| Oil-immersed transformers are the leading equipment for changing voltage levels in transmission and distribution systems above 35 k V,and their stability and reliability play a vital role in the safe operation of the power grid.The losses generated by transformers under long,high-load operations are transferred to the transformer as heat.Overheating inside the transformer accelerates the aging of the winding insulation material,leading to deformation and cracking of the insulation structure and failure of the insulation system.Transformer hot spot temperature is a significant factor limiting the overload capability of transformers and an essential parameter for calculating the relative aging rate of insulation materials.However,the winding hot spot temperature and hot spot location change with the transformer load and operating environment.It is difficult to practically obtain the hot spot temperature under a high voltage and strong magnetic field environment.In order to achieve the analysis of hot spot temperature and hot spot location of oil-immersed transformers,the heat transfer model and thermal resistance of oil-immersed insulating paper,the convective heat transfer model,and convective heat transfer coefficient of oil-immersed windings are investigated in this paper.On this basis,the spatial heat path model of oil-immersed transformers is constructed by combining the structural characteristics of transformer windings and the heat transfer medium thermal properties parameters.The main work is as follows:A multilayer rough and thin layer heat transfer model is proposed for the effects of rough and thin layers of insulating paper,multilayer stacking,and oil immersion state on the heat transfer of windings.The multilayer rough and thin layer heat transfer model is applied to calculate the contact heat transfer thermal resistance of insulating paper-insulating paper and insulating paper-copper magnetic wire in the oil-immersed condi-tion.The model is validated by measuring the oil-immersed insulating paper’s contact heat transfer thermal resistance at different temperatures through a designed steady-state dual heat flow meter experiment.At 70?C,the experimentally measured contact thermal resistance of insulating paper-copper and the contact thermal resistance of insulating paper-insulating paper are 1.92×10-4m2·K/W and 2.68×10-4m2·K/W,respectively,and the values calculated by the model proposed in this paper are 1.83×10-4m2·K/W and2.59×10-4m2·K/W,respectively.At 60?C～80?C,the thermal resistance of the insulating paper-copper contact calculated by the rough thin-layer heat conduction model is 6.64%in error with the experimentally measured contact thermal resistance.The thermal resis-tance of the insulating paper-insulating paper contact calculated by the multilayer rough thin-layer heat transfer model is 4.46%in error with the experimentally measured contact thermal resistance.The winding convective heat transfer characteristic number equation is constructed for the effect of the transformer winding surface on the boundary layer fluid.The equations are constructed considering the effects of transformer oil channel structure,boundary layer oil-flow state,insulation paper,and thermal oil properties on convective heat transfer.The characteristic number equation is applied to calculate the winding con-vective heat transfer coefficient,and the critical parameters of the characteristic number equation and the natural convective heat transfer coefficient of the winding are obtained utilizing a designed steady-state circular tube experiment.For characteristic lengths of3 cm and characteristic temperatures of 45.5?C,the convective heat transfer coefficients calculated from the steady-state circular tube experiment,the winding convective heat transfer characteristic number equation,and the empirical characteristic number equation are 128.99 W/（m2·K）,131.38 W/（m2·K）,123.01 W/（m2·K）.At a characteristic temper-ature of 55.0?C,the convective heat transfer coefficients calculated from the steady state circular tube experiment,the winding convective heat transfer characteristic num-ber equation,and the empirical characteristic number equation are 144.38 W/（m2·K）,141.12 W/（m2·K）,132.14 W/（m2·K）respectively.The convective heat transfer coeffi-cients calculated by the winding convective heat transfer characteristic number equation and the empirical characteristic number equation have a smaller error in the winding convective heat transfer characteristic number equation compared to the experimental values.A spatial heat path model for oil-immersed transformers is proposed by integrating the internal structure of oil-immersed transformers and the thermophysical properties of transformer oil.The model takes the thermal resistance of the insulation paper and the convective heat transfer coefficient of the winding as critical parameters.To solve the problem of difficult temperature measurement of oil-immersed transformer windings,a fully dielectric fiber-Bragg grating（FBG）temperature sensor is developed and embedded inside the high-voltage windings.This method improves the accuracy of the winding hot spot temperature measurement.The validity of the spatial heat path model is verified by combining the winding temperatures obtained from the transformer temperature rise test.In the rated power temperature rise test,the hot spot temperature measured by the FBG sensor is 82.3?C,with the hot spot location on the 10th winding.The hot spot location calculated by the spatial heat path model is on the 10th winding section,with a hot spot temperature of 83.3?C,a difference of 1.0?C from the hot spot temperature measured by the FBG temperature sensor.The hot spot temperature calculated by the empirical thermal model is 87.7?C,which differs from the sensor-measured hot spot temperature by 5.4?C.The hot spot temperature calculated by the oil-immersed transformer spatial heat path model is less inaccurate than that calculated by the empirical thermal model. |
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