Porosity Strategies to Optimize Temperature Distribution in Oil‑Directed Power Transformers
P.Quintanilla(1,2), E.Sanchez(1), T.Laneryd(1), A.Ortiz(2) 1. R&D Department, Hitachi Energy, Cordoba (Spain) 2 Department of Electrical Engineering, E.T.S.I.I.T., University of Cantabria (Spain) 2026-06-27

Abstract

Power transformers rely on efficient oil circulationto ensure adequate cooling of their active parts and to prevent excessive thermal stresses that may compromise insulation life. In forced oil-directed (OD) cooling regimes, the flow distribution through winding ducts and oil guides plays a critical role in controlling local temperatures. However, conventional oil guide designs may promote flow stagnation and recirculation zones, particularly in the regions immediately above the oil guides, leading to the formation of localized hot spots on upper winding discs. In the cases where these effects happen, the cooling design is not optimum and affect to the lifecycle of the electrical machine.

This work presents a numerical investigation of oil guides modeled as porous media to mitigate these adverse flow phenomena. The porous approach is employed to replicate the hydraulic behavior of perforated oil guides, where discrete holes allow partial oil penetration through the guide cross-section. Given the two-dimensional nature of the computational domain as a practical compromise between accuracy and computational cost, the porous media formulation provides an effective and physically consistent approximation of the three-dimensional perforation effects.

Computational fluid dynamics (CFD) simulations are conducted to evaluate the impact of the porous guide representation on oil velocity fields and temperature distributions within the winding region. The results demonstrate a significant reduction in stagnation zones and recirculating flow structures, accompanied by a more uniform oil distribution and lower local temperatures in critical areas. The proposed methodology offers a practical and computationally efficient framework for improving thermal performance in OD-cooled power transformers.

Key words: Hot-Spot, Mineral Oil, Porosity, Power Transformer.

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