An adaptive virtual capacitive droop for hybrid energy storage system in DC microgrid

Hybrid energy storage system (HESS) is an integral part of DC microgrid as it improves power quality and helps maintain balance between energy supply and demand. The battery and supercapacitor of HESS differ in terms of power density and dynamic response and appropriate control strategies are required to share power among these storage elements. This paper presents an adaptive virtual capacitive droop for supercapacitor in HESS. A switched-controlled capacitor (SCC) is used in series with a supercapacitor to change capacitance droop coefficient which affects the power absorbed and released by the storage element. The SCC control uses state of charge (SoC) balancing mechanism to improve charge profile by reducing the peak and settling time. Moreover, SCC is fully soft switched to reduce switching losses. A detailed design of controller is discussed for HESS. Compared to the conventional supercapacitor control where the SoC balancing is ensured in secondary control, the proposed scheme improves SoC profile of supercapacitor and ensures that it rapidly contributes power during transients of the microgrid. Simulations performed in PSCAD/EMTDC highlight the superior performance of this control over the conventional scheme.