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Condition Monitoring

Condition Monitoring. CTs , CVTs and LAs. Current Transformer. EHV CURRENT TRANSFORMERS IN POWERGRID NETWORK. DEAD TANK - HAIR PIN DESIGN - EYE BOLT DESIGN (B) LIVE TANK TYPE. CTs installed in Substation. Insulation Grading. Capacitance and Tan Delta Measurement.

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Condition Monitoring

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  1. Condition Monitoring CTs, CVTs and LAs

  2. Current Transformer

  3. EHV CURRENT TRANSFORMERS IN POWERGRID NETWORK • DEAD TANK - HAIR PIN DESIGN - EYE BOLT DESIGN (B) LIVE TANK TYPE

  4. CTs installed in Substation

  5. Insulation Grading

  6. Capacitance and Tan Delta Measurement • CTs with Test Tap- Ungrounded Specimen Test mode (UST) • CTs without Test Taps – Grounded Specimen Test (GST) mode with jumpers disconnected • Values to be monitored w.r.t. factory/ pre-commissioning values • Sudden change in measured values indicate faster deterioration of insulation. • Precautions: P1/P2 to be shorted. Porcelain surface to be thoroughly cleaned. Test Tap to be reconnected to Earth after the Test

  7. Capacitance and Tan Delta Measurement – Contd. • Connection of Test Tap to be ensured otherwise it may lead slow arcing in the soldering area and insulation may fail in due course of time. • Measurement of Tan Delta of C2 (insulation between last foil on which test tap wire is soldered to the ground) to be carried out. • Measurement in GSTg mode with P1/P2 terminal guarded.

  8. EFFECT OF MOISTURE IN PAPER INSULATION

  9. FACTORS AFFECTING TAN DELTA MEASUREMENT • TEMPERATURE TAN T0 = TanT0 e(T-T0) Where,  = Temperature coefficient = (ln TanT - ln TanT0)/ (T - T0)

  10. Current Transformers (Paper/Oil) up to 245 kV - IT range 1. Oil filling plug 2. Dome 3. Nitrogen filling valve 4. Collar 5. Primary terminal 6. Porcelain insulator 7. Insulated primary 8. Cover plate for tank 9. Tank 10. Secondary cores

  11. Current Transformers (Paper/Oil) up to 420 kV - IT range 1. Dome 2. Nitrogen filling valve 3. Primary terminal 4. Collar 5. Porcelain insulator 6. Primary conductor with insulation 7. Adaptor cylinder 8. Secondary cores 9. Base 10. Oil drain plug

  12. Live tank Current Transformers (Paper/Oil) 1. Diaphragm bellows 2. CT cores 3. HT primary terminal 4. Primary conductor assembly 5. Head housing 6. Core housing 7. Porcelain shell 8. Bushing tube 9. Capacitive grading layer 10. Secondary terminal blocks 12. Ground pad 13. Secondary terminal box 14. Base assembly 15. Sealing plate

  13. CONSTRUCTION OF CT

  14. CTs (Live Tank)installed in Substation

  15. CONSTRUCTION OF CT

  16. CT Failures • About 30 nos. CTs have failed due to poor impregnation and paper wrapping at works. • About 90 nos. CTs have failed due to pre-mature ageing of almost all makes • 1 no. CT failed after repair at site. • Moisture entry due to N2 gas leakage

  17. Primary Insulation Failure due to moisture entry

  18. Connection of Test Tap

  19. Condition Monitoring of CTs • Capacitance and Tan Delta Measurement • Insulation Resistance Measurement • N2 Gas Monitoring • Dissolved Gas Analysis- SOS

  20. DISOLVED GAS ANALYSIS • After experiencing few failures of CTs, DGA of CT oil introduced as Conditioning Monitoring of all CTs of POWERGRID. • Ist sample after one month of commissioning • 2nd sample after 7 months of commissioning • 3rd sample before expiry of warrantee period • Further sample will be based on requirement

  21. DISOLVED GAS ANALYSIS cont.. • All DGA results are to be analysed to know the internal condition of CT. If there is any increase in trend of fault gases/ratios, the same shall be taken up with manufacturer immediately with intimation to RHQ & CC/ OS • The H2 Gas content on any CT shall not be more than 50ppm as per latest circular dtd 05.12.2008 of CC/OS

