SF6 gas is used in High Voltage and Extra High Voltage Circuit Breaker. For better understanding of why SF6 is chosen, we will go through the chemical and dielectric properties of SF6.
Chemical Properties of SF6:
5) The metallic fluorides are good dielectric materials hence are safe for electrical equipment.
Dielectric Properties of SF6 Gas:
1) Dielectric strength of SF6 at atmospheric pressure is 2.35 times that of air, it is 30% less than that of dielectric oil used in oil circuit breakers.
2) At higher pressure the dielectric strength of the gas increases. At pressure about 3 kg/cm2 the dielectric strength of SF6 gas is more than that of dielectric oil. This property permits smaller clearances and small size of equipments for the same kV.
3) The SF6 gas is electronegative gas. The molecules of electronegative gases have an ability to attract, hold free electrons and form negative ions. The negative ions being heavy and practically immobile they do not flow easily. Hence, the dielectric strength of electronegative gases is more than that of air.
4) Electronegativity of the gas gives lower arc-time constant. The time required for the medium to regain its dielectric strength after final current zero is called arc-time constant. The arc-time constant of SF6 gas is of the order if a few Microseconds.
Arc Extinction in SF6 Circuit Breakers:
1) During the arcing period, SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and radial dissipation.
2) Due to its electronegativity, and low arc-time constant, the SF6 gas regains its dielectric strength rapidly after the final current zero; the rate of rise of dielectric strength is very high.
3) The basic requirement in arc extinction is not primarily the dielectric strength, but high rate of recovery of dielectric strength. In SF6 gas, the electrons get attached with molecules to become ions. Thereby, the dielectric strength is quickly regained.