SF6 gas sensors for gas-insulated switchgears

The main application area of the non-combustible gas sulfur hexafluoride (SF6) is the insulation of high voltage switchgear, transformer and cable. Although sulfur hexafluoride is non-toxic and non-flammable, its global warming potential is about 24,000 times that of carbon dioxide (GWP =23,900). In addition, sulfur hexafluoride is odorless, heavier than air, and can accumulate in the event of SF6 leaks in low-lying areas of gas-insulated switchgear. Without reliable SF6 gas sensors monitoring, there is a risk of suffocation for workers.

SF6 gas sensors

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 JXCT 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.

SF6 sensors
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 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 SF6 gas sensors from JXCT, 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.

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