Research On The Measurement Error Of Hot Spot Temperature Of Oil-immersed Transformer Windings

Oil-immersed transformer is one of the important equipments in power supply and distribution system.With the rapid development of China's electric power industry,the construction of the national electric power transmission network has made gratifying achievements.The annual installed capacity has increased year by year,and the transmission system plays an increasingly important role in national production and life.The higher the transformer grade,the greater the probability of transformer failure.Because of the complex internal structure of transformer,the internal temperature distribution is uneven,which leads to the performance degradation of insulation system and threatens the service life and safe operation of transformer.In order to ensure the safe and stable operation of power transmission system,it is necessary to ensure that the hot spot temperature of large oil-immersed power transformer is within the safe range for a long time.Establishing and optimizing the thermal circuit model of transformer temperature field is an important way to analyze the hot spot temperature of transformer winding at present,and more accurate measurement of the hot spot temperature of transformer is the key to improve the thermal circuit model and improve the accuracy.According to the national standard GB/T 1094.7-2008,the method of hot spot measurement of oilimmersed transformer is to place several temperature sensors inside the gasket within the predicted hot spot position to measure the temperature of transformer winding.The highest temperature measured is regarded as the hot spot temperature of transformer.Considering the heat transfer structure of transformer,insulating paper and gasket are bad conductors of heat,there must be temperature difference between gasket temperature measuring point and winding temperature,which is not mentioned in national standard GB/T 1094.7-2008,which will inevitably affect the accuracy of transformer hot spot temperature measurement.The main contents of this paper are as follows:(1)Based on the actual transformer structure,this paper analyses the heating environment of gasket temperature measuring point,chooses the calculation model from the angle of two-dimensional steady heat transfer,establishes the boundary node XII discrete equation,calculates the theoretical value of oil-immersed transformer temperature measuring point by Gauss-Seidel iteration method,completes the theoretical calculation model of gasket temperature measuring point,and realizes the heat transfer model of gasket temperature measuring point.(2)An experimental platform of oil-immersed transformer simulator is built to measure the hot spot temperature under steady-state conditions according to the method described in the national standards,and to measure the surface temperature of windings,the boundary oil temperature of transformer tank and the ambient temperature.The experimental results are analyzed.The heat transfer model parameters are set according to the temperature characteristics of the oil-immersed transformer simulation experimental device and each heat transfer node.The theoretical data are calculated and compared with the experimental data.(3)By thermoelectric analogy method,a simulation model for calculating winding temperature based on gasket temperature measurement point is established.The simulation data are compared with the experimental data to verify the accuracy of the conclusions.The function form between error and winding temperature is analyzed,error data are fitted,and the relationship between error and winding temperature is obtained.By means of theoretical analysis,practical experiment and simulation,this paper studies the error of winding hot spot measurement method described in national standard GB/T 1094.7-2008.In the process of implementation,the theoretical calculation model of gasket temperature measurement point is established;the thermal circuit model based on gasket temperature measurement point is established;the conclusion that there is a real temperature difference between winding temperature and gasket temperature measurement point is drawn.The results show that when the winding temperature is 77 ?,the calculation results of heat transfer model of temperature measuring point are 74.7 ? and the experimental temperature of temperature measuring point is 74.9 ?.With the increase of the experimental temperature,the temperature difference between the temperature of measuring point and winding becomes larger gradually;the maximum relative error is 4.8% when the experimental value is compared with the calculated value,which has a good application prospect.