Study Of Short-circuit Strength And Stability Of Windings In Large Power Transformer

The dissertation focuses on the study of major scientific and technological project in Liaoning Province "EHV large power transformers Integrated Design and Simulation Analysis System". Based on the summary of relevant literature, the dissertation deals with the transient electromagnetic field in large power transformers, surge electromagnetic force, short-circuit strength of winding and the impact of repeated short-circuit deeply. A winding short-circuit modal analysis model and the impact of the cumulative deformation of the elastic-plastic model is established. A few achievements with values on theory and engineering have been gained, theoretical basis are provided to improve capacity for transformer windings to withstand short-circuit. The full text mainly consists of four parts.In the first part, the field-circuit coupled problem of three-dimensional transient electromagnetic field of flux density distribution is studied. According to the structural characteristics of large capacity transformer, the application parameters design language of ANSYS software are used for the second development, where field-circuit coupled finite element model of three-dimensional transient electromagnetic field for transformers under short-circuit and no-load conditions is established, By using this model, Many accidental conditions of magnetic field distribution in large transformers are studied, and laws of transient magnetic flux density in per unit are given.In the second part ,when transformer is switching-on with no-load, inrush current distribution and electromagnetic force is analyzed and calculated. As non-linear of transformer core magnetic permeability, when transformer is switching-on with no-load, outside the fault of secondary winding is removed again, it may have transient inrush which is compared with the short-circuit current. The resulting mechanical force will cause great harm on the transformer winding, changing turn insulation strength and the dynamic stability, so the no-load of switching simulation model is built, the characteristics of surge electromagnetic force is analyzed, where differences between short-circuit current and inrush current are found, That's the reason why homeostasis loads need calibrating.In the third part, the power transformer windings axial short-circuit electromagnetic force and strengthand and radial stability issues are studied. Axial dynamic analysis model for experimental modal is established, which is used to analyze relations of natural frequency and pro-compression, the conditions of pre-compression are given to avoid resonance, combined with numerical methods to calculate disk unit displacements by impacting of axial short-circuit. The axial stability criterion is proposed. According to the structural characteristics of windings, distribution of spacer and strip, radial vibration process defects and other complex factors, the radial vibration simulation model is established, which is used to analyze stability of test model, measures are provided to guarantee radial short-circuit strength.In the fourth part, the power transformer winding deformation cumulative impact of multiple short-circuit are carefully dealt with. The cumulative impact of repeated short-circuit deformation of laminated cylindrical shell with stiffeners model is built, the impact of accumulated deformation of repeated short-circuit of transformer winding is calculated with large deformation elastic-plastic theory. Displacement distribution of the impact on repeate short-circuit of the winding and plastic deformation of the load are given, the simulation results show that paper of the model and the theoretical formula have great significance, which is for power transformer windings of the elastic-plastic deformation analysis and the stability judgement of winding.