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Circuit Breakers
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Air Blast Circuit Breakers (ABCB)-Construction,Working

Principle of Air Blast Circuit Breakers(ABCB)

The Air Blast Circuit Breaker is an external extinguishing energy type in which the high pressure air is employed for an arc extinction.The auxiliary compressed air system of an air blast circuit breaker shown below.in this article you will find Air Blast Circuit Breakers (ABCB)-Construction,Working



An auxiliary compressed air system of an air blast circuit breaker consists of a fixed contact and a moving contact, enclosed in an arc extinction chamber. Under normal operating Conditions, both the contacts are closed. Whenever a fault occurs, high currents are induced which raises the temperature. When the air is submitted into the arc extinction chamber the air pushes away the moving contact establishing an arc. The air inside the arc extinction chamber will have high pressure than the atmospheric pressure. The air blast cools the arc and sweeps away all the ionized gases along with it. Because of this, the dielectric builds up rapidly, between the contacts, which prevents the reestablishment of arc. Thus, the flow of current is interrupted.

Air Blast Circuit Breakers are two types 
1.Axial Blast Air Circuit Breaker
2.Cross Blast Air Circuit Breaker


The difference between two types of Air Blast Circuit Breakers is just air flow direction with respect to the arc.in axial type air flows in direction of arc.In cross blast air flow direction perpendicular with respect to arc.

Axial Blast Air Circuit Breaker

Cross Blast Air Circuit Breaker

Construction of a Typical EHV Air Blast Circuit Breaker

A typical EHV air blast circuit breaker is shown in figure, which consists of the following components.

1. Air reservoir
2. Hallow insulator columns
3. Double arc extinction chamber
4. Valves
5. Current carrying conductors.




1. Air Reservoir

In air reservoir, the air of high pressure with an atmospheric pressure of 20-30 atm is stored. The air is taken from at: silt compressed air system.

2. Hallow Insulator Columns

The hallow insulator columns are mounted on air reservoir with valves provided at base. It carries air from reservoir to an arc extinction chamber.

3. Double Arc Extinction Climber

The are extinction chamber is mounted on the top of the hallow insulators columns. It consists of a fixed contact and a moving contact with a spring mechanism. Depending upon the pressure of air the moving contact opens (or) closes the ail outlet valves.

4. Valves

The purpose of valves is used to supply the air from the air reservoir to hallow insulator columns and an arc extinction chambers. This is happened when the valves are kept open. Similarly when the valves are closed it stops supplying the an insulator columns and are extinction chambers.

5. Current Carrying Conductors

The current carrying conductors links all the arc extinction chambers in series and the poles of neighboring equipment.

Working Operation of Air Blast Circuit Breakers

When a breaker receives a signal either pneumatic or electrical the rod connected to valves gets opened. During tint instant. the air enters into the hallow insulator column and further enters into arc extinction chambers. Because of high pressure in are extinction chamber, the pressure increases on the moving contact and becomes more than the spring pressure. Hence. the contacts are separated. Because of high pressure on moving contact, it travels a short distance against the spring pressure and closes the air outlet valves. As a result-the high pressure air in the arc extinction chamber is not let to go out However. at the arcing period the air goes out through the air outlet valves with all ionized gases.

Later, if the valves are closed. then the air in the insulator column is let into the atmosphere, due to which the pressure on the moving contact is dropped to atmospheric pressure. Hence. the moving contact closes over the fixed contact by virture of its spring pressure; in the applications of high voltage installations, air blast circuit breakers are very much preferred and by using these circuit breakers the tire hazards are eliminated.

Advantages of Air-blast Circuit Breakers

1.Due to absence of oil in air-blast circuit breaker, risk of  fire and maintenance of oil is eliminated.

2.Deterioration of oil is eliminated and the arcing products produced are completely removed.

3.Fast operation is possible and suitable for indoor installation.

4.Suitable for rapid reclosing at successive operation and has a very high rupturing capacity.

5.It is easily accessible. It has relatively less weight.

