Oil Circuit Breaker & Air Blast Circuit Breaker

Oil Ciruit Breaker:

• Insulating oil like Transformer oil is used as arc quenching medium.
• Transformer oil has very good dielectric strength ie, 110kV/cm.
• In oil circuit breaker the contacts of the breaker are made to separate within an insulating oil.

Working Principle :

During the normal operating conditions, the contact of the oil circuit breaker is closed and carry the current. When the fault occurs in the system, the contacts of the breaker are moving apart under the insulating oil, and an arc is struck between the contacts.

Due to this arc, a large amount of heat is liberated, and a very high temperature is reached which vapourises the surrounding oil and dissociate it into a substantial volume of gaseous hydrogen at high pressure.The hydrogen gas occupies a volume of about one thousand times that of the oil decomposed. The oil is there fore, pushed away from the arc and expanding hydrogen gas bubble surrounds the arc and adjacent portions of the contacts. Hydrogen gas has high heat conductivity and cools the arc

The arc extinction is facilitated by the following process,

1) The hydrogen gas bubble generated around the arc cools the arc column and aids the deionisation of the medium between the contacts.

2) The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc path.

3) As the arc lengthens due to the seperating contacts, the dielectric strength of the medium is increased. The result is that arc is extinguished and circuit current interrupted.

Advantages of Oil circuit breaker :

1. The oil has a high dielectric strength and provides insulation between the contact after the arc has been extinguished.
2. The oil used in circuit breaker provides a small clearance between the conductors and the earth components.
3. The hydrogen gas is formed in the tank which has a high diffusion rate and good cooling properties.

Disadvantages of Oil circuit breaker :

1. The oil used in oil circuit breaker is inflammable and hence, cause a fire hazard.
2. There is a risk of formation of explosive mixture with air.
3. Due to decomposition of oil in the arc, the carbon particles is generated which polluted the oil and hence the dielectric strength of the oil decreases.

Types of Oil circuit Breaker:

Bulk oil circuit breaker

Minimum oil circuit breaker

Air Blast Circuit Breaker (ABCB)

• High pressure air blast as an arc quenching medium.
• Also called compressed air circuit.
• ABCB used compressed air/ gas .as the arc interruption medium.
• Air blast cools the arc and sweeps away the arcing products to the atmosphere.
• Increase the dielectric strength of medium between contacts and prevents from re establishing the arc.so the arc extinguished immediately.

Types of Air blast circuit breaker: Depending upon the direction of air blast in relation to the arc, classified into three,

1. Axial blast type( air blast is directed along the arc path)

2) Cross blast type(air blast is directed at right angle to  the arc path)

3) Radial blast type (air blast is directed at radially at different position to  the arc path)

Construction:

• Fixed & Moving contacts : held in closed position by spring pressure under normal condition within the arcing chamber.
• Arcing Chamber is connected to the air reservoir tank with an air valve. This valve remains closed under normal operations, but opens automatically by tripping impulse occurs on the system.
• An additional compressed plant is connected to air reservoir for maintain sufficient capacity high pressure air production.
• Series isolator : Normal air clearance for working voltage

Working:

when any fault occures in the system, the tripping impulse causes the air valve which connects the breaker reservoir to the arcing chamber. The high pressure air entering the arcing chamber pushes the moving contact against the spring pressure. The moving contacts is seperated and an arc is struck. At the same time high pressure air blast flows along the arc and takes away the ionised particle along with it., so the arc is extinguished and current flow is interrupted.

Noted that , the contact separation required for interruption is gnerally small(1.75 cm)such a small gap may constitute inadequate clearance for the normal service voltage.so an series isolated switch is used for the normal air clearance for the operating voltage, it opens immediately after fault interruption .

Advantages of ABCB:

• Risk fire is eliminated
• Faster operation
• Pollution less
• Less maintenance & simple assembly
• Contact gap is small so reduce the size of device
• Suitability for repeated operation
• Small arcing time-less burning of contacts
• Energy supplied for arc extinction is independent of the current to be interrupted