Dielectric Dissipation Factor (DDF) and Resistivity
These characteristics are very sensitive to the presence in the oil of soluble polar contaminants, ageing products or colloids. Changes may be motivated even when contamination is so slight, so as to be undetectable by chemical methods. Acceptable limits for these characteristics depend largely upon the type of apparatus and application.
However, high values of dissipation factor may influence the power factor and/or the insulation resistance of transformer windings. There is generally a relationship between DDF and resistivity at elevated temperature with resistivity decreasing as DDF increases. It is normally not required to conduct both tests on the same oil.
Useful additional information can be obtained by measuring resistivity or DDF at both ambient and at higher temperature such as 90 °C. A satisfactory result at 90 °C coupled with an unsatisfactory value at lower temperature is an indication of the presence of water or degradation products perceptible in the cold, but generally at a tolerable level. Unsatisfactory results at both temperatures indicate a greater extent of contamination and that it may not be possible to restore the oil to a satisfactory level, by reconditioning.