Calculation On The Windings Short-Circuit Force Of Power Transformer

The poor mechanical strength of transformer windings is one of factors that cau-se short-circuit fault. So the accurately calculated on the transformer windings short-circuit force is to improve the ability to withstand short-circuit impact.As a case study on a150MVA power transformer with off-circuit-tap-changingwinding in this paper, the2D axis-symmetrical FEM model on LV winding three-ph-ase short-circuit at the exit conditions is presented with the field-circuit couplingmethod, which considers oil duct size, disk discontented turn, turn insulation andother factors. Short-circuit test is simulated based on the coupling model, and theshort-circuit impedance of transformer is solved directly based on short-circuitimpedance definition. The feasibility of model and calculation method is proved bycomparing the calculated results and the measured ones. Based on this analysis, thewinding short-circuit electromagnetic force is calculated. Firstly, the rule of short-circuit current with time and the distribution of disc current density are calculatedand analysed. Secondly, the transformer leakage magnetic field and the distributionof disc magnetic flux density in different tap are obtained. Finally, the short-circuitelectromagnetic force is solved based on the lorentz force formula.The damage of windings under short-circuit electromagnetic force depends gre-atly on winding’s natural frequency and corresponding mode shape, both constituteits modal.The finite element model of disc beam element was established. the datashowed that the influence of natural frequency characteristic have great influence onthe axial dynamic response of winding. Hence the axial vibration model of windingis created, which considers the different stiffness coefficients of insulation spacerbetween disc. Firstly, the winding natural frequency under different axial prestressesis calculated. Secondly, applying axial short-circuit electromagnetic force to discelement and solving the winding end displacement based on Newmark method, therule of winding vibration under different prestresses can be discussed. Meanwhile,based on the winding end displacement don’t appear amplification under the impactof maximum axial short-circuit electromagnetic force in disc. The suitable axial prestress of winding would be determined. The validity of model and the accuracy ofcalculation method is proved by comparing the calculated results and actual one.Finally, the vibration displacement and dynamic force of disc were calculated.Moreover, the frequency spectra of dynamic force for disc was analyzed.After obtaining the short-circuit of every discs, the method to calculate windingmechanical strength is presented. The windings short-circuit strength of the discsunder the largest stress is checked.