By Michael Valenti, Senior Editor

For years, utilities have concentrated on detecting, measuring, and reducing the emissions of the fossil fuels they burn to make electricity. More recently, they have taken aim at the unintended release of gases that leak from equipment in the power grid.

For instance, the GasVue GT20 laser imaging camera made by Laser Imaging Systems Inc. of Punta Gorda, Fla., detects leaks of insulating gases in substation circuit breakers. A catalytic sensor-based system developed by Gas Tech Inc. of Newark, Calif., enables power companies to analyze transformer gases on-site to diagnose transformer malfunctions.

GasVue is a laser-based gas detection system originally developed for chemical plants, oil refineries, and aircraft. It now helps utilities identify leaks of sulfur hexafluoride at their substations. SF₆ is widely used as an insulator in electrical equipment, particularly in high-voltage circuit breakers and switchgear. The gas is an ideal dielectric, or nonconductor of electricity, but although it is chemically inert, has been assessed as a contributor to the greenhouse effect. Recently, the U.S. Environmental Protection Agency in Washington has begun requesting electric utilities to voluntarily reduce SF6 leaks at their plants. Aside from environmental considerations, there are strong economic reasons for detecting SF6 leaks as soon as possible; a single bottle holding 115 pounds of the gas can cost $1,500.

Because SF₆ is colorless, utilities have traditionally relied on soap bubble or "sniffer" testing to locate SF6 leaks. This involves de-energizing the equipment, and spraying liquid soap on it. Gas leaks create bubbles. Sniffers are gas detection units that vacuum air samples from equipment to detect the presence of SF6. Both techniques require the leak inspector to be in close proximity to the equipment, which often means excessive climbing and reaching if the equipment is off the ground. In addition to being labor-intensive, soap bubble and sniffer testing are time-consuming and costly.

A decade ago, Tom McRae, a physicist working at Lawrence Livermore National Laboratory in Livermore, Calif., developed a laser-based detection system to pinpoint fugitive emissions from chemical plants and oil refineries. These so-called GasVue devices emit a wavelength of laser light that will be absorbed by a target gas. An integral infrared camera captures the light reflected from the target and displays it on a screen. Leaks show up as black. The GasVue records its findings on videotape.

In 1988, McRae left government service, formed Laser Imaging Systems, and received exclusive licensing privileges for the GasVue technology from the U.S. Department of Energy. The first two GasVue cameras were the MG30 and the TG5. The MG30 is a shoulder-mounted device that can be tuned to detect different gases, making it popular with chemical processors such as Union Carbide. It is capable of detecting leaks up to 50 feet away. The TG5 is a tripod-mounted version that is designed to detect SF6 used by aviation manufacturers, including Boeing and Lockheed Martin, to pressurize integral fuel lines in aircraft. The TG5 is more sensitive, but has a shorter detection range, about 15 feet.

"When SF₆ became an issue in the last couple of years, we began to demonstrate the TG5 in power plant substations," said McRae. "It became apparent that we would need to develop a new GasVue camera that combined the mobility and range of the MG30 with the SF6 detecting capability of the TG5 to service this application."

The Electric Power Research Institute of Palo Alto, Calif., the leading science and technology manager for the power industry, asked Laser Imaging Systems to develop a GasVue camera to identify substation SF₆ leaks and supported its development. A key challenge was shielding the new GasVue camera to function in the strong electrical and magnetic fields found in utility substations. As a result, the GasVue GT20 was launched commercially, and is now used by more than two dozen utilities, including Illinois Power, Entergy, Texas Utilities, Public Service Electric & Gas in New Jersey, and service companies including ABB and the National Grid of Britain.

Leak inspectors hoist the 16-pound cameras to their shoulders, aim at the equipment under study, and activate the GT20s by pressing a button. Any leaks are indicated in the display on the unit's video screen in real time, and are recorded on videotape so that maintenance personnel can pinpoint them for repair.

By using the GasVue camera, Eskom technicians in South Africa can inspect live power substation busbars for insulating gas leaks up to 50 feet away.

Eskom, the Johannesburg-based national utility that generates 98 percent of South Africa's electricity, uses the GasVue to inspect its substation equipment. The main benefits GasVue provides are "first, the ability to perform leak detection on live, in-service apparatus, and second, the dramatic reduction in time necessary to detect a leak site," said Luke van der Zel, chief consultant at Eskom.

