Distributed Secondary Control of Energy Storage Units for SoC balancing in AC Microgrid

This paper introduces a new distributed secondary control (DSC) method for distributed energy storage units (DESUs) in an islanded alternative current (AC) microgrid (MG). Dynamics of distributed storages for the extended time duration are not taken into account in the traditional hierarchical control of MG. Thus, it is challenging to control the DESUs with various levels of stored energy represented by the state of charge (SoC). The storage units can utilise their full power capacity after converging to a common SoC to mitigate the generation and demand variations in the MG. SoC depletion of DESUs with lower initial SoC occurs faster than those with higher initial SoC by using the traditional P-f droop control and then their capacities are no longer accessible. Furthermore, applying the droop control to match the SoC of DESUs causes the deviation of frequency and voltage from their reference values. However, restoration of the MG frequency using the conventional DSCs disrupts the SoC-balancing. The designed DSC can achieve simultaneous frequency/voltage regulation, power sharing and SoC-balancing as well as removing the centralized communication. The proposed method is evaluated in the established Matlab/Simulink model and the results validate the effectiveness of the proposed method.