By Thomas F Armistead, Consulting Editor

Here comes the sun, and your plant is in its cross-hairs. The solar energy that sustains life on this planet cycles through pulses of activ­ity roughly every 11 years. The puls­es are evident in a crescendo of sun­spots, flares, and other eruptions as the orb builds toward what astrono­mers call the solar maximum. Then the eruptions wane, returning in another dozen years or so.

When one of the eruptions is aimed directly at Earth, the results can be catastrophic. Loss of reactive power is the most likely outcome from a severe solar storm centered over North America, according to a report released in 2012 by the North American Electric Reliability Corp. Significant losses of reactive power could lead to voltage instability and, if not identified and managed appropriately, power-system voltage collapse could occur. In response to the findings, NERC is developing and gradually implementing new standards for the continent’s bulk-power system.

How bad could it be? On Mar 9, 1989, the Kitt Peak National Obser­vatory on the Tohono O’odham Reser­vation in Arizona reported a powerful flare on the sun. The next day, an explosion on the sun ejected a cloud of electrically charged particles 36 times as large as Earth directly toward our home planet. The cloud arrived at 2:44 a.m. (Eastern), March 13.

The wave of electrons and protons washed over and around Earth’s magnetic field, was channeled into the magnetic field lines, converged at the poles, and induced electric cur­rents at the higher latitudes (Fig 1). Where the ground was conductive, the geomagnetically induced cur­rents flowed harmlessly through the ground.

But the resistive igneous bedrock of the Canadian Shield forced the current to seek easier conductance (Fig 2). It entered the electric grid through ground wires and propa­gated throughout Hydro-Québec’s system. Within 92 seconds it brought down almost the entire grid, knock­ing 21,500 MW off line. The blackout lasted more than nine hours, affected 6 million people, and cost $2 bil­lion.

Quebec’s system was the most obvious victim of that event, but bulk-power systems throughout North America, including the US, experi­enced more than 200 related trans­former and relay problems. The most serious of those was the loss, from overheat­ing and permanent insulation dam­age, of a $12-million, 22-kV genera­tor step-up transformer at the Salem Nuclear Plant, Hancocks Bridge, NJ (Fig 3). Even with a spare transform­er available, the plant was offline for 40 days, entailing millions of dollars in lost power sales and replacement power purchases, in addition to the replacement cost of the transformer.

Playing defense

To forestall a repetition or worse outcome, in January 2015, NERC filed TPL-007-1, “Transmission System Planned Performance for Geomagnetic Disturbance Events.” The standard requires North American ISOs and utilities to perform state of-the-art vulnerability assessments of their systems and equipment for potential impacts from a severe, once-a-century benchmark geomagnetic disturbance event (GMD) and to mitigate identified impacts. Mitigation could include changes in system or equipment design or the installation of hardware to monitor or reduce the flow of geomagnetically induced currents (GIC). TPL-007-1 was approved by the Federal Energy Regulatory Commission in September 2016 (Fig 4).

The requirements are designed to be implemented over a five-year period, said Mark Olson, NERC senior engineer in reliability assessments. Entities began implementing the standard in 2017 and must take several steps leading to completion of the vulnerability assessments and mitigation plans by 2022. At FERC’s direction, NERC is developing a revision to be labeled TPL-007-2. It will enhance the benchmark GMD event used in the vulnerability assessments and is due for completion by May 2018.

Another NERC reliability standard, EOP-010-1, was approved by FERC in June 2014 and took effect in early 2015. The EOP-010-1 standard requires grid operators to have procedures that can be put in place to reduce impacts of severe GMD events.

Anticipation

The National Oceanic and Atmospheric Administration (NOAA), which operates the National Weather Service, has a Space Weather Prediction Center in Boulder, Colo, which monitors solar activity. And like the National Weather Service, which forecasts hurricanes by observing precursors such as tropical depressions in the Atlantic, the Space Weather Center tracks the development of sunspot groups to forecast GMDs, said Bill Murtagh, program coordinator for the center.

When large, complex sunspot groups emerge, “they may create eruptive activity called mass ejections that can have impacts here on Earth,” he said. “Isolated, complex sunspot clusters will produce coronal-mass ejections (CME) of significance that will create sizeable GMDs here on Earth.”

