Oil cooling for disk-type transformer windings-part 1: theory and model development

Based on a finite control volume analysis, a coupled thermal model is developed to investigate the two-dimensional hot-spot temperature field in the disks for oil-filled transformer windings. The model consists of two submodels: the heat conduction submodel and the nonisothermal hydraulic submodel. The heat conduction submodel for the temperature distribution in the winding disks includes a nonuniform heat-generation rate, and axisymmetry of the disk geometry. A successive over-relaxation method is adopted to accelerate numerical convergence. Taking into account the temperature-dependent fluid properties, the nonisothermal hydraulic model can predict the flow and temperature distributions in the fluid. The cooling oil flow is treated as a thermally developing laminar flow for the determination of the local heat-transfer coefficient, due to the low-flow Reynolds numbers (20-200), high oil Prandtl numbers (150-50), and relatively short flow channel length encountered in the oil-filled transformers. The two submodels are coupled through the heat-transfer boundary condition, and an iterative method is used to solve the coupled thermal model.