Specifications

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cleaned of all accumulated sludge, muck and dirt. Take care to clean up the drain plug, and
wash the interior of the tank as well as the cooling tubes, first with old oil and again with new oil.
If there are any cracked or badly welded joints they may be attended to. Thoroughly clean all the
gasket surfaces and keep new gaskets ready. Check acidity of oil, if excessive replace it by
fresh oil. Sludge to be removed from the core and coil assembly . The sludge should be scraped
off with fibre or wooden strips. Oil ventilation ducts should be specially cleared. By applying a jet
of oil under pressure the interior parts of the coil assembly could be flushed and all traces of
sludge removed.
Wedges which are usually provided to hold the coil assembly in position should be driven in
to take up any slackness. Large transformers are often equipped with adjustable screws bearing
on clamping rings placed on the outerside of the end coils, so that any slackness due to shrinking
of coils could be taken up. These screws should be tightened up. Shrinkage and settlement
usually takes place within the first few months of the commissioning of a transformer. If the coils
are not rigidly in position, it will lead to repeated movement of the coils as well as the layers and
turns, which in turn will cause abrasion and wear and ultimately breakdown of insulation. To
overcome this, modern transformer coils are often pre-shrunk. The first available opportunity
should be taken, preferably after six months or one year after commissioning, to drive in the
wedges and tighten up the pressure screws where they are provided. Core clamping bolts and
check nuts should be tightened, as also all bolted electrical joints. Attention should be given to
clean up the tap-changing switch contacts and mechanism. After a careful check the core
should be lifted and lowered into the tank, internal connections put back, fresh oil filled in and top
cover bolted in position. Before closing the cover, make sure that no spanners or tools are
inadvertently left inside. The oil may then be circulated through an oil purifier and the transformer
dried out until all moisture has been driven out. The transformer may then be subjected to as
many tests as feasible depending upon the test facilities available. A load test would be particularly
useful. Where an industry, railway or electric supply authority has a large number of transformers
to maintain, it is obviously desirable to make a periodical overhaul (POH) programme, maintaining
a few spare transformers of different capacities for being sent out to work in place of other
transformers to be brought into the central workshop one after another for POH. The work load
on the workshop will be uniform throughout the year and no transformer will be left neglected
for long periods until it breaks down.
QUESTIONS
Part - A
Choose the Correct Answer (1 Mark)
1. Abnormal rise in temperature of oil in transformer may be due to
A) Over voltage B) Transformer losses
C) Overloading D) Failure of circuit breaker.
2. During drying process of transformer the temperature should not exceed
A) 50
0
C B) 90
0
C C) 120
0
C D) 30
0
C