Impact of Ageing on Thermal Properties of Power Transformer Insulating Liquids

This paper presents the results of experimental research analysing the impact of ageing on the thermal performance of several commonly used insulating liquids with different chemical compositions, including naphthenic mineral inhibited and uninhibited oil, hydro-carbon based biodegradable oil, gas to liquid oil and synthetic ester liquid. Among the liquid thermal parameters, viscosity has a dominant effect on the thermal performance of power transformers. Viscosity of the insulating fundamentally depends on temperature and chemical composition and can be formulated using the viscosity-temperature correlation equation. While viscosity-temperature of various unused insulating liquids correlation are commonly available, research and data on ageing effects of the viscosity-temperature correlations are limited. The paper presents measurements of kinematic viscosity temperature dependency of six new transformer insulating liquids before and after standard laboratory artificial in the temperature range from −20 °C to 100 °C. The viscosity-temperature correlations are formulated with Vogel-Fulcher-Tammann equation, which has been identified as the most accurate method of correlation in comparison to the other commonly used equations. Correlations between the liquid type, chemical parameters, kinematic viscosity and temperature are analysed and discussed.