Sulfur Hexafluoride (SF6) Circuit Breaker
Introduction:
In the field of circuit breakers, sulfur hexafluoride (SF6) circuit breakers distinguish themselves through superior performance. Unlike traditional alternatives such as oil, air or vacuum circuit breakers, SF6 circuit breakers ensure faster arc extinction, faster recovery voltage build-up and increased dielectric strength. Let us learn in depth about the definition, working principle and key properties of SF6 gas.
Definition:
SF6 circuit breakers use SF6 gas as a medium to extinguish the electrical arc. SF6, a colorless, odorless, non-toxic gas, has remarkable dielectric and arc-quenching properties. It has become the preferred choice over traditional mediums such as oil and air in high-voltage (HV) systems.
Working principle of SF6 Circuit Breaker:
During the arcing period, SF6 gas flows axially along the arc, removing heat through axial convection and radial dissipation. This process reduces the diameter of the arc, and the turbulent flow assists in extinguishing the arc. SF6 circuit breakers employ a puffer piston principle, where gas is forced through a converging-diverging nozzle to effectively extinguish the arc.
Properties of SF6 gas:
Physical and Chemical Properties: SF6 is colourless, non-toxic and non-flammable. It is chemically stable, ensuring longevity in switchgear.
Density and thermal conductivity: SF6 is five times denser than air, and its high thermal conductivity helps effectively cool current-carrying parts.
Dielectric Strength: SF6 has superior dielectric strength, which ensures effective insulation even when diluted with air.
Recombination and Reactivity: SF6 recombines rapidly after the source of spark is removed. It is chemically stable, preventing reactions with structural materials.
Arc Extinction:
SF6 circuit breakers excel in arc extinction due to their fast dielectric strength recovery. The gas flows through a nozzle, reaching supersonic speed in the divergent part, reducing the diameter of the arc to almost zero at current zero. Fresh SF6 gas quickly restores dielectric strength, aided by its electronegativity and turbulent flow.
Construction of SF6 Circuit Breaker:
The SF6 circuit breaker consists of an interrupter unit and a gas system. The interrupter unit consists of dynamic and static contacts, an arcing probe and interrupting nozzle. The gas system employs a closed circuit with repair and recovery of SF6 gas after each operation to ensure cost-effectiveness.
Advantages of SF6 Circuit Breaker:
SF6 circuit breakers offer several benefits:
1. Excellent insulation and arc-extinguishing properties.
2. Non-flammable and chemically stable gas.
3. Reduction in electrical clearance.
4. Compact, maintenance-free and cost-effective.
5. Reliable performance in various atmospheric conditions.
6. Noiseless operation and minimal contact corrosion.
7. Extended maintenance interval (4-10 years).
8. No overvoltage problem during arc extinction.
9. Versatility in handling various electrical duties.
10. Adequate overload margin due to superior heat transfer capability.
Challenges or Disadvantages in SF6 Circuit Breaker:
Despite their benefits, SF6 circuit breakers face some challenges:
1. Gas leakage is occurring due to sealing problem.
2. There is a risk of asphyxiation if SF6 leaks, although it is non-toxic.
3. Toxicity of arcade SF6 gas.
4. Harmful effects of moisture flow.
5. Requirement of higher energy levels in puffer-type breakers.
6. Requirements for special facilities in gas transportation and handling.
In conclusion, SF6 circuit breakers represent a technological leap forward, providing efficient and reliable power interruption while requiring careful operation and maintenance to minimize associated challenges.
BRIEF SUMMARY
• SF6 circuit breakers use SF6 gas to extinguish the electrical arc, which provides better dielectric and arc-quenching properties than oil and air.
Working principle:
• SF6 gas flows axially along the arc, reducing the arc diameter through axial convection and radial dissipation. The puffer piston principle aids in effective arc quenching.
Properties of SF6 gas:
• SF6 gas is colorless, odorless, non-toxic and non-flammable with high density and thermal conductivity. It exhibits superior dielectric strength and chemical stability.
Arc Extinction:
• SF6 circuit breakers excel in arc extinction by rapidly recovering dielectric strength. The flow of gas through the nozzle reduces the arc diameter to almost zero at current zero.
Construction:
• The circuit breaker consists of an interrupter unit with moving and fixed contacts, an arcing probe and an interrupting nozzle. A closed gas circuit ensures efficient repair and recovery.
Advantages:
• SF6 circuit breakers provide excellent insulation, are non-flammable, compact and reliable in a variety of conditions. They require minimal maintenance, operate without noise, and have longer maintenance intervals.
Challenges:
• Challenges include gas leakage, risk of asphyxiation, toxicity of arcing SF6 gas, harmful effects of moisture, high energy requirements in puffer-type breakers and the need for special facilities for gas management.
Conclusion:
• SF6 circuit breakers represent a technological leap forward, ensuring efficient power interruption with the associated challenges requiring careful operation and maintenance.