Effect of dielectric strength of mineral oil on mixing with a natural ester oil as a polar contaminant under AC stress

Ester oils are considered as suitable substitutes for conventional mineral oil for transformer applications due to good environmental performance and higher fire point compared to mineral oils. The electronegative property of the oxygen atom in the ester oil group makes ester oil more polar than mineral oil. Ester oils also have higher permittivity and lower resistivity with respect to mineral oil. Hence, it would be of interest to predict the influence of polar contamination in a non-polar nature of mineral oil. In this paper, 10% of natural ester oil is mixed as a polar contaminant in mineral oil to evaluate its effect on AC dielectric strength. Initially, AC dielectric strength of mineral oils and natural ester oil are predicted as a reference for various oil gaps under power frequency voltage conditions. Both naphthenic and paraffinic based mineral oil grade (uninhibited and inhibited oils) are utilized to consider the effect of inhibitors and aromatic contents of mineral oil in natural ester oil on AC dielectric strength. The breakdown voltages are determined by rising voltage method for nonuniform field (Sphere-needle) configurations. Weibull distribution functions are used to analyze the experimental data and to estimate the breakdown voltage with risk of 1% failure probability. Finally, finite element method (FEM) is used to predict the maximum stress in mineral oil, natural ester oil and mixture oil.