Liquid-immersed transformer insulation system thermal verification: A case for revised sealed tube test

This article describes the development and evolution of test methods for evaluation of the thermal class of liquid-immersed transformer insulation systems. Various methods recognized by industry standards are briefly described, with the focus on a revised procedure for the sealed tube test method. An example of the product evaluation using this method is presented. Recent industry discussions on the thermal performance of cellulose in ester liquids, resulted in a need for performing thorough studies on this subject. It was agreed within IEC Technical Committee 14 "Power Transformers" that comprehensive studies shall be made. It is likely that the sealed tube test methodology discussed in this article could be used for developing more systematic conclusions on cellulose-ester thermal performance.