Geomagnetically Induced Currents, Transformer Harmonics, and Reactive Power Impacts: Observed Response of a Single‐Phase Transformer in New Zealand

Geomagnetic storms represent a space weather hazard to power transmission networks due to the effects of induced geo-electric fields within the conducting surface of the Earth. These drive electric currents in power transmission lines which can flow to ground through the neutral-ground connections of transformers. Geomagnetically induced currents (GIC) can negatively impact the operation of high voltage transformers through asymmetric half-cycle transformer core saturation. In this study GIC measurements, derived effective current, and reactive power responses (Q) of the single-phase bank autotransformer, T4, operating at 220 kV in the Halfway Bush substation, Dunedin, New Zealand have been analyzed over the period 2013–2017. During 8 elevated GIC events linear enhancements of reactive power consumption (Qcon, MVAr) occurred, even with comparatively low levels of GIC (i.e., <7 A). This is consistent with transformer core saturation where there is little tolerance or “headroom” to GIC in the design for single-phase bank transformers. Reactive power measurements show high variability. The removal of the non-GIC variability is difficult and introduces uncertainty into the identification of GIC-driven responses. In this analysis we consider two techniques to isolate GIC-induced reactive power responses. We find that transformer T4 exhibited a reactive power response of ∼0.115 MVAr/A. The results are a factor of 2–3 larger than MVAr changes in a nearby three-phase, three-limb autotransformer, aligning with previous research. The results are consistent with, but less extreme than modeling studies found for transformer units operating at significantly higher voltages.