Real-Time Performance Verification of a Commercial Transformer Differential Relay under GIC Conditions

Transformer differential protection employs a harmonic blocking (HB) function that blocks the relay operation when harmonic levels, specifically second harmonics, in transformer differential currents are high. Such a condition can occur when a transformer experiences part-cycle core saturation caused by geomagnetically induced current (GIC). Offline studies on transformer differential relays in the literature show that these relays are likely to remain blocked by the HB function for in-zone faults during the GIC. In this paper, hardware-in-the-loop (HIL) tests are conducted to analyze the response of a commercial differential relay to the faults during the GIC. The experimental results show that the relay fails to operate for in-zone faults during the GIC, regardless of the configuration of the employed HB function. This study concludes that the performance of the existing commercial relays should be experimentally verified and proposes a solution to immunize the transformer protection schemes against GIC consequences.