Sun to Earth: time for standards?

Mitigating the impact of severe geomagnetic storms on high-voltage power grids

By William A. Radasky

Since the geomagnetic storm in March 1989 that produced a power system blackout in the Province of Québec, Canada, considerable scientific work has been undertaken in North America, Europe and Asia to understand fully the effects of the Sun’s activity on the Earth. Significant progress has been made, and questions are now being asked about the power industry’s response to the possibility of future storms. It may be time to consider the development of standards to mitigate this high impact – but low probability – threat.

Solar flares on the Sun's surface (Photo: NASA) Solar flares on the Sun's surface (Photo: NASA)

Sun to the Earth

The Sun produces many types of high-energy photon and particle emissions throughout its eleven-year cycle. Sunspots, representing areas of intense magnetic activity, are easily observable and vary in number during each solar cycle. Heat flow from the sun is reduced at the spots’ locations, resulting in a cooler temperature. Periodically these sunspots become active, resulting in flares and CMEs (coronal mass ejections). The CMEs are of primary import to the Earth’s high voltage power grids, as they represent massive numbers of energetic charged particles (electrons and protons) that are ejected from the Sun. Some of these particles return to the Sun’s surface due to the intense magnetic fields present. Those heading for the earth typically take 1-3 days to reach its surface, as they travel more slowly than the speed of light.

Large geomagnetic storms at the Earth’s surface can occur at nearly any point within the solar sunspot cycle. This is because the CMEs typically emanate from a single sunspot, so the strength of a geomagnetic storm reaching the Earth does not depend on the total number of spots present on the Sun at that instant. Recent historical data shows that the occurrence of large storms is more likely several years after a peak in sunspot numbers, as opposed to at the time of the peak of the sunspot cycle.

The high-energy charged particles in the solar wind that are captured in the Earth’s magnetic field descend to altitudes of approximately 100 km in the magnetic Northern and Southern latitudes. There they are completely ionized and result in a horizontal flow of charge in the ionosphere. This structure is known as the Electrojet. Since the enhanced solar wind fluctuates in time with charged particles, this Electrojet also fluctuates in time, resulting in a current pulse with rise times of seconds and a pulse width of thousands of seconds. There may be many pulses of current over the period of a day, depending on the strength of the CME.

The time-varying current in the ionosphere creates a time-varying magnetic field that possesses the same time history as the current flow in the ionosphere (fig. 1). This magnetic field couples electromagnetically to the grounded high voltage grid (through the transformer neutrals), where the actual "ground" is found at depths greater than 500 km. Thus the loop is very large, and even small fluctuations in geomagnetic field can create a significant voltage difference between the ends of a transmission line. Given the very low resistance design for high voltage power lines (to reduce the resistive loss of power), large quasi-dc (nearly dc as compared to the power frequency) currents will flow in the power lines, creating saturation conditions in high voltage transformers. When these transformers are saturated, the magnetic field linkage between the transformer cores will be deformed; the reactive load of each transformer will increase suddenly and there is a possibility of hot spot formation leading to potential wiring damage if the saturation condition lasts long enough. In addition, high levels of even harmonics will be generated that can cause problems in series capacitors in high voltage networks. Finally, the reactive load increase of hundreds of transformers at the same time can lead to voltage collapse.

Modeling power grids

Computation of the GICs (geomagnetically induced currents) and their impact begins with the variation of the geomagnetic field across the power grid of interest (an example of a geomagnetic field disturbance at one instant in time is shown in fig. 2). Using layered ground models for the region, the electrical fields are computed. These fields are then coupled to a "dc" model of the entire power grid (at high voltage) to which the resistances in each line, transformer and grounding grid have been applied. The electric fields are coupled to all lines simultaneously for each time increment to obtain the current flow in every high voltage transformer. These currents are then used to estimate the reactive power demand in each transformer, the level of harmonics generated and whether a transformer is at risk of damage. These types of calculations have been performed for power grids throughout the world, including in the US, the UK, Norway, Sweden and Japan. Validation on past geomagnetic storms has been performed and published, using ground-based magnetometer data to provide the inputs for each calculation, and the calculations of currents flowing in transformer neutrals have been compared to measurements of GICs with excellent accuracy.

What information is missing?

Given a particular distribution of geomagnetic field disturbances in time and space, it is possible to accurately compute the flow of GIC within a complex power network. While this is good news, some important pieces of information are still missing.

First it is important to determine the levels of worst-case storms or, better still, a probabilistic distribution of levels of storm and their likelihood. This would allow the power industry to determine cost-effective protection strategies to deal with the problem. The US and European space agencies, NASA and ESA, are working on this problem, trying to determine why particular CMEs at the Sun are more powerful than others and how often powerful storms may occur in the future. It is important to realize that only a limited amount of detailed data is available concerning the Sun’s activity levels and the particles that are ejected (only made possible by the advent of satellites). To establish statistics regarding 100-year storms based on 50 years of data will not be easy.

Second, when storms occur, the time-dependent characteristics of the geomagnetic field disturbances on the Earth vary considerably, and it is necessary to determine which characteristics are the most dangerous for the power grid. While a sudden rise in the geomagnetic field may create high levels of GIC, if these currents last only a short time (minutes), is this as important as a lower-level storm that lasts for hours? This requires the accumulation of more magnetometer data during many different types of geomagnetic storms.

Third, more work is needed to determine if particular transformer designs are more susceptible to GIC with respect to both reactive power and hot-spot heating possibly leading to damage. While there is an understanding that certain transformer designs are affected more by GICs, more work is needed to make transformers less susceptible. There is also the problem that many transformers have been exposed to previous storms, thus shortening their lifetime. To understand this part of the problem, transformer design studies as well as testing and monitoring of transformers are needed.

The need for standardization

While part of the geomagnetic storm problem is understood, other aspects are evolving. Although the role of standards is not clear in trying to establish the level of a worst-case storm or a 1-in-a-hundred year storm, other portions of the problem do lend themselves to standardization.

The characteristics of the ground-based geomagnetic storm environment need to be standardized, and a preliminary effort is underway in Cigré (Council on Large Electric Systems) C4 to evaluate approximately 30 years of world magnetometer data to determine the basic waveform characteristics of B-[magnetic] fields at different latitudes on the Earth. While this effort will not result in a worst-case environment, it will provide some information regarding the typical levels of B-fields that can be expected over time. With the use of power grid modelling techniques, these B-field waveform characteristics can then be "translated" into GIC levels in power lines.

A second potential area for standardization is to develop design requirements for transformers to tolerate particular levels of GIC, and to determine whether protected designs might be appropriate for use in high GIC regions of the Earth. A related area is the development of appropriate test methods to ensure that such designs are properly verified.

Finally there are designs of equipment to protect existing power grid transformers from the GIC, including, for example, the placement of high-speed switching capacitors into the neutrals of large transformers. While some prototypes for these capacitors have been built, there is a need for test standards to ensure that these blocking capacitors achieve their performance specifications.

Solar flares on the Sun's surface (Photo: NASA) Solar flares on the Sun's surface (Photo: NASA)
Fig 1. Coupling process of the geomagnetic fields to long power lines Fig 1. Coupling process of the geomagnetic fields to long power lines
Fig 2. Geomagnetic disturbance conditions for the geomagnetic field in nT at 15:39 UT on 28 Oct 1991 Fig 2. Geomagnetic disturbance conditions for the geomagnetic field in nT at 15:39 UT on 28 Oct 1991