8.4 Effects on ground

Space weather effects are seen even on ground, as strong currents in the ionosphere can induce currents in long baseline conductor systems on ground. Affected systems include electric power transmission networks, oil and gas pipelines, telecommunication cables and railways systems. In power grids, these geomagnetically induced currents cause saturation of transformers, which tends to distort and increase the excitation current. The excitation current in turn induces harmonics in the electricity, unwanted relay trippings, large reactive power consumption, or voltage fluctuations, which can lead to black-outs or to permanent damage of transformers (Kappenman, 1996).

In buried pipelines, geomagnetically induced currents and the associated pipe-to-soil voltages contribute to corrosion and disturb corrosion control surveys and protection systems (Boteler, 2000). Telecommunication devices may experience overvoltages. As optical fibre cables do not carry induced currents, space weather risks on telecommunication equipment are probably smaller today than they were earlier. However, metal wires are still used in parallel with optical cables for the power to repeat stations.

The first observations of space weather-associated induced currents were made already in early telegraph systems more than 150 years ago (Boteler et al., 1998). In general, being related to auroral phenomena, the geomagnetically induced currents are a high-latitude problem, affecting most regions such as the Scandinavian countries, Russia, and Canada. The amount of induced current in a system is not directly related to the distance to the auroral currents; the ground resistivity and the particular network configuration and its resistances have an important contribution in determining the size of the disturbance. This makes the observed induced current values highly variable from site to site and from system to system. The current magnitudes that are a potential risk for a power transmission system are highly dependent on transformer design and on other engineering details of the network. This means that any space weather hazard assessment must include the details of the engineering solutions in the estimates.

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