A Harmonic Time-Current-Voltage Directional Relay for Optimal Protection Coordination of Inverter-Based Islanded Microgrids

Inverter-interfaced distributed generators (IIDGs) are characterized by their low fault current contributions that impose a major challenge for microgrid protection. This article proposes a new overcurrent protection strategy that utilizes a third harmonic voltage generated by the IIDG controller during short-circuit faults. By generating harmonic voltages, a new current flow layer in the harmonic domain is created during faults and is independent of the fundamental fault current, which is limited by IIDGs. A harmonic time-current-voltage directional relay is developed based on the measured harmonic currents and voltages at the relay location to ensure optimal protection coordination (OPC) for isolated microgrids. The OPC with the proposed relay characteristics is formulated as a nonlinear program and equipped with a harmonic short-circuit current calculation method to determine the optimal relay settings. The 9-bus Canadian test system and the IEEE 33-bus system are adopted to validate the operation of the proposed OPC. Simulation results demonstrate that the proposed harmonic time-current-voltage directional relay is capable of detecting the direction of the fault current and maintaining precise protection coordination without the need for communication.