Statistics on geomagnetically-induced currents in the Finnish 400 kV power system based on recordings of geomagnetic variations

A method to estimate geomagnetically-induced currents (GICs) in a power grid based directly on magnetic recordings is presented. To determine the electric field on the earth's surface for the calculation of GICs based on available magnetic data, a model is used in which the geomagnetic variation field propagates as a vertical plane wave in the earth and the earth is a half-space with a constant conductivity. To get statistically reliable results, the earth's conductivity is allowed to be a function of the geomagnetic K-index.