Oil Circuit Breaker

We already discussed Vacuum Circuit Breaker and SF6 Circuit Breaker. In modern power systems these two types of circuit breakers are mainly used for high voltage application. While vacuum breakers are mainly used for voltage upto 38 kV, SF6 breakers are used starting from distribution voltage at 11 kV upto 765 kV and 1200 kV level.  Although the use of oil breaker has reduced very much one can still find oil CB in many installations. So I liked to write a little about oil circuit breaker in one article.

Oil Circuit Breakers (OCB) can be categorised into two types. One is Bulk Oil Circuit Breaker (BOCB) and the other type is Minimum Oil Circuit Breaker (MOCB). MOCB type is also called as Low Oil Circuit Breaker.

Bulk Oil Circuit Breaker (BOCB)

The Bulk Oil CB design is very simple (Fig-A). In Fig-A the arc control device between the fixed and moving contacts is not shown, so making the sketch even simpler. This type of circuit breaker uses a steel tank containing oil and the contacts are immersed in the oil. The steel tank is earthed (dead tank type). In this type construction the oil requirement is more as the oil is required to provide insulation to the contacts from the steel tank and insulation between the contacts(in open state). The oil also serves as the medium for extinguishing the arc formed when the moving contact separates from fixed contact. When the contacts separate, arc is formed between the contacts. The arc gives rise to formation of gas in the oil which initiates oil circulation. This phenomena helps in extinguishing the arc so breaking the circuit. For higher voltage this very simple principle cannot be much effective. So an arc control device is usually used to facilitate arc extinction process.

The BOCB is available as single tank type or three tank type. Usually for lower voltage use, below 38 kV, single tank type is adopted with barrier between the phases.  For higher voltage application three separate tanks are used.

Minimum Oil Circuit Breaker

If you visit an old substation, having BOCB installed, you immediately recognise the oversize Circuit Breaker. As explained above, BOCB is large in size and requires more space. 

Minimum Oil Circuit Breakers (MOCB) require less oil as the purpose here is only to extinguish the arc and not for providing insulation to the contact. Arc interruption takes place inside the Interrupter. The whole system is placed inside the porcelain housing. Because of this insulating porcelain the insulation requirement of contacts is reduced very much. This is the reason of its smaller size. As such the MOCBs are of  live tank outdoor type design and mainly used for voltage levels above 38 kV.

The oil circuit breakers have some severe disadvantages. The main disadvantage is that the OCB can explode causing harm to the personnel and other equipment of the system. The tank type design is very bulky so making it difficult for transportation and handling and requires more space. The OCB requires more maintenance in comparison to vacuum and SF6 breakers. Irrespective of these few disadvantages the OCBs are not going to vanish within few years.