Study On Calibration Models And Evaluation Technology For Short Circuit Withstand Ability Of Power Transformer

It is inevitable to occur short circuit for a power transformer in the operation process. The transformer will be damaged under the huge short-circuit force if the ability to withstand short circuit is poor, so it is necessary to calculate its short circuit withstand ability.In this paper, the distribution characteristics of the leakage magnetic field and short-circuit force are obtained based on the theory of leakage magnetic field of power transformer, and the deformation forms and stress calculation models are derived. It is proposed that calibration of the anti-short circuit ability is to make sure that none of the stress would exceed the allowable values in the case of short circuit. Based on the idea, the calibration model is put forward, and main methods of calibration are given. Finally, the proposed model's effectiveness is verified after the check of short-circuit force and natural frequency.The distribution law of leakage magnetic field and short circuit force are obtained by building the two-dimensional axisymmetric finite element model. During postprocessing, the short-circuit impedance calculated by the energy method is consistent with the design value, also the electrodynamics' force is compared, which verify the leakage magnetic field models' accuracy.Transformer windings will exhibit compression, tension and bending under the short circuit force. Main mechanical stresses are calculated according to the results of short circuit electrodynamics' force, and the mechanical stress calculation model of the power transformer is established according to material mechanics, which provides an effective parameter to reflect the mechanical performance.Based on the elastic dynamics theory, and according to the elastic support equivalent principle of radial pressboards, a 2D model to calculate axial vibration natural frequency of power transformer winding is presented. The 3D model to calculate natural vibration frequency can be simplified as a 2D one because the support of pressboard on the winding is same. It is tested that results of the 2D model are consistent with those of 3D model, but the former gets much higher calculation efficiency. Influence factors of the natural frequency are analyzed from the aspect of winding structure and methods to avoid axial resonance are put forward.