The work of diagnostic systems in operating modes leads to the need to take into account the influence of physical processes in the transformer on the results of measuring diagnostic parameters. This process is associated with a change in load and the corresponding heating and cooling of the transformer. The research and comparison of the new refined thermal models of the transformer is carried out and the possibilities of their use for determining the basic values of the diagnostic parameters of the transformer are analyzed. To evaluate the work of the models examined, the thermal processes in the transformer were modeled. Determination of temperatures using all models, except for model IEC 60076-7, provides for the solution of nonlinear differential equations. To solve this problem, the corresponding functions of the mathematical packages were used. It is shown that the most expedient is the use of the refined thermal model of the transformer, which takes into account the influence of the moisture content of the oil on the thermal characteristics of the transformer when the operating modes of the transformer change.