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  Different Methods Transformer Drying Out

Introduction to Different methods of drying out a transformer

The major difficulty in drying out a transformer is not drying the oil- this is fairly easily done through passing it two times or three times by an appropriate filter- it is the removal of moisture absorbed through the windings. This is rather a time consuming process in a new transformer to be commissioned for the 1st time, and might last from a few days for a small transformer to 3 or 4 weeks for a large unit. And during this period the temperature has to be kept among the 800 to 850 C irrespective of surrounding air temperature differences. A very important condition to be made sure during the period is that the oil temperature never goes beyond the limit of 900C, as it may critically damage the insulation. Purification of the oil can be completed when the transformer is in service on light load.

There are mainly two methods of drying out that are with the tank dry or with the tank filled with oil.

(a)  With the tank dry that is with the core and windings in position but with no oil, the heat needed for drying may be generated in two ways:

(i) Through blowing in hot air by the transformer tank.

The air is blown in the tank by an appropriate opening at the bottom of the tank like the drain pipe or radiator pipe outlet. An air outlet has to be left at the top through removing the explosion vent / manhole cover. The inlet air should be at a temperature that is not less than 850 or more than the 1000C. Heating elements of 15 KW (kilo watt) capacity will be enough for a tank capacity of 10 m3 (350 cu.ft.). The outlet air temperature should be not less than 650 to 750C (1400 to 1670 F). This technique is fairly reliable and the drying time along with reasonably dry air and ambient temp., is approximately 4 days for an 11 kV unit and 15 days for a large 220 kV unit. The benefit of this technique is that the coil is not a subject to high temperatures for long periods like in other methods. Moisture in the windings is rapidly removed due to low humidity of hot air.

(ii) Through short circuiting the secondary winding and applying a decreased voltage on the primary, like in the heat run test. The top cover should be maintained open for free flow of air. This technique is not completely satisfactory since the temperature distribution in the windings will be not even because of nonexistence of oil, and is not suggested except for small transformers and while there is no another means available for drying. Though, this method might be adopted in conjunction with method (i) above to decrease the burden on the air heaters. The winding temp, through the resistance method should never be allowed to exceed 900C.

(b) Along with the tank filled with oil, heat could be generated in three dissimilar ways:

i) Through short-circuit method as given in (a) (ii) above.

ii) Through circulating oil through a suitable purifying plant.

iii) Through connecting various immersion type heaters and letting them into the transformer tank.

Application of vacuum very much accelerates the drying-out process. Vacuum might be applied directly into the transformer tank through connecting a vacuum pump generating at least 28 in. of vacuum, by an appropriate outlet, given that the tank is particularly designed to withstand the full air pressure (15lbs/sq. in.) on the exposed surfaces of the tank. Otherwise vacuum type purifiers should be employed.

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