Disadvantages of Air-blast Circuit Breakers

1.The construction of air-blast circuit breaker is very complicated.

2.Compressed air system consisting of electric motor, air compressors etc., are required for installation, which increases the cost.

3.They are very sensitive to variations in RRRV (Rate of Rise of Recovery Voltage).

4.Maintenance of plant requires highly trained personnel.

5.Cost of operation is high.
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Current Chopping Phenomenon-Circuit Breakers

What is Current Chopping?

While interrupting highly inductive current, like no-load current of transformer, the rapid deionization of contact space and blast effect may cause current interruption before its natural zero. Such an'interruption of current before its natural zero is termed as “current chopping”.This phenomenon is more pronounced in case of air-blast circuit breakers which exerts the same deionizing force for all currents within its short-circuit capacity. Even though, the instantaneous value of current being interrupted may be less than the normal current rating of the breaker, it is quite dangerous from the point of view of overvoltages which may result in the system.

Current Chopping

Let, 
L = Inductance of the system 
C = Capacitance of the system 
i =Instantaneous value of arc current
V = Instantaneous value of capacitor voltage (which appears across the breaker when it opens)

The electromagnetic energy stored in the system at the instant before interruption is 1/2(Li²) As soon as the current is interrupted the value of  i becomes zero. But, the  electromagnetic energy stored in the system [1/2(Li²)] cannot become zero instantaneously and so it is converted into electrostatic energy [1/2(CV²)] as the system has some capacitance.

According to the principle of energy conversion we have,

1/2(Li²)=1/2(CV²)

V=i√(L/C)

This theoretical value of V is called as “prospective Voltage or Arc Voltage”. If this voltage is very high when compared with the gap withstanding voltage, then the gap breakdowns and so "the arc restrikes. Again the current is chopped (interrupted) because of high quenching force and so, restriking occurs. This process repeats until the current is suppressed finally without any restrike and this occurs near current zero as shown in the figure.


In actual proactive the voltage across the breaker does not reach dangerously high prospective values of voltage. It is due to the fact that as soon as the breaker voltage increases beyond the gap withstanding voltage, it breaks down and the arc restrikes due to which the voltage across breaker falls to a very low value of arc voltage which can also be seen in the figure. Hence, it can be said that the arc is not an undesirable phenomenon and instead it protects the power system from severe stress on insulation due to overvoltages.



Current Chopping


In order to reduce the phenomenon of current chopping, the overvoltages are to be reduced. This is possible by connecting voltage— grading (or non—linear) resistors across the circuit breaker contacts during arc interruption. In medium voltage systems, an RC surge absorber is connected across line and ground in between the inductive load and the circuit breaker. As a result, the RC combination absorbs the overvoltages.
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Arc in Circuit Breaker | Electric Arc

Arc Phenomenon

Whenever a fault occurs on the system, the contacts of the circuit breaker are opened. As soon as a fault or short circuit occurs on the system, there is a heavy in rush of current through the contacts liberating heat, causing the temperature rise. Due to temperature rise, electrons are emitted from the cathode. This happens during the period of separation. The air surrounding the contacts already consists of electrons, positive ions and neutral molecules. These electrons emitted from the surface of cathode get ionized with the air. When the contacts are opened, due to ionization an arc is struck in between. A small p.d exists between the contacts and is responsible for maintaining the arc. The current continues to flow as the low resistance path is provided by an arc. It is clear from above analysis that current flow entirely depends upon the resistance of arc.

Arc Extinction

To extinguish the arc, the factors responsible for are maintenance are to be taken into consideration.

1. The are persists mainly because of the p.d developed between the contacts which is small but sufficient to maintain the arc.

2. Moreover, the ionization process which is taking place between the electrons emitted from cathode and the electrons and positive ions present in air helps the surrounding air to fill with ionized particles.

Principle of Arc Extinction

1. Firstly, the p.d between the contacts is decreased, so that the arc is avoided.
It is possible [by moving the contacts of a CB to a certain distance where p.d fails to build-up for the maintenance of are.

