Air Blast Circuit Breaker Working


This post is basically based on Air blast circuit breaker working,here you can find out the entire mechanism of air blast circuit breakers with the help of circuit diagram and supportive theory.

In the air blast circuit breakers the arc interruption  takes place to direct a blast of air, at high pressure and velocity, to the arc. Dry and fresh air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased.Consequently the arc may be interrupted at the first natural current zero. In air blast circuit  breakers, the contacts are surrounded by compressed air. When the contacts are opened the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process.A compressor plant is necessary to maintain high air pressure in the receiver.

The air blast circuit breakers are especially suitable for railways and arc furnaces, where the breaker operates repeatedly. Air blast circuit breakers is used for interconnected lines and important lines where rapid operation is desired. In air blast circuit breaker (also called compressed air circuit breaker) high pressure air is forced on the arc through a nozzle at the instant of contact separation. The ionized medium between the contacts is blown away by the blast of the air. After the arc extinction the chamber is filled with high pressure air, which prevents restrike. In some low capacity circuit breakers, the isolator is an integral part of the circuit breaker. The circuit breaker opens and immediately after that the isolator opens, to provide addition gap.

In the air reservoir there is a high pressure air stored between 20 to 30 kg/cm2. And that air is taken from compressed air system. On the reservoir there are three hollow insulator columns mounted with valves at their base. On the top of the hollow insulator chambers there are double arc extinguishing chambers mounted . The current carrying parts connect the three arc extinction chambers to each other in series and the pole to the neighboring equipment. since there exist a very high voltage between the conductor and the air reservoir, the entire arc extinction chamber assembly is mounted on insulators. Since there are three double arc extinction poles in series, there are six breakers per pole. Each arc extinction chamber consists of one twin fixed contact. There are two moving contacts. The moving contacts can move axially so as to open or close. Its opening or closing mechanism depends on spring pressure and air pressure.

The operation mechanism operates the rods when it gets a pneumatic or electrical signal. The valves open so as to send the high pressure air in the hollow of the insulator. The high pressure air rapidly enters the double arc extinction chamber. As the air enters into the arc extinction chamber the pressure on the moving contacts becomes more than spring pressure and it causes the contacts to be open.The contacts travel through a short distance against the spring pressure. At the end of contacts travel the part for outgoing air is closed by the moving contacts and the entire arc extinction chamber is filled with high pressure air, as the air is not allowed to go out. However, during the arcing period the air goes out through the openings and takes away the ionized air of arc.

While closing, the valve is turned so as to close connection between the hollow of the insulator and the reservoir. The valve lets the air from the hollow insulator to the atmosphere. As a result the pressure of air in the arc extinction chamber is dropped down to the atmospheric pressure and the moving contacts close over the fixed contacts by virtue of the spring pressure. the opening is fast because the air takes a negligible time to travel from the reservoir to the moving contact. The arc is extinguished within a cycle. Therefore, air blast circuit breaker is very fast in breaking the current.Closing is also fast because the pressure in the arc extinction chamber drops immediately as the value operates and the contacts close by virtue of the spring pressure.

Air Blast Circuit  Breaker ratings are :

12 KV, 40 KA

22 KV, 40KA

145 KV, 40 KA, 3 cycles

245 KV, 40 KA, 50 KA, 2.5 cycles

420 KV, 40 KA, 50 KA, 63.5 KA, 2 cycles

Advantages:
How air blast circuit breaker is better than oil circuit breaker:
1. The growth of dielectric strength is so rapid that final contact gap needed for arc extinction is very small. this reduces the size of device.
2. The risk of fire is eliminated.
3. Due to lesser arc energy, air blast circuit breakers are very suitable for conditions where frequent operation is required.
4. The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil is replacement is avoided.
5. The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted.
6. The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction that in oil circuit breakers, thus resulting in less burning of contacts.
Disadvantages:
1. Considerable maintenance is required for the compressor plant which supplies the air  blast.
2. Air blast circuit breakers are very sensitive to the variations in the rate of restriking voltage.
3. Air blast circuit breakers are finding wide applications in high voltage installations.Majority of circuit breakers for voltages beyond 110 kV are of this type.
This is a video of Air Blast Circuit Breaker Testing :

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