Thermal Simulation of Underground Power Distribution Transformers

This paper presents the differential equations that model the heat flow of power transformers located in underground vaults. The solution procedure results in the hot spot winding temperature and its time variation through the daily load cycle. A set of simulations is carried out to evaluate the transformer´s thermal behavior during overload and contingency conditions. In such simulations the loss of life is estimated to objectively evaluate the technical and economical performance of the power transformer. The air flow in the ventilation system is considered, to search the optimal conditions of heat transfer to the external environment. The paper also considers the conditions in which the forced ventilation system can be switched off.