Control of the Modular Multilevel Converter for variable-speed drives

This paper presents a control strategy of the entire frequency range operation for Modular Multilevel Converter (MMC), especially focusing on variable speed drive of an AC machine. The structure of MMC essentially requires energy balancing control so as to mitigate the voltage pulsation of each cell capacitor in converter arms. In the proposed control strategy, two operation modes are employed. One is a low frequency operation mode for start-up and low speed operation of the AC machine, and the other is a normal frequency operation mode from medium to rated speed of the AC machine. To reduce the pulsation, this paper proposes the energy balancing control strategies at each operation mode. Theoretically, the energy balancing control of the capacitors is prone to be unstable at low frequency operation. In order to prevent the instability, a special control strategy is introduced. The strategy exploits a common mode voltage and a circulating current with high frequency component in low frequency operation mode. With the proposed control scheme, the speed control range of the AC machine driven by MMC can be down to zero speed without instability of voltage of the cell capacitors. Experimental results for the energy balancing control are shown to demonstrate the effectiveness of the proposed control strategy.