Long‐Term Geomagnetically Induced Current Observations From New Zealand: Peak Current Estimates for Extreme Geomagnetic Storms

Abstract

Geomagnetically induced current (GIC) observations made in New Zealand over 14 years show induction effects associated with a rapidly varying horizontal magnetic field (dBH/dt) during geomagnetic storms. This study analyzes the GIC observations in order to estimate the impact of extreme storms as a hazard to the power system in New Zealand. Analysis is undertaken of GIC in transformer number six in Islington, Christchurch (ISL M6), which had the highest observed currents during the 6 November 2001 storm. Using previously published values of 3,000 nT/min as a representation of an extreme storm with 100 year return period, induced currents of ~455 A were estimated for Islington (with the 95% confidence interval range being ~155–605 A). For 200 year return periods using 5,000 nT/min, current estimates reach ~755 A (confidence interval range 155–910 A). GIC measurements from the much shorter data set collected at transformer number 4 in Halfway Bush, Dunedin, (HWB T4), found induced currents to be consistently a factor of 3 higher than at Islington, suggesting equivalent extreme storm effects of ~460–1,815 A (100 year return) and ~460–2,720 A (200 year return). An estimate was undertaken of likely failure levels for single‐phase transformers, such as HWB T4 when it failed during the 6 November 2001 geomagnetic storm, identifying that induced currents of ~100 A can put such transformer types at risk of damage. Detailed modeling of the New Zealand power system is therefore required to put this regional analysis into a global context.

Transformer‐Level Modeling of Geomagnetically Induced Currents in New Zealand's South Island

“…Comparisons of the modeled GICs in this study with observations of GICs made by Transpower (Rodger et al, ) show a reasonable agreement with the reported typical relationship between GIC magnitude at HWBT4L and ISLT6H. Rodger et al () compared the observed GICs at ISLT6 and HWBT4 during five large space weather events occurring between 17 March 2013 and 22 June 2015. In these events the peak GIC flowing through HWBT4 was ~3 times that flowing through ISLT6H.…”

“…We have focused on key locations that Manus Mac et al () and Rodger et al () report as having high levels of observed GICs and have experienced documented disruption during geomagnetic storms in the past (Marshall et al, ). Transformer number 6 at ISL (ISLT6) shows relatively high observed GICs during geomagnetic storms.…”

“…Transformer number 6 at ISL (ISLT6) shows relatively high observed GICs during geomagnetic storms. (Note, ISLT6 is called M6 in Manus Mac et al () and Rodger et al () based on an old Transpower naming convention, but we are calling it T6 for consistency with other transformer names.) Archived observations started at ISLT6 in November 2001, and thus, ISLT6 is one of the longest monitored transformers in the South Island network.…”

“…Transpower New Zealand Ltd have measured GICs through multiple individual transformers within substations (Manus Mac et al, ; Rodger et al, ). It is important to note that these GIC observations are of the GIC flowing through individual transformers (transformer‐level GIC) rather than a total measure of GIC to ground for each substation (substation‐level GIC).…”

“…We calculate both substation‐level and transformer‐level GIC using the transformer‐level network representation. We compare the GICs calculated from the original substation‐level network representation with the transformer‐level network and the ratios of observed GICs in transformers at Islington (ISL) and HWB substations reported by Manus Mac et al () and Rodger et al (). We demonstrate the importance of this transformer‐level modeling to GIC calculations through this comparison.…”