Analysis Of Flow-thermal Coupling Of Oil-immersed Power Transformer Windings Based On Equivalent Thermal Conductivity Method

As an important power equipment in the power system,power transformers are responsible for power transportation and voltage conversion.Their operation has a significant impact on the power system,so the stable operation of the transformer is a necessary condition for the stable operation of the power system.When the hotspot temperature is too high,the insulation life of the transformer is reduced,and the life of the transformer will also be reduced.Analysis of transformer temperature rise and hotspot issues can not only provide corresponding references for power transformer design,but also predict transformer insulation life and transformer service life.Therefore,it is necessary to accurately and effectively analyze transformer temperature rise and hotspot issues.This thesis firstly introduces the current research status at home and abroad in terms of transformer multi-field coupling numerical calculation,dimensionless least squares finite element theory,and equivalent thermal conductivity theory.In this thesis,a hybrid algorithm is used to analyze the flow-heat coupling problem of oil-immersed power transformer winding.When solving the flow field problem,the fluid field governing equations are transformed into discrete equations by the dimensionless least squares finite element method.Row-column equilibrium method and Jacobian preprocessing conjugate gradient method are used to reduce the stiffness matrix condition number.When solving the temperature field problem,on the basis of the Galerkin finite element method,the upside-down formula is introduced for numerical calculation.This thesis takes the single partition winding model as an example.By comparing the simulation results of the Matlab hybrid algorithm with the simulation results of the Fluent software,the influence of the grid accuracy on the flow field results and the effect of the inter-turn insulation on the temperature field results are analyzed.The insulating layer of oil-immersed power transformer windings is very thin,and the mesh size needs to be small when modeling the transformer windings,resulting in a large number of meshes.Therefore,in order to reduce the amount of calculation and ensure the accuracy of the simulation,this thesis intends to use the equivalent thermal conductivity method to calculate the axial equivalent thermal conductivity and radial thermal conductivity of the transformer winding.By establishing a single-zone winding model and an eight-zone winding model,and comparing and analyzing the Fluent simulation results of the split-turn model and the two equivalent models,the accuracy of the two equivalent thermal conductivity methods is verified.Finally,on the large-scale oil-immersed power transformer winding experiment platform,the results of the hybrid algorithm proposed in this thesis and the experimental measurement results are compared and analyzed,and the effectiveness of the equivalent thermal conductivity method is verified.At the end of the thesis,a numerical simulation model of converter transformer is established,and the flow field distribution and temperature field distribution of the transformer are analyzed based on the proposed equivalent thermal conductivity method.The simulation analysis shows that the hotspots in the windings of the line-side winding and the valve-side winding are located at 92% of the winding height,which conforms to the improved hotspot temperature calculation model based on the winding heat distribution.