Geomagnetically induced currents in the Finnish power transmission system Geomagnetically induced currents (GICs) are a source of inconvenience when flowing in manÐmade conductors like power transmission systems. In order to confirm and generalize results obtained in previous works, a oneÐyear GIC project was carried out in coÐoperation between the Finnish Meteorological Institute and Imatran Voima Oy power company. During the project GICs were measured at four transformer sites and in one transmission line.
In addition to the 400 kV power transmission system, the 220 kV system is taken into account, now for the first time. The model for the earth's conductivity and ionospheric currents is a complex composite model, which permits an inversion from GICs to the geoelectric field. The model is flexible and changes as a function of the geomagnetic activity. Using the model the largeÐscale structure of the geoelectric field is monitored based on the GIC measurements during a geomagnetically active event.
A new method for calculating the impulse responses between GIC at a site and arbitrary number of geomagnetic data recordings from different observatories is introduced. The method allows GIC at a certain site to be monitored using the geomagnetic measurements without any further knowledge of the power system or the conductivity structure of the earth.
New statistics on geomagnetic variations and GIC occurrence probabilities are presented. The main result of the work are the GIC probabilities for every Finnish highÐvoltage station. The results are presented as diagrams containing estimations for three different geophysical situations: active years, quiet years and averaged over sunspot cycles. The validity of the results obtained are thoroughly discussed.
Since the work joins the practical and theoretical knowledge of the subject, the methods and ideas presented in this particular work are easily adapted in any other situation. The validity of the results does not depend on the complexity of the earth's electromagnetic structure or power system configuration.
Suggestions for future work, measuring projects and method enhancements are given.
Finnish Meteorogical Institute, Geophysical publications No. 32, 101 pp.