SF6 circuit breaker uses SF6 (Sulfur Hexafluoride) gas as an air quenching medium. This gas is electro-negative and it has high dielectric strength to absorb free electrons. This is very effective for high voltage applications ranging from 33kV to 800kV.
It consists of moving and fixed contacts enclosed in a chamber called an arc interruption chamber filled with SF6 gas. The connection is given to an SF6 gas reservoir. A valve mechanism allows the flow of gas from the reservoir to flow towards the arc interruption chamber.
Working Principle of SF6 Breaker
The operation of the SF6 circuit breaker is based on self-blast and compressing technique. As per the construction diagram, when the fault occurs, the main contacts are separated. The current flowing through the arcing contacts makes no electric arc is hit.
The displacement of the moving contacts is synchronized 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 free electrons in the arc to form relatively immobile negative ions. This is arcing contact separation.
The loss of electrons in the arc builds up enough dielectric strength to extinguish the arc(Circuit breaker open). This process reduces the pressure of the SF6 gas up to 3kg/cm^2. Thus, a low-pressure reservoir stores the gas. This low-pressure gas is pulled back to the high-pressure reservoir for re-use.
Properties of SF6 Circuit Breaker
Physical properties
- No color, no odor, non-toxic and noninflammable gas
- Excellent heat transfer property
Chemical properties:
- Chemically inert
- Chemically stable at atmospheric pressure and temperature
- Non-corrosive on all metals at ambient temperatures
Electrical properties:
- High dielectric strength (2.5 times greater than air)
- Arc-interrupting capacity
- The dielectric constant is independent of the frequency of the applied voltage
Advantages
The advantages of SF6 mechanical circuit breaker over vacuum circuit breaker are
- Low maintenance cost
- There is no risk of fire as it is non-inflammable
- The gas has good insulating properties
- Less arcing time
- The dielectric strength of SF6 gas is 2 to 3 times that of air and can interrupt much larger currents.
- Noiseless operations
- Ideal for interrupting current
- Suitable for outdoor installation
Disadvantages of SF6 Circuit Breaker
- The cost of gas is high
- Needs cleaning periodically
- Re-condition of the SF6 gas after every operation
Applications of SF6 Circuit Breaker
The main applications of choosing an SF6 circuit breaker are:
- Power distribution and control
- Line protection for power circuits.
- For the protection of transformers
- Capacitor blocks
- Rectifier circuits.
Other than the above applications of SF6, over headlines and cables use this circuit breaker, motor over-voltage protection, capacitor switching to limit high currents (for voltages greater than 27kV), Shunt reactors, etc.
Comparison Between SF6 Gas and Vacuum
SF6 or Vacuum circuit breaker? Which one is best for breaking and making fault currents in load circuits. Let’s discuss the difference between SF6 and vacuum circuit breakers.
| Parameter | SF6 Circuit Breaker | Vacuum Circuit breaker |
|---|---|---|
| Arc interruption | Uses self-blast interruption to extinguish arc at high and low currents | Forms charge carrier to extinguish short circuit currents. |
| Electrical Endurance | Maintenance free | Maintenance free |
| Switching over voltages | Capable by using soft interruption | Uses contact materials for switching |
| Capacitor | Uses to limit current | Synchronous control with voltages greater 27kV |
| Motor Switching | Protects over-voltage limit | For small motors (currents less than 600 Amperes) |
| Technique | Auto puffer | Auto puffer |
| Malfunction | Due to over-pressure values | Implosion effect |
| Usage | The SF6 circuit breaker is used in high-frequency applications | The vacuum Circuit breaker is used in low-frequency applications (16 Hz) |
Both SF6 and vacuum circuit breaker (VCB) are suitable for medium voltage switching. The choice of frequent or soft switching influences the user preferences in choosing the particular circuit breaker.
Conclusion
An SF6 breaker is a medium voltage circuit breaker that operates at low pressure. It has a good arc extinguishing ability and high dielectric strength to absorb free electrons. Hence arc put outs quickly due to the loss of conducting electrons.
Though this type of circuit breaker can be used from medium (132kv) to high voltage (800kv) applications, the cost of this type of circuit breaker is extremely high.
