Geomagnetically induced current
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Geomagnetically induced currents (GIC), affecting the normal operation of long technological conductor systems, are a manifestation at ground level of space weather. During space weather events (or geomagnetic storms) Earth's near space current systems experience large spatiotemporal variations reflected also in the variations of the Earth’s geomagnetic field. These variations induce currents (GIC) in conductors operated at the surface of Earth. Electric transmission grids and buried pipelines are common examples of such conductor systems. GIC can cause problems such as increased corrosion rate of pipeline steel and damaged high-voltage power transformers. Although this article discusses only the GIC aspect of the ground level of space weather, geomagnetic storms can also affect, for example, geophysical exploration surveys and oil and gas drilling operations.
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[edit] Background
The Earth’s geomagnetic field varies over a wide range of timescales. The longer-term variations, typically those occurring over decades to millennia, are predominantly the result of dynamo action in the Earth’s core. However, geomagnetic variations on timescales of seconds to years also occur, due to dynamic processes in the ionosphere, magnetosphere and heliosphere. These changes are ultimately tied to variations associated with the solar activity (or sunspot) cycle and are manifestations of ‘Space Weather’.
The fact that the geomagnetic field does respond to solar conditions can be useful, for example in investigating Earth structure using magnetotellurics, but it also creates a hazard. This geomagnetic hazard is primarily a risk to technology, at least under the Earth’s protective atmospheric blanket (for recent reviews see, e.g., Lanzerotti, 2001; Pirjola et al, 2005). Here we discuss the ground effects of space weather, but we note that space weather also impacts other technologies, for example, those associated with airlines, Earth-orbiting satellites, GPS, radio communication systems, and unmanned space missions. Astronaut health during space weather events, for example during extended stays on the International Space Station, and on any future Moon and Mars missions, continues to be a prime consideration for national and international space agencies.
[edit] The Risk to Ground Infrastructures from Geomagnetically Induced Currents
A time-varying magnetic field external to the Earth induces electric currents in the conducting ground. These currents create a secondary (internal) magnetic field. As a consequence of Faraday's law of induction, an electric field measurable at the surface of the Earth is induced associated with time variations of the magnetic field. The surface electric field causes electrical currents, known as geomagnetically induced currents (GIC), to flow in any conducting structure, for example, a power or pipeline grid grounded in the Earth. This electric field, measured in V/km, acts as a voltage source across networks.
Examples of conducting networks are electrical power transmission grids, oil and gas pipelines, undersea communication cables, telephone and telegraph networks and railways. GIC are often described as being quasi direct current (DC), although the variation frequency of GIC is governed by the time variation of the electric field. For GIC to be a hazard to technology, the current has to be of a magnitude and occurrence frequency that makes the equipment susceptible to either immediate or cumulative damage. The size of the GIC in any network is governed by the electrical properties and the topology of the network. The largest magnetospheric-ionospheric current variations, resulting in the largest external magnetic field variations, occur during geomagnetic storms and it is then that the largest GIC occur. Significant variation periods are typically from seconds to about an hour, so the induction process involves the upper mantle and lithosphere. Since the largest magnetic field variations are observed at higher magnetic latitudes, GIC have been regularly measured in Canadian, Finnish and Scandinavian power grids and pipelines since the 1970s. GIC of tens to hundreds of Amperes have been recorded. However GIC have also been recorded in countries at mid-latitudes during major storms. There may even be a risk to low latitude nations especially during a storm sudden commencement because of the high, short-period, rate of change of the field that occurs everywhere on the dayside of the Earth.
GIC have been known since the mid-1800s when it was noted that telegraph systems could run without power sometimes during geomagnetic storms, described at the time as operating by means of the “celestial battery”, while at other times they were completely inoperative (Boteler et al, 1998). However technological change and the growth of conducting networks have made the significance of GIC greater and more pervasive in modern society. We therefore describe the GIC hazard in detail in two of the best-studied networks, power grids and pipelines. The technical considerations for undersea cables, telephone and telegraph networks and railways are similar. However fewer problems are known, or have been reported in the open literature, about these systems. This suggests that the hazard is less today, or that there are reliable methods for equipment protection.
[edit] GIC in power grids
Modern electric power transmission systems consist of generating plant inter-connected by electrical circuits that operate at fixed transmission voltages controlled at transformer substations. The grid voltages employed are largely dependent on the path length between these substations and 200kV-700kV system voltages are common. There is a trend towards higher voltages and lower line resistances to reduce transmission losses over longer and longer path lengths. However low line resistances produce a situation favourable to the flow of GIC. Power transformers have a magnetic circuit that is disrupted by the quasi-DC GIC: the field produced by the GIC offsets the operating point of the magnetic circuit and the transformer may go into half-cycle saturation. This produces a harmonic-rich AC waveform, localised heating and leads to high reactive power demands, inefficient power transmission and possible mis-operation of protective measures. Balancing the network in such situations requires significant additional reactive power capacity (Erinmez et al, 2002). The magnitude of GIC that will cause significant problems to transformers varies with transformer type. Modern industry practice is to specify GIC tolerance levels on new transformer purchases.
On 13 March 1989 a severe geomagnetic storm caused the collapse of the Hydro-Quebec power grid in a matter of seconds as equipment protection relays tripped in a cascading sequence of events (Bolduc, 2002). Six million people were left without power for nine hours, with significant economic loss. Since 1989 power companies in North America, the UK, Northern Europe and elsewhere have invested time and effort in evaluating the GIC risk and in developing mitigation strategies. GIC risk can, to some extent, be reduced by capacitor blocking systems, maintenance schedule changes, additional on-demand generating capacity, and, ultimately, shedding of load. However, these options are expensive and sometimes impractical. The continued growth of high voltage power networks, for example in North America and in mainland Europe, is leading to a higher risk. This is partly due to the increase in the interconnectedness at higher voltages; connections in terms of power transmission to grids in the auroral zone, and commercial considerations that see grids run closer to capacity than was the case historically.
