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Study of circuit breakers

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This presentation gives the simple understanding of all circuit breakers used in thermal power plants for different operating voltages.

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Study of circuit breakers

  1. 1. Circuit Breaker: It is a device designed to close/make or open/break the electrical circuit under abnormal and normal conditions automatically and also manually. Duties of a circuit breaker are:  It carries the full load current continuously without overheating and damage.  It opens and closes the circuit on load.  It makes and breaks the normal operating current of magnitude up to which it is designed for (automatically).  It makes and breaks the circuit the normal operating current (manually for switching purpose). Moving Contact Fixed Contact Moving Contact Fixed Contact
  2. 2. How does a circuit breaker operate automatically?
  3. 3. Fixed Contact Moving Contact Arc Insulating Fluids commonly used for circuit breakers are: Air at atmospheric pressure Compressed Air Oil producing Hydrogen for Arc Extinction Ultra high Vacuum Sulphur Hexa Floride
  4. 4. Arc Resistance depends on factors like: Degree of Ionization Length of the arc Cross section of arc Methods of Arc Quenching: 1) High Resistance Method: Arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. 2) Low Resistance Method: Dielectric Medium between the contacts is built up more rapidly so that the arc fails to restrike and the current will be interrupted.
  5. 5. High Resistance Method:  Lengthening of arc  Reducing the cross section of the arc  Splitting the arc Low resistance Method:  Lengthening the gap  High pressure in the vicinity of the arc  Cooling the dielectric medium  Blast Effect Significant Terms associated with Circuit Breakers:  Arc Voltage  Restriking Voltage  Recovery Voltage  Breaking Capacity  Making Capacity  Short Time Capacity
  6. 6. Pros of Oil Circuit Breakers: It absorbs the arc energy to decompose the oil into gasses which have excellent cooling properties. It acts as an insulator and permits smaller clearance between live conductors and earthed components. Cons of Oil Circuit Breakers: Oil is inflammable there is a risk of fire. Oil may form an explosive mixture when it comes in contact with air. Arcing products like carbon remain in the oil and its quality deteriorates with successive operations. This requires periodic checking and replacement of oil. Rating: Bulk Oil CB= Up to 12 KV Minimum Oil CB= 13.6 KV to 146 KV
  7. 7. Pros of Air Blast Circuit Breakers: Risk of fire is totally eliminated. The size of the circuit breaker is reduced. High speed operation and suitable for frequent operation. Negligible maintenance. Cons of Air Blast Circuit Breakers: Air insulation is lesser than other medium like as oil etc. Considerable Maintenance required for the compressor plant which supplies air blast. Rating: 110 KV & above
  8. 8. Pros of SF6 Circuit Breakers: Very Sort Arcing Time Can Interrupt very large currents due to high dielectric strength. Noiseless Operation due to its closed/sealed gas circuit. No problem of moisture. No risk of fire as SF6 is inflammable. Low Maintenance Cost. Cons of SF6 Circuit Breakers: SF6 Circuit Breakers are costly due to high cost of SF6. Since SF6 gas has to be reconditioned after every operation of the breaker additional equipment is needed for this purpose. Rating: 3.6 KV to 760 KV
  9. 9. Advantages of VCB: Compact & reliable. Less Maintenance and quite in operation. Withstand Lightning Surges. Small Power for control mechanism. Perfect breaking of any heavy fault current. No risk of Fire. Routine tests conducted on VCB:  Voltage Test with different frequencies.  High Voltage Test  Timing Test  Penetration Test  Insulation Test
  10. 10. Conclusion: Therefore by using appropriate circuit breaker for appropriate operating voltages we can provide protection for busbar and electrical equipments like Power transformers, generators, C.T.’s , P.T.’s, wave traps etc. from different faults that occur in power system.

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