A Study on the Cooling Performance of 18.62 MVA ONAF Transformer with Mineral and Vegetable Oil as Cooling Medium

Abstract

The transformer is an essential device in the power grid system, which guarantees electric energy's safe and stable transmission. It enables electric energy to operate more economically and safely in the grid. The modern power network is developing rapidly, and the capacity of the power grid continues to increase. Compared with the natural oil circulation transformer, the forced oil circulation transformer has an external fan with a better heat dissipation effect. Studies have shown that the most critical factor affecting the insulation life of oil-immersed transformers is the hot spot temperature of the windings. Therefore, it is essential to study the internal temperature field distribution of oil-immersed transformers. This study uses the finite volume method combined with the Fluent software to perform numerical simulation analysis on the three-dimensional temperature field and hot spot temperature of the 18.62 MVA ONAF large oil-immersed transformer produced by HanChang Company. Firstly, the transformer's internal structure was simplified reasonably, and an ONAF transformer simulation analysis model was established. Secondly, it compares and analyzes the similarities and differences between the temperature field and the oil flow inside the transformer when mineral oil and vegetable oil are used as the cooling medium. The feasibility of replacing mineral oil with vegetable oil as the cooling medium of oil-immersed transformer is discussed. Finally, the IEC mathematical model is compared with the simulation result, which verifies the reliability of the simulation result. The research results show that in the high and low voltage windings, the temperature is unevenly distributed, and the temperature at the bottom and top is relatively low, the temperature in the middle is rather high, and the temperature of the low voltage winding is higher than of the high voltage winding. In addition, the hot spot temperature of the two windings rises gradually along the axial direction. It then gradually decreases, and the hot spot temperature appears in the middle of the low-voltage winding. By comparing mineral oil and vegetable oil when used as a cooling medium, this article finds that when mineral oil is used as a cooling medium, the transformer's internal hot spot temperature and top oil temperature are lower than vegetable oil. At the same time, when mineral oil is used as a cooling medium, the internal oil flow velocity is greater than that of vegetable oil. However, because vegetable oil has better physical properties, it is feasible to replace mineral oil as a cooling medium. In summary, these studies in this article have specific guiding significance for optimizing the oil-immersed transformer structure and selecting the cooling medium and provide a basis for the positioning of the optical fiber to measure the hot spot temperature of the transformer winding.