This paper takes a typical 220 kV three-phase three-limb oil-immersed transformer as\nan example, this paper building transient field-circuit coupled model and 3D coupled magneto\n-fluid-thermal model. Considering a nonlinear Bââ?¬â??H curve, the magneto model uses the field-circuit\ncoupled finite element method (FEM) to calculate the magnetic flux distribution of the core and the\ncurrent distribution of the windings when the transformer is at a rated current and under direct\ncurrent (DC) bias. Taking the electric power losses of the core and windings as a heat source,\nthe temperature inside the transformer and the velocity of the transformer oil are analyzed by\nthe finite volume method (FVM) in a fluid-thermal field. In order to improve the accuracy of the\ncalculation results, the influence of temperature on the electrical resistivity of the windings and\nthe physical parameter of the transformer oil are taken into account in the paper. Meanwhile, the\nconvective heat transfer coefficient of the FVM model boundary is determined by its temperature.\nBy iterative computations, the model is updated according to the thermal field calculation result\nuntil the maximum difference in hot spot temperature between the two adjacent steps is less than\n0.01 K. The result calculated by the coupling method agrees well with the empirical equation result\naccording to IEC 60076-7.
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