Abstract
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.