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Solar Flares Threaten Utilities
[ 录入者:admin | 时间:2010-05-28 18:03:42 | 作者: | 来源: | 浏览:2326次 ]

Solar Flares Threaten Utilities


 


Trouble In the Sky!
Solar Activity May Cause Problems for Utilities

By Ron Behrens, P.E.,
The Hartford Steam Boiler Inspection and Insurance Company

Are you ready for GICs (Geomagnetic Induced Currents)? As if hurricanes, earthquakes and fears about Y2K bugs weren't enough, many scientists indicate that we have entered the peak period for solar flares and solar storms (January through April 2000). These storms could result in disruptions of satellite communications and less reliable electric power.

Other researchers predict that potentially serious activity will extend through 2001 and the early years of the new decade. This pattern of sun activity recurs every 11 years. The odd-numbered cycles have a record of being more active and intense than the even-numbered cycles. The current cycle is "Cycle 23," and it just started its solar flare peak activity — perhaps the highest in history.
Danger from the Sun
What does this mean? Satellites might have trouble, including spinning out of orbit. Communications may be blocked, including pagers, television, wireless telephones, radio and global positioning signals (GPS). Even astronauts could be at risk. Geomagnetic induced currents in power transmission lines, particularly in susceptible areas such as the Northeastern United States, may result in large-area electric power blackouts. Large, expensive utility transformers could be permanently damaged. For utilities, any power interruptions due to solar storms will put pressure on the remaining grid. Chicago, New York City and other areas of North America already had their share of interruptions in the summer of 1999 without needing any added surprises from solar activity.

But it’s not all doom and gloom. U.S. utilities, owners of large transformers and insurers have never before had the sophisticated advance warning tools they have today. This article discusses when and where GICs might occur, and lists several steps that will help utilities and satellite owners eliminate the potential for loss from GICs and solar-related disturbances.

What Are GICs?

Solar flares usually originate from solar active regions also associated with sunspots. Geomagnetic effects from flares occur when the flare accelerates waves of material and radiation energy into interplanetary space away from the sun and toward the earth. This solar wind package travels the 93 million miles through space and can impact the earth's magnetic field.

The earth's magnetic field then undergoes a period of fluctuations. These fluctuations can induce electric currents within conductive material on or near the earth's surface — e.g., ocean salt water, mineral ore and soil, and long utility transmission lines in areas where the ground soil is rocky and highly resistive to current flow. These events are collectively known as geomagnetic disturbances.

The delay time between the flare and disturbance arrival can be anywhere from one to four days. The variations can be measured by ground-based and satellite magnetometers, but lead time for loss prevention action is very short.

Damaging Earth Currents

The earth is a conducting sphere with a corresponding magnetic field. When solar plasma is spit our way, it flexes the earth's magnetic field, and this can induce voltages (and currents in closed circuits).

Perhaps you remember a science project where a wire was moved through a strong magnetic field and a voltage was detected between the ends of that wire. The same thing happens with geomagnetic disturbances, except the wire is a power company's transmission line, and instead of moving the wire, the magnetic field is moving while the transmission line remains stationary.

The frequency of the science project voltage depended on how fast you could reverse the direction you moved the wire. During severe geomagnetic storms, values of 2 to 10 volts per mile can be induced in transmission lines with corresponding GICs in excess of 100 amperes. The frequency of GICs is very low (one to a few milliHertz) compared to our normal line frequency of 60 Hertz — and that's part of the problem, at least for large transformers.

I've probably oversimplified the mechanics of earth's reaction to solar disturbances. The truth is, the physics are still not fully understood. Geomagnetic storms subside gradually in most cases, with an average duration of 24 hours. But new storms will come and go, especially during the most violent and disruptive part of the 11-year cycle (January 2000 through April 2000).

Why Are GICs a Concern?

The GICs that are especially undesirable are those that end up on utility transmission lines. In the United States, there are about 500,000 miles of bulk transmission lines. Electric power systems become exposed to GICs through the grounded neutrals of wye-connected transformers at the ends of long transmission lines. The low frequency of the GICs saturates the transmission transformer's steel core.

Can you remember 25 or 50 Hertz motors? If so, one physical feature that becomes obvious is that for a given horsepower, and all other items being equal, a 25 or 50 Hertz motor was much larger in physical size — it had more steel laminations. Without the extra steel laminations it would run too hot.

