Scheduled Power Control and Autonomous Energy Control of Grid-Connected Energy Storage System (ESS) With Virtual Synchronous Generator and Primary Frequency Regulation Capabilities

This paper presents a combined control scheme for the grid-connected energy storage system (ESS). There are two control modes: the power control mode for the charging or discharging condition and the energy control mode for the stand-by condition. In the power control, a derivative element is added in the inertial power-frequency droop as a simple power system stabilizer to enhance the system damping. The energy control is developed from the power control by considering the energy storage dynamics. During system disturbances, both control modes are able to provide autonomous grid support. The small-signal model and the large-signal transient model of the ESS with the proposed control scheme are derived. As a result, the output power and/or the stored energy of ESS is in proportional to the frequency deviation and will be restored to the pre-disturbance condition. By selecting appropriate control parameters, the stored energy is controlled within a safe range constrained by the frequency limits without manual interventions. The primary frequency regulation with both power-frequency and energy-frequency proportional relationships is realized by the energy control, which coordinates with the grid secondary frequency regulation. The control performance and the derived energy model are validated by detailed time-domain simulation studies.