APTP Articles

Monitoring and Maintenance of In-Service Mineral Insulating Liquid

For reliable operation of liquid-filled electrical equipment, the monitoring and maintenance of the insulating liquid is essential. Naphthenic mineral oil is the most widely used insulating liquid for cooling and insulation in liquid-filled electrical equipment. The characteristics of the oil, supplied as unused, may change during service life. Therefore, the oil quality should be regularly monitored during its service life. 

In many countries power companies and electrical power authorities have established codes of practice for this purpose. In general, these cover monitoring guidelines and corrective actions depending on the oil status. If a certain amount of oil deterioration or contamination is exceeded, then the possibility and risk of premature failure should be considered. While the quantification of the risk can be very difficult, a first step involves the identification of potential effects of increased deterioration. 

IEC60422 is the guide for supervision and maintenance of mineral insulating oils. This standard is currently under revision to take account of changes in oil and equipment technology and has due regards, for the best practices currently in use, worldwide. Changes are also being made to use current methodology and comply with requirements and regulations affecting safety and environmental aspects.


Oil in electrical equipment deteriorates in service due to the conditions of use. The reliable performance of the oil in the insulation system depends on the basic characteristics of the oil, which may affect the performance of the equipment. A variety of processes occur, some inter-related, which degrade the oil. These processes include oxidation; contamination by water, particles, or fibers; electrical arcing or discharge; and local or general overheating.

Oxidation of the oil will occur in all equipment where it is in contact with air. In such equipment, oxidation of the oil will occur gradually and naturally over many years. Oxidation is accelerated as operating temperatures of the oil increase. Oxidation can also be accelerated by the presence of catalysts such as metals or metallic compounds. The cumulative effects of oxidation on the oil are to darken the oil, produce water and acids, and in the extreme, produce sludge. The production of water and acids can lead to corrosion of metal surfaces, particularly above the oil surface and can attack cellulose material, resulting in loss of mechanical strength and possible breakdown.

If the oil reaches an advanced state of oxidation, insoluble products are generated, which result in sludge formation in the oil. The produced sludge is soluble in the oil depending on the type of oil but it will precipitate once the saturation point is reached. Generally, sludge is more soluble in naphthenic oil and less soluble in paraffinic oil. The sludge deposits itself on windings and other parts, blocking ducts and reducing cooling efficiency. The overheating of the oil and windings, and the oxidation of the oil increase cumulatively, leading to possible breakdown.

Early warning of the onset of oil oxidation is provided by monitoring of the colour and appearance of the oil, regular testing for acidity levels in the oil, monitoring moisture levels in the oil, and visual inspection during maintenance for signs of sludgy deposits on internal surfaces of equipment.

About the Author

Dr B Pahlavanpour, Dr Ed Casserly and Juan Acosta - Ergon