VACUUM CIRCUIT BREAKERS
Vacuum breakers are additionally comparable in development like alternate sorts of breakers, with the exception of that the breaking medium is vacuum and the medium fixed to guarantee vacuum. The utilization of vacuum for circular segment elimination has the accompanying preferences:
1)Vacuum has high dielectric quality.
2)When contacts get open amid operation in AC frameworks, the current is hindered right away present zero moment which causes the dielectric quality between the contacts to develop at a high rate than that can be accomplished with other circuit breakers.
3)Ideally vacuum is such a dielectric in which curve ought not persevere.
When the vacuum circuit breaker operates, the fixed contact and moving contact shown in the figure separates. Metal Vapor is then released from the contacts which fills the vacuum between the contacts and thus maintains the arc. Metal vapor formed is actually known as plasma which is the combination of electrons and positive metal ions. After the first current zero instant is reached the plasma vanishes and the medium regains its dielectric strength. The arc will now get extinguished as only vacuum is left between the contacts.
Because it is virtually impossible for electricity to flow in a vacuum, the early designs displayed the ability of current chopping i.e. switching off the current at a point on the cycle other than current zero. This sudden instantaneous collapse of the current generated extremely high-voltage spikes and surges into the system, causing failure of equipment.
Another phenomenon was pre-strike at switch on. Due to their superior rate of dielectric recovery, a characteristic of all vacuum switches was the production of a train of pulse during the closing operation.
Although of modest magnitude, the high rate of rise of voltage in pre-strike transients can, under certain conditions produce high-insulation stresses in motor line end coils. Modern designs have all but overcome these problems. In vacuum circuit breakers, operating speeds are also much higher which, together with contact materials that ensure high dielectric strength at a small gap, have ensured that prestrike transients have ceased to become a significant phenomenon. These have led to the use of vacuum breakers more common in modern installations.
ADVANTAGES
•VCB is self contained does not need filling of gas or oil. They do not need auxiliary air system, oil handling system etc. No need of periodic refilling.
•No emission of gas, pollution free
•Non explosive
•Silent operation
•Large number of operation on load or short circuit .suitable for repeated operating duty ,long life
•Suitable for capacitor switching ,cable switching, industrial switching
•Constant contact resistance
•Constant dielectric
DISADVANTAGES
•For interruption of low magnetizing currents in certain range, additional surge suppressors are required in parallel with each phase of a VCB
•It require high technology for production
•It is more expensive.
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Vacuum breakers are additionally comparable in development like alternate sorts of breakers, with the exception of that the breaking medium is vacuum and the medium fixed to guarantee vacuum. The utilization of vacuum for circular segment elimination has the accompanying preferences:
1)Vacuum has high dielectric quality.
2)When contacts get open amid operation in AC frameworks, the current is hindered right away present zero moment which causes the dielectric quality between the contacts to develop at a high rate than that can be accomplished with other circuit breakers.
3)Ideally vacuum is such a dielectric in which curve ought not persevere.
When the vacuum circuit breaker operates, the fixed contact and moving contact shown in the figure separates. Metal Vapor is then released from the contacts which fills the vacuum between the contacts and thus maintains the arc. Metal vapor formed is actually known as plasma which is the combination of electrons and positive metal ions. After the first current zero instant is reached the plasma vanishes and the medium regains its dielectric strength. The arc will now get extinguished as only vacuum is left between the contacts.
Because it is virtually impossible for electricity to flow in a vacuum, the early designs displayed the ability of current chopping i.e. switching off the current at a point on the cycle other than current zero. This sudden instantaneous collapse of the current generated extremely high-voltage spikes and surges into the system, causing failure of equipment.
Another phenomenon was pre-strike at switch on. Due to their superior rate of dielectric recovery, a characteristic of all vacuum switches was the production of a train of pulse during the closing operation.
Although of modest magnitude, the high rate of rise of voltage in pre-strike transients can, under certain conditions produce high-insulation stresses in motor line end coils. Modern designs have all but overcome these problems. In vacuum circuit breakers, operating speeds are also much higher which, together with contact materials that ensure high dielectric strength at a small gap, have ensured that prestrike transients have ceased to become a significant phenomenon. These have led to the use of vacuum breakers more common in modern installations.
ADVANTAGES
•VCB is self contained does not need filling of gas or oil. They do not need auxiliary air system, oil handling system etc. No need of periodic refilling.
•No emission of gas, pollution free
•Non explosive
•Silent operation
•Large number of operation on load or short circuit .suitable for repeated operating duty ,long life
•Suitable for capacitor switching ,cable switching, industrial switching
•Constant contact resistance
•Constant dielectric
DISADVANTAGES
•For interruption of low magnetizing currents in certain range, additional surge suppressors are required in parallel with each phase of a VCB
•It require high technology for production
•It is more expensive.
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