A Decentralized SoC Balancing Technique for Precharging Series Energy Storage Systems

Energy storage systems (ESSs) are widely installed in the present power system. When multiple ESSs are connected in series, proper power sharing is needed to avoid the unbalance SoC issue: some ESSs are not fully charged while others have been overcharged. Due to geographical restriction, there is usually no communication link between different ESSs. In this case, each ESS only knows its local information, such as the state of charge (SoC), output current, and voltage. It is therefore more challenging to achieve the ESS SoC balancing during the pre-charging state. To address this issue, a decentralized SoC balancing control based on complex virtual impedance has been proposed in this paper. The proposed method controls the ESS as a virtual complex impedance, whose impedance angle is correlated with the real-time SoC value. Through this effort, the active power of ESS can be dynamically regulated during the entire pre-charging process to autonomously realize SoC balancing. Finally, hardware-in-loop (HIL) simulation results of a series ESS system are provided for verification.