ABSTRACT
This chapter discusses the techniques used for the Geomagnetically Induced Currents (GIC) observation in different countries, describes the historical background and achieved results, gives examples of extreme registered GICs, and provides an overview of the physical causes. The GIC measurement equipment is normally installed in the power transformers grounding cables, since no current flows over the grounding cable in the normal balanced state. The differential magnetometer method (DMM) targets the GIC estimation in a power line. The GIC is evaluated from the difference between two magnetic field measurements: directly underneath the power line, and at a distance from the power line, where the magnetic field of the GIC induced in the transmission line has a negligible effect. The GIC amplitudes measured at the beginning of the recordings and the nowadays data exhibit clear differences in values. The problem of GMD impact on power grid operation goes beyond the territory of a single country.
