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

This paper is aimed at the comparison of the estimated lifetime of insulation systems using mineral oils to that with alternative liquids, biodegradable and ester oils, under thermal stress. The different types of oil are aged in 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, Dissolved Gas Analysis (DGA), furanic compounds and methanol. End of life criteria is based on paper mechanical properties. Experimental results show that sealed conditions prevent oil from aging (no oxidation nor hydrolysis) and that the use of ester and biodegradable oils could improve the aging behavior of paper. These results are to be confirmed with the full study by establishing Arrhenius law for each liquid.