Geomagnetically Induced Currents, Transformer Harmonics, and Reactive Power Impacts of the Gannon Storm in May 2024

Geomagnetically induced current (GIC) measurements made at two 3 phase, 3 limb transformers, operating in the Halfway Bush substation in Dunedin, New Zealand have been analyzed during the May 2024 Gannon Storm. GIC measurements were combined with very low frequency radio wave AC harmonic measurements made nearby, and reactive power measurements made at key points in the substation. This study focuses on the 11 May, 00–14 UT period when geomagnetic activity was high and the 220 kV transformers, T6 and T3, experienced multiple short periods where GIC > 50 A in each transformer, maximizing at 113 A. During high GIC periods linear enhancements of even order AC harmonic intensity were identified, particularly for the 2nd and 4th harmonics, consistent with asymmetric half-cycle transformer core saturation. Reactive power consumption (Qcon, MVAr) increased linearly when GIC levels were >30 A, consistent with the enhancement of even order AC harmonics due to transformer core saturation >30 A DC. Transformer T6 exhibited a reactive power response of 0.038 MVAr/A, while for T3 it was 0.026 MVAr/A. Simple extrapolation of these findings to extreme storm modeling of the New Zealand high voltage grid suggests that an additional ∼200–350 MVAr of generation would be required to compensate for peak increased reactive power consumption at 19 of the most affected sites during a Carrington-level event. Such additional power requirements are likely to be within the capabilities of the power generation network.