FDS Response of Insulating Papers Impregnated with Various Fluids

A preliminary investigation on the dielectric response at different frequencies of the solid insulating system of transformers in mineral oil and natural ester is presented. FDS measurements have been performed on new scale model winding specimens with vapor-phase dried solid insulation when impregnated in three different insulating oils: an uninhibited naphthenic mineral oil and two typical natural esters for transformers. Two different types of insulating paper (standard Kraft paper and a Thermally Upgraded Kraft paper) were used as solid material. Comparative FDS results indicate that the dielectric response curves of Kraft and TUK paper-specimens overlap, when impregnated in with the same type of insulating oil. This confirms that new papers show the same dielectric behavior when the same drying pre-treatment is applied. Instead, for the same solid insulation in different insulating oil, the greater affinity for water of esters with respect to mineral oil heavily influences the dielectric dissipation loss at low frequency.