CFD study of the impact of the deviation on the mass flow inlet on winding hot-spot temperature of a power transformer using mineral and ester oils

This work presents a study where a deviation on the mass flow rate in adjacent sectors of the winding is considered. A CFD model was developed for this study, where heat transfer between sectors is enabled due to the lack of thermal symmetry, and the goal of this study is to determine whether this effect can be neglected or not. To carry out this study, a Low Voltage Winding of a 100 MVA power transformer was selected. This transformer has been manufactured and tested by BEST Transformers, and the results obtained from the heat run test were used to validate the CFD model. The study also considered an alternative dielectric liquid, natural ester, in order to estimate if the different thermal and hydraulic conditions can affect to the relevance of the studied effect. The CFD simulations have been carried out with ANSYS Fluent. The results showed a deviation of 3-4% of the heat dissipated due to the non-symmetric conditions with a 10% of mass flow deviation, which is the heat transfer between the adjacent sectors. This effect is identical when considering mineral oil or natural ester. The results shows there is a small impact of the winding temperatures due to this effect, and that it can be considered negligible.