-Construction of Oil Circuit Breaker:

Oil circuit breaker is very easy in construction. It consists of current carrying

contacts enclosed in a strong, weather-tight earth metal tank and the tank is filled
with transformer oil. The oil is both acts as an arc extinguishing medium and as an
insulator between the live part and earth.
At the top of the oil, air is filled in the tank which acts as a cushion to control the
displaced oil on the formation of gas around the arc and also to absorb the
mechanical shock of the upward movement of oil. The breaker tank is securely
bolted for carrying out the vibration caused on interrupting very high current. Oil
circuit breaker consists gas outlet which is fitted in the tank cover for the removal
of the gases.
-Working Principle of Oil Circuit Breaker:
During the normal operating conditions, the contact of the oil circuit breaker is
closed and carry the current. When the fault occurs in the system, the contacts of
the breaker are moving apart, and an arc is struck between the contacts.
Due to this arc, a large amount of heat is liberated, and a very high temperature is
reached which vaporises the surrounding oil into gas. The gas, thus liberated
surrounds the arc and its explosive growth around it displace the oil violently. The
arc is extinguished when the distance between the fixed and moving contact
reaches a certain critical value, depends on the arc current and recovery voltage.

The oil circuit breaker is very reliable in operation, and it is very cheap. The most
important feature of oil circuit breaker is that no special devices are used for
controlling the arc caused by moving contact

-Such circuit breakers have the following advantages:

I. The pressure chamber is relatively cheap to make.
ii. Length for the critical gap is reduced.
iii. Arcing time is reduced.
iv. The breaking capacity of the circuit breaker is increased.
1-Plain Break Oil Circuit Breakers:
The plain oil circuit breaker is very simple in construction. It consists of current
carrying contacts enclosed in a strong (in order to withstand the large gaseous
pressure developed owing to dissipation of large energies), weather tight (to keep
moisture out), earthed metal tank and immersed in oil, called the transformer oil.
The oil acts both as an arc extinguishing medium and as an insulator between the
live parts and earth.
At the top of the oil air is filled in the seal vessel to serve as cushion to
accommodate the displaced oil on formation of gas around the arc and also to
absorb the mechanical shock of the upward movement of oil. The breaker tank is
securely bolted to an adequate foundation to bear out the vibrations caused on
interrupting very heavy currents.

-The main features that have an important bearing upon the

performance of a plain break oil circuit breaker are:
1. Critical length of break.
2. Speed of contact movement. In order to quench the arc earlier the speed
should be high and this is the reason that two break circuit breakers are
preferred.
3. Head of oil above contacts, which is governed by the operating voltage of the
breaker.
4. The clearance between the live contacts and the earthed pressurized tank.
All the factors contribute to increased rupturing capacity are a considerable head
of oil and large clearances; for this reason large rupturing capacity tends to require
large circuit breakers. The plain break oil circuit breakers are widely employed on
low voltage dc and ac circuits. Such circuit breakers are not considered suitable for
short-circuit rating exceeding 250 MVA at 11 kV. Also such breakers are not
considered suitable for high speed interruption, therefore, these cannot be
employed for auto-reclosing.
2-Arc Control Oil Circuit Breakers:
In case of a plain break oil circuit breaker, there is no control over the arc other
than increase in length caused by the moving contact. However, it is necessary and
desirable that final arc extinction should occur while the contact gap is still short.
For this purpose, some arc control is to be provided and the breakers provided with
arc control are called the arc control circuit breakers.

-Arc control breakers are of two types:

1. Self-Blast or Self-Generated Pressure Oil Circuit Breakers:
In such breakers use of pressure generated by the arc itself is made in speeding up
the movement of the oil into contact space immediately after the current zero. The
high pressure generated by the arc causes an immediate displacement of oil into
the space between the contacts after the arc current goes to zero. This is achieved
by surrounding the contacts by a pressure chamber or pot. The pressure developed
by the oil depends upon the value of current interrupted.
2. Externally Generated Pressure or Forced Blast or Impulse Type Oil Circuit
Breakers:
In the self-blast oil circuit breakers, the arc itself generates the pressure required
to force the oil across the arc path. The major drawback of such breakers is that
arcing times tend to be long and inconsistent with fault currents considerably less
than rated currents. It is due to generation of reduced gas at low values of fault
currents. This problem is overcome in forced-blast oil circuit breakers in which the
necessary pressure is developed by external mechanical means independent of the
magnitude of the fault currents to be interrupted.
In this type of circuit breaker, the oil pressure instead of being created by the arc is
created mechanically by the piston-cylinder arrangement. The movement of the
piston is mechanically coupled to the moving contact, thus automatically oil
pressure is generated and thus high speed interruption is attained.
Circuit breakers have also been built up which combine the self-blast and forced
blast.
3-Low Oil or Minimum Oil Circuit Breakers:
The design and special features of what may be described as the bulk oil type of
circuit breaker for voltage from the lowest up to 380 kV. But in case of bulk oil
circuit breakers the quantity of oil required reaches very high figures with the
increase in system voltage. For example, a 110 kV, 3,500 MVA breakers may need
8 to 12 thousand kg of oil, while a breaker of the same rating output for 220 kV may
need 50 thousand kg of oil.
These large quantities of oil are subject to the carbonization,sludging , etc., which
occurs due to arc interruption and other causes, reducing (in time) the insulating
properties and requiring regular maintenance. Not only this but also the expenses,
tank size and weight of the breaker are increased. In case of a floor mounted large
bulk oil circuit breaker, access to the contact system is not easy and is obtained
only through access parts in the tank side after the oil has been pumped away to
storage.
In bulk oil circuit breakers the oil performs two functions. Firstly, it acts as an arc
extinguishing medium and secondly it insulates the live parts from earth.
Fortunately the quantity of oil actually required for arc extinction is only about one-
tenth of the total, the rest being used for insulation. This fact led to the
development of low oil content or live tank circuit breaker in which the arc chamber
contains a minimum quantity of oil and is mounted on a porcelain insulator to
insulate it from the earth, the arc chamber itself is, in fact of bakelite paper
enclosed in porcelain, so that such a circuit breaker appears as in Fig. 10.10, the
lower porcelain being the support and the upper porcelain enclosing the contacts.
Such type of a circuit breaker has the added advantage that it requires less space
than the bulk oil type, an important feature in large installations. Such circuit
breakers are less suitable where very frequent operation occurs because degree of
carbonisation produced in the small volume of oil is far more severe than in the
conventional bulk oil circuit breakers and this leads to deterioration in the dielectric
strength across the contact system in the open position. As regards quenching of
the arc the oil behaves identically in bulk oil as well as in low-oil content circuit
breakers. The methods of arc extinction, therefore, are basically same in both
types, the difference lies in the application of these methods to achieve the desired
arc extinction by various constructions of arc control device or interrupters.
Reverting to Fig. 10.10 it will be seen that the upper arcing chamber comprises a
synthetic resin bonded paper cylindrical enclosure within a porcelain insulator. An
annular space between these is filled with oil as an insulating medium but, again,
this is physically separated from that in the arcing chamber and thus cannot be
contaminated.

-However, it suffers from the following drawbacks when compared

with a bulk oil circuit breaker:
(i) Increased degree of carbonisation due to smaller quantity of oil.
(ii) Rapid deterioration of dielectric strength of oil due to high degree of carbonisation.

(iii) Difficulty in removal of gases from the contact space in time.