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

Outages may impact power grids, e.g., lines or generation forced out of service. Thus, solving the optimal protection coordination (OPC) problem for the main network topology results in protection miscoordination under contingencies. Further, the limited fault currents of inverter-interfaced distributed generators (IIDGs) require a highly sensitive and reliable protection scheme. This paper proposes an overcurrent protection scheme for islanded microgrids powered by droop-based IIDGs. The inverter controller is modified to include a virtual impedance-fault current limiter (VI-FCL) to protect inverter switches from overcurrent and limit IIDG fault currents. The VI-FCL is designed such that it adapts to fault severity. OPC of directional overcurrent relays (DOCRs) is achieved using a two-stage optimization algorithm. The first Stage calculates the short-circuit currents involving various fault resistances. Next, constraints on the DOCRs operation times are formulated for each topology resulting from an N-1 contingency and the main topology. The OPC problem is formulated as a constrained nonlinear programming problem. Finally, the second Stage is dedicated to obtaining the DOCRs optimal settings. The OPC method is tested on a radial microgrid that is part of a Canadian urban distribution system.