When a coronal mass ejection (CME), a huge plasma cloud originating from the Sun, hits the Earth, it causes a space weather storm. Electrons in the Earth's magnetosphere cascade into the polar regions, creating a current that flows along the auroral oval. When these currents change in time an electric field will be induced in the ground. The electric field will act as a celestial battery driving currents in the ground and in man-made conductors such as communication cables, pipelines electric power transmission grids and railway equipment. These currents, which thus constitute ground effects of space weather, are known as geomagnetically induced currents (GICs).
Normally part of a power system consist of Y-configurated transformers connected to 3-phase transmission lines in a network. When the AC supply is balanced and there are no geomagnetic disturbances the three phase currents add to zero in the neutral point of the transformer and no current is flowing to ground. GIC is, however, not balanced in the same way, and so it flows between the transmission lines and the ground through the transformer windings. The currents do not cancel in the neutral point and this can cause severe problems to the transformers and the network.
GICs have frequencies < 1 Hz, which are very small compared to the 50 or 60 Hz AC in the power system. These currents can then be treated as near-DC. When geomagnetically induced currents flow through the transformer winding they will produce extra magnetisation during the half-cycles when the AC is in the same direction. The magnetic flux spreads out producing eddy currents that can cause hot spots which can damage the transformer. During the half-cycle saturation the magnetising current becomes distorted and increases significantly resulting in increased harmonics in the AC waveform. The enhanced content of harmonics can cause incorrect operation of protective relays, misoperation of equipment and may thus lead to disconnection of power lines. The increased reactive power demand together with misoperation of protective relays may cause a collapse of the power system.
The history of GICs dates back about 150 years. The first observations were made in early telegraph devices and later on in power systems. GICs have been recorded by the power industry during many years. The first documented case, occurred during a severe geomagnetic storm on Easter Sunday, March 24, 1940. On the US East coast many disturbances were noted. The most damaging GIC-event so far took place in March 1989. The entire province of Quebec experienced a blackout lasting about nine hours. The Hydro-Qúebec power company lost more than 21 500 MW. A large transformer at a nuclear plant on the US east coast was damaged. On October 30, 2003, there was a power failure in Malmö in southern Sweden which caused an outage of 50000 customers lasting about one hour.