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.

Mineral oil has been frequently used in most of high voltage transformers. However, this oil has started to be replaced by vegetable oils. Additionally, different authors have found that mineral oil-based nanofluids have a potential in improving insulating properties of transformer oil. In order to improve the characteristics of a commercial vegetal oil, this work has used TiO2 nanoparticles to obtain a nanofluid whose ageing behavior at 150ºC for 300 hours has been studied.

Part of the work was performed during secondments between BEST Transformers and the University of Cantabria executed in the framework of the BIOTRAFO project ''Raising knowledge and developing technology for the design and deployment of high performance power transformers immersed in biodegradable fluids '', H2020-MSCARISE-2018'', H2020-MSCARISE- -823969, 2019

Abstract.Transformers have an insulation system based on dielectric oil and paper. Several factors as shortcircuits, high load, or overheating influence the aging of the insulating system. Thus, these materials are decomposed during transformer operation, and its state determines the status of the transformer. The degree of polymerization of dielectric paper is an indicator of its real condition. The concentration of furanic compounds in oil offer an estimation of paper degradation. Moreover, these compounds are easy to measure from oil samples. This paper review equations which relate one or more properties of the insulating system with the life expectancy of the transformer. These equations were used in a fleet of transformers in order to estimate the accuracy of these mathematical models. Results show that these methods achieve the most reliable estimation when the transformers have operated less than 8 years. Nevertheless, when the transformers are older, the estimation of lifetime based on furanic compounds is quite erratic and should be improved.