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Testing of transformer

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Testing of transformer

  1. 1. Testing of Transformer K.N.Srinivasan
  2. 2. Traceabllity Accuracy Precision Probability Std. deviation Standards Accreditation
  3. 3. Insulation tests Short ckt test Power rating Other tests
  4. 4. Overview A MODERN TRANSFORMER IS A COMPLEX ELECTROMAGNETIC APPARATUS TRANSFORMERS SERVE FOR TRANSMISSION AND DISTRIBUTION OF ELECTRICAL ENERGY TRANSFORMERS (DISTRIBUTION) ARE THE LAST MAJOR LINK BETWEEN UTILITY & CONSUMER
  5. 5. Introduction TRANSFORMERS ARE STATIC ALTERNATING CURRENT MACHINES PRINCIPLE IS BASED ON MUTUAL INDUCTION BETWEEN WINDINGS THROUGH A COMMON CORE OIL-IMMERSED AND DRY TYPES POWER AND DISTRIBUTION TYPES SPECIAL TYPES
  6. 6. Transformer on load ϕ V1 I0 E2 I1I’2 I2 E = - 4.44 φmf N
  7. 7.  Measurement of winding resistance  Measurement of voltage ratio  Polarity test  Measurement of no-load loss & no-load current  Measurement of load-loss & short-circuit impedance  Measurement of insulation resistance  Dielectric tests Routine tests
  8. 8.  Measurement of winding resistance  Measurement of voltage ratio  Polarity test  Measurement of no-load loss & no-load current  Measurement of load-loss & short-circuit impedance  Measurement of insulation resistance  Dielectric tests  Temperature-rise test Type tests
  9. 9.  Dielectric test  Short-circuit test  Measurement of zero-sequence impedance of three phase transformers  Measurement of acoustic noise level  Measurement of harmonics of the no-load current  Measurement of power taken by the fans & oil pumps Special tests
  10. 10.  Dielectric test  Short-circuit test  Measurement of zero-sequence impedance of three phase transformers  Measurement of acoustic noise level  Measurement of harmonics of the no-load current  Measurement of power taken by the fans & oil pumps Special tests
  11. 11. • Separate source voltage withstand test • Induced over voltage withstand test • Impulse voltage withstand test Dielectric test
  12. 12. Terminals of windings under test connected to HV terminal of the source . All other windings, core, frame and tank are earthed. Duration of test is 60 secs. Highest system voltage(kV) Power frequency voltage(kV) 1.1 3 3.6 10 7.2 20 12 28 24 50 36 70 52 95 72.5 140 Separate source voltage withstand test
  13. 13.  This test is useful in determining layer insulation and inter turn insulation of windings.  HV is kept open & twice the rated voltage at twice the rated frequency is applied to the LV for a duration of 60 secs. Induced over voltage withstand test
  14. 14. Testing facilities • Impulse generator • Discharge circuit • Recording system • Voltage divider for measurement Standard Impulse wave Impulse withstand test
  15. 15. Wave shape  Standard wave shape of impulse voltage is 1.2/50µsec.  t1 is the rise time = 1.2µsecs.  t2 is the tail time = 50 µ secs.  Tolerance on t1 is ±30% & ±20% on t2. Impulse generator  Impulse wave is generated by discharge of capacitors charged in parallel into a wave shaping circuit.
  16. 16. Connection Fig.7.3.1.Connection diagram lmpulse Application : (Negative Polarity). Oscillograms : 7.3.1 Reduced full wave 100 kV (Peak) 7.3.2. 100% full wave 170 kV (Peak) 7.3.3 Reduced Chopped wave 103 kV (Peak) 7.3.4. 100% chopped wave 170 kV (Peak) 7.3.5. 100% chopped wave 170 kV (Peak) 7.3.6. 100% full wave 170 kV (Peak) 7.2.7. 100% full wave 170 kV (Peak)
  17. 17. Oscilograms
  18. 18. Oscilograms contd.
  19. 19. Oscilograms contd. Fig 7.3.3 Connection diagram Generator capacitance 0.0625 uF Series resistance 160 ohms Tail resistance 1040 ohms Voltage divider 1200 pF in parallel with 100 ohms Impulse applications : (Negative Polarity) Oscillogram No. 7.3.15. Reduced full wave 43 kV (Peak) 7.3.16. 100% full wave - 75 kV (Peak) 7.3.17. 100% full wave 75 kV (peak) Comparison of oscillograms 15 & 16 indicates that the second voltage wave oscillogram has collapsed and the corresponding point of the neutral current there are high frequency oscillations. This indicates an arc over across major portion of the transformer winding as a result of failure of the transformer insulation.
