Long term geomagnetically induced current observations from New Zealand: peak current estimates for extreme geomagnetic storms

Geomagnetically induced current (GIC) observations made in New Zealand over 14years show induction effects associated with a rapidly varying horizontal magnetic field (dB(H)/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 3,000nT/min as a representation of an extreme storm with 100year return period, induced currents of 455A were estimated for Islington (with the 95% confidence interval range being 155-605A). For 200year return periods using 5,000nT/min, current estimates reach 755A (confidence interval range 155-910A). 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 460-1,815A (100year return) and 460-2,720A (200year 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 100A 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.