Dynamic Fault Detection and Protection Strategies for Medium-Voltage Networks Supplied by Grid-Forming Inverter Sources

Abstract

In recent years, high penetrations of inverter-based resources are posing significant challenges to the medium-voltage networks, in which protection schemes based on high fault currents and unidirectional power flow may not perform as expected. This paper proposes a dynamic fault-detection and relay-coordination scheme for a medium-voltage network with high penetration of grid-forming inverter sources. A detailed 33 kV system model comprising six battery energy storage system (BESS) feeders and a four-distributed-load model was built in DIgSILENT Power Factory and tested under various grid-connected and islanded system conditions using the complete short-circuit method. Four simultaneous fault checks, including sequence component analysis, symmetrical voltage variation, superimposed current with voltage restraint, and current waveform analysis, are used to detect the fault in a specific part of the medium-voltage network. After-fault detection, dynamic pickup scaling and relay blocking are coordinated through IEC 61850 GOOSE and DNP3 so only the closest unblocked relay or relay pair trips. The dynamic pickup settings are adjusted considering the ratio of fault levels in the conventional system versus the inverter-based resources-fed medium-voltage system. Simulation results show successful overcurrent coordination retention even when inverter fault current limitation is set at 1.3 p.u. or lower with 10% generation margin. The proposed scheme allows traditional relays with existing infrastructure to function correctly in fully inverter-dominated medium-voltage systems without any synchronous backup. The novelty is the integration of fault confirmation, pickup scaling and a blocking scheme with retention of an independently operating local backup. Compared to fixed grid-connected settings, the proposed scheme recovers islanded-mode pickup values while maintaining primary–backup grading margin for the relay.