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CTs

Randhir Singh. CTs. Approximate Population of Substation Equipments. Transformers 300 Nos. Reactors 325 Nos. Circuit Breakers 2100 Nos. Current Transformers 5200 Nos. Capacitive Voltage Transformers 3200 Nos. Lightning Arresters 3000 Nos. Isolators 5000 Nos.

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CTs

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  1. Randhir Singh CTs

  2. Approximate Population of Substation Equipments Transformers 300 Nos. Reactors 325 Nos. Circuit Breakers 2100 Nos. Current Transformers 5200 Nos. Capacitive Voltage Transformers 3200 Nos. Lightning Arresters 3000 Nos. Isolators 5000 Nos.

  3. Current Transformers

  4. Population of CTs in POWERGRID

  5. Types of CTs Dead Tank Design Hair Pin Design Eye Bolt Design Live tank Design

  6. Hair Pin Design 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 IT 400 Cross section Hair-Pin design

  7. Internal details 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 Eye bolt design

  8. Active Part Manufacturing IT range

  9. Live Tank CTs

  10. Dead tank CTs

  11. CT Standards IEC 60044 – 1 IS 2705

  12. CT Design Core Material – The main aim is to give high accuracy with low saturation factor. Core Material is made of CRGO Silicon steel

  13. CT accuracies As per IEC-60044(1) Metering Core – ±0.2 or 0.5% at rated Currents Protection Cores – ± 1% at rated current

  14. Accuracies as per IEC-60044-1

  15. Protection Cores

  16. Factors for Protection 1. Accuracy Limiting Factor/composite error For e.g if the class designation is 5P20 20 is the Accuracy limiting factor which signifies that when 20 times the rated primary current is applied the composite error of 5P( +/- 5%) is maintained. Typical Class designations are 5P10, 5P20, 10P10, 10P20 etc.,

  17. Ratio Error Ratio Error = (KnIs- Ip)*100/ Ip Kn = Rated transformation ratio Ip = Actual primary current Is = Actual secondary current

  18. Instrument Transformer Error Primary Ip Ip K : Phase error K.Is :Ratio error Secondary Is Ip Is TC : K= Current transformer :

  19. Phase Angle Error The difference in Phase between the Primary and Secondary current vectors

  20. Knee Point Voltage 10% increase in Voltage will lead to 30% or more increase in Current.

  21. Insulation Levels For Windings having Um greater than 300kV, the rated insulation level is determined by rated switching and lightning impulse withstand voltage For voltages < 300kV, insulation levels are decided by lightning impulse and power-frequency withstand voltages

  22. Insulation Levels

  23. Creepage Distances

  24. Pre Commissioning Tests • Polarity Test • Magnetization Curve Test • Ratio Test • Primary Current Injection Test • Secondary Current Injection Test

  25. Reasons of 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 2 no. CT failed after repair at site. Moisture entry due to N2 gas leakage

  26. Primary Insulation Failure due to moisture entry

  27. Raipur CT failure (AREVA)

  28. Insulation puncture

  29. Failure of Primary Insulation

  30. Condition Monitoring Checking of Bellow expansion - M Visual Inspection for leakages - M Tan Delta Measurement – 2Y Thermovision Scanning - Y Nitrogen Pressure Checking – 2Y DGA testing of Oil - SOS

  31. CT TESTING –TAN DELTA

  32. CT Insulation

  33. 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

  34. Capacitance and Tan Delta Measurement – Contd. Connection of Test Tap to be ensured otherwise it may lead to 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.

  35. CVTs

  36. Population of CVTs in POWERGRID

  37. CVT Construction Details

  38. Capacitor stack Inductive VT

  39. CVT Construction Details There are 280 – 300 elements in C1 & C2 C1 will be about 260 to 280 elements C2 will be 15 to 20 elements Ratio of C1/ C2 is 20 to 22 400/ 20 = 20kV (Tap Voltage)

  40. Compensating Reactor Compensating Reactor is provided to compensate for the phase displacement in Capacitor elements wL = 1/w (c1+c2) L = 1/ w2 (c1+c2)

  41. Ferro Resonance Ferro resonance in CVTs is due to the Capacitance in Voltage Divider in series with the inductance of the Transformer and series reactor. This circuit is brought to resonance by various disturbances in the network that may saturate the iron core of the transformer, over heat electro magnetic unit and lead to insulation breakdown.

  42. Ferro Resonance Circuit Ferro resonance circuit is provided in CVT Secondary to suppress Ferro resonance oscillations There can be active or passive Ferro resonance circuits It can be RLC circuit (ABB) or RL circuit (CGL) or Resistance (BHEL, WSI, AREVA)

  43. CVT Secondary Voltage CVT Secondary Voltage v = k * V * C1/ (C1+C2) V – Primary Voltage k – Secondary Transformer Transformation ratio Note: Puncturing of C1 – Secondary Voltage will increase Puncturing of C2 – Secondary Voltage will decrease

  44. CVT VA ratings As per POWERGRID specifications, VA ratings for core-1, core-2 and core-3 are 50VA, 50VA and 50VA respectively. Earlier CVTs it was 200/ 200/ 50VA CVT accuracies are guaranteed if connected burdens are within 25% to 100% of the rated burdens In POWERGRID, with static meters and static/ numerical relays, connected burdens are 10 to 20 VA in each core which are very low as compared to earlier rated burdens.

  45. PD Measurement

  46. REASONS OF FAILURES OF CVTs WRINKLES ON ALUMINUM FOIL POOR SOLDERING QUALITY POOR QUALITY OF PAPER(LOCAL SOURCE) PINHOLES IN BELLOWS SNAPPING OF BELLOW CONNECTION OVERHEATING OF DAMPING RESISTOR SHORTING OF TRANSFORMER CORES FAILURES OF FR CIRCUIT COMPONENTS RUSTING OF COUPLING BOLTS (BETWEEN FLANGE AND EMU TANK) RUSTING OF FLANGE

  47. Reasons of CVT Failures LOOSENESS OF CORE BOLTS SNAPPING OF CONNECTION BETWEEN PRIMARY WINDING AND COMPENSATING REACTOR FAILURE OF VARISTORS PROVIDED IN SECONDARY ENTRY OF MOISTURE IN CAPACITOR STACKS POOR GASKET QUALITY ALMOST ALL COMPONENTS OF CVTs HAVE SHOWN FAILURE TREND

  48. Rusting of EMU Tank

  49. EMU Tank Transformer Winding shorted

  50. Top Bellow with uneven surface and soldered material

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