A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application. For higher voltage vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called vacuum interrupter.
The vacuum interrupter consists of a steel arc chamber in the center symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10– 6 bar.
The material used for current carrying contacts plays an important role in the performance of the vacuum circuit breaker. Cu/Cr is the ideal material to make VCB contacts. Vacuum interrupter technology was first introduced in the year 1960. But still, it is a developing technology.
As time goes on, the size of the vacuum interrupter is being reduced from its early 1960’s size due to different technical developments in this field of engineering. The contact geometry is also improving with time, from but contact of early days it gradually changes to spiral shape, cup shape and axial magnetic field contact. The vacuum circuit breaker is today recognized as most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies.
Working of Vacuum Circuit Breaker
When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of the 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 a 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).
Principle of Vacuum Circuit Breaker
The working principle of the Vacuum Circuit Breaker is that once the circuit breaker is opened inside the contact vacuum, an arc can be generated between the contacts by ionization of the metal vapor in the contact. But the arc can be easily extinguished. This is because fast electrons, ions, and metallic vapor are generated in the outer part of the circuit breaker contact, so that dielectric strength can be achieved quickly.
The most important feature of a vacuum is that once an arc is generated inside the vacuum, it can be quickly extinguished due to the rate of rapid improvement in the dielectric strength of the vacuum.
Contact Materials of Vacuum Circuit Breaker
Vacuum Circuit Breaker contact materials must adhere to the following properties.
Contact resistance must be low.
The electrical conductivity must be high to pass normal load currents without overheating of any kind.
Increases thermal conductivity to quickly dissipate large amounts of heat generated during arcing.
The thermionic function should be high to allow the destruction of the starting arc.
The trend must be less welded.
Low current chopped level.
Resists high arc capacity.
Boiling points should be high to prevent arc erosion.
Gas content must be given below to ensure long service life.