Transformer design considerations utilising natural ester oils.
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As modern day transformer designs are gradually moving towards an eco-friendly alternate insulation medium, this dissertation is focussed on the modelling of a typical 88/6.6 kV 20 MVA high voltage winding in both FEMM and COMSOL software packages to where the thermal and dielectric the performance of mineral and ester oil were assessed. For the dielectric analysis, Finite Element Model (FEM) software was utilised and electric field stress measurements (long and short gap) were taken between the windings, paper and pressboard for both ester and mineral oil. Results showed that for natural ester oil, the electrical stresses were concentrated in the paper as opposed to mineral oil where the stresses are more concentrated in the oil. Also a comparison between thermally upgraded (higher permittivity) and standard kraft paper yielded a 1 kV/mm reduction in electric stress for the upgraded kraft paper. This means that transformers with natural ester oil will require more insulation on the windings and greater distances between live and ground paths. For the thermal analysis, the material used was mineral oil and ester oil, with the latter having a higher viscosity. The results indicate that the cooling of the transformer is highly dependent on the velocity of the fluid (thermal transfer through convection). The study showed that for the same inlet velocity of fluid the velocities differed slightly due to the viscosity, but they ultimately had a similar thermal performance. The entire transformer, including radiators, would need to be considered to determine the correct flow and thermal performance for the ester.