In this paper, a double-quadrant state-of-charge (SoC) based droop control method for distributed energy storage system (DESS) is proposed to reach the proper power distribution in autonomous DC microgrids. Since DESS is commonly used in DC microgrids, it is necessary to achieve the rational power sharing in both charging and discharging process. In order to prolong the lifetime of the energy storage units (ESUs) and avoid the overuse of a certain unit, the SoC of each unit should be balanced and the injected/output power should be gradually equalized. Droop control as a decentralized approach is used as the basis of the power sharing method for distributed energy storage units (DESUs). In the charging process, the droop coefficient is set to be proportional to the nth order of SoC, while in the discharging process, the droop coefficient is set to be inversely proportional to the nth order of SoC. Since the injected/output power is inversely proportional to the droop coefficient, it is obtained that in the charging process, the ESU with higher SoC absorbs less power, while the one with lower SoC absorbs more power. Meanwhile, in the discharging process, the ESU with higher SoC delivers more power, and the one with lower SoC delivers less power. Hence, SoC balancing and injected/output power equalization can be gradually realized. The exponent n of the SoC is employed in the control diagram to regulate the speed of SoC balancing. It is found that with larger exponent n, the balancing speed is higher. MATLAB/Simulink model comprised of three ESUs is implemented, and the simulation results are shown to verify the proposed approach.
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