Abstract: Power transformers are electrical machines which are one of the most critical components in power distribution systems worldwide. Their failure generally is due to the degradation of its insulating system (insulating oil and paper insulation). This deterioration is produced by electrical, mechanical, chemical and thermal stresses which power transformers suffer during their operational lifetime. When dielectric paper has aged severely it loses its tensile strength and its capability to withstand electrical faults decreases considerably. This paper describes an end of life study carried out on a distribution transformer. This analysis has used the tensile index of the dielectric paper to estimate the temperature distribution in the windings of the electric machine. The tensile index is a mechanical parameter associated with the state of the paper insulation that protects the conductors of the windings. By knowing the value of this magnitude for a new and for an aged paper, and the period of operation of a transformer, the temperature distribution along the height of the windings can be estimated. These results and the load rate that a transformer has endured during its life can provide useful information for future designs or for similar transformers still in operation.

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