Insulation systems for liquid-immersed transformers - new materials require new methods for evaluation

Insulation systems for liquid-immersed transformers have been essentially unchanged for the last 100 years, using combinations of mineral oil and cellulosic based papers and boards. Previous papers have outlined initial work in the development and then validation of a dual-temperature method for the evaluation of the thermal capability of insulation systems. This paper will provide a comparison of this method to historical methods, showing very similar results for the dual-temperature model as compared to industry data which make up the traditional life curves for upgraded and non-upgraded cellulose papers. Transformers for high speed trains, windmills and those utilizing alternative fluids, however, all use insulation systems and materials which are different from the mineral-oil cellulose systems. In particular, these insulation systems utilize hot spot temperatures above those used with traditional insulation systems. These new transformer systems often have materials of dissimilar thermal capability, making some of the past methods less suitable for evaluation of the thermal capability of the insulation system. This paper will provide results of the evaluation of new insulation systems described above under a variety of test methods. Equipment manufacturers and users will better understand the relative thermal capability of their equipment utilizing these new insulation systems.