SF6 or Sulfur Hexafluoride gas is inbariably used in electrical switchgear as an arc quenching nedium for its some suitable features and recycling usability.
Why SF6 is better arc interrupting medium
SF6 is hazard free
Sulfur hexa fluoride is an inert, nontoxic, colorless, odorless, tasteless, and nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. It is about five times as dense as air. SF6 is used in GIS – gas insulated system at pressures from 400 to 600 kPa absolute.
Wide range of pressure
The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. SF6 has two to three times the insulating ability of air at the same pressure.
SF6 is about 100 times better than air for interrupting arcs. It is the universally used interrupting medium for high-voltage circuit breakers, replacing the older mediums of oil and air.
Repeated recycle of SF6 gas
SF6 decomposes in the high temperature of an electric arc or spark, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS. There are some reactive decomposition byproducts formed because of the interaction of sulfur and fluorine ions with trace amounts of moisture, air, and other contaminants.
The quantities formed are very small. Molecular sieve absorbents inside the GIS enclosure eliminate these reactive byproducts over time.
Easy contain and transporting of SF6 gas
SF6 is supplied in 50 kg gas cylinders in a liquid state at a pressure of about 6000 kPA for convenient storage and transport.The SF6 in the equipment must be dry enough to avoid condensation of moisture as a liquid on the surfaces of the solid epoxy support insulators because liquid water on the surface can cause a dielectric breakdown. However, if the moisture condenses as ice, the breakdown voltage is not affected. So dew points in the gas in the equipment need to be below about 108C.
Easy detection of contamination or foreign materials in Gas
Small conducting particles of millimeter size significantly reduce the dielectric strength of SF6 gas. This effect becomes greater as the pressure is raised past about 600 kPA absolute. The particles are moved by the electric field, possibly to the higher field regions inside the equipment or deposited along the surface of the solid epoxy support insulators—leading to dielectric breakdown at operating voltage levels.
Cleanliness in assembly is therefore very important for SF6 using switchgear. Fortunately, during the factory and field power frequency high-voltage tests, contaminating particles can be detected as they move and cause small electric discharges (partial discharge) and acoustic signals—they can then be removed by opening the equipment. Some switchgear equipment is provided with internal ‘‘particle traps’’ that capture the particles before they move to a location where they might cause breakdown. Most switch gear assemblies are of a shape that provides some ‘‘natural’’ low electric-field regions where particles can rest without causing problems.
SF6 is a strong greenhouse gas that could contribute to global warming. At an international treaty conference in Kyoto in 1997, SF6 was listed as one of the six greenhouse gases whose emissions should be
reduced. but as this gas has limited use plus it is used by recycle- it does not contribute that much to pollution.