A circuit breaker in which SF6 under
pressure gas is used to extinguish the arc is called SF6 circuit
breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric,
arc quenching, chemical and other physical properties which have proved its
superiority over other arc quenching mediums such as oil or air.
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Constructionof SF6 Circuit Breaker.
An sulphur hexaflouride
(SF6) circuit breaker consists of fixed and moving contacts enclosed in a chamber. The chamber is called arc
interruption chamber which contains the sulphur hexafluoride (SF6) gas.
This chamber is connected to sulphur hexaflouride (SF6) gas reservoir.
WorkingPrinciple of SF6 Circuit Breaker.
In the closed position of the
breaker, the contacts remain surrounded by sulphur hexaflouride gas (SF6) gas at a pressure of about 2.8 kg/cm2. When
the breaker operates, the moving contact is pulled apart and an arc is struck
between the contacts. The movement of the moving contact is synchronized with
the opening of a valve which permits sulphur
hexaflouride gas (SF6) gas at 14 kg/cm2
pressure from the reservoir to the arc interruption chamber.
The high pressure flow
of sulphur hexaflouride gas (SF6) rapidly absorbs the free electrons
in the arc path to form immobile negative ions which are ineffective as
charge carriers. The result is that the medium between the contacts quickly
builds up high dielectric strength and causes the extinction of the arc. After
the breaker operation (i.e. after arc extinction), the valve is closed by the
action of a set of springs.
Advantage of SF6 circuit breaker.
SF6 circuit breakers have the following
advantages over conventional breaker.
1. SF6 gas has excellent insulating, arc
extinguishing and many other properties which are the greatest advantages of SF6 circuit breakers.
2. The gas
is non-inflammable and chemically stable. Their decomposition products are
non-explosive and hence there is no risk of fire or explosion.
3. Electric
clearance is very much reduced because of the high dielectric strength of SF6.
4. Its
performance is not affected due to variations in atmospheric condition.
5. It
gives noiseless operation, and there is no over voltage problem because the arc
is extinguished at natural current zero.
6. There
is no reduction in dielectric strength because no carbon particles are formed
during arcing.
7. It
requires less maintenance and no costly compressed air system is required.
8. SF6 performs various duties like clearing
short-line faults, switching, opening unloaded transmission lines, and
transformer reactor, etc. without any problem.
Disadvantages of SF6 circuit breakers.
1. SF6 gas is suffocating to some extent. In the
case of leakage in the breaker tank, the SF6 gas being heavier than air and hence SF6 are settled in the surroundings and lead to
the suffocation of the operating personnel.
2. The
entrance of moisture in the SF6 breaker tank is very harmful to the
breaker, and it causes several failures.
3. The
internal parts need cleaning during periodic maintenance under clean and dry
environment.
4. The
special facility requires for transportation and maintenance of quality of gas.
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