A top-oil thermal model for power transformers that consider weather factors

The aim of this work is to develop a top-oil thermal model based on the thermal-electrical analogy and heat transfer principles that captures the thermal influence of prevailing winds and solar radiation which can be generally applied to large power transformers. The key improvements of the proposed thermal model are calculating the heat transfer coefficient of the radiator on the air side using the Nusselt number of combined forced and natural convection, and including the solar radiation as an addition heat source. The proposed model is validated with the operational measurements of 3 transformers that are comprised of a 120/240-MVA unit and two 90/180-MVA units. The results are also compared with the predictions based on the IEEE-Annex G model. The proposed model is more accurate over windy and summer periods as expected