2. Secondly, the ionized particles present between the contacts and surrounding air are to be swept out either by cooling or by passing gases which are highly electronegative in nature.


Circuit Breaker Failure & Arc Formation Video

video

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Minimum Oil Circuit Breakers (MOCB)-Contruction,Working

Construction of Minimum Oil Circuit Breaker

The simplified constructional diagram of a Minimum Oil Circuit Breaker (MOCB) is shown in the figure. It consists of two oil filled chambers namely upper chamber and lower chamber, which are separated from each other.

The are extinction process is carried out in the upper chamber. So, it is called as an arc extinction chamber or current interruption chamber of Minimum Oil Circuit Breaker (MOCB).This chamber houses an arc control device, an upper fixed contact and a ring shaped lower fixed contact. The are control device is fitted to the upper to the upper fixed contact.The moving contact slides through the lower fixed contact such that a physical (or electrical) maintained between them. The entire assembly of upper fixed contact. lower fixed contact and arc control device is enclosed in a glass fiber enclosure which is surrounded by oil.

Minimum Oil Circuit Breakers (MOCB)-Contruction,Working

The oil present in the lower chamber does not involve the arc extinction process and instead it is used only for insulation purpose. So, the lower chamber is also known as dielectric supporting chamber. Both the upper and lower chambers are individually enclosed with the cylindrical shaped synthetic resin bonded papers within the porcelain insulators.

The operating rod which is permanently fixed to the moving contact is connected to the operating mechanism which provides vertical motion in order to make and break the circuit.

Operating Principle of Minimum Oil Circuit Breaker

Whenever the moving contact is drawn out of the. hollow structure of fixed contact, under current carrying conditions, an arc is drawn between them. As the contacts are present in the dielectric oil, the oil surrounding the arc attains a high temperature and thus decomposes the releasing gases. The gases formed will expand and so the pressure inside the chamber rises.Hence,the gases will move upwards by which the contacts cools down and the arc splits. 

The gap between the contacts will be filled with fresh oil. This oil will also be decomposed and the released gases will expand. So. the pressure will be increased again. Hence, the gases will move upwards and again fresh oil will enter.

Minimum Oil Circuit Breakers (MOCB)-Contruction,Working

This flow of fresh oil through the gap between the contacts will continue until the final arc extinction which occurs near . current zero instant. In order to achieve sufficiently high dielectric strength immediately after current zero instant. either of the following method is implemented.

(i) Forcing fresh oil into the gap between the contacts with the help of piston action produced by the piston attached to the moving contact.

(ii) Maintaining high pressure on the oil inside the circuit breaker with the help of an inert gas. By this the fresh oil will enter into the gap between the contact from all directions and the gases moves upwards.

Applications of Minimum Oil Circuit Breaker

1. For indoor applications, minimum oil circuit breakers can be used upto 36 kV.
2. For outdoor applications, minimum oil circuit breakers can be employed upto the line voltages of 220 kV.
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Vacuum Circuit Breakers (VCB) Construction,Operation

A vacuum circuit breaker is the suitable for mainly medium voltage application circuit breaker where the arc quenching takes place in vacuum.The major parts of Vacuum Circuit Breakers circuit breaker are breaker contacts. vapour condensing shields, metallic bellows. end flanges and enclosure.The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.

Construction of Vacuum Circuit Breakers

1. Breaker Contacts
Some of the alloys used as vacuum circuit breaker contacts are copper-bismuth, silver-bismuth, silver—lead and copper-lead. Usually. the contacts have large disc-shaped faces with large stem. The disc faces contain spiral segments so that, the arc current produces axial magnetic field. Such a geometry causes the plasma of the arc ’to move rapidly over the contact surface and hence minimizes the metal evaporation and erosion of the contacts due to are.

2. Vapour Condensing Shield or Metallic Shield or Spotter Shield
These are supported on an insulated housing and are placed in between the contacts and the enclosure such that they enclose the contact region. It prevents the metal vapour (released during arcing) reaching the enclosure and condensing on it. So. condensing of metal vapour takes place on these shields only. Usually, this shield is made of stainless steel.

