Analysis Of Leakage Magnetic Field And Short Circuit Electrodynamic Force In Transformer Windings

With the increasing scale of power grid,the short-circuit fault of power transformer threatens the stability and reliability of the whole power system more and more seriously,which has gradually attracted the attention of relevant researchers.In practical operation,transformer windings are damaged by short-circuit electrodynamic force impact,which accumulates gradually through multiple short-circuit shocks.Once this potential safety hazard breaks out,it will cause serious destruction to the power grid.Therefore,further revealing the influence of transformer short-circuit electrodynamic force on windings has a positive guiding role in its structure optimization and short-circuit tolerance,and also helps to ensure the safe and stable operation of the power grid.Firstly,three-dimensional short-circuit models of single-phase transformer and three-phase transformer are established by ANSYS.Three-phase symmetrical short-circuit model is adopted for three-phase transformer.The distribution of leakage magnetic field,axial and radial short-circuit force of transformer winding under short-circuit is analyzed by Maxwell module.Secondly,the model is further refined,on the premise of neglecting the temperature change,not only the supporting effect of pads and struts on the winding,but also the stress-strain curve of the plastic material of the winding are considered.Through the coupling of electromagnetic field and force field,the deformation and stress distribution of windings under single short circuit loading and unloading are analyzed.At the same time,the modal analysis of windings is carried out to reveal their natural frequencies and modes under short-circuit impact,and the differences of natural frequencies between internal and external windings,before and after short-circuit and under different end constraints are compared to ensure that the natural frequencies of windings are not near the short-circuit force frequencies,so as to reduce the possibility of accidents such as resonance,and improve the life and reliability of the structure.Finally,the finite element method is used to analyze the effect of multiple short-circuit impulses on the accumulative deformation and stability of transformer coils,and the simulation results are verified by comparing with the actual transformer short-circuit test results.According to the simulation results,the radial magnetic leakage and axial force mainly act on the end of winding,which aggravates the ampere-turn imbalance between inner and outer windings.The axial magnetic leakage and radial force mainly act on the middle part of winding.The low-voltage coil is compressed inwards and the high-voltage coil expands outwards,especially in the upper and middle part of the coil in the iron core window.After short-circuit impact coil,residual deformation and residual stress will occur,and the dynamic stability of the winding will be reduced.Although the stability of the winding can be improved by increasing pre-tightening force and end reinforcement,multiple short-circuit will make its deformation accumulate gradually,and the probability of resonance will rise,which will ultimately lead to the structural damage of the transformer.The simulation results are verified by transformer short-circuit test,and the trend of short-circuit force is gradually slowed down.