- Acidity Test Definition: The acidity test of transformer oil measures the amount of potassium hydroxide (KOH) needed to neutralize the acid in the oil.
- Causes of Acidity: Acidity occurs due to oxidation, especially when oil contacts air, and is accelerated by heat and metals like iron and copper.
- Effects of Acidity: Higher acidity lowers oil resistivity, increases the dissipation factor, and can harm transformer insulation.
- Acidity Test Kit Components: The test kit includes rectified spirit, sodium carbonate solution, a universal indicator, test tubes, and volumetric syringes.
- Testing Procedure: The procedure involves adding specific amounts of rectified spirit, sodium carbonate, and a universal indicator to the oil and observing the color change to determine acidity.
Causes of Acidity in Insulating Oil
The insulating oil in a transformer may occasionally come into contact with air, for example while opening any blanking or through leaks in the oil tanks or associated pipelines. Because of this contact, the oxidation reaction takes place in the transformer oil, accelerated further by temperature and by the presence of catalysts like iron, copper and dissolved metallic compounds in the transformer oil.
Effects of Acidity in Insulating Oil
Increased acidity in the oil lowers its resistivity and raises the dissipation factor. Excessive oxidation leads to more sludge and can damage the paper insulation in transformer windings.
Acidity Test Kit
We can determine the acidity of transformer insulating oil with a simple portable acidity test kit. It consists of one polythene bottle of rectified spirit (ethyl alcohol), one polythene bottle of sodium carbonate solution and one bottle of universal indicator (liquid). It also includes clear, transparent test tubes and volumetrically scaled syringes.
How do we measure Acidity of Insulating Oil?
The acidity of insulating oil is measured by the amount of KOH (in milligrams) needed to neutralize the acid in a specific amount of oil (in grams). For example, if the oil has an acidity of 0.3 mg KOH/g, it means 0.3 milligrams of KOH is needed to neutralize 1 gram of the oil.
Principle of Acidity Test of Insulating Oil
Adding alkali to the oil changes its acidity based on the amount of acid present. If the alkali added equals the acid present, the oil’s pH will be 7 (neutral). More alkali makes the oil alkaline (pH 8-14), while less leaves it acidic (pH 0-6). The universal indicator shows different colors for different pH levels, allowing us to determine the oil’s acidity visually.
Procedure of Acidity Test for Insulating Oil
- First, we have to take exactly 1 gram of insulating oil. We normally do this by drawing 1.1 milliliters of the oil to be tested into the provided volumetric syringe, since 1.1 milliliters of oil is taken as 1 gram of oil.
- Before the test, we have to extract the acid dissolved in the oil. We do this by adding exactly 1 ml of rectified spirit (ethyl alcohol) to the test sample, because the acid produced in mineral oil is highly soluble in alcohol.
- After shaking the test sample well, we add 1 ml of sodium carbonate solution to the sample. Sodium carbonate is the most suitable alkali for this purpose because its characteristics change little when it comes into contact with the atmosphere during use.
- Finally, after re-shaking the sample mixture, we add 5 drops of universal indicator to the sample.
Observation
The color of the sample oil after mixing rectified spirit, sodium carbonate and universal indicator, describes the value of acidity of the oil sample in mg KOH / g as follows.
