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Geomagnetically induced current (GIC) can seriously disrupt the operation of power transmission grids over a wide area. If a GIC event damages key components such as power transformers, the effects may continue for months or even years.
The phenomenon of GIC is well documented in technical sources but not necessarily well-known or well-understood outside specialist circles. The primary source of GIC events is activity on the sun’s surface, in the form of sunspots and solar flares.
Solar flares produce coronal mass ejections, x-rays and charged particles that form a plasma cloud—a gust of solar wind—that can reach the earth in as little as eight minutes. Depending on its orientation, the magnetic field produced by the electric currents within the plasma cloud can interact with the earth’s magnetic field, causing it to fluctuate, and resulting in a geomagnetic storm.
The distances over which these effects are felt can be quite large. The field, then, essentially behaves as an ideal voltage source between remote neutral ground connections of transformers in the power system, causing a GIC to flow through these transformers, the connected power system lines and the neutral ground points.
Power system risk
The susceptibility of a power system to geomagnetic storms—and therefore, GIC—varies and depends on a number of contributing elements. These include:
• The characteristics of the transformers on the system. These serve as the entry and exit points for the GICs. Relevant factors are:
—Transformer winding configuration: Any transformer with a grounded-star connection is susceptible to a quasi-DC current flowing through its windings; an autotransformer, where the high- and low-voltage windings are partly shared, permits GIC to pass through the high-voltage power lines, but a delta-wye (delta-star) transformer does not.
—Transformer core construction. The design of the core determines the magnetic reluctance of the DC flux path, which influences the magnitude of the DC flux shift that will occur in the core. Three-phase transformers with a three-leg core are the least vulnerable to GIC because they have an order of magnitude higher DC reluctance in the core-tank magnetic circuit than transformers with other types of core. Most GIC problems are associated with single-phase core- or shell-form units, three-phase shell-form designs and three-phase five-leg core form designs.
—Transformer ground construction: Transformers on extra-high voltage (EHV) transmission systems are particularly vulnerable as these systems are very solidly grounded, creating a low-resistance preferential path for GIC. Additionally, EHV transformers are usually not three-phase, three-leg core form designs.
• The geographical location of the power system. The closer the power system is to the earth’s magnetic poles, the nearer it is likely to be to the auroral electrojet currents and consequently, the greater their effect. The earth’s magnetic poles, however, don’t coincide exactly with its geographical poles. This means in the US, for example, East-coast geographic mid-latitude locations are more vulnerable than the equivalent West-coast geographic locations, as the former are closer to the magnetic pole.
• Ground conductivity. Power systems in areas where ground conductivity is poor are more vulnerable to the effects of geomagnetic activity because any geomagnetic disturbance will produce a larger gradient in the earth surface potential it induces into the ground; and also because the high ground resistance encourages more current to flow through alternative paths such as power transmission lines.
• Orientation of the power system lines. The gradient of the earth surface potential is usually though not invariably, greater in the east-west direction than the north-south direction.
• The length of the power system lines. The longer the transmission lines, the greater their vulnerability. This was demonstrated in March 1989 when power systems operated by Hydro Quebec were ravaged by a GIC event. The Hydro Quebec system includes generators that are 1,000 km away from the main populated load centres.
• The strength of the geomagnetic storm. The more powerful the storm, the greater the intensity of the auroral electrojet currents, and the closer these are likely to be to the equator. The impact of GIC on transformers and power systems is well understood in general terms. However, because so many variables influence vulnerability, it is almost impossible to predict in quantitive terms the impact of a GIC event on a particular power system. In fact, most attempts at quantification to date have essentially been anecdotal.
Jill Duplessis is global technical marketing manager and editor with Megger. This article is an edited version of the original, which appeared in the November 2015 edition of Electrical Tester Magazine, published by Megger.
Megger designs and manufactures portable electrical test equipment. Megger products help you install, improve efficiency, reduce cost and extend the life of your or your customers’ electrical assets. To learn more visit www.megger.com
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