Specifications

181
N.B. Application of vacuum greatly accelerates the drying-out process. Vacuum may be
applied directly into the transformer tank by connecting a vacuum pump producing at
least 28 in. of vacuum, through a suitable outlet, provided that the tank is specially
designed to withstand the full air pressure (15lbs/sq. in.) on the exposed surfaces of
the tank. Alternatively vacuum type purifiers should be used.
16.5 Time of drying-out operation
Whatever be the method employed, the drying-out operation should be continued until the
transformer oil samples from the top and bottom of the tank show high di-electric strength and
the windings high insulation resistance. The actual time required depends upon many factors,
such as the condition of the windings, the amount of moisture it has absorbed, the type and size
of purifying plant, temperature of the oil which it can maintain etc., and may vary from about a
day for a small transformer to as much as a month for a large unit.
An important point to remember is that raising the temperature, will no doubt increase the
rate of drying, but simultaneously it also increases the thermal decomposition of the cellulose of
the paper insulation, which actually produces water vapour. Heating for longer time at temperatures
above 90
0
C is harmful and should be avoided. It is in this context that drying-out under vacuum
is of great value, since it reduces the time required for drying. One test which confirms if the
drying-out has been well done is to allow the transformer to cool down and test a sample of the
oil after a week or fortnight. If its Breakdown Value (BDV) is high as also the insulation resistance,
after making due allowance for the temperature at which the tests are conducted, it is a clear
proof that the windings are quite dry.
16.6 The qualities required for good transformer oil :
The specifications for transformer oil are fully covered by IS : 335-1993. For instructions for
maintaining the insulating oil IS Code of Practice No. 1866 may be referred.
Briefly, the following characteristics are desired :
i) Mineral oil grade B should be used.
ii) It should have a high di-electric strength, i.e. not less than 40 kV in drums and 30 kV in
the transformer tank.
iii) It should contain negligible moisture content. A simple test which shows the presence
of moisture is the
‘
crackle
’
test. This could be done in two ways: Pour a small quantity
of oil sample into a test tube and heat up rapidly under a Bunsen burner. There should
be no cracking. Another method, a half inch iron rod heated to dull red heat is dipped
into a sample of oil kept in a clean vessel. Crackling shows presence of moisture.
iv) The oil should be perfectly clear and pale in colour. Cloudiness indicates presence of
moisture or impurities like sludge or rust, reddish tinge indicates presence of asphalt,
and green colour indicates presence of copper soaps.
v) Acidity content should be very low, as it will cause precipitation of sludge and corrosion