A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called SF6 circuit breaker. SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air. The SF6 circuit breaker is mainly divided into three types
- Non-puffer piston circuit breaker
- Single- puffer piston circuit breaker.
- Double-puffer piston circuit breaker.
The circuit breaker which used air and oil as an insulating medium, their arc extinguishing force builds up was relatively slow after the movement of contact separation. In the case of high voltage circuit breakers quick arc extinction properties are used which require less time for quick recovery, voltage builds up. SF6 circuit breakers have good properties in this regards compared to oil or air circuit breakers. So in high voltage up to 760 kV, SF6 circuit breakers is used.
Properties of Sulphur hexafluoride Circuit Breaker
Sulphur hexafluoride possesses very good insulating and arc quenching properties. These properties are
- It is colourless, odourless, non-toxic, and non-inflammable gas.
- SF6 gas is extremely stable and inert, and its density is five times that of air.
- It has high thermal conductivity better than that of air and assists in better cooling current carrying parts.
- SF6 gas is strongly electronegative, which means the free electrons are easily removed from discharge by the formation of negative ions.
- It has a unique property of fast recombination after the source energising spark is removed. It is 100 times more effective as compared to arc quenching medium.
- Its dielectric strength is 2.5 times than that of air and 30% less than that of the dielectric oil. At high pressure the dielectric strength of the gas increases.
- Moisture is very harmful to SF6 circuit breaker. Due to a combination of humidity and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted) which can attack the parts of the circuit breakers.
Construction of SF6 Circuit Breakers
SF6 circuit breakers mainly consist of two parts, namely (a) the interrupter unit and (b) the gas system.
Interrupter Unit – This unit consists of moving and fixed contacts comprising a set of current-carrying parts and an arcing probe. It is connected to the SF6 gas reservoir. This unit consists slide vents in the moving contacts which permit the high-pressure gas into the main tank.
Gas System – The closed circuit gas system is employed in SF6 circuit breakers. The SF6 gas is costly, so it is reclaimed after each operation. This unit consists low and high-pressure chambers with a low-pressure alarm along with warning switches. When the pressure of the gas is very low due to which the dielectric strength of gases decrease and an arc quenching ability of the breakers is endangered, then this system gives the warning alarm.
Working Principle of SF6 Circuit Breaker
In the normal operating conditions, the contacts of the breaker are closed. When the fault occurs in the system, the contacts are pulled apart, and an arc is struck between them. The displacement of the moving contacts is synchronised with the valve which enters the high-pressure SF6 gas in the arc interrupting chamber at a pressure of about 16kg/cm^2.
The SF6 gas absorbs the free electrons in the arc path and forms ions which do not act as a charge carrier. These ions increase the dielectric strength of the gas and hence the arc is extinguished. This process reduces the pressure of the SF6 gas up to 3kg/cm^2 thus; it is stored in the low-pressure reservoir. This low-pressure gas is pulled back to the high-pressure reservoir for re-use.
Now a day puffer piston pressure is used for generating arc quenching pressure during an opening operation by mean of a piston attached to the moving contacts.
Advantage of SF6 circuit breaker
SF6 circuit breakers have the following advantages over conventional breaker
- SF6 gas has excellent insulating, arc extinguishing and many other properties which are the greatest advantages of SF6 circuit breakers.
- The gas is non-inflammable and chemically stable. Their decomposition products are non-explosive and hence there is no risk of fire or explosion.
- Electric clearance is very much reduced because of the high dielectric strength of SF6.
- Its performance is not affected due to variations in atmospheric condition.
- It gives noiseless operation, and there is no over voltage problem because the arc is extinguished at natural current zero.
- There is no reduction in dielectric strength because no carbon particles are formed during arcing.
- It requires less maintenance and no costly compressed air system is required.
- SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, and transformer reactor, etc. without any problem.
Disadvantages of SF6 circuit breakers
- SF6 gas is suffocating to some extent. In the case of leakage in the breaker tank, the SF6 gas being heavier than air and hence SF6 are settled in the surroundings and lead to the suffocation of the operating personnel.
- The entrance of moisture in the SF6 breaker tank is very harmful to the breaker, and it causes several failures.
- The internal parts need cleaning during periodic maintenance under clean and dry environment.
- The special facility requires for transportation and maintenance of quality of gas.