Geomagnetic Storm Impacts on the High-Voltage Power Grid:

Current understanding and mitigation concepts

William A. Radasky Metatech Corporation

Goleta, California USA wradasky@aol.com

Abstract—This paper summarizes the current understanding

of geomagnetic storms and their impacts on the power grid. Accomplishments over the past 23 years are reviewed, and the remaining aspects to be resolved are mentioned. The second part of the paper describes the basic mitigation concepts that can be used to minimize or eliminate these impacts through operational and/or protective measures.

Keywords—geomagnetic storms; coronal mass ejection (CME); geomagnetically-induced current (GIC); high-voltage power grid

I. INTRODUCTION This paper reviews the understanding of the impacts of

geomagnetic storms on high-voltage (V > 100 kV) transmission power grids from both a modeling and a data point of view. Papers over the past 23 years [1]-[11], [14] have indicated great progress in this understanding, although there are still some aspects to be resolved in order to have confidence that we can predict all types of impacts on power grids.

The second objective of this paper is to describe several methods to mitigate the impacts of geomagnetic storms on the high-voltage power grid including both operational measures and hardware protection.

It should be noted that Smart Grid aspects have not been evaluated in this paper, but it is clear that significant voltage disturbances and harmonics will make it more difficult to operate a more complex power system in the future.

II. SOLAR ACTIVITY The Sun produces many types of high-energy emissions

through its eleven-year cycle. These include well-known flares and coronal mass ejections (CMEs). Flares are very dynamic and they often (but not always) precede a CME, which ejects charged particles into space. As these particles travel slower than the speed of light, it typically takes 1-3 days for them to arrive at the Earth (if they are Earth directed).

Fig. 1 shows a summary of the value of the averaged monthly sunspot number for the last 70 years and compares them to a rough measure of geomagnetic activity measured at the Earth. The Ap index shown in the figure is a magnetic field variation parameter that is averaged over many observatories during a 24-hour period. Since geomagnetic field impacts on the power grid can occur in less than a few

minutes, the severity of the storms for power grids described by the Ap index is not considered to be precise.

It is interesting to observe in Fig. 1 that large geomagnetic storms at the Earth’s surface can occur at nearly any time within the sunspot cycle. This is because the CMEs are typically launched from the Sun at the location of a single sunspot, and the strength of the resulting geomagnetic storm at the Earth does not appear to depend on the number of sunspots present at the time of the CME.

Fig. 1. Plot of the averaged monthly sunspot number and the occurrence of “large” geomagnetic storms [10]

The 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. They are then ionized and create a horizontal flow of charge in the ionosphere as shown in Fig. 2. This structure is known as the Electrojet. Since the enhanced solar wind is fluctuating in time with the arrival of charged particles, this electrojet is also fluctuating in time on a time scale of seconds. This ionospheric current generates a magnetic field with the same time dependence as the current. It should also be noted that the vector direction of the current changes with time over time scales of seconds, resulting in magnetic field vectors at the Earth’s surface that also shift with time.

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Fig. 2. Coupling process of the geomagnetic fields to long power lines [10]

III. MODELING OF THE POWER GRID COUPLING

A. Basic Grid Modeling Approach Over the past 20 years, scientific and engineering work has

established that fluctuations in the solar wind created from coronal mass ejections (CMEs), coronal holes and even sudden storm commencement (SSC) events can produce fluctuating magnetic fields at the Earth’s surface. In order to model the induced quasi-dc currents in a power grid, it is necessary to know (or estimate) the variation of the magnetic field with time and location over the power grid. A time-varying magnetic field induces an electric field in the Earth based on the ground conductivity profile to a depth of 700 km. The electric fields must be coupled to each segment of high-voltage power line and all of the line and transformer resistances (including the grounding grid for the transformers) must be modeled. The next step is to compute the impact of the flow of the quasi-dc current in each transformer in terms of creating reactive power and harmonics and to determine whether a transformer hot spot will be formed, potentially damaging a particular transformer. All of these aspects will be discussed in more detail below.

B. Time Dependence of the B-field In the early 1990s this author and colleagues set up high

data rate measurements in a high-voltage substation north of Duluth, Minnesota to evaluate the relationship between the incident geomagnetic field fluctuations, the electric fields induced in the Earth, and the currents flowing in the local high voltage grid. Direct measurements of the magnetic field in 3 dimensions, the horizontal electric field in the North and South directions and the current flow in two high-voltage transformers were measured. Through the modeling of the Earth’s conductivity in the region, it was possible to develop a validated model that predicted the electric fields and the geomagnetically induced currents (GICs) in the power system within about 10% [2]. An important conclusion was that time resolutions of ~2 seconds or smaller were needed to accurately compute the electric fields and the GIC flow in transformers. This was an important discovery because the standard magnetic field measurement time step made by most observatories in the early 1990s was 60 seconds.

