This type of circuit breaker uses an air blast to slake the medium to break the circuit. The air blast swipes away the arcing manufacture to the atmosphere. This type of circuit breaks are used were system voltage is above 245kV, 420kV and even more.
Air Blast Circuit Breakers:
Advantages of Air Blast Circuit Breakers:
An air blast circuit breaker has the following advantages above an oil circuit breaker:
(i) The risk of fire is eliminated.
(ii) The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expenditure of regular oil replacement is avoided.
(iii) The growth of dielectric strength is so fast that final contact gap needed for arc extinction is very small. This minimizes the size of the device.
(iv) The arcing time is very small due to the fast build-up of dielectric strength between contacts. Therefore, the arc energy is only a fraction of that in oil circuit breakers, thus resulting in less fiery of contacts.
(v) Due to smaller arc energy, air blast circuit breakers are very suitable for conditions where the dense operation is required.
Disadvantages of Air Blast Circuit Breakers:
The use of air as the arc slake medium offers the following disadvantages :
(i) The air has relatively contemptible arc extinguishing properties.
(ii) The air blast circuit breakers are very sensitive to the variations in the rate of rising of restriking voltage.
(iii) great maintenance is required for the compressor plant which supplies the air-blast.
Construction of Air Blast Circuit Breaker
The figure shows the development of a typical air blast device. Each interrupter consists of a porcelain insulator, mounted on the air inlet manifold, with the exhaust chamber fixed at the opposite end of the porcelain.
The exhaust chamber consists of casting with a curved hood for weather protection and slots on the underside for the directional exhaust of the compressed air to the atmosphere.
The moving contact assembly structure of a chromium-copper contact coupled to two pistons by means of an insulated tie rod that moves inside the contact tube. The main current in the moving contact is transferred to the contact tube by means of transfer contact fingers.
An arcing point is provided at the end of the moving contact. The moving contact is maintained in a normally closed position with the fixed contact by springs. The details of construction of the interrupter head vary with its interruption rating.
Figure 1 shows the closed position of the interrupter. Figure 2 shows contacts in a partially open position with arcing between fixed and moving contact. Figure 3 shows contacts in a fully open position with arc extinguished.
The interrupting ability of the air blast circuit switchgear is usually increased by increasing the normal pressure range. Generally, the pressure level is around 30 to 35 bars. In order to maintain the insulation level and credibility of operation, it is also necessary for the condition of the air to be very dry.
Types of Air Blast Circuit Breakers
- Axial Blast Type – air blast is conducted along the arc path.
- Cross Blast Type – air blast is conducted at right angles to the arc path.
- Radial Blast Type – air blast is conducted radially