This case study describes how proactive maintenance practices saved a power transformer. During routine maintenance, oil samples from three identical transformers were tested. One transformer's (Transformer #2) test results showed abnormally high ethylene levels, over 8 times the limit. Upon further inspection of Transformer #2 after de-energizing it, a washer was found resting on the core, causing sparking during insulation resistance testing. Removing the washer resolved the issue. The conclusion recommends carefully checking for foreign objects before commissioning transformers and conducting regular diagnostic tests to detect problems early.

1. www.systemprotection.in +91-265-2225137
ISO 9001:2015
ISO 45001:2015 (OH & S)
Certified Company
Case Study
How Proactive Electrical Maintenance
team saved a Power Transformer

2. Your valuable 10
minutes may avoid
breakdown of
Electrical Equipment
in your plant
Today, we are going to present a case study
of ‘POWER TRANSFORMER’, which we never
heard off or thought that such even case
may happen in plants but happened in one.
However, we wish to share the experience,
so that the problem Maintenance Engineers
faced in that plant, may not be faced by
others or can be avoided.
Power transformer of rating 31.5 MVA, 66/11
kV is used for power distribution to utilities
through an 11 kV switch-gear, they were
having three Transformers of identical
rating, in which all three transformers are in
charged condition & the transformer which
we are talking about is used as a standby
transformer.
This case study shows, how Pro-active
Engineers, preventive Maintenance &
Routine testing activities can save your
Electrical Equipments and Revenue worth
crores.
The transformers were in charged
condition, predominantly the 1st and 3rd
transformers were in a loaded condition
comparative with the 2nd one. Almost one
year passed, all three transformers were
functioning normally, with a load of 10 to
20% of its rated capacity.
www.systemprotection.in +91-265-2225137

3. Incident of Fault :
As a yearly Preventive maintenance activity,
the all 3 transformers maintenance has
been carried out, everything was found in
order, the mineral oil samples of all three
transformers were collected and has been
sent to the nearest Lab for analysis of DGA,
BDV & water content.
The Maintenance team didn’t take any final
verdict on shutting down the transformer,
for more assurance, they have sent the oil
samples to another four labs just for
checking the actual increase in C2H4 by
authenticating with their own lab results,
the samples were sent to two private labs,
one to OEM and one sample to ERDA, all
the results were not the same, moreover
there is a significant change in C2H4 as
compared with others.
The DGA test results of Transformer#2 was
found abnormal compared with the past
test results (first result before charging), the
C2H4(Ethylene) content was found
immense, as per IEEE C57.104 standards it
should be less than 50 ppm. But where has
in the test result, the content of C2H4 was
found around 402 ppm, almost 8 times the
specified limit as per IEEE.
Below is the
results of Oil test
www.systemprotection.in +91-265-2225137

4. COMPARISON OF
DGA RESULTS
The Transformer is de-energized & OEM
representative has been called out to the
plant location to further scrutinize the case,
the site team performed some electrical
tests to ensure the healthiness, the test
results were found in order.
While conducting Insulation resistance test
the voltage is notwithstanding/ building up
normally, all of a sudden, the team noticed
some chattering sound from inside the
transformer, all the team members were
inspecting the transformer from all three
sides of inspection window, but the
chattering sound/ spark is not accurately
visible.
The oil has been drained out of the
transformer, the manhole covers were
removed, the core yoke and tank shorting
point disconnected/separated, IR test
conducted between the separated points
(Core & yoke frame) with respect to ground,
a voltage of 2500 Volts is applied to check
the healthiness of insulation.
www.systemprotection.in +91-265-2225137
As there is a complication to enter inside
the transformer due to the space concern
between the body and the core-coil
assembly, the inspection window in three
sides was opened for conducting the above
test.
OEM recommended dispatching the
transformer to their works for further
investigation, the transformer is dispatched
from the site and the site team also
reached to OEM works for inspection.

5. CCA OF THE SAID
TRANSFORMER
The core has been lifted from the body and
visual inspection performed, no
abnormalities found, Insulation resistance
between core & yoke frame conducted @
2500 volts, chattering sound observed.
www.systemprotection.in +91-265-2225137
Finally, case was concluded, one washer
found resting on the bottom of a center
limb of core and touching the yoke frame
resulting in sparking/chattering.
Pic of the washer which was resting in the bottom
of the center limb.
OMG! From where this washer came inside the
transformer?
We can't judge this now, however, this
washer may have gone inside the
transformer in OEM workplace itself or else
in site during installation of the transformer,
we may say that it was sheer negligence or
poor workmanship. But this can happen
After removal of washer Insulation
resistance test conducted the values were
found more than 500 Mega-ohms.
The transformer has been packed and
testing has been done, everything found
normal, the same has been commissioned
and taken in service.
Conclusion that can
be drawn from this
Incident :
Conclusion which maintenance team of
that plant came up with, which may not be
feasible, but if they have done it, they could
have saved lot of time and money.
Below is the conclusion drawn by the
maintenance team of that plant are as
below

6. Points to be considered before & after
commissioning of the Transformer (related to
this case study) :-
1. After receipt of the transformer at your
premises, don’t forget to check the
insulation resistance of Core & Yoke frames
earthing point individually with respect to
the ground/ body.
www.systemprotection.in +91-265-2225137
4. DGA / Oil test report should be cross
verified (if the report is not within
prescribed limits) with a minimum of One
NABL accredited labs, before concluding.
2. If these points are not available outside
the transformer body, then the manhole
cover to be removed and all these three
points to be isolated and insulation
resistance to be conducted as stated in
point no 1.
3. DGA to be carried out before charging,
after charging (no load 24hrs), in a loaded
condition (after 24 hrs) & after a month,
where online DGA is not available.
5. While erecting transformer and its
accessories, one should be very cautious of
handling materials and one must double
check that nothing is fell inside the
transformer tank, before boxing up & oil
filtration/ charging.
Hope it will help you, also share this with other
Electrical Engineers you know.
Any other suggestions are welcomed.

7. CONTACT INFORMATION
301/2/306, Silverline Complex,Opposite BBC
Tower,Sayajigunj,Vadodara - 390 005,
Gujarat, India.
bdm@systemprotection.in
www.systemprotection.in
+91-265-2225137