when contacts of a breaker are opened in vacuum, the interruption occurs at first current-zero with dielectric strength between the contacts, building up at a rate thousands of times higher than that obtained with other circuit breakers
Principle :
The production of arc in a vacuum circuit breaker and its extinction can be explained as follows:
When the contacts of the breaker are opened in vacuum (10-7 to 10–5 torr), an arc is produced between the contacts by the ionization of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours electrons and ions produced during are rapidly condense on the surface of the breaker contacts, resulting in quick recovery of dielectric strength.
We should consider the feature of vacuum as an arc quenching medium. As soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.
The parts of a typical vacuum circuit breaker. It consists of fixed contact, moving contact and are shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows.
This enables the permanent scaling of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours on the inside surface of the outer insulating cover.
Working :
When the breaker operates, the moving contact separates from thefixed contact and an arc is struck between the contacts. The production of arc is due to the ionization of metal ions and depends very much upon the material of contacts.
The arc is quickly extinguished because the metallic vapours, electrons and ions produced during are are diffused in a short time and seized by the surfaces of moving and fixed-members and shields.
Since vacuum has very fast rate of recovery of dielectric strength the arc extinction in vacuum breaker occurs with a short contact separation (say 0.625 cm.)
Advantages :
1.They are compact, reliable and have longer life.
2.There are no fire hazards.
3.There is no generation of gas during and after operation.
4.They can interrupt any fault current. The outstanding feature of a VCB is that
5.it can break any heavy fault current perfectly just before the contacts reach the definite open position
6.They require little maintenance and are quite in operation.
7.They can successfully withstand lightning surge,
8.They have low arc energy.
9.They have low inertia and hence require smaller power for control mechanism
Applications : For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance-free circuit breakers should prove a definite advantage, Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited rating of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas.
