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Analysis of two charging modes of battery energy storage system for a stand-alone microgrid

In this paper, two charging modes of battery energy storage system (BESS) for a stand-alone microgrid are analyzed. The stand-alone microgrid system consists of 50kW BESS, 50kW diesel generator (DG) and controllable loads, where BESS is composed of 115kWh battery bank and 50kW DC-AC inverter. The operation modes of the stand-alone microgrid system are divided into four modes, and BESS is connected to DG to charge an insufficient SOC (State of Charge) of battery bank through two charging modes. Charging mode I is that BESS performs constant voltage constant frequency (CVCF) control as main source and DG operates in active power control. On contrary, charging mode II is that DG performs CVCF control as main source and BESS is charged by constant current constant voltage (CC-CV) method. The operation of BESS is similar to grid-connected characteristic in the charging mode II, where BESS is charged from DG for increase of SOC. PR voltage + P current control in the stationary reference frame for charging mode I and PI + R control in the synchronous reference frame is applied for charging mode II. Stability of two charging modes is analyzed by using root locus in the discrete time z-domain. Demonstration site is constructed and performance of the proposed two control methods is verified through experiment, where THD of charging current is 1% in charging mode I and is 2.2% in charging mode II.