Energy Security

Mitigating EMP and GMD Risks for Power Grid Resilience

Risks posed by geomagnetic disturbance (GMD) and electromagnetic pulse (EMP) events add challenges to an already complex power system environment. But they remain crucial to address. How prepared is the power grid to withstand these powerful natural and man-made disturbances?

On the evening of May 10, grid operator PJM Interconnection activated a rare geomagnetic disturbance (GMD) action after it observed “persistent geomagnetically induced current (GIC)” at multiple stations within its 13-state footprint. While part of a specific set of PJM operating procedures, the action marked a 14-hour period of high alert before it was downgraded to a GMD warning, which persisted for two more days.

The measure responded to a historic solar storm stemming from a large sunspot cluster that the National Oceanic and Atmospheric Administration (NOAA) had warned could rise to G4 and G5 levels on the Planetary K-index (Kp scale).

Sunspots (Figure 1) and other solar phenomena can produce large clouds of plasma that can induce electric currents in the ground and on high-voltage transmission lines. These currents can flow up from the ground or down into the earth through grounded grid equipment, mainly power transformers, PJM explained. “High levels of these ground-induced currents can cause conditions in power transformers that can result in stressed system operating conditions and potentially lead to blackouts,” it said.

1. NASA’s Solar Dynamics Observatory (SDO) captured this image of an X5.8 solar flare peaking at 9:23 p.m. EDT on May 10, 2024. The image shows a subset of extreme ultraviolet light that highlights the extremely hot material in flares. Source: NASA

NOAA uses the Kp scale as a physical measure denoting the severity of a geomagnetic storm event, essentially rating disturbances in the Earth’s magnetic field. The larger the K-index (7+), the more active the Earth’s magnetic field becomes due to a storm from the sun. The smaller the index (1-2), the quieter it is.

Associated G-scale values categorize K-index values to indicate the severity of a solar storm, much like hurricanes and tornadoes. A G1 storm is “minor” but may still prompt weak power fluctuations. In contrast, a G5 storm is “extreme” and has the potential to cause “widespread voltage control problems and protective system problems,” raising the threat of damage to transformers and grid system collapse or blackouts. While of lesser intensity, G4 storms are “severe,” pointing to “possible widespread voltage control problems” and potentially affecting some protective systems that could trip out key grid assets.

“To help anticipate problems, PJM’s members have installed special equipment to detect and measure GICs,” PJM explained. If NOAA issues a warning for a potential geomagnetic storm with a severity of K7 or greater, PJM initiates a GMD warning (see sidebar), alerting all generation members. If GIC measurements exceed operating limits (in amperes) at two or more monitored transformers, the grid operator initiates an action that re-dispatches generation to control the GMD transfer limits.

How Do EMPs and GMDs Affect the Grid?

While electromagnetic pulse (EMP) and geomagnetic disturbance (GMD) events aren’t frequently included in the array of threats currently assailing the North American bulk power system, their wide-ranging nature poses cross-sector risks. Though both phenomena create magnetic disturbances, their characteristics are very different.

GMDs Are Naturally Occurring. GMDs occur when the Earth is subjected to changes in energized particle streams emitted by the Sun. Coronal mass ejections (CMEs), which are eruptions of charged particle plasma from the Sun’s corona that can bombard the Earth within as little as 14 hours. When CMEs deposit energy into the Earth’s magnetosphere, radiation belts, and ionosphere, the Earth’s magnetic field is affected. This can induce currents, known as geomagnetically induced currents (GICs), in long electrical conductor systems such as electric power transmission and distribution lines (Figure 2).

2. Geomagnetically induced currents (GIC) on the grid. Source: U.S. Department of Energy

“GMDs take the form of slowly changing the Earth’s magnetic fields by inducing electric fields (relative to 60 Hertz [Hz]) along the Earth’s surface,” the U.S. Department of Energy explains. “For a relatively strong storm, the geoelectric field intensity might be only several volts per kilometer, but this could result in a difference of hundreds of volts between the ends of a long-distance line. The fields change so slowly that the induced voltage is practically constant from the perspective of the power grid (i.e., near-direct current [DC]).” Transformers and substations, however, require alternating current (AC) to function, so when this quasi-DC current flows through this equipment, it can disrupt their operation. And, if the quasi-DC currents are large enough, they may cause voltage collapse.

“DC currents that are very high, of long duration, and/or persistent can cause thermal damage to transformers,” the agency notes. “The quasi-DC GIC through the windings will slowly (over seconds or minutes) push a transformer into deep asymmetric saturation. Deep saturation creates severe internal thermal and mechanical stress in transformers. For grid AC voltage calculations, the saturation current is assumed to be mostly lagging; therefore, it can be approximated in a conventional AC power flow program (that utilities already have) as a very large GIC-dependent reactive power demand. The power flow program uses these reactive powers to resolve AC bus voltages and currents. Transformer saturation over a wide area can reach a point where bus voltages collapse, creating a blackout.”

