Autonomous Control for Cooperative Operation Between Energy Storage Systems

This paper proposes an autonomous control scheme for power-sharing between energy storage systems (ESSs). The targeted ESS consists of multiple distributed ESSs, all connected to the same DC grid. Importantly, the proposed control method does not require communication between the distributed ESSs and solves the existing power-sharing problems during a communication failure. The autonomous control scheme is based on conventional voltage droop control but features manipulating the reference voltage with a fixed virtual resistance. Each ESS manipulates the reference voltage command value and cooperatively shares the power between any number of ESSs. Operational problems occur when combining autonomous control with an existing state-of-charge (SOC) based mode-switching control, which toggles charge and discharge modes according to the SOC threshold of the battery. There is a possibility that all ESSs are in the same mode, then they cannot share the power and face the risk of system failure. Furthermore, the SOC of each ESS is unbalanced with no power-sharing as it only depends on its power generation and consumption. The resulting difference in charge and discharge cycle causes the difference in the life cycle of the batteries between the ESSs. To solve this problems, this paper additionally proposes a control method for a power-sharing operation that averages the SOC of each ESS without mode switching. Accordingly, the basic control law and control method of autonomous control are derived, and the amount of power-sharing is quantitatively considered, and the proposed control method is experimentally validated.