Research On Coupled Fluid-thermal Field Of 2D Oil-immersed Transformer Based On Triangular Mesh

Power transformers are key components for energy conversion and transmission in power grid.Its fault will bring great economic losses to power system.The highest temperature in the winding,usually referred to as the hot-spot temperature,is of the greatest interest because the insulation paper at the hot-spot undergoes the severest thermal ageing,and determines the life expectancy of the transformer insulation.Therefore,the primary objective of transformer thermal design is to control the hot-spot temperature rise over the ambient temperature within certain limit.For liquid-immersed power transformers,the hot-spot temperature rise over the ambient temperature is controlled by the winding geometry,power loss distribution,liquid flow rate and liquid properties.The calculation of temperature rise of oil-immersed power transformer is a coupling problem of electromagnetic field,flow field and temperature field.In order to calculate the temperature field of oil immersed transformer accurately,firstly,the testing platform has been established for measuring the dynamic viscosity of transformer oil.This paper focuses on 25~#and 45~#insulating oil used in oil-immersed transformers.The kinematic viscosity of the insulating oil at different temperatures was first tested.Thermal accelerated aging tests of 25~#insulating oil was conducted in laboratory conditions.Based on the test data,the software was used to fit the test data to obtain the function relation between the viscosity and the temperature of the transformer oil.Then,a general numerical method based on unstructured mesh for flow field and temperature field coupled problems is presented in this paper.The calculation domain of this method is divided into three-node triangular elements.The velocity distribution in the oil duct is obtained by solving the governing equation of fluid field used control volume finite element method(CVFEM).Then,the uniform governing equation of temperature field for the oil fluid and winding is solved by Streamline upwind/Petrov-Galerkin formulations(SUPG).The flow rate increase method is implemented to deal with the backflow happened at the outlet boundary.The choice of the SUPG stabilization parameter suitable for linear triangular elements is proposed.The proposed coupled method is applied to calculate the temperature distribution of the winding in an oil-immersed power transformer,and the results of the proposed method are compared with software Fluent.It shows that there is a good agreement between the two studies for the mass distribution throughout the pass and the average disc temperature rise for each disc,which proves the accuracy and validity of the proposed method.Finally,the effect of temperature effect on loss,eddy current loss,heat source distribution,inlet velocity conditions,thermal aging of oil,the height of the vertical oil duct and the paper insulation around each conductor on the flow and temperature distributions in a disc-type transformer winding is investigated using the proposed coupled method.The analysis results can be provided as references for optimum design of oil-immersed transformer cooling system.