Jill Duplessis - Global technical marketing manager
Geomagnetically Induced Current (GIC) has the potential to seriously disrupt the operation of the power transmission grid over a wide area. And, if a GIC event damages key components like power transformers, the effects may continue to be felt for months or even years. This article examines what GIC is and why it occurs, before discussing its effects in general terms. A subsequent article will further explore the impact of GIC on the power grid, with particular reference to the vulnerability of transformers.
The phenomenon of GIC is well documented in technical sources but is not necessarily well-known or well-understood outside specialist circles. For this reason, and to provide a sound foundation for the rest of the material that will be presented in these articles, it is worthwhile starting with a brief examination of GIC and its causes.
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, resulting in a geomagnetic storm.
Geomagnetically Induced Current (GIC) is produced when auroral electrojet currents that flow in circular paths around the earth’s geomagnetic poles at altitudes of around 100 km become energised by the arrival of a plasma cloud. This energisation results in slow, time-varying fluctuations in the earth’s normally unvarying magnet field.
In accordance with Faraday’s law of induction, these magnetic field fluctuations induce currents in the earth’s surface which, in turn, give rise to potential differences – ESPs (earth surface potentials) – between grounding points. 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 a power system, causing a GIC to flow through these transformers, the connected power system lines and the neutral ground points.
The susceptibility of a power system to geomagnetic storms – and hence GIC – varies and depends on a number of contributing elements. These include:
- The characteristics of the transformers on the system, as these serve as the entry and exit points for the GICs. Relevant factors are:
- 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.
- 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 winding configuration: Any transformer with a grounded-wye (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. (See Figure 1).
- Ground conductivity: Power systems in areas where ground conductivity is poor (see Figure 2) 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.
- 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. Note, however, that the earth’s magnetic poles do not coincide exactly with its geographical poles. This means that in the USA, for example, Eastcoast geographic mid-latitude locations are more vulnerable than the equivalent West-coast geographic locations, as the former are closer to the magnetic pole.
- 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 all-too-convincingly demonstrated in March 1989 when power systems operated by Hydro Quebec in Canada 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 quantitative terms the impact of a GIC event on a particular power system. In fact, most attempts as quantification to date have essentially been anecdotal.
The next article in this series will examine in more detail the first order effects of GIC on transformers and the second order effects on the power system. It will also discuss how current transformers and protective relays are likely to be affected by GIC, as well as briefly reviewing regulations introduced by the Federal Energy Regulatory Committee (FERC) and National Energy Reliability Committee (NERC) in the USA in response to the threats posed by GIC.