Pure sulphur hexafluoride gas is inert and thermally stable. The Sf6 gas is an electro-negative gas and has a string tendency to absorb free electrons. It possess good arc quenching and insulating properties which make it suitable for use in circuit breakers. Sulphur hexafluoride remains in a gaseous state upto a temperature of 9° C. Its density is about 5 times that of air and the free heat convection is 1.6 times as much as that of air. Apart from being inert gas, SF6 gas is non-inflammable, non-poisonous and odourless.
The contacts of the circuit breaker are opened in a high pressure flow of SF6 gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas 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 SF6 circuit breakers have been found to be very effective for high power and high voltage service.
SF6 gas Properties:
Sulphur Hexa fluoride (SF6 ) has unique properties, such as
- High dielectric strength.
- Non-reactive to other components of circuit breaker.
- High heat transfer property ( about 1.6 times of that of air).
- Low thermal time constant.
- Fast recombination property.
SF6 circuit breaker Construction:
SF6 circuit breaker consists of fixed and moving contacts enclosed in a chamber containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contacts of circuit breaker are opened, the valve mechanism permits a high pressure 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 to permit the SF6 gas to let out through these holes 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 SF6 gas is costly, it is recommended and reclaimed by suitable auxiliary system after each operation of the circuit breaker.
Working of SF6 circuit breaker:
In the closed position of the circuit 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 synchronised with the opening 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 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, the valve is closed by the action of a set of springs.
Advantages of SF6 circuit breaker:
- Sf6 circuit breakers have very short arcing time due to the superior arc quenching property of SF6.
- These circuit breakers can interrupt much larger currents as the dielectric strength of SF6 is 2 to 3 times that of air.
- This type of circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.
- There is no moisture problem as the closed gas enclosure keeps the interior dry.
- There is no risk of fire in such breakers because SF6 gas is inflammable.
- There are no carbon deposits so that tracking and insulation problems are eliminated.
- These breakers have low maintenance cost, light foundation requirements and minimum auxiliary equipment.
- Since SF6 breaker is totally enclosed and scaled from atmosphere, they are particularly suitable where explosion hazard exists example coal mines.
Disadvantages of SF6 circuit breaker:
- SF6 breaker is costly as SF6 gas is costly.
- Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.