  22. DISOLVED GAS ANALYSIS cont.. • .

  23. CVT Schematic

  24. Capacitor Voltage Transformers (Paper/Film/Oil) up to 765 kV 1. Oil level indicator (optional) 2. Expansion device 3. Capacitor units 4. Insulating oil 5. Porcelain insulator 6. Sealing 7. Electromagnetic unit 8. Low voltage terminals box/ HF terminal 9. Series inductance 10. Medium voltage transformer 11. Damping circuit against ferroresonance effects

  25. Failure of Capacitor Element in

  26. Constraint in Cap. Measurement on CVT • Capacitor divided unit needs to be disassembled from EMU Tank otherwise due to presence of stray cap. to ground measurement of Capacitance will be inaccurate • Best alternate is to periodically measure voltage output of all 3 phases simultaneous

  27. Simultaneous Voltage monitoring of CVT in three phases cont… • CAPACITANCE DIVIDER Ratio ={(C1+C2)/C1} • C1= PRIMARY CAPACITOR • C2= SECONDARY CAPACITOR • IN THE EVENT OF AN ELEMENT FAILURE IN C2, CAPACITANCE, C2 INCREASES • CAPACITANCE DIVIDER RATIO INCREASES • MV TAP/INTERMEDIATE VOLTAGE REDUCES

  28. Simultaneous Voltage monitoring of CVT in three phases cont… • IN THE EVENT OF AN ELEMENT FAILURE IN C1, CAPACITANCE C1 INCREASES • CAPACITANCE DIVIDER RATIO REDUCES • MV TAP/INTERMEDIATE VOLTAGE INCREASES • HENCE, SECONDARY VOLTAGE INCREASES

  29. Simultaneous Voltage monitoring of CVT in three phases cont… • NO. OF C2 ELEMENTS ARE LESSER • e.g. 20: HENCE CHANGE IN C2 DUE TO ONE ELEMENT FAILURE IS (1/20)x100=5% • NO. OF C1 ELEMENTS ARE MORE • e.g. 90: HENCE CHANGE IN C1 DUE TO ONE ELEMENT FAILURE IS (1/90)x100=1%

  30. Simultaneous Voltage monitoring of CVT in three phases • Secondary Voltage Measurement – variation in voltages indicates shorting/ puncturing of capacitor elements.

  31. Simultaneous Voltage monitoring of CVT in three phases cont… • Drift in secondary voltage • Upto +/- 0.5V - Healthy - Monitor 6 monthly • +0.5 to +0.8 - To be monitored - 3 monthly • +0.8 to +1.2 - Close monitoring - monthly • +1.2 to +2.0 - Close monitoring - 15 Days • Above +2.0 V - Alarming - Replacement • -0.8 to -4.0V - Close monitoring - 15 Days • Less than -4.0V - Alarming - Replacement

  32. Arrester Fundamentals How the Arrester Operates … Vs= System Overvoltage Ia = Arrester Discharge current Va = Voltage Across Arrester L = Inductance per unit length C = Capacitance per unit length Zo = Surge Impedance of the system

  33. Arrester Fundamentals V I Characteristics

  34. Arrester Fundamentals • Minimize all separation distances • L1, L2, LA affected the arrester discharge voltage • ‘d’ affects the magnitude and frequency of voltage oscillation at the transformer • Protective distance: • D = U – Up X V • 2 x s LA D = Protective distance V = Velocity of propagation of overvoltage wave = 300 mts / micro sec S = FOW steepness of the incident overvoltage in kV/ micro sec

  35. Arrester Fundamentals • Energy Considerations • Mogard Arrester Capabilities

  36. Ageing of Metal Oxide Surge Arresters • Normal Operating Voltage causes ageing of ZnO Blocks • Temporary O/V, Switching O/V and Lightning O/V may cause overloading of all or some of the ZnO blocks • External Pollution may cause non-linear voltage distribution. Accelerated ageing caused by internal PDs • The increase in Resistive Leakage Current may bring the arrester to Thermal instability and complete Arrester Breakdown

  37. Basic Circuit of LCM

  38. Equivalent Circuit of LA

  39. . • Thank You for your kind attention please

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