What is an Air blast Circuit Breaker?
A circuit breaker that uses high-pressure(pressure of 30 kg/cm2) compressed air or gases for arc extinction purposes in an arc quenching medium is called an air blast circuit breaker.
Principle of Air blast Circuit breaker
An air blast circuit breaker uses compressed air as an arc quenching medium. Compressed air is stored in the tank. when the fault occurs fixed and moving contacts are separated and the arc is produced between the contacts. During this blast valve of the breaker are opened and a high-pressure air blast cools down the arc and sweeps away ionized particle of air due to the arc.
This rapidly increases the dielectric strength of the medium between the fixed and moving contacts and prevents from restriking of the arc. by this way arc is extinguished and the current is interrupted.
Construction of Air blast Circuit breaker
An air blast circuit breaker consists following major components
1. Air receiver
2. Air Valve
3. Arcing Chamber
4. Fixed and moving Contacts
5. Hollow insulators
6. Closing spring
1. Air receiver
The Air receiver is a storage tank that stores high-pressure compressed air. the air receiver unit is connected to the air compressor unit which provides compressed air to the air receiver. the pressure of air in the air receiver is 20-30 atm.
2. Air valve
The air valve is connected between the arcing chamber and the air receiver. when the fault occurs in the system this valve gets opened and provides high-pressure air into the arcing chamber.
3. Arcing chamber
The arcing chamber consists of fixed and moving contacts and closing. the arcing chamber is connected to the air receiver through the air valve.
4. Fixed and moving contacts
The fixed and moving contacts are made of chromium-copper martial. during normal conditions fixed and moving contacts remain in closed condition due to force of closing spring.
5. Hollow insulators
The hollow insulators are made from glass or porcelain material. Arcing chamber and fixed, moving contacts are enclosed inside the hollow insulators. hollow insulators insulates arcing chamber and fixed moving contacts from the earth.
6. Closing spring
The closing spring is used to keep fixed and moving contacts in closed conditions during normal conditions.
Working of Airblast Circuit Breaker
During normal operating conditions, fixed and moving contacts remain in closed condition due to the force of the closing coil.
When falt occurs, fixed and moving contacts are separated from each other, and an arc is produced between them. the separation of fixed and moving contacts is done by air blast produced by the opening of the blast valve which is connected between the air receiver and arcing chamber.
The air blast cools down the arc and sweeps away the ionized particle of air into the atmosphere that is produced due to the arc.
The dielectric strength of arcing medium is increased which is prevents the restriking of the arc. by this way arc extinguished and the current interrupted.
The separation between the fixed and moving contacts is very small 1.75 cm. this small gap is very improper clearance for normal service voltage. so isolating switch is included with a circuit breaker for more protection and safety.
Types of Air Blast Circuit breaker
Depending upon the direction of flow of compressed high-pressure air around the arc between the fixed and moving contacts air blast circuit breaker is classified as bellows
1. Axial blast air circuit breaker
2. Cross blast air circuit breaker
3. Radial blast air circuit breaker
1. Axial blast circuit breaker
During the normal operating conditions, the fixed and moving contacts are held in a closed condition with help of a closing spring as shown in the below figure. In an axial blast, circuit breaker air blast is directed along the arc path.
The Air reservoir is connected to the arcing chamber through a controllable air valve. The air valve controls the flow of high-pressure compressed air into the arcing chamber. these air valves remain in closed positions during normal conditions.
During fault conditions, a tripping impulse is produced and this tripping impulse gave the command to the air valve and the air valve opened.
The air valve connects the air reservoir to the arcing chamber and high-pressure air enters into the arcing chamber and pushes away the moving contacts aginst the closing spring.
The moving contacts are separated from fixed contacts and an arc is produced between the contacts, and a high-pressure air blast flows along the arc. Due to high-pressure air blast, ionized particles from the arc are taken away from arcing chamber.
In this way arc in the air blast, the circuit breaker is extinguished and the fault current is interrupted.
2. Cross Blast Air Circuit Breaker
In Cross blast air circuit breaker, an air blast is directed right angle to the arc as shown in the figure. Cross blast of air lengthen the arc and forces the arc into the arc splitters or arc chutes for arc extinction purposes.
The cross blast circuit breaker consists of arc splitters and baffles. Arc splitter lengthen the arc and increase the resistance of the arc and baffles cool down the arc.
When a fault occurs, fixed contacts separate from moving contacts, and an arc is produced between the contacts. High-pressure cross blast air forces the arc towards the arc splitters which is lengthening the arc and baffles cool down the arc.
In this way, the arc is extinguished and the current is interrupted in the cross blast circuit breaker.
3. Radial Blast Circuit Breaker
In radial blast circuit breaker, air blast directed radially along arc path. breaking of circuit breaker employed by double air blast is called Radial air blast circuit breaker.
During the fault condition, contacts are separated and the arc is produced between the fixed and moving contacts. The air blast flows at great speed axially along the arc. It is reduced the heat of the arc and the arc is extinguished and the current is interrupted.
Advantages of Air blast Circuit breaker
1. In an Air blast circuit breaker, the risk of fire is eliminated.
2. In air blast circuit breaker operating speed high.
3. In air blast circuit breaker arcing products are completely removed by the air blast. In oil circuit breaker oil is deteriorated after successive operations and regular replacement of oil is required. as compared to oil circuit breaker air blast circuit breaker has less maintenance cost.
4. Air blast circuit breakers are useful for frequent operations.
5. In air blast circuit breaker arcing time is very small due to rapid build-up of dielectric strength between the contacts.
6. Air blast circuit breaker cheap because we are using air as arc extinction medium.
7. Air blast circuit breakers are easily accessible and weight is relatively less in the case of air blast circuit breakers.
8. Air Blast circuit breaker allows high-speed arc interruption.
9. Air blast circuit breaker has a high rupturing capacity.
10. Resistance switching employed in air blast circuit breaker which is reduced growth of restriking voltage.
Disadvantages of Air blast Circuit breaker
1. Air compressor required regular maintenance which supplies the air blast.
2. The air blast circuit breaker is very sensitive to variations rate of rise of restriking voltage.
3. Current chopping phenomenon occurred in the air blast circuit breaker.
Current chopping: Current chopping is the phenomenon in which arcing current is forcibly interrupted before natural current zero occurs. The current chopping phenomenon occurs mostly in Vacuum circuit breakers and air blast circuit breakers.
4. Air blast circuit breaker requires additional equipment like motor, air compressor, etc which is increase the cost of the breaker.
5. frequent operations are taking place in air blast circuit breakers so a higher capacity air compressor is required.
6. There are chances of air leakage from air pipes joints.
Applications of Air blast Circuit breaker
1. It is used for arc furnaces industry and railway traction systems because they are useful for repeated duty.
2. In air blast circuit breakers are used 132Kv to 400 Kv systems with breaking capacity of 7000mva.
3. Air blast circuit breakers are mostly used in plants where the risk of fire or explosion is more.
4. Air blast circuit breaker used in electricity sharing system up to 15kv.