Electrical power transmission networks are protected and controlled by High Voltage Circuit Breaker inside electrical grid substaion. 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:
- overloading (excessive currents not due to faults)
- transformer faults
- short-circuit faults between phases
- short-circuit faults to earth
Classification of High Voltage Circuit Breaker
These High Voltage Circuit Breaker are available for indoor or outdoor applications and High-voltage breakers are broadly classified by as follows.
- Oil circuit breakers (OCBs)
- Vacuum circuit breaker (VCB)
- SF6 Circuit Breakers
- Air circuit breakers
Brief detail of above types are given in bellow .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 arc-quenching medium we have to consider advantage of oil so followings are advantages of oil as an arc-quenching medium.
- Oil is very good insulator
- Has a high dielectric strength
- Cold oil is able to act as an insulator
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.
Oil circuit breakers are comprises of a tank filled with insulating oil. There are 2 kinds of OCBs. Those are.
- Oil Circuit breakers
- 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 bushings
Risk factors of Circuit Breakers
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Type of Circuit Breakers
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Risk Factors
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Oil Circuit Breakers
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High Risk
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Vacuum Circuit Breakers
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Less Risk
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SF6 Circuit Breakers
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Very Less Risk
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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 High Voltage Circuit Breaker
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.
- Good heat transfer quantity.
- Pressure can be checked easily
- Short arcing time
- Compartmented SF6 HV load-break isolating switch
