HESS power distribution control strategy for DC microgrid based on droop method

Hybrid energy storage system can compensate for the fluctuation of bus power which is caused by the change of output power of power generating unit and load in DC microgrid. In the existing hybrid energy storage coordinated control strategy, using traditional voltage droop method to control the battery to suppress non-high frequency components of bus power fluctuation, and the integral droop method is used to control the super capacitor to suppress the high-frequency fluctuation component of the power fluctuation. The effect of this strategy is similar to that of a filter, which automatically divides the ripple power into high-frequency and non-high-frequency ripple components. However, when the voltage droop method is applied, the difference of the battery SOC in the parallel hybrid energy storage unit is not taken into consideration, and the non-high frequency fluctuation component borne by the battery is not redistributed, which is not conducive to avoiding overcharge and over-discharge of the battery. It is considered that the voltage sag coefficient will be affected by SOC, the voltage droop coefficient is divided by the nth power of the SOC to obtain a new voltage droop coefficient, thereby obtaining an improved voltage droop method in which the voltage droop coefficient is related to the SOC, and then it is combined with the integral droop method to get a new coordination control strategy. Simulation and experimental results show that the proposed control strategy can re-distribute the non-high frequency power according to the SOC of the battery while suppressing the bus power fluctuations, so as to achieve SOC equalisation of the batteries in different energy storage units, the bus voltage fluctuation is small during the entire process.