Investigation of Liquid Temperatures and Velocities at Winding Inlet in Natural Cooled Transformers Through Complete-Cooling-Loop Based CFD Simulations and Experiments

The liquid temperatures and velocities are of great importance for estimating the hot-spot temperature (HST) within the transformer windings. For liquid natural cooled power transformers (ON/KN), the liquid temperatures and velocities can only be obtained by modelling the complete-cooling-loop (CCL), which refers to the insulating liquid circulation between the windings and the radiators. In this paper, a CCL based computational fluid dynamics (CFD) model was developed for determining the liquid temperatures and velocities in the natural cooling mode. The validities of the CCL CFD simulations were verified by conducting experiments under different loading conditions, at different thermal heads and of different insulating liquids. The experimentally verified simulation results showed that the top liquid temperature increases exponentially against the power loss, whereas the bottom liquid temperature increases linearly against the power loss. The liquid velocity is in an approximately linear relationship against the square root of the product of the power loss and the thermal head. Moreover, the thermal performances of different insulating liquids were investigated. The dominating material property for the liquid thermal performance of an ON/KN transformer is the dynamic viscosity. For different liquids, the closer the dynamic viscosity, the better matching of the liquid temperatures and velocities.