Testing Transformer Oil- MBT Electric

Testing the acidity of transformer oil.

Table of content: 

1. Specification of transformer oil

2. Reasons for creating acidity in transformer mineral oil

3. Effect of acidity in transformer Mineral oil

4. How to test the acidity of transformer oil?

5. Principle of testing the acidity of transformer oil

6. Procedure for testing the acidity of transformer oil


Transformer oil is used for transformer insulation and cooling purposes. The transformer oil acidity test is an essential test to analyze the oxidation of transformer oil. An acidity test is conducted to measure the acid content in transformer oil. Transformer oil acidity testing is performed using a transformer oil acidity meter.

 1. Specifications of transformer oil

Any change in the characteristics of the transformer oil can lead to transformer failure. The specifications of transformer oil are given below.


S.N. Characteristics of  Transformer oil Value

Density Max

0.89 gm/Cm2 at 29.50

Kinematic viscosity Cst at 27°C

27Cst Maximum

3 Inter-facial tension at 27°C Newtons/M 0.04 Minimum

Neutralization value

       a)Total acidity mg

0.03mg KOH/gm Max.
5 b) Inorganic acidity / Alkalinity Nil

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)

8 Flashpoint Pansky/marten (Closed) 140°C Min

Pour point

—6°C  Maximum
10 Dielectric Dissipation vector (Ten deltas) 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

12 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 transformer oil rating given above indicates that some defect has developed in the transformer insulation system.


Advantages of transformer oil

2. Reasons for creating acidity in transformer mineral oil

Sometimes transformer oil is exposed to air. This situation occurs when there is an oil leak from the tank or the pipeline of the transformer. Transformer oil oxidizes when exposed to air. Oxidation continues to accelerate due to high temperatures and catalysts such as iron, copper and metal compounds present in the transformer oil.


3. Effect of acidity in transformer Mineral oil

The resistivity of transformer oil is 35 x 1012 Ohm-cm and 1500 x 1012 Ohm-cm at 90°C and 27°C, respectively. The resistivity of transformer oil is reduced as oxidation increases. The resistivity of transformer oil decreases with increasing acidity causing paper destruction. The increased acidity also increases the dissipation factor (tanδ) of the transformer. The rate of deposit formation also increases as the acidity of the transformer oil increases.


4. How to test the acidity of transformer oil? - testing transformer oil 

When acidic material is neutralized, alkaline material is added to it. Neutralization of the effect can be checked by the amount of base material added to the acidic material. The same principle is used to calculate the acidity of transformer oils.

KOH in a specific amount is added to the transformer oil sample to neutralize the acidic effect of the transformer oil. The acidity of healthy transformer oil is 0.3 mg KOH/gram. That means 0.3 milligrams of KOH is needed to neutralize 1 gram of insulating oil.


5. Principle of testing the acidity of transformer oil

Neutral solutions have a P.H. value of 7. Acids have a P.H. value of less than seven and alkaline substances have a P.H. value of more than 7. The acidity of common oils is 0.3 mg KOH/gram. When alkali is added to transformer oil, it may remain acidic if alkali is added to neutralize the acidic effect of transformer oil.


Transformer oil can achieve a neutral state of P.H. value 7 if 0.3g of alkali is added in 1g transformer oil. If alkali is added to transformer oil more than 0.3 mg in 1 gram of transformer oil, the oil may reach a P.H. value greater than 7 and the transformer oil becomes alkaline.


The P.H. value of transformer oil should be below 7. It must have acidic character. The transformer oil acidic nature can be visualized by observing the color of the bands when it is dipped in transformer oil


Related article: 

sealed type transformer

three phase transformer

types of transformer


6. Procedure for testing the acidity of transformer oil

1 gram of transformer oil is taken to test its acidity. 1 ml of transformer oil was taken with a volumetric syringe.


Acids present in transformer oil are very soluble in alcohol. So the transformer oil soluble acid was extracted by adding 1 ml of rectified spirit (ethyl alcohol).


After shaking the test sample, 1 ml of sodium carbonate is added to the sample.


Then we add 5 drops of universal indicator to the sample.


After adding the multimeter indicator, the solution will change color and the color chart will show the amount of acid contained in the insulating oil.

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