The study of predictive control of temperature in oil-filled transformers is performed in this work, In order to Study transformers thermal loss of life, complex models taking into account electrical and thermal characteristics are required. Moreover, the precision of thermal models is dependent upon the exactitude of the parameters. The work presented in this article shows that, through electromagnetic similitude laws, for an homogeneous series of trails formers with different rated powers, the main parameters required for the thermal model call be obtained. Different methodologies to estimate thermal parameters with data from standardised heat-run tests are compared. The foremost advantage of our alternative methodology is its compactness, since parameters are obtained only From the knowledge of transformer rated power. Theoretical results were compared with data from transformer manufacturers and the good agreement between both validates theoretical results is accomplished.