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CB Operating Principles By G. Eswar Rao, AE, Khammam

CB Operating Principles By G. Eswar Rao, AE, Khammam. Primary Functions of Circuit Breakers. Carry Rated Current at rated Voltage and rated Frequency Interrupt rated current at rated Voltage and Frequency

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CB Operating Principles By G. Eswar Rao, AE, Khammam

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  1. CB Operating Principles By G. Eswar Rao, AE, Khammam

  2. Primary Functions of Circuit Breakers • Carry Rated Current at rated Voltage and rated Frequency • Interrupt rated current at rated Voltage and Frequency • Interrupt fault currents depending on the fault level of that particular location. • Maintain rated dielectric (power and impulse voltages) in open condition.

  3. CB Operation

  4. Type of CBs

  5. Dielectric Strength of different mediums

  6. CB Interrupting Chamber

  7. Main and Arcing Contacts

  8. Circuit Breaker Trip Operation

  9. Circuit Breaker Operation

  10. Erosion of the Contacts

  11. Ionization of SF6 gas

  12. Ideal Interruption of Current

  13. Resistive Current Switching

  14. Capacitive Current Switching

  15. Inductance Circuit

  16. Reactor Switching

  17. Recovery Voltage and build up of dielectric strength of the medium Dielectric Strength TRV

  18. Unsuccessful arc quenching

  19. Dielectric Strength of Medium • Depends on: • Contact speed • Gap distance • Arcing electrode shape- arcing wear will play a larger role in determining the behaviour of the CB • Field distribution within the contact gap

  20. PIR Operation

  21. Operation of BHEL make CB Stage1: Both PIR and Main Contact are Open Stage 2: PIR is closed whereas Main Contact is Open Stage 3: Main Contact Closes Stage 4: PIR Opens out Stage 5: Both Main and PIR in Open Condition

  22. Operating Mechanisms • Hydraulic • Pneumatic • Spring • Pneumatic Spring • Hydraulic Spring

  23. SF6 Circuit-Breakers - evolution since 1974 Evolution of Tripping Energy

  24. CB Parameters • Operating Timings • Trip • Close • Pole Discrepancy • Electrical Timings • Mechanical Timings

  25. CB Duty Cycles • O-0.3sec-CO-3min-CO • CO-15sec-CO • Close Time- max-150ms and 200 ms for 400kV and 220kV respectively • Trip Time – 2 Cycle CBs, Mechanical and Electrical Time

  26. CONDITION ASSESSMENT TECHNIQUES • ON LINE CONDITION ASSESSMENT TECHNIQUES • OFF LINE CONDITION ASSESSMENT TECHNIQUES

  27. ON LINE CONDITION MONITORING TECHNIQUES • SF6 GAS PRESSURE MONITORING • TRIP COIL SUPERVISON • AUXILIARY CONTACTS OPERATING TIMINGS • CONTACT SPEED MEASUREMENT BY INSTALLING TRANSDUCERS

  28. OFF LINE CONDITION ASSESSMENT TECHNIQUES • OPERATING TIMINGS OF MAIN AND AUXILIARY CONTACTS-1 Y • DEW POINT MEASUREMENT OF SF6 GAS-4Y • DYNAMIC CONTACT RESISTANCE MEASUREMENT-2Y • TAN DELTA MEASUREMENT OF GRADING CAPACITORS- 4Y • TRIP/CLOSE COIL CURRENTS MEASUREMENT-1Y • STATIC CONTACT RESISTANCE MEASUREMENT-2Y

  29. Factors affecting Testing • Reliable operation of Testing Instrument • Pollution • Moisture/ Relative Humidity • Temperature • Electromagnetic Field • Human Error

