Research On Key Parameters Of Convective Heat Transfer Between Transformer Oil And Windings

Hot-spot temperature is one of the important factors to determine the service life of transformer.It is of great theoretical and practical significance to analyze the hot-spot temperature of transformer for the normal operation of transformer.The thermal circuit model method is the most commonly used method in engineering at present.How to establish accurate thermal circuit model and set reasonable model parameters is a hot issue in current research.Accurately obtaining the convective heat transfer coefficient between transformer oil and insulating paper is the key to defining the thermal resistance parameters of solid-liquid interface in the model.At present,the commonly used method in engineering is based on the empirical formula(characteristic number correlation formula)obtained by experimental methods,However,the values of C and n parameters in the formula are based on experience and estimation.When the convection heat transfer coefficient is obtained,due to the inaccuracy of the parameter value,there is a certain deviation between the convection heat transfer coefficient obtained by the empirical formula and the actual value.In this paper,a natural oil circulation cooling transformer is taken as the research object,and the key parameters of convective heat transfer between transformer oil and winding are studied.The main research contents are as follows:(1)An experimental device is designed based on the steady-state circular tube method to simulate the oil flow environment of insulating paper under the natural convection condition in the transformer.The convective heat transfer coefficient between transformer oil and insulating paper is measured under different power levels by collecting and processing the experimental data.The rated power varies from 0.14W to 0.455W(the ambient temperature is constant to 22℃).The h_i ranges from17.7W/(m~2K)to 20.62W/(m~2K).The convection heat transfer coefficient model is obtained from the empirical formula.According to the model,the experimental data is nonlinearly fitted.The fitting degree is 87.35%,and the key parameter C and n in the model are obtained with the solution of 1.04 and 0.24 respectively.(2)Based on the thermoelectric analogy method and the transformer internal space structure,the distributed parameter heat circuit model of transformer windings is established.The simulation is carried out by using MATLAB software.In simulation1,the convective heat transfer coefficient between transformer oil and insulating paper is calculated by key parameter based on experiments convective heat transfer coefficient model.In simulation 2,the convective heat transfer coefficient between transformer oil and insulating paper is calculated by empirical parameter convective heat transfer coefficient model.Simulated 1 temperature,simulated 2 temperature at 1times rated power,and simulated 1 temperature and simulated 2 temperature at 1.3times rated power are obtained respectively.(3)Verifying the key parameters C,n in the convective heat transfer coefficient model based on the steady-state tube method.A temperature-rise test platform is built.the temperature information of the key points in the transformer is collected by using the fiber bragg grating temperature sensor.The measured temperature is compared with two simulation temperatures obtained in Chapter 3.The results show that the deviation between the maximum temperature of simulation 1 and simulation 2 and the measured temperature is 1.2℃and 1.7℃at 1 times the rated power.At 1.3 times rated power,the maximum temperature of simulation 1 and simulation 2 deviates from the measured temperature by 1.3℃and 1.8℃respectively.The simulation 1 temperature distribution curve is closer to the measured temperature distribution curve than the simulation 2temperature distribution curve,which verifies the accuracy of the key parameters C and n obtained by the steady-state tube method.