SF6 Detection and Monitoring

Monitoring Sulfur Hexafluoride (SF6) in the Power Utility Industry

LumaSense offers quality solutions to monitor SF6 to the utility industry. For the past thirty years, the amount of Sulfur Hexafluoride (SF6) used in the Power Utility Industry has been increasing. The gas is used mainly as an insulator for electrical switch-gear and control-gear rated for voltages greater than 1kV, but is also used in cables, tubular transmission lines, electrostatic generators and transformers (in preference to insulating oils because of their flammability problems). Although the gas's chemical and physical properties make it very attractive for the applications above, unfortunately, SF6 is one of the most potent "Greenhouse" gases with a Global Warming Potential twenty-five thousand times greater than that of carbon dioxide. And its presence in the atmosphere is increasing at a rate of eight percent every year.

Safeguarding the environment is one of today's hot-topics, and a gas such as SF6 attracts attention from the regulators. However, the few alternatives that can be used instead of this gas have their own problems, so it is expected that new regulations will allow the continued use of SF6, but that a strict monitoring regime must be enforced. In addition to its greenhouse effects, the processes of arcing, sparking discharging etc., cause SF6 to produce by-products. These by-products degrade the insulating qualities of the gas, which result in the gas either having to be replaced or refined. In addition to this, the most dominant by-product, Thionyl Fluoride (SOF2), decomposes to SO2 and HF in the presence of water. These by-products can be particularly damaging to the health and safety of the workforce.

Quality of Equipment and SF6 Gas

In order to meet the final test standards, as well as uphold any new regulations that may be enforced to enable the continued use of SF6, a manufacturer of equipment to the power utility industry will require a monitoring system. The system must be accurate, stable and reliable, and be able to measure the total concentration of the gas in a monitored enclosure where the switch gear is tested. Although only the total amount leaked is required by the standards, it is advantageous to be able to pin point the leaks. This makes it easier to improve the gas-tightness of the equipment.

Monitoring Installed SF6 Equipment

To safeguard the environment against leaks from installed equipment, future regulations may require that monitoring systems are installed at transformer and switch-gear stations. The system must have low detection limits and maintain its stability over long periods of time. A multipoint sampling system would offer fuller coverage, and a system that can operate unattended for long periods of time would release valuable manpower resources to deal with other tasks.

It was found that a Multi-gas Monitor 1312 could meet the demands both for monitoring in the field and for finished equipment approval tests. The monitor with its photoacoustic (PAS) measurement technique could take advantage of SF6's ability to absorb infrared light. With the correct narrow band optical filter installed and the monitor correctly calibrated, the Multi-gas Monitor is capable of measuring accurately over a wide dynamic range. This enables manufactures to not only monitor the presence of SF6, but to measure it quantitatively. The monitor can be moved around without any loss in accuracy and without needing to be recalibrated. This enables test engineers to locate areas that are a cause for concern.

Conclusion

Manufacturers found that the time required for their approval tests could be reduced considerably, thereby boosting productivity by as much as a factor of four.

In order to monitor a "farm" of switch gears, the monitor is connected to a multiplexer. Samples are drawn from up to twelve different points via tubes, which are connected to the inlet of the multiplexer. The outlet from the multiplexer is fed to the monitor, which is calibrated to a detection limit low enough (e.g. 5 ppb) to meet the customers' specification.

The system is able to monitor unattended for extended periods of time. The monitor's extended self-test routines maintain the reliability of the results, which are available on-line or can be dumped when required.

The customer soon discovered that the system not only monitored for leaks, it was also able to detect low levels of gas, thus safeguarding both the environment and the health and safety of the workers while the equipment is operating.