Simulation Study On Temperature Rise Characteristics Of 35kV Transformer Based On Iron Loss Distribution

Oil-immersed distribution transformer as an important part of the distribution network,its safety and stability is directly related to power supply reliability of the distribution network.As the uneven distribution of heat within the transformer,it is prone to occur the phenomenon of local overheating,which will accelerating the aging of the internal insulation of transformer,or making heavy damage to the transformer insulation,and finally causing serious accidents.Therefore,it is of great significance for the safe and reliable operation of the power grid to study the temperature-fluid field of the transformer,and then to reduce the hot spot temperature of the transformer,improve the heat dissipation of the transformer and reduce the aging speed and accident rate of the insulation from the aspects of structure and material.Based on the simulation results,the quantitative relationship between the excess loss and the domain wall bowing degree is established by using the magnetic domain theory and the 1-D nonlinear eddy current field model.Then,the 3-D transformer model based on the laminated core structure was constructed,and the magnetic field distribution in the core was obtained.The Bertotti's iron loss three-stage model was used to calculate the iron loss distribution of the main area and seam area under the three forms of seam area,and the excess in the model was instead by quantitative relationship above.Then,using the multi-physics simulation software COMSOL Multiphysics and based on the actual distribution transformer(core stack series N=2)to build 2-D axisymmetric temperature-fluid field original model,that is,regardless of the loss distribution of the core,which is regarded as a uniform heating element.At the same time,considering the loss distribution in the core,build the improved model.The results are compared with the data of the factory temperature rise test.It is found that the simulation results of the improved model are closer to the factory test values.The temperature rise characteristics of mineral oil transformer and vegetable oil transformer under the laminated structure(core stack series N=3,N=4)are further calculated by using the improved model.It is found that the temperature of the two transformers decreased when the number of layers N increased.That is to say changing the laminated structure of the core can improve the temperature rise characteristics of the transformer.Finally,the temperature rise characteristics of oil-based silica nanofluids were analyzed by using the improved model.It is found that the temperature of mineral oil-based silica nanofluids transformer and vegetable oil-based silica nanofluids transformer also decreases with the increase of N series.At the same time,under the same core structure,temperature rise characteristics of mineral oil-based silica nanofluids transformer is the best.And the temperature rise characteristics of vegetable oil is the lowest,which still meet the national standard.In this thesis,two methods of improving the temperature rise characteristics of transformers are proposed:one is to change the laminated structure of the transformer core to optimize the iron loss distribution;the second is to use the oil-based silica nanofluids instead of the traditional transformer oil to increase the thermal conductivity of oil.These two methods can provide some theoretical and practical significance for the design and manufacture of transformers.