SF6 CIRCUIT BREAKERS
Sulphur-hexaflouride (SF6) is an inert insulating gas, which is becoming increasingly popular in modern switchgear designs both as an insulating as well as an arc-quenching medium. SF6 has the following advantages :
1)Very good insulating strength
2)High affinity for electrons which ultimately give rise to high dielectric strength to SF6
3)Heat Transfer-ability much higher than air
4)Chemically stable without any chance of carbon decomposition
For medium- and low-voltage installations, the SF6 circuit breaker remains the same construction wise as that for oil and air circuit breakers mentioned above, except for the arc interrupting chamber which is of a special design, filled with SF6.
In an SF6 circuit-breaker, the current continues to flow after contact separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally de-ionizes the contact gap and establishes the dielectric strength required to prevent a re-strike.
As SF6 has high affinity for electrons whenever a free electron collides with the neutral gas molecule negative ions are formed. The electron is absorbed by the neutral gas molecule. The following reactions are involved:
SF6 + e- === SF6-
SF6 + e- === SF5- + F
The negative ions formed ash shown above will be heavier as compared to free electrons and under a given electric field the ions will not attain sufficient energy to initiate cumulative ionization in the gas. In this way the gas de-ionizes and establishes the dielectric strength required to prevent a re-strike.
Sulphur-hexaflouride (SF6) is an inert insulating gas, which is becoming increasingly popular in modern switchgear designs both as an insulating as well as an arc-quenching medium. SF6 has the following advantages :
1)Very good insulating strength
2)High affinity for electrons which ultimately give rise to high dielectric strength to SF6
3)Heat Transfer-ability much higher than air
4)Chemically stable without any chance of carbon decomposition
For medium- and low-voltage installations, the SF6 circuit breaker remains the same construction wise as that for oil and air circuit breakers mentioned above, except for the arc interrupting chamber which is of a special design, filled with SF6.
In an SF6 circuit-breaker, the current continues to flow after contact separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally de-ionizes the contact gap and establishes the dielectric strength required to prevent a re-strike.
As SF6 has high affinity for electrons whenever a free electron collides with the neutral gas molecule negative ions are formed. The electron is absorbed by the neutral gas molecule. The following reactions are involved:
SF6 + e- === SF6-
SF6 + e- === SF5- + F
The negative ions formed ash shown above will be heavier as compared to free electrons and under a given electric field the ions will not attain sufficient energy to initiate cumulative ionization in the gas. In this way the gas de-ionizes and establishes the dielectric strength required to prevent a re-strike.
ADVANTAGES
- they have very short arcing time due to greater arc quenching property of SF6.
- the dielectric strength of SF6 gas is 2 to 3 times greater than that of air thus making they can interrupt larger currents.
- the closed gas circuit makes their operation noiseless and keep the interior dry avoiding any possible moisture problem.
- since SF6 is non inflammable gas there ain't any risk of fire and no carbon deposits
- they have low maintenance cost, light foundation requirements and minimum requirement auxiliary equipment.
- SF6 breakers are totally enclosed and sealed from atmosphere hence are very suitable in the places where explosion hazard exists.
DISADVANTAGES
- these breakers are costly because SF6 is expensive .
- SF6 gas has to be reconditioned after each and every operation of the breaker and for which other additional devices are required.
APPLICATION
SF6 breakers have been used for voltages 115kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.
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