ABSTRACT

This contribution presents the early results of a R&D collaboration established between the University of Cantabria and the Power Transformer Manufacturer EFACEC. This paper tests two different techniques of steady state thermal modelling applied to power transformer windings, Computational Fluid Dynamics (CFD) and Thermal-Hydraulic Network Modelling (THNM). The state of the art of thermal modelling demonstrates that these techniques have been used to calculate both average and hotspot winding temperatures by solving the winding temperature and flows profiles within the winding. THNM models have worse accuracy than CFD in the predicted results. The improvement of these THNM models is a topic of study in transformer thermal modelling. The first goal of this paper is to test the accuracy of a new calibrated THNM model of a disc-type winding immersed in mineral oil. Then, this THNM model is tested with esterbased liquids, such as a natural ester and a synthetic ester, to determine if it can be applied to these liquids without further calibrations. Finally, the cooling performance of both type of liquids is compared using only the THNM model results. The results of this work show that the THNM model developed herein gives good estimations of temperatures compared to those obtained with CFD for both types of liquids. Also, the use of alternative fluids leads to lower temperatures when considering the same oil flow rate and temperature as inlet boundary condition.

No abstract

In this paper, an experimental and numerical research was conducted to study the aging effect on the cooling performance of a natural ester habitually used in power transformers. A monitored experimental platform has been used to observe the temperature increases with the aging of the ester. Three samples with three aging levels (fresh oil, aged oil and aged oil with paper) were tested at three load levels, C=0.72, C=1 and C=1.3 (9 tests). The two aged samples (6 liters each) were aged in an accelerated way within an oven: 1 week at 150 ºC. Also, some physicochemical properties of the insulation system were determined. In the case of the liquid, viscosity, moisture, acidity, dissipation factor (90ºC) were measured. The degree of polymerization and the moisture were measured in paper samples. A 2D numerical model has been developed with COMSOL Multiphysics to replicate the thermal-hydraulic behavior of the samples in the platform. That is, the idea is to determine the temperature and velocity distributions of the different samples in order to analyze their differences and to compare their cooling capacity. At first sight, the aging affects negatively to the cooling capacity of the new alternative liquid based on natural esters.

The thermal behavior of an industrial Low Voltage non-segregated three-phase busduct was analyzed by means of the comparison of a 3D numerical model with experimental results. This model has been carried out using COMSOL Multiphysics, software based on finite element method. The numerical model replicates the short-circuit test, using the same geometry configuration and the boundary conditions of the laboratory in which this assay was carried out. The standard IEC 61439 was applied, both in test and model, in order to obtain the steady state temperatures in several parts of the busbar system. As a result of the data comparison can be concluded that the experimental test was replicated by the numerical model with sufficient accuracy. The temperature differences between simulation results and those of the heating tests were in a narrow range. On the other hand, a sensitivity analysis was carried out with the intention to study the influence of sensors positioning on the temperature measurement in the laboratory test, thus concluding no high precision was needed in the location of the temperature meters. As a final conclusion of this study, it is needed to point out that the numerical model has the enough exactness to be used in the first steps of the busbar design.