Hot spot temperature for 154 kV transformer filled with mineral oil and natural ester fluid

This paper deals with the hot spot temperature of a transformer when filled with mineral oil, and later on, with natural ester fluid. 14 optical fiber sensors were installed in the 154 kV, 15 MVA transformer filled with mineral oil, with the hot spot temperature measured during the heat run tests. The transformer was retrofilled with natural ester fluid, after which the hot spot temperatures were measured again. For the rated power, the hottest spot temperature was 105.9 °C when the transformer was filled with mineral oil and 129.4 °C when it was filled with natural ester fluid. The hot spot temperature increased by 23.5 °C in the transformer filled with natural ester fluid. This paper applies the hot spot temperature measured using optical fiber sensors to the life curve in natural ester by McShane et al. The life curve of the thermally upgraded kraft paper in natural ester fluid was compared with that in mineral oil, as suggested by the IEEE guide. The life of the transformer filled with natural ester fluid was similar to that filled with mineral oil. Therefore, transformers filled with natural ester fluid do not require increase of cooling pump capacity and surface area of the radiator or reduction of the load to obtain the same life as that of the mineral oil.