What is Sludge in Transformer Oil?

Sludge is a precipitate or mixture of impurities and acid in transformer oil. The impurities are in the form of semi-solid in a transformer oil. Sludge formation in the transformer is due to the oxidation reaction, damage of sludge insulation,  decomposition if oil and insulation in transformer oil.

Increasing of moisture in transformer oil can form sludge due to the reason of reducing the dielectric strength and corrosion. Acid can damage the windings of the transformer. It is too easy to reclaim the oil by removing the precursors of sludge through the use of activated clay or activated aluminum.

How does the Sludge form?

Degradation of oil and oxygen are the cause of sludge formation. Peroxide, the organic acids, alcohols, aldehyde, ketones, lacquers, and the aromatic compound are the early precursors of sludge. If these materials attack to the part of the transformer like paper, iron, and copper then it creates a semi-solid material which is called sludge.

Interfacial Tension:

Tension is a force. When some oxidized contaminate like varnish, paint, and etc. are present in transformer oil, the film strength of the transformer will be lower.  If the of interfacial tension decreases then it indicates that the accumulation of contaminating and oxidizing products.

Neutralization number:

Neutralization number of oil is the measurement of acid and alkaline present in the transformer oil. The measurement of neutralization number in the transformer is simple like a titration. The unit of the neutralization number in transformer oil is ASTM D 974, mg KOH/g. High neutralization number (acidity) of oil indicates that the oil-contaminated like varnish, soaps, paints, and or other foreign matter.

Pentane-insoluble Sludge:

Pentane-insoluble sludge is also known as doble. Pentane-insoluble test conducted by mixing of the sampling oil with a constant amount of hydrogen solvent, pentane. This mixture takes 16-24 hours for cooling and testing. The mixture will be kept in a cool and dark place. After the cooling of mixture observes visually for tiny solid particles of gelatinous clumps. The solid particles of gelatinous clump form from the soluble sludge which precipitate due to the addition of hydrocarbon solvent.


At last, we can conclude that the sludge test in transformer oil is very important for the transformer. Before the filtration or dehydration sludge test is necessary for the transformer. By the help of sludge test, we can identify the impurities, acid, semi-solid, and etc. in the transformer.