Drying of oil-
- If the dielectric strength of oil is low, then the oil is treated in a filtering plant and drying plant will increase its dielectric strength.
- The oil should be drawn from near the bottom and deliver back into the transformer tank at the top.
- This process is repeated until the oil will have good dielectric strength.
Drying of cores and coils-
This can be done with or without removing the oil from transformer.
Drying in oil-
- In this method one winding is short circuited and low voltage is applied to other winding. So as to circulate 70-80% of full load current through the shorted winding.
- To prevent loss of heat through cooling tubes, the oil level must be just below the top of the tube.
- The valve in the pipe leading to radiator must be closed.
- All air vent should be kept open, to escape freely the hot air. The voltage and current should be so adjusted that temperature of oil does not exceed 80°. Otherwise, serious damage can be caused by overheating.
Drying without oil-
- By internal heat
- By external heat
- Drying under vacuum
- Drying in oven
By internal heat-
This is the short circuit method where voltage is required only 0.5-1.5% of normal voltage. The temperature should not exceed 80-90°. This method is not very satisfactory because drying process is slow and superficially.
By external heat-
In this method hot air at about 115°c is blown through the transformer. This method depends largely upon efficient circulation of hot air through the various opening and ducts in the coil.
Drying under vacuum-
The temp of oil is raised to 80-85°c gradually over a period of 24 hours, hot oil is drained and vacuumed of the order of 635-710 is created with a vacuum form. The transformer is maintained in this condition until satisfactory insulation reading is obtained. It is a quick and most effective method.
Drying in oven-
- The cores and windings of medium size transformer can be conveniently dried in a drying oven.
- The temperature of oven is maintained at 85-90°c and hot air is allowed to escape through the vents at the top.
The drying is continuing until the required results are obtained.