The Oil Flow Distribution And Thermal Field Research In Natural Oil Circulation Power Transformers

As the single capacity and voltage level of power transformers become higher and higher, the first problem is that the loss of the transformer becomes larger and larger, and causing the local overheating problem. Meanwhile, the oil immersed cooling way is mainly adopted by large power transformer, and oil-immersed transformer with natural oil circulation cooling way is more than 80 % in order to prevent oil flow electrification. The main motion of flow for natural oil circulation transformer is the thermal buoyancy lift compared with oil-forced circulation power transformer, the cooling path of winding and heat sink self-contained, then the heat exchange is more complex, and the hot-spot temperature-rise of transformer is mainly determined by the losses and internal oil flow. Therefore, there is important theoretical and practical significance to investigate the oil flow distribution and thermal field research in natural oil circulation power transformer. The thorough research and systematic investigation are proposed from three aspects, including the calculation of losses, the calculation and analysis of oil flow properties and the research of hot-spot temperature-rise, the main work is as follows:(1) For the calculation of transformer losses and the local overheating problems, the three-dimensional(3-D) frequency domain nonlinear eddy current field calculation model which considers the skin effect is established, the magnetic leakage field and stray losses of structure parts in transformer are computed, and compared with the experimental data to verify the correctness of the proposed approach; the influence of the novel plate embedded magnetic shielding upon the magnetic leakage field and stray losses of metal structure parts is analyzed comprehensively, on the basis of magnetic leakage field and losses analysis, the 16 sets optimization analysis measures of plate embedded magnetic shielding geometry is investigated, including the thickness of plate embedded magnetic shielding and the distance between the plate embedded magnetic shielding and winding. The findings of this study provide the basic guidelines for the technology industrialization of plate embedded magnetic shielding transformer.(2) For the internal heat transfer and flow characteristics of natural oil circulation transformer, the oil flow model of winding area in natural oil circulation transformer is established by finite volume method, the oil flow distribution and oil flow characteristics under various operating conditions are analyzed. The effect is researched to determine the motion between the single winding and the whole model, then the calculation model is more reasonable and the computation accuracy is improved. The influence of different block number, different batten thickness upon the oil flow characteristics of winding is comprehensive considered, the basic rules are given for the optimization of transformer winding area.(3) The 3-D finite volume model of the transformer winding area for temperature-rise calculation is established, the hot-spot temperature-rise of winding area is determined based on electromagnetic field, fluid field and thermal field coupling approach. The influence of temperature upon material properties is considered in this method, the magnetic-fluid-thermal coupling phenomenon is reasonable description, and the calculation results of hot-spot temperature-rise are consistent with the experimental ones. On this basis, the influence of different structure upon temperature-rise of winding area is studied, including different block number and different batten thickness, the findings give significant references for the transformer thermal design.(4) The whole oil flow model of transformer is established, the whole oil flow characteristics of transformer is obtained by porous media model simplified, the convective heat transfer coefficients of structure parts surface are obtained, then the electromagnetic field and thermal field coupling analysis is used to determine the size and location of hot-spot temperature-rise in structure parts, and compared with the experimental data, this analysis gives novel idea in the calculation of convective heat transfer coefficient and the determination of hot-spot temperature-rise in structure parts.(5) Three kinds of cooling oil model of embedded magnetic shielding are presented, the temperature and oil flow properties are analyzed, this gives some basic references for the cooling design of embedded magnetic shielding; the optimization design of the whole magnetic circuit and the winding eara oil circuit for a prototype is computed by the design methods in this paper, the structure parts and winding temperature-rise are improved in final, and the optimization transformer has passed the test evaluation.