SF6 Circuit Breaker

SF6 Circuit Breaker
SF6 circuit breaker
In such circuit breaker SF6 gas is used as the arc quenching medium. SF6 is an electro-negative gas and has strong tendency to absorb free electrons.The contacts of the breaker are open in the high pressure. Flow of SF6 gas and arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gap to form relatively immobile ions.The loss of conducting electron in the arc quickly builds up enough insulating strength to extinguish the arc. SF6 circuit breaker has been found to be very effective for high power and high voltage service.

Construction:-

Constructional details of SF6 circuit breaker
Constructional details of SF6 circuit breaker
Above figure shows the parts of typical SF6 circuit breaker. It consists of fixed and moving contacts enclosed in a chamber(called arc interruption chamber) containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contact of breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber. The moving 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 to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of the fixed contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is costly, it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

Working:-

In the closed position of the breaker, the contacts remain surrounded by 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 operating of a valve which permits SF6 gas at 14 Kg/cm2 pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF6 rapidly absorb the free electrons to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly build up high dielectric strength and causes the extinction of the arc. After the breaker operation i.e., after arc extinction valve is closed by the action of a set of spring.

Advantages :-

  1. Due to the superior quenching property of SF6 such circuit breaker have very short arcing time.
  2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
  3. The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike air blast circuit breaker.
  4. The closed gas enclosure keep the interior dry so that there is no moisture problem.

Disadvantages:-

  1. SF6 circuit breaker are costly due to high cost of SF6 gas.
  2. SF6 gas has to be reconditioned after every operation of the breaker. Hence additional equipment is required for this purpose.
There are many advantages of using SF6 circuit breaker, only few have listed above.

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