Regular transformer oil analysis is useful in monitoring the condition of engines, turbines and other oil lubricated equipment. The same can be said for transformer oils used to insulate many transformers and other electrical distribution equipment. The analysis of insulating oils provides information about the oil, but also enables the detection of other possible problems, including contact arcing, aging insulating paper and other latent faults and is an indispensable part of a cost-efficient electrical maintenance program.

Transformer Reliability

Over the past 20 years “Just in time delivery” has changed Preventative maintenance from a necessary item of expenditure to a strategic tool in managing your production. Extreme reliability is demanded of electric power distribution equipment, and even though the failure risk of a transformer and other oil-filled electrical equipment is small, when failures occur, they inevitably lead to high repair costs, long downtime and possible safety risks.

By measuring the physical and chemical properties of oil, in addition to the concentrations of certain dissolved gases, a number of problem conditions associated with either the oil or the transformer can be determined and down time can potentially be avoided. The following are some common tests performed on electrical insulating oils.

Moisture Content

One of the most important functions of transformer oil is to provide electrical insulation. Any increase in moisture content can reduce the insulating properties of the oil, This is of particular importance with fluctuating temperatures because, as the transformer cools down, any dissolved water will become free, which results in poor insulating power.

Acid Number

Transformer oils are oxidized under the influence of excessive temperature and oxygen, this results in an increase in Acid Number, due to the formation of carboxylic acids. Further oxidization can result in sludge and varnish deposits. In the worst-case scenario, the oil canals become blocked and the transformer is not cooled well. Oil degradation also produces acids, which tend to reduce the insulating properties of the oil. An increase in Acid Number often goes hand-in-hand with a decrease in dielectric strength and increased moisture content.

Dielectric Strength

The dielectric strength of transformer oil is defined as the maximum voltage that can be applied across the fluid without electrical breakdown. Because transformer oils are designed to provide electrical insulation, any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function.

Power Factor

The power factor of insulating oil is the ratio of true power to apparent power. In a transformer, a high power factor is an indication of significant power loss in the insulating oil, usually as a result of contaminants such as water, oxidized oil and cellulose paper degradation.

Dissolved Gas Analysis (DGA)

Dissolved gas analysis (often referred to as DGA), is used to determine the concentrations of certain gases in the oil such as nitrogen, oxygen, carbon monoxide, carbon dioxide, hydrogen, methane, ethane, ethylene and acetylene. The concentrations and relative ratios of these gases can be used to diagnose certain operational problems with the transformer. For example, high levels of carbon monoxide may indicate thermal breakdown of cellulose paper, while high hydrogen, in conjunction with methane may indicate a corona discharge within the transformer.


Furan derivatives are a measure of the degradation of cellulose paper. When the paper ages its degree of polymerization reduces, so its mechanical strength decreases. The degree of polymerization can only be determined directly by taking a sample of paper, a very complex operation and almost never performed in practice. However, the degree of polymerization of the paper can be directly related to the concentration of furan derivatives in the oil.

Just like machinery oil analysis, electrical insulating oil analysis can play a vital role in preventing unscheduled outages in electrical distribution equipment by determining the condition of the equipment itself, and other vital components including the condition of the oil and the cellulose paper insulation. For all critical oil-filled electrical equipment, including transformers, regular, routine oil analysis should be the cornerstone of any PM program.


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