Statistical analysis of AC breakdown voltage of cellulose paper enhanced with aramid immersed in mineral oil and synthetic ester liquid

This article presents the results of the studies on AC breakdown voltage of new, 0.18 mm in thick, insulation cellulose paper enhanced with aramid. The samples of dried paper were immersed with mineral oil and synthetic ester liquid respectively, and then tested in uniform (plane-plane) electrode configuration as per IEC 60243 Standard. The rapid rise tests were applied in both cases for breakdown voltage determination. The results, in the form of 15 values of breakdown voltages, were analyzed using Normal and Weibull distribution functions. For both distribution functions 5% as withstand voltage and 50% breakdown probabilities were evaluated. Then dielectric strength (expressed in kV/mm) of the insulation paper was calculated. Comparing the results this was stated that aramid-enhanced cellulose paper fulfills the requirements for papers used in power transformers according to the specified standard. The values concerning uniform field were in both case higher than 75 kV/mm (for 50% breakdown probability). Taking into account high thermal class of the paper analyzed (Thermal Class 140 °C in ester liquid and Thermal Class 130 °C in mineral oil) it may be applied without any doubts in the transformer applications required high temperatures, especially when using this paper combined with ester liquid.