An SOC-Based Bidirectional Virtual DC Machine Control for Energy Storage Systems in Offshore Isolated Island DC Microgrids

In order to achieve the state of charge (SOC) balance of distributed energy storage systems (ESSs) in offshore isolated island DC microgrids and enhance the inertia and damping characteristics of DC microgrids, an SOC-based bidirectional virtual DC machine (VDCM) control is presented. The control proposed has the following three improvements. Firstly, the improved VDCM control removes the power loop and torque loop on the basis of ensuring inertia and damping characteristics. Secondly, the inertia time constant and damping constant, which are related to the bus voltage difference and the change rate of bus voltage difference, are introduced to improve bus voltage dynamic performance. The proposed method overcomes the shortcoming in that traditional VDCM control cannot adaptively suppress bus voltage oscillation according to the bus voltage fluctuation amplitude. Thirdly, an improved SOC-based VDCM virtual armature resistance (VAR) is proposed to achieve rapid SOC balance. The improved SOC-based VDCM VAR is an SOC-related function added to the initial VAR. This enables the proposed VAR to have a certain adaptive ability, which can adjust the energy absorbed or released by energy storage units according to the SOC state. Compared with other controls, the control proposed takes less time to achieve SOC balance. Finally, the effectiveness and superiority of the scheme are proven by simulation cases.