types of circuit breaker ...............................

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1. Power System Protection and Switch Gear Prof. Sushma Pujari Dept. of EEE(C0-Ed) Sharnbasva University, Kalaburagi

2. Air blast circuit breaker

3. Construction At the bottom, there is a tank which is called air reservoir, and this air reservoir is connected with an air valve. On the air reservoir tank, there are three hollow insulator columns. On the top of each insulator column there is double arc extinction chamber. The current carrying parts are connected to the arc extinction chamber in series. The assembly of entire arc extinction chamber is mounted on insulators as there exists large voltage between the conductors and air reservoir. The main arc extinction phenomenon is done in the double arc extinction chamber

4. Working An auxiliary compressed air system is required by this type of circuit breaker. This system will supply air to the reservoir of the breaker. During the fault operation, the air is allowed to enter in the extinction chamber which pushes away the moving contacts. As we know that when contacts get separated then an arc is struck between them. This arc has to be interrupted as soon as possible. So, in this circuit breaker the contacts are separated under the action of air blast, this air blast will take away the ionized gases and arcing products with it and finally helps in extinguishing the arc. This is very frequent operation, hence the arcing time is very less in air blast circuit breakers.

5. Types of air blast circuit breaker 1) Axial blast type  – In this type, the blasting of air is done directly along the path of arc, 2) Cross blast type- in this type, the blasting of air is directed at right angle to the arc path,

6. Axial blast air circuit breaker

7. Construction: The arcing chamber is connected to the air reservoir with an air valve as shown. The fixed contact and moving contact are held in closed position by the piston and the spring pressure under normal conditions. The air valve remains closed under normal condition, but it opens automatically by the tripping impulse when any fault occurs in the system. A series isolator is also connected in the axial blast type to give the sufficient voltage clearance after the breaking operation.

8. Working: When any fault occurs in the system, the tripping impulse causes the air valve to open automatically and it connects the air reservoir to arc extinction chamber. The high pressure compressed air now move into the extinction chamber where fixed and moving contacts are in closed position with the spring pressure. Now, as the arc extinction chamber is now filled with compressed air so, it will exert pressure on moving contact against the spring pressure. When the air pressure becomes more than the spring pressure then, moving contact will separate and an arc will be struck between moving and fixed contact. If you see closely then you will find that the pressure of air blast is acting axially to the arc and it will decrease the area of cross-section of the arc.

9. Cont…. We know that resistance is inversely proportional to area of cross-section, it means when area of cross section decreases then the resistance will increase, in other words dielectric strength of the path will increase. As, the current is flowing through the arc but due to increase in resistance, current flow will stop, also the arcing products (ionized gases) are taken away with the blast of air to the atmosphere. Hence, arc is extinguished. In such circuit breakers, the contact separation required for interruption is very small (1.75 cm or so). Such a small gap may constitute inadequate clearance for the normal service voltage.

10. Cross blast air circuit breaker

11. Construction Air reservoir filled with compressed air is connected to the arc extinction chamber with the help of air valve. The Arc extinction chamber contains fixed and moving contacts. Arc splitters and baffles are also connected as shown in the figure. These Arc splitters are used to increase the length of the arc whereas baffles are used to provide necessary cooling.

12. Working when the moving contacts are withdrawn due to fault in the system, an arc is struck between the moving and fixed contacts. Now, the high pressure air which is coming at right angle to the arc will force the arc into a chute consisting of arc splitters and baffles. These arc splitters will increase the length of arc as shown in figure. As we know that resistance is directly proportional to the length. It means if the length of arc increases, then resistance (or dielectric strength) of the path will also increase. The increment in length of arc will increase the dielectric strength of the path and finally it will interrupt the flow of current. Hence, the arc is extinguished. Since the blast pressure is independent of value of fault current, so it is very efficient at low currents also. Here in cross blast air circuit breaker, a series isolating switch is not necessary, because the final gap for interruption is good enough to give normal insulation clearance.

