Thermal evaluation optimization analysis for non-rated load oil-natural air-natural transformer with auxiliary cooling equipment

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.