Quality Tests Performed on Insulated Oils

Hi there! We have discussed in a previous article the theory behind the electrical breakdown in liquids applied to insulated oils inside power transformers.

We found that contaminants inside the oil are the main cause for electrical breakdown inside oils, therefore since mineral oils degrade over time, it is expected to eventually see contaminants inside the oil.

In order to ensure that the quality of the oil remains within acceptable values, it is necessary to develop quality tests to determine if it is necessary to schedule maintenance or not.

Mineral Oil Inside Power Transformers

The main function of mineral oil is to act as electrical insulation between different potential points inside the transformer. Moreover, it is also essential for thermal dissipation by convection and acts as a protection of the transformer tank against air and humidity.

The dielectric strength of the oil should be between 50-300 kV/cm.


Quality Tests On Insulated Oils

It is important to perform the quality tests before the transformer is connected to the electrical system, as it will act as a reference for future occasions. Once scheduled preventive maintenance is reached, there are several tests that need to be done. They can be classified as:

  • Humidity Test

The portion of humidity is measured in parts per million (ppm). This reveals the percentage of humidity within a specific volume of oil. The maximum expected value should be below 40 ppm in used oils, while on new oils should be less than 35 ppm.


  • Interfacial Tension

This test quantifies the attraction forces between the molecules inside the oil. Two liquids with very different interfacial tensions can be translated into insoluble liquids, such as oil and water. If there are contaminants (moisture, mud) then the interfacial tension of the oil reduces when compared to water. The maximum expected value in used oils is 20 dinas/cm.


  • Breakdown Voltage

The test simply implies to apply a voltage over a sample of the oil until the electrical breakdown is reached. Voltage is elevated with a rate of 3kV/min and the separation between electrodes is 2.54 mm. The minimum breakdown voltage should be 24kV in old oils, and 30kV in new ones.


  • Gas Chromatography

Gases are normal inside a transformer, especially at the operation temperature values that it can reach. However, deep concentrations of specific gases can be indicators of malfunction. The gas chromatography allows us to determine the portion of these gases inside the transformer and jump to conclusions based on the results.

Here we can see the normal and abnormal values of several gases inside a transformer.


Gas Normal Abnormal
Hydrogen Less than 150 ppm Above 1,000 ppm
Methane Less than 25 ppm Above 80 ppm
Ethane Less than 10 ppm Above 35 ppm
Ethylene Less than 20 ppm Above 100 ppm
Carbon Monoxide Less than 500 ppm Above 1,000 ppm
Carbon Dioxide Less than 10,000 ppm Above 15,000 ppm


Among some common causes for the appearance of these gases we can find:

  • Electrical discharges that create great volumes of hydrogen, methane, ethane, and ethylene.
  • Thermal decomposition of the oil to produce hydrogen, methane, and ethylene.
  • Thermal decomposition of the cellulose to produce carbon monoxide, carbon dioxide, and water.

Regularly practicing these tests ensures us to keep control of the performance of our transformer to take preventive corrections if needed. In the case that the oil is not within the established quality standards, then it must be passed through a filter to restore its electrical properties.


Have you ever tried this kind of tests? If yes, was it effective? Comment and share below!

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