Transformer Differential Protection During Geomagnetic Disturbances: A Hybrid Protection Scheme and Real-Time Validation Tests

The transformer differential relays utilize harmonic blocking modules that are prone to block the relay operation for internal faults under the geomagnetically induced current (GIC) flow during geomagnetic disturbances (GMDs). In this paper, by running hardware-in-the-loop (HIL) tests on an advanced commercial differential relay, it is proven that the relay fails to operate for the in-zone terminal and turn faults due to the undesired harmonic blocking under the GIC condition, with and without current transformer (CT) saturation. A new protection scheme is proposed to ensure the relay unblocking during the GIC. The proposed scheme utilizes a hybrid GIC detection method that identifies the GIC conditions based on the raw/unfiltered waveforms and second harmonic phasors of differential currents. The performance of the waveform-based part of the hybrid method is improved by an average-based algorithm detecting the GIC by monitoring the average, maximum, and minimum values of differential current waveforms. The proposed scheme's dependability and security are extensively investigated in GIC- and non-GIC-based scenarios, including internal and external faults and inrush currents. Furthermore, the real-time tests prove the effectiveness of the proposed scheme in unblocking the relay for the in-zone faults while the transformer is subjected to the GIC.