SOC Balancing Strategy for Distributed Energy Storage Units in Isolated DC Microgrids Considering Capacity Differences

In an islanded DC microgrid, there is a problem of slow state of charge (SOC) equalization between distributed energy storage units (DESUs) with different capacities. To address this issue, a fast SOC equalization strategy for DESUs, which accounts for capacity differences, is proposed. First, the SOC equalizer constructs a relationship between the droop coefficient and SOC using a power function. By selecting appropriate equalization adjustment coefficients, the droop coefficient can be adaptively controlled, thereby accelerating SOC equalization. Then, the virtual droop equalizer is introduced to mitigate the impact of line impedance on current distribution accuracy by simply adjusting the PI controller, which improves the precision of current distribution. Additionly, the selection range of control parameters for this strategy is determined by using a system stability analysis. Finally, a DC microgrid hardware-in-the-loop experimental platform is developed. The experimental results, compared with those from existing literature under various operating conditions, demonstrate that the proposed control strategy improves the speed of SOC equalization and realizes the rapid recovery of bus voltage.