Study on Calculation Method of Cooling Oil Flow Rate Allocation for Natural Oil Circulation Transformer

For the natural oil circulation power transformer, the hot spot temperature and winding temperature should be within prescribed limits so as to ensure its life and reliability. Temperature rise of inner windings not only depends on the transformer loss, but relates to oil flow closely. Therefore, whether the oil flow of transformer and oil flow distribution rules among the three windings can be predicted or not, relates to success or failure of the winding temperature rise research. According to the structural characteristics of transformer, a three-dimensional computational model including windings and external radiators was built by a three-dimensional modeling software. Initially, from the view of flow, the flow resistance of natural oil circulation power transformer windings has been calculated and analyzed using the CFD methodology, and the resistance characteristic curves about high voltage windings, middle voltage windings, low voltage windings, radiators and pipelines were fitted respectively by the least square method. And then, on the basis of above, the porous media model was applied to simplify the integral model of transformer so as to build a three-dimensional coupled porous media-pure fluid computational model. Meanwhile, the author calculated porosities and linear resistance coefficients of high voltage, middle voltage and low voltage windings. At the end of this paper, to obtain the relationship of oil flow distribution among three windings, the pressure loss was calculated by numerical simulation. Compared with the theoretical calculation results which was based on the part of experimental data, the numerical calculation results are in agreement with these data. And the error between them is within 6 percent. Therefore, the feasibility and accuracy of calculation method, which was used to calculate oil flow of natural oil circulation power transformer windings by the porous media model, has been verified as well. There are two kinds of conventional method to calculate oil flow, including an empirical method and an experimental method. On the one hand, although the former kind of method can be applied conveniently, it cannot consider all the factors so that the accuracy cannot be guaranteed. On the other hand, experimental method to calculate the flow can present high accuracy, but the cost of carrying out experimental study is high. Therefore, this paper presents another calculation method aimed to the flow resistance and oil flow by CFD. And the calculation method presented has higher calculation accuracy compared to above. Meanwhile, the new method provides a theoretical reference about oil flow distribution for the design of the power transformer.