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An Evaluation of the Frequency Independence Assumption of Power System Coefficients Used in Geomagnetically Induced Current Estimates

A common assumption used when estimating geomagnetically induced currents (GICs) in a power system given a time series of nearby direct measurements or indirect estimates of the horizontal geoelectric field components Ex(t) and Ey(t) on Earth's surface is that the system is resistive. That is, the approximation GIC(t)=aoEx(t)+boEy(t) (Model 1) is used, where ao and bo are frequency‐independent power system coefficients. A first test of this assumption is made using GIC measurements in a 187 kV transformer connected to a ∼100 km power line in Memanbetsu, Japan and geoelectric field measurements made at the Memanbetsu Magnetic Observatory ∼9 km away. A second model (Model 2) is obtained using a frequency domain generalization of Model 1: GIC(ω)=a(ω)Ex(ω)+b(ω)Ey(ω). The coefficients a(ω) and b(ω) are shown to be frequency‐dependent, and this model provides significantly better estimates of the measured GIC than Model 1. Based on results using a simulated geoelectric field, it is suggested that the measurement‐derived frequency dependence of the system coefficients may be explained by spatial variations in the spectrum of the geoelectric field over the spatial extent of the power system. It is also shown that further improvements over Model 2 can be made using frequency‐dependent models with the geomagnetic field as an input.