Restricted Earth Fault Protection in Low-Impedance Grounded Systems With Inverter-Based Resources

A restricted earth fault (REF) protection element provides sensitive detection of ground faults near the neutral of a transformer or a generator. The sensitivity gains from the REF element are especially more valuable when the equipment is low-impedance grounded when the transformer phase differential element (87T) is ineffective. However, the REF element faces several challenges in low-impedance grounded systems, which are becoming more prevalent in inverter-based resource (IBR) plants. Dependability of the REF element may be compromised if the zero-sequence current at the transformer terminal is capacitive for an internal ground fault. Security of the REF element may be jeopardized for external faults, such as a phase-to-phase-to-ground or an evolving fault with current transformer (CT) saturation. Using field events from low-impedance grounded IBR plants and simulations, this paper discusses the challenges and design improvements to an REF element for low-impedance grounded systems. CT selection criteria and setting guidance for reliable operation of the REF element are provided. Finally, the coverage provided by the REF element is compared with the 87T element to highlight its benefits in low-impedance grounded systems.