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

• Published in: Space Weather Vol. 15; no. 11; p. 1447
• Main Authors: Rodger, Craig J, Mac Manus, Daniel H, Dalzell, Michael, Thomson, Alan W P, Clarke, Ellen, Petersen, Tanja, Clilverd, Mark A, Divett, Tim
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.11.2017
• Subjects: Confidence intervals; Correlation; Electric currents; Extreme weather; Geomagnetic storms; Geomagnetism; Magnetic fields; Magnetic induction; Magnetic storms; Phase transitions; Regional analysis; Regional planning; Storm effects; Storms; Transformers; New Zealand
• ISSN: 1539-4964; 1542-7390
• DOI: 10.1002/2017SW001691

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. Key Points Analysis of a 14 year data set of GIC in a transformer in Islington, New Zealand, shows peaks correlated with local H' Peak GIC values are very poorly correlated with global geomagnetic indices (ap, Kp, and aa*), and weakly correlated with local ak index values Estimated peak GIC at Islington for a 100 year return period geomagnetic storm is ~155-605 A, and ~155-910 A for 200 years