Study of effects of changes of earthing resistances on geomagnetically induced currents in an electric power transmission system

Geomagnetically induced currents (GIC) are a potential source of problems in ground-based technological conductor systems. Power grids, which are discretely-earthed networks, constitute an example. When calculating GIC in a system the low frequencies involved enable a dc treatment. In principle, the computation of GIC can be carried out exactly based on geoelectric data if the configuration and resistances of the network are known. In practice, however, the parameters often involve uncertainties. This especially concerns earthing resistance data, partly because they are usually only known at ac frequencies and partly because lower-voltage networks that are neglected when considering a high-voltage system may affect them. To investigate the influence of changes of earthing resistances on GIC, we present results from several computations. Although the calculations refer to the Finnish 400 kV power grid the observations also have more general relevance. It is seen that the impact of possible small uncertainties in the earthing resistances is not large regarding practical applications, in which knowledge of the level of GIC, rather than of precise values, is needed. It is also shown that additional resistances in the earthings provide a possible means to decrease GIC risks. In addition, the results indicate that interactions between different stations, i.e., the effects of off-diagonal elements in the earthing impedance matrix, are minor.