C. Importance of the Ground Conductivity Depending on the location of the power grid of interest, the

ground conductivity can vary with depth in different ways.

For some examples of incident geomagnetic fields, the peak electric fields may vary as much as a factor of 7. While 1-D profiles have been used with good results [2], there are some effects that may occur due to strong horizontal changes such as when an ocean boundary occurs and the sea floor drops rapidly [14].

D. Grid Modeling Fig. 3 indicates an example of a regional view of a single

time step computation during a relatively small geomagnetic storm in Japan. It illustrates the orientation and relative magnitude of the electric field at a particular time point, and the direction and magnitude of currents flowing out of (green) and into (red) transformers in substations (size of circles indicates the relative strength of current flow). This type of grid modeling requires that every line and transformer in the region be modeled including the vector direction of each segment of line. This is crucial since a real geomagnetic B-field rotates over time frames of interest, which causes the electric field to also rotate with time. The notion that the magnetic fields are always oriented north-south and the electric fields are oriented east-west is incorrect (see the example in Fig. 3). Also it is important to understand that an evaluation of a constant electric field in a given direction is not a reliable way to model the coupling to a complex power grid, as the currents induced are dependent on the vector relationships.

Fig. 3. Grid modeling display for Central Japan on November 6, 2001 with the instantaneous electric field vectors shown [10]

E. Validation of GIC Calculations In the development of geomagnetic storm models over the

past 20 years, it is important to perform end-to-end validations to ensure that one has captured all of the important details of the Earth conductivity and the modeling of the power network itself. Fig. 4 illustrates a typical comparison at a substation in Minnesota for a small storm. The measured geomagnetic fields from nearby magnetometers are used as the inputs for the calculation. While the GIC comparison is not exact, it is clear that the main features of the computed induced current in a 400 kV transformer are reproduced reasonably well.

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Fig. 4. Observed transformer neutral GIC and calculated GIC (in amperes) at the Forbes substation on February 21, 1994 [10]

F. Impact of Grid Voltage Level In the process of performing calculations over many years,

it became obvious that the largest GICs were induced in the highest voltage networks in the United States. The reasons for this can be explained quite easily.

Fig. 5 illustrates a very simple network consisting of a single straight three-phase circuit with a length of 100 km and a hypothetical induced peak electric field of 1 V/km.

Fig. 5. A simple transmission line geometry that shows the typical grounding scheme for a high-voltage transmission line and illustrates how the GIC will flow [10]

As indicated in Fig. 5, the current that flows in the network will depend on the voltage induced between the neutral grounding points of the transformers and the resistance in the network (the lines themselves, the transformers and the grounding grid).

Using average power line data for typical voltage levels and adding in other smaller resistances for the transformer windings and the grounding, one can compute the current flowing on average as a function of transmission network voltage. This is shown in Fig. 6.

Fig. 6. Average level of GIC in amperes/phase induced by a 1 V/km geomagnetic storm electric field in a 100 km long straight transmission line for estimated levels of circuit resistance typical for different transmission voltages [10]

Fig. 7. Relationship between GIC and reactive power demand in MVars for 3 different single-phase transformers [10]

G. Possible Damage of High Voltage Transformers With validated computational models it is possible to

evaluate very large grids against possible worst-case geomagnetic storms. Fig. 8 illustrates the possible impact on the three high-voltage US power grids due to a 1 in 100 year geomagnetic storm [10]. This calculation estimated that a mid-latitude storm with a dB/dt magnitude of 4800 nT/min could occur, and that EHV transformers would be at risk of hot-spot damage for GICs larger than 90 A/phase. The black lines illustrate the areas of likely blackout and that 365 EHV transformer were at risk of damage.

Fig. 8. Estimated impact of a mid-latitude 4800-nT/min storm [10]

While high-quality data do not exist regarding the exact level of failure of EHV transformers due to GIC threats, it is clear that some transformers have failed in the past at levels below 90 A/phase and others have survived this level. It is likely that the design type, the age and the historical stress on a transformer are contributing factors to hot-spot damage. Studies on this issue are underway at transformer manufacturers to increase the understanding of failure mechanisms and levels for EHV transformers.

IV. MITIGATION OPTIONS There are two main methods of mitigation to be considered

to prevent power grid blackouts and transformer damage from GICs. These include operational and hardening measures.

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A. Operational Measures All operational measures depend to some extent on the

ability to determine if a severe geomagnetic storm is about to arrive. Over the past decade, the NOAA Space Weather Prediction Center [12] has improved its ability to evaluate solar activity to determine whether a CME is Earth-directed and the likely time of arrival at Earth. However, the strength and locations on the Earth likely to be affected are not as well understood in advance. Given this information, however, it is possible for power companies to anticipate the arrival of a geomagnetic storm and to take actions to reduce the possibility of voltage collapse. These items could include:

1. Notification of system operators to increase situational awareness

2. Reduction of load due to industrial agreements 3. Increase in generator spinning reserve 4. Reduction of power transfers

It is noted however, that for each power grid operator, the best operational procedures will vary, and it is important that assessments of the impacts of different types of storms be evaluated and training of the operators be accomplished.

