The document summarizes a failure analysis of a dry type cast resin medium voltage/low voltage transformer. It describes the chronological events of the failure, a visual inspection showing damage to the windings, and analyzes the causes of failure which were likely partial discharges that damaged the insulation of the medium voltage windings. The conclusions recommend replacing the failed windings, checking other windings for damage, and modifying the electrical layout to avoid power loss to half the building if another failure occurs.

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. 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. Power Transformers Design Principles
INTRODUCTION

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. Electrical layout

6. Main characteristics of the transformer

7. Power Transformers Design Principles
CHRONOLOGICAL EVENTS

8. Chronological events
from 7.30 am to 9.30 am July 14th 2008

9. Main chronological events
#1
Q6 to Q21 were opened
to prepare for the
permanent disconnection
of some LV cables

10. Main chronological events
#2
The two main LV
switches (Q1 and Q3)
were opened

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. Main chronological events
#4
The lines are
disconnected and the
terminals of cables are
extracted

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. Main chronological events
#6
The on site maintenance
personnel perceive a
buzzing (“frying”) noise
and two blows in fast
sequence.

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. 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. 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. 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. Failure analysis
of a dry type cast resin MV/LV
transformer
VISUAL INSPECTION

20. Visual inspection
W and U windings:
black smoke traces.

21. Visual inspection
MV-LV channel:
electric arc traces

22. Visual inspection Lower part of LV-MV channel:
overheating traces

23. Visual inspection
Parts of winding
below the lower part
of winding W

24. Failure analysis
of a dry type cast resin MV/LV
transformer
CAUSES OF FAILURE

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. MV Windings
Cast resin section type
Coils axially separated by insulating
spacers

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. 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. Failure analysis
of a dry type cast resin MV/LV
transformer
CONCLUSIONS

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. 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. 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. 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