed without excessive cell capacitor voltage ripples. The experimental results verify that stable operation is guaranteed for both a 15r/min operation with 40% step load torque and the entire frequency operation with varying load torque.