On Currents Induced in Power Transmission Systems During Geomagnetic Variations

Theoretical estimation of the electric field occurring at the earth´s surface during a time variation in the geomagnetic field is discussed. Numerical computations, which have been made assuming an idealized behaviour for the magnetic field or using measured magnetic values, indicate the existence of horizontal electric fields in the order of 1 V/km in connection with geomagnetic disturbances. Calculation of quasi-direct currents produced in an arbitrary conductor network (e. g. a power transmission system) by a geomagnetically-induced electric field is discussed. Numerical results concerning the Finnish 400 kV grid are given. It is seen that currents in the order of 100 A can be expected to flow through transformers during geomagnetic disturbances. The largest current measured in the Finnish recordings so far has been 165 A. These recordings carried out since 1977 are also described in this paper.