EMPs Are Man-Made. EMPs involve the detonation of a nuclear weapon at high altitude or in space—at more than 30 kilometers above the Earth’s surface. Sometimes referred to as high-altitude electromagnetic pulse (HEMP) events, these events are typically categorized by pulse intensity. E1—a “fast pulse”—events consist of intense, short-duration EMPs characterized by a rise time of 2.5 nanoseconds and amplitude on the order of tens of kilovolts per meter (kV/m), and up to 50 kV/m at the most severe location on the ground. E2 events, which are similar to lightning, have field pulse amplitudes of about 0.1 kV/m, lasting 1 microsecond to 10 milliseconds. E3 events exhibit a “slow pulse”—at very low frequencies of below 1 Hz with amplitudes on the order of tens of volts per kilometer (V/km), lasting 1 second to hundreds of seconds.

According to EPRI, E1 EMP fields “can be quite large,” but the area of coverage depends on where the nuclear weapon explodes. “For example, a detonation at 200 km can affect a circular area of on the order of 3 million square miles. However, not all areas included within the circular region experience the maximum electric field, and strength of the field falls off with distance from the ground zero location,” EPRI says. Still, its impact can be severe. If it couples to overhead lines and cables, it can expose connected equipment to voltage and current surges, and radiate equipment directly. Potential impacts include moderate disruption or damage of electronics, including digital protective relays (DPRs), communication systems, and supervisory control and data acquisition (SCADA) systems, and they could affect “large areas such as an electrical interconnection.” E2 EMPs do not couple to overhead lines or cable, though they could couple to conductors through the air like an E1. Still, because the amplitude is low, “impacts to the transmission system are not expected to occur,” EPRI says.

E3 EMPs, perhaps, pose the most insidious threats to the grid, given that they induce low-frequency (quasi-DC) currents in transmission lines and transformers. “The flow of these geomagnetically induced currents (GICs) in transformer windings can cause magnetic saturation of transformer cores, which causes transformers to generate harmonic currents, absorb significant quantities of reactive power, and experience additional hotspot heating in windings and structural parts,” EPRI warns. “Potential impacts of E3 EMP on the bulk power system can include voltage collapse (regional blackout) and transformer damage due to additional hotspot heating.”

Glaring GMD Vulnerabilities

May’s heightened solar activity, with real implications for the electric system, is notable given the reliability concerns associated with the natural phenomena. While the power industry has been aware of GMD risks since the 1940s, its vulnerability to GMD events was prominently illustrated by a solar storm in March 1989 that prompted a severe GMD event that struck Quebec’s power grid and caused a blackout that lasted for nine hours (Figure 3).

3. A March 1989 solar storm resulted in the complete loss of a 1,000-MVA generator step-up (GSU) transformer connected to the 500-kV transmission grid at the Salem nuclear power plant in New Jersey. This image shows the extensive internal damage that occurred to the 22-kV low-voltage windings in the transformer. One expert suggested that within two years of the storm, 11 nuclear plants recorded failures of GSU transformers that were likely linked to the event. Courtesy: Metatech Corp.

That storm, which produced two periods of intensity that registered K9, induced DC ground current that saturated transformers and generated even-order harmonic currents that caused seven static compensators on Hydro-Quebec’s 735-kV network to trip or shut down. The events gave rise to system instability that culminated in the separation of 9.5 GW of generation from the adjacent La Grande generating stations and cascaded into a system collapse “within seconds,” according to a report from the North American Electric Reliability Corp. (NERC). In the months after the event, the industry suffered “an increased number of failed transformers,” some sources suggest.

Given GMD’s potential impact, grid operators, generators, and transmission owners are required to comply with reliability standards issued by the North American Electric Reliability Corporation (NERC) in 2014. As PJM noted, “In 2016, FERC [Federal Energy Regulatory Commission] approved an additional NERC reliability standard that identified a ‘benchmark GMD event’ against which asset owners and grid planners are required to assess their equipment and develop and implement mitigation plans.” This standard was further modified in 2018 to include a “supplemental GMD event” for additional assessment.

In 2021, NERC, in collaboration with EPRI and utilities, completed a GMD research work plan to better understand the risks posed by severe GMDs. This assessment confirmed the validity of using the benchmark GMD event for vulnerability assessments, refined regional conductivity models, and improved tools for assessing transformer thermal impacts. PJM stated, “That work affirmed the adequacy of the standards in place in assessing and reducing the risk of GMDs to the electric grid.”

