A voltage-shifting-based state-of-charge balancing control for distributed energy storage systems in islanded DC microgrids

Abstract

This paper presents a distributed secondary level control strategy for battery energy units (BEUs) parallel in a DC microgrid. The control structure is divided into two layers. The primary control layer is implemented with a droop control. The voltage-shifting term is generated by the secondary control layer, where an information state factor λ is introduced to meet the bus voltage restoration, state-of-charge (SOC) balancing and accurate current sharing. A sigmoid function with a balancing adjustment factor is adopted to improve both SOC balancing accuracy and speed. When each information state factor in the system converges to the average value, accurate current sharing according to the SOC values of BEUs and capacities is attained, and the average output voltage is consistent with the nominal voltage of microgrid. Compared with existing methods, the proposed method has no requirement on a complex control structure and only two variables need to be transferred between adjacent converters, which reduces the information traffic and the communication burden. Moreover, the proposed nonlinear control strategy effectively avoids unnecessary cyclic charge or discharge between BEUs and inhibits circulating currents. The proposed method also has the ability of plug and play, since SOC balancing among BEUs with different capacities is considered, and, in the communication layer, the dynamic average consensus algorithm is utilized to obtain both average SOC and average λ. The performance of the proposed control method is confirmed in simulation and experiment.