Merits and Demerits of SF6 Circuit Breaker

5. Description of different Circuit Breaker
5.4  Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear
5.4.5 Merits and Demerits of SF6 Circuit Breaker
  1.  Outdoor EHV SF6 CB has less number of interrupters per pole than ABCB and MOCB. It is simple, less costly, maintenance free compact.
  2. The gas is non-inflammable and chemically stable. The decomposition product are not explosive. Hence there is no danger of fire or explosions.
  3. Same gas is re-circulated in the circuit. Hence requirement of SF6 gas is small in the long run. No replacement is required for at least five years.
  4. Ample overload margin. For the same size of conductors, the current carrying ability of SF6 CB is about 1.5 times that of ABCB because of superior heat transferability of SF6 gas.
  5. The breaker is silent and does not sound, like ABCB, during operation.
  6. The sealed construction avoids the contamination by moisture, dust, sand, etc.
  7. No cost for compressed air system as in ABCB.
  8. Maintenance required is minimum. The breaker may need maintenance once in four to ten years.
  9. Ability to interrupt low and high fault currents, magnetizing currents, capacitive currents, without excessive over voltage. Sf6 gas CB can perform the various duties like clearing short line faults, opening unloaded transmission lines, capacitor switching, transformers, reactor switching. etc. much smoothly
  10. Excellent insulating, arc extinguishing, physical and chemical properties of SF6 gas is the greatest advantages of SF6 breakers.
  11. No frequent contact replacement. Contact corrosion is very small due to inertness of SF6 gas. Hence, contacts do not suffer oxidation.
  12. No over voltage problems. Due to particular properties of SF6 gas, the arc is extinguished at natural current zero without current chopping and associated over voltage originating in circuit breakers
  13. Simplicity of the interrupter chamber which does not need an auxiliary chamber for breaking.
  14. Autonomy provided by the puffer technique.
  15. The possibility to obtain the highest performances, up to 36 KA, with a reduced number of interrupting chambers.
  16. Short break time of 2 to 2.5 cycles.
  17. High electrical endurance, allowing at least 25 years of operation without reconditioning.
  18. Possible compact solutions when used for GIS or hybrid switchgear.
  19. Integrated closing resistors or synchronized operations to reduce switching over voltages.
  20. Reliability and availability.
  21. Low noise level.
5.4.6 Some Demerits of SF6 Circuit Breaker
  1. Sealing problems arise due to the type of construction used. Special materials are necessary in construction. Imperfect joints lead to leakage of gas.
  2. Arced SF6 gas is poisonous and should not be inhaled or let-out.
  3. Influx of moisture in the system is very dangerous to SF6 gas circuit breakers.
  4. The double pressure SF6 CB is relatively costly.
  5. The internal parts should be cleaned thoroughly during periodic maintenance, under clean, dry environment.
  6. Special facilities are needed for transporting the gas, transferring the gas, and maintaining the quality of the gas. The deterioration of quality of the gas affects the reliability of the SF6 circuit breaker.
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