Saturday, March 26, 2011

Merits and Demerits of Air Blast Circuit Breaker

5. Description of different Circuit Breaker
5. 2. Air-Blast Circuit-Breaker
5.2.4 Merits and Demerits of Air Blast Circuit Breaker
Merits :
  1. Can be used at high pressure.
  2. Reliable operation due to external source of extinguishing energy.
  3. Free from decomposition.
  4. Clean, non-inflammable. 
  5. Air is freely available everywhere. 
  6. Fresh medium is used every time. Hence the breaker can be repeatedly operated, if designed for such duty.
  7. At high pressure the small contact travel is enough. 
  8. The same are serves purpose of moving the contact and arc extinction. 
  9. High speed of operation. The compressed air moves very fast and brings about the opening operation. The arcing time is also short. Hence total breaking time is short operation mechanism of air blast circuit breaker are pneumatic. The arcing time is almost exactly 0.01 second, i.e 1/2 cycle plus another 1/2 cycle  for operation the contacts. Hence breaker speed of the order of 2 cycles can be achieved.  This makes the circuit breaker suitable for important lines because high speed opening and auto-reclosure can improve system stability. 
  10. Rapid auto-reclosure The circuit breaker can be given rapid auto-reclosure feature. The manufacturer gives such a provision at additional cost. The ABCB  is easy to reclose because the reclosure is by spring pressure against reduced air pressure. 
  11. Clean service. No need of maintenance as of oil
  12. Unit type construction gives advantage in design, manufacturer and testing
  13. Very high breaking capacities and service voltage can be obtained by connection more number of units in series. Hence for all extra high voltage and high breaking capacities of today air-blast circuit breakers are used, e.g. 420 KV, 63.5 KA, 2 cycles
  14. Suitability for repeated operation,  The fresh air is used every time. Hence the breaker can be used for repeated operation if designed for duty. This is not the case with oil circuit breaker.
Demerits :
  1. Complex design of arc extinction chambers and operating mechanisms, problems for switchinfg over voltages are reduced by reclosing resistors.
  2. Auxiliary high pressure air system is necessary. The cost can be justified if there several breakers in the switching yard. For a single breakers the cost of auxiliary compressed air system would be too high.
5.2.5 Maintenance aspects
       Maintenance of ABCB is comparatively easier than that of tank type oil circuit breaker and minimum oil circuit breaker. This is because there is no oil, which needs regular testing and purefication. Secondly, the units can be easily deassembled, checked and reassembled. The assembly of various units is similar and easy. The operating mechanism can be easily dismantled and reassembled. 
       The major problem in air blast installation is the leakage from compressed air system and from the pipe connection. The leakage takes place from the threaded joints or from mating parts joined by means of nut-bolts.
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