Geomagnetically Induced Currents and Harmonic Distortion: High time Resolution Case Studies

High time resolution (1‐5 s) magnetometer, geomagnetically induced current (GIC), and mains harmonic distortion data from the Halfway Bush substation in Dunedin, New Zealand are analyzed. A recently developed technique using VLF radio wave data provides high resolution measurements of mains harmonic distortion levels. Three case studies are investigated, each involving high rates of change of local geomagnetic field, but with different timescales of magnetospheric driver mechanisms, and different substation transformer configurations. Two cases of enhanced GIC during substorm events are analyzed, and one case of a storm sudden commencement. Time delays between magnetic field fluctuations and induced transformer currents are found to be ~100 s for substorm events, but only ~20 s for the storm sudden commencement containing higher frequency variations. Boxcar averaging of the magnetic field fluctuations using running windows of ± 2 minutes leads to spectral power profiles similar to those of GIC profiles, with reduced power at frequencies >0.003 Hz (periods < 5 minutes). Substantially lower mains harmonic distortion levels were observed after the removal of the single phase bank transformer, HWB T4, from the high voltage configuration at Halfway Bush. No systematic time delay was found between GIC variations and mains harmonic distortion levels. The power spectra of magnetic field fluctuations and GIC variations during the sudden storm commencement with no harmonic distortion showed low levels of low frequency power (<0.003 Hz, periods >5 minutes). This low frequency component of the magnetic field power spectrum appears necessary for mains harmonic distortion to occur.