Using the three-parameter Weibull distribution in assessment of threshold strength of pressboard impregnated by different liquid dielectrics

This study presents the results of the measurements of electrical strength of pressboard specimens impregnated with three different liquid dielectrics: mineral oil, natural ester and synthetic ester. In the experiment carried out in accordance with IEC 60243 standard, two types of pressboard specimens of thicknesses 0.5 and 1 mm, respectively, were considered. The data obtained during the measurements in the form of breakdown voltages were used for calculation of the electrical strength. Both standard-based approach where median value was taken into account in calculations and statistical approach using three-parameter Weibull distribution function where location parameter V-0 was considered were applied. The results obtained on the basis of both approaches showed that both the specimens impregnated with mineral oil and ester liquids gave the satisfactory results within the electrical strength determined, which in all cases was higher than minimum required for the impregnated pressboards. The differences indicating slightly better impregnation properties of mineral oil, when the same impregnation conditions are used, are noticeable especially in the case of location parameter of Weibull distribution function. However, these differences are small and in relation to the requirements for the impregnated pressboards may be treated as negligible.