3. Metallic Bellows
The two ends of metallic bellows of vacuum circuit breaker are welded to lower end flange and the moving contact. They are used to permit the movement of contact inside the sealed construction. They are usually made of stainless steel. In order to perform satisfactory repeated operations. the design of bellows plays a significant role.

4. End Flanges
The end flanges of vacuum circuit breaker support the fixed contact. sputter shield, outer insulating enclosure, metallic bellows and their protective shield. They are made of non-magnetic metal.

5. Enclosure or Outer Envelope
Generally, it is made up of glass because glass is non-porous, impermeable insulating material which can retain high Vacuum. Also, it is quite easy to join it to the end flanges.


Operating Principle,Working of Vacuum Circuit Breakers

The contact faces of vacuum circuit breaker do not have perfectly smooth surfaces instead they have certain micro projections. At the instant just before the contact separation, the entire current through the circuit breaker will be concentrated in these projections as they form the last point of contact. As these micro projections will have a very small area of cross-section, the resistance offered by them will be high. So, these projections will become hot spots due to resistive heating. These hot spots supply sufficient quantity of vapour (or electrons) for the arc formation. So, an arc is drawn between the contacts. In addition to thermionic emission, electron emission can also be due to field emission, secondary emission by positive ion bombardment, secondary emission by photons and pinch effect. From the above, it can be said that the arc electrons and ions Will not come from the vacuum but they are being emitted from the electrodes.

The contacts will release sufficient metal vapour even for very low value of current so that the vacuum arc is stable in order to avoid current chopping. Therefore, the arc current interruption will take place only at current-zero instant. At current zero, the cathode spot disappears within ions and 3 after this the steady state dielectric strength will be recovered quickly. Thus the restriking of arc will not occur.

Applications of Vacuum Circuit Breakers

1. Capacitor bank switching
2. Reactor switching
3. Transformer switching
4. Line dropping.

Advantages of Vacuum Circuit Breaker Over Conventional Type Circuit Breakers

1. Simple construction. 
2. Self contained i.e., no need of periodic refilling of gas or oil.
3. Compact size.
4. Low power requirement for making and breaking operations.
5. Pollution free.
6. Long life
7. Non-explosive.
8. Suitable for repeated operating duty.
9. High speed’of dielectric recovery.
10. Silent operation.
11. Low maintenance.
12. Capable of interrupting highly inductive and capacitive currents without restriking.


Tags:vacuum circuit breaker principle,vacuum circuit breaker pdf,vacuum circuit breaker working wikipedia. 
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Sulphur Hexaflouride (SF6) Circuit Breakers

Sulphur Hexaflouride (SF6) Circuit Breakers

A circuit breaker in which uses  sulfur hexafluoride or SF6 gas as arc extinguished medium is known as SF6 circuit breaker.SF6 is a very good dielectric material which can resist high potentials.SF6 gas is used as the arc quenching medium.Sulfur Hexaflouride (SF6) is an electro-negative gas and has a strong tendency to absorb free electrons. Whenever circuit breaker opens it's contacts an arc will form between the two contacts at the same time with high pressure SF6 gas will be rapidly pumped on the arc.The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions.The SF6 circuit breakers are very effective for high power & high voltage operations.



Construction of SF6 Circuit Breakers:-

SF6 circuit breaker consists of fixed and moving contacts enclosed in  arc interruption chamber.which contains SF6 gas.This arc interruption chamber is connected to SF6 gas reservior. When the contacts of breaker are opened, the valve mechanism permits a high pressure SF6 gas from the reservoir to flow towards the arc interruption chamber.

The fixed contact,moving contact are hollow cylindrical arrangements. The moving contact contains rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF6 gas is costly, it is recycled & reused by suitable auxiliary system after each operation of the breaker.



