State-of-charge balance control and safe region analysis for distributed energy storage systems with constant power loads

This paper presents a fully distributed state-of-charge balance control (DSBC) strategy for distributed energy storage system (DESS). In this framework, each energy storage unit (ESU) processes the state-of-charge (SoC) information from its neighbors locally and adjusts the virtual impedance of the droop controller in real-time to change the current sharing. It is shown that the SoC balance of all ESUs can be achieved. Due to the virtual impedance, the voltage deviation of the bus occurs inevitably and increases with load power. Meanwhile, the widespread of the constant power load (CPL) in the power system may cause instability. To ensure the reliable operation of DESS under the proposed DSBC, the concept of the safe region is put forward. Within the safe region, DESS is stable and the voltage deviation is acceptable. The boundary conditions of the safe region are derived from the equivalent model of DESS, in which the stability is analyzed in terms of modified Brayton-Moser's criterion. Both simulations and hardware experiments verify the accuracy of the safe region and the effectiveness of the proposed DSBC strategy.