The Calculation And Analysis Of Stray Loss And Thermal Problem In High Impedance Transformers

With the installed capacity of power grid gradually increased,in order to limit the short circuit current of power system and enhance the stability of power system operation,more and more high impedance transformers are applied to the construction of strong power grid.Compared with conventional transformers,the leakage flux density of high impedance transformer is larger and unevenly distributed,which increase the loss of structural parts.For foil winding,the asymmetrical distribution of leakage flux generate crowding effect,resulting in excessive loss density in the end of winding and speeding up the aging of insulation.With the rapid development of power electronics technology,a large number of nonlinear components are used in power system to improve energy efficiency,but bring the problem of harmonic pollution.The presence of higher harmonics makes the leakage flux distribution through the structural member more complex and the stray loss further increased.Therefore,it is of practical significance to deeply study the stray loss and thermal problem of high impedance transformer.Firstly,the high impedance transformer is considered as the research object,the model of which is SCBH15-2500/10/0.48.And three-dimensional calculation model is established by simplifying the research object.Finite element method is used to calculate the model by Magnet software and then get the results,including the magnetic density distribution in core,leakage flux density distribution and the loss value of structural parts and winding.The short circuit impedance of high impedance transformer is respectively calculated by magnetic circuit method and energy method.Through the calculation of team problem 21 b model,the calculative accuracy of magnetic flux density and loss by Magnet software is verified.Secondly,based on the field-circuit coupled method,the three-dimensional nonlinear time-harmonic field solver is used to calculate the loss of structural parts.The eddy current loss of foil winding is respectively calculated by analytical method and finite element method and then the results are compared and analyzed.The frequency domain analysis method is adopted to calculate the loss of high impedance transformer under different high order harmonic condition and the order of harmonics that have greater impact on the loss are analyzed.Finally,the heat dissipation process of high impedance transformer and the computational principle of fluid field and temperature field are set forth.Based on indirect coupling method,heat generation rate of unit volume in each component of high impedance transformer is applied as thermal load to the calculation model.Ansys CFX software is used to solve the multi-physics field of high impedance transformer and the distribution of fluid field and temperature field is analyzed and the temperature rise of high voltage winding under different tapping voltage is calculated.