General information on ground effects of space weather (GIC)

 

Space weather effects on ground systems
Power systems
Effects on power system transformers
Reduction of the effects on power systems
Pipelines
Other systems affected
List of reported magnetic disturbances that have affected electrical systems at the earth's surface

 

Space weather effects on ground systems

Large electric currents are continuously flowing in the magnetosphere and ionosphere. When hitting the magnetosphere, a disturbance in the solar wind produces a change in the current system, in which the magnetospheric-ionospheric coupling plays an important role. The geomagnetic field brings the disturbance in particular to high latitudes resulting in visible auroras and in an intense ionospheric current system.

The variations of magnetospheric and ionospheric currents are seen as geomagnetic disturbances or storms at the Earth’s surface. In accordance with the basic electromagnetic theory, a geomagnetic variation is accompanied by a geoelectric field. The structure and intensity of the geoelectric field is greatly dependent on the Earth’s conductivity structure, smaller conductivity implies larger electric fields. Although the auroral electrojet system is of particular importance concerning geomagnetic disturbances, similar effects may also be experienced at lower latitudes.


Movie of ionospheric equivalent currents and GIC for April 6-7, 2000 geomagnetic storm. [Animation AVI MPEG 19 MB] High-voltage power transmission systems are affected by geomagnetic disturbances.

 

The geoelectric field implies the existence of voltages between different points at the Earth’s surface. For example, there is a voltage between the grounding points of two transformers, and a current will flow in the power transmission line connecting the transformers. Such a current is known as a geomagnetically induced current (GIC). Besides power systems, GIC flows in other technological conductors, like oil and gas pipelines, telecommunication cables and railway equipment, causing problems for the normal operation of the systems affected. Bulk of the scientific work on ground effects of space weather has been about GIC on power systems and pipelines, although the first GIC observations in the mid-1800’s occurred in telegraph systems.

 

Animation of the 90 degrees rotated time derivatives vectors of the horizontal ground magnetic field (indicating the direction and the magnitude of the geoelectric field) and GIC in the Scottish and Finnish technological systems (circles). [Animation QuickTime 2 MB]

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Power systems


It is not only the geoelectric field that dictates the GIC magnitudes in a power system but also the geometrical and structural details have a significant influence. Usually GIC greatly vary from site to site in a power system. In general, transformers located at corners of a power system suffer from large GIC values. Also, long transmission lines carry larger GIC.
The problems caused to power grids are due to a half-cycle saturation of transformers resulting from GIC. This means that a transformer which normally operates with a very small exciting current starts to draw an even hundred times larger current which results in a large asymmetry, and the transformer operates beyond the design limits.

 

High-voltage power transformer in New Jersey. Figure credits: John Kappenman, Metatech.
 

Effects on power system transformers

A saturated transformer: 1.) consumes large amounts of reactive power, which decreases the capability of the ac transmission of the system, and the voltage tends to get lower, 2.) generates a lot of harmonics in the electricity, which may lead to false relay trippings of the protective devices and also to additional losses in various equipments, 3.) causes increased magnetic flux in a transformer, and it may take paths not designed to carry a magnetic flux. This causes excess heating in the transformer, and localized hot spots may appear. In the worst case the final consequence can be a permanent damage of the transformer.

In terms of harmful effects, the largest GIC event was experienced on March 1989 when collapse of the Canadian Hydro-Quebec power systems left the province without power for several hours.

Damaged transformer windings. Figure credits: John Kappenman, Metatech. Damages in North America during the March 1989 storm.

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Reduction of the effects on power systems

The flow of GIC can be blocked by using series capacitors in transmission lines or in earthing wires of transformers. After the March 1989 event, Hydro-Québec has, as a part of an extensive study and mitigation procedure, installed several series capacitors in their system. However, the use of series capacitors is expensive and also technically not straightforward.

