Circuit Breaker : Types
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Circuit breakers are switching devices that can make, carry, and break electric currents under both normal and abnormal circuit conditions. They contain fixed and moving contacts that remain closed during normal operation but open automatically during faults to interrupt the fault current. When contacts open under fault conditions, an arc is produced that must be quickly extinguished. Different circuit breakers use various mediums like oil, air, vacuum, or SF6 gas to rapidly quench the arc through cooling and increasing dielectric strength between contacts. Common types of circuit breakers include oil, vacuum, air blast, and SF6 breakers that vary based on voltage level, switching speed, maintenance needs, and arc quenching method.
Circuit Breaker : Types
- 2. Contents Introduction Operating mechanism Types of circuit breakers Air Blast circuit breaker Vacuum circuit breaker Oil circuit breaker SF6 circuit breaker Conclusion
- 3. Introduction A circuit breaker is a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions. It is also capable of making and carrying currents for a specified time and breaking currents under specified abnormal circuit conditions, such as those of a short circuit.
- 5. Circuit Breakers A circuit breaker is a piece of equipment which can (i) make or break a circuit either manually or by remote control under normal conditions (ii) break a circuit automatically under fault conditions (iii)make a circuit either manually or by remote control under fault conditions
- 6. Circuit Breakers IEEE definition: “A device designed to open and close a circuit by nonautomatic means, and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.”
- 7. Operating Principle Circuit Breaker consists of two contacts: Fixed contact. Moving contact. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
- 8. Operating Principle When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time .
- 9. Fixed contact Moving contact ARC Fixed contact Moving contact ARCARC IS QUENCHED BY MEDIUM IN A CIRCUIT BREAKER OPERATING PRINCIPLE OF BREAKER
- 10. Arc Phenomenon During the separation of contacts, due to large fault current and high current density at the contact region the surrounding medium ionizes and thus a conducting medium is formed. This is called the ARC. During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts.
- 11. Arc Phenomenon The arc resistance depends upon the following factors : Degree of ionization Length of the arc Cross-section of arc Factors responsible for the maintenance of arc between the contacts. These are : p.d. between the contacts ionized particles between contacts
- 12. Arc Extinction
- 13. Arc Extinction Temperature zones in arc
- 14. Methods of Arc Extinction There are two methods of extinguishing the arc in circuit breakers viz. High resistance method. Low resistance or current zero method High resistance method. In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. The resistance of the arc may be increased by : (i) Lengthening the arc. (ii) Cooling the arc. (iii) Reducing X-section of the arc. (iv) Splitting the arc.
- 15. Methods of Arc Extinction Low resistance or Current zero method. This method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking inspite of the rising voltage across the contacts. All modern high power a.c. circuit breakers employ this method for arc extinction. There are two theories to explain zero current method Recovery Rate Theory Energy Balance Theory
- 16. Methods of Arc Extinction In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another half-cycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and the current will be interrupted.
- 17. Methods of Arc Extinction The rapid increase of dielectric strength of the medium near current zero can be achieved by : (a) causing the ionized particles in the space between contacts to recombine into neutral molecules. (b) sweeping the ionized particles away and replacing them by unionized particles. The de-ionization of the medium can be achieved by: (i) lengthening of the gap. (ii)high pressure. (iii)cooling. (iv)blast effect.
- 18. Restriking voltage. It is the transient voltage that appears across the contacts at or near current zero during arcing period. At current zero, a high-frequency transient voltage appears across the contacts and is caused by the rapid distribution of energy between the magnetic and electric fields associated with the plant and transmission lines of the system. This transient voltage is known as restriking voltage . The current interruption in the circuit depends upon this voltage. If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for another half-cycle. On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted Restriking voltage
- 20. Recovery voltage. It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of the circuit breaker after final arc extinction. It is approximately equal to the system voltage. When contacts of circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated sufficiently apart and dielectric strength of the medium between the contacts attains a high value due to the removal of ionized particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by the restriking voltage. Recovery voltage
- 21. Recovery voltage. Consequently, the final arc extinction takes place and circuit current is interrupted. Immediately after final current interruption, the voltage that appears across the contacts has a transient part (See Fig. 19.1). However, these transient oscillations subside rapidly due to the damping effect of system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is known as recovery voltage. Recovery voltage
- 22. Recovery Rate Theory-By slepain Slepian’s Theory of Arc Extinction •Arc extinction process is a race between dielectric strength and restriking voltage •Residual column of ionized gas exists after current zero •If Dielectric strength builds up faster than Restriking voltage- arc extinguishes. •This theory incomplete –compares restrike voltage and dielectric strength –does not cover arcing phase
