This paper presents a fluid-thermal coupled analysis method to compute the temperature distribution in a 31.5 MVA/110 kV oil natural air natural (ONAN) three-phase three-limb transformer. The power losses of windings and core are measured by load-loss test and no-load test respectively. The convective heat transfer process, radiation and oil flow inside the transformer are investigated by finite volume method (FVM). In order to validate the feasibility and accuracy of the presented method, the temperature measuring system based on fiber Brag grating (FBG) sensor is constructed for the temperature rise test of the 31.5 MVA/110 kV ONAN transformer. The simulation results deduced from the proposed method agree well with experimental data. This model can be applied to optimizing design and load scheduling.

This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as an example, this paper building transient field-circuit coupled model and 3D coupled magneto -fluid-thermal model. Considering a nonlinear B–H curve, the magneto model uses the field-circuit coupled finite element method (FEM) to calculate the magnetic flux distribution of the core and the current distribution of the windings when the transformer is at a rated current and under direct current (DC) bias. Taking the electric power losses of the core and windings as a heat source, the temperature inside the transformer and the velocity of the transformer oil are analyzed by the finite volume method (FVM) in a fluid-thermal field. In order to improve the accuracy of the calculation results, the influence of temperature on the electrical resistivity of the windings and the physical parameter of the transformer oil are taken into account in the paper. Meanwhile, the convective heat transfer coefficient of the FVM model boundary is determined by its temperature. By iterative computations, the model is updated according to the thermal field calculation result until the maximum difference in hot spot temperature between the two adjacent steps is less than 0.01 K. The result calculated by the coupling method agrees well with the empirical equation result according to IEC 60076-7.