Sulfur Hexafluoride (SF6) gas Properties
Sulfur Hexafluoride (SF6) gas is polpular because of its dielectric and Insuation properties. The structural geometry of Sulfur Hexafluoride (SF6) gas molecule has Octahedral Geometry, containing suphur atom at the center connecting 6 Fluorine atoms around sulphur atom. Sulfur Hexafluoride (SF6) gas finds wide applications in Electrical Industy because of the dielectric and insulation properties it posesses. SF6 gas aslo finds applications in Medical field and used also as Tracing Compound. But majority of the applications lies in electrical industry and Sulfur Hexafluoride (SF6) is used as Dielectric and Arc Quenching medium in Circuit Breakers and in Switch gears. SF6 Circuit Breakers and SF6Switchgears become more popular in High Voltge Engineering present days because of better arc quenching ability and compactness. SF6 gas is used as Insulating medium between live parts of the High Voltage equipment. So the clearances required between live parts reduces thus making the power system compact. This Compact was applied in Gas Insulation Substations where Sulfur Hexafluoride (SF6) gas plays a major role in reducing the substation to 1/10 th of the conventional substation and making the substation Compact.
Sulfur Hexaflouride gas is prepared by burning coarsely crushed sulphur in fluoride gas. The gas obtained contains other fluorides such as S2F10, SF4 along with SF6 gas. These Other Fluorides are purified from the SF6 gas later and gas is stored and transported in liquid form.
Sulfur Hexaflouride (SF6) gas Properties:
Physical, Chemical dielectric Properties and Arc Quenching Characteristics Of Sulfur Hexafluoride (SF6) gas is given as:
- Sulfur Hexaflouride (SF6) gas is Colourless, Odourless, non toxic and non inflamable gas
- Sulfur Hexaflouride (SF6) gas is extremely stable and inert.Density of Sulfur Hexaflouride (SF6) gas is 5 times higher than air.
- Sulfur Hexaflouride (SF6) gas thernal conductivity (Heat Transfer ability) is about (2 to 2.5 times) higher than air and assists in better cooling of current carrying parts. So heat will be dissipated
- Thermal Time Constanst of Sulfur Hexaflouride (SF6) gas is low and as a result the presurre it should be stored and used are relatively smaller as compared to the air.
- Sulfur Hexaflouride (SF6) gas is chemically stable upto 500 degre centigrate and doesnot react with the structural material up to 500 degree Centigrade.
- Moisture is very harmful to the properties of Sulfur Hexaflouride (SF6) gas. In the presence of moisture hydrogen fluoride is formed during the arcing which can attack the mettalic and Insulating parts of the Circuit Breakers.
- The gas Starts Liquefying at certin low temperatures. The temperature at which Sulfur Hexaflouride (SF6) gas changes to liquid state depends on the pressure. With higher pressure temperature increases. To avoid the liquefraction of Sulfur Hexaflouride (SF6) gas the temperature of the gas should be maintained above certain level. For 15 atm pressure Sulfur Hexaflouride (SF6) gas starts to convert to liquid state at 10 degree Centigrade. In case of high pressure heaters are provided to maintain gas temperature at 16 Degree Centigrade.
- Sulfur Hexafluoride (SF6) gas has strong electronegative property i.e, free electrons that are produced during arcing phenomenon (Free electrons are produced because of ionisation of atoms surrounding the arcing contacts) readily removed from the discharge by the formation of negative ions which are formed by the interaction of sf6 gas molecule with the electrons. This negative ions formed are relatively heavier and immobile compared to the free electrons. Because of Strong Electronegative property, the time constant of the Sulfur Hexafluoride gas (SF6) is very low (less than 1μSec) and the rate of rise of dielectric strength is very high. So SF6 Circuit breakers can withstand severe RRRV.
- Dielectric Strength of SF6 gas at atmospheric pressure is 2.35 times that of air and 30% less than the dielectric oil. At normal pressure dielectric oil has more dielectric property than the SF6 gas. At higher pressures dielectric strength of the gas increases. At pressure about three times the atmospheric pressure the dielectric strength is more than the oil. This property permits smaller clearances and small size of the equipment for the same voltage.
- Sulfur Hexaflouride (SF6) gas not only possess good dielectric strength but also fast recombination after the source energizing the spark is removed. This property of SF6 makes it very efficient in arc quenching . SF6 is approximately 100 times as effective as air with respect to arc quenching.
- SF6 gas maintain high dielectric strength even when diluted by the air. 30% SF6 gas and 70% air by volume has a dielectric strength twice that of air at the same pressure.