When a fault occurs, the breaker contacts open in oil, an arc is struck between them. Due to the heat of arc, the surrounding oil of the contacts evaporates and hydrogen gas is produced at high pressure. The oil is pushed away from the arc and a hydrogen gas bubble surrounds the arc region and adjacent portion of contacts.
The arc is extinguished by two processes. Firstly, the hydrogen gas cools the arc due to its high heat conductivity. Secondly, the gas sets up a violent movement in oil and forces it into the space between the contacts.
Thus arcing products are eliminated from the arc path and arc interruption takes place when the arc current goes to zero. As a result of which arc is extinguished and circuit current is interrupted.
Following are the main two types of oil circuit breaker:
- Bulk oil circuit breakers
- Low oil circuit breakers
Bulk Oil Circuit Breaker
In these types of oil circuit breaker, a large quantity of oil is used. These are also known as dead tank type oil circuit breaker because the tank is held at ground potential. These can be further divided into:
- Plain Break Oil Circuit Breakers
- Arc Control Oil Circuit Breakers
Plain Break Oil Circuit Breaker Construction
The construction of plain break oil circuit breaker is very simple. It consists of current carrying contacts immersed in transformer oil and enclosed in a strong earthed metal tank.
The oil acts as an insulator between live parts and earthed tank as well as arc extinguishing medium.
The oil tank is not fully filled with oil but some space is left for air at the top. Air serves as a cushion and provides space for the displaced oil on the formation of gas around the arc. It also absorbs the mechanical shock produced due to upward movement of oil.
The breaker tank is tightly bolted to a foundation to bear out vibrations produced at the time of operation of the circuit breaker.
A sufficient head of oil above the arcing contacts is necessary to provide enough oil pressure at the arc and to prevent the occurrence of the chimney effect. A chimney of gas from the arc to the oil surface is produced which comes in contact with the earthed tank. If this gas is of low dielectric strength, an arc will strike between contacts and earthed tank. It will result in serious consequences.
Hence a sufficient amount of oil should always exist between the live parts and the tank. A gas outlet from the tank is absolutely necessary and some form of vent is made in the tank cover.
The figure shows a double break plain oil circuit breaker. It provides two breaks in series. The two breaks in series provide rapid arc lengthening. For successful interruption of fault current, a comparatively long arc length is essential so that the turbulence in the oil produced by the pressures generated by the arc may assist in quenching it.
Oil Circuit Breaker Working
Normally the fixed and moving contacts remain closed and the breaker carries the normal circuit current. When a fault occurs, moving contacts are pushed down by the protective system and an arc is struck between the contacts and a large amount of heat is produced which vaporizes the surrounding oil into gas. This gas surrounds the arc and displaces the oil violently.
The arc is extinguished when the distance between the fixed and moving contacts reaches a certain value. The speed of the break should be as high as possible because the sooner arc is extinguished the smaller energy is released in the breaker and the less mechanically strong a breaker will have to be designed.
The main problem in a double break plain oil circuit breaker is that unequal voltage distribution across the breaks which results in unequal sharing of the total interrupting duty. Following are the main features that have an important effect on the performance of plain break circuit breaker:
- critical length of break
- the speed of contact movement
- head of oil above contacts, it depends on the operating voltage of the breaker
- the clearance between the live parts and the earthed tank
Among the above-mentioned factors, the rupturing capacity of the plain break circuit breaker mainly depends upon the head of oil and large clearances. That is why large rupturing capacity requires large circuit breakers.
These circuit breakers have become obsolete now. The plain oil circuit breakers were widely employed on low voltage AC and DC circuits upto 250 MVA at 11 kV.