Acidity Test of Transformer Oil

The transformer oil is used for insulation and cooling purpose in a transformer. Acidity Test of Transformer oil is very important test  for analyzing the oxidation of transformer oil. The acidity test is conducted to measure the acidity content in the transformer oil. Transformer oil Acidity Test is carried out using a transformer oil acidity kit.

Transformer oil acidity kit

Any variation in transformer oil characteristics may lead transformer failure. The specifications of the transformer oil are as given below.

Characteristics       of  Transformer oil
Density Max
0.89 gm/Cm2 at 29.50 C
Kinematic viscosity Cst at 27°C
27Cst Maximum
Inter-facial tension at 27°CNewtons/M
0.04 Minimum
Neutralization value
a)Total acidity mg
0.03mg KOH/gm Max.
b) Inorganic acidity / Alkalinity
Oxidation stability
a) Neutralization value after oxidation for 16 hrs at 100°C
b) Total sludge after oxidation for 164 hrs at 100°C
0.40mg KOH/gm maximum
0.10% by weight maximum
Electric strength (Break down voltage)
a) New untreated oil
b) After treatment
30kv Minimum (RMS)
60kv Minimum (RMS)
Flash point Pansky/marten (Closed)
140°C Min
Pour point
6°C  Maximum
Dielectric Dissipation vector (Ten delta) at 90°C
0.002 Max
Specific resistance (Resistivity)
A) at 90°C
B) at 27°C
35 x 1012    Ohm-cm Minimum
1500 x 1012 Ohm-cm Minimum
Water content (PPM)
50 PPM Maximum
Ageing characteristics after accelerated ageing (open breaker method with copper catalyst
a) Specific resistance (Resistivity)
1) at 27°C
2) at 90 °C
b) Dielectric Dissipation vector (Ten delta at 90°C
c) Total acidity
d) Total sludge value
2.5 x 1012 Ohm-cm Minimu
0.2 x 1012 Ohm-cm Minimum
0.2 Max
0.05mg KOH/gm max
0.05% by weight maximum

Any deviation  in the above given transformer oil parameter shows that some defect has developed in the transformer insulation system.

Reasons of Acidity in transformer Mineral oil

Sometimes the transformer oil comes into contact with air. This condition happens when there is leakage of oil from the transformer tank or pipeline. The transformer oil gets oxidized when it comes into contact with air. The oxidation process further accelerates because of high temperature and the catalysts like iron and copper and metallic compounds mixed in the transformer oil.

Effects of Acidity in transformer Mineral oil
The resistivity of the transformer oil is 35 x 1012 Ohm-cm and 1500 x 1012 Ohm-cm    at 90°C and  27°C respectively. The resistivity of the transformer oil gets decreased with an increased oxidation. The decreased resistivity of the transformer oil with an increased acidity cause paper destruction. Increased acidity also increase the dissipation factor (tanδ) of the transformer. The rate of sludge formation also gets increased with increased acidity of the transformer oil.

How to measure Acidity of  transformer oil?

When the acidic material is to be neutralized the alkali material is added to it. The neutralization of the effect can be examined by the amount of base material added to the acidic material. The same principle is used to calculate the acidity of the transformer oil. The substance required to measure the acidity of transformer oil is potassium hydroxide. It is also commonly referred to as caustic potash.

The potassium hydroxide (KOH) in a specific quantity is added in the transformer oil sample  to neutralize the acidity effect of the transformer oil. The acidity of the healthy transformer oil is 0.3 mg KOH/gram. It means 0.3 milligram of KOH is required to neutralize 1 gram of insulating oil. 

Principle of Acidity Test of Transformer oil

The neutral solution has PH value of 7. The acidic material has PH value less than 7 and, the alkali material has  PH value above 7. The acidity of the normal oil is 0.3 mg KOH/gram. When alkali material is added to the transformer oil it may remain acidic if the alkali added to neutralize the acidity effect of the transformer oil.

The transformer oil may attain neutral state of PH value 7 if the 0.3 gram alkali is added in the 1 gram transformer oil. If the alkali added to the transformer oil is more than 0.3 mg in 1 gram transformer oil, the oil may attain PH value more than 7 and transformer oil becomes alkaline.

The PH  value of the transformer oil must be below 7. It must have acidic properties. The acidic nature of the transformer oil can be visualized by observing the colour of the strips when it is dipped in the transformer oil. 

Procedure of Acidity Test of Transformer oil

  •  1 gram of transformer oil is taken to test its acidity. 1 millilitre   of transformer oil is taken by volumetric syringe.
  •  The acid present in the transformer oil is highly dissoluble in  the alcohol. So, the dissolved acid in the transformer oil is   extracted by adding 1 ml of rectified spirit(ethyl alcohol).
  •  After shaking the test sample, 1 ml of sodium carbonate is   added in the sample.
  •  Then we add  5 drops of universal indicator in the sample.
  • After adding the universal indicator  the solution changes its  color and the colour chart will show how much acid is contained in the insulating oil.colour chart for determining acidity of transformer oil
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  2. Transformer Oil Types and Properties
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