SF6 sensors for gas-insulated switchgears (GIS)

The main area of application for the non-flammable gas sulphur hexafluoride (SF6) is insulation of electrical high-voltage switchgears, transformers and cables. Although sulphur hexafluoride is non-toxic and non-flammable, its global warming potential is about 24,000 times higher than carbon dioxide (GWP=23,900). That makes it the most potent of all greenhouse gases, which is why its use is subject to the most stringent environmental regulations. In addition, sulphur hexafluoride is odourless and heavier than air, so that it can accumulate in case of SF6 leaks in low-lying areas of the gas-insulated switchgear. Without reliable SF6 monitoring with SF6 gas sensors there would be a danger of asphyxiation for the workers.

Advantages of sulphur hexafluoride in gas-insulated switchgears

SF6 insulation allows safe operation of high- and medium-voltage systems in close areas, since insulation with sulphur hexafluoride requires much smaller safety clearances. Non-dispersive infrared gas sensors (NDIR sensors) from smartGAS are ideal for measuring the SF6 gas quality and for detecting even very small quantities of the harmful gas.

Gas-insulated switchgears with high-voltage switches are used worldwide. The advantage over air-insulated switchgears is their compact design. Since the protective gas sulphur hexafluoride has a dielectric strength three to four times higher than air and therefore effectively prevents sparking or arcing, air-insulated switchgears with the size of a football field can theoretically be installed in the basement of a single-family home, by using SF6 insulation. In addition, spark gaps in a sulphur hexafluoride atmosphere are quenched much more quickly than in air. This makes sulphur hexafluoride indispensable in metropolitan areas with high electrification in small areas in order to guarantee safe operation of high- and medium-voltage switchgears and stable supply of the urban infrastructure.

Sulphur hexafluoride: SF6 gas sensors monitor gas quality

During the operation of gas-insulated switchgears the purity of the SF6 decreases and the protective function of the SF6 gas is reduced. This is the result of arcs that occur during switching of the high-voltage switchgears and cause chemical transformation of the sulphur hexafluoride molecules. Gas-insulated switchgears are furthermore never absolutely leakproof. Leaks in seals, flanges, etc. allows SF6 to emerge and allows air to enter. The loss rate of sulphur hexafluoride is generally about 0.5 percent by volume of SF6 per year. These leaks must be eliminated immediately, since sulphur hexafluoride is extremely harmful to the environment.

Depending on the operating duration SF6 must therefore be checked and replaced regularly. To measure the purity within the gas-insulated switchgear a very small SF6 gas volume is  measured with a FLOWEVO SF6 gas sensor for 100 Vol.-%. Based on the measurement it can be determined whether the sulphur hexafluoride must be replaced.

SF6 gas-insulated switchgears can also be equipped with stationary SF6 monitors, which issue an alert as soon as a critical SF6 concentration in the room air is exceeded. The measuring ranges of such SF6 gas measuring systems are normally between 1000 and 1500 ppm. 
For use in maintenance and the search for leaks, portable SF6 leak detectors with measuring ranges of 50 ppm or 2000 ppm are used.

Optical measuring processes, such as those used in NDIR gas sensors from smartGAS, are superior to other measuring methods with respect to life cycle, selectivity and maintenance expenses. Optical NDIR gas sensors operate on the basis of a purely physical process in order to measure the concentration of the target gas by means of the amount of absorption of its specific wavelengths in the infrared spectrum.

SF6 gas sensors for every application

With the SF6 sensors of the FLOWEVO series with measuring ranges of 100 Vol.-%, 2000 ppm, 1000 ppm and 50 ppm smartGAS offers technologies for monitoring SF6 gas quality, SF6 monitoring of room air and SF6 leak detection. The NDIR based FlowEVO SF6 sensor (100 Vol.-%) precisely measure the purity or SF6 gas quality in gas-insulated switchgears, transformers and cables.

The compact NDIR type FlowEVO SF6 sensor (2000 ppm) is typically used in portable gas metres or stationary SF6 leak detectors for checking compliance with the SF6 concentration in the ambient air (MAC = 1000 ppm) of gas-insulated switchgears. Other areas of application are gas reservoirs, power plants and transformer stations, as well as systems for the manufacture of semiconductors.

The compact and lightweight FLOWEVO SF6 sensor (50 ppm) is also installed in portable devices for SF6 gas measurement or stationary SF6 detectors for intermittent or continuous measurement of minuscule SF6 leaks. The high sensitivity and very small lower detection limit enable the 50 ppm sensor to detect even minimal sulphur hexafluoride leaks with absolute reliability.

Another typical application for the highly sensitive FLOWEVO SF6 sensor (50 ppm) is detection of SF6 in environmental simulations. Since sulphur hexafluoride is not present in the atmosphere and non-flammable and non-toxic, it is used in very small concentrations as a tracer gas for real-time analyses of the spreading of fire gases or hazardous substances in tunnels, subways, buildings or ships.

Product brochure SF6

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