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.
