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Sulphur Hexafluoride (SF6) Circuit Breakers - Construction, Working and Advantages

In such breakers, sulphur hexaflouride gas (SF6) is used as the arc quenching medium. 

The sulphur hexaflouride gas (SF6) is an electronegative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of sulphur hexaflouride (SF6) gas and an arc is struck between them. The gas capture the conducting free electrons in the arc to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. 

The sulphur hexaflouride (SF6) circuit breakers have been found to be very effective for high power and high voltage service.

SF6 CBs are available for all voltages ranging from 144 to 765 kV or even above, continuous currents up to 8000 A, and symmetrical interrupting ratings up to 63 kA at 765 kV and 80 kA at 230 kV

Construction of 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 hexaflouride (SF6) gas. This chamber is connected to sulphur hexaflouride (SF6) gas reservoir. A valve mechanism is there to permit the gas to the arc interruption chamber. When the contacts of breaker are opened, the valve mechanism permits a high pressure sulphur hexaflouride (SF6) gas from the reservoir to flow towards the arc interruption chamber. 

The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides. The holes permit the sulphur hexaflouride gas (SF6) gas to let out through them after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since sulphur hexaflouride gas (SF6) gas is costly, it is reconditioned and reclaimed using suitable auxiliary system after each operation of breaker.

SF6 gas CBs are of either the dead tank design
for outdoor installations or live tank (or modular design) for outdoor installations, and increasingly, dead tank breakers are integrated in SF 6 -insulated substations for indoor or outdoor installations.

The SF6 Circuit Breakers are available as in following designs.

  1. live tank, 
  2. dead tank, 
  3. or grounded tank designs. 

The live tank means the interruption happens in an enclosure that is at line potential. Such SF6 CB has an interrupter chamber that is mounted on insulators and is at line potential. An interrupter with such a modular design can be connected in series to operate at higher-voltage levels.

The dead tank means that interruption takes place in a grounded enclosure and CTs are located on both sides of the break (i.e., interrupter contacts). In such CBs, the interruption maintenance takes place at ground level and its seismic withstand is better than CBs with the live tank designs.

However, they require more insulating gas in order to provide the proper amount of insulation between the interrupter and the grounded tank enclosure. The modular dead tank CB has been especially developed for integration of SF6 insulated substation systems.

The grounded tank means that interruption happens in an enclosure that is partially at line potential and partially at ground potential. The evolution of the grounded tank CB design is the result of installing a live tank CB interrupter into a dead tank CB design.

Working of SF6 CB

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 synchronised 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.

Advantages of SF6

Due, to the superior arc quenching properties of sulphur hexaflouride gas (SF6) gas, the sulphur hexaflouride gas (SF6) circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below : 
  1. Due to the superior arc quenching property of sulphur hexaflouride gas (SF6), such circuit breakers have very short arcing time.
  2. Since the dielectric strength of sulphur hexaflouride (SF6) gas is 2 to 3 times that operation due unlike of air, such breakers can interrupt much larger currents.
  3. The sulphur hexaflouride gas (SF6) circuit breaker gives noiseless operation due its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.
  4. The compact design of SF6 gas CBs substantially reduces space requirements and building installation costs.
  5. SF6 gas CBs can handle all known switching phenomena.
  6. SF6 gas CBs perfectly can adapt to environmental requirements since they have completely enclosed gas system that eliminates any exhaust during switching operations. 
  7. Contact separation in SF6 gas CBs is minimum due to dielectric strength provided by the high pressure SF6 .
In general, the only disadvantage of the SF6 Circuit Breakers is their relatively high costs, which have been somewhat coming down in recent years.