Long‐term geomagnetically induced current observations in New Zealand: Earth return corrections and geomagnetic field driver

Abstract

Transpower New Zealand Limited has measured DC currents in transformer neutrals in the New Zealand electrical network at multiple South Island locations. Near‐continuous archived DC current data exist since 2001, starting with 12 different substations and expanding from 2009 to include 17 substations. From 2001 to 2015 up to 58 individual transformers were simultaneously monitored. Primarily, the measurements were intended to monitor the impact of the high‐voltage DC system linking the North and South Islands when it is operating in “Earth return” mode. However, after correcting for Earth return operation, as described here, the New Zealand measurements provide an unusually long and spatially detailed set of geomagnetically induced current (GIC) measurements. We examine the peak GIC magnitudes observed from these observations during two large geomagnetic storms on 6 November 2001 and 2 October 2013. Currents of ~30–50 A are observed, depending on the measurement location. There are large spatial variations in the GIC observations over comparatively small distances, which likely depend upon network layout and ground conductivity. We then go on to examine the GIC in transformers throughout the South Island during more than 151 h of geomagnetic storm conditions. We compare the GIC to the various magnitude and rate of change components of the magnetic field. Our results show that there is a strong correlation between the magnitude of the GIC and the rate of change of the horizontal magnetic field (H′). This correlation is particularly clear for transformers that show large GIC current during magnetic storms.

Space Weather Quarterly Volume 14, Issue 4, 2017

“…For the New Zealand region, Mac Manus et al (2017) found that the time variation of South Island GIC was typically well correlated with H 0 measured by the Eyrewell magnetic observatory. In order to study the occurrence and properties of GIC in New Zealand, we investigate periods in which large |H 0 | values were observed at Eyrewell.…”

“…A detailed description of the New Zealand GIC measurements in the South Island has been previously reported (Mac Manus et al, 2017). The following section is a brief summary, and the reader is directed to the earlier study for a more complete explanation.…”

“…The following section is a brief summary, and the reader is directed to the earlier study for a more complete explanation. The process by which this data set is corrected to remove stray earth return currents and any calibration offsets was given in detail by Mac Manus et al (2017). Once these return currents, which can be >10 A in some locations, were removed, a substantial GIC data set was produced.…”

“…In addition, we note that as shown in Figure 1 the Islington substation is close to the Eyrewell magnetic observatory, described below. All the GIC observations reported here are after the corrections described in Mac Manus et al (2017) have been undertaken.…”

“…As GICs are closely linked to the rate of change of the horizontal magnetic field component we focus on the time periods with significant H 0 and examine how these events contrast with global and local disturbance indices. We focus on measurements made at transformer number 6 in Islington, Christchurch (ISL M6), which had the highest observed currents during the 6 November 2001 storm (Mac Manus et al, 2017;Marshall et al, 2012). This allows us to estimate likely GIC magnitudes expected for this location during extreme storms.…”