Calculation And Analysis On The Windings Short-Circuit Strength In Large Power Transformers

After summarizing relevant literatures, 2D transient axisymmetric electromagnetic field in large transformers and winding short-circuit strength are studied deeply in the thesis. A few achievements with values on theory and engineering have been gained and a software package practical in engineering is developed. Referential rules are provided to improve abilities for transformer windings to withstand short-circuit.The significance and purpose of the calculation of short-circuit strength of the large power transformers are introduced in the beginning of the thesis, and then the development of the leakage magnetic field of the transformer, the winding short-circuit strength, the stability problem of transformer are discussed. In the part of the calculation of the leakage magnetic field, numerical calculation are set up based on the theory of finite element method, and considering the saturation of the silicon sheet and the steel construction, the non-sine varieties of the current, 2D transient axisymmetric electromagnetic field in large transformers under short-circuit is calculated, the distribution of electromagnetic field is obtained.In the part of electromagnetic force, radial force and axial force are calculated by Lorentz. In the transition process of short-circuit, short-circuit current and leakage magnetic field are all to change continuously. Therefore, the short-circuit force which is generated by the interactivity of the short-circuit current and leakage magnetic field are dynamic force actually. The calculation of the dynamic force considers some factors in the thesis, such as the mechanical property of various insulation material, inertia force, elastic force and the friction force which is acting on the displacement of the structures. Combined with the numerical calculation method, the disk unit displacement under the axial impulsive force is calculated. In the part of the calculation of the winding short-circuit strength, the radial buckling depends on a lot of the complex element, such as the structure form of the windings, the number of the bracing structure, the axial vibration, fabrication process. With disc-, helical-and layer-type windings having conductors consisting of strands or non-bonded CTCs, the corresponding critical force is calculated. That is to say, the possibility of the axial buckling is checked.The distributions of the magnetic flux density, electromagnetic force, and winding short-circuit strength, as well as the displacement and the dynamic force of the disk unit under the axial impulsive force are calculated, such as SZ11-31500/66 type and SFSZ11-31500 type in the thesis. The result indicated that their windings strength satisfies the request.Based on above studies, a software package is developed which can be use to analyze 2D transient axisymmetric electromagnetic field and winding short-circuit strength for transformers of various voltage classes, capabilities and kinds of structures under operation and fault conditions. The software package is based on windows platform and has good pre and post processing functions.