Explain construction and working of SF6 circuit breaker.

1 Answer

Answer :

Construction: A sulphur hexafluoride (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 hexafluoride (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 hexafluoride (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 arcing horn. The moving contact is also a hollow cylinder with rectangular holes in the sides. The holes permit the sulphur hexafluoride 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 a copper-tungsten arc-resistant material. Since sulphur hexafluoride gas (SF6) gas is costly, it is reconditioned and reclaimed using the suitable auxiliary system after each operation of the breaker.

Working of SF6 circuit breaker: 

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Gas is compressed by the moving cylinder and is released through the nozzle and rapidly absorbs the free electrons to extinguish the arc formed. The moving cylinder (1) connected to moving contact (2) against the fixed piston (5). Due to relative motion between (1) and (5) the gas gets compressed in enclosure (6) and is released through nozzle for arc extinction. This happens by puffing action. At current zero the diameter becomes too small and arc gets extinguished. 

OR

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Double pressure type breaker: 

Here the gas is made to flow from area P1 to P2 through a convergent-divergent nozzle. The flowing gas covers the arc. In the divergent section the speed of the gas is very high and carries away most of the heat and absorbs free electrons from the periphery of arc that results in the reduction of diameter of arc, which becomes nearly zero at current zero, leading to arc being extinguished. Finally the gas enters the contact space increasing the dielectric strength.

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