SF6 CIRCUIT BREAKER





 In the SF6 circuit breaker, the current-carrying contacts operate in sulfur hexafluoride gas is known as an SF6 circuit breaker. It is an excellent insulating property and high electro-negativity. It can be understood that, the high affinity of absorbing free electrons. The negative ion is formed when a free electron collides with the SF6 gas molecule; it is absorbed by that gas molecule. The two different ways of attachment of electron with SF6 gas molecules are

SF6 + e = SF6
SF6 + e = SF5- + F

The negative ions which are formed will be much heavier than a free electron. Therefore, when compared with other common gases overall mobility of the charged particle in the SF6 gas is much less. The mobility of charged particles is majorly responsible for conducting current through a gas. Hence, for heavier and less mobile charged particles in SF6 gas, it acquires very high dielectric strength. This gas good heat transfer property because of low gaseous viscosity. SF6 is 100 times more effective in arc quenching media than an air circuit breaker. It is used for both medium and high voltage electrical power system from 33KV to 800KV.

SF6 Circuit Breakers Advantages:

  • Excellent insulating, arc extinguishing, physical and chemical properties of SF6 gas is greater advantage of SF6 circuit breakers
  • The gas is non-inflammable and chemically stable. The decomposition products are non-explosive i.e, rhere is no risk of fire or explosion
  • Electrical clearances are very much reduced because of high dielectric strength of SF6
  • Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to the air-blast circuit breaker and minimum oil breaker. Outdoor SF6 circuit breaker is simple, comparatively cheaper in cost, maintenance free and compact
  • Its performance is not affected due to variation in atmospheric conditions
  • It gives noiseless operation it does not make sound like air-blast circuit breaker during operation
  • No frequent contact replacement-arcing time is small owing to outstanding arc quenching properties of SF6 and therefore contact erosion is less. Hence contacts do not suffer oxidation
  • Therefore is no reduction in dielectric strength of SF6 since no carbon particle is formed during the arcing
  • Minimum maintenance. The breaker may require maintenance once in four to ten years
  • The sealed construction avoids the contamination by moisture, dust, sand etc. No costly compressed air system is required as in the case of air blast circuit breaker
  • Same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.
  • No over voltage problem. The arc is extinguished at natural current zero without the current chopping and associated over-voltages originating across the circuit breaker terminals
  • The SF6 gas circuit breaker can perform various duties like clearing short line faults, opening unloaded transmission lines, capacitor switching, transformer reactor switching etc without any problem
  • Ample overload margin. For the same size of the conductors the current carrying capacity of the SF6 circuit breakers is about 1.5 times that of the air blast circuit breakers because superior heat transfer capability of the SF6 gas

SF6 Circuit Breaker Diadvantages:

  • Imperfect joints leading to leakage of the SF6 gas. Continuous monitoring devices are required
  • SF6 gas is suffocating to some extent. In case of leakage in the breaker tank SF6 gas being heavier than the air settles in the surroundings and may lead to suffocation of the operating personnel. However it is not poisonous
  • Arced SF6 gas is poisonous and should not be inhaled
  • The internal parts need thorough cleaning during periodic maintenance under clean and dry environment. Dust of Teflon and sulphides should be removed
  • Special facilities are required for transportation of gas, transfer of gas an maintenance of quality of the gas. The deterioration of quality of gas affects the performance and hence reliability of the SF6 circuit breaker

SF6 Circuit Breakers Applications:

SF6 Circuit breakers are mostly employed for High voltage applications. Because of the above said features and properties of SF6 gas they are mostly employed in switching duties related to high voltage and extra high voltage applications



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