Transformer oil is used for the cooling and insulating purpose in the transformer. The windings of the transformer are insulated with cellulose and surrounded by oil for the proper insulation and cooling. To make the insulation proper it must be dry.

If there will be moisture in the transformer or in the transformer oil it will decrease the dielectric strength, increase the risk of bubble formation at high temperature, and can condense during the cool down in transformer oil. The solubility of water in oil depends upon on the temperature of the oil. Moisture of oil can be calculated in ppm ( parts per million ). It is a unit to measure the moisture in the oil.

The solubility of water in Oil as proportional to the Temperature

Temperature of oil Presence of moisture in oil (ppm)
0°C 22
10°C 36
20°C 55
30°C 83
40°C 121
50°C 173
60°C 242
70°C 331
80°C 446
90°C 592
100°C 772

Effects of moisture in Transformer oil:

We can say that moisture is a big enemy of a transformer. Moisture is very harmful to the power equipment. It can create damage for both insulating oil and paper insulation in the transformer. If the paper insulation loses its mechanical strength due to the moisture then paper insulation cannot be returned to its original condition.

Source of water or moisture in transformer oil:

The moisture can be entered in different sources. Like, internal sources and atmosphere. Water can be entered through the breather of silica gel and through the help of damaged power equipment (bad gasket, cracked insulation, and etc.). If the transformer oil can get out, then water can get in into the transformer. Water or moisture can generate from the internal sources. Degradation of paper and oil produces water.

Prevent the effect of moisture from Transformer Oil:

  • Repair the leakage area of the transformer.
  • Make sure that the oil tank is totally sealed.
  • And sure that the gap above the oil is filled with nitrogen.
  • Recycle or purify the oil at a correct time.

Test of water content in Transformer Oil:

Karl Fisher Titration is the method to measure the water content in transformer oil. To make the experiment we need a container with full of solution and two platinum electrodes. The electrodes must be immersed in the solution. In this method, H2O react with Iodine(I2), Sulfur dioxide(SO2), alcohol(CH3OH), and an organic base (C5H5C) in an organic solvent. The ions of iodide are decomposed into the platinum electrode. After the depolarization of the ions, it will indicate the end point of Fisher reaction in the solution. And this reaction is:

H2O + I2 + SO2 + 3C5H5N → 2C5H5NHI + C5H5NSO3


In this reaction, the ions Iodide are produced by the process of electrolysis to take part in the reaction. The iodide ions are totally consumed in this reaction as long as the water molecules present in the solution. After some time there will be no water molecules to react in this reaction and the reaction will stop at this time.

This reaction follows the rules of ‘Faraday law of electrolysis’. Anyone can easily calculate the actual mass of iodine participated in the reaction.

From the first reaction, we get that 1 mole of iodine reacts with 1 mole of water. As per the experiment, you can calculate the weight of iodine and determine the exact quantity of water present in the transformer oil.


Calculation of moisture content in transformer oil:

The formula of the percentage of moisture content in transformer oil is:

Moisture content (%) = (W2-W3/W2-W1)* 100

Where; W1= weight of the container with cover

W2= weight of the container (before the sample dry)

W3= weight of the container (after the sample dry)


To make the transformer long lasting we have to extract the moisture from the transformer oil. Water is very important for our life but in this case, water is the destroyer of the transformer.