A Study on Mechanisms and Modeling of Insulation Resistance Variation According to Battery Stray Capacitance and SOC in Large Scale ESS

Recently, due to the significant growth of RES(renewable energy source), the capacity of ESS (energy storage system) interconnected to RES is increasing rapidly. Especially, it is mandatory to install IMD (insulation monitoring device) in order to monitor insulation resistance of ESS and detect faults such as ground fault. However, there are no specific regulations on numerous IMD being operated in actual ESS, and particularly the issues that insulation resistance dramatically varies depending on the stray capacitance and SOC (state of charge) of batteries are being occurred in large scaled ESS. Therefore, in order to operate ESS in a stable manner, this paper performs surveys on actual measurement data of ESS insulation resistance, which varies according to the operation environment and conditions in distribution system. Based on the evaluation on influence of stray capacitance and SOC to insulation resistance from the survey results, this paper proposes a mechanism of variation in insulation resistance depending on the stray capacitance and SOC of battery and performs modeling of DC distribution system with ESS to evaluate the variation characteristics of insulation resistance according to each mechanism. From the simulation result based on the proposed mechanism and modeling, it is confirmed that the leakage current and CMV (common mode voltage) increases along with stray capacitance and SOC but insulation resistance decreases. Therefore, in order to operate large scaled ESS in a stable manner, it is confirmed that proper measures to enhance insulation level depending on the capacity and SOC of battery and mitigate leakage current and CMV.