Inter-Arm State-of-Charge Balancing Control Based on Arm Valley Voltage Adjustment in MMDTC-BESS

This paper addresses a critical challenge in Modular Multilevel DC-link based T-type Converter (MMDTC) battery energy storage systems: the inherent power symmetry between upper and lower arms that prevents natural state-of-charge (SOC) balancing. A novel inter-arm SOC balancing control strategy is proposed that precisely modulates arm valley voltage characteristics—both amplitude and duration—to create controlled power differentials between arms. The theoretical analysis establishes a quantitative relationship between arm valley voltage width and inter-arm power difference, demonstrating that SOC balancing time is inversely proportional to arm valley voltage width. A key advantage of the proposed approach is its adaptive regulation capability: during steady-state operation, closed-loop feedback maintains arm valley voltage width within a narrow range through real-time SOC differential assessment, ensuring dynamic SOC equilibrium while minimizing impact on output power quality. The strategy works effectively in both charging and discharging modes, with comprehensive control logic for various operational scenarios. Finally, simulation and experimental results confirmed the effectiveness of the proposed control strategy.