Research On Eddy Current Loss Calculation And Temperature Rise Distribution Of Large Power Transformer

With the increasement of capacity of large-scale power transformer, the leakage magnetic field gradually becomes strong and stray losses in metal structural parts get serious. The local overheating problem produced by non-uniform distributed stray losses threatens the safe operation of the power transformer. It is difficult to get an accurate loss distribution in that the traditional analytical method has more uncertain experience factors. Due to the complexity of the structure of large transformer, ferromagnetic materials with nonlinear, anisotropic properties, especially the existence of “large size and small penetration”, the computing scale is too large to get reasonable grid section division. It makes a very big difference between stray losses calculated by general commercial electromagnetic field Finite Element Analysis software and experimental results. With the increasement capacity of the transformer, the problem is becoming serious. Therefore, it is significant to calculate stray losses and their distribution accurately for transformer design.An analysis method which combined Finite Element Method(FEM) and analytical method to calculate eddy current losses in large power transformer tank walls and other metal parts is presented in this paper for improving the accuracy of the stray losses calculation. The process of finite element discretization is transformed into a continuous mathematical function. The analytical method may eliminate certain error produced by discretization and linear element of FEM at a certain extent. Firstly, the perpendicular magnetic flux density has been obtained at discrete points on the surface of the tank by the FEM. Double Fourier series is adopted to express the obtained magnetic flux density by analytical formulae. The coefficients of the analytical expressions are determined by a least mean error with curve fitting technique and optimization algorithm. Based on the electromagnetic theory and Maxwell equations, the eddy current losses are obtained by the analytical formulae. Secondly, the other analysis methods which combined FEM and analytical method are discussed as well. Based on the magnetic flux density and eddy current density on the surface of metal structural parts which have been obtained by the FEM, at a low computational cost, the stray losses of power transformer will be calculated by the analytical method with taking the effect of skin depth of metal parts into account.The validation of these methods is verified by the TEAM Problem 21(Model B). And the proposed methods are applied to calculate stray losses in metal parts of a DFZ-28.333MVA/230 kV single-phase power transformer. Besides, the computional accuray and the scale of stray losses in metal parts of these analysis methods are compared with the FEM, which are using the surface impedance method or local refined technology.Taking the material properties affected by temperature into consideration, the two-way electromagnetic-thermal coupling analysis method is used to calculate the stray losses and temperature distribution on tank walls and other metal structural parts of power transformer. And propriate measures are adopted to reduce stray losses and eliminate the local hot spots in the metal parts of power transformer, for example: slotting in the tie-plate.