Explain the Working Principle of Oil Circuit Breaker (OCB)

What is an Oil Circuit Breaker?

An oil circuit breaker, also known as an OCB, is a type of circuit breaker that use insulating oil as the dielectric medium in order to safely break the circuit and quench the arc. 

Insulating oil, which is typically transformer oil since it has a higher dielectric strength than air, is the oil that is used. 

Because of the heat generated by the arc, the oil is vaporized, which results in the formation of a hydrogen gas bubble that surrounds the arc. 

During the zero-crossing, the arc is extinguished as a result of the gas bubble being compressed by the pressure of the oil, which increases the dielectric strength of the gas bubble.

Oil Circuit Breaker Diagram

Construction of an Oil Circuit Breaker

The structure of this type of circuit breaker is easy. 

It contains current-carrying contacts that are housed in a sturdy metal tank. 

The tank is filled with transformer oil. The transformer oil serves as an insulator and arc extinguishing medium between the existing element and the earth.

At the peak of transformer oil, air can be pumped into the tank, which serves as a pad for managing the transferred oil and the formation of gas in the arc zone. 

It absorbs the mechanical shock from the upward movement of oil. The oil tank in this breaker needs to be fastened to withstand the vibration created by interrupting the high flow of electricity. 

This features a gas outlet that is fitted to the oil tank cover to eliminate the gasses.

Working Principle of an Oil Circuit Breaker

When a fault occurs, the current-carrying contacts get separated within the insulating oil. 

As the contacts split, the large voltage difference between them ionizes the medium around it, resulting in an arc. The intense heat produced by the arc evaporates the oil around the contacts. 

The oil decomposes mostly into hydrogen gas, with traces of methane, ethylene, & acetylene. The degraded gases produce a gas bubble around the contacts.

Two processes are principally responsible for arc extinction.

• For starters, since hydrogen gas possesses high thermal conductivity (heat conductivity), it cools the arc and helps deionize the medium across the contacts.
• Second, the gas creates turbulence in the oil, pushing it into the gap between contacts and removing the arcing substances from the arc path.

As the outcome, the arc is extinguished & the circuit current is disconnected.

The hydrogen gas decomposes into atomic form, producing enormous heat that raises the temperature of arc. As a result, oil vaporization increases. The volume of gas created is 1000 times that of the degraded oil. 

High temperatures cause the gas bubble’s volume to rapidly grow. The surrounding oil inside an enclosed container exerts significant pressure on the gas bubble. 

Because of compression, the ionized medium around the contacts begins to de-ionize. 

As the pressure increases due to the heat of the arc, the medium rapidly de-ionizes, increasing its dielectric strength. The arc is extinguished at the next current zero crossing.

What are two Functions of Oil in a Circuit Breaker?

1. Provide insulation between the oil circuit breakers. 
2. Provide cooling for the contacts. Quenching arc.

Standards of Oil Circuit Breaker (OCB)

• IEEE C37.013 – IEEE C37.013 specifies the design & performance of AC high-voltage circuit breakers, particularly ones that use oil as an interrupting medium.
• IEC 62271-100 – IEC 62271-100 defines the specifications for high-voltage alternating current circuit breakers, including oil circuit breakers.
• ANSI C37 – The ANSI C37 series comprises specifications for high-voltage circuit breakers, a few of which may apply to oil circuit breakers.

Maintenance of Oil Circuit Breakers

All types of circuit breakers require maintenance. Similarly, it is necessary to inspect and replace both the oil and the contacts. When a short circuit happens, the circuit breaker is tripped. 

Arcing can occasionally cause damage to contacts. As a result, the dielectric oil can get carbonized inside the contact region, reducing its dielectric strength and breaking capacity. 

As a result, breaker maintenance is required to inspect and replace the oil and contacts.

Check the following points before checking the circuit breaker:

1. Examine the interior parts and arcing contacts. When it develops a short circuit, the contacts need to be replaced.
2. Check the coil’s dielectric strength.
3. The breaker’s surface must be cleaned and carbon deposits removed using a dry, firm fabric.
4. Check the level of oil.
5. Tripping and closing mechanisms must be tested.
6. Furthermore, the cooling impact of the oil & gas bubbles assists in arc quenching.

