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Transformer failure case study

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Dry type transformer failure case study

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Transformer failure case study

  1. 1. Angelo Baggini, angelo.baggini@unibg.it, Bergamo University - Engineering Department Via Marconi 5, 24044 Dalmine (BG) – Italy Failure analysis of a dry type cast resin MV/LV transformer
  2. 2. Failure analysis of a dry type cast resin MV/LV transformer 1. Introduction 2. Chronological events 3. Visual inspection 4. Causes of failures 5. Conclusions
  3. 3. Power Transformers Design Principles INTRODUCTION
  4. 4. Installation Site and other • Where: Office building of a bank group - Milan city center (Italy) • When: following planned maintenance works that required the whole installation to be turned off • Consequence: half the building being without power for 2 days
  5. 5. Electrical layout
  6. 6. Main characteristics of the transformer
  7. 7. Power Transformers Design Principles CHRONOLOGICAL EVENTS
  8. 8. Chronological events from 7.30 am to 9.30 am July 14th 2008
  9. 9. Main chronological events #1 Q6 to Q21 were opened to prepare for the permanent disconnection of some LV cables
  10. 10. Main chronological events #2 The two main LV switches (Q1 and Q3) were opened
  11. 11. Main chronological events #3 The back panels of the main LV distribution board are opened in order to carry out the disconnection operations.
  12. 12. Main chronological events #4 The lines are disconnected and the terminals of cables are extracted
  13. 13. Main chronological events #5 Once the disconnection is completed the network is restored The main LV switch of the transformer 2 (Q3) is closed while Q6 to Q21 remain open.
  14. 14. Main chronological events #6 The on site maintenance personnel perceive a buzzing (“frying”) noise and two blows in fast sequence.
  15. 15. Main chronological events #7 The two MV switches of the transformer (Q4 and Q5) trip, while the main MV switch (Q2) does not trip
  16. 16. Main chronological events #8 and #9 In order to understand which transformer was out of order, the technicians on site try to rearm the MV switch of the transformer 2 (Q5). Once (Q5) is closed the main MV switch (Q2) trips immediately, but not the main MV switch of the electrical company (ENEL).
  17. 17. Main chronological events #10 to #11 The MV switch is restored (Q2). The transformer 1 is put into service, by closing Q4, without any problem.
  18. 18. Main chronological events #12 to #13 The enclosure of transformer 2 is opened, following prescribed procedures. Technicians on site found considerable damage to phase W of transformer 2
  19. 19. Failure analysis of a dry type cast resin MV/LV transformer VISUAL INSPECTION
  20. 20. Visual inspection W and U windings: black smoke traces.
  21. 21. Visual inspection MV-LV channel: electric arc traces
  22. 22. Visual inspection Lower part of LV-MV channel: overheating traces
  23. 23. Visual inspection Parts of winding below the lower part of winding W
  24. 24. Failure analysis of a dry type cast resin MV/LV transformer CAUSES OF FAILURE
  25. 25. LV Windings Sheet (or slab) type • The windings were constituted by thin Cu sheet with height almost equal to the height of the column • radially wounded coils were separated by insulating sheets
  26. 26. MV Windings Cast resin section type Coils axially separated by insulating spacers
  27. 27. Causes of failures • consequence of a low number of coils inside the MV winding is an high layer voltage • partial discharges have damaged the insulation of the MV windings and accelerated its breakdown • The overvoltage, associated with the reconnection of the transformer at the end of the maintenance operation, was sufficient to trigger a breakdown through the weakened insulation • the earlier failure of the same transformer supports the conclusion.
  28. 28. Causes of failures • the discharge between the LV and MV windings was not the primary cause of failure as the air insulation gap (channel MV/LV) was sufficiently large to withstand the most severe overvoltage to which the transformer could reasonably have been expected to be exposed; • the structure of the network and the peak values of the impulse which the transformer has been designed to withstand indicate that the transformer would not have been affected by the over-voltage if the insulation system had not been previously damaged; • degradation of the insulating system by over-stressing, such as that assumed here in order to explain the failure does not enable the prediction of the time of the inevitable dielectric failure • a degraded transformer could remain in service under these conditions for as long as several years or for just a few months. What is certain is that failure will occur.
  29. 29. Failure analysis of a dry type cast resin MV/LV transformer CONCLUSIONS
  30. 30. Conclusions • To agree with the transformer manufacturer that all MV windings and the failed LV winding would be replaced. Taking into account the fact that the fault is a result of a construction defect, even though it became apparent some years into service life and considering the age of the transformer, the costs should be shared accordingly between supplier and customer. • During dismantling of the transformer the conditions of the other windings (not currently inspected) should be checked, with particular attention to the central phase which may have been partially affected by the fault on phase W. • This solution would result in a practically new transformer at a modest cost.
  31. 31. Additional conclusions • To avoid the risk of a repetition of the same problem, it would be wise to evaluate the possibility of replacing all the MV windings of the other transformer (T1), since it is based on the same electromagnetic design. Of course, even in this case the benefits are the same as indicated for the unit currently out of service. • The electrical layout should be modified to avoid the situation of having half the building out of service in the event of a single fault. This could be achieved by adopting a double radial scheme or by providing the ability to feed both LV busbars from a single transformer (at reduced total load or increasing the capacity of each transformer).
  32. 32. Failure analysis of a dry type cast resin MV/LV transformer 1. Introduction 2. Chronological events 3. Visual inspection 4. Causes of failures 5. Conclusions
  33. 33. Thank you | Presentation title and date For more information please contact Angelo Baggini Università di Bergamo Dipartimento di Ingegneria Viale Marconi 5, 24044 Dalmine (BG) Italy email: angelo.baggini@unibg.it ECD Engineering Consulting and Design Via Maffi 21 27100 PAVIA Italy

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