Numerical Evaluation of the Directed Oil Cooling System of a Mobile Power Transformer

Abstract Power transformers represent an important part of the capital investment in transmission and distribution substations. The cooling of the windings (electrical coil) depends on the convection of heat, enhanced by the forced circulation of oil through the windings and heat exchangers. The forced circulation of oil combined with the forced circulation of air in the heat exchangers is usually found in mobile transformers, whose compact structure is a challenge in terms of heat transfer rates. An improper design or fabrication problem associated with the assembling of the cooling system may result in an inefficient exchange of heat, which may lead to transformer failure from overheating or a reduction of the life span. In this context, the present work proposes a 2D mathematical model to simulate the winding cooling system of a 138x69-34x13.8 kV 25 MVA Mobile Power Transformer with the objective of investigating the causes of overheating. The model is implemented in CFD Ansys-Fluent® version 17.0 and validated with experimental data. It is evaluated the velocity and temperature distribution, and the identification of the hot spots on the transformer operating considering the nominal conditions for oil flow rate, inlet temperature, and power dissipated. The hot spot temperatures are compared with the current Brazilian Association of Technical Standards - NBR 5356-2: 2007 Power Transformers Part 2: Heating, 2007. After, some geometric and/or operational constraints are artificially imposed to the transformer. Their impact on the velocity and temperature fields, as well as the hot spot temperatures, are mapped in order to verify if they still respect the standard´s temperature limits and if imposed constraints may lead to the transformer failure. The numerical results demonstrated that the geometric imperfections of the disks, guides, or axial cooling ducts dimensions directly affect the oil flow and the temperature distributions, as well modify the position and greatly increase the temperature of the hottest spot in the winding, even extrapolating the ABNT NBR 5356:2 standard in some situations. This way, the proposed mathematical model can be used as a consistent tool for understanding the thermal behavior and, to investigate and map the occurrence of failures in mobile transformers.