B. Hardening Measures The main method of protecting or “hardening” a high-

voltage power grid is to prevent the flow of quasi-dc current flow into the neutrals of the transformers. Kappenman was instrumental in evaluating the effectiveness of adding either resistors or capacitors into the neutral of transformers to reduce or block the flow of GIC in a high voltage network [9]. While this appears to be a simple approach, it is important that the transform neutral is able to pass high currents to ground if a fault or other transient such as from lightning occurs. In addition the operation of relays during transients may depend on the grounding situation.

Fig. 9 illustrates the basic design of such a GIC “blocking” device with the ability to bypass large currents due to AC fault currents. Note that in this design, the resistor module will not block the GIC but will reduce the GIC current due to the addition of resistance in the DC circuit shown in Fig. 5. If a capacitor is used instead of a resistor, then it could be possible to block the DC. In either case one could add a relay-actuated switch (as shown) to keep this blocking device out of the neutral circuit most of the time. When there was an alert of an arriving geomagnetic storm, then this switch could be closed.

In the United States the Federal Energy Regulatory Commission (FERC) has evaluated the issues of mitigation measures for the geomagnetic disturbance (GMD) and has required NERC to develop both operational and hardening standards to protect the U.S. power grid from future storms [13]. The development of these standards will be very important to the ability of high-voltage power grids to withstand a severe geomagnetic storm.

Fig. 9. Schematic of a Neutral Resistor and Bypass Device (NRBD) in a transformer neutral [9]

REFERENCES [1] J. G. Kappenman and V. D. Albertson, “Bracing for the geomagnetic

storms,” IEEE Spectrum Magazine, pp. 27-33, March 1990. [2] W. A. Radasky, M. A. Messier, J. G. Kappenman, S. Norr and R.

Parenteau, “Presentation and analysis of geomagnetic storm signals at high data rates”, 1993 IEEE International Symposium on EMC, August 1993, pp. 156-157.

[3] J. G. Kappenman, L. J. Zanetti and W. A. Radasky, “Space weather from a user’s perspective: geomagnetic storm forecasts and the power industry,” EOS Transactions of the American Geophysics Union, Vol. 78, No. 4, pp. 37-45, January 28, 1997.

[4] J. G. Kappenman, L. J. Zanetti and W. A. Radasky, “Geomagnetic storms can threaten electric power grids,” Earth in Space, American Geophysics Union, Vol. 9, No. 7, pp. 9-11, March 1997.

[5] J. G. Kappenman, W. A. Radasky, J. L. Gilbert and I. A. Erinmez, “Advanced geomagnetic storm forecasting: a risk management tool for electric power operations,” IEEE Plasma Society Special Issue on Space Plasmas, Vol. 28, #6, pp. 2114-2121, December 2000.

[6] I. A. Erinmez, S. Majithia, C. Rogers, T. Yasuhiro, S. Ogawa, H. Swahn and J. G. Kappenman, “Application of modelling techniques to assess geomagnetically induced current risks on the NGC transmission system,” CIGRE Paper 39-304, 2002.

[7] J. G. Kappenman and W. A. Radasky, “Too important to fail: the looming threats of large geomagnetic storms and other high-altitude disturbances with modern electric power grids may produce significant damage to critical infrastructure,” Space Weather Journal, 18 May 2005.

[8] “Severe Space Weather Events – Understanding societal and economic impacts workshop report,” National Research Council, 2008.

[9] W. A. Radasky and J. G. Kappenman, “Protection recommendations to deal with severe electromagnetic threats to the U.S. power grid,” Meta-R-325, Metatech Corporation, May 2010.

[10] W. A. Radasky, “Overview of the impact of intense geomagnetic storms on the U.S. high voltage power grid,” IEEE EMC Symposium, Long Beach, August 2011, pp. 300-305.

[11] J. L. Gilbert, W. A. Radasky and E. B. Savage, “A technique for calculating the currents induced by geomagnetic storms on large high voltage power grids,” IEEE EMC Symposium, Pittsburgh, August 2012, pp. 323-328.

[12] National Weather Service: Space Weather Prediction Center, www.swpc.noaa.gov.

[13] “Final rule on reliability standards for geomagnetic disturbances,” Federal Energy Regulatory Commission, Docket No. RM12-22-000, Item No. E-5, May 16, 2013.

[14] J. L. Gilbert, “Modeling the effect of the ocean-land interface on induced electric fields during geomagnetic storms,” Space Weather Journal, Vol. 3, S04A03, doi: 10.1029/2004SW000120, 2005.

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