Thermally upgraded insulation in transformers

This paper will review the development of thermally upgraded paper. It will discuss the various chemical modifications developed and give the current processes used today. It will review current standards activities which deal with thermally upgraded insulation systems and their impact on the recommended loading practices of transformers. It will also review the effect of thermal upgraded insulations on diagnostic techniques such as gas-in oil analysis of furanic compounds and their indication of insulation degradation. Finally recent aging tests will be presented which correlate aging rates as a function of the degree of thermal upgrading measured by the nitrogen content of the paper