Calculation of core hot-spot temperature in power and distribution transformers

Accurate calculation of the value of the core hot-spot temperature at the design stage is becoming very critical in order to ensure that hydrogen generation that can be caused by oil-film degradation at core hot-spot temperatures as low as 110-120°C is limited. Presented are two calculation methods developed for this purpose. Results of the FEM calculation of the location and value of the hot spot temperature in cores of three-phase, three-limb cores of power and distribution transformers show excellent agreement with tested values on a number of transformers. Insight learned from this analysis was used to enhance a previously available empirical method. This simplified calculation method is appropriate for everyday use and, hence, is suitable for design calculations. This method was shown to have again very good accuracy in the range of 2°C for most of the cases. The simplified calculation method was also shown to provide a much greater accuracy than the original empirical method.