Evaluation of the Dielectric and Short-Circuit Capabilities of Naturally Aged Power Transformers

The paper summarizes the results obtained in a series of dielectric and short-circuit tests on power transformers of the 120-161 kV/10-20 MVA class, recently removed from Hydro-Québec´s transmission and distribution system. These results show that after many (17 to 27) years of normal service, signs of insulation aging can be observed. The short-circuit withstand capability of the transformers tested was satisfactory up to their full rated short-circuit level however. Some unexpected results were also obtained in tests with switching impulses, which yielded lower withstand capability than in tests with standard lightning impulses.