Capacitor Voltage Balancing Control of Modular Multilevel Converters with Energy Storage System by Using Carrier Phase-Shifted Modulation

Modular Multilevel Converters (MMC) are used in large scale battery energy storage system by integrating batteries into submodule. But batteries will affect submodule power, making it more difficult to control submodule capacitor voltage. Firstly, this paper analyzes the impact of batteries on capacitor voltage. It has opposite characteristic in discharging and charging mode. In discharging mode, capacitor voltage is self-balancing. But in charging mode, capacitor voltage is easy to be unbalanced. Then a modulation ratio factor is defined to control submodule power through changing the AC component of submodule terminal voltage. On this basis, a close-loop based voltage balancing control method is proposed. Different from traditional method, arm current is not needed here. In order to improve the dynamic performance, modulation ratio factor is used as feedforward. Finally, the analysis and control method are verified through MATLAB simulation and a single phase experiment prototype.