Chapter / 132
Ground Effects of Space Weather
Space weather effects on electric power transmission grids and pipelines
Risto Pirjola, Ari Viljanen, Antti Pulkkinen, Sami Kilpua1*, Olaf Amm
Finnish Meteorological Institute, Geophysical Research Division
P. O. Box 503, FIN-00101 Helsinki, Finland
phone: +358-9-19294652, fax: +358-9-19294603, email:
*1Now at:
GeoForschungsZentrum Potsdam
Telegrafenberg, D-14473 Potsdam, Germany
Abstract Space storms produce geomagnetically induced currents (GIC) in technological systems at the Earth’s surface, such as electric power transmission grids, pipelines, communication cables and railways. Thus GIC are the ground end of the space weather chain originating from the Sun. The first GIC observations were already made in early telegraph equipment about 150 years ago, and since then several different systems have experienced problems during large magnetic storms. Physically, GIC are driven by the geoelectric field induced by a geomagnetic variation. The electric and magnetic fields are primarily created by magnetospheric-ionospheric currents and secondarily influenced by currents induced in the Earth that are affected by the ground conductivity. The most violent magnetic variations occur in auroral regions, which indicates that GIC are a particular high-latitude problem but lower-latitude systems can also experience GIC problems. In power networks, GIC may cause saturation of transformers with harmful consequences extending from harmonics in the electricity to large reactive power consumption and even to a collapse of the system or to permanent damage of transformers. In pipelines, GIC and the associated pipe-to-soil voltages can enhance corrosion and disturb corrosion control measurements and protection. Modelling techniques of GIC are discussed in this paper. Having information about the Earth’s conductivity and about space currents or the ground magnetic field, a GIC calculation contains two steps: the determination of the geoelectric field and the computation of GIC in the system considered. Generally, the latter step is easier but techniques applicable to discretely-earthed power systems essentially differ from those usable for continuously-earthed buried pipelines. Time-critical purposes, like forecasting of GIC, require a fast calculation of the geoelectric field. A straightforward derivation of the electric field from Maxwell’s equations and boundary conditions seems to be too slow. The complex image method (CIM) is an alternative but the electric field can also be calculated by applying the simple plane wave formula if ground-based magnetic data are available. In this paper, special attention is paid to the relation between CIM and the plane wave method. A study about GIC in Scotland and Finland during the large geomagnetic storm in April 2000 and another statistical study about GIC in Finland during SSC events are also briefly discussed.
Keywords Geomagnetically induced currents, GIC, geoelectric field, geomagnetic disturbances, geoelectromagnetics, plane wave, complex image method
1. Introduction
At the Earth's surface, space weather manifests itself as geomagnetically induced currents (GIC) flowing in long conductors, such as electric power transmission networks, oil and gas pipelines, telecommunication cables and railways systems. In power grids, GIC cause saturation of transformers, which tends to distort and increase the exciting current. It in turn implies harmonics in the electricity, unwanted relay trippings, large reactive power consumption, voltage fluctuations etc., leading finally to a possible black-out of the whole system, and to permanent damage of transformers (Kappenman and Albertson, 1990; Kappenman, 1996; Erinmez et al., 2002b; Molinski, 2002).
In buried pipelines, GIC and the associated pipe-to-soil voltages contribute to corrosion and disturb corrosion control surveys and protection systems (Boteler, 2000; Gummow, 2002). Telecommunication devices have also experienced GIC problems (Karsberg et al., 1959; Boteler et al., 1998; Nevanlinna et al., 2001). As optical fibre cables do not carry GIC, space weather risks on telecommunication equipment are probably smaller today than previously. However, it should also be noted that metal wires are used in parallel with optical cables for the power to repeat stations. There are not many studies of GIC effects on railways, and to the knowledge of the authors of this paper, the only publicly and clearly documented case has occurred in Sweden where GIC resulted in misoperation of railway traffic lights during a geomagnetic storm in July 1982 (Wallerius, 1982). (A private communication with a Russian scientist indicates that space weather has caused problems in Russian railway systems, too.)
