The air blast circuit breaker is the type of circuit breaker where high pressure (at a pressure of 30 kg/cm2) air-blast is used for arc extinction. So they seek applications in the high voltage (132 kV and above) transmission system and interconnected networks with breaking capacity up to 7500 MVA.
Principle of Air Blast Circuit Breaker (ABCB) :
The air blast is an external extinguishing energy type in which the high-pressure air is employed for arc extinction in the circuit breaker. The air blast circuit breaker consists of a fixed contact and a moving contact, enclosed in an arc extinction chamber.
Under normal operating conditions, both the contacts are closed. Whenever a fault occurs, high currents are induced which raises the temperature. When the air is submitted into the arc extinction chamber the air pushes away the moving contact establishing an arc. The air inside the arc extinction chamber will have high pressure than the atmospheric pressure.
The air blast cools the arc and sweeps away all the ionized gases along with it. Because of this, the dielectric builds up rapidly, between the contacts, which prevents the reestablishment of the arc. Thus, the flow of current is interrupted.
Construction of Air Blast Circuit Breaker (ABCB) :
A typical air blast circuit breaker is shown in the below figure. The major components of the air blast circuit breaker are,- Air reservoir
- Hallow insulator assembly
- Arc extinction chamber
- Valves
- Current carrying conductors.
Air Reservoir :
The air reservoir is a tank that stores the air at high pressure (at atmospheric pressure of 20-30atm). It is connected to an auxiliary compressed air system that provides high-pressure air to the reservoir.
Hallow Insulators Assembly :
These are mounted on the air reservoir that connects the air reservoir to the arc extinction chamber for the supply of high-pressure air from the air reservoir to the arc extinction chamber. Values are provided at the base of the hallow insulators that control the flow of high-pressure air. They also provide insulation to the other equipment from the high voltage lines.
Arc Extinction Chamber :
The arc extinction chamber is mounted on the hollow insulator assembly. The making and breaking of the circuit are carried out in the arc extinction chamber. It consists of fixed and moving contacts with a spring mechanism. The moving contact moves away and closes to the fixed contact depending upon the air pressure.
Valves :
These are provided at the base of the insulators that regulate the flows of high-pressure air from the air reservoir to the arc extinction chamber. When there is a fault in the circuit the values get opened with the help of an operating rod connected to the pneumatic operating mechanism.
Current Carrying Conductors :
The current carrying conductors link all the arc extinction chambers in series and the poles of neighboring equipment.
Operation of Air Blast Circuit Breaker :
Under normal conditions, the breaker contacts present in the arc extinction chamber are in contact with each other. Under fault conditions, the breaker receives a tripping signal and this makes the valves open by operating rod connected to it. When the valves get opened, the high-pressure air enters into the arc extinction chamber through the hallow insulator assembly.
Once the high-pressure air enters into arc extinction chamber, it causes to increase the pressure on the moving contact inside the arc extinction chamber. Hence the moving contact gets separated from the fixed contact.
At the time of separation, an arc is drawn between the two contacts. The arc is formed due to the ionization of air present between the contacts and this ionized air is taken out by the high-pressure air through the air outlet valves.
Thus the arc formed between the two contacts gets extinguished. After short distance travel by the moving contact, it closes the air outlet valves. As a result, the high-pressure air inside the arc extinction chamber is not let to go out and it maintains the dielectric strength between the contacts that it avoids the restriking of the arc.
Later after the successful breaking of the circuit and clearance of the fault, the valves get closed and the pressure on the moving contact gets dropped to atmospheric pressure. Hence, the moving contact gets moved back (close to the fixed contact) by virtue of its spring pressure and thus making the circuit again.
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 shows 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 shown in figure 1.
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 are 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 as shown in figure 2.
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 restriking 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.
Factors Influencing Performance of Air Blast Circuit Breaker :
The factors which influence the performance of an air blast circuit breaker are air pressure, the distance between contacts, contact material, mass flow, area of cross-section of exit hole, circuit severity, and resistance switching.
Air Pressure :
At higher air pressures, high-speed operation is possible. For a similar contact structure, the breaking capacity increases with the operating pressure.
Distance Between Contacts :
For a particular optimum distance between the contacts, excellent arc extinguishing conditions are obtained. For distances greater than or less than this optimum distance, the arc extinction process is affected which in turn affects the breaking capacity of the circuit breaker.
Contact Material :
By using contact materials of high boiling point, burning, erosion, and flame emission are reduced. Hence, the breaking capacity will be increased by using alloys like tungsten copper. Also, regular replacement of the contacts can be avoided for the entire life span of the breaker.
Area of Cross-section of Exit Hole :
An exit hole is a hole through which the hot and decomposed gases exit. As the arcing chamber's temperature rises severely in a very small time duration, the exit hole must have a sufficient area of cross-section so that fresh air can enter into the arc chamber with high speed. Thus, the arc extinguishes in a short time. Therefore, by increasing the area of the cross-section of the exit hole, the breaking capacity increases.
Circuit Severity :
The circuits with lower MVA ratings are said to be more severe than higher MVA rating circuits. It is because the blast effect is constant for all values of current due to which the rate at which the circuit breaker's gap recovers its dielectric strength will be inversely proportional to the amount of ionization present. It means that the circuits with a low MVA rating will produce a High Rate of Rise of Restriking Voltages (RRRV). The breaking capacity reduces with an increase in RRRV.
Resistance Switching :
In resistance switching, the resistance is connected in parallel with the arc of the circuit breaker which helps in damping out the high-frequency oscillations during the arc extinction process. As a result, the magnitude of RRRV and critical value of shunt resistance is reduced giving rise to the transient free response. Hence, shunt resistance increases breaking capacity and minimizes the chances of restriking of the arc.