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Answer Posted / abuhuraira munir
Sulphur 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 of dielectric strength of oil at 3 kg/cm2. SF6
is now being widely used in electrical equipment like high
voltage metal enclosed cables; high voltage metal clad
switchgear, capacitors, circuit breakers, current
transformers, bushings, etc. The gas is liquefied at
certain low temperature, liquefaction temperature increases
with pressure.
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 fluorides such as S2F10,
SF4 and must be purified further SF6 gas generally supplier
by chemical firms. The cost of gas is low if manufactured
in large scale.
Medium-voltage circuit breakers rated between 1 and 72 kV
may be assembled into metal-enclosed switchgear line ups
for indoor use, or may be individual components installed
outdoors in a substation. Air-break circuit breakers
replaced oil-filled units for indoor applications, but are
now themselves being replaced by vacuum circuit breakers
(up to about 35 kV). Like the high voltage circuit breakers
described below, these are also operated by current sensing
protective relays operated through current transformers.
The characteristics of MV breakers are given by
international standards such as IEC 62271. Medium-voltage
circuit breakers nearly always use separate current sensors
and protection relays, instead of relying on built-in
thermal or magnetic overcurrent sensors.
Medium-voltage circuit breakers can be classified by the
medium used to extinguish the arc:
Vacuum circuit breaker
SF6 is an inert, nonflammable, non-toxic and non-ozone
depleting insulating medium.
SF6 has three times the dielectric strength of air at
atmospheric pressure. This can be further increased by
increasing the pressure.SF6 consists of very large
molecules and can be enclosed without notable losses for
the complete service life of a switchgear installation
(approx. 40 years).
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