Air Blast Circuit Breaker

In this air blast circuit breaker, it uses a high-pressure air as the arc quenching medium. In this type of circuit breaker when the contacts are separated, high-pressure air is forced on the arc through a nozzle.

It cools the arc and the arcing products are blown away by the blast of the air. The chamber is filled with high-pressure air after the extinction of arc which prevents restrike.

  

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. 

    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. 

Working of Air Blast Circuit Breaker:

When a breaker receives a signal either pneumatic or electrical the rod connected to valves gets opened. During tint instant, the air enters into the hallow insulator column and further enters into arc extinction chambers. Because of high pressure in are extinction chamber, the pressure increases on the moving contact and becomes more than the spring pressure. Hence. the contacts are separated. Because of high pressure on moving contact, it travels a short distance against the spring pressure and closes the air outlet valves. As a result-the high-pressure air in the arc extinction chamber is not let to go out However. at the arcing period the air goes out through the air outlet valves with all ionized gases.

    Later, if the valves are closed. then the air in the insulator column is let into the atmosphere, due to which the pressure on the moving contact is dropped to atmospheric pressure. Hence. the moving contact closes over the fixed contact by virtue of its spring pressure; in the applications of high voltage.

Types of Air Blast Circuit Breaker:

Depending on the type of flow of blast of compressed air around the breaker contacts the air blast circuit breakers are classified into three types. They are,

·         Axial blast air circuit breaker

·         Cross blast air circuit breaker

·         Radial blast air circuit breaker

• Axial Blast Air Circuit Breaker:

In axial blast type air circuit breakers, the air flows at high speed axially along the arc. The below figures show the axial blast air circuit breaker. Whenever it is required to open the contacts of the circuit breaker, high-pressure air is entered into the arcing chamber which pushes the movable contact against spring pressure.

As soon as the contacts get separated an arc is drawn between them and it is subjected to high-pressure air. Due to the forced convection, a considerable amount of heat is taken away from the arc periphery. So, the arc diameter reduces, and the core temperature increases.

Hence, the steepness of the temperature gradients established in the arc will increase and so the heat losses also increase. Near the current zero instant, the arc diameter will become narrow and finally, the arc will be completely extinguished at the current zero instant. The gap between the contacts is filled with fresh air. Meanwhile, the movable contact closes the exit hole.

Hence, the air pressure inside the arc chamber increases. This high-pressure air inside the arc chamber will have high dielectric strength and hence it will withstand the transient restriping voltage.

• Cross Blast Air Circuit Breaker:

In cross blast type circuit breaker, the air flows at high speed across the arc. So the arc is subjected to high pressure. The below shows the schematic diagram of the cross-blast air circuit breaker. Fixed contacts shown in the figure above consist of copper fingers surfaced with silver placed between the insulating blocks. The moving contact consists of a copper blade that is connected near the arc splitter plates. The capacity of the circuit breaker depends on the number of splitter plates used.

When the arc is forced on the arc splitter plates its length increases, due to this appreciable resistance is introduced in the arc. So, there is no need for resistance switching in this type of circuit breaker. This type of breaker is used to interrupt high currents.

 

• Radial blast air circuit breaker:

In this radial type, the flow of air is longitudinal along the arc. Breaking by employing a double blast is called a radial blast circuit breaker. The air blast flows radially into space between the contacts.

The contacts are separated and an arc is formed between them. The air flows at a great speed axially along the arc. It causes the removal of heat and the arc is reduced to a value at current zero.  Thus, the arc is extinguished. The flow of fresh air builds the dielectric strength between them.

Protective Devices of Air Blast Circuit:

Pressure Relief Device: Air blast circuit breakers are equipped with pressure relief devices, such as rupture discs or relief valves, to prevent excessive internal pressure buildup. If the pressure inside the breaker exceeds a certain threshold, these devices release the excess pressure to protect the equipment from damage.

Arc Chutes: Arc chutes are designed to guide and cool the arc produced during circuit interruption. They are typically made of materials with high arc-quenching properties. Arc chutes create a longer path for the arc, facilitating effective cooling and reducing the energy and temperature of the arc.

Compressed Air System: The compressed air system provides the necessary air pressure to extinguish the arc. It includes components like compressors, air receivers, and control valves. The system should be properly designed and maintained to ensure an adequate and reliable air supply.

Overcurrent Protection: Air blast circuit breakers have built-in overcurrent protection to safeguard the power system against excessive currents. This protection can be achieved through various devices, such as current transformers and protective relays, which detect abnormal current levels and send signals to trip the circuit breaker.

Trip Circuit Supervision: To ensure proper operation, air blast circuit breakers employ trip circuit supervision. This feature continuously monitors the integrity of the trip circuit, which includes the control wiring, trip coils, and associated devices. Any faults or abnormalities in the trip circuit are detected and can prevent the breaker from closing or tripping unintentionally.

Mechanical Interlocks: Mechanical interlocks are used to prevent incorrect operations and ensure proper sequencing of circuit breaker operations. They can prevent the breaker from being closed or opened when certain conditions are not met, helping to maintain the safety and integrity of the power system.

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