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.
Any variation in transformer oil characteristics may lead transformer failure. The specifications of the transformer oil are as given below.
S.N
|
Characteristics of Transformer oil
|
Value
|
1.
|
Density Max
|
0.89 gm/Cm2 at 29.50 C
|
2.
|
Kinematic viscosity Cst at 27°C
|
27Cst Maximum
|
3.
|
Inter-facial tension at 27°CNewtons/M
|
0.04 Minimum
|
4.
|
Neutralization value
a)Total acidity mg
|
0.03mg KOH/gm Max.
|
b) Inorganic acidity / Alkalinity
|
Nil
|
|
5.
|
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
|
6.
|
Electric strength (Break down voltage)
a) New untreated oil
b) After treatment
|
30kv Minimum (RMS)
60kv Minimum (RMS)
|
7.
|
Flash point Pansky/marten (Closed)
|
140°C Min
|
8.
|
Pour point
|
—6°C Maximum
|
9.
|
Dielectric Dissipation vector (Ten delta) at 90°C
|
0.002 Max
|
10.
|
Specific resistance (Resistivity)
A) at 90°C
B) at 27°C
|
35 x 1012 Ohm-cm Minimum
1500 x 1012 Ohm-cm Minimum
|
11.
|
Water content (PPM)
|
50 PPM Maximum
|
12.
|
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
Effects of Acidity in transformer Mineral oil
How to measure Acidity of transformer oil?
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.
1 thought on “Acidity Test of Transformer Oil”