A SOC control method for reducing capacitor voltage fluctuations in MMC-BESS

The Modular Multilevel Converter-Battery Energy Storage System (MMC-BESS) possesses considerable potential for diverse applications. However, when controlling the State-of-Charge (SOC) balancing among various energy storage units, the utilization of circulating currents often leads to increased voltage fluctuations in the capacitors of individual units. To address this issue, a submodule battery SOC balancing control strategy is proposed, which does not require additional control measures on the MMC. Instead, SOC balancing is achieved through the switching control of bidirectional DC/DC converters. Initially, the power transfer relationships within the MMC-BESS are derived, and the principles of battery SOC balancing are studied based on its mathematical model. By solely adjusting the duty cycles of the bidirectional DC/DC converter switches, the achievement of SOC balance among the three phases and within the individual phase during the charging and discharging of the submodule is accomplished without any adverse impact on the capacitance of the submodule. Finally, the performance of the proposed controller is successfully validated through the construction of a main circuit on the Typhoon HIL402 Hardware-in-the-Loop experimental platform.