Directional Overcurrent Relays Coordination Scheme for Protection of Microgrid

The changes in operating states of microgrid cause significant variations in fault currents that may affect the coordination of directional over current relays (DOCRs). In this paper, an adaptive protection scheme is proposed for coordination of DOCRs in microgrids having two connection points with external grid. The proposed scheme is designed optimally to handle the coordination problem in various operating states such as radial and mesh along with grid connected and islanded modes of microgrid. The coordination problem is formulated as non-linear programming constrained problem and solved to find optimal settings of DOCRs by minimizing operating times of both primary and backup relays. The proposed scheme is tested on 7-bus microgrid in presence of both synchronous and converter based distributed energy resources (DERs). Further, obtained results are validated on Hardware-in-loop (HIL) setup using real time digital simulator (RTDS), digital relay and CMS-356 (voltage and current amplifier). From both simulation and hardware results, it is noticed that the proposed scheme effectively handles coordination problem in various operating states of microgrid.