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In this paper, a coordinated state-of-charge (SOC) control strategy for the energy storage system operating under microgrid islanded mode to stabilize the frequency and voltage was proposed. The proposed SOC control loop makes up of PI controller, which uses a SOC state of the energy storage system (ESS) as an input and an auxiliary reference value of secondary control as an output. The SOC controller changes the auxiliary reference value of secondary control to charge or discharge the ESS. To verify the proposed control strategy, PSCAD/EMTDC simulation study was performed. The simulation results show that the SOC of the ESS can be regulated at desired operating range without degrading of stabilizing control performance by proposed coordinated SOC control method.