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 800 to 850 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 900C, 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.
Procedure
(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 850 nor more than 1000C. Heating elements of 15 KW capacity 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 C (1400 to 1670 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 900 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)
ii)above. by circulating oil through a suitable purifying plant.
iii)by connecting several immersion type heaters and letting them into the transformer tank.
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.
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 900C 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.
Different methods of purifying and drying-out transformer oils
There are primarily three types of oil purifiers in common use:
- Centrifugal purifiers such as the ‘De Laval’ type.
- Filter pack type such as the ‘Streamline’ purifiers.
- Filters using activated earth media.
The purpose of oil purification is to remove from it, all contaminants such as water, carbon deposits, dirt, sludge, dissolved moisture and gases. In transformer oils the most important quality to be preserved is the di-electric strength. This is severely affected by the presence of water either in the free state or as dissolved moisture. Transformer oils and the insulating materials used in the transformer windings are hygroscopic, i.e. tend to absorb moisture, which may enter into the transformer tank due to defective breathers, gaskets or by addition of untreated make up oil.
Circuit breaker and switch, oils get carbonised on account of the tremendous heat produced in the electric arc before it is interrupted. Dissolved air and excessive heating (due to arcing) cause oxidation of the oil and formation of heavy sludge deposits at the bottom of the tank and on the surface of the windings. It is essential to remove all these impurities at least once in two years by filtering the oil, in order to maintain the equipment in a healthy state
