Transformers

 

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. Now the National Oceanic and Atmospheric Administration is warning the electric-power industry to prepare for unexpected events, because the next solar maximum is forecast to peak between 2012 and 2014.

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 time), 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 on the Canadian Shield, the resistive igneous bedrock forced the current to seek easier conductance (Fig 2). It entered the electric grid through ground wires and propagated throughout Hydro-Quebec’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 six million people, and cost $2 billion.

During the cold war, alarms occasionally were sounded in the news about the threat of attack by a lone hostile missile over the US, which with a thermonuclear explosion could emit an enormous high-altitude electromagnetic pulse, frying every piece of electronics in a multistate area and bringing our entire electricity- and electronics-dependent society to its knees. The feared attack never came from a hostile power, but the March 1989 solar eruption provided an object lesson in the vulnerability of North America’s electric power systems to such exotic incidents.

Quebec’s system was the most obvious victim of that event, but power systems throughout North America, including the US, experienced more than 200 related transformer and relay problems. The most serious 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.

A new maximum

“Space weather” is not unusual, and the storm that blacked out Quebec is far from being the most powerful on record. Now the sun is ramping up to a new maximum, which will produce a new wave of solar magnetic storms, threatening power systems globally. The solar maximum is accompanied by an increase in the frequency of two space weather events—solar flares and coronal mass ejections (CME).

Solar flares blast X-rays into space along with masses of protons and electrons traveling nearly at the speed of light. A CME is an immense, billion-ton bubble of plasma and magnetic fields travelling at speeds exceeding a million mph. These eruptions occur all over the sun’s surface, and almost always are directed into empty space.

Occasionally, however, a flare or CME that occurs on the surface facing the Earth strikes our home planet more or less directly. When that happens, the solar flare can disrupt short-wave communications; the CME creates a geomagnetic storm that engulfs the Earth, triggering spectacular auroras in the skies over the higher latitudes and an Earth-level geomagnetic disturbance (GMD) that can cripple modern electric systems.

GMDs of varying intensity have always struck Earth, but they have become especially destructive since the mid-19th century, when the invention of the telegraph led to the development of a system of wires that offered a path for geomagnetically induced current (GIC). Most often cited are the superstorms of May 1921 and September 1859.

The 1921 storm, considered a 100-year storm, generated auroras seen in Samoa, 13 degrees off the geomagnetic equator. In the US, it virtually ended telegraph service east of the Mississippi River and disrupted service in at least six cities in the West all the way to San Francisco.

The 1859 storm was the largest ever recorded. Known as the Carrington event for the British astronomer, Richard Carrington, who witnessed the white-light solar flare that generated it, the superstorm created havoc in the young telegraph network. Some operators were able to use their systems without batteries thanks to the induced current in the wires. Others scrambled to escape the fires that broke out in their offices when their wires melted and sparks from their keys ignited papers.

Those events occurred when electric power systems were far less developed and interconnected than they are now. Our understanding of the GMD threat to the bulk electricity system is still limited, said Mark Lauby, VP/director of reliability assessment and performance analysis for the North American Electric Reliability Corp (NERC), Princeton, NJ. The system’s vulnerability has grown with the increasingly interconnected and sophisticated grid, as well as with the burgeoning use of consumer electronics of all kinds (Fig 4).

The 1989 storm that brought down the Quebec grid and damaged a transformer in New Jersey was the alarm that awoke the industry to the threats from solar weather. It has been intensively studied for its lessons. In January 2011, NERC launched a task force to determine the industry’s baseline risk. Transformers now are “more robust,” said Lauby, but we still lack a statistical basis for defining a 100-yr storm and the theoretical maximum storm. Working out those definitions is part of the task force’s mission.

   Analysis of Arctic ice cores suggests that the Carrington event was a 500-yr storm, but astronomers say storms half as strong occur at about 50-yr intervals. Today, instead of telegraph systems, satellites will be among the most affected systems, in addition to the electric-power grid, which acts as an antenna with multiple ground points providing a path for GIC into the system. A report by the National Academy of Sciences estimated that the cost of recovering from a Carrington event today could be $1-$2 trillion, and recovery and restoration could take four to 10 years.

