Coupled CFD and electromagnetic analysis of an ONAN distribution transformer cooled with mineral oil and biodegradable esters

This work describes the electromagnetic and coupled thermo-fluid dynamic analysis of an oil-natural air-natural (ONAN) distribution transformer with the main objective of studying the changes in the heat dissipation performance when the fluid employed to cool the machine is a biodegradable ester instead of mineral oil. The distribution transformer under analysis has a rated power of 315 kVA with a voltage ratio of 13.2 kV / 0.4 kV. The heat losses in the magnetic core and the windings are computed with the ANSYS Maxwell software on suitable finite element meshes. These heat losses are then transferred as volume heat source terms to appropriate finite volume meshes which are used to compute the heat conduction in the core and windings, then the heat dissipates to the oil by convection, afterwards to the walls of the oil tank and to the radiators panels and finally to the surrounding air. In the thermo-fluid dynamic model, the natural convection of the fluid flow is taken into account using a temperature-dependent density. Moreover, the heat conduction through the metal sheet of the oil tank and the radiators panels are considered. The coupled thermo-hydraulic problem is solved with Code_Satume software. The experimental data of the total power losses provided by the unit manufacturer were employed to validate the electro- magnetic model. Because the thermo-fluid dynamic numerical model is computationally expensive, only one quarter of the transformer is modelled. In addition, equivalent anisotropic thermal conductivity in the core and windings are calculated to simplify the heat conduction model in the active parts. The differences in the temperature distribution and oil flow observed between the transformer working with a standard mineral oil and a biodegradable ester are discussed. This work is carried out as part of the EU Horizon 2020 BIOTRAFO project.