In this circuit breaker, sulphur hexaflouride ( SF6 ) gas is used as the arc quenching medium. The SF6 gas is an electro negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF6gas 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 are very effective for high power and high voltage service.
Construction: Fig 15 shows the parts of a 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 contacts of 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, its reconditioned and reclaimed by a suitable auxiliary system after each operation of the breaker.
Working: In the closed position of the breaker the contacts remained 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 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 a 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 over oil and air circuit breakers:a. Due to superior arc quenching property of SF6 , such breakers have very short arcing time
b. Dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
c. Gives noiseless operation due to its closed gas circuit
d. Closed gas enclosure keeps the interior dry so that there is no moisture problem
e. There is no risk of fire as SF6 is non inflammable
f. There are no carbon deposits
g. Low maintenance cost, light foundation requirements and minimum auxiliary equipment
h. SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazard exists
Disadvantages:A. SF6 breakers are costly due to high cost of SF6
B. SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is
required for this purpose
Applications: SF6 breakers have been used for voltages 115kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.