Oil Circuit Breakers

 Oil Circuit Breakers

In such circuit breakers, some insulating oil (i.e., transformer oil) is used as an arc quenching medium. The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of hydrogen gas at a high pressure. This large volume of the hydrogen gas pushes the oil away from the arc.

In an oil circuit breaker, the arc quenching process is entirely dependent on arc energy generated. The arc drawn across the contacts is contained inside the interrupting pot, and thus the hydrogen gas formed by the vaporized oil (gas) is also contained inside the chamber. As the contacts continue to move, and the moving contact rod separates itself from the orifice of the chamber, an exit similar to a nozzle allows escape of the hydrogen gas trapped inside the interrupting chamber. The escaping high pressure hydrogen gas, having a high thermal conductivity, takes away the heat, thus making the contact gap cool and free from ionization, immediately after current zero.

However, the oil breakers have prolonged arcing times, due to insufficient vapor pressure generated at lower interrupting currents. Periodic monitoring of the oil dielectric conduction and its maintenance is important for the effective operation of medium oil circuit breaker. However, maintenance required for oil circuit breakers is of low technology.

Figure : Oil Circuit Breaker

The arc extinction is facilitated mainly by two processes:

1- Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionization of the medium between the contacts.

2- Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. This results in extinguishing the arc and as a result the circuit current is interrupted.

Advantages:

As an Arc extinguishing medium oil has the following advantages:

1- Oil absorbs the arc energy to produces hydrogen gas during arcing. The hydrogen has excellent cooling properties and helps extinguish the arc. (In addition to hydrogen gas, a small proportion of methane, ethylene, and acetylene are also generated in oil decomposition.)

2- The oil provides insulation for the live exposed contacts from the earthed portions of the container.

3- Oil provides insulation between the contacts after the arc has been extinguished.

4- The oil close to the arc region provides cooling surface.

Disadvantages:

1- Oil is inflammable and may cause fire hazards. When a defective circuit breaker fails under pressure, it may cause an explosion.

2- The hydrogen generated during arcing, when combined with air, may form an explosive mixture.

3- During arcing, oil decomposes and becomes polluted by carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement.

Types of Oil Circuit Breakers

The oil circuit breakers find extensive use in the power systems. They can be classified with the reference to the quantity of oil used. The types are:

1- Bulk Oil Circuit Breakers
2- Minimum Oil Circuit Breakers

a) Bulk Oil Circuit Breakers

These circuit breakers use a large quantity of oil. The oil has to serve two purposes:

1. It extinguishes the arc during opening of contacts.
2. It insulates the current conducting parts from one another and from the earthed tank.

In the bulk oil circuit breakers, the interrupting unit is placed in a tank of oil at earth potential and the incoming and outgoing conductors are connected through insulator bushings.

b) Minimum Oil Circuit Breakers

These circuit breakers use a small quantity of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts insulated by air or porcelain or organic insulating material.

In these circuit breakers, the oil requirement can be minimized by placing the interrupting units, in insulating chambers at live potential, on an insulator column.