Thursday, April 7, 2011

Merits and Demerits of Vacuum Circuit Breaker

5. Description of different Circuit Breaker
5.5 Vacuum circuit breaker
5.5.4 Merits of Vacuum Circuit Breakers
       The vacuum switchgear has been successfully developed and is gaining rapid popularity. The vacuum switches are likely to be popular for wide range of applications. These switches devices have several merits such as :
  1.  VCB is self contained  and does not need filling of gas or oil. They do not need auxiliary air system, oil handling system, etc. No need for periodic refilling.
  2. No emission of gases, pollution free.
  3. Modest maintenance of the breaker, no maintenance of interrupters. Hence economical over long period.
  4. Breakers forms a unit which can be installed at any required orientation. Breaker unit is compact and self contained.
  5. Non-explosive
  6. Silent operation.
  7. Large number of operation on load, or short circuit. Suitable for repeated duty.
  8. Long life of the order of several hundred operations on rated normal current.
  9. Constant dielectric. There are no gas decomposition products in vacuum and the hermetically sealed vacuum interrupter keeps out all environmental effect.
  10. Constant contact resistance. In vacuum, the contacts cannot be oxidized, a fact which ensures that their very small resistance is maintained through their life.
  11. High total current switched. Since contact piece erosion is small, rated normal interrupted current is up to 30.000 times; and rated short circuit breaking current is on the average of a hundred times.
       The above reasons, together with the economic advantages offered, have boosted acceptance  of the vacuum arc quenching principle.

5.5.5  Demerits of Vacuum Circuit Breakers
  1.  The vacuum interrupter is more expensive than the interrupter devices in other types of interrupters and its cost is affected by production volume. It is uneconomical to manufacturer vacuum interrupters in small quantities.
  2. Rated voltage of single interrupter is limited until very recently to about 36√3 = 20 KV above 36 KV, tow interrupters are required to be connected in series. This makes the breaker uneconomical for voltage rated above 36 KV.
  3. Vacuum interrupters required high technology for production.
  4. In the event of loss of vacuum, due to transient damage or failure, the entire interrupter is rendered useless. It cannot be required at site.
  5. For interrupter low magnetizing currents, in certain range, additional surge suppressors are required in parallel with phase of a VCB.
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