Thermal Analysis of Natural Cooling Type Distribution Transformer Retrofilled With Natural Ester Oil

Retrofilling distribution transformers with natural ester has attracted increasing interest among the utilities. Many retrofilled transformers have layered winding structures and natural cooling mode. The thermal performance of such types of transformers has not been thoroughly studied. To ensure the temperature rise in a layered winding type retrofilled transformer complying with IEC-60076-14, it is necessary to analyze the temperature distribution inside the transformer. In this article, the temperature distribution of a layered winding natural cooling type distribution transformer is investigated under a retrofilling scenario. The computational fluid dynamics (CFD) analysis is performed to compare the cooling performance of the transformer filled with mineral oil and that of the transformer filled with natural ester oil. The temperature distribution along the height of the transformer winding is investigated through fiber optic sensors. Moreover, the dimensional analysis method is applied to understand the influence of several parameters (oil characteristics and dimension of cooling ducts) on temperature distribution and hot spot factor. The findings of this article can support the decision-making of retrofilling of layered winding natural cooling mode transformers.