Control Strategy Research on Charge Balancing of Hybrid Energy Storage Units with Islanded Operation Mode of DC Microgrid

When DC microgrid operates in islanded mode, the hybrid energy storage units (HESUs) can maintain voltage stability and energy balance if load fluctuates or distributed generation (DG) output fluctuates by charging and discharging. To improve the stability and duration of microgrid operation in islanded mode, a hybrid energy storage system (HESS) of Nickel-Metal-Hydride battery and supercapacitor is adopted in this paper. Based on DC-DC conversion circuit, Nickel-Metal-Hydride battery compensates for the low-frequency part of power, and supercapacitor compensates for the high-frequency part of power. Aiming at the unbalanced state of charge (SOC) caused by uneven power distribution of HESUs, an improved droop control strategy considering line impedance is designed. The power distribution of each unit is adjusted in real time by detecting the SOC and the state of converter output current in each hybrid energy storage unit (HESU), and finally the SOC of each unit is consistent. A hybrid energy storage microgrid system is built based on Matlab/Simulink, and the proposed control strategy is verified. The results show that the control strategy can stabilize the DC bus voltage not only in normal operation but also under one HESU fault, and realize the balanced distribution of current load among the units.