Properties and Application of Good Transformer Oil and Agents which Contaminated Transformer Oil

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Properties and Application of Good Transformer Oil and Agents which Contaminated Transformer Oil

Properties and Application of Good Transformer Oil and Agents which Contaminated Transformer OilProperties and Application of Good Transformer Oil and Agents which Contaminated Transformer Oil

Transformer oil is used as di-electric coolant in transformers, switch gears, starters, regulators etc. The satisfactory performance of such equipment largely depends on the conduction and quality of transformer oil. The fresh oil has a pale clear yellow colour. The dark cloudy appearance indicates deterioration and impurities in it.

Following should be the properties of a good insulating oil.

Properties of a good insulating oil :

  1. Appearance : The appearance should be pale-clear-yellow colour, free suspended matters, and it should be transparent. Cloudy appearance shows impurities and moisture in it.
  2. Density : As per I.S., the density of insulating oil should not be more than 0.89 gm/Cm3.
  3. Viscosity : The low viscous oil flows in all voids, holes and places in the machine and provides good insulation and fast cooling. Low viscosity means higher fluidity.
  4. Electric strength : Di-electric strength of oil should be higher to withstand high voltage surges. Less dielectric strength indicates the presence of free moisture in the oil. It should be filtered. The di-electric strength should not be less than 30 kV with 2.5 mm standard gap of testing this oil. 
  5. Specific resistance : It should be as high as possible. If the specified resistance of oil is less than 1 x 1012 Ω cm at 27°C it should be rejected. If the value lies between 1 x 1012 to 10 x 1012 the oil should be reconditioned by filtering.
  6. Purity : The presence of impurities such as sulphur is harmful. It causes deterioration of metallic parts coming in contact with it. A sludge is formed due to it. The pure oil is free from impurities.
  7. Sludging : Due to heat and oxidation, the sludge is formed. Sludge is a formation of hydrocarbons which are semi-solids. It affects cooling process and machine becomes hot. Its effect can be reduced with the use of conservator tank in transformer.
  8. Water Content : Presence of water/ moisture reduces di-electric strength of insulating oil. It should be removed by filtering. The oil should be moisture free.
  9. Pour point : The oil should not freeze other wise a problem is created in Buchcholz’s relay in transformer.
  10. Flash point : For safety the flash point of oil should not be less than 160°. The flash point is the temperature above which the vapour of oil on its surface starts igniting.
  11. Fire point : This should be above 200°C. This is the temperature at which oil starts igniting and burns continuously.
  12. Acidity : Acid present in the oil is very harmful. The oil should be reconditioned if acidity is from 0.5 to 1 mg. KOH/gr and oil should be rejected if it is more than 1 mgKOH/gr. Acidity corrodes the metallic parts. Transformer oil is a liquid insulation meant for cooling insulating and quenching purposes.

Applications of a good insulating oil :

The transformer oil is used in

  1. Minimum oil circuit breaker.
  2. Bulk oil circuit breaker.
  3. Oil fuse switches.
  4. Rheostats.
  5. Starters/controllers.
  6. Regulators.
  7. ON-load tap-changers.
  8. Isolators.
  9. Transformers.

Agents which contaminate Transformer oil

Agents :

As discuss in the previous section the oil should be pure and free from impurities. But following different agents contaminate the oil and it becomes impure and loses its di-electric and insulating properties.

  1. Water
  2. Dirt/Dust
  3. Solid particles/sledges
  4. Fibers of various origins
  5. Sulphur
  6. Acid
  7. Gases
  8. Grease
  9. Carbons
  10. Ketons
  11. Hydrocarbons

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