Download electrical high voltage circuit breakers

ELECTRICAL HIGH VOLTAGE CIRCUIT
BREAKERS
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By mplgmg
132kV High Voltage circuit breaker
Electrical power transmission networks are protected and controlled by high-voltage breakers. In
substations the protection relay scheme can be complex, protecting equipment and busses from
various types of overload or ground/earth fault. Electrical protection should be provided against
the following abnormal conditions:
1.
2.
3.
4.
overloading (excessive currents not due to faults)
transformer faults
short-circuit faults between phases
short-circuit faults to earth
These breakers are available for indoor or outdoor applications and High-voltage breakers are
broadly classified by as follows.
1. Oil circuit breakers (OCBs)
2. Vacuum circuit breaker (VCB)
3. SF6 Circuit Breakers
4. Air circuit breakers
Brief detail of above types are given in bellow
Oil Minimum Circuit Breakers (OMCB)
Nowadays OMCBs are very popular in sub stations and power stations. In oil minimum circuit
breakers the current breaker takes place inside of the interrupter. The enclosure of the interrupter
is made of insulating material like porcelain. Hence the clearance between the live parts and the
enclosure can be reduced and lesser quantity of oil require for internal insulation. This type of
circuit breaker is used widely in outdoor sub station, rated even up to 66 kV. The type of oil
circuit breaker is bulky.
Vacuum circuit breaker (VCB)
In this breaker vacuum is used as insulator and as an arc-quenching medium VCB also used as a
high voltage control switch. There is no oil. Therefore will not occurring any fire problems in
this kind of circuit breaker. Vacuum circuit breakers in particular are useful for controlling and
protecting electrical systems. Vacuum circuit breakers offer many advantages as compared to
other types of circuit breakers.
Air Circuit Breaker (ACB)
ACB is used against very high current. Air circuit breakers are commonly used in electrical
distribution systems. Air circuit breakers which include operating mechanisms that are mainly
exposed to the environment. ACB is also protecting for over load, short circuit and earth fault.
Most of air circuit breakers are use in indoor type substations.
SF6 Circuit Breakers
Sulfur hexafluoride (SF6) gas is an alternative to air as an interrupting medium. SF6 is a
colorless nontoxic gas, with good thermal conductivity and density approximately five times that
of air. The principle of operation is similar to the air blast breakers, except that the SF6 gas is not
discharged into the atmosphere. A closed circuit completely sealed and self-contained
construction is used. SF6 Circuit Breakers are mostly use in Indoor type Primary sub stations in
Sri Lanka and other countries because of its convenience.SF6 Gas has good qualities as bellow.
1.
2.
3.
4.
Good heat transfer quantity.
Pressure can be checked easily
Short arcing time
Compartmented SF6 HV load-break isolating switch
Circuit breakers have their contacts immersed in oil. Current disconnect takes place in oil which
cools the arc developed and thereby cool the arc. There are three poles in this oil circuit breaker
which use to protect 3 phases. The poles of small oil circuit breakers can be placed in one oil
tank however; the large high-voltage circuit breakers have each pole in a separate oil tank. The
oil tanks in oil circuit breakers are normally sealed. The Electrical connections between the
contacts and external circuits are made through porcelain bushings. If we are using oil as an arcquenching medium we have to consider advantage of oil so followings are advantages of oil as
an arc-quenching medium.
1. Oil is very good insulator
2. Has a high dielectric strength
3. Cold oil is able to act as an insulator
Oil circuit breakers are comprises of a tank filled with insulating oil. There are 2 kinds of OCBs.
Those are.
1. Oil Circuit breakers
2. Oil – Minimum Circuit breakers (OMCB)
In OCBs there are fixed and movable contacts. It has 3 porcelain bushings, 3-phase line current
set to fixed contacts. When the circuit breaker is closed, the line current of each phase penetrate
the tank by means of a porcelain bushing, flow through the first fixed contact to the second fixed
contact, and then out by second bushing .
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