الأربعاء، 2 فبراير 2011

Vacuum Circuit Breakers

 Vacuum Circuit Breakers

In such circuit breakers, the vacuum is used as the arc quenching medium. The vacuum circuit breaker takes the advantage of non-sustainability of electric arc in vacuum, and employs the principle of contact separation under vacuum where there is no ionization due to medium. The initial arc caused by field and thermionic emissions during the contacts separation, will die away soon, as there is no further ionization because of vacuum.

The degree of vacuum in these circuit breakers is in the range from 10-7 to 10-5 torr. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. When the contacts in the vacuum circuit breakers are opened in vacuum, an arc is produced between the contacts by the ionization of metal vapors of contacts. However, the arc is quickly extinguished because the metallic vapors, electrons and ions produced during arc rapidly condense on the surface of the circuit breaker contacts.

Figure : A Vacuum Circuit Breaker
Vacuum circuit breakers are cost-effective in the medium voltage range. Vacuum interrupters are sealed units and maintenance-free for 10,000 normal load operations.

Construction
The vacuum circuit breaker consists of fixed contact, moving contact and arc shield mounted inside the 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 vapors falling on the inside surface of the outer insulating cover.

Figure : Cross Sectional View of Vacuum Circuit Breaker
Applications
For a country like Pakistan, where distances are quite long and accessibility to remote areas are 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.
This technology has been found to be most suitable for medium voltage application though the experimental interrupters for the 72.5 kV and 145 kV have been developed, they were not found to be commercially viable.

Advantages
1. The vacuum circuit breakers are compact in size and have longer lives.
2. Operating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances.
3. Because of the very low voltage across the metal vapor arc, energy is very low. (Arc voltage is between 50 and 100V.)
4. Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapor re-condenses on the contact, contact erosion is extremely small.
5. There is no generation of gases during and after the circuit breaker operation.
6. The outstanding feature of these breakers is that it can break any heavy fault current perfectly just before the contacts reach a definite open position.
7. They can successfully withstand lightning surges.
8. It is now possible to produce cost-effective VI (vacuum interrupter) designs with electrical lives that exceed the required mechanical life of the circuit breakers, and that will even be able to satisfy a recent requirement of extended shortcircuit operating life.

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