Multiple time scale optimal operation of MMC battery energy storage system

The battery energy storage systems (BESS) using modular multilevel converter (MMC) as the grid interfacing converter could integrate multiple independent battery stacks with flexible control capability by assuming the unique circulating current control scheme. Traditionally, the separate battery stacks in MMC are controlled to have the same state of charge (SOC) for maximizing the operational capacity, which however is not the optimal operation strategy when considering the health of battery stacks due to the inherent inconsistence among batteries. This paper therefore proposes a multiple time scale operational principle for optimizing the capacity and lifetime of MMC battery energy storage system, which includes three-layer operational schemes with their own control objectives. In specific, the proposed operational strategy would distribute the charging/discharging power per battery stack according to the estimated healthy statuses during a long term period and regulate the SOCs of all battery stacks under a medium time scale, meanwhile the MMC will control its circulating currents and output currents in terms of the desired operational efficiency and output quality under the real time scale. Matlab simulations and experimental results both verified the performance of the proposed operational strategy.