Abstract
Conventional directional overcurrent relays (DOCRs) may fail to protect microgrids formed using inverter-based distributed generation (IBDG) units, as IBDGs have limited fault current contribution. Maloperation of DOCRs can be due to: 1) inability to distinguish load and fault currents; 2) incorrect fault direction identification; or 3) miscoordination of primary and backup DOCRs. This article proposes employing harmonic current injection capability of IBDGs to design a novel harmonic DOCR (HDOCR). In the proposed method, during fault conditions, each IBDG injects a harmonic current with an order different from the ones injected by its neighboring IBDGs. The magnitude of this current is modulated by an equivalent impedance that is locally computed at the terminals of each IBDG. New time-harmonic-current characteristic, directional element, and protection coordination optimization model are devised for the proposed HDOCRs. The proposed protection scheme is applicable to radial and ring microgrids with multiple IBDGs and does not require communication. Simulation studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults.