Global Temperature Rise Calculation Of Oil-immersed Power Transformer

The power transformer is one of the most important equipments in the power transmission and transformation network.The reliability of the power transformer directly determines whether the entire power grid can operate safely and stably.In recent years,with the rapid development of the power industry,the capacity and load of power transformers have been continuously increased,and losses have been increasing.As a result,the temperature inside the transformer has become too high,causing deterioration of the coolant and insulation aging.According to statistics,more than 80% of the end of transformer life is due to insulation aging due to excessive temperature rise in the transformer.Therefore,the calculation of the loss and temperature rise of large power transformers is particularly important.In view of the difficulty in designing the cooling structure of the large-capacity oil-immersed power transformer and the difficulty in predicting the internal temperature,this paper studies the heat transfer characteristics of the oil-immersed transformer based on the thermal network method.According to the actual operation of the oil-immersed transformer,the repetitiveness of the heat dissipation of the three-phase winding is considered,and only the thermal network model of the onephase winding and the related structure is established.The relevant thermal parameters in the thermal network model are solved according to the knowledge of heat transfer.The prototype electromagnetic calculation model was established and calculated using the finite element method as the heat source condition in the thermal network model.Analyze the oil circuit structure in the winding area and establish its fluid calculation model.Field-circuit coupling method is used to calculate the overall oil circuit,and the fluid characteristics of each section oil path are analyzed locally to obtain the flow resistance-velocity relationship of each section.On the whole,after the network is connected in series,the operating point of the pump is determined by calculation,and the flow rate of each segment is obtained.Substituting it into the thermal network model.Using the first law of thermodynamics,iteratively realizes the coupling of temperature and fluid and electromagnetics,and the average temperature and temperature rise of the transformer are obtained.Finally,taking the high-voltage winding as an example,the effects of the position of the oil-retaining plate,the number of guide zones,and the height of the horizontal oil passage on the hot spot temperature of the high-voltage winding are considered,and the results are analyzed to optimize.Taking a 240000 kVA and 330 kV oil-immersed power transformer as an example,this paper calculates the prototype by using the method described in this paper,and analyzes the calculation results of thermal network method and numerical analysis by comparing the temperature rise test.The results show that the error of the thermal network model is within the allowable range and is simple and fast.This method can provide the basis for future heat transfer research and temperature rise prediction.