Evaluation of Transformer Loading A bove Nameplate Rating

This article presents the evaluation of transformer overload capabilities based on variables that affect the functional life of the power and distribution transformers. The combined effect of thermal overloads, mechanical stresses due to transient overvoltage, and other parameters relating to components other than windings must be taken into account. The basic criterion, which limits the transformer load capabilities, is the temperature of the winding and insulation. Several studies to measure the quantitative loss of transformer life due to the effect of thermal aging have been carried out since 1930. Loading beyond nameplate rating and cumulative loss of transformer functional life have been the basic considerations in the well-practiced IEC 354, 1981 ANSI C57.91, and NEMA Guides for loading oil immersed power and distribution transformers with 65°C average winding temperature rise.