When transmission transformers are exposed to a GIC component, they are likely to overheat, even if the low frequency portion is only a small, almost insignificant portion of normal line current. When a transformer saturates, it becomes a source of harmonics. It also increases the inductive VARs (Volt-Amperes Reactive) power drawn by the transformer, and there is a high likelihood of stray leakage flux, eddy current losses, and excessive localized heating.

High harmonic levels can overload capacitors and interfere with operation of power system protective relays. Protective devices such as overload and voltage balance on capacitors and compensators may trip them off line, creating a domino effect.

VARs are a measure of the non-usable power drawn by the system and transformer. For utilities, the increased VAR draw/swing is measured in MVAR (Millions of VAR) and can cause problems with electric generator's VAR limits. Worst case is generator shut down.

According to Robert J. Ringlee and James R. Stewart of Power Technologies, in their Institute of Electrical and Electronics Engineers (IEEE) paper, "Effects of Geomagnetic Disturbances on Power Transformers," if a transformer core saturates, the eddy loss density on transformer steel surfaces may rise to 30 watts per square inch, nearly the thermal flux density of an electric range element! This has a detrimental effect on the insulation systems within the transformer, both liquid and solid. Combustible gases also would be generated within the transformer tank.

Blackouts and Communication Failures

In 1989, during the last solar cycle, geomagnetic disturbances damaged utility-owned electrical equipment in Virginia and the eastern United States. Also, 6 million people were left in the dark by a March 13 blackout in the Hydro Quebec system. The U.S. system came close to experiencing a similar outage. Less severe solar storms impacted earth in 1989, 1991 and 1992. In 1998, a smaller solar storm was blamed for bringing down the Galaxy 4 satellite, halting news transmissions and electronic pagers (45 million) across North America for days.

Large extra high voltage transformers (e.g. 500 kilovolts) static VAR compensators, and relay systems are the most susceptible to adverse influence due to GICs.

The presence of even small levels of GIC (20 Amps or less) can cause a large transmission transformer to saturate. The saturation of the core steel results in stray flux that can produce severe transformer heating. In an incident related to the March 1989 storm, a 500 kV, 1200 MVA utility-owned generator step-up transformer at a nuclear plant in New Jersey was damaged beyond repair. Cause was reported as hot spot overheating due to stray flux.

In relay and protection systems, geomagnetic disturbances can cause false operation (harmonic currents are misinterpreted by the relay as a fault or overload condition), failure to operate, and slower than desired operation. Static VAR compensators are essential on many power company system grids for voltage control and system stability. With their loss, voltage would drop and frequency may increase, and the system would be unstable. Certain transmission lines would trip to try to stabilize the system. If this did not occur, the entire grid might collapse and result in a system blackout. Assuming no major equipment damage, power could probably be restored over a 12-hour period.

Bringing everyone back on line is not always as easy as it sounds, even if there were no equipment damage. Typical in-rush currents for start-up are 600 percent the normal loads. In addition, blackouts are likely to cause transients voltage spikes that stress and weaken the system components, such as circuit breakers, transformers, and generators. In some cases, it may take days before power is fully restored.

Loss Prevention Is Key

Transformers are failing at an above-average rate in areas of the United States particularly prone to geomagnetic disturbances, according to John G. Kappenman, in charge of Transmission Power Engineering at Minnesota Power, Duluth, Minn. Also, the mean time-to-failure is shorter in the GIC susceptible regions of the United States. This is a major concern to insurance companies that insure transformers, generators, and other electrical equipment for utility companies.

"Our engineering risk model takes into consideration the geographic location of large transformer units," said Hartford Steam Boiler's Matthew Glennon, assistant vice president for HSB Loss Control. "We have always taken a proactive approach to loss prevention, and this is no exception. Our energy unit works with utilities on a daily basis; that's their specialty. They are very aware of the potential impact."

Of particular concern to utilities and insurers are GIC events that last for several hours over several days. The extended period (and deteriorating heating effect) increases the likelihood of insulation damage and premature failure. Thermal damage is cumulative, and that would support the findings of above average failure rates in GIC prone areas.

