Moisture-Dependent Dielectric Indicators for Retrofilling Criteria from Mineral Oil to Natural Ester and Palm Fatty Acid Ester in Kraft, TUK, and DPE Paper Insulation Systems

The dielectric performance of power transformer insulation systems is strongly influenced by the type of cellulose paper, the impregnating dielectric liquid, and the paper moisture content. This study evaluates the moisture-dependent dielectric response of three cellulose insulating papers, Kraft, Thermally Upgraded Kraft, and Diamond Printed Enhanced, impregnated with three dielectric liquids: mineral oil, a rapeseed-based natural ester, and a Palm Fatty Acid Ester. Paper samples were first conditioned at controlled moisture contents ranging from 1% to 5%, then impregnated under stabilized conditions, and finally characterized by Frequency Domain Spectroscopy. The results show that increasing the moisture content produces a systematic rise in the dielectric dissipation factor, indicating higher dielectric losses and reduced insulation performance. The relative dielectric benefit of the ester liquids was strongly dependent on paper type, moisture content, and frequency. This paper-dependent behavior was particularly pronounced for Kraft and Thermally Upgraded Kraft papers. Finally, laboratory-derived moisture-dependent 50 Hz tan δ crossover points were identified as dielectric indicators for retrofilling assessment. These findings provide comparative dielectric information for the investigated paper–liquid insulation systems and may support preliminary retrofilling assessment under controlled laboratory conditions.