Thermal Aging Comparison of Alternative Liquids for Oil-Paper High-Voltage Insulation in Current Transformers

This article is aimed at the comparison of the long-term behavior of insulation systems using mineral oils to that with alternative liquids, biodegradable hydrocarbons, and ester oils, under thermal stress. The different types of oil are aged in the presence of enameled copper and Kraft paper under sealed conditions. Oil degradation is monitored by relevant aging markers, such as water content, breakdown voltage, dielectric dissipation factor (DDF), and acidity. Paper degradation is monitored through the tensile strength. The end-of-life criterion is based on paper mechanical properties. The experimental results confirm that the sealed conditions prevent oil from aging (no oxidation nor hydrolysis) and that the use of biodegradable hydrocarbon oils could be similar to that of mineral oil and that the use of esters could improve the aging behavior of paper.