Why SF6 Gas used in HV/EHV Circuit Breaker?

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.

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.

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