In such breakers, sulphur hexafluoride (SF) gas is used as the are quenching medium. The SF, is a electronegative gas and has a strong tendency to absorb free electrons.
The contacts of the breaker are opened in a high pressure flow of SF, gas and an are 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 conduction electrons in the are quickly builds up enough insulation strength to extinguish the are. The SF6 circuit breakers have been found to be very effective for high power and high voltage service.
Properties of Sulphur-Hexafluoride Gas :
1.This gas is extremely stable and inert, and its density is five times that of air
2.It is non-toxic and non-flammable.
3.It is an ideal medium for circuit interruption, Its properties make it an ideal insulating and arc quenching medium.
It has a thermal conductivity higher than that of air and helps in better cooling of current canying parts.
It has exceptionally low reactivity and does not attack metals, plastics etc.
Its dielectric strength at atmospheric pressure is 235 times that of air and 30% less than oil. At three times the atmospheric pressure, it is more than that of oil.
The gas at atmospheric pressure can interrupt currents of the order of 100 times the value of those that can be interrupted in air with a plain breaker interrupter
The low are time-constant (S I us) and its capacity to absorb free electrons due to its electromagnetic character makes an excellent medium for arc quenching.
Types of SF, circuit breakers :
There are two types of SF, circuit breakers as below:
1.Double pressure SF
2.circuit breakers.
Single – pressure Puffer-piston type SF, circuit breakers.
Double-Pressure SF, Circuit Breakers:
This is the early design of SF circuit breakers. Its operating principle is similar to that of air-blast circuit breakers. In this type of a circuit breaker, the gas from a high-pressure compartment is released to the low pressure compartment to extinguish the arc.
Because of its complicated design and construction, and its need for various auxiliaries such as gas compressors, filters and control devices, this type of circuit breakers have become obsolete,
Single-pressure Puffer-Piston Type SF, Circuit Breaker:
This type of circuit breaker are also sometimes called single pressure or impulse type SF circuit breakers. In this type breaker gas is compressed by a moving cylinder system and is released through a nozzle to quench the arc. This type is available in the voltage range 3.6 kv to 765 kV.
puffer-type breaker in closed position. The moving cylinder and the moving contact are coupled together. When the contacts separate and the moving cylinder moves, the trapped gas is compressed. The trapped gas is released through a nozzle and flows axially to quench the arc
