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The aging of the epoxy-paper insulation of an oil-impregnated transformer was studied through partial discharge analysis. Accelerated electrical aging tests were carried out on samples of transformers windings, with and without paper insulation. Partial discharge (PD) measurements according to standard IEC 60270 were performed periodically during aging, and partial discharge phase patterns were recorded throughout the aging test. Some representative PD phenomena were obtained from the experimental results. The evolution of typical PD patterns is reported until breakdown.