Loss Estimates in the Millions

Until 1989, these losses were probably investigated and erroneously attributed to overheating causes other than GICs as the root cause. In fact, there is no long-term evidence of GIC-caused transformer problems. However, these facts remain:

The failure frequency of generator step-up transformers is much higher in the GIC susceptible regions of the United States than elsewhere.
The mean time-to-failure is also shorter in the GIC susceptible regions of the United States.
Conservative cost estimates of equipment replacement due to GIC events put the total damage to the industry at upwards of $100 million. Also, the Oak Ridge National Laboratory assessed the potential economic damage of a widespread blackout in the Northeast United States as a result of a slightly more severe storm than the March 1989 storm at $3 billion to $6 billion.

Where GIC Damage Might Occur?

GICs are more likely to occur in regions of the country with low earth conductivity; for example, areas with igneous (high resistivity) rock. The Northeast United States is prone to GICs, in particular the New England states down through the Chesapeake Bay area.

From an equipment standpoint, the principal concern is extra high voltage grounded-wye transformers. Three-phase core form transformers are less prone to GIC induced saturation than three-phase shell-form transformers. But both core-form and shell-form single-phase transformers are susceptible to GIC induced saturation. Transformer damage and risk of failure rises with prolonged operation under saturation conditions.

Early Warning Systems

The National Oceanic and Atmospheric Administration's Space Environment Center (SEC) in Boulder, Colo., continuously monitors solar-geophysical activity. Alerts, warnings, and forecasts concerning the state of the space environment, daily predictions and data summaries are available from the SEC in real time on a variety of communication networks.

In August of 1997, NASA’s Advanced Composition Explorer satellite was launched that detects bursts of solar energy. It is positioned 1 million miles upstream of the earth's magnetic field and sends warnings back to earth, giving about an hour's notice. This will give power companies time to align circuits to minimize or avoid damage from electrical surges.

Most large utilities now get these warnings, as well as updated forecasts on the probability of magnetic disturbances. The SEC also issues 27-day and long-range forecasts. Canada and the United Kingdom have similar warning programs. Although little data was available in 1989, today huge databases in Japan, France and the United States are updated hourly with the latest satellite data and from ground-based observation stations worldwide. We've come a long way.

Partial Sample Geophysical Activity Forecast from SESC/USAF for October 4, 1999:
"THE GEOMAGNETIC FIELD IS EXPECTED TO RANGE FROM QUIET TO ACTIVE LEVELS FOR THE NEXT THREE DAYS. TWO CORONAL HOLES, WHICH ENLARGED SINCE LAST ROTATION, ARE MOVING INTO FAVORABLE POSITION FOR THEIR HIGH SPEED STREAMS TO IMPACT THE EARTH."

Steps to Protect Against GICs

Capacitor and compensator protection circuits can be adjusted to make operation more reliable during magnetic storm activity. Utilities also can monitor transformer neutral current to initiate a critical alarm so the transformer can be removed from service to prevent overheating and possible catastrophic failure. But loss prevention action must be fast to avert any physical damage.

In the Hydro Quebec chain of events, the resulting blackout had a total elapsed time of only one and a half minutes. It is a good example for proactive action rather than a reactionary plan. Still, the addition of inexpensive GIC monitoring to better assess the root cause of equipment damage and/or incidents is a good idea.

In addition, satellite operators can power down equipment or send corrective signals to their spacecraft that are in the path of incoming energy waves. Much of today's satellite communications technology is relatively new and has not been exposed to maximum solar activity. Time will tell how vulnerable they are to solar storm disturbances.

Want More Information?

Visit these web sites for in-depth studies and current solar conditions:

"Lessons Learned from Solar Cycle 22 and Outlook for Cycle 23," an IEEE paper by John Kappenman, May 1996: (www.mpelectric.com/storms/index.htm). Many of the compiled facts and figures come from Kappenman’s paper.

The Space Environment Center, National Oceanic and Atmospheric Administration, U.S. Department of Commerce (www.sel.noaa.gov/index.html). Check out today's space weather. This page has hundreds of links, including international partners, with extensive information about solar weather.

Ron Behrens, a director of Loss Control for The Hartford Steam Boiler Inspection and Insurance Company in Chicago, has more than 23 years of insurance and engineering experience. He is a licensed Professional Engineer (Illinois) and earned an Electrical Engineering degree from Valparaiso University. Ron also received the Associate in Loss Control Management designation from the Insurance Institute of America, is a certified infrared thermographer, a member of the IEEE and the National Society of Professional Engineers (NSPE).

 
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