A communication-free active unit protection scheme for inverter dominated islanded microgrids

Large-scale integration of different renewable energy resources introduces significant challenges to the protection of microgrids (particularly those that may operate in islanded mode), including variable and low fault levels, difficulty in operation of main and backup protection with their coordination, and bidirectional power flow during faults. As a solution to such challenges, this paper presents a novel active protection strategy for the inverter dominated islanded microgrids that coordinates protection actions with the inverter control strategy. The proposed scheme dictates specific actions from the inverter interfaced distributed generator (IIDG) controller to inject specific harmonic components into the microgrid during the fault. Relays throughout the network detect and analyse the injected harmonic components to identify the faulted section, and take appropriate isolating actions, without any requirement for relay-to-relay communication. The scheme achieves selectivity and coordination using definite time delay settings. To verify the performance of the scheme, a realistic microgrid model incorporating the proposed protection strategy has been developed in MATLAB Simulink, where a wide range of fault scenarios have been simulated with variations in fault location, type, fault resistance, line impedance, and different combinations of IIDGs (including with and without connection of a synchronous generator). Additionally, case studies using a real-time digital simulator (RTDS) platform have also been conducted to validate the performance of the proposed solution in real-time, with multiple relays implemented as hardware prototypes running on the OPAL-RT platform — thereby demonstrating the system operation in a hardware-in-the-loop (HiL) configuration. It is shown that the scheme is highly effective in detecting and isolating faults, with proper discrimination, stability and provision of backup, under all investigated scenarios.