  30. Typical Operating Links

  31. Operating Timings

  32. SOURCES OF MOISTURE IN CB • GAS HANDLING – FILLING AND EMPTYING THE CB. AIR MAY BE LEFT DURING EVACUATION WHICH SHALL ADD TO IMPURITIES IN SF6 GAS. EVACUATION TO BE DONE UPTO 1 mbar (CIGRE-WG23) • EXUDATION OF MOISTURE CONTAINED IN ORGANIC INSULATING MATERIALS. • PERMEATION THROUGH SEALED SECTIONS

  33. DEW POINT MEASUREMENT

  34. DEW POINT MEASUREMENT • IF SF6 GAS CONTAINS MOISTURE, IT IS EASILY HYDRATED TO PRODUCE HIGHLY REACTIVE H2SO3 AND HF(HYDROGEN FLUORIDES). • AS THESE CHEMICALS CAUSE DEGRADATION OF INSULATION AND CORROSION IN THE INTERRUPTING CHAMBER, MONITORING OF MOISTURE CONTENT IN SF6 GAS IS VERY IMPORTANT.

  35. CONTAMINATION OF SF6 GAS BY MOISTURE

  36. DECOMPOSITION OF SF6 GAS ARCS DURING TRIPPING OPERATION LEADS TO A SUBSTANTIAL EROSION OF THE CONTACT AND INSULATION MATERIALS BY THE HOT ARC. THE MAIN CAUSE FOR SF6 GAS DECOMPOSITION IS THE REACTION OF THESE EROSION PRODUCTS WITH FRAGMENTS OF THERMALLY DISSOCIATED SF6 AND OTHER TRACE GASSES SUCH AS OXYGEN AND WATER VAPOUR. (A) Cu + SF6 = CuF2 + SF4 – COPPER POWDER (B) W + 3SF6 = WF6 + 3 SF4 - TUNGSTEN (C) CF2 + SF6 = CF4 + SF4 - ERODED PTFE ( A CF2 POLYMER)

  37. EFFECTS OF CONTAMINATION • DECOMPOSITION PRODUCTS ARE CORROSIVE AND MAY AFFECT THE SURFACE INSULATION BY THE FORMATION OF CONDUCTIVE LAYERS ALONG THE INSULATORS. • HUMIDITY/MOISTURE IS VERY DANGEROUS WHEN IT CONDENSES ON THEM IN LIQUID FORM CAUSING REDUCED INSULATION STRENGTH • CUF2, WO3, WO2F2 ORIGINATE FROM CONTACT EROSION. • HEALTH RISK DUE TO BYTOXIC PRODUCTS LIKE SO2 ETC.

  38. Insulation Equivalent Circuit

  39. Thank You for your kind attention please

  40. Condition Monitoring through DCRM

  41. Healthy Contacts of CB after 16 years

  42. Contact Condition of CB after 16 Years

  43. Some CB contacts even after 16 years of service

  44. Eroded finger contact after 7-8 years

  45. PROBLEMS IN CB AFTER 7-8 YEARS

  46. NEED FOR DCRM • During normal contact resistance measurement, healthiness of main contacts is evaluated Req = R*r/(R+r)= r/(1+r/R), r = Main contact R = Arcing contacts • DCRM signatures/finger prints indicate true condition of CB arcing contacts. • Arcing contact play vital role during CB operation

  47. DYNAMIC CONTACT RESISTANCE MEASUREMENT (DCRM) • CONTACT RESISTANCE MEASUREMENT DURING CLOSING AND TRIPPING OPERATION. C-O DELAY TIME ABOUT 300MS. • 100 AMP CURRENT IS INJECTED THROUGH CB CONTACTS. • VOLTAGE DROP AND CURRENT IS MEASURED TO COMPUTE CONTACT RESISTANCE. • VARIATION IN FINGER PRINT OF DCRM INDICATE PROBLEM IN ARCING AND MAIN CONTACTS.

  48. Incoming Flange Outgoing Flange Main Contact, Moving Main Contact, Fixed Fixed contact Holder Arcing Contact, Moving Arcing contact, Fixed Moving contact Holder A Look at Contact Assembly…

  49. Basic connection arrangement for DCRM Measurement

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