5. Description of different Circuit Breaker
5.4 Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear
5.4.1 Introduction
Sulpher hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure and is more than that of dielectric oil at pressure of 3 kgf/cm2 . This gas is now being very widely used in electrical equipment like high voltage metal enclosed cables; high voltage metal clad switchgear, capacitors, circuit breakers, current transformers, bushings, etc. This gas liquefies at certain low temperatures, the liquefaction temperature increases with pressure. This gas is commercially manufactured in many countries and is now being extensively used by electrical industry in Europe, U.S.A. and Japan.
Several types of SF6 circuit breakers have been developed by various manufacturers in the world, for rated voltage from 3.6 to 760 KV. However, at present they are generally preferred for voltages above 72.5 KV.
SF6 gas insulated metal clad switchgear comprises factory assembled metal clad, substation equipment like circuit breakers, isolators, earthing switches, bus bars, etc. These are filled with SF6 gas . Such sub-stations are compact and are being favored in densely populated urban areas.
Sulphur hexafluoride gas is prepared by burning coarsely crushed roll sulphur in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of sulphur. The steel box is made gas-tight. The gas thus obtained contains other fluoride such as S2F10 , SF4 and must be purified further. SF6 gas is generally supplied by chemical firms. The cost of gas is low if manufactured on a large scale.
The gas is transported in liquid from in cylinders. Before filling the gas, the circuit breaker is evacuated to the pressure of about 4 mm of mercury so as to remove the moisture and air. The gas is then filled in the circuit breaker. The gas can be reclaimed by the gas-handling unit.
There are tow types of SF6 circuit breakers:
1. Single pressure puffer type SF6 circuit
In which the entire circuit breaker is filled with SF6 gas at single pressure ( 4 to 6 kgf/cm2) . The pressure and gas flow, required for arc extinction, is obtained by piston action.
2. Double pressure type SF6 circuit breaker
In which the gas from high pressure system is released into low pressure system over the arc during the arc quenching process. This type has been superseded by puffer type.
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