The South African utility is a member of EPRI and previously used soapy water and halon sniffers to detect SF6 leaks from its gas insulated substation, or GIS, busbars. Eskom began using GasVue in 1998 and quickly learned of its benefits. "We knew there was a leak in a 50-meter GIS section, and located it using the GasVue camera, which saved us about a week of inspection time," explained Van der Zel. GasVue enables Eskom to inspect live parts of outdoor substation apparatus from ground level without the cost of an outage, and saving about one day of inspection per piece of equipment.

In addition to allowing quick inspection of long substation bus runs and live equipment above head level, the GasVue demonstrated its ability to detect very low leak rates of SF₆ that would be difficult to pinpoint using other detection techniques, Van der Zel said.

Another GasVue user is the East Kentucky Power Cooperative in Winchester, Ky., which serves the electrical needs of two-thirds of the state through its 17 member systems. "We relied on low-air alarms to identify switchgear equipment leaking insulation gas so that we could refill it, but had no way to pinpoint the leaks for repair. We contracted EPRI to perform inspections using the GasVue in the fall of 1999," said Paul Dolloff, an electrical engineer and technology consultant at the co-op.

With GasVue's accuracy, the cooperative was able to pinpoint and verify the leaks in the inspected switchgear. "The results of the GasVue give us the advantage of knowing the exact nature of the leaks, indicating what repairs are needed," explained Dolloff. "This will be a great help to us when we request bids for repairing these leaks. Now, both EKPC and the repair contractor will know exactly what each repair will entail. This provides more accurate estimates of repair cost and outage time, which is important for scheduling outages, and enables us to order the necessary repair equipment ahead of time."

While GasVue aids power companies in tracking sulfur hexafluoride leaks, the GT-Transformer Gas Tester was designed by Gas Tech to help utilities test transformers for the presence of combustible gases that can indicate transformer malfunctions.

Malfunctioning electrical transformers in power plants and substations can generate flammable gases such as methane, ethane, ethylene, acetylene, propane, and propylene in their inert headspaces. These gases are created by the electrical and thermal stresses placed on the transformer oil and solid, cellulose-based insulation, which causes both oil and insulation to degrade abruptly, forming these explosive gases.

East Kentucky Power Cooperative technicians use the GasVue camera to investigate sulfur hexafluoride leaks on substation equipment, improving the estimates for repair costs.

These gases pose a fire hazard if they build up sufficiently to support combustion, which is one of two reasons that substation operators are on the lookout for them. "The other reason for detecting transformer gases is that identifying the type and concentration of gas helps determine whether the transformer is experiencing any problems and how to fix them," explained Steve Lockridge, marketing manager at Alfa Transformer Co. in Fort Smith, Ark. "For example, a high volume of acetylene could indicate that the transformer is arcing due to degradation of insulation, or because a connection is loose." Alfa sells, repairs, and rents a variety of electrical transformers.

Utilities and their service companies have traditionally searched for transformer gas generation by collecting samples of transformer oil and sending it to a laboratory that would analyze it for dissolved gas content. This was a messy, time-consuming, and expensive process. Gas Tech developed its GT-Transformer Gas Tester to perform quick, real-time detection and measurement of transformer gases.

The GT testing system was commercially launched in June last year, and is a redesigned version of a previous Gas Tech transformer gas detector. The major challenges in improving the GT were understanding exactly how power transformers are configured, determining which target gases and ranges of detection are required, and how to best configure the instrument for sampling a transformer, according to Bruce Holcom, a chemical engineer and engineering manager at Gas Tech.

"Steve Peluffo of Gas Tech found answers to these questions by working with transformer owners who were using dissolved gas analysis as their primary test method," said Holcom. Peluffo also studied various protocols in place, such as the ones from the Institute of Electrical and Electronics Engineers and the U.S. Bureau of Standards, to ensure that the GT test system readings of the vapor phase of a transformer could be correlated to results from a dissolved gas analysis.

The GT system is equipped with a dilution fitting, located on the front of the instrument, to introduce oxygen into the gas sample line to ensure proper operation of the catalytic combustible gas sensor. This is necessary because the transformer headspace is typically blanketed with nitrogen or some other inert gas.

The operator attaches the GT test system to the sample opening in the transformer headspace and fills a three-liter Tedlar gas collection bag, closing the valve when the bag is filled. The operator then attaches the filled sample bag to the probe on the GT tester, so the internal sample pump will draw gas into the instrument for analysis.

The catalytic combustion sensor reacts with any combustible gases in the sample and sends its output to a microprocessor that converts the reaction to a reading of gas concentration, typically expressed in parts per million, or percent lower explosive limit. The reading is shown on the unit's liquid crystal display.


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