In my thesis I engaged in diagnostics of oil-paper insulated transformers. The measurements were taken of a model transformer. I examined the temperature dependence of the dielectrical properties of insulation in two ways. First, I measured the resistance of the coil insulations, then the slopes of the decay and return voltages, using the voltage response method.
From measured data I computed the activation energy of conductivity and polarization of the coils. The activation energy of conductivity is more than twice as much as of polarization. Consequently, conductivity depends on temperature more, than polarization does.
The dielectrical properties of oil-paper insulation are very much influenced by the water that is in the system. The water content of oil and of paper depends on temperature. At higher temperatures water streams from the paper to the oil. As the temperature increases, equilibration is faster, so in the transformer that is heated and then cooled, it takes much more time to reach former equipoise.
Therefore, the dielectrical properties of the transformer are different before and after heating. To determine the coherence, further investigation is necessary.