One of the most important functions of a transformer oil is to provide electrical insulation. Any increase in moisture content can reduce the insulating properties of the oil, which may result in dielectric breakdown.
Just like industrial oils, transformer oils are oxidized under the influence of excessive temperature and oxygen, particularly in the presence of small metal particles which act as catalysts, resulting in an increase in Acid Number, due to the formation of carboxylic acids.
The dielectric strength (ASTM D300-00) of a 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 under high electrical fields, any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function.
The power factor (ASTM D924) of an 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 polar 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 (ASTM D3612). The concentrations and relative ratios of these gases can be used to diagnose certain operational problems with the transformer, which may or may not be associated with a change in a physical or chemical property of the insulating oil.