Virtual DC machine-based distributed SoC balancing control strategy for parallel battery storage units in DC microgrids

The state-of-charge (SOC) balance among battery storage units (BSUs) and bus voltage stability are key issues for DC microgrids. This paper proposes a novel distributed SoC balancing control strategy based on the virtual DC machine (VDCM), which is expected to be effective. A hierarchical control structure that consists of two control layers is developed for SoC balancing control. In the primary control layer, the voltage and current double closed-loop control with the VDCM is applied, which brings inertia and damping that enhance the stability of the system. An armature electromotive force (EMF) compensation as the output of the secondary control layer is added to the armature equation of the VDCM for achieving precise proportional current-sharing. In the secondary control layer, an improved SOC balancing strategy is proposed, which accelerates the dynamic SoC balancing between BSUs. In contrast with strategies based on the consensus algorithm, this approach significantly reduces communication pressure since BSUs only exchange information with adjacent units, which means no global information is required. Finally, a DC microgrid model and a hardware-in-the-loop (HIL) experimental platform are established to verify the effectiveness of the proposed strategy. Results show that the proposed strategy can achieve rapid dynamic SoC balancing, inertia injection, and damping control simultaneously under various operating conditions. When compared to other advanced SoC balancing strategies, the control strategy proposed in this paper achieves a faster SoC balancing speed.