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Abstract
A disc-type winding of an oil-immersed power transformer is modeled with Computational Fluid Dynamics. Different approaches are implemented to evaluate the feasibility of the Boussinesq approximation: (i) constant fluid properties, (ii) variable viscosity and thermal diffusivity and (iii) temperature-dependent fluid properties. Temperature and flow distributions are reconstructed and put into relation with physical phenomena and model assumptions. Their comparison suggests that numerical results are fairly sensitive to the thermophysical model as long as the buoyancy force is a relevant component of the flow. Nonetheless, all the cases converge to very close predictions of the hot-spot value and location, with possibly positive implications for the use of reference parameters when deriving flow and heat transfer correlations for this topic.
Highlights
Temperature and coolant flow distribution are fundamental aspects in the design and operation of oilimmersed power transformers
Concluding remarks The feasibility of the Boussinesq approximation when modeling disc-type windings of oil-cooled transformers has been investigated by means of Computational Fluid Dynamics (CFD)
The Boussinesq approximation cannot be considered valid in its canonical formulation throughout the temperature range of the problem, as dictated by the relations used for the thermophysical properties of the oil
Summary
Temperature and coolant flow distribution are fundamental aspects in the design and operation of oilimmersed power transformers. Estimating the value and location of the maximum temperature occurring in winding paper insulation, the so-called hot-spot, is of primary importance since it dictates transformers reliability and life expectancy [1]. International Standards [1, 2] give practical indications for the calculation of the hot-spot temperature during steady state or transient operation. Those methods often prove to be quite unreliable [3, 4], since they are based on average temperatures of the windings and of the oil measured in a few specific points of the cooling circuit.
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