Averages of geomagnetically induced currents (GIC) in the Finnish 400Â kV electric power transmission system and the effect of neutral point reactors on GIC

Geomagnetically induced currents (GIC) in technological systems are ground effects of space weather. In electric power transmission networks, GIC may produce problems by saturating transformers. Thus, GIC flowing into and from the Earth through transformers are more important in practice than GIC in transmission lines. In this paper, we show by considering a simple idealised model and by numerical computations about the Finnish 400 kV system that, on the average, earthing GIC, i.e. transformer-neutral-to-ground GIC, are smaller than line GIC, which should thus be regarded as a good observation from the practical point of view. It should, however, be noted that GIC greatly vary from site to site in a system and from system to system. Installing neutral point reactors in earthing leads of transformers, as has been done in Finland, tend to decrease GIC due to their large resistance. Computations of GIC presented in this paper quantitatively demonstrate the effects of reactors on GIC in the Finnish power system. It is seen that earthing GIC magnitudes decrease at the stations with reactors but the average decrease is not very large. Stations without a reactor usually experience an increase of GIC when reactors are installed at other stations.