GIC have a long history since the first observations were already made in early telegraph systems about 150 years ago (Boteler et al., 1998). In general, GIC is a high-latitude problem, which is supported by the fact that the most famous destructive GIC event occurred in the Hydro-Québec power system in Canada (Czech et al., 1992; Bolduc, 2002). GIC values in a system are, however, not directly related to the proximity of the auroral zone but the ground resistivity and the particular network configuration and resistances also affect. GIC values usually greatly vary from site to site and from system to system. Furthermore, GIC magnitudes that are a potential risk for a power transmission system are highly dependent on transformer structures and on engineering details of the network. For example, the largest GIC measured in the Finnish 400 kV power system is about 200 A but Finland’s transformers have not experienced GIC problems (Elovaara et al., 1992; Lahtinen and Elovaara, 2002). Probably, the largest GIC anywhere and ever measured is 320 A in Sweden during the geomagnetic storm in April 2000 (Erinmez et al., 2002b). The value of 600 A in Sweden mentioned by Stauning (2002) is evidently not correct (private communication with a Swedish engineer).
There are engineering means which may be used for preventing harmful GIC in a system. For example, the dc-like GIC cannot flow through series capacitors installed in power transmission lines. However, determining the locations of capacitors in a power grid is not straightforward (Erinmez et al., 2002a; Pirjola, 2002). Thus, the flow of GIC cannot easily be blocked in a system, and efforts should be concentrated on estimating expected GIC magnitudes at different sites and on forecasting large GIC events.
The horizontal geoelectric field induced at the Earth’s surface drives GIC. Therefore, model developments in GIC research should aim at calculating the geoelectric field. After knowing this field, the determination of GIC in a system is a simpler task although a discretely-earthed power grid and a continuously-earthed buried pipeline require different techniques (Lehtinen and Pirjola, 1985; Pulkkinen et al., 2001).
As described by Faraday's law, the geoelectric field is induced by a temporal variation of the magnetic field during a geomagnetic disturbance or storm. Both the magnetic field and the electric field are primarily produced by ionospheric-magnetospheric currents, but they also have a secondary contribution from currents in the Earth affected by the Earth’s conductivity structure. In principle, knowing the space currents and the Earth's conductivity permits the determination of the electric and magnetic fields at the Earth's surface by using Maxwell's equations and appropriate boundary conditions. Such a straightforward method is presented by Häkkinen and Pirjola (1986). In practice, however, the ionospheric-magnetospheric currents and the conductivity of the Earth are not known precisely, and even if they were known, the exact formulas would not allow fast enough computations needed for forecasting purposes. The complex image method (CIM) has shown to be a suitable technique for geoelectromagnetic calculations because it is accurate and fast (Boteler and Pirjola, 1998; Pirjola and Viljanen, 1998).
The simplest relation between surface electric and magnetic fields is obtained by making the plane wave assumption, which rigorously means that the primary electromagnetic field originating from space current is a vertically-downwards propagating plane wave. Assuming further that the Earth has a layered structure and operating in the frequency domain, the electric field is obtained by multiplying the magnetic field by the surface impedance. It has been shown that the assumption of a vertical plane wave need not be strictly fulfilled for the plane wave technique to work in practice (Cagniard, 1953; Wait, 1954; Dmitriev and Berdichevsky, 1979; Wait, 1980). Thus, the plane wave method provides a good tool for the calculation of the geoelectric field if magnetic data are available.
In Section 2, we summarize the methods to be used for determining the geoelectric field and for computing GIC. Special attention is paid to the relation between the plane wave technique and CIM. There are a great variety of different space current systems which can produce a significant magnetic disturbance, a geoelectric field and GIC in technological systems. The spherical elementary current system (SECS) method is a novel useful tool for determining (equivalent) ionospheric currents from ground magnetic observations during different space weather events (Amm, 1997; Amm and Viljanen, 1999; Pulkkinen et al., 2003a). A step forwards in understanding GIC processes and forecasting them is to classify space weather events by considering their GIC impacts. In Section 3, we summarize a study of GIC during the large magnetic storm in April 2000, and briefly discuss observations of GIC in the Finnish natural gas pipeline during sudden storm commencements (SSC), which are global geomagnetic disturbances.
