Grid connection power control and the SOC balancing strategy for lithium battery energy storage systems

In the new generation of lithium battery energy storage systems， battery modules are cascaded into modular power converters to form battery power modules， which then output in series to integrate into a DC grid. In charging mode， independent current control by each module controller can lead to unstable currents and diverging bus-side voltages. To solve this problem， the paper proposes a series-type I-P droop control strategy that incorporates the parallel droop concept to achieve stable power distribution. Additionally， it introduces a proportional balancing strategy based on I-P droop control， inspired by traditional proportional balancing methods. To enhance the balancing speed， the paper analyzes the operating points of the battery power modules and optimizes the design of the voltage division coefficient， proposing an improved balancing strategy that decouples power control from balancing control. Finally， the paper conducts a simulation analysis of a cascaded modular system consisting of six battery modules. The results demonstrate that the proposed series-type droop control can achieve stable power distribution， and with the inclusion of the balancing strategy， both stable power and state of charge （SOC） balancing can be achieved. Under the same balancing metrics， the improved balancing strategy significantly reduces the balancing time compared to the proportional balancing strategy.