The power conversion system (PCS) for energy storage uses a virtual synchronous generator (VSG) control system that simulates the rotation and excitation characteristics of synchronous motors, so as to achieve the applications of energy storage in the field of power system frequency regulation. Under frequency modulation conditions, electrochemical batteries frequently charge and discharge, and they often require the PCS to operate under reduced load. In this case, the power difference can cause changes in the power angle and voltage of the VSG, resulting in circulating current within the equipment. A variable-parameter virtual impedance scheme coupled with the power change rate is proposed to match the equivalent impedance with the variation in PCS power. In response to the problem of poor output voltage anti-interference capability caused by changes in the energy storage power, a second-order linear auto-disturbance rejection control method including an error robust integral tracking module is put forward, so as to achieve the stable output voltage from the energy storage. Simulation results based on PSCAD show that the matching of variable-parameter virtual impedance with the power variation of PCS can effectively suppress circulating current within the equipment, and the second-order linear auto-disturbance rejection control can effectively improve the voltage control link’s capability of suppressing voltage fluctuations, thereby enhancing the robustness of the energy storage VSG control system.