Specifications

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16. MAINTENANCE OF TRANSFORMERS
16.1 Introduction
The transformer is one of the most reliable items of electrical equipment, requiring relatively
little attention; yet often even the minimum of attention is not given, they also sometimes
breakdown because of neglect. The programme of inspection and maintenance is governed by
the size of the transformer, place of installation, whether indoors or outdoors, if in a substation is
it manned or unattended, the operating conditions and so on. The degree of attention required
depends greatly upon how heavily or lightly the transformer is loaded. The intervals of inspection
are indicated as : hourly, daily, weekly, monthly, 3 monthly and 6 monthly, yearly, two yearly and
five yearly. In major installations, using air blast cooling or water cooling a daily check should be
made of the ancillary installations like air blowers, water pumps and the connected protective
devices, on-load tap changers, etc.
16.2 Action to be taken if the oil temperature rises unduly :
Excessive oil temperature is the result of overloading or inadequate cooling. Switch on a
larger unit for operation or put another unit in parallel to share the load or take steps to reduce
the load on the transformer. Check whether all the cooling systems are working properly. Excessive
temperature reduces life and therefore, if oil temperature rises unduly for any reason the
transformer should be put out of service immediately, but the cooling fans, if any, should continue
to run until the oil temperature falls to normal.
16.3 Points to be checked if the oil level tends to fall down:
Transformer oil may leak at several points, i.e. oil level gauge, cork packing below the top
cover, oil conservator connection, drain cock, gasket, bolts where a cable box is bolted in, and
welded joints. Every oil leak should be traced to its source and remedial action taken to stop the
leak. If gasket leak cannot be stopped by tightening the bolts, the gaskets should be renewed.
The best material for gaskets is cork-rubber sheet of 5 mm thick. The surfaces between which
the gasket is provided should be quite flat and smooth, perfectly clean and free from scale, old
paint, remains of glue and old gasket, grease or oil. The bolt tightness should be checked
periodically. Sometimes slight oil leakage occurs at the welded joints. The exact point of leakage
outside the tank can be discovered first by cleaning the surface thoroughly with a grease solvent
or petrol or denatured alcohol, and then coating the surface with a thin a layer of chalk, cement
paste in water or white wash, and allowing it to dry. Leakage of oil is then readily revealed by the
dark patch it forms. Formation of a few drops over a period of time is not of any consequence
and occurs in most transformers. If it is bad, the cracked or spongy weld may be repaired by
welding in a metal patch. Take care to guard against any explosions. If a hole exists in a casting
it may be drilled and a tight brass plug driven in. It is no use using shellac as a filler as it shrinks
considerably when it dries. After completing the repairs, the oil level should be brought to required
level and a careful note made of the oil level for verification at the next inspection.