A different approach to mitigate GIC problems is provided by the possibility of forecasting geomagnetic storms and GIC. When power utilities are warned about an approaching geomagnetic storm they can take different actions. These include the reduction of the loading in the system, which gives a larger margin, ensuring that the possible series capacitors are operating properly, and being prepared for possible problems. All actions are costly, so unnecessary predictions should not exist. This and the fact that GIC cannot be predicted reliably yet emphasize the importance of a continuous research on forecasting methods.

Animation of the computed geoelectric field and GIC in the Finnish system during April 7, 1995 storm. [Animation QuickTime 4.4 MB] Map of power systems in Northern Europe. Interconnected power systems make GIC problems propagate across state boundaries.

Pipelines

Buried oil and gas pipelines are prone to corrosion, which may occur at points where an electric current flows from the metal into the surrounding earth. To avoid corrosion, pipelines are covered by an insulating coating and equipped with a cathodic protection system that tries to prevent electric currents leaving the pipeline.

GIC flowing along pipelines are accompanied by voltages between the pipeline and the Earth. Voltage variations related to GIC can easily exceed the cathodic protection potential making the protection thus invalid. Today’s coatings cause larger GIC-related pipe-to-soil potentials, thus significantly increasing the risk of corrosion at defects in the coating. How much pipe-to-soil voltages induced by space weather effects really increase the corrosion rate of a pipeline is anyhow still a somewhat open question.

Besides a direct contribution to corrosion, geomagnetically induced pipe-to-soil voltages are a nuisance when measuring cathodic protection parameters and making control surveys. The measurement results may be completely incorrect and thus lead to erroneous conclusions.


GIC measured in the Finnish pipeline and the time derivative of the north component of the geomagnetic field measured at Nurmijärvi during the “Bastille Day” geomagnetic storm on July 15, 2000.
Animation of the modeled GIC (blue curve) and pipe-to-soil potential (red curve) in the Finnish natural gas pipeline during March 31, 2001 geomagnetic storm. [Animation Quick Time 6.4 MB]. Animation credit: Mikko Kuitunen.
Similarly to a power system, the magnitudes of GIC along a pipeline network and of pipe-to-soil voltages depend both on the geophysical situation and on the details of the network. In general, the pipe-to-soil voltages are larger at and near inhomogeneities of the system, such as ends, bends and branches of the pipeline, changes in the material or size of the pipeline, or variations in the Earth’s conductivity.

In general, space weather risk has not been investigated as much in pipelines as in power systems. Recent investigations containing more sophisticated tools for model calculations are improving the situation.
Animation of the measured GIC in the Finnish pipeline and observed auroras at Abisko during the geomagnetic storm on February 18, 1999. [Animation QuickTime 5 MB]

Other systems affected

In principle, all long conductors experience GIC. The first observations of GIC were made in telegraph equipment more than 150 years ago. Many times since then, telesystems have suffered from overvoltages, interruptions in the operation and even fires caused by GIC flowing through the equipment. Optical fiber cables generally used in telecommunication nowadays do not carry GIC at all. However, their use does not totally remove the problem because the voltage to amplifiers is fed by a metallic cable that may suffer from GIC.

Submarine phone cables lying at ocean floors form a special category of telesystems affected by geomagnetic disturbances. Their length implies that the voltages induced on such cables during geomagnetic storms are easily hundreds, even thousands, of volts leading to possible problems.


First effects of GIC were experienced on telegraph equipment. Image about the installation of a sea cable. Figure credit: AT&T.

 

On railways geomagnetic induction may cause unexpected voltages resulting in misoperations of equipment. During the magnetic storm in July 1982, such a voltage made traffic lights turn red without any train coming in Sweden.

Magnetic surveys are used for example in oil and gas exploration. The measurements concern changes of the magnetic field, so there is a problem of separating space weather-related variations from the desired spatial variations. Scheduling surveys for periods when disturbances are forecast to be small could be a solution.
Magnetic surveys used in oil and gas exploration are disturbed by space weather related geomagnetic variations. Railway signalling systems in Sweden were affected by a geomagnetic storm in July 1982. Figure credits: Andrew Pam.