What is the Power Factor of MIDEL ester fluids?


Power factor is the ratio of resistance current to capacitance current in an insulation system.

In new mineral oil transformers power factor is used to determine the level of insulation dryness. For operating transformers, changes in power factor can indicate increased levels of moisture or other contaminants in the insulation system. There are several variables that impact power factor measurements, even for new, essentially dry uncontaminated units, including:

  • the ratio of solid to fluid insulation

  • the type of dielectric fluid

  • winding design

  • temperature at time of measurement

Understanding MIDEL Power Factor

The dielectric dissipation factor of ester-based liquids is higher than that of mineral oil, due to the polar chemical structure of the ester. This in turn leads to higher power factor values for clean, dry insulation than would normally be seen with mineral oil.

Distribution transformers

Experience gained during the testing of transformers and published research has shown that the power factor of a new distribution transformer filled with MIDEL liquid will be around 50% higher than the same design with mineral oil.

Power transformers

In power transformers the liquid to solid insulation ratio will be higher and therefore the ester liquid will have more influence on the power factor value. It should be expected that the power factor will be approximately doubled after retrofill in a power transformer.


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Retrofilling transformers with ester fluids