Space-weather forecasts don’t yet meet the AccuWeather standard, so predictions are more art than science. A panel of experts sponsored by the National Oceanic and Atmospheric Administration (NOAA) convened in 2009 on solar disturbances predicted sun-spot activity would be below normal, peaking in 2013. Some panel members, however, still predicted the coming solar max could produce a storm similar to the Carrington event. “There is no correlation between sunspots and CME intensity,” said Richard Lordan, a senior technical executive at the Electric Power Research Institute (EPRI), Palo Alto, Calif. Furthermore, “There are no organizations that I know of that predict the severity of the storms for a given cycle.”

There are two kinds of predictions of space weather threats, said Lordan. NOAA issues an alert when a CME occurs if it is likely to affect electrical systems. That gives about a day’s warning. The other prediction comes from the ACE space satellite, located at the gravitational center between Earth and sun, about 1 million miles out. When the CME passes, it monitors its characteristics and the information provides 30-60 minutes of warning for informed preparations by system and powerplant operators.

At risk

Transformers are the generation infrastructure most at risk from a geomagnetic disturbance (Sidebar 1), 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 EPRI Journal.

One lesson the industry has learned from the 1989 Quebec storm is that operators now widen the operational region of sensitive equipment to withstand a wider range of voltages, said Lordan. When the generator step-up transformer at a powerplant receives a GIC, it can produce harmonics creating imbalance in the rotor of a generator, he said. The immediate effect is small, but it can accelerate equipment aging.

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.

“The industry probably needs to look at how to harden its systems against this disturbance,” said John Kappenman, a leading expert on GMD and owner of Storm Analysis Consultants, Duluth, Minn. System hardening requires blocking a GIC from entering the transformer in the neutral-to-ground connection, he said. In 2010, while he was with Metatech Corp, Goleta, Calif, Kappenman prepared a report for Oak Ridge National Laboratory in Tennessee describing the alternatives.

Inserting a capacitive device in the transformer neutral-to-ground connection would block all dc flow, but could risk impedance changes and ferroresonance concerns on the network, it said. A simpler, low-ohmic resistor in the neutral-to ground connection in all transformers on the power grid would achieve only partial GIC reduction, but would be more reliable and less costly.

The resistor would reduce GIC by 60% to 70% for a parts-and-installation cost of about $8000, Dobbins said. “It’s relatively easy to install and thus prevent those effects.” Retrofitting all the most critical transformers in the US would cost $150 million to $200 million, he said. That cost compares favorably to the alternative possibility of losing a transformer. That is a potential problem because the manufacturing capacity for such large equipment is not available on the scale that might be required in the aftermath of a serious superstorm, he continued.

Some companies are developing the technology and have demonstrated the hardware. Phoenix Electric Corp, Boston, Mass, developed a GIC blocking device with Kappenman in the mid-1990s. In an EPRI-sponsored project in 1997, Phoenix installed two prototypes in transformers owned by Minnesota Power and simulated an injection of direct current into a 500-kV transformer.

“Those transformers didn’t like it,” said Stephen Simo, Phoenix SVP. The project’s main aim was to test the operation of the bypass in the neutral-to-ground connection that would pass an ac surge to ground before it could damage the transformer, while still blocking dc flow, said George Sweezy, Minnesota Power’s supervising engineer for system operation and performance.

The project lasted three years. Sweezy admits GIC is a concern for his utility, located as it is in a northerly latitude, but says series capacitors now installed on the region’s 500-kV transmission grid also protect the transformers.

Phoenix Electric has not installed any of the devices since then, but is reviewing the design and evaluating the market for them, said Georgia Beyersdorfer, manager of marketing. “Scientists believe the problem has grown dramatically,” said Simo. If another coronal mass ejection blasts the Earth, “we are not prepared to lose transformers in the grid.”  

