“Geomagnetically induced currents” (GICs) in ground-based networks are a manifestation of space weather and a potential source of problems to the systems. Exact matrix equations, which are summarized in this paper, for calculating GICs in a power system were formulated in the 1980s. These equations are reformulated here to show how they lead to a direct relation between GICs and the horizontal geoelectric field. Properties of the matrices, which enable the derivation of physical features of the flow of the GICs in a power system, are considered in detail. A specific aim of this study was to show that the ratio of the GICs obtained at a station under two different geophysical situations is independent of the total earthing resistance at that particular station. The behavior of the elements of the transfer matrix between geovoltages and GICs confirms the earlier result that it is sufficient to make a calculation of a small grid around the site if the GIC values at one site are of interest. Using the developed technique, I have calculated the GICs of the old configuration of the Finnish 400-kV grid.
