Improved capacitor voltage balancing control for multimode operation of modular multilevel converter with integrated battery energy storage system

Modular multilevel converter with integrated battery energy storage system (MMC-BESS) has been proposed for energy storage requirements in high-voltage applications with large-scale renewable energy resources. The MMC-BESS is essentially a three-port converter which can transfer energy between any two of the ac port, dc port, and BESS, constituting multimode operation of the system. Different from conventional MMCs, the state-of-charge (SOC) inconsistency among batteries would magnify the submodule capacitor voltage unbalance issue in the MMC-BESS. In this study, an improved capacitor voltage balancing method applicable for multimode operation of MMC-BESS is proposed by adjusting ac and dc modulation indexes simultaneously. After that, the ratio of ac and dc modulation indexes is optimised to enhance the tolerance of unbalanced power among SMs within phase arm, greatly improving the SOC equalisation rate. Based on the proposed capacitor voltage balancing method, the control structure is given and the dynamic model is conducted for the analytic design of the closed-loop controller. Finally, the simulations and experimental results validate the effectiveness and feasibility of the proposed control strategy.