Evaluation of transformer loading above nameplate rating

The paper presents the evaluation of transformer overload capabilities base d on variables that affect the functional life of the power and distributio n transformers. The combined effect of thermal overloads, mechanical stress es due to transient overvoltage, and other parameters relating to component s other than windings must be taken into account. The basic criterion, whic h limits the transformer load capabilities, is the temperature of the windi ng and insulation. Several studies to measure the quantitative loss of tran sformer life due to the effect of thermal aging have been carried out since 1930. Loading beyond nameplate rating and cumulative loss of transformer f unctional 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 degrees C average winding temperature ris e.