The consensus among climate scientists is that the atmosphere's natural insulating property — the greenhouse effect — is enhanced by a rise in the concentration of gases released as a result of human activities.

Among these so-called greenhouse gases are carbon dioxide (CO₂), methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulphur hexafluoride (SF₆). In sufficient quantities, these gases are responsible for increases in global warming. These gases are targeted for attention by the Kyoto Protocol, in an effort to reduce their impact on the world's climate. For its part, the United Kingdom agreed to reduce its gas emissions to 12.5% below 1990 levels by between 2008 and 2012. These targets are important to National Grid because of its electrical equipment that, under abnormal circumstances, may emit these gases.

Controlling SF₆

Its high dielectric strength, chemical and thermal stability, and nontoxic and nonflammable properties make SF₆ gas an excellent dielectric for high-voltage equipment. However, authorities identify SF₆ as a potential threat to the environment. It is a potent greenhouse gas with about 24,000 times the global-warming potential as compared to CO₂. Because of its extremely stable chemistry, SF₆ emissions accumulate in the atmosphere for hundreds of years. It is National Grid's only process-related emission within the guidelines on reporting greenhouse gases.

The effective management of SF₆ usage relies on the accurate determination of the amount of gas required to replenish leakage losses. National Grid's objective is to minimize the release of the gas into the atmosphere. To this end, procedures and systems, whose purpose is to control and manage its use, produce information that feeds directly into the work-management system. The efforts to reduce gas losses use mobile gas-handling equipment (Fig. 1) to recycle the gas during maintenance and use new special-purpose weighing trolleys (Figs. 2 and 3) to provide accurate weighing of the gas. Together with the frequency of topping up, this information gives an estimate of leakage rates for prioritizing leak detection and repair work.

The EPRI gas view camera (Fig. 4) is an improvement in the technology for identifying leaks and makes leak repair a priority during maintenance. The device is an infrared camera with an infrared laser tuned to a strong absorption wavelength of SF₆ at 10.5μ. The laser scans across the field of view, producing a gray-scale picture on a standard TV monitor on which clouds of SF₆ gas appear as black.

Gas leaks, assessed using the camera at 50 locations during the year 2000, revealed that certain types of equipment, such as current transformers, suffer from several similar types of leaks. These leaks, occurring mainly from the filling valves, account for nearly half the number of leaks. In addition, other leakage areas have been found at pipework couplings and large GIS trunking flanges.

Gas Leak Rectification

The majority of leaks found on small pipework fittings are handled successfully by merely tightening them. Field personnel use two other techniques when tightening is not effective. The first involves the application of a thin film of jointing compound such as Loctite 542. One word of caution, use great care to ensure that excess compound does not enter the pipes. The second technique encapsulates the leaking fitting. However, it's difficult to prevent gas bubbles from forming a leakage path through the sealing material before it cures. This is especially true if the procedure is carried out when the pipe is under usual gas pressure. Note that the future breaking of these joints is difficult.

Leaks can occur at GIS enclosure flanges where the most common cause is corrosion between the mating faces. For flange leaks, there are two main options. The most reliable option is to remove the gas, dismantle the enclosure, clean the surfaces, replace the O rings and seals, then remake the joint. This technique, virtually guaranteed to be successful, is expensive and time consuming because it involves taking an outage and dismantling a considerable section of the GIS. The other option, as with pipework connectors, is to attempt a repair without dismantling by encapsulating the leak (Figs. 5 and 6).

The environment continues to be high on National Grid's agenda, with routine gas testing performed at six yearly intervals to test for moisture, oxygen content and acidity using suitable direct-reading instruments.

National Grid has instituted a verified and comprehensive environmental management system and has gained ISO 14001 certification across all of its U.K. operations, which have enabled National Grid to chart its performance more effectively. National Grid has met many of the targets set in previous years, particularly with respect to SF₆ gas losses and is working to develop new methods to detect, minimize and rectify these losses.

Bill Wilkinson is the Technical Services Manager in Network Engineering, within the transmission business of the National Grid Co. plc, which is responsible for the engineering and maintenance policy for transmission assets in England and Wales. He has some 37 years of experience in transmission plant maintenance, has a degree in electrical engineering, is a chartered electrical engineer and is a member of the Institute of Electrical Engineers. He is currently responsible for the management of a team of engineers providing engineering advice, maintenance documentation and the development of maintenance practices to field staff.