Autonomous Power Management of Distributed Energy Storage Systems in Islanded Microgrids

In this paper, an autonomous power management strategy is proposed for distributed energy storage units deployed in islanded microgrids with photovoltaic (PV) and droop controlled units. The proposed strategy offers controlled and selective prioritization of the charging/discharging actions while coordinating with PV and droop units to maintain power balance in the microgrid. The selective priority allows battery units with low state-of-charge (SOC) to keep charging, at a reduced or a full rate, while other battery units meet the demand. This is in contrast to the autonomous techniques in the literature where all units can only charge/discharge simultaneously. This is achieved by employing the proposed adaptive piecewise P/f characteristics, with breakpoints that enable seamless transition between power control and frequency regulation. The charging/discharging decisions and the amount of power exchanged with the microgrid, is determined autonomously at each unit using local measurements only. Also, the strategy offers a unidirectional shutdown mechanism where, below a certain SOC limit, it modifies the structure of the P/f characteristics so the unit cannot discharge, yet it can still charge anytime surplus generation becomes available. Simulation results from a detailed switching model, and also an averaged model, for prolonged simulation times, are used to validate the proposed strategy.