SF6 circuit breaker uses sulphur hexafluoride gas as an arc quenching medium. SF6 gas is used because it has high dielectric strength which is acts as a good arc quenching medium. SF6 is an electro-negative gas and has a high capacity to absorb free electrons.
SF6 circuit breakers contact are opened in the high-pressure flow of SF6 gas and arc is stuck between breakers contact. The free electrons in the arc are suddenly absorbed by gas to form negative immobile ions and the arc gets quench rapidly.
SF6 circuit breaker is manufacture for voltage rating from 6.6Kv to 760Kv also is an inert, non-toxic, non-flammable gas. SF6 has dielectric strength 2.35 times that of air at atmospheric pressure.
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Properties of SF6 Gas
Physical properties
- SF6 gas is colorless, odorless
- It is a non-toxic and non-flammable gas
- Nonhazardous
- SF6 gas is heavy having density 5 times that air at a 20-degree temperature
- Has excellent heat transfer property
Chemical properties of SF6 Gas
- SF6 gas is chemically stable at standard atmospheric pressure and temperature
- Chemically inert gas
- Non corrosive at ambiant tempreture
- Electro negative gas
- It recombines in a short time after arc extinguish
Dielectric Properties
- The dielectric strength of SF6 gas is 2.35 times that of air at atmospheric pressure.
- At higher pressure dielectric strength of gas increases.
Construction Of SF6 Circuit Breaker
SF6 circuit breaker is filled with SF6 gas at pressure 5Kaf/cm^2. Sulphur hexafluoride breaker consists of two-part mainly interrupter unit and gas unit.
The below diagram shows the construction of a puffer type SF6 circuit breaker.
The moving cylinders attached to the movable conductor against the fixed piston. As shown in the diagram of the Sulphur hexafluoride breaker with a fully closed position of the interrupter and there is the relative movement between the fixed piston and movable cylinder. Due to this gas is compressed between this contact. This compressed gas in the cavity is released through the nozzle during the arc extinguishing process.
Working Principle Of SF6 Circuit Breaker
Normally breakers contact are closed position when the fault occurs in the power system. The contacts get separated and arc stuck between the fixed contact and movable contact during contact separation, compressed SF6 gas in cavity is released over the arc and it reduced arc diameter by axial conversion and radial dissipation. At current zero arc diameter becomes too small and arc get extinguish.
Advantages Of SF6 circuit breaker
The advantages of the SF6 circuit breaker is as follows
- The size of the Sulphur hexafluoride breaker is smaller than a conventional circuit breaker of the same rating.
- SF6 gas is non-inflammable and chemically stable hence no danger of fire.
- Required gas is small due to the same gas is recirculated in the circuit.
- Does not make a sound during operation as an air blast circuit breaker due to its closed gas circuit.
- Closed construction avoids contamination of moisture, dust, etc.
- Required less maintenance.
- It has the ability to interrupt low and high current.
- No contact replacement required due to small contact corrosion
Disadvantages
Disadvantages of SF6 circuit breaker
- Sealing problem arises due to type of construction
- If the joint not make perfect gas may be leak.
- SF6 breaker is costly due to the high cost of SF6 gas
- Arced SF6 gas is poisonous and should not be inhaled.
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Switchgear-Protection
