Research On The Leakage Magnetic Fields And Local Overheating Due To The Magnetic Shunt And Heavy Current Leads In Large Power Transformers

Based on references involved in magnetic shunt and leakage magnetic flux due to heavy current leads of a large power transformer, key technique and technical path are founded. The problem belongs to large numerical calculations of 2d or 3d open boundary eddy current fields. To be practicability, not only precision must be higher, but also memory and CPU time must be saved in the calculating method. Geometric transformation method and T_I ψ-φ_m method are thought as proper methods in solving the problem.The mathematical models of the geometric transformation method are first established and deduced. A convenient and applied geometric transformation function, which is suit for interface of quadrangle or hexahedron, is proposed. The coefficients of rigidity matrix in Kalerkin FEM are given. The program of the geometric transformation method can be accomplished after the occurred FEM one is simply modified. The inverse function of the geometric transformation method and formula of magnetic field intensities from transformation region to external one are given. Magnetic field intensity of any point in the external region can be solved by means of the inverse function and the formula. By calculating a 2d open boundary field of an infinite length lead, the geometric transformation method is proved to be correct. Compared with the truncation method in calculation precision, CPU time and memory, the geometric transformation method is superior to the truncation method.T_I ψ-φ_m method of less variable and higher precision is proposed, at the case of electric vector potential T and total magnetic scalar potential ψ in eddy current regions, reduced magnetic scalar potential φ_m in non-eddy current regions. The governing equations of T_I ψ-φ_m method and their discrete mathematical models are given. Because the magnetic intensities aren't the sum of source field and induced one, the cancellation problem of two great numbers are avoided. As the fields due to winding currents are solved by Bio-Sofa law, the generation of FEM meshes may not be restricted by the shape of the winding. By calculating TEAM workshop problem 21 model B with or without aluminum screen, T_I ψ-φ_m method and 3d geometrical transformation method are proved to be correct.A magnetic shunt effect on leakage magnetic flux at the winding end in a 720MVA transformer is studied by means of programmed software according to theabove methods. The leakage magnetic flux at the winding end is analyzed at the case of different magnetic shunt size. Size of the magnetic shunt influence on the leakage magnetic flux in the clamp and the oil tank is discussed. The size of the magnetic shunt are given, when maximal loss density in the clamp can be reduced and magnetic flux densities in the magnetic shunt can't be reached the saturated value.The overheating and eddy current loss in the tank wall and the ascending flange of bushing are studied at lead current of 10~30kA by means of the programmed software. The some effects on improving the overheating and loss are calculated and analyzed, such as the wall material, the thickness of copper screen, the lead current, the distance from the lead to the wall, the branch number of parallel connection of the leads. The loss density in the tank wall can be effectively reduced, when the branch number is two. It is no use exceeding two branches. The wall of steel with copper screen is better than the one of non-magnetic steel. The cover of nonmagnetic steel is superior to the one of steel.