Abstract
Faults in inverter-based islanded microgrids can be a formidable protection challenge due to the limited fault current contribution of inverters. This paper designs a selective and sensitive protection scheme by injecting a pattern of up to three synthetic harmonics utilizing the flexibility of existing inverter-based distributed generation (IBDG) controllers (called injection pattern [IP]). The measured pattern (MP) by each relay is the result of the combination of IPs from IBDGs at different locations in a microgrid. To ensure that relays measure different MPs for forward and reverse faults, an optimization model is formulated to set and minimize the total number of IPs by IBDGs. A pattern-based directional element is proposed based on the cosine similarity measure between the MP and the predefined characteristic pattern (CP) of each relay. CPs are set offline as the normalized MPs of forward faults. Moreover, a new direction-comparison relaying scheme is devised, which comprises of permissive overreaching transfer trip (POTT) and direction zone-interlocking (DZI) for primary and remote backup protection, respectively. Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults and different microgrid topologies.