Specifications

180
16.4 Different methods of drying out a transformer :
The main problem in drying out a transformer is not drying the oil- this is quite easily done
by passing it twice or thrice through a suitable filter- it is the removal of moisture absorbed by the
windings. This is quite a time consuming process in a new transformer to be commissioned for
the first time, and may last from a few days for a small transformer to 3 or 4 weeks for a large
unit. And throughout this period the temperature has to be maintained between 80
0
to 85
0
C
irrespective of surrounding air temperature variations. A vital condition to be ensured throughout
the period is that the oil temperature never exceeds the limit of 90
0
C, as it may seriously damage
the insulation. Purification of the oil can be done while the transformer is in service on light load.
There are basically two methods of drying out, i.e. with the tank dry or with the tank filled
with oil.
(a) With the tank dry, i.e. with the core and windings 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 85
0
nor more than 100
0
C. Heating elements of 15 KW capacity will be sufficient for
a tank capacity of 10 m
3
(350 cu.ft.). The outlet air temperature should be not less than
65
0
to 75
0
C (140
0
to 167
0
F). This method is quite reliable and the drying time with
reasonably dry air and ambient temp., is about 4 days for an 11 kV unit and 15 days for
a large 220 kV unit. The advantage of this method is that the coil is not subjected to high
temperatures for long periods as in other methods. Moisture in the windings is quickly
removed because of low humidity of hot air.
(ii) By short circuiting the secondary winding and applying a reduced voltage on the primary,
as in the heat run test. The top cover should be kept open for free flow of air. This
method is not entirely satisfactory because the temperature distribution in the windings
will be uneven due to absence of oil, and is not recommended except for small
transformers 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 temp., by the resistance method should never be permitted
to exceed 90
0
C.
(b) With the tank filled with oil, heat could be produced in three different ways:
i) by short-circuit method as given in (a) (ii) above.
ii) by circulating oil through a suitable purifying plant.
iii) by connecting several immersion type heaters and letting them into the transformer
tank.