Self-Adaptive and Optimal SOC Balancing Control for High Voltage Transformerless Battery Energy Storage System

State of charge (SOC) balancing is significant for high voltage transformerless (HVT) battery energy storage system (BESS) to utilize their full energy capacity. However, traditional methods face challenges in selecting proper balancing coefficient, resulting in either insufficient equalization speed due to conservative parameter, or battery overcurrent and SMs over-modulation with too large value. To overcome this challenge, the mechanism of overcurrent and overmodulation is revealed by theoretical analysis and four maximum coefficients corresponding four critical boundary conditions are deduced. Therefore, an optimal SOC balancing control with self-adaptive equalization coefficient is proposed in this article and is adaptive. 35kV/25MW/50MWh experimental results validate the effectiveness of the proposed control and its performance is compared with respect to conservative methods.