Oil Circuit Breaker

 

        An oil circuit breaker is a traditional type of circuit breaker. For arc extinguish in this transformer used oil so it is called oil transformer. It has separate contact. The primary function this contact is to separate insulating oil. When fault or problem occurs, this includes good comparable properties to air, which opens in the lower part of the breaker contact oil.

        The oil in oil circuit breakers (OCBs) serves two purposes. It insulates between the phases and between the phases and the ground, and it provides the medium for the extinguishing of the arc. When electric arc is drawn under oil, the arc vaporizes the oil and creates a large bubble that surround the arc. The gas inside the bubble is around 80% hydrogen, which impairs ionization. The decomposition of oil into gas requires energy that comes from the heat generated by the arc. The oil surrounding the bubble conducts the heat away from the arc and thus also contributes to deionization of the arc.

            When the arc strikes between the two breaking contacts, the heat from the arc dissolves the oil around it, and the high pressure separates some gaseous hydrogen. The unique feature of this circuit breaker is its low cost, reliable operation, and ease of use.

Construction:

This type of circuit breaker is simple to build. It has current-carrying contacts that are encased in a sturdy metal tank. The tank is filled with transformer oil here. The transformer oil acts as an insulator and arc extinguisher between the existing element and the earth.

        The air can be filled within the tank that acts as a pad to maintain the transferred oil on the formation of gas in the region of the arc at the peak of the transformer oil. It helps to absorb the mechanical shock of rising oil movement. The oil tank in this breaker would be bolted to absorb the vibration caused by interrupting the high current flow. This includes a gas outlet installed in the oil tank cover for gas elimination.

Working Principle:

When the oil circuit breaker contacts are opened under oil and an arc is formed between them. The arc’s heat evaporates the surrounding oil, dissociating it into a large volume of gaseous hydrogen at high pressure.

In normal operation of this circuit breaker, the contact in the breaker will be closed as well as carries the current. Once the fault happens within the system, then the contacts will move apart & an arc will be struck among the contacts.

                        

Because of this arc, a huge amount of heat will be released & high temperature can be achieved to vaporize the nearby oil to gas. So this gas will be surrounded by the arc & its unstable increase around it will move the oil violently.

            The arc will be turned off once the space between the contacts like fixed & moving arrives at a certain critical value. It mainly depends on the recovery voltage & arc current. The operation of this circuit breaker is extremely reliable & cheap. The main feature of this circuit breaker is, there is no particular devices are used while controlling the arc which is caused by moving contact.

Oil Circuit Breaker are generally of two types and they are

1) Bulk Oil type or the Dead tank type (OCB)

2) Minimum Oil Circuit Breaker (MOCB) 

Bulk Oil type or the Dead tank type (OCB):

                 Bulk Oil type or the Dead tank type (OCB) The bulk oil circuit breakers are those which use large volumes of oil for two specific purposes. These oils need large tanks which solve the purpose of arc quenching chamber.

The oil is used for

a) Providing insulation from the live current carrying contacts

b) Serving as a medium of arc quenching.

The tank, that houses the current carrying contacts, is at ground potential. When the moving contacts open to initiate the circuit breaking process, an arc is struck inside the oil medium. Due to oil heating, it vaporizes to produce hydrogen and other hydrocarbon gases. These gases generate high pressure in the vicinity of the arc and cool it. Subsequently, the arc can’t sustain itself and gets quenched.

 Minimum Oil Circuit Breaker (MOCB):

        These types of circuit breakers utilize oil as the interrupting media. However, unlike bulk oil circuit breaker, a minimum oil circuit breaker places the interrupting unit in insulating chamber at live potential. The insulating oil is available only in interrupting chamber.

The features of designing MOCB is to reduce requirement of oil, and hence these breakers are called minimum oil circuit breaker. As the volume of the oil in bulk oil circuit breaker is huge, the chances of fire hazard in bulk oil system are more. For avoiding unwanted fire hazard in the system, one important development in the design of oil circuit breaker has been introduced where use of oil in the circuit breaker is much less than that of bulk oil circuit breaker.

        It has been decided that the oil in the circuit breaker should be used only as arc quenching medium, not as an insulating media. In this type of circuit breaker, the arc interrupting device is enclosed in a tank of insulating material which as a whole is at live potential of system. This chamber is called arcing chamber or interrupting pot.  

        The gas pressure developed in the arcing chamber depends upon the current to be interrupted. Higher the current to be interrupted causes larger the gas pressure developed inside the chamber, hence better the arc quenching. But this puts a limit on the design of the arc chamber for mechanical stresses. With use of better insulating materials for the arcing chambers such as glass fiber, reinforced synthetic resin etc, the minimum oil circuit breaker are able to meet easily the increased fault levels of the system.

 

In a minimum oil circuit breaker, the arc is drawn across the current carrying contacts is contained inside the arcing chamber. Hence the hydrogen bubble formed by the vaporized oil is trapped inside the chamber. As the contacts continue to move, after its certain travel an exit vent becomes available for exhausting the trapped hydrogen gas. There are two different types of arcing chambers, available in terms of venting. One is axial venting and other is radial venting. In axial venting, gases (mostly Hydrogen), produced due to vaporization of oil and decomposition of oil during arc, will sweep the arc in axial or longitudinal direction.

Let’s have a look on working principle Minimum Oil Circuit Breaker with axial venting arc chamber. The moving contact has just been separated and arc is initiated in MOCB. The ionized gas around the arc sweeps away through upper vent and cold oil enters into the arcing chamber through the lower vent in axial direction as soon as the moving contact tip crosses the lower vent opening and final arc quenching in minimum oil circuit breaker occurs.

            The cold oil occupies the gap between fixed contact and moving contact and the minimum oil circuit breaker finally comes into open position. Whereas in case of radial venting or cross blast, the gases sweep the arc in radial or transverse direction. The axial venting generates high gas pressure and hence has high dielectric strength, so it is mainly used for interrupting low current at high voltage. On the other hand, radial venting produces relatively low gas pressure and hence low dielectric strength so it can be used for low voltage and high current interruption. Many times, the combination of both is used in minimum oil circuit breaker so that the chamber is equally efficient to interrupt low current as well as high current. These types of circuit breaker are available up to 8000 MVA at 245 KV.