2. MODELLING THE GEOELECTRIC FIELD AND GIC
2.1 Calculation of the geoelectric field
2.1.1 Plane wave model
GIC are usually considered in systems located in a limited area. Therefore models used in this connection have a regional character permitting the use of a flat-Earth model. The standard coordinate system has its xy plane at the Earth’s surface with the x , y and z axes pointing northwards, eastwards and downwards, respectively. Let us assume that the primary electromagnetic field originating from ionospheric and magnetospheric sources is a plane wave propagating vertically downwards and that the Earth is uniform with a permittivity e, a permeability m and a conductivity s. Considering a single frequency w (i.e. the time dependence is exp(iwt)), it is simple to derive the following relation between the y component of the electric field Ey and the x component of the magnetic field Bx at the Earth’s surface (or similarly between Ex and By):
(1)
where the propagation constant k is given by
(2)
In geoelectromagnetics always swe, and m can be set equal to the vacuum value m0. Thus
(3)
and
(4)
Inverse-Fourier transforming equation (4) into the time domain yields
(5)
where the time derivative of Bx(t) is denoted by g(t). Equations (4) and (5) show that the electric field decreases with an increasing Earth conductivity. This indicates that GIC should be taken into account in particular in resistive areas. This conclusion is, however, not self-evident since the ground conductivity also has an influence on earthing resistances of a power system thus affecting the GIC flow (Pirjola and Viljanen, 1991). It is seen from equation (5) that the electric field at a given moment t is not only related to the time derivative of the magnetic field at the same moment but earlier values also affect with a decreasing weight (the square root factor in the denominator). The inverse-Fourier transform may, of course, be performed for the exact formula (1) as well leading to an expression which approximately reduces to equation (5) (Pirjola, 1982, p. 23).
If the Earth is not uniform but has a layered structure the term mw/k in (1) has to be replaced by the (plane wave) surface impedance Z = Z(w) (see e.g. Wait, 1981, pp. 43-55), so that
(6)
If the Earth’s structure also depends on the x and y coordinates, as for example in coastal areas, the situation becomes much more complicated, and the independence of x and y of the fields disappears.
Equations (4) and (6) form the basis of the magnetotelluric sounding method of the conductivity structure of the Earth (Cagniard, 1953). A lot of discussion has concerned the validity of the plane wave assumption of the primary field (e.g. Mareschal, 1986; Pirjola, 1992) since a vertical plane wave is certainly not true near a concentrated ionospheric current, like an auroral electrojet. It, however, appears that the magnetotelluric equations (4) and (6) are applicable to a wide range of events.
2.1.2 Transfer function between horizontal electric and magnetic fields
Let us now assume that the primary electromagnetic field incident on a uniform Earth depends on the x coordinate (but for simplicity not on y), which is the case if, for example, an auroral electrojet is modelled by an east-west line current. Considering a single frequency w and making a spatial Fourier transform from the x coordinate to the wavenumber b, equation (6) is satisfied with
(7)
where k is given by formula (2) (Pirjola, 1982, p. 51). Setting b equal to zero in equation (7) gives the surface impedance included in equation (1) as expected.
Inverse-Fourier transforming into the x domain yields
(8)
Using the convolution theorem, this can be written as
(9)
where the transfer function is given by
(10)
Equations (8), (9) and (10) do not presume that the Earth is uniform but any layered structure is possible. The reference Wait (1981, pp. 43-55) mentioned in Section 2.1.1 is associated with a wavenumber-dependent surface impedance, i.e. not only with the plane wave case. The treatment included in equations (8), (9) and (10) is analogous to that used for calculating the electric field at the seafloor in terms of the surface magnetic field in the two-dimensional case (Pirjola et al, 2000).
In the case of a uniform Earth, a substitution of equation (7) into (10) gives
(11)