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

摘要

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 very low frequency (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 similar to 100 s for substorm events, but only similar to 20 s for the storm sudden commencement containing higher-frequency variations. Boxcar averaging of the magnetic field fluctuations using running windows of +/- 2 min leads to spectral power profiles similar to those of GIC profiles, with reduced power at frequencies >0.003 Hz (periods 5 min). This low-frequency component of the magnetic field power spectrum appears necessary for mains harmonic distortion to occur.