Abstract
In early May 2024 ~6 Coronal Mass Ejections were launched toward the Earth in short succession. This triggered the G5 “Gannon” geomagnetic storm lasting from ~17 UT on 10 May 2024 to ~9 UT on 12 May 2024. Auroral displays were seen around the world including at lower latitudes than previous geomagnetic disturbances this century. Magnetic field variations measured at multiple sites in New Zealand exceeded values observed over the past 30 years. In 2022, following many years of industry-research collaboration, an updated and enhanced “All of New Zealand” Geomagnetically Induced Current (GIC) mitigation strategy was developed. This strategy involves targeted line disconnections to reconfigure the network. The goal of this approach is to reduce GIC magnitudes and durations at the most at risk transformers, while still maintaining the continuous supply of power. Following training of control room staff, this strategy was declared operational in mid-2023. Once disturbance levels reached the G5 threshold during the Gannon storm Transpower control room staff followed the planned procedure and implemented the mitigation strategy for the first time. There was no impact to New Zealand's electrical supply from this storm. GIC was measured at more than 70 transformers throughout the New Zealand power grid. Peak GICs observed after the mitigation strategy was in place reached ~113 A at a transformer in the city of Dunedin. Without mitigation, modeling shows that a peak GIC of ~200 A through the neutral would have occurred, exacerbated by the tripping of a transformer at a neighboring substation.