Testing for Liquid Power Factor in Mineral Oil - Part 1

Liquid power factor is an outstanding tool for evaluating in-service transformer oil. The test is valuable for acceptance testing of new oil from a supplier, and for evaluating conditions in newly installed equipment. For in-service oil, there are several adverse conditions that can be discovered from the liquid power factor results.

New, clean, and dry transformer oil starts out with a very low liquid power factor, typically <0.05%. As the oil ages or becomes contaminated, the liquid power factor increases. Liquid power factor is usually run in the laboratory at two temperatures, 25°C and 100°C because each temperature provides unique direction in what is happening with the fluid. If an abnormal value for liquid power factor is obtained during testing, the respective trends of these two values in the past may be used to help diagnose the conditions that could be causing the abnormal values.

The concept behind the test is actually quite simple. When an insulating liquid such as transformer oil is subjected to an alternating current field, the oil experiences dielectric losses. These losses cause two effects. The resulting current is deflected slightly out of phase with the AC field that has been applied, and the energy of the losses is dissipated as heat. Liquid power factor (or a closely related measurement, dissipation factor, which is similarly interpreted) is calculated from direct measurement of these dielectric losses – the lower these losses, the better.

The standard method that we follow in the laboratory is ASTM Method D924, which may quantify either power factor or dissipation factor of an insulating liquid. Liquid power factor is calculated as the sine of the loss angle – the amount of current deflection due to dielectric loss – while the dissipation factor is the tangent of the same loss angle. Some test standards refer to the dissipation factor as tan δ because the loss angle is designated as δ in the vector diagram. Values may be expressed as either a decimal number or as a percentage, such as 0.001 or 0.10%. Typically, in the management of electrical equipment and insulating oils where these dielectric losses are very low, we use percent values for liquid power factor.

Note that the calculated values for liquid power factor and for dissipation do not differ by very much until you get into the larger decimal values for each. At a calculated liquid power factor (four significant figures) of 10.00%, the dissipation factor would be 10.05%.

Contamination of the oil by moisture or by other contaminants will increase the liquid power factor. The aging and oxidation of the oil will also elevate liquid power factor values. Therefore, this is an extremely useful test because almost everything “bad” that can happen to the insulating oil will cause the liquid power factor to increase. Running the test at two temperatures allows for some further diagnostics concerning the cause(s) of the abnormal power factor.