Transformer Differential Protection Under Geomagnetically Induced Current Conditions

Solar activities result in fluctuations in the Earth’s magnetic field, referred to as geomagnetic disturbances (GMDs). During the GMDs, geomagnetically induced currents (GICs) flow through the power transformers with grounded neutrals. The transformers subjected to the GIC experience part-cycle core saturation creating significant magnetizing current harmonics, including dominant second harmonics. The GIC-caused harmonics can pose issues for protection systems, such as transformer differential relays. The majority of real-world transformer differential relays employ harmonic blocking (HB) modules to prevent malfunctions during transformer energization leading to inrush currents containing high levels of second harmonics. Considering that the GIC-related transformer current second harmonics can be significant, the differential relays are prone to be blocked during the GMDs and remain blocked for the faults occurring in their protection zones, which may damage the transformers and threaten the system’s resiliency.

This dissertation conducts in-depth time-domain analyses on the performance of differential relays, equipped with various HB logics, for internal/in-zone faults under the GIC conditions. It is revealed that regardless of the employed HB logic, such relays are unwantedly blocked for the faults with specific ranges of fault resistances in the presence of GIC, and therefore, fail to trip for the faults. To address this issue, different unblocking schemes are proposed, which can detect the GIC flow in the transformer and override the blocking signals, generated by the relay’s HB module, during the internal faults. The effectiveness of the developed schemes is extensively investigated to ensure (i) dependability for the co-existence of internal faults and GIC and (ii) security during the GMDs in the absence of faults as well as the transformer energization and external faults, especially under the current transformer (CT) saturation.

Moreover, detailed real-time hardware-in-the-loop (HIL) tests verify that a commercial transformer differential relay fails to operate for terminal and transformer turn faults owing to the harmonic blocking during the GIC when the post-fault differential current fundamental component decreases below certain levels. Furthermore, it is shown that the relay equipped with a proposed unblocking scheme is effectively unblocked in real time and operates in the internal fault cases where the commercial relay cannot trip.