In this paper, an experience is presented showing repowering an oil-submerged power transformer. The most efficient option is the redesign and repowering a transformer 20/25 MVA, 115/34.5 kV, Y/Y/D to increase the power from 20 to 30/35 MVA. This experience considers methodological aspects, preliminary tests, redesign and construction of main winding and compensation winding; final tests factory and validation of the methodology were carried out. The implementation of the methodology was conducted through a computational tool and the validation was conducted by testing at the factory laboratory of Industries Explorer, Duitama, Colombia.