Interharmonic Differential Relay with a Soft Current Limiter for the Protection of Inverter-Based Islanded Microgrids

Inverter-interfaced distributed generators (IIDGs) introduce a major challenge for the protection of inverter-based islanded microgrids (IBIMs) due to their limited fault current contribution. This paper proposes a new strategy for the protection of IBIMs, founded on two novel aspects including a soft current limiter and an interharmonic differential relay. In addition to limiting the fault current, the proposed current limiter performs an amplitude modulation of the output fault current from the IIDG which occurs only during fault conditions. This modulation produces interharmonic currents with specific signatures that flow towards the fault. The process of injecting interharmonics is spontaneously initiated by a subset of the IIDGs upon the fault occurrence without a need for a fault detection scheme. Thereafter, based on the locally detected interharmonics, an interharmonic differential relay requiring low bandwidth communication is proposed to identify and isolate the fault. The proposed strategy has been validated using PSCAD/EMTDC. The results confirm the ability of the proposed protection strategy to protect IBIMs considering different fault types, locations, and values of the fault resistance without the need for additional fault detection components or high-bandwidth communication which may constraint the applicability and reliability of the protection schemes.