Testing of Transformer oil


Testing of Transformer oil:-

Testing of Transformer oil is necessary for the obtain breakdown strength of oil.oil is used in transformer is neutral oil. Which has very high dielectric strength.

Oil used in transformer as the insulating material.but transformer oil is reduced in its breakdown strength by presence of moisture contents.oil is also provide cooling to the apparatus of the transformer. Breakdown voltage of transformer oil is also known as the dielectric strength of transformer.

Breakdown voltage is measure by the keeping two electrode sphere in oil. Both sphere is separate by some distances. test cell is made from transparent glass. Oil which is breakdown voltage is to measure is take into the test cell. Its shown in fig,

Oil Testing

Here, two transformer are used, one is auto transformer and other is high voltage transformer.
Take two stainless steel polished lectrode sphere of diameter 12.5 mm are kept in test cell. Maintain 2.5 mm gap between they both electrode sphere. They sphere are connected to the high voltage winding of high voltage transformer. And by usig auto transformer variable voltage is supplied to the primary of high voltage transformer. Given voltage is to the primary of auto transformer is about 230 V AC and 50Hz frequency. Here, circuit breaker is connected between the supply voltage and auto transformer for the safety purpose.

Testing procedures :-

  • First fill the sample oil which is need to tested in the vessels of the test cell.
  • Two test electrodes with a few distance of 2.5 mm are surrounded by the dielectric oil.
  • Then test voltage is applied to the electrode sphere¬† and continuous increase the voltage up to the breakdown voltage.
  • After that applied voltage is reached to maximum level of breakdown voltage and finally breakdown is occur. Breakdown is occur in form of electric arc between the two electrodes.
  • Then after this , when arc is ignition. By the testing device test voltage is switched off automatically.
  • Testing devices will measure the breakdown voltage and report it.
  • The value of breakdown voltage is reported in form of RMS value.
  • Finally transformer oil test is completed.
  • Dielectric strength of the oil is measured uder the test.

Oil should has the permissible limit of dielectric strength. Which are following as,
Equipment rated voltage is less then 70 KV and breakdown voltage is above 30KV. For 70 to 170 KV is above 40KV and for above 170KV is above 50KV respectively.


If any query or suggestion about Testing of Transformer oil please comment below or Email on sohal@electricalidea.com.


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