13. Advantages Cheapness and free availability of the interrupting medium, chemical stability, and inertness of air High speed of operation Elimination of fire hazard Short and consistent arching time and therefore less burning of contacts Less maintenance Suitable for frequent operation

14. Disadvantages It is very sensitive to restriking voltage. Maintenance of compressor is required. Problem of current chopping Produces high level of noise when air is discharged to an open atmosphere

15. Oil circuit breaker

16. Advantages: Arc energy is absorbed in decomposing of oil The gas formed which is mainly hydrogen, has high diffusion rate and high heat absorption, thus provides good cooling properties The oil has high dielectric strength and provides insulation between the contacts after the arc has been finally extinguished and there has been time for the oil to flow into the gap between contacts Cooling oil presents the cooling surface in close proximity to the arc The oil used (such as transformer oil) is a very good insulator and allows smaller clearance between live conductors and earth components

17. Disadvantages: Oil may be flammable and can cause fire hazards, if a defective oil circuit breaker should fail under pressure and cause an explosion There is a risk of formation of explosive mixture with the air Due to the decomposition of the oil in the arc, the oil becomes polluted by carbon particles, which reduces its dielectric strength. Hence periodical maintenance and replacements are required

18. Plain break oil circuit breaker

19. Self generated pressure oil circuit breaker Arc is utilized to generate a high pressure in a chamber known as explosion pot( pressure chamber or arc controlling device) The pot is made up of insulating material and is placed in the tank

20. Plain explosion pot

21. Cross- jet explosion pot

22. Self compensated explosion pot

23. SF6 Circuit breaker Non-puffer type SF6 circuit breaker

24. Puffer type SF6 circuit breaker

25. SF6 circuit breakers Advantages: Due to the superior arc quenching property of SF6, such circuit breakers have very short arcing time. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such breakers can interrupt much larger currents. The  SF6 circuit breaker  gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere, unlike the air blast circuit breaker. The closed gas enclosure keeps the interior dry so that there is no moisture problem.  There is no risk of fire in such breakers because SF6 gas is non-inflammable. There are no carbon deposits so that tracking and insulation problems are eliminated. The  SF6 Circuit breakers  have a low maintenance cost, light foundation requirements and minimum auxiliary equipment. Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazard exists e.g., coal mines. 

26. SF6 circuit breakers Disadvantages: SF6 breakers are costly due to the high cost of SF6.  Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.  Problem of perfect sealing, there may be a leakage of SF6 gas because of imperfect joints internal parts should be cleaned thoroughly during periodic maintenance under clean and dry environment

27. Vacuum circuit breaker

28. In a vacuum circuit breaker, the vacuum of the order of 10 -5   to 10 -7   torr is used as an arc quenching medium. (1 torr = 1mm of Hg). Basically, vacuum is a pressure below atmospheric pressure where no gas is present. These breakers have highest insulating strength than any other medium. Vacuum circuit breaker does the interruption of current at the first current zero. It means it interrupts the current in the half cycle.

29. Construction It has an outer envelope which is made up of glass joined with end caps. This glass shield is used to examine the breaker after operation, if the colour becomes milky white then it indicates that the breaker is losing its vacuum. Two contacts named as fixed contact and moving contact are placed as shown in the figure. The main arcing process is done in between these two contacts. The moving contact is connected with metallic bellows which are made up of stainless steel. These bellows are used to move the moving member up and down and a spring mechanism is connected with the bellows to operate them. Sputter shield made up of stainless steel is present to prevent metal vapour from reaching outer envelope. The ceramic envelope is also present to insulate the chamber

30. Working of vacuum circuit breaker Initially, the fixed contact and moving contact both are connected to each other and the current flows through them. But as soon as any fault occurs in the system, the moving contact start separating from the fixed contact and arc phenomenon occurs between them. Vacuum circuit breakers have a different way of arc generation. The arc is generated due to vaporisation of metal vapours from the contact surface. A contact surface has large number of microscopic projections and when current reaches to these projections. Now, we know that, after disconnecting the contacts the last point of current will be the contact surface which results in high current density at these points and due to which resistive heating occurs. Due to excessive heating, the metal ions starts vaporising from the contact surface When these ions come in between of two contacts, then they provide a path to the current and current starts flowing which we see as an arc. This is how arc generates in these breakers.