The Research On Winding Strength Of Large Power Transformer Under Short Circuit And Reclosing Conditions

In this paper, the computation method of impact current in larger power transformer winding after reclosing is studied, when the conditions of sudden short-circuit in the system can’t be eliminate at a short time by summarizing relevant literatures, and the project is based on "National Natural Science Foundation of China". The transient current has been simulated by considering reclosing time, phase angle, transformer cores remanence, and other factors. The J-A hysteresis model is cited to analyze and calculate residual flux of transformer. The magnetic field and electromagnetic force distribution of windings has been analyzed by using numerical methods, while considering power transformer winding structure and design features. The strength of winding disc is been computed at the same time. The windings dynamic electromagnetic force is computed through dynamic analyzed method, and it can be seen as the stress load on winding discs. Mechanical properties and stability of transformer winding under repeated short-circuit impact force after reclosing had been studied.The impact current transient process of power transformer is analyzed, and the influence on short-circuit current of residual flux of transformer has been given. J-A hysteresis model is applied to calculate remanence of transformer after initial short-circuit by analyzing the characteristics of the existing transformer hysteresis model. The mathematical expression of J-A hysteresis model has been acquired by discriminating and analyzing energy expression and differential equations of various existing models. The model is certified correctly by experimental data. The J-A hysteresis model has been quoted to transformer remanence calculation, and the residual flux of transformer is calculated finally. The transient short-circuit current of transformer after reclosing has been simulated and calculated according to the results.The distribution of leakage field in transformers of different winding forms is analyzed. The difference of circular magnetic field to radial and axial magnetic field in spiral winding transformer has been emphatically analyzed, then the current density and the spiral angle of the impact of the three kinds of magnetic field has been given. Through calculation, the short circuit current of transformer after reclosing ia acquired, and the magnetic field distribution of high and low vltage winding under reclosing condition is worked out. Comparing the axial and radial flux density of the initial short circuit and reclosing condition, the magnitude and distribution of short-circuit electromagnetic force after reclosing in winding disks is calculated at last.Throug the calculation of axial and radial electromagnetic force of transformer winding after reclosing, the inherent frequency of winding is analyzed, and the axial and radial short-circuit strength of discs is computed. The displacement distribution characteristic of winding under the action of axial and radial force respectively is provided, and its distribution character of after initial short circuit and reclosing is compared. The elastic modulus of windings with influence of temperature is analyzed, and the axial intensity of transformer under the circumstance is been calculated. The results of radial limited force of discs and the change of radial limited displacement after reclosing short-circuit condition is been given.The transient process of transformer after reclosing is analyzed, and the model of axial dynamics analysis for analytical calculation is established, then the transient process of displacement of the winding under the action of axial short-circuit electromagnetic force on condition of reclosing is provided. According to the structure features of winding and the distribution rule of spacer and bracing, and the analytical model of radial vibration is established. The calculation result is analyzed and verified with application of initial short experimental model, mearsures to guarantee the dynamic stability of reclosing winding is provide.