  20. 20. Oscillograms 7.3.15 to 7.3.17
  21. 21.  MINOR INSULATION -conductor turns, layers ● enamel, paper ● oil ducts, paper cylinders  MAJOR INSULATION- winding, core ● press board cylinders, oil ducts BUSHINGS - porcelain , condenser
  22. 22. Power & distribution transformers must be constructed to withstand mechanical stresses caused by external faults. These stresses are produced due to electromagnetic forces as a result of very high current in the windings during short-circuits. Short-circuit test
  23. 23.  Prior to the short-circuit, transformer is subjected to routine tests. Asymmetrical current  The peak current that transformer is required to withstand = Isc(peak)=K Isc Dynamic ability to withstand short-circuit Value of K : x/r 1 1.5 2 3 4 5 6 8 10 >14 K 1.51 1.64 1.76 1.95 2.09 2.19 2.27 2.38 2.46 2.55
  24. 24. Short-circuit may be a) Preset or b) Post set Duration of each test being 0.5 sec. ( > 5 MVA 0.25 sec) For 1φ Transformers No. of tests = Three one test with tap at highest voltage ratio, one test in principal taping & one test in lowest voltage ratio Short-circuit testing procedure
  25. 25. 3φ Transformers  No. of tests = Nine  Three tests with tap at highest voltage ratio, three tests in principal taping & three tests in lowest voltage ratio Tolerances on  Asymmetrical current = ±5 %  Symmetrical current = ±10 %  Test duration = ±10 %
  26. 26. Oscillogram of dynamic short-circuit test on 3Phase 11/0.433kV 500kVA Distribution transformer
  27. 27.  All the routine tests shall be repeated  Dielectric test shall be at 75%  Transformer shall be untanked  Reactance measured after s.c. test shall not differ by more than 2% for circular coils & 7.5% for non-circular coils. Analysis of the results SC Test contd.
  28. 28. This is to ascertain that transformer & its cooling arrangements are effectively designed so that temp. rise of winding & cooling medium does not exceed the permissible limits. Temp. rise limits for Dry type transformers Class of insulation Temp. rise °C A 50 E 65 B 70 F 90 H 115 C 140 Temperature rise test
  29. 29. Temp. rise limits for oil immersed transformers Part Temp. rise °C External cooling medium Air Water Winding 55 60 (temp.rise by resistance method) Top oil 50 55 (temp.rise by thermometer method)
  30. 30. a) Direct loading b) Back-to-Back method c) Short-circuit method Loading methods Duration of test Top oil temperature-rise does not vary more than 10 C/hour during 4 consecutive hourly readings
  31. 31. Partial Discharge Tests 100pC 500pC
  32. 32. Sound pressure level 20 log10 d/0.00002 dB d Newton per square meter Noise Level
  33. 33. meter
  34. 34. Highest voltage for equipment Um kV (r.m.s. value) Rated short duration power frequency withstand voltage kV (r.m.s. value) Rated lightning impulse withstand voltage kV (peak value) 3.6 10 20 40 7.2 20 40 60 12 28 60 75 95 17.5 38 75 95 24 50 95 125 145 36 70 145 170 52 95 250 72.5 140 325 123 (185) 230 450 550 145 (185) 230 275 (450) 550 650 170 (230) 275 325 (550) 650 750 245 (275) (325) 360 395 460 (650) (750) 850 950 1050
  35. 35. Highest voltage for equipment Um kV (r.m.s. value) Standard switching impulse withstand voltage -------------------------------------- ------------------- Longitudinal Phase-to-earth Phase-to-phase Insulation (ratio to the (note 1) phase-to-earth kV kV peak value) (peak value) (peak value) Standard lightning impulse withstand voltage kV (peak value) 300 750 750 1,50 850 950 750 850 1,50 950 1050 362 850 850 1,50 950 1050 850 950 1,50 1050 1175 420 850 850 1,60 1050 1175 950 950 1,50 1175 1300 950 1050 1,50 1300 1425 525 950 950 1,70 1175 1300 950 1050 1,60 1300 1425 950 1175 1,50 1425 1550 765 1175 1300 1,70 1675 1800 1175 1425 1,70 1800 1950 1175 1550 1,60 1950 2100

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