To understand the flow of GIC in power grids and therefore to advise on GIC risk, analysis of the quasi-DC properties of the grid is necessary (Lehtinen and Pirjola, 1985). This must be coupled with a geophysical model of the Earth that provides the driving surface electric field, determined by combining time-varying ionospheric source fields and a conductivity model of the Earth. Such analyses have been performed for North America, the UK and in Northern Europe. However the complexity of power grids, the source ionospheric current systems and the 3D ground conductivity make an accurate analysis difficult (see Thomson et al, 2005). By being able to analyse, post-event, major storms and their consequences we can build a picture of the weak spots in a given transmission system and even run hypothetical event scenarios.
Grid management is also aided by space weather forecasts of major geomagnetic storms. This allows for mitigation strategies to be implemented. Solar observations provide a 1-3 day warning of an Earth-bound coronal mass ejection (CME), depending on CME speed. Following this, detection of the solar wind shock that precedes the CME in the solar wind, by spacecraft at the Langrangian L1 point, gives a definite 20-60 minutes warning of a geomagnetic storm (again depending on local solar wind speed). However, the magnitude and accurate time of arrival of a CME after shock detection is unknown, although there is much research and model development within the space weather community. Given the demands on managing power grids more accurate information would have high value.
[edit] GIC hazard in pipelines
Major pipeline networks exist at all latitudes and many systems are on a continental scale. Pipeline networks are constructed from steel to contain high-pressure liquid or gas and are covered with special coatings to resist corrosion. 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. Cathodic protection rectifiers are used to maintain pipelines at a negative potential with respect to the ground. This minimises corrosion without allowing any chemical decomposition of the pipe coating and the operating potential is determined from the electro-chemical properties of the soil and Earth in the vicinity of the pipeline. The GIC hazard to pipelines is that GIC cause swings in the pipe-to-soil potential, increasing the rate of corrosion during major geomagnetic storms (Gummow, 2002). GIC risk is not, therefore, a risk of catastrophic failure, rather the reduced service lifetime of the pipeline, or parts of it.
Pipeline networks are modelled in a similar manner to power grids, for example through distributed source transmission line models that provide the pipe-to-soil potential at any point along the pipe (Boteler, 1997; Pulkkinen et al., 2001). These models need to take into account complicated pipeline topologies that include bends and branches as well as electrical insulators, or flanges, that electrically isolate different sections of the network. From a detailed knowledge of the pipeline response to GIC, pipeline engineers can understand the behaviour of the cathodic protection system even during a geomagnetic storm, when pipeline surveying and maintenance may often be suspended.
[edit] Further reading
- Bolduc, L., GIC observations and studies in the Hydro-Quebec power system. J. Atmos. Sol. Terr. Phys., 64(16), 1793-1802, 2002.
- Boteler, D. H., Distributed source transmission line theory for electromagnetic induction studies. In Supplement of the Proceedings of the 12th International Zurich Symposium and Technical Exhibition on Electromagnetic Compatibility. pp. 401-408, 1997.
- Boteler, D. H., Pirjola, R. J. and Nevanlinna, H., The effects of geomagnetic disturbances on electrical systems at the Earth’s surface. Adv. Space. Res., 22(1), 17-27, 1998.
- Erinmez, I. A., Kappenman, J. G. and Radasky, W. A., Management of the geomagnetically induced current risks on the national grid company’s electric power transmission system. J. Atmos. Sol. Terr. Phys., 64(5-6), 743-756, 2002.
- Gummow, R. A., GIC effects on pipeline corrosion and corrosion-control systems. J. Atmos. Sol. Terr. Phys., 64(16), 1755-1764, 2002.
- Lanzerotti, L. J., Space weather effects on technologies. In Song, P., Singer, H. J., Siscoe, G. L. (eds.), Space Weather. American Geophysical Union, Geophysical Monograph, 125, pp. 11-22, 2001.
- Lehtinen, M., and R. Pirjola, Currents produced in earthed conductor networks by geomagnetically-induced electric fields, Annales Geophysicae, 3, 4, 479-484, 1985.
- Pirjola, R., Kauristie, K., Lappalainen, H. and Viljanen, A. and Pulkkinen A., Space weather risk. AGU Space Weather, 3, S02A02, doi:10.1029/2004SW000112, 2005.
- Thomson, A. W. P., A. J. McKay, E. Clarke, and S. J. Reay, Surface electric fields and geomagnetically induced currents in the Scottish Power grid during the 30 October 2003 geomagnetic storm, AGU Space Weather, 3, S11002, doi:10.1029/2005SW000156, 2005.
- Pulkkinen, A., R. Pirjola, D. Boteler, A. Viljanen, and I. Yegorov, Modelling of space weather effects on pipelines, Journal of Applied Geophysics, 48, 233-256, 2001.
- Pulkkinen, A. Geomagnetic Induction During Highly Disturbed Space Weather Conditions: Studies of Ground Effects, PhD thesis, University of Helsinki, 2003. (available at eThesis)
[edit] External GIC links
- GICnow! Service by Finnish Meteorological Institute
- Ground Effects Topical Group of ESA Space Weather Working Team
- Article by J. Kappenman on GIC in power grids
- Metatech Corporation's GIC site
