Influence of Slope on Thermal Index Calculations During Accelerated Aging Tests

Accelerated aging studies are used to determine the thermal class of insulation systems. For liquid filled transformers these tests are completed according to IEEE CS7.100. The Industry-proven system within this standard has a defined minimum life expectancy curve with a specified influence of temperature (slope) and end of life point. When evaluating a candidate system according to this standard, accelerated aging must be completed at a minimum of three temperatures in order to create a minimum life expectancy curve for that system which can be compared to that of the industry-proven system. Due to the extrapolation of results, the slope of the minimum life expectancy curve can have a significant impact on the calculated thermal class of the candidate insulation system. This paper presents accelerated aging data for thermally upgraded Kraft paper aged with seven different liquids, five mineral oils and two natural esters, and discusses three methods of defining the slope of the minimum life expectancy curve and the subsequent impact of each method on the thermal index calculation results.