Decomposition of the Hot-Spot Factor

This paper describes the application of the finite-elements method for determining the distribution of losses over windings and a detailed thermal-hydraulic network model for determining the value of the hot-spot factor and the hot-spot temperature. The influence of nonuniform losses in the conductor of the windings and the influence of nonuniform cooling are clearly separated. A detailed discussion of the components included in the hot-spot factor is given in this paper, enabling proper understanding of this complex and important quantity. The case study transformer was a 100-MVA oil power transformer with oil-directed water-forced cooling. The method was applied within the scope of the project of increasing the power of a hydro power station by 12%. The task was to calculate the temperatures and to recommend whether the reconstruction of the transformer is necessary. The developed calculation method and software were hitherto used for transformer design (in the production or the reconstruction stage of transformers) and it is the first application to give an answer as to whether it is possible to increase the power of an existing transformer. In this specific application, necessary onsite measurements for determining the characteristics of the coolers were performed.