Synthetic Harmonic Distance Relaying for Inverter-Based Islanded Microgrids

Faults in inverter-based islanded microgrids can be a formidable protection challenge due to (i) low fault current magnitudes, (ii) compromised fault current phase angles, and (iii) bidirectional flow of fault currents. This paper proposes a protection scheme that disregards the fault signals altogether. Instead, it relies on decoupled synthesized signals introduced only during fault conditions. This scheme is achieved by exploiting the existing inverter-based distributed generation (IBDG) controllers to inject synthetic harmonic voltages and currents. These synthetic signals are measured locally by digital relays in the microgrid to develop a novel synthetic harmonic distance relay (SHDR). Apart from the utilization of high-order harmonic signals that enhance SHDR reactance reach, further reach improvement is achieved via the introduction of line reactance magnifiers (LRMs). Transient studies in PSCAD/EMTDC verify the performance of the proposed scheme under various faults, contingencies, and different microgrid configurations.