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-

  1. By internal heat
  2. By external heat
  3. Drying under vacuum
  4. 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.

Sachidananda Dhar

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Harish Sawla · September 2, 2018 at 6:32 am

Sir, question: How to calculate harmonic factor to avoid damages of transformer in putting before the operation in replacement

    Sachidananda Dhar · September 2, 2018 at 7:00 am

    I am unable to write here equation, because it is a comment section. So i already send you details about harmonic to your mail. Please check your mail.
    Thank you.

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