Sulfur hexafluoride is a non-toxic, inert, and nonflammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. Sulfur hexafluoride (SF6) is the most common insulation gas in high voltage technology because of the electron attachment (electron affinity.)SF6 is around five times the density of atmospheric air at mean sea level, and has no color, odor, or taste. SF6 is almost purely water-insoluble, and as with all other gases, its solubility decreases as the temperature of the water increases.
SF6 Gas Pressure SF6 gas is enclosed in GIS systems at pressures between 400 to 600 kPa absolute. This pressure range is ideal, to prevent the gas from condensing into a liquid at the lowest potential temperatures to which the equipment may be subjected to.SF6 is a gas in which the speed of sound propagation is about three times less than in air, (at atmospheric pressure.) The sound of the interruption of the arc will thus be quieter in SF6 than in air.
Toxic Breakdown Gases from SF6HF - Hydrofluoric (HF) acid, is a toxic breakdown product of SF6. It is typically a gas, with a boiling point of approximately 19°C. Symptoms of exposure are delayed. Due to its high reactivity, it does not have a long half-life.SO2 - Sulphur dioxide (SO2), a toxic breakdown product of SF6. It is typically a gas (boiling point approximately -10°C).SOF2 - Thionyl fluoride (SOF2), a toxic breakdown product of SF6.It decomposes over time in the presence of moisture into SO2 and HF. It is a colorless gas with a pungent odor (similar to rotten eggs), condensing at approximately -43°C.
Gas Recycling To control gas leakage from a GIS system, the SF6 is completely contained within the sealed enclosures, allowing for full reclamation and recycling. By following the present international guidelines for the use of SF6 in electrical equipment, the contribution of SF6 to global warming can be kept to less than 0.1% over a 100-year period, when guidelines are stringent followed by GIS personnel.
Condensation as MoistureThe SF6 gas in the enclosures must be dry enough to prevent condensation of moisture onto the surfaces of the epoxy support insulators, as moisture on the surface can cause a dielectric breakdown.
Gas Dew Points; Condensation as IceCondensation as Ice - when the moisture condenses as ice, the breakdown voltage is not affected.Gas Dew Points – Thus gas dew points should remain tained below approximately –10°C. Additionally, levels below 1000 ppmv of moisture are typically specified, and easily obtainable when careful gas handling is practiced.
Required Gas Purity - fortunately, the purity of the SF6 needs to only be above 98% as established by international standards, so a simple field test of gas purity using commercially available percentage SF6 meters will qualify the used SF6 for reuse.