Avaliação do sistema de arrefecimento por óleo dirigido de um transformador de potência móvel por meio de dinâmica dos fluidos computacional

Abstract

Power transformers represent an important part of capital investment in transmission and distribution substations. The cooling of the windings (electric coil) depends on the heat exchange, which can be intensified by the forced circulation of the cooling oil throughout the windings as well as in the external heat exchangers. An ineffective heat exchange between the cooling oil and the windings can shorten the lifespan of the transformer or even lead to its sudden failure due to overheating. The development of a mathematical model, and its solution throughout a numerical method, that involves the mass, momentum and energy conservation equations, contributes to the evaluation of the fluid dynamics behavior, as well to the improvement of the transformer´s design in order to prevent and identify manufacturing and/or assembling issues that may impact on the cooling system performance. Thus, the present work proposes a numerical study of the winding cooling system of a 138x69-34x13.8 kV 25 MVA Mobile Power Transformer (named TM1), which will be allowed evaluating the thermal performance of the heat exchange system operating under rated conditions and under irregular flow conditions or geometric issues. A two-dimensional steady state mathematical model is proposed to simulate the fluid flow and heat transfer between the oil and the windings. To obtain the numerical solutions, the Ansys-Fluent® software was used. The validation of the mathematical and computational analysis is initially based on with experimental results obtained in the literature. After validation, the specific study of TM1 is started, initially considering the nominal values for oil flow rates, temperatures, and dissipated power/heart. Subsequently, some restrictions and imperfections in relation to the input and geometric parameters are artificially imposed to the model, and their results allow us to evaluate their impact on the thermal performance of the TM1. Among the simulated cases, it was found that some geometric deviations and secondary imperfections of the manufacturing process can directly affect the oil flow and temperature fields, as well the location of the so-called hot spots in the winding, exposing certain areas to temperatures above of the admissible. In this way, for each disturbance imposed, a mapping of the possible vulnerabilities of the winding is obtained.