A novel reduced switching-frequency voltage-balancing strategy for modular multilevel converters

Capacitor voltage balancing with a relatively low switching frequency is one of the main concerns for the design of modular multilevel converter (MMC) controller. This paper proposes a novel reduced switching-frequency voltage-balancing strategy. Firstly, SMs in each arm are divided into several groups. Then, the required inserted SMs are assigned to those groups, and a developed principle is presented to keep the amount of required SMs invariable for each group as far as possible. Finally, it attempts to maintain the switching states of SMs in each group unaltered by a designed rule. The effectiveness of proposed strategy is validated by a 21-level MMC model in PSCAD/EMTDC environment. Simulation results demonstrate that the switching frequency can be reduced significantly with an allowable deviation of capacitor voltages.