Geomagnetically induced currents in the Finnish 400 kV power transmission system

Abstract

Faraday's law of induction implies that an electric field is connected with a time variation of the geomagnetic field. The electric field drives electric currents in conductors according to Ohm's law. The induced currents and voltages are usually sources of inconvenience to technical systems such as power transmission grids and pipelines. In an a.c. power system, a geomagnetically induced current (GIC) is a quasi-d.c. current and may cause saturation of transformers leading to disturbances in the operation of the system or even to permanent damage of transformers. Although Finland lies partly in the auroral zone, where the question of geomagnetic induction in technical systems is of particular importance, no significant effects of GICs have been found in the Finnish power system (yet). Research on GICs in the Finnish 400 kV grid containing both measurements and theoretical estimations has, however, continued since 1977. The quasi-d.c. current flowing from a transformer neutral into the Earth is recorded. The highest current observed so far is 165 A (10-s mean value), but values > 100 A are relatively scarce. Theoretical estimates indicate that GICs as high as several hundreds of amperes, i.e., in the order of the 50 Hz rated phase currents of the transformers, might be expected to occur in the Finnish 400 kV grid at times. This paper deals with the measurements and theoretical calculation of GICs. The Fourier spectrum of the recorded GIC is also briefly discussed.