Air Blast Circuit Breakers
These circuit breakers employ a high pressure air-blast as an arc quenching medium. The contacts are opened in a flow of air-blast established by the opening of the blast valve. The air-blast cools the arc and sweeps away the arcing products of the atmosphere. Consequently, the arc is extinguished and flow of current is interrupted.
Whenever current at high voltages needs to be interrupted, more breaking units are used, in series. Dry and clean air supply is one of the most essential requirements for the operation of the air-blast circuit breakers.
Figure : Air-Blast Circuit Breaker
In addition, other gases such as Nitrogen, Carbon dioxide, and Hydrogen can also be used. But air is preferred because of the fact that the Carbon dioxide tends to freeze, and the hydrogen gas is very expensive.
This type of circuit breaker has been used earlier for open terminal HV applications, for voltages of 245 kV, and 400 kV up to 765 kV, especially where faster breaker operation was required.
This type of breaker has been used for special applications wherein several super thermal power stations located in close vicinity to each other have been inter-connected. These one-cycle breakers from the inter-connecting link which, in the event of a fault in an outgoing feeder of a particular station, isolates the station from the neighboring ones thereby limiting the fault level for the feeder breaker.
The interrupting capability of air circuit breaker is usually increased by increasing the normal pressure range. Normally, the pressure level is around 30 to 35 bars. In order to maintain the insulation level and reliability of operation, it is also necessary for the condition of the air to be very dry.
Currently, however, SF6 circuit breakers have practically eliminated the use of this technology.
Advantages:
1. The risk of fire is eliminated in these circuit breakers.
2. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations. So the expenditure of oil replacement is avoided in air-blast circuit breakers.
3. The size of these breakers is reduced, as the dielectric strength grows so rapidly that final contact gap for the arc extinction is very small.
4. Due to the rapid growth of the dielectric strength, the arcing time is also very small. It causes less burning of oil. The arc energy is also very small fraction of that in oil circuit breakers.
5. The arc extinction is facilitate by the high pressure air, and is independent of the fault current to be interrupted.
2. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations. So the expenditure of oil replacement is avoided in air-blast circuit breakers.
3. The size of these breakers is reduced, as the dielectric strength grows so rapidly that final contact gap for the arc extinction is very small.
4. Due to the rapid growth of the dielectric strength, the arcing time is also very small. It causes less burning of oil. The arc energy is also very small fraction of that in oil circuit breakers.
5. The arc extinction is facilitate by the high pressure air, and is independent of the fault current to be interrupted.
Disadvantages:
1. These circuit breakers are very sensitive to the variation s in the rate of rise of restriking voltage.
2. The air-blast is supplied by the compressor plant that needs considerable maintenance.
2. The air-blast is supplied by the compressor plant that needs considerable maintenance.
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