The current 11-year solar cycle began in December 2008 and reached solar maximum in April 2014, Murtagh said. “In the waning stages of the solar cycle, we get quite a few coronal-hole high-speed solar wind streams.” Coronal holes are large areas on the sun with lower magnetic fields allowing increased solar wind, he said.

“Solar-wind instruments will typically measure in backgrounds around 400 kilometers per second. One of the high-speed streams of solar wind associated with coronal holes will sweep past the Earth and buffet the Earth’s magnetic field for a couple of days, sometimes up to about 600 km/sec, and that’s a moderate-level storm.”

At this stage of the cycle, we should expect an increase in “moderate-level GMD, but we will continue to see isolated strong GMD associated with sunspot groups,” Murtagh said. “We typically see a couple of big clusters emerge over the course of these waning years at the three- to five- or six-year point.” A big sunspot group emerged in early September, he added.

The next solar maximum is expected in 2025, but Murtagh would not predict the intensity of the ramp-up to the maximum. “The 11-year cycle in itself is obviously quite predictable,” but the GMDs are not. He compares predicting GMDs with predicting hurricanes at the beginning of hurricane season. “Sometimes you get two or three hurricanes and other times you can get 20 hurricanes in a season. In the sunspot cycles, we’ve seen very big cycles, like Solar Cycle 19 back in the 1950s and early 1960s, and then we’ve seen this cycle, which is actually the smallest cycle since the first decade or two of the 1900s.”

Geomagnetically induced currents are the terrestrial events caused by GMDs. “When a coronal-mass ejection with its own magnetic field that gets shot out from the sun hits Earth, it hits Earth’s magnetic field, so we get two magnets coming together,” Murtagh said. “The interaction of the magnetic fields ends up inducing currents above us in the atmosphere, above the ionosphere, and those currents above manifest themselves on Earth in the form of geomagnetically induced currents.”

Earthly effects

Earth is affected only when a CME is directed at Earth, but there is no way, given the state of the science, to forecast when a CME directed at Earth will occur. Instead, Murtagh expresses the situation in probabilities. If he detects a very large sunspot cluster in the middle of the sun, “my probability of an eruption of a CME to occur is at 80%.” Probability is as much as he can do with the science he has. Being able to identify a pre-eruptive signature that would allow more precise forecasting is one of the industry’s “holy grails,” he said. “We’re not there yet.”

Like the National Weather Service, Murtagh produces routine daily forecasts. If sunspot activity indicates an 80% probability, NOAA would announce that level of probability, as a local weather station might announce a 50% chance of a thunderstorm. But when the threat reaches a certain threshold, he starts putting out alerts and warnings, just as a weather station might announce a tornado watch or tornado warning.

“We do it the same way. That product would go out as a forecast, saying, ‘80% chance of an eruption today.’ Then when the eruption occurs, and especially when we see a CME that’s Earth-directed, we issue a ‘watch product,’” which, like a weather watch, alerts recipients to the potential for an active threat.

NERC asked EPRI for assistance in responding to FERC’s request for validation of the GMD standard, said Rob Manning, EPRI’s VP of transmission and distribution infrastructure, and the research organization is working with the electric utility industry to evaluate the standard in further depth, doing calculations and validations and technical basis for the standard.

A geomagnetic disturbance created by a solar flare has only one characteristic: geomagnetically induced currents, primarily on the transmission system, Manning said. It manifests itself as heating in the transformer cores, depending on how high the current gets, because it saturates ac transformer cores. The primary danger from a solar-flare-type occurrence would be the loss of transformation in the switchyard. A secondary outcome would be harmonics in the plant. “If you’re protected against harmonics in general, you’re probably pretty well protected against GIC,” he said.

EPRI sponsors Sunburst, a system of sensors deployed around North America, which measures GICs on the grid and reports them to utilities. “We can tell you at any time what the actual GIC current flows are,” Manning said.  “With a solar flare, we generally believe we would have adequate time to isolate transformers that are potentially susceptible to damage.”

Unless you get a particularly catastrophic solar flare, it would be relatively slow heating, so you would have several minutes to several hours to take a transformer offline, which would then completely protect it from GIC.

