SoC-based dynamic power sharing method with AC-bus voltage restoration for microgrid applications

In a microgrid system, distributed energy storage units are commonly employed as the energy buffers. In this paper, a dynamic power sharing method based on the state-of-charge (SoC) of each energy storage unit is proposed. Droop control is employed as the basic control strategy for the distributed energy storage units. By using the proposed method, the coefficients in the conventional droop method are adjusted according to the SoC of each energy storage module. The modules with higher SoC delivers more active power, while those with lower SoC delivers less. Meanwhile, the reactive power is equally shared in the energy storage system. The relationship between the droop coefficient and SoC are studied deeply and the small signal model is developed to verify the stability of the control system. It is found that the active power sharing speed becomes faster with higher exponent of SoC. At the same time, in order to restore the AC-bus voltage, secondary control is employed to eliminate the deviations of the voltage frequency and amplitude caused by the droop control, with the droop coefficients adjusting according to the SoCs. The model of the secondary control scheme for SoC-based droop method is developed and its stability is discussed. The theoretical analysis is demonstrated by both simulation and experimental results.