SoC balancing method for energy storage systems in DC microgrids using simplified droop control

Abstract

DC microgrids adopt energy storage units to maintain the dynamic power balance between distributed power systems and the load. For DC microgrids in small-scale applications including residential microgrids, to ensure the coordination of the state of charge (SoC) and load current sharing among each of the energy storage units, an improved SoC-balanced control method without interconnection communication is proposed in this paper. The proposed method applies an adaptive droop control expression with a specific SoC-function to regulate its reference voltage in both the charging and discharging processes of the energy storage units. Thus, the balance of the SoC and the load current is achieved autonomously. This method can reduce the bus voltage deviation and weaken the impact of the output current on the bus voltage variation, especially for low-voltage DC microgrids. Moreover, the sampling of the output current is avoided, and both the cost and complexity of controller design are significantly reduced. In addition, a function curves analysis method is proposed to analyze the speed of the SoC balancing and the DC bus voltage deviation, which gives instruction in the choice of the adjustment factor in the adaptive droop equation. A mathematical description of the operating process and a small signal model of the proposed method are established to evaluate the system feasibility and stability. A laboratory-scale DC microgrid is built to verify the proposed method. Finally, simulation and experimental results are presented.