In a briefing laying out the potential impacts of the solar storms in May, however, Lt. Col. Tommy Waller, who serves as president and CEO of the Center for Security Policy and is co-director of the nationwide Secure the Grid Coalition, suggested current mitigation may not be adequate. “The March 1989 Solar Storm that damaged the above transformer and blacked out Quebec is regarded as a ‘40-year’ solar storm (i.e. the moderate type that strikes earth roughly every 40 years),” he noted. “The 1859 Carrington Solar Storm is considered a much more powerful ‘100-year’ storm.” And while it is with “statistical certainty that the earth will experience another Carrington-class or larger event in the future,” NERC protection standards are “so low that it requires no action, no actual hardware mitigation for the utility industry to stop those ground-induced currents,” he said. Waller advocated for the installation of more effective technologies, such as neutral ground blockers, which could block the harmful currents that would otherwise travel through transmission lines and damage transformers.

EMP Threat Real but Limited

In the power industry, which is currently grappling with a “hypercomplex” risk environment as the energy transition gains ground, the prevalence of solar storms has generally overshadowed electromagnetic pulses (EMPs), another insidious threat with wide-ranging implications for the grid. EMPs, sometimes called HEMPs (high-altitude electromagnetic pulses), result from the detonation of a nuclear weapon at high altitude or in space. While the prospect of a debilitating EMP event has been at the center of doomsday discussions for decades (and not without fair reason), a three-year EPRI study on HEMP impacts concluded in 2019 did not support notions that they could cause crippling blackouts that could last for many months to years.

The research specifically suggested that while the combined effects of the initial and late pulses could trigger a regional service interruption, they would not trigger a nationwide grid failure. “Recovery times are expected to be similar to those resulting from large-scale power interruptions caused by other events provided that mitigations specific to the initial pulse are deployed. Possible damage to large power transformers was found to be minimal,” EPRI said.

The findings were thought significant, given the outcome of limited EMP field tests. These include the 1962 Starfish Prime high-altitude burst nuclear test 900 miles from Hawaii, which knocked out 36 strings of streetlights and set off burglar alarms, and tests by the former Soviet Union in Kazakhstan in 1961 and 1962, which reportedly caused damage to communications systems, the power supply, and safety devices. William Graham, chairman of the former EMP Commission, established by Congress in 2001, in July 2017 concluded in a report that “even for a low-yield 10 to 20 kiloton weapon, the EMP field should be considered for unprotected U.S. systems.”

As with all threats, the power industry has taken crucial steps to assess and act on potential EMP risks, leveraging a “defense-in-depth approach.” Industry champion the Electricity Subsector Coordinating Council (ESCC) acts as the primary liaison between the federal government and the power industry, focusing on deploying advanced tools and technologies, ensuring timely communication of actionable intelligence, and coordinating incident response efforts. According to the Edison Electric Institute, other efforts span risk prioritization and protection including engineering redundancy to avoid single points of failure, regular exercises, and planning such as GridEx, ClearPath, and Cyber Guard to test and improve response capabilities for different emergency scenarios.

Common Risk Mitigation Strategies

Still, scenarios involving GMD and EMP events have unveiled new opportunities for risk mitigation. A commonly cited best practice to mitigate risks from both threats involves hardening.

The Pacific Northwest National Laboratory (PNNL) and EPRI, in separate reports, recently highlighted the necessity of electromagnetic shielding to protect critical components. Recommendations include using shielded control/signal cables with proper grounding, modifications to substation control houses to enhance shielding properties, and employing conductive concrete for control houses. A specific guard against EMPs should also involve the hardening of transmission control centers using EMP-resistant materials, EPRI said.

The research groups also urged effective grounding and bonding practices to ensure GICs are safely dissipated. For EMP risk mitigation, EPRI suggested employing fiber optics-based communication and protection systems to alleviate electromagnetic interference. For GMDs, devices like neutral blocking devices (NBDs) and GIC reduction devices (GRDs) may be effective at blocking or reducing the flow of GICs into transformers and prevent impacts from part-cycle saturation. Another sound approach involves the deployment of low-voltage surge protection devices (SPDs), such as metal oxide varistors (MOVs) and hybrid SPDs, to shield electronic equipment from voltage surges caused by EMP and GMD events.

Among effective operational strategies, the research group underscored the importance of effective monitoring and early warning systems. That includes the continuous monitoring of space weather and real-time GIC data collection. Ensuring redundancy through spare parts is also another good practice. Keeping an inventory of critical spare parts, such as digital protective relays (DPRs) and high-voltage circuit breakers, may prove essential for quick replacement in case of damage from EMP or GMD events.

Finally, experts generally agree that the industry’s push to ensure adequate transformer spares is a step in the right direction. Gripped by a recent crisis that has increased manufacturing lead times, supply chain constraints, and the high costs of transformers, the power industry is already cultivating voluntary collaborative efforts for sharing spare equipment, like the Spare Transformer Equipment Program (STEP) and Grid Assurance. Utilities also generally have mutual aid agreements or other informal sharing efforts.

Sonal Patel is a POWER senior editor.

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