Sulphur Hexafluoride (SF6) Circuit Breaker
SF6 circuit breakers use sulfur hexafluoride gas for arc quenching and are widely used in high voltage and high power systems. Let's explore their working, construction, advantages, and applications.
Key Features of SF6 Circuit Breaker
- SF6 gas is used as the arc quenching medium.
- It is an electro-negative gas with a strong tendency to absorb free electrons.
- When contacts open, an arc forms and SF6 rapidly captures electrons to extinguish it.
- SF6 breakers are efficient and widely used for high voltage service.
Construction of SF6 Circuit Breaker
- Comprises fixed and moving contacts enclosed in an arc interruption chamber filled with SF6 gas.
- This chamber connects to a high-pressure SF6 gas reservoir.
- Upon operation, SF6 flows into the chamber through a valve mechanism.
- Contacts are made of arc-resistant copper-tungsten material.
- SF6 gas is reclaimed and reused after each operation to reduce costs.
Working of SF6 Circuit Breaker
- Contacts remain surrounded by SF6 gas at 2.8 kg/cm² in the closed state.
- When the breaker operates, an arc is created between separating contacts.
- Simultaneously, SF6 at 14 kg/cm² is released into the arc chamber.
- SF6 absorbs free electrons, forming negative ions and extinguishing the arc.
- After arc extinction, the valve closes via spring action.
Advantages of SF6 Circuit Breakers
- Very short arcing time due to superior arc quenching.
- Can interrupt large currents due to high dielectric strength.
- Noiseless operation and sealed gas system.
- No moisture-related issues due to closed construction.
- Non-inflammable gas — no fire hazard.
- No carbon deposits; fewer insulation issues.
- Low maintenance cost and minimal auxiliary equipment required.
- Ideal for hazardous environments like coal mines.
Disadvantages of SF6 Circuit Breakers
- Costly due to the high price of SF6 gas.
- Requires auxiliary equipment to recondition gas after every operation.
Applications of SF6 Circuit Breaker
These breakers are used in systems requiring:
- Voltages ranging from 115 kV to 230 kV.
- Interrupting capacities up to 60 kA.
- Power ratings between 10 MVA to 20 MVA.
- Fast operating times — less than 3 cycles.
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Frequently Asked Questions (FAQs)
What is the role of SF6 gas in circuit breakers?
SF6 gas acts as an excellent arc quenching medium due to its high electronegativity, helping to quickly extinguish the arc by capturing free electrons.
Why is SF6 circuit breaker preferred over air or oil breakers?
SF6 circuit breakers offer shorter arcing times, higher interrupting capacities, and are noiseless and maintenance-friendly compared to air or oil breakers.
What are the disadvantages of SF6 circuit breakers?
They are costly and require additional equipment to recondition and reclaim SF6 gas after each operation.
Where are SF6 circuit breakers commonly used?
They are used in high voltage substations, power stations, and industrial applications — especially where reliability and compactness are important.