Working of SF6 Circuit Breakers:-

In the initial position of the breaker, the contacts remain surrounded by SF6 gas at a pressure of about 2·8-3.2 kg/cm2. The working of SF6 Circuit Breaker of first generation was quite simple it is some extent similar to air blast circuit breaker. Here SF6 gas was compressed and stored in a high pressure reservoir. During operation of SF6 circuit breaker this highly compressed gas is released through the arc in breaker and collected to relatively low pressure reservoir and then it pumped back to the high pressure reservoir for re utilize.

The high pressure flow of SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc.After the breaker operation, the valve is closed by the action of a set of springs.

Advantages of SF6 Circuit Breakers:-

Due to the superior arc quenching properties of SF6 gas, the SF6 circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below :
(i) Due to the superior arc quenching property of SF6, such circuit breakers have very short arcing time.
(ii) Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
(iii) The SF6 circuit breaker gives noiselss operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.
(iv) The closed gas enclosure keeps the interior dry so that there is no moisture problem.
(v) There is no risk of fire in such breakers because SF6 gas is non-inflammable.
(vi) There are no carbon deposits so that tracking and insulation problems are eliminated.
(vii) The SF6 breakers have low maintenance cost, light foundation requirements and minimum auxiliary equipment.
(viii) Since SF6 breakers are totally enclosed and sealed from atmosphere, they are particularly suitable where explosion hazard exists e.g., coal mines.

Disadvantages of SF6 Circuit Breakers:-

(i) SF6 breakers are costly due to the high cost of SF6.
(ii) Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is requried for this purpose.

Applications of SF6 Circuit Breakers:-

A typical SF6 circuit breaker consists of interrupter units each capable of dealing with currents upto 60 kA and voltages in the range of 50—80 kV. A number of units are connected in series according to the system voltage. SF6 circuit breakers have been developed for voltages 115 kV to 230 kV, power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.


Tags:SF6 Circuit Breakers,SF6 Circuit Breakers working,SF6 Circuit Breakers construction.
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Plain Break Oil Circuit Breakers

Plain Break Oil Circuit Breakers

A plain-break oil circuit breaker involves the simple process of separating the contacts under the whole of the oil in the tank. There is no special system for arc control other than the increase in length caused by the separation of contacts. The arc extinction occurs when a certain critical gap between the contacts is reached.

Plain Break Oil Circuit Breaker Construction

The plain-break oil circuit breaker is the earliest type from which all other circuit breakers have developed. It has a very simple construction. It consists of fixed and moving contacts enclosed in a strong weather-tight earthed tank containing oil up to a certain level and an air cushion above the oil level. The air cushion provides sufficient room to allow for the reception of the arc gases without the generation of unsafe pressure in the dome of the circuit breaker. It also absorbs the mechanical shock of the upward oil movement. Fig.1 shows a *double break plain oil circuit breaker. It is called a double break because it provides two breaks in series.


Plain Break Oil Circuit Breaker Working

Under normal operating conditions, the fixed and moving contacts remain closed and the breaker carries the normal circuit current. When a fault occurs, the moving contacts are pulled down by the protective system and an arc is struck which vaporizes the oil mainly into hydrogen gas. The arc extinction is facilitated by the following processes :

(i) The hydrogen gas bubble generated around the arc cools the arc column and aids the deionization of the medium between the contacts.
(ii) The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc path.
(iii) As the arc lengthens due to the separating contacts, the dielectric strength of the medium is increased. The result of these actions is that at some critical gap length, the arc is extinguished and the circuit current is interrupted.

Fig.1

Disadvantages Of Plain Break Oil Circuit Breakers

(i) There is no special control over the arc other than the increase in length by separating the moving contacts. Therefore, for successful interruption, long arc length is necessary.
(ii) These breakers have long and inconsistent arcing times.
(iii) These breakers do not permit high speed interruption.

Due to these disadvantages, plain-break oil circuit breakers are used only for low-voltage applications where high breaking-capacities are not important. It is a usual practice to use such breakers for low capacity installations for voltages not exceeding 11 kV.

Tags:oil circuit breaker operating mechanism,Operation of Plain Break Oil Circuit Breakers
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