Bidirectional Three-Port Converter for Modular Multilevel Converter-Based Retired Battery Energy Storage Systems

Using retired power batteries in battery energy storage systems (BESS) is beneficial for environmental protection and cost reduction. Modular multilevel converter (MMC) is the most promising structure in power conversion systems (PCS) integrated with retired power batteries. However, in MMC-based BESS, inter-submodule (SM) active power disparity is inevitable. It will lead to an increase in fundamental circulation and capacitor voltage ripple, limiting the usable capacity of the battery. This paper proposes a bidirectional three-port converter (BTPC) configuration suitable for MMC-based BESS. It ensures that retired batteries can set up charging/ discharging strategies according to their states-of-health (SOH) and state-of-charge (SOC), while maintaining power balance at each port of the MMC. This configuration facilitates the integration of a large number of retired batteries into the grid via the MMC converter. In light of the BTPC configuration, the battery charge/discharge control strategy and power balance strategy are proposed. Simulation and experimental results are obtained on a single-phase system, verifying the effectiveness of the proposed converter and the corresponding control strategy.