- 23. Energy Balance Theory Cassie’s Theory of Arc Extinction •Arc consists of column at uniform temp, well defined boundary •Uniform distribution of energy in column, temp remains constant •Arc cross section adjusts itself to accommodate current •Power dissipation proportional to column cross-section •Energy equation •Breakdown occurs if power fed in arc > power loss
- 24. Arc Extinction in Oil Arc extinction in OIL •Arc decomposes dielectric oil •Gasses produced increase chamber pressure •Flow of gasses channelized through vents •Arc gets extended into vents cooled by flowing gases •Gas contains 70% hydrogen -good dielectric strength •Contact area filled with fresh dielectric for arc extinction •Chamber may be pressurized with inert gas
- 25. Arc Extinction in Vacuum Arc extinction in VACUUM •Current leaves contact from small intensely hot spots •Metal vaporizes from spots •Vapor constitutes the plasma in vacuum arc •Rate of vapor emission -current in arc •At current zero plasma may vanish •Vacuum has very high dielectric strength-though arc may not restrike
- 26. Arc Extinction in Air Blast Arc extinction in AIR-BLAST •Air flows from high pressure reservoir during arc extinction process •Flow rate governed by throttle diameter of nozzle, pressure difference, nozzle profile •Almost supersonic speed of air flow-rapid reduction of arc diameter •Arc does not reappear after final current zero
- 27. Arc Extinction in SF6 Gas Arc extinction in SF6 gas •SF6 atoms and molecules attracts electrons, forms ‘–ve’ ions •‘–ve’ ions heavier than electrons-resistance of plasma increases rapidly •Gas flows through nozzle over arc - takes the heat away • Medium regain dielectric strength rapidly
- 28. Classification of Circuit Breaker
- 29. According to their location 1) Outdoor Circuit Breaker 2) Indoor Breaker Based on External Design 1) Dead Tank Type 2) Live Tank Type
- 30. According to the voltage level of installation types of circuit breaker are referred as 1) Ultra High Voltage( Above 765 KV) 2) Extra High Voltage (300KV to 765 KV) 3) High Voltage Circuit Breaker(66KV to 220 KV) 4) Medium Voltage Circuit Breaker (1KV to 52 KV) 5) Low Voltage Circuit Breaker(Less than 1KV)
- 31. Based on Medium used for ARC Quenching Oil Circuit Breakers Vacuum Circuit Breakers Air Blast Circuit Breakers SF6 Circuit Breakers
- 32. OIL CIRCUIT BREAKER It is designed for 11kv-765kv. These are of two types • BOCB (Bulk oil Circuit Breaker) • MOCB (Minimum oil Circuit Breaker) The contacts are immersed in oil bath. Oil provides cooling by hydrogen created by arc. It acts as a good dielectric medium and quenches the arc.
- 33. Advantages: Oil has good dielectric strength. Low cost. Oil is easily available. It has wide range of breaking capability. Disadvantages: Slower operation , takes about 20 cycles for arc quenching. It is highly inflammable , so high risk of fire. High maintenance cost.
- 34. VACCUM CIRCUIT BREAKER It is designed for medium voltage range (3.3-33kv). This consists of vacuum of pressure (1*10-6) inside arc extinction chamber. The arc burns in metal vapor when the contacts are disconnected. At high voltage, it’s rate of dielectric strength recovery is very high. Due to vacuum arc extinction is very fast. The contacts loose metals gradually due to formation of metal vapors.
- 35. Advantages: Free from arc and fire hazards. Low cost for maintenance & simpler mechanism. Low arcing time & high contact life. Silent and less vibrational operation. Due to vacuum contacts remain free from corrosion. No byproducts formed. Disadvantages: High initial cost due to creation of vacuum. Surface of contacts are depleted due to metal vapours. High cost & size required for high voltage breakers.
- 36. AIR BLAST CIRCUIT BREAKERS This operates using high velocity blast of air which quenches the arc. It consists of blast valve , blast tube & contacts. Blast valve contains air at high pressure. Blast tube carries the air at high pressure & opens the moving contact attached to spring. There is no carbonization of surface as in VCB. Air should be kept clean & dry to operate it properly.
- 38. Advantages: High speed operation as compared to OCB. Ability to withstand frequent switching. Facility for high speed reclosure. Less maintenance as compared to OCB. Disadvantages: Little moisture content prolongs arcing time. Pressure should be checked frequently for frequent operation. Risk of fire hazards due to over voltages. It can’t be used for high voltage operation due to prolonged arc quenching.
- 39. SF6 CIRCUIT BREAKERS It contains an arc interruption chamber containing SF6 gas. In closed position the contacts remain surrounded by SF6 gas at a pressure of 2.8 kg/cm2 . During opening high pressure SF6 gas at 14 kg/cm2 from its reservoir flows towards the chamber by valve mechanism. SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions to build up high dielectric strength. It also cools the arc and extinguishes it. After operation the valve is closed by the action of a set of springs. Absorbent materials are used to absorb the byproducts and moisture.
- 41. Advantages: Very short arcing period due to superior arc quenching property of SF6 . Can interrupt much larger currents as compared to other breakers. No risk of fire. Low maintenance, light foundation. No over voltage problem. There are no carbon deposits. SF6 breakers are costly due to high cost of SF6. SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose. Disadvantages:
- 42. CONCLUSION: Therefore, we conclude that circuit breaker is the most essential part of the electrical networks as it protects every device from damage. It helps us to detect the fault and area affected by it. Nowadays vacuum and SF6 circuit breakers are widely used due to their reliable and fast operations.