| With the steady development of power industry, the rated capacity and the electromagnetism load of large power transformer are constantly improving. If the related design is not appropriate, the loss in transformer will exceed the allowed rage, or the heat dissipation performance will not meet the requirements.As a result, the transformer temperature rise will be too high, the insulation will age, and also the physical and electrical properties of the internal cooling oil will change. Finally, the short-circuit accident may be caused by these factors.Therefore, the calculation of the loss and the temperature rise in large power transformer is particularly important.Aiming at the complex structure of the internal cooling system in large-scale power transformers and the difficulty on temperature accurate prediction, a fluid network model is presented, which is based on the structural characteristics of the oil circuit and the analysis of winding cooling features. In this model, the axial repeatability of the winding oil loop is taken into consideration, and the cooling circuit is divided into several sections, which contain winding, cooler and connection structures. Through the calculation of local flow fields by CFD,the flow resistance curve and the cooling characteristics are analyzed, and the global temperature field distribution can be determined based on network coupling.The main contents of this academic dissertation are as follows:1. The fluid network model of large-scale power transformer is established,and the deficiency of the IEEE standard temperature calculation model is discussed from the aspect of mathematics and heat transfer.2. The 3D core-winding electromagnetic calculation model of prototype is built. In this model, the magnetic field distribution change is considered,which is caused by the metal structures and the oil gap of core. At the same time, the coreis built by lamellar elements according to actual sizes. In the following simulation, the resistance loss and the eddy current loss of winding are calculated by the finite element method, which is used as the heat source of winding.3. In the network flow rate calculation, the fluid field model is established which contains the main structure of oil loop, according to the actual structure of prototype. The entire cooling circuit is divided into segmented oil channels, and local analysis is taken to the fluid properties of every segmented oil channel.After these analyses, the relationship between the flow rate and the flow resistance of each part can be achieved. Through network calculation, the working point of the transformer oil pump and the flow rate distribution of the oil circuit are determined.4. As the axial repeatability of the winding oil loop is used in simplification of the winding oil circuit model, the winding oil channels with different structures are piecewise modeled, and the required parameters of the fluid network can be concluded. In this model, the actual structures, such as the block,the stay and the insulating paper are considered. The winding is constructed in the form winding wire plates along the axial direction, according to the actual size. Through local thermal analysis, the average heat transfer coefficient of each winding wire plates is achieved. Then the temperature distribution and hottest point of winding are determined combined with the fluid network and the formulas put forward.Taking a 240000 k VA oil-immersed transformer as an example, the author calculated the prototype winding temperature by the method of this academic dissertation. In order to verify the effectiveness of the proposed method, the prototype test is taken. The results prove that: the model size is reduced and the calculation time is saved by using the method of this academic dissertation,in the case of accurate temperature prediction. At the same time, the presented method can be used in temperature calculation of transformers with other structures,because of the universality of this method. |
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