High-voltage direct-mounted energy storage systems are an ideal solution for constructing large-capacity battery energy storage power stations. The state of charge (SOC) imbalance between bridge arms and submodules can reduce the system’s capacity utilization and

in severe cases

lead to risks of battery overcharging or overdischarging. This paper proposes a multi-operational-condition adaptive rapid SOC balancing control strategy for delta-connected high-voltage direct-mounted battery energy storage system (BESS). First

the necessity of inter-phase and intra-phase SOC balancing strategies and balancing speed for improving capacity utilization in delta-connected high-voltage direct-mount BESS is analyzed. Based on multiple constraint boundaries—including battery C-rate

device current stress

and modulation ratio—we theoretically derive the analytical relationship between the maximum allowable balancing coefficients and operational conditions. This approach fully exploits the topology’s balancing capability while achieving adaptive changes under multiple operating conditions. Second

addressing the coupled limitations of inter-phase and intra-phase balancing capabilities

we analyze the coupling relationships among coefficient boundaries and propose adaptive coefficients to achieve decoupled control. Finally

a 10 kV/8.66 MW simulation model is developed in MATLAB/Simulink for validation. Results demonstrate that the proposed strategy achieves rapid SOC balancing under varying conditions without exceeding current or modulation ratio limits

effectively accelerating both inter-phase and intra-phase SOC balancing rates.