Optimal Protection Coordination of Islanded Microgrids Utilizing an Adaptive Virtual Impedance Fault Current Limiter

Inverter-interfaced distributed generators (IIDGs) have limited fault current contributions compared to those of synchronous-based DGs. These low fault currents impose challenges on microgrid protection. This paper proposes a sensitive protection scheme for islanded microgrids that adopt droop-based IIDGs as the primary power source. Virtual impedance-fault current limiters (VI-FCLs) are employed in the inverter control scheme to limit IIDGs fault currents and protect inverter switches from overcurrent. The VI-FCLs are adapted to provide sensible fault currents. A two-stage optimization method is proposed to achieve optimal protection coordination (OPC) of directional overcurrent relays (DOCRs). Stage I is devoted to short-circuit currents calculation while involving different adaptive VI-FCL characteristics. In Stage II, the OPC problem is formulated as a nonlinear programming problem to obtain the minimum relays' total operating time while satisfying protection coordination constraints and solved to obtain the optimal DOCRs set groups. The performance of the proposed protection scheme is tested on a radial microgrid. The results confirm that the proposed protection scheme successfully maintains protection coordination using a single set group for a range of fault resistances.