The geoelectric fields drive currents in the ground and in man-made conductor networks, such as power grids, communication cables, oil and gas pipelines and railway equipment. A common name for these currents is geomagnetically induced currents (GIC) and they are the ground end of the space weather chain. GIC are mainly (but not only) a high-latitude phenomenon since geomagnetic disturbances and geoelectric fields are the largest and most frequent in these areas.
The power industry is the branch that has had most problems with GIC. A power grid consists of 3-phase transmission lines with “Y”- or “Delta”-connected transformers. The former transformer type always has a grounding whereas the latter does not. During normal loads the currents add up to zero at the neutral point. The earthing offers a path for GIC to enter the power grid. The currents flow along the transmission lines and then back to the ground at other transformer stations. Compared to the 50/60 Hz frequency used for household electricity, GIC can be treated as a direct current (< 1 Hz). A schematic GIC path between two transformers is shown below.
Problems may arise when the flow of GIC in the transformer winding creates a DC magnetic field that can lead to saturation of the core. This results in a non-linear operation of the transformer. The magnetising current increases during every half-cycle resulting in an excessive amount of harmonics. In a saturated transformer, the magnetic flux can spread out through structural members producing eddy currents, which in turn may cause hotspots, possibly with permanent damage.
Protective relays may malfunction resulting in parts of the system being disconnected. Together with increased reactive power demands these effects may cause a collapse of the whole system. The degree of disturbance depends on several factors, such as the geographical location of the power grid, the ground conductivity, the design of the grid and the transformers and the load situation.
GIC may also affect pipelines. Pipeline networks are constructed from steel, and contain liquids or gas with corrosion resistant coatings. Weathering and other damage to the pipeline coating can result in the steel being exposed to moist air or to the ground, causing localised corrosion problems.
A cathodic protection system is used to minimise corrosion by maintaining the steel at a negative potential with respect to the ground. The operating potential is determined from the properties of the soil and Earth in close vicinity of the pipeline. During a geomagnetic storm, GIC may cause fluctuations in the pipe-to-soil potential, increasing the rate of corrosion. However, GIC risk is not a risk of catastrophic failure, but a reduced service life of the pipeline.
Railway equipment have also been affected in the past due to GIC. In the early 30′s, GIC were observed in the Swedish railway. At that time GIC caused the track relays to attract incorrectly, and thereby indicating the track to be free. Due to this dangerous construction, reconstruction began in the 50′s to eliminate this effect.
However, effects have occurred later on the signaling system, but without any severe consequences. One event occurred during the geomagnetic storm, in the night between 13 and 14 July 1982, when the traffic lights turned red without any obvious reason in a railway section of about 45 km in length in the southern part of Sweden. After a while the lights turned green and back to red again later. The reason was that the geoelectric field affected the relays.
In recent years, there have also been some cases where the geoelectric field have caused some disturbances on the Russian railway system.
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