Comparison between variable Circuit Breaker (ACB, OCB, MOCB, ABCB, VCB)
ACB (Air circuit breaker)
There are mainly two types of ACB are available.
- Plain air circuit breaker.
- Air blast Circuit Breaker.
Some specific advantages:
- There is no chance of fire hazard caused by oil.
- The breaking speed of circuit breaker is much higher during operation of air blast circuit breaker.
- Arc quenching is much faster during operation of air blast circuit breaker.
- The duration of arc is same for all values of small as well as high currents interruptions.
- As the duration of arc is smaller, so lesser amount of heat realized from arc to current carrying contacts hence the service life of the contacts becomes longer.
- The stability of the system can be well maintained as it depends on the speed of operation of circuit breaker.
- Requires much less maintenance compared to oil circuit breaker.
Some specific disadvantages:
- In order to have frequent operations, it is necessary to have sufficiently high capacity air compressor.
- Frequent maintenance of compressor, associated air pipes and automatic control equipment is also required.
- Due to high speed current interruption there is always a chance of high rate of rise of re-striking voltage and current chopping.
- There also a chance of air pressure leakage from air pipes junctions.
Uses of Air Circuit Breaker:
It is used for protection of plants
It is used for common protection of electrical machines
It used for protection of transformers, capacitors and generators.
Air circuit breaker is also used in Electricity sharing system and NGD about 15kV
Also used in Low as well as High voltage and Currents applications.
Vacuum Circuit Breaker or VCB
Advantages of VCB
- The vacuum interrupters have long life.
- Unlike oil CB (OCB) or air blast CB (ABCB), the explosion of VCB is avoided. This enhances the safety of the operating personnel.
- No fire hazard.
- The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for repeated operation.
- Vacuum circuit breakers are almost maintenance free.
- Due to the rapid gain of dielectric strength of vacuum interrupter, the separation required between the moving contact from fixed contact is of the order of few millimetre. This makes the VCB compact.
- VCB is light weight.
- No exhaust of gas to the atmosphere.
- Quiet operation.
Disadvantages of VCB
- The main disadvantage of VCB is that it is uneconomical for use of VCB at voltages exceeding 38 kV. The cost of the breaker becomes prohibitive at higher voltages. This is due to the fact that at high voltages (above 38 kV) more than two numbers of interrupters are required to be connected in series.
- Advance technology is used for production of vacuum interrupters.
SF6 Circuit Breaker
SF6 Circuit Breakers Advantages:
- Excellent insulating, arc extinguishing, physical and chemical properties of SF6 gas is greater advantage of SF6 circuit breakers
- The gas is non-inflammable and chemically stable. The decomposition products are non-explosive i.e, rhere is no risk of fire or explosion
- Electrical clearances are very much reduced because of high dielectric strength of SF6
- Outdoor EHV SF6 circuit breaker has less number of interrupters per pole in comparison to the air-blast circuit breaker and minimum oil breaker. Outdoor SF6 circuit breaker is simple, comparatively cheaper in cost, maintenance free and compact
- Its performance is not affected due to variation in atmospheric conditions
- It gives noiseless operation it does not make sound like air-blast circuit breaker during operation
- No frequent contact replacement-arcing time is small owing to outstanding arc quenching properties of SF6 and therefore contact erosion is less. Hence contacts do not suffer oxidation
- Therefore is no reduction in dielectric strength of SF6 since no carbon particle is formed during the arcing
- Minimum maintenance. The breaker may require maintenance once in four to ten years
- The sealed construction avoids the contamination by moisture, dust, sand etc. No costly compressed air system is required as in the case of air blast circuit breaker
- Same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.
- No over voltage problem. The arc is extinguished at natural current zero without the current chopping and associated over-voltages originating across the circuit breaker terminals
- The SF6 gas circuit breaker can perform various duties like clearing short line faults, opening unloaded transmission lines, capacitor switching, transformer reactor switching etc without any problem
- Ample overload margin. For the same size of the conductors the current carrying capacity of the SF6 circuit breakers is about 1.5 times that of the air blast circuit breakers because superior heat transfer capability of the SF6 gas
SF6 Circuit Breaker Diadvantages:
- Imperfect joints leading to leakage of the SF6 gas. Continuous monitoring devices are required
- SF6 gas is suffocating to some extent. In case of leakage in the breaker tank SF6 gas being heavier than the air settles in the surroundings and may lead to suffocation of the operating personnel. However it is not poisonous
- Arced SF6 gas is poisonous and should not be inhaled
- The internal parts need thorough cleaning during periodic maintenance under clean and dry environment. Dust of Teflon and sulphides should be removed
- Special facilities are required for transportation of gas, transfer of gas an maintenance of quality of the gas. The deterioration of quality of gas affects the performance and hence reliability of the SF6 circuit breaker
Oil Circuit Breaker (OCB)
Advantages of Oil Circuit Breaker (OCB)
As an Arc extinguishing medium oil has the following advantages:
1. Oil produces hydrogen during arching. The hydrogen helps extinguish the arc.
2. The oil provides insulation for the live exposed contacts from the earthed portions of the container
3. Oil provides insulation between the contacts after the arc has been extinguished
Disadvantages of Oil Circuit Breaker (OCB)
1. Oil is inflammable and may cause fire hazards. When a defective circuit breaker fails under pressure, it may cause an explosion
2. The hydrogen generated during arcing, when combined with air, may form an explosive mixture
3. During arcing, oil decomposes and becomes polluted by carbon particles, which reduces its dielectric strength. Hence, it requires periodic maintenance and replacement.