Transformer External Fault Detection Under Geomagnetically Induced Currents

During geomagnetically induced currents (GICs), transformer differential relays are likely to remain blocked for internal faults. This issue is pinpointed by extensive studies in literature, and several solutions have been proposed to unblock the relays for the internal faults under the GIC flow in transformers. Nonetheless, such unblocking schemes require an external fault detection (EFD) technique to ensure security for some external faults during the GIC and current transformer (CT) saturation. The GIC leads to pre-fault CT saturation, which can make the transformer external fault detection challenging. This paper proposes an EFD method for transformer differential protection, especially under GIC conditions with pre-fault CT saturation. The developed EFD approach's effectiveness is verified in various fault scenarios, with and without the GIC. Moreover, case studies show higher fault detection capability of the proposed method compared with an existing differential protection's EFD logic during the GIC-caused CT saturation.