Electromagnetic And Thermal Field Coupled Modeling And Experimental Research In Dry-type Transformer

The problem of heating and cooling of large power transformers is the multiphysics coupling process of electromagnetic fields,temperature fields and fluid fields.The mathematical description and numerical implementation of the coupled field problem are complicated considering not only the nonlinearity and anisotropy of the material,but also the external conditions on material properties,such as temperature and stress.Efficient numerical modelling of the electromagnetic and thermal problems in electrical engineering depends very closely on accurate electromagnetic fields solution.Indeed,the geometric symmetry of large power transformers is almost non-existent,geometric entities(several meters)coexist with thin-walled cells(insulation paper thickness is not more than 0.3mm),which caused certain difficulties in the establishment of 3D thermal field model and mesh generation.The leakage magnetic flux generated by large-capacity transformers forms stray losses in metal structural parts,the research on stray loss is of great significance for energy saving and consumption reduction,avoiding possible local overheating and guiding the practical product design of transformer.In this thesis,a 2500 kVA epoxy resin cast dry-type transformer is taken as the research object.Based on engineering electromagnetics and heat transfer,the transformer loss calculation and temperature rise prediction are carried out using the magneto-thermal coupling method.Particular content is below:(1)Based on the numerical theory of electromagnetic field,the two-dimensional(2-D)and 3-D magnetic field calculation models of dry-type transformers is established by using finite element analysis software Ansoft Maxwell.The distribution characteristics and influence of spatial leakage magnetic field generated by transformer windings are calculated and analyzed systematically.(2)The measurement data of 2-D magnetic property for 30P120 oriented silicon steel sheet is carried out.The Bertotti loss separation model is applied to obtain the core loss considering the nonlinearity and anisotropy of the steel sample,the influence of technological holes on lamination core loss is compared and analyzed.The potential set based A-φ-A model is used to realize the non-uniform eddy current distribution and stray loss calculation on foil windings and structural based on 3-D load eddy current field electromagnetic simulation.(3)The magneto-thermal-fluid coupling model is established and the nodes loss density is coupled into the 3-D thermal field as heat sources.Considering the effect of temperature on material loss property,the convective heat transfer process is simulated accurately,and the calculation of transformer temperature rise and the prediction of hot spots is realized.The influence of the insulating barrier mounted between high and low voltage windings on the temperature rise is analyzed and compared.(4)No-load and Full-load transformer temperature rise test are carried out,the results indicate that the error between the calculated values and test results was ± 6%,The distribution rule of temperature rise was in good agreement with the results of testing.The two-way multi-physical field coupling method adopted in this paper provides loss prediction method and calculated data in the initial stage of product development,which is of great significance to the design and manufacture of transformer products.