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In the authors' previous work (1997), an original thermal model of transformers with ONAN cooling was developed. The model takes into account the influence of nonlinear thermal characteristics in transient thermal processes; instead of exponential functions and time constants, the numerical solution of differential equations is used. The model delivers one characteristic temperature in copper (solid insulation) and one characteristic temperature in oil. In this paper, the analysis stresses the definition of the hot-spot temperature of the solid insulation by using easily measurable quantities. Then, the parameters of the model can be precisely determined from inexpensive measurements in a short-circuit heating experiment and the model can deliver the hot-spot temperature. The experimental base of this research are the measurements on a 630-kVA, 3 × 10 kV/3 × 6 kV ONAN transformer equipped with 112 temperature sensors (102 inside the central positioned 10-kV winding).