Research On Electromagnetic Characteristics And Cumulative Effect Of Large Transformer Windings Under Multiple Short Circuit Conditions

The role of large power transformer in transmission and distribution network is important.The safety and reliability of transformer is the key to the stable operation of smart power grid and energy internet.The main reason for the increase of failure rate of power transformer is that,as the capacity of the power system increases,the transformer winding can not withstand the corresponding increase of the fault impact load.In order to solve the problem of insufficient power transformer winding strength,a follow-up work expands based on the national natural science foundation project "short time interval multiple short circuit impact conditions".The cumulative effect mechanism of transformer winding strength damage under multiple impact conditions is studied,as a great significance to improve the resistance of power transformer windings.The main contents of this dissertation are divided into five parts.The electromagnetic characteristics of the transformer and the calculation method of current are studied under multiple impact condition.Firstly,in order to clarify the causes of multiple impact damage of transformer windings,the simulation model of transformer core transient magnetic flux is established and the conditions of multiple impact damage are analyzed and enumerated.Then,the fault current which is an important factor to generate the impact load of windings is studied,and the influence law of excitation parameters on fault current under multiple impact conditions is consideried.In this way,the mechanism of the magnetization process of the transformer under multiple impact conditions is described.On the basis of the above theoretical researches,the accurate calculation method of current in multiple short circuit conditions is established.Lastly,base on the calculation method,the main electromagnetic factors of multiple impact damage of transformer windings are analyzed quantitatively through practical transformer products.The distribution characteristics of the leakage magnetic field and the impact load are studied.Different from previous studies,the numerical calculation method of the vortex region is introduced to study the strength of transformer windings.It is assumed and verified that the distribution of leakage magnetic field of three-phase transformer winding in multiple impact conditions is interchangeable.By establishing a 3D finite element model of transformer,the distribution characteristics of leakage flux density in different regions of windings are analyzed by using the magnetic field calculation method.Enumerating the assessment stress in the national standard of transformer short-circuit strength,the corresponding analytical calculation model is established.Combined with the actual test data of transformer products,the main stress types of transformer winding plastic deformation are analyzed.The elastic-plastic calculation model of transformer winding strength is established.The mechanical problems in the cumulative effect of transformer winding under multiple impact conditions are discussed in this dissertation.The stage research model of the cumulative effect of transformer winding is designed.First stage,the numerical calculation model of winding strength is established based on the distribution characteristics of the leakage magnetic field and impact load of transformer winding under multiple impact conditions.Then,the main structural factors of the displacement of the transformer windings are studied.The influence law of structural parameters on the deformation of transformer windings is analyzed by means of more comprehensive winding wire gauges,and an analytical calculation model for elastic plastic deformation of transformer winding is established.The cumulative effect of transformer winding strength under multiple impact conditions is studied.Sencond stage,the numerical model of winding strength in the springback stage is established.The influence of structural parameters and impact load on the residual deformation of windings is analyzed by means of elastic-plastic deformation and springback phase calculation model.Then,the numerical calculation model of residual stress of transformer winding with equivalent residual deformation is established to the detect influence rule of impact on the strength of transformer winding.Combined with the actual test data of transformer products,the variation law of the cumulative effect of transformer winding under multiple impact conditions is analyzed.The experimental study on the cumulative effect of transformer winding under multiple impact conditions is designed and verified.In order to simulate the multiple impact conditions,the corresponding wiring,the loading scheme of impact current and the layout plan of the flux leakage and stress measurement sensor are designed.The experimental study on the cumulative effect of transformer winding under multiple impact conditions is verified by comparing the measured data and the calculation results,and the model is modified.