Thermal aging effects on the moisture equilibrium curves of mineral and mixed oil-paper insulation systems

Moisture equilibrium curves are well accepted methods for moisture estimation of insulation paper in field transformers. The thermal aging effects on moisture partition diagrams of mineral oil-paper and a new aging-resistant mixed oil-paper insulation system were studied. The purpose was to assess the moisture content accurately, particularly in transformers used for long-term operations. Thermal aging experiment was conducted at 130 °C for 80 days, and moisture equilibrium curves at five different aging degrees and three different temperatures were constructed. Results showed that under the same oil moisture content, the moisture content in paper decreased as the aging condition aggravated. The mass fraction of moisture content in oil and paper was calculated at each aging degree. Comparative results revealed that the moisture absorption capacity of insulation paper had an age-related decreasing tendency, whereas the moisture absorption capacity of insulating oil had an age-related increasing tendency. Moisture partition diagrams of mineral and aging-resistant mixed oil-paper samples were compared. Results showed that with the same oil moisture concentration, the moisture of insulation paper impregnated in mixed oil was remarkably lower than that impregnated in mineral oil.