Advantages of Oil Circuit Breaker

1. It expends a smaller quantity of oil.
2. A high dielectric strength is possessed by oil.
3. When the oil in the breaker continues to decompose, it will take up the energy of the arc.
4. With less room, the risk of fire might be decreased.
5. Moreover, maintenance also reduced.

Disadvantages of Oil Circuit Breaker

1. It makes use of a less quantity of oil, which results in an increase in the amount of carbonization.
2. It is difficult to remove the gasses that are contained within the contact gap.
3. Because of the significant amount of carbonization, the oil’s dielectric strength will decrease in a short amount of time.
4. The arcing time is quite lengthy.
5. Ensure that there is no disruption caused by high-speed
6. It is possible to control the interruption of the arc by taking into account the length of the arc.
7. By use of the air, it is capable of forming any volatile mixture.

When do we use Oil Circuit Breaker?

Oil circuit breakers (OCBs) are generally utilized in high-voltage applications that require reliable current interruption. They are used in systems with voltages ranging from 33 to 220 kV and are ideal for outdoor switchyards & substations.

The main reasons for employing oil circuit breakers are:

• Oil is a suitable medium for quenching the arc formed after a current interruption, preventing equipment damage.
• OCBs are appropriate for high-voltage transmission & distribution networks because of their capacity to manage enormous currents and voltages.
• In large substations with high fault levels, OCBs are a dependable and long-lasting option because oil provides insulation between the breaker contacts.
• Oil circuit breakers are commonly used in outdoor substations because they can resist harsh weather conditions and provide insulation & arc protection.

While newer technologies, including as SF6 and vacuum circuit breakers, are replacing OCBs in modern grids because to their higher efficiency and lower maintenance requirements, OCBs are still used in older installations and places where modernization is underway.

Types of Oil Circuit Breaker

There are 2 primary types of oil circuit breakers.

1. Bulk oil circuit breakers (BOCB)
2. Minimum Oil Circuit Breaker (MOCB)

1). Bulk Oil Circuit Breaker 

This type of circuit breaker requires a significant amount of insulating oil. The oil is used to quench arcs and to isolate the live and earth components of the breaker.

BOCB can be categorized into two types:

1. Plain Break Oil Circuit Breaker
2. Arc Control Oil Circuit Breaker

Plain Break Oil Circuit Breaker– This type of BOCB has no control over the arc other than to increase its length by separating the contact points. For the most part, they are straightforward & simple to design.

Arc Control Oil Circuit Breaker– This BOCB has a specialized arc control technology that effectively quenches the arc. Arc Control Oil circuit breakers are split into two distinct types:

• Self-Blast Oil Circuit Breaker: In this type of OCB, the arc is controlled internally, such as by using the arc’s energy to extinguish itself.
• Forced Blast Oil Circuit Breaker: This type of OCB uses external techniques to control the arc.

2). Minimum Oil Circuit Breaker (MOCB)

Minimum Oil Circuit Breaker- This type of oil circuit breaker utilizes only a small amount of oil to quench the arc.

What is Arc Quenching?

The term “arc quenching” describes the metal transfer or arc stopping. It is important to take caution when breaking high voltages and to reduce the current flowing when the contact opens in order to reduce the transfer of metal.

How Arc Extinguishing is Achieved in Oil Circuit Breaker?

The oil circuit breaker’s arc extinguishing chamber generates a high-speed airflow from the gas produced by the oil’s decomposition to extinguish the arc.

Advantages of Oil as an Arc Quenching

• After the arc is quenched, the oil offers insulation between the contacts due to its high dielectric strength.
• The oil utilized in circuit breakers maintains a small gap between the conductors & the earth components.
• The tank produces hydrogen gas that possesses a high diffusion rate & excellent cooling qualities.

Disadvantages of Oil as an Arc Quenching

• The oil utilized in oil circuit breakers is combustible and hence poses a fire hazard.
• There is a possibility of producing an explosive combination with air.
• Carbon particles are created during the degradation of oil in arc, polluting the oil and reducing its dielectric strength.

Applications of Oil Circuit Breakers

Oil circuit breakers are utilized in high voltage applications that include power grids, substations, powerlines, & transmission & distribution systems with voltages up to 220 kV.

Where is Minimum Oil Circuit Breaker used?

Minimum oil circuit breakers operate very rapidly and are utilized in transmission networks & substations. These circuit breakers are live-tank in design and use minimal oil.