Effects of different drying procedures on the composition of a liquid impregnated solid insulation

Although solid and gaseous insulations have become increasingly important during the last decades, the application of insulating liquids together with cellulosic paper is still essential for many kinds of components like power transformers. A dominant risk, reducing the strength of such insulations, is water, thus sometimes drying procedures are required to extend lifetime and operation reliability. Actually transformer drying is in mast cases performed as a maintenance action where it is tried to remove water by vacuum and heat. Such a procedure has to be performed offline and also comprises the risk of insulation endangerment. In this contribution some basic investigations on the reasons for the observed tendency for failures after a conventional drying of the solid insulation of an aged transformer are presented. Therefore the composition of an impregnated cellulosic insulation paper has been recorded during impregnation, accelerated aging and a drying as it is performed for a nonimpregnated insulation.