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Purpose and Procedure of Transformer Oil testing

In this article we will discuss the topic transformer oil testing. What is the purpose of transformer oil. Which equipment are needed to perform transformer oil testing. What is the purpose of this test. How to perform the test and what will be the required results.

 Step by step Procedure of Transformer oil testing according to IEC 60156 standard.

Purpose of Transformer Oil

Insulating oil fill in the main tank of transformer. It has two purposes. One is to provide insulation between the HT and LT winding and between ground and windings. It also provides cooling to the transformer. Oil circulates between different parts of transformer. It carries heat from the windings to the radiator of transformer. Depending upon the type of cooling, heat is removed from transformer either by natural air or forced air cooling. Some Engineers equates the oil of transformer to the blood of human body. So if the condition of transformer oil is up to the mark, transformer can work properly and safely. Therefore some tests are conducted on regular basis to verify the condition of transformer oil.

Purpose of Transformer Oil testing

Among different tests, testing the dielectric strength test of transformer oil is one of main test which was carries out to verify the insulating properties of oil under high voltage. Dielectric strength of Transformer must be above the threshold value. However this strength can be reduced by contaminants such as water, sediment and conducting particles. There can different reasons of these particles and contaminants which is beyond the scope of this article. These particles are mostly conducting which tend to reduce the dielectric strength of oil. Weakening of dielectric strength can result in leakage current between HT and LT winding. In extreme cases it can result in the spark and may be short circuit. One indication of healthiness of transformer oil is its dryness and cleanliness. However this does not necessarily indicates the absence of all contamination.

Difference between Meggering and oil test.

Usually students and yound graduate confuse these tests. Although both are insulation tests but meggering is done to find insulation between HT & LT winding and between both windings and body (Ground). On the other hand transformer oil testing is done to find dielectric strength of oil. To understand the procedure of meggering please refer to my article how to megger transformer.

Equipment to perform Transformer oil test

The equipment is used to measure the dielectric strength of transformer must have the following provisions.

  • Capable of supplying high voltage.
  • Arrangement to hold sample of oil.
  • Visibility of spark.
  • Voltmeter.
  • The equipment should be capable to increase the voltage at rate of 2kV/sec.

Typical transformer oil testing device is shown the figure.

Equipment for transformer oil testing

It can be seen that it contains transformer oil cup, a voltmeter and high voltage provision. There are certain rules or requirements oil cup.

  • The air gap between two electrodes in oil cup must have air gap of 2.5mm according to IEC 60156 standard.
  • The gap between electrodes must be adjustable.
  • The ends of electrodes must be concave in shape.

transformer oil test cup

Desired results of transformer oil testing

  • High voltage should be applied between 1 min to 5 min.
  • New transformer oil must withstand upto 70KV voltage.
  • Used transformer oil should withstand 40 to 50 kV voltage. s

Transformer oil testing procedure

  • Take free oil from transformer main tank in glass bottle. Don’t used PET material or plastic bottle for oil sample.
  • Connect the equipment with grounding wire. Then connected with 220VAC, 50Hz or 110VAC, 60Hz according vender recommendation.
  • First set the air gap between the electrodes, it must 2.5 mm according IEC 60156 standard.
  • Then fill oil cup with sample oil until the electrodes fully dip in oil. Close the safety cover of the cup.
  • There will be start button on test set and display showing voltage. Press the test start button, test will start and voltage will increase at the rate of 2kV/sec, that can be observed on display.
  • The voltage will increase until breakdown of oil occurs, at this point voltage will be stop to increase. Note the reading from display it first breakdown voltage reading of the transformer oil.
  • Remove safety cover of cup. The shake the oil with help clean small wooden stick.
  • Now wait 10 minutes for next reading. After 10 minutes same procedure may repeat. Similarly take six reading and take average value of six reading.
  • For used oil acceptance value is 40 ~ 50 kV while for new oil 70kV is limit.

(Courtesy to Engr Akhtar hussain shah for his expert opinion and guidance on this topic.)





About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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