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3.3.2.2. Overhaul Of Transformers
a) Overhaul of a transformer is normally undertaken either if it is faulty or
at the end of 7-10 years by way of periodic maintenance. This can be
done in the Central Repair Shop which is a covered shop having full
facilities including a core lifting bayu with a crane. Before commencing
the work ensure that spare gaskets of proper quality are available.
Drain out the oil, disconnect all leads, remove manhole covers where
required. The EHV and 25kV bushings are then carefully removed out
and stored well protected in a safe place. Then remove the core by
means of the lifting hooks and place on shop floor over a trestle in a
large receptacle into which oil can drain out.
b) If the transformer has been opened up because of any internal fault,
make a careful note of colour of transformer oil, arc-marks, carbon
deposits, charring of insulation, condition of the windings, unusual
odour and other abnormalities which would all help in ascertaining the
cause of the failure. If a coil has been burnt out, the whole transformer
will have to be completely dismantled and then the damaged coil
replaced with a new coil. In the case of the traction transformers, the
replacement of the damaged coil is best done in the Manufacture’s
works where necessary facilities and staff with the requisite skills are
available.
c) Arrange for the interior of the transformer tank to be thoroughly cleaned
of all accumulated debris, sludge, etc and wash with fresh oil. Remove
the drain plug, lightly polish the valve seat and renew the oil-tight
gasket round the spindle so that when assembled the plug is fully oil
tight; the same remarks apply to the oil sampling valve, if provided.
Opportunity should also be taken to plug or weld up any small blow
holes through which oil seepage was observed earlier. Finally paint the
exterior of the tank if necessary after thoroughly cleaning it up of all
paint work, rust and traces of oil and dirt.
d) If the coil assembly is lifted up after 5, 10 or more years of service,
considerable amount of sludge formation would have occurred on all
parts of the transformer, i.e. at the bottom of the tanks, metal work of
the transformers, windings and inter-spaces between windings. All
these should be scrapped off carefully with a wooden or fibre wedge
without causing any damage to the windings. Traces of the sludge left
over in inaccessible places are best removed by directing a thin jet of
transformer oil under pressure using small oil purifier. At the same time
the old surface contamination should be brushed and washed down,
until; the clear surface of the winding is exposed.
e) Care should be taken to protect the windings against ingress of
moisture particularly during inclement weather. Care should also be
taken by wiping off body sweat with a towel. The windings should also
be kept warm by surrounding the open windings by a number of infrared
lamps or by other means.
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