With the increase of power load, the overheating problem of oil-immersed transformer in operation cannot be ignored. Therefore, it is necessary to propose reasonable heat dissipation enhancement measure for oil-immersed transformer in operation and study the influence of the measure on the internal temperature of transformer. The heat transfer process of oil-immersed transformer is analyzed, and the air-side heat transfer coefficient is pointed out to be the important parameter limiting the heat dissipation capacity of the transformer; In order to improve heat dissipation capacity, the measures based on cooling fans and falling film are proposed and the calculation methods for the air-side heat transfer coefficient are conducted respectively; Moreover, the top-oil temperature and hot-spot temperature under two measures are compared by the thermal-fluid coupling model of oil-immersed transformer. For the transformer operating at rated load and the ambient temperature of 25?C, after the cooling fans are adopted, the top-oil temperature and hot-spot temperature can be reduced by 22.4?C and 20.7?C; while after the falling film is adopted, the top-oil temperature and hot-spot temperature can be reduced by 31.2?C and 28.7?C, respectively. The results show that the falling film is a more effective heat dissipation enhancement measure, which can meet the higher load demand of oil-immersed transformer.
Obtaining an accurate hot-spot temperature of oil-natural air-natural (ONAN) transformer is important for evaluating its operation status and load capacity. To meet the overload requirement, the cooling fan is usually added to the operating ONAN transformer as the auxiliary cooling equipment. However, the hot-spot temperature evaluation methods in this case lacked discussion currently. To address this issue, an improved hot-spot temperature evaluation method for the ONAN transformer considering the thermal resistance on the air side of radiator was proposed in this paper. Subsequently, the calculation method for the key parameter of the improved method (the convective heat transfer coefficient on the air side of radiator) was conducted. The accuracy and superiority of the improved method compared with the IEEE loading guides were verified by the manufacturer's software. Finally, the maximum steady-state load of the ONAN transformer with cooling fan was achieved by the improved method. The results indicate that for the case of non-rated load, the improved method can provide a more accurate thermal evaluation of ONAN transformer under different cooling modes than the IEEE loading guides. The method proposed in this paper can provide a reference for selecting the cooling fan parameters and formulating the load control strategy of the ONAN transformer.
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