Air blast circuit breaker is used to interrupt heavy currents and for the extinction of arc with the help of blast of air.
Air blast circuit breakers employ high pressure air blast as an arc quenching medium. The high pressure air blast cools the arc and sweeps away the arcing products to atmosphere.
Now, one question arises here that, why only air is used for arc quenching instead of other gases such as Nitrogen, Carbon di-oxide or hydrogen ?
So the answer of not using above mentioned gases over air is as follows,
Nitrogen- as nitrogen gas has almost similar circuit breaking properties as of air, so, we don’t use Nitrogen in air blast circuit breakers.
Carbon-di-oxide – carbon di oxide gas tends to freeze, hence it is not used.
Hydrogen – Hydrogen gas best cooling properties and it is the most suitable gas for circuit breaking operation (arc quenching), but due to it,s very high cost, we don’t use hydrogen gas.
In air blast circuit breakers, highly compressed air is used, so these breakers are also called compressed air circuit breakers.
These breakers are most suitable for high voltages and majority of these are used for voltages beyond 110kV.
Construction of Air blast circuit breakers
Pictorial diagram of air blast circuit breakers is given below and you will see such figure in substations. Let’s understand the construction.
At the bottom, there is a tank which is called air reservoir, and this air reservoir is connected with an air valve.
On the air reservoir tank, there are three hollow insulator columns. On the top of each insulator column there is double arc extinction chamber.
The current carrying parts are connected to the arc extinction chamber in series.
The assembly of entire arc extinction chamber is mounted on insulators as there exists large voltage between the conductors and air reservoir.
The main arc extinction phenomenon is done in the double arc extinction chamber. We will see the arc extinction phenomenon in the types of air blast circuit breaker.
Working of air blast circuit breakers
An auxiliary compressed air system is required by this type of circuit breaker. This system will supply air to the reservoir of the breaker.
During the fault operation, the air is allowed to enter in the extinction chamber which pushes away the moving contacts.
As we know that when contacts get separated then an arc is struck between them. This arc has to be interrupted as soon as possible.
So, in this circuit breaker the contacts are separated under the action of air blast, this air blast will take away the ionized gases and arcing products with it and finally helps in extinguishing the arc.
This is very frequent operation, hence the arcing time is very less in air blast circuit breakers.
Types of air blast circuit breaker
According to the type of flow blast of air, two major types are –
- Axial blast type – In this type, the blasting of air is done directly along the path of arc, as shown in figure below.
- Cross blast type-in this type, the blasting of air is directed at right angle to the arc path, as shown in the figure below.
we will now discuss both these types in detail.
Axial blast air circuit breaker-
This is the most common type of ait blast circuit breaker employing axial flow of blast air for extinction of arc, as shown in the fig. below
The arcing chamber is connected to the air reservoir with an air valve as shown. The fixed contact and moving contact are held in closed position by the piston and the spring pressure under normal conditions.
The air valve remains closed under normal condition, but it opens automatically by the tripping impulse when any fault occurs in the system.
A series isolator is also connected in the axial blast type to give the sufficient voltage clearance after the breaking operation.
When any fault occurs in the system, the tripping impulse causes the air valve to open automatically and it connects the air reservoir to arc extinction chamber.
The high pressure compressed air now move into the extinction chamber where fixed and moving contacts are in closed position with the spring pressure.
Now, as the arc extinction chamber is now filled with compressed air so, it will exert pressure on moving contact against the spring pressure.
When the air pressure becomes more than the spring pressure then, moving contact will separate and an arc will be struck between moving and fixed contact.
If you see closely then you will find that the pressure of air blast is acting axially to the arc and it will decrease the area of cross-section of the arc.
We know that resistance is inversely proportional to area of cross-section, it means when area of cross section decreases then the resistance will increase, in other words dielectric strength of the path will increase.
As, the current is flowing through the arc but due to increase in resistance, current flow will stop, also the arcing products (ionized gases) are taken away with the blast of air to the atmosphere. Hence, arc is extinguished.
In such circuit breakers, the contact separation required for interruption is very small (1.75 cm or so). Such a small gap may constitute inadequate clearance for the normal service voltage.
Therefore, an isolating switch is incorporated as the part of this circuit breaker which open immediately after fault interruption and provides necessary clearance for insulation.
Cross blast air circuit breaker-
In this type of circuit breaker, the air blast is directed at right angles to the arc. We will see the construction and working of cross blast air circuit breakers below.
Air reservoir filled with compressed air is connected to the arc extinction chamber with the help of air valve.
The Arc extinction chamber contains fixed and moving contacts. Arc splitters and baffles are also connected as shown in the figure.
These Arc splitters are used to increase the length of the arc whereas baffles are used to provide necessary cooling.
when the moving contacts are withdrawn due to fault in the system, an arc is struck between the moving and fixed contacts.
Now, the high pressure air which is coming at right angle to the arc will force the arc into a chute consisting of arc splitters and baffles.
These arc splitters will increase the length of arc as shown in figure. As we know that resistance is directly proportional to the length. It means if the length of arc increases, then resistance (or dielectric strength) of the path will also increase.
The increment in length of arc will increase the dielectric strength of the path and finally it will interrupt the flow of current. Hence, the arc is extinguished.
Since the blast pressure is independent of value of fault current, so it is very efficient at low currents also.
Here in cross blast air circuit breaker, a series isolating switch is not necessary, because the final gap for interruption is good enough to give normal insulation clearance.
The various advantages of air blast circuit breakers are,
- High speed of operation is achieved.
- Risk of fire is eliminated.
- The growth of dielectric strength is very fast so that final gap need for the arc extinction is very less. This reduces the size of device.
- Arcing time is very small due to rapid build-up of dielectric strength between contacts. Therefore, arc energy is also very small which finally results is lesser burning of the contacts.
- They provide frequent operation due to lesser arc energy.
- This type of circuit breaker needs less maintenance.
The various disadvantages of air blast circuit breaker are,
- It is necessary to have a compressor with sufficient capacity of high pressure air.
- There is a possibility of air leakages at pipe fittings.
- It is very sensitive to restriking voltage.
- Maintenance of compressor is required.
- In arc furnace.
- In traction purpose.
- Range of voltages (110kV to 400kV) with breaking capacity of 700MVA.
This is all about air blast circuit breaker. Comment down your views and doubts.
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