A preliminary study on the dielectric response at different frequencies of the solid insulation system of transformers in mineral oil and natural esters was conducted. Frequency domain spectroscopy (FDS) measurements were performed on new samples of scaled model windings with solid insulation dried in vapor phase when impregnated with three different insulating oils: an uninhibited naphthenic mineral oil and two typical natural esters for transformers. As solid insulation, two different types of insulation paper were used (standard Kraft paper and thermally upgraded Kraft paper).

Comparative FDS results indicate that the dielectric response curves of Kraft paper and thermally upgraded Kraft (TUK) paper samples overlap when impregnated with the same type of insulating oil. This confirms that the new papers exhibit the same dielectric behavior when the same drying pretreatment is applied. However, for the same solid insulation in different insulating oils, the higher water affinity of esters compared to mineral oil significantly influences the dielectric dissipation loss at low frequencies.