In vacuum circuit breakers, vacuum is used as the arc quenching medium.
Vacuum offers the highest insulating strength. So it has far superior arc quenching properties than any other medium.
For example, 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.
The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable.
Principle of Vacuum Circuit Breaker
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 ionisation of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength.
The salient feature of vacuum as an arc quenching medium is that 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.
Construction of Vacuum Circuit Breaker
Typical parts of vacuum circuit breaker are shown in the figure .
It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber.
The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing 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 falling on the inside surface of the outer insulating cover.
Medium Voltage Vacuum Circuit Breaker
Working of Vacuum Circuit Breaker
The working of Vacuum circuit breakers is briefly explained below,
When the breaker operates, the moving contact separates from the fixed 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 vapors, electrons and ions produced during arc 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 a vacuum breaker occurs with a short contact separation (say 0.625 cm).
Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV. Even with limited ratings say 60 to 100MVA, they are suitable for the majority of applications in rural areas.