By Rich Heidorn Jr.
A Government Accountability Office report on geomagnetic disturbances released last week found a lack of consensus on how much of a risk they pose to the U.S. electric grid, in part because of limited modeling capabilities.
GMDs, which occur when the sun ejects charged particles that change Earth’s magnetic fields, can cause geomagnetically induced currents (GIC) that produce voltage instability and damage connected equipment.
Although such coronal mass ejections occur regularly, GAO said there have been only four GMDs worldwide since 1932 that significantly affected the grid with large-scale service disruptions or equipment damage. The only instances in the U.S. were GMDs in March and September 1989 that damaged four single-phase transformers at one power plant, with no loss in electric service.
“The magnitude of potential damages from a large GMD is not fully understood, in part because there have been few examples worldwide of GMDs that have caused equipment damage or large-scale blackouts,” GAO said. “Determining how GMDs will interact with and harm the electric grid is challenging because the magnitude of the ensuing GIC is influenced by several factors. The reaction of specific components of the electric grid to GIC and its secondary effects is also challenging to accurately model.”
GAO said there are “key gaps” in the understanding of variables that impact severity, such as data on local geoelectric fields. The U.S. Geological Survey has only 14 ground-based observatories measuring local magnetic fields.
“The relatively sparse coverage of magnetic observatories, particularly in the contiguous United States, limits the ability to monitor GMD in areas without magnetic observatories,” GAO said. “Even when the GMD is measured at nearby magnetic observatories, Earth resistivity required to calculate the geoelectric field … is often the dominant source of uncertainty in GIC calculations. … Earth resistivity varies by about a factor of 10,000 within a Midwest region otherwise described by a single, one-dimensional ground resistivity model.”
Because extreme GMDs are rare, researchers have attempted to extrapolate the impact of extreme events from available data on moderate events. But GAO said, “Researchers at Los Alamos National Laboratory found that the probability of extreme events is not accurately described by statistical models of historical records.”
The worst-case scenarios from a solar-induced GMD — or an electromagnetic pulse produced by the detonation of a nuclear device 25 to 250 miles above Earth’s surface — sound like the stuff of disaster movies.
“A large GMD might have long-term, significant impacts on the nation’s electric grid,” GAO said. “Given the interdependency among infrastructure sectors, such a disruption to the electric grid could also result in potential cascading impacts on fuel distribution, transportation systems, food and water supplies, and communications and equipment for emergency services, as well as other communication systems that utilize electrical infrastructure.”
But the auditors said recent research suggests that the worst GMDs might have only limited impact. “The most persuasive studies we reviewed concluded that the most likely effects of a large GMD would be service interruptions that are neither long-term nor large-scale,” GAO said.
Two National Laboratory studies that evaluated the impact of an extreme GMD event on the Eastern and Western interconnections concluded “that the disconnection or loss of transformers experiencing high GIC would avoid equipment damage and maintain grid stability. … It is possible to use operating procedures or GIC-blocking technologies to protect transformers and grid stability.”
NERC cited operational procedures such as increasing operating reserve margins, modifying protective relay settings and removing vulnerable equipment from service.
A study by an unnamed electric power supplier “concluded that failures in generators or capacitors are unlikely during a 100-year storm,” GAO added.
NERC’s Geomagnetic Disturbance Task Force concluded that the most likely worst-case system impacts from a severe GMD event would be voltage instability and potential blackouts. But GAO noted that “blackouts that originate in the transmission grid in the absence of substantial equipment damage are generally restored within three days and often much sooner.”
FERC, NERC Actions
GAO’s findings on the limited data echo frustrations FERC and the Department of Energy have expressed.
In 2016, DOE said traditional power system planning models are flawed because they do not include substation grounding or transformer configuration details, which are essential to modeling GIC flows.
In November, FERC approved NERC’s revised GMD reliability standard, which broadens the definition of GMDs, requires grid operators to collect certain data and imposes deadlines for corrective actions (RM18-8, RM15-11-003). (See Revised NERC GMD Standard Approved.)
The standard seeks to create a benchmark for estimating the impact of a large GMD. But GAO said “conducting such estimates is challenging because the wide variety in transformers, including model, age and power capacity, could lead to significant variability in the effects [of] GIC on specific transformers.”
At FERC’s direction, NERC has joined with the Electric Power Research Institute to develop a research plan to improve the benchmark GMD event and Earth resistivity models.
An October 2016 executive order by President Barack Obama directed DOE and the Department of Homeland Security to develop a plan to test and evaluate technology that could mitigate the effect of GMDs. The GAO report came in response to a request by the Senate Committee on Homeland Security and Governmental Affairs to examine the availability of such technologies and the challenges of using them.
DOE told the auditors that it completed a plan for a pilot program to test commercially available technology in April and has hired contractors to implement the plan.
The GAO researchers reported that three-phase transformers may be less vulnerable than single-phase units, but it said the larger, heavier three-phase units present shipping challenges.
GAO said series capacitors, used to improve the transfer capability of long transmission lines, can also block GIC. “However, care must be exercised in placing series capacitors in the electric power transmission system because blocking GIC in one section of the grid can affect GIC flow in other sections of the electric power transmission system. Therefore, it is necessary to evaluate the effect of series capacitors in sections of the electric power transmission system on other sections of the electric power transmission system before they are installed,” GAO said.