Differential frequency protection scheme based on off-nominal frequency injections for inverter-based islanded microgrids

Protection is considered one of the major challenges associated with the operation of islanded microgrids with high penetration of inverter-based distributed generation (IBDG). As stated in the IEEE Std. 1547.4, due to the limited injected fault current levels by IBDG, conventional overcurrent relays might not be capable of detecting and clearing faults. Thus, this paper proposes a novel protection strategy for inverter-based islanded microgrids which relies on injecting an off-nominal frequency, through the DG interface, during fault conditions. A differential protection scheme that is based on the differences in the current frequency components, measured at both ends of a feeder section, is proposed to detect and isolate faults. This paper also presents the structure and control logic of the proposed relay for differential protection. The proposed protection scheme is implemented and tested on a distribution system equipped with IBDG, modeled to operate in an islanded mode. Simulation results are presented to highlight the effectiveness of the proposed protection scheme in detecting and isolating faults for inverter-based islanded microgrids.