Transformer absorbs moisture sometimes which is very harmful to its health. Presence of moisture can be detected by two following tests:
dielectric strength test of oil
insulation resistance test of the winding
Checking Insulation Resistance of Winding
Insulation resistance of winding should be checked with a 1000 V megger, the voltage should be applied for one minute. All the windings except the winding under test should be earthed during this test. The insulation resistance should not be less than the recommended value for the corresponding temperature.
At room temperature, the insulation resistance may be high even though the winding is not completely dry. In such a case the insulation resistance drops rapidly when the winding gets hot. Insulation resistance is halved for every 10° C rise in temperature. Therefore for a proper comparison of insulation resistance, all readings should be taken at nearly the same temperature.
If the insulation resistance of the winding is satisfactory but the dielectric strength of the oil is low then the treatment of oil in filtering and drying plant will improve its dielectric strength.
If the insulation resistance of winding is lower than the specified value then the core and winding have to be dried.Transformer moisture removal process can be performed with or without removing the oil from the transformer.
Transformer Drying Process in Oil
In this method of drying, one winding is short-circuited and low voltage usually of the order of 5% for even less is applied to the other winding to circulate 70 to 80% full load current through the shorted winding.
To prevent loss of heat through cooling tubes the oil level may be lowered just below the top of the tubes. If there is a radiator the valve in the pipe leading to the radiator may be closed.All air vent should be kept open to allow the free escape of hot air. Inspection and manhole cover if any may also be removed during the drying process. The tap changing switch should be so set as to have all the windings in the circuit.
The voltage and current should be so adjusted that the temperature of the oil does not exceed 80oC and the transformer is in no case subjected to a temperature beyond 85oC as damage can be caused by overheating. The tank cover should be covered with lagging during the drying process to prevent condensation of moisture on the inside.
Simultaneous filtration of oil through a plant will help in the drying process.It should be noted that the drying process may take several days to obtain satisfactory results.
Dehydration of Transformer Without Oil
It can be accomplished by one of the four methods listed below:
By Internal Heat: This method is similar to the short circuit method already discussed except that the voltage required to be applied will be only 0.5 – 1.5% of normal voltage.The temperature measured by the resistance method should not exceed 80 – 90oC.The method is not very satisfactory as it results in slow and superficial drying only.
By External Heat: In this method, hot air about 115 oC is blown through the transformer if the core is to be dried in the tank itself. The success of the method depends largely upon the efficient circulation of hot air through the various openings and ducts in the coils.
Drying Under Vacuum: It is quick and at the same time the most effective method of drying provided the transformer tank can withstand the external pressure resulting from a vacuum of 635 mm or more inside.
The temperature of the oil is raised to 80 – 85oC gradually for 24 hours;hot oil is then drained and vacuum of the order of 635 – 710 mm is created with a vacuum pump.The transformer is maintained in this condition until the temperature drops to 50oC.The whole process is repeated until satisfactory insulation readings are obtained.
Drying In Oven: The core and windings of medium-sized transformers can be conveniently dried in a drying oven.The temperature of the oven is maintained at 85 – 90oC and hot air is allowed to escape through the vents at the top. The drying is continued until the required results are obtained.
Measurement of Insulation Resistance During Drying
The success of transformer moisture removal process will depend upon the accurate measurement of insulation resistance at regular intervals of say 2 hours throughout the drying process at, particularly constant temperature.
The insulation resistance may fall at first as the moisture spreads and work its way outwards or it may rise gradually in the beginning and rapidly as the drawing proceeds.The drawing should be continued until the insulation resistance remains constant for at least 12 hours. Plotting of a curve with resistance as ordinate and time as abscissa will help incorrect interpretation of the results.
Precautions During Transformer Drying Process
Drawing of a transformer must be done under continuous and competent supervision. The Transformer should never be left unattended during the drying process.
Careful observation of temperature is essential during drying as high temperature can result in damage to the insulation.
No spark and smoking should be allowed in the vicinity of a transformer being dried.
Low voltage hand lamps should be used for inspection of the tanks from inside.
On the completion of transformer moisture removal process, the transformer should be filled with tested oil only with the help of a filter plant through the drain or sample valve located near the bottom and not from the top or through the conservator.
All the vents should be open during filling. Filling the tank in this way will ensure the escape of all air from within the tank and prevent the formation of air pockets.The tank should be gently stroke with a mallet as the oil is filled. The rate of filling should be reduced as the oil level approaches the top cover Oil may be filled under partial vacuum if the tank is designed to withstand vacuum.
The transformer should be kept energized for 6 hours at no load at the end of which air cork should be opened to allow the released air to escape.