Phoenix’s new blocking device can be activated by the utility when a solar weather forecast warns that GIC is expected. The default setting is in the bypass mode, but the utility can set the dc blocking mode and bypass to operate in microseconds if the device detects an increase in ac larger than 8 kV, said Simo.

After the bypass operation, the utility can return the device to blocking operation, much as it would reclose circuit breakers after a surge (Fig 5). Simo said he thinks the device will be ready for market about mid 2012.

Two other US utilities queried for this article said they are aware of the potential threat from CME, but they have not invested in equipment to harden their systems. “Geographically, we’ve seen very little. We just really haven’t encountered” GIC, said Susan Gallager, spokeswoman for Ameren Corp, St. Louis, Mo. “We track solar activity and subscribe to services that monitor and forecast this activity,” but “we haven’t experienced any difficulty in our recent history,” she said.

Southern Company, Atlanta, also takes a low-key approach. “Solar flares are most likely to impact high-latitude regions of the Northern Hemisphere, which are not inside Southern’s service territory,” said spokeswoman Stephanie Kirijan. The utility “continues to investigate how increased magnitudes of solar flare events might ultimately impact electric grid operations,” and is working with both EPRI and NERC on studies of GMD effects, she said.

Seeking shelter from the storm

Ontario Power Generation, the largest generation utility in the Canadian province of Ontario, is well aware of the problem, being a neighbor to Hydro-Québec and, like that utility, sitting on the Canadian Shield. Ontario’s power grid is operated by Hydro One and is controlled by the Independent Electricity System market operators.

Gian di Giambattista, OPG’s director of emergency management and business continuity, said “the transformer is the most critical piece of equipment” that requires protection from GMD in OPG’s inventory, and he is focused on understanding what happens within transformers when they occur. He said OPG is working with NERC and its Ontario industry partners on ways to protect powerplant equipment.

Citing company policy on security, he declined to say how OPG protects its equipment from hazards and threats. “I can substantiate that since the 1989 [solar magnetic disturbance] cycle there has been a lot of R&D and the understanding how to mitigate them is much better today,” he added.

In May 2011, however, a NERC advisory warned, “While the impacts of geomagnetic disturbances in the Northern Hemisphere have been primarily observed in the northern latitudes, a severe GMD can reach the central and southern portions of the US.”

In such an event, NERC would call on generation operators “to increase real and reactive reserves to preserve system integrity” through measures such as reducing generator loading, evaluating the generator redispatch mix to be implemented, and bringing on line equipment such as generators, synchronous condensers, and static VAr compensators that can provide reactive power (Sidebar 2).

Since 2008, the Dept of Homeland Security, NERC, and other stakeholders have actively studied and consulted on the potential threat from the coming solar max. The NERC task force on GMD now “is investigating and fully vetting bulk power system reliability implications of geomagnetic disturbances, assessing available studies and associated models, and developing interconnection-wide technical solutions that are complete and accurate,” wrote Gerry Cauley, NERC President and CEO, in a letter to industry in June 2011.

The task force is trying to determine what actually constitutes a 100-yr storm and what is the theoretical maximum of a GMD, said Mark Lauby, NERC VP/director of reliability assessment and performance analysis. The 1921 storm has been called a 100-yr event, but that is “speculative,” he said. “We have little working data before 1989.” The challenge to the industry is that our ability to forecast storms is low, he said. The GMD task force is scheduled to complete its final report and issue recommendations in 1Q/2012. “They will probably be suggesting additional work,” he said.

NASA and EPRI are working on improving the short-term forecasting capability and have completed study of a forecasting system called Solar Shield. “One of the major near-future updates will include capability to model and forecast GIC also at lower geomagnetic latitudes,” said Antti Pulkkinen, associate research scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md. “We are also supporting NERC Geomagnetic Disturbance Task Force activity in terms of providing geophysical estimates for 100-yr GIC events. These results can be used in further engineering analyses carried out by power engineers.”  CCJ