  Geomagnetically induced currents (GICs) in power systems are conventionally
modelled as direct currents, based on their commonly described quasi-DC nature.
For more representative power system dynamic response modelling, it is
necessary to model GICs using low-frequency AC. After analysing spectra of
geomagnetic field measurements and GICs in several power systems, preliminary
results indicate that the GIC spectra depend on the magnitude and frequency
spectrum of the geomagnetic disturbance, the earth conductivity and the
electrical characteristics of the network. In this paper, analysis based on
measured GIC power spectra suggests that the dominant frequencies to use in GIC
modelling and simulation are typically below 50 mHz. The associated error
introduced by only using these frequencies is also quantified. The results have
implications for modelling the geoelectric fields inducing GICs, dynamic models
of power system response and the need for GICs and their associated driving
fields to be measured at intervals not exceeding 10 s.
