Drying
of Electrical Transformer Insulation | New
transformers are almost invariable sent fully dried out, filled with oil and
fitted with a breather. Sometimes large transformers are sent out without oil,
with the cooling coils separately packed, to reduce weight and facilitate
handling. The cooling tube outlets are the sent blanked off sealed,
the tank itself being filled with dry gas, either air or nitrogen under
pressure, so that the windings may be perfectly dry when received; it will then
be necessary only to mount the cooling tube and fill the tank with good
transformer oil, preferably by the vacuum suction process. Drying out period in
the field will then be greatly reduced. Read the article about transformer noise reduction and operation too.
Drying of Electrical Transformer Insulation
The main
problem in drying out a transformer is not so much drying the oil-this is quite
easily done by passing it twice or thrice through a suitable filter-as the
removal of moisture the windings. This is quite a time consuming process in a
new transformer to be commissioned to 3 or 4 weeks for a large unit. And
throughout this period the temperature has to be maintained between 800 to 850
C irrespective of ambient air temperature variations.
A vital
condition to be ensured throughout the period is that the oil temperature never
exceeds the limit of 900 C, as it may seriously damage the insulation. The
amount of heat radiated from a transformer tank is quite appreciable especially
in cold cover round the transformer if it is installed outdoors and also to lag
the sides and the cooling tubes to reduce the radiation losses.
In electrical substation having a large number and different types of transformers which have to be periodically
dried out, it is worthwhile installing a purifier in the substation premises
and providing a piping system permanently so that purification of the oil could
be carried out on any transformer at any time desired, from the central plant,
by making just a few flexible conduit pipe connection. Incidentally purification of the
oil may proceed while the transformer is in service on light load. Coming to
the methods of drying out, there are basically two i.e. with the tank dry or
with the tank filled with oil.
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Drying of
Electrical Transformer Insulation
|
With the
tank dry i.e. with the core and winding in position but without oil, the heat
required for drying may be produced in two ways:
(i) By
blowing in hot air through the transformer tank.
The air is
blown into the tank through a suitable opening at the bottom of the tank such
as the drain pipe or radiator pipe outlet. An air outlet should be left at the
top by removing the explosion vent or manhole cover. The inlet air should be at
a temperature not less than 850 nor more than 1000 C. Some manufacturers offer
special driers comprising a centrifugal type air blower with built in air
filters and heating elements of 15 KW capacity, which will be sufficient for a
tank capacity of 10 m3 (350 CU.FT.).
The outlet
air temperature should be not less than 650 to 750 (1400 to1670F). This method
is quite reliable and the drying time with reasonably dry air and ambient
temperature, is about 4 days for an 11kV unit and 15 days for large 220 kV unit.
This advantage of this method is that the oil is not subjected to high
temperatures for long period as in other methods. Moisture in the windings is
quickly removed because of low humidity of hot air.
(ii) By
short circuiting the secondary windings and applying a reduced voltage on the
primary, as in the heat run test. The top cover should be kept open for free
ventilation. This is not entirely a satisfactory method because the temperature
distribution in the windings will be uneven due to absence of oil, and is not
recommended except for small transformer and when there is no other means
available for drying. However, this method may be adopted in conjunction With
method (i) above to reduce the burden on the air heaters. The winding
temperature, by the
resistance
method should never be permitted to exceed 900 C.
(b) With
the tank filled with oil, heat could be produced in three different ways:
(i) By
short circuit method outlined above.
(ii) By
circulating oil through a suitable purifying plant.
(iii) By
connecting several immersion type heaters and letting them into the transformer
tank.
N.B.
Application of vacuum may be applied directly into the transformer tank by
connecting a vacuum pump production at least 28 in. of vacuum, through a
suitable outlet, provided that the tank is specially designed to withstand the
full air pressure (15 lbs/sq.in.) on the exposed surfaces of the tank.
Alternatively vacuum type purifiers should be used.
Final
Word
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of Electrical Transformer Insulation. Incase of any doubt please
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