The net result from that might be an unscheduled shutdown of the generating plant if there were a solar flare and the operators were able to confirm two things: one, that they are seeing GIC currents flow in the Sunburst system and two, the transformer is showing additional heating. If you see those two factors, that might be enough to shut down the plant to save it. You can reboot it back after the solar flare is past.”

Distributed energy resources, energy storage, and other latter-day evolutions in the grid have not materially modified the risks of exposure to GIC. “The primary susceptibility to GIC currents is very long lines, radial lines,” Manning said. A very long radial line can accumulate GIC, and it can only go to the end of that line, where it can damage equipment. Microgrids and distributed solar power are not likely to be susceptible to GIC.

At risk

Transformers are the generation infrastructure most at risk from a GMD (side­bar), but other equipment also is exposed. “Potential effects include overheating of auxiliary transformers; improper operation of relays; heating of generator stators; and possible damage to shunt capacitors, static VAR (volt-ampere, reactive) compensators, and filters for high-voltage dc lines,” added an article in the spring 2011 issue of the EPRI Journal. 

Powerplant operators have at least two main ways to protect their equip­ment from damage in a geomagnetic storm, said Buddy Dobbins, director of machinery breakdown in the Risk Engineering Dept of Zurich Services Corp, Schaumburg, Ill: They can take equipment offline when NOAA issues a warning of a CME or they can hard­en transformers against geomagnetically induced current.

Completion of Reliability Standard TPL-007-2 by NERC and approval of the standard by FERC by 2022 will provide transmission planners with new tools to help ensure the bulk-power system’s ability to withstand and mitigate the effects of potentially damaging solar activity. CCJ

What happens in a geomagnetic disturbance

A report by the North American Electric Reliability Corp (NERC), Princeton, NJ, explained, “As the solar particles arrive at Earth, they cause rapid fluctuations of Earth’s geomagnetic field. This, in turn, produces an induced earth-surface potential and geomagnetically induced currents, or GIC. GIC appears as a quasi-dc current (an ac waveform with a period of several minutes), and for all intents and purposes, appears as dc to the bulk electric system.”

GIC is substantial, said Richard Lordan, a senior technical executive at the Electric Power Research Institute, Palo Alto, Calif. If you could insert a voltage probe in the East Coast and another in the West Coast, they could reveal a voltage differential across the US of as much as 10 V/km, he said.

“The consequences of this dc current is [sic] to drive transformer cores into saturation,” the NERC report continued. “This, in turn, causes significant heating from stray flux, increases VAr losses that depress system voltages, and can damage the transformer itself. Core saturation can also generate harmonic distortion that impacts other elements in the electric system.”

Those impacts “can jeopardize the integrity of the bulk electric systems” by causing overcurrent relays to trip capacitor banks, or static VAr compensators to trip for overcurrent or overvoltage protection, said the NERC report.

Tripping a large quantity of reactive resources during a GMD can further depress system voltages already reduced by transformer VAr losses. The result could be to precipitate a multiple-contingency incident in an electric system mostly designed only for single-contingency operation. In a word, it would cause an immense, long-lasting, extremely costly blackout.

As the loss of the Salem Nuclear Plant’s transformer attests, equipment damage is one possible consequence of such an event. NERC explained, “Depending on the location and concentration of stray flux internal to the transformer, heating of the oil, and relative equipment health, hot spots can emerge on tank or core locations, internal windings, and other structures within the transformer and can damage the transformer insulation systems.

“Further, during saturation, the reactive demands increase proportional to the transformer operating voltage (that is, a 765-kV transformer will produce twice the MVAr demand as a 345-kV transformer with the same level of geomagnetically induced currents), and it emits substantial amounts of both even and odd harmonics making traditional relaying challenging.”

Heat damage or torsional damage could also appear on the generator/turbine shaft system, said John Kappenman, a leading expert on GMD and owner of Storm Analysis Consultants, Duluth, Minn. “Protective systems on the generator may not be well suited to detect this threat.”

Really big CMEs affecting Earth are “quite infrequent,” said Kappenman, but “as we see more [sun] spot activity, we will see more coronal mass ejections from those spots.” Big CMEs actually occur all the time, he notes, but they don’t affect Earth unless they are directed at us. “It’s a bit of a Russian roulette game with the sun that one is playing here,” he said.