Review of Kraft paper/natural ester fluid insulation system aging

We examine the results of recent Kraft paper/natural ester fluid insulation system sealed tube aging studies. The initial rates of cellulose degradation of Kraft and thermally upgraded Kraft in mineral oil or natural esters are similar. Subsequent degradation in natural ester occurs at a slower rate. This appears to be true for both plain and thermally upgraded Kraft. Based on early sealed tube aging results, the insulation unit life of the natural ester/thermally upgraded Kraft insulation system is estimated to be 130 degrees C. Accelerated aging of single phase distribution transformers filled with natural ester also show a reduction using a thermally upgraded Kraft/natural ester insulation system, meeting criteria for 120 degrees C hottest spot temperature operation.