Dielectric and mechanical assessment of cellulosic insulation during transformer manufacturing

Due to the impact of cellulose of paper insulation on transformer life, it is imperatire to remove moisture from the oil and the solid insulation. Several techniques have been implemented during manufacturing of power transformers to reduce water content in transformers. These drying processes can involve different costs and time, and they can damage the insulation paper. In this work, a drying process has been implemented in the laboratory trying to simulate the most aggressive conditions that can be suffered by the paper in transformer manufacturing in a real industry. Once the moisture content of papers was lower than 0.5%, the effect of the drying process on paper degradation was evaluated using the analysis of mechanical and dielectric properties and the degree of polymerization. Different commercial papers were studied to quantify the possible degradation induced by the drying process. The results of the mechanical strength study showed a reduction on the degree of polymerization from 1100 to 850 after 4 days of drying. The dielectric analysis of the samples showed different behavior in one of the solids evaluated and it was also found a decreased hygroscopic capacity of degraded samples in comparison with new samples.This research is under the National Research Project “Gestión del Ciclo de Vida de Transformadores Aislados con Fluidos Biodegradables” (PID2019-107126RB-C22/ AEI /10.13039/501100011033). The authors also wish to thank “Fundación Iberdrola” for its financial support for the research project: “Análisis de las Propiedades Dieléctricas de Aislamientos Sólidos Impregnados con Líquidos Dieléctricos”. C. Méndez would like to acknowledge the Spanish Ministry of Science, Innovation and Universities for the financial support for the FPU grant (FPU19/01849)

Study of the impregnation of power-transformer cellulosic materials with dielectric ester oils

The application of alternative dielectric oils as esters in power transformers is hindered by the lack of knowledge regarding their properties and respecting which are the best techniques to ensure their proper performance. In this sense one of the fields needing an impulse is the impregnation processes of transformers cellulosic materials with these alternative oils, currently impregnated in most of the cases with mineral oils. This paper studies the impregnation behavior of eight usual dielectric solids, with two esters and a traditional mineral oil. Empirical equations of the impregnation evolution with time have been obtained, from these the rigid cellulosic materials present in the transformers and the viscosity of the dielectric oils have been identified as the key materials and properties to consider during impregnation. An adaptation of the current impregnation processes to the alternative oils have been proposed by increasing their temperature from ambient temperature up to 61-74°C, depending on the viscosity of the oil used.This work was supported in part by the Spanish Ministry of Science and Innovation by the National Research Project Asset Management of Biodegradable-Fluid-Based Transformers under Grant PID2019-107126RB-C22/AEI/ 10.13039/501100011033, in part by the Universities and Research Council of the Government of Cantabria by the Grant ‘‘Biodegradable Fluids in Electrical Power Transformers: Solid Dielectric Impregnation and Thermal Modeling with Thermal Hydraulic Network Models (THNM)’’ under Grant VP32, 2019-2, and in part by the University of Cantabria through the Industrial Doctoral Program 2016, Scholarship DI13

Analysis of the Impregnation Process of Cellulosic Materials by Ester-Based Insulating Liquids

RESUMEN: El aislamiento eléctrico de los transformadores de potencia en baño de aceite se constituye de la combinación de un líquido dieléctrico y sólidos celulósicos, que deben ser completamente impregnados por el líquido dieléctrico. Esto garantiza que las cavidades presentes en el aislante sólido sean rellenadas por el líquido, confiriendo así alta rigidez dieléctrica al conjunto y evitando descargas parciales que puedan acelerar considerablemente el deterioro del aislamiento. Los aceites minerales son los líquidos dieléctricos más utilizados para este propósito, a pesar de dos grandes inconvenientes: su bajo punto de inflamación y peligrosidad medioambiental (ecotoxicidad y baja biodegradabilidad). Para hacer frente a estas dos importantes limitaciones, desde hace años se han desarrollado distintos líquidos dieléctricos alternativos entre los que se encuentran los ésteres naturales y sintéticos. Sin embargo, estos líquidos poseen una mayor viscosidad que perjudica el proceso de impregnación. Surge así la necesidad de estudiar el proceso de impregnación con ésteres, aumentando el conocimiento existente y obteniendo datos cuantitativos valiosos para la industria, para facilitar la sustitución del aceite mineral por estos fluidos alternativos. En este trabajo se han realizado en el laboratorio distintos ensayos experimentales de impregnación de tres líquidos dieléctricos (aceite mineral, éster sintético y éster natural) en diversos materiales celulósicos y a diferentes temperaturas. Para complementar estos datos se ha medido el efecto de la temperatura en la tensión superficial y viscosidad de los líquidos dieléctricos, sus principales propiedades con influencia en la impregnación. La impregnación con los ésteres es considerablemente más lenta que con el aceite mineral, como consecuencia de su mayor viscosidad. Elevando la temperatura se consigue acelerar la impregnación al reducirse la viscosidad, aunque afecta negativamente a la presión capilar. Con los datos recopilados se ha realizado un estudio comparativo de las características de impregnación de diferentes combinaciones de sólido y líquido dieléctrico, y se han planteado ecuaciones para estimar el tiempo necesario para completar la impregnación de cada una de estas combinaciones en función de la temperatura de impregnación. Los resultados de impregnación han sido comparados con un modelo analítico elaborado a partir de los datos experimentales obtenidos en el laboratorio, observándose buena concordancia entre ambos. Finalmente, para completar el estudio se ha planteado un primer acercamiento a la elaboración de un modelo numérico del proceso de impregnación. Este modelo se ha construido con la herramienta COMSOL Multiphysics y permite observar la influencia de la temperatura en la capilaridad de los distintos líquidos dieléctricos estudiados.ABSTRACT: The electrical insulation of oil-immersed power transformers is a combination of insulating liquid and cellulosic solids, which requires full impregnation in the insulating liquid. This ensures that the air cavities present in the solid insulator are filled by the liquid, conferring high dielectric strength to the assembly and avoiding partial discharges that can significantly accelerate the deterioration of the insulation. Mineral oils are most commonly used the insulating liquids for this purpose, despite two major drawbacks: low fire point and environmental hazard (ecotoxicity and poor biodegradability). To deal with these two important limitations, different alternative insulating liquids have been developed, including natural and synthetic esters. However, these liquids have a higher viscosity that impairs the impregnation process. This motivates the need to study the impregnation process with esters, bolstering existing knowledge and obtaining valuable quantitative data for the industry, to facilitate the replacement of mineral oils with these alternative liquids. In this work, different experimental tests of impregnation of three insulating liquids (mineral oil, synthetic ester and natural ester) in various cellulosic materials and at different temperatures have been carried out in the laboratory. To complement these data, the effect of temperature on the surface tension and viscosity of insulating liquids has been measured, as these are the main insulating liquid properties that affect the impregnation. Impregnation with esters is considerably slower than with mineral oil, as a consequence of its higher viscosity. Raising the temperature can accelerate the impregnation by reducing the viscosity, although it negatively affects the capillary pressure. With the data collected, a comparative study of the impregnation characteristics of different combinations of solid and insulating liquid has been carried out, and equations have been proposed to estimate the time required to complete the impregnation of each of these combinations at different impregnation temperatures. These impregnation results are consistent with an analytical model developed from the experimental data obtained in the laboratory. Finally, to complete the study, a first approach to the elaboration of a numerical model of the impregnation process has been proposed. This model, built using the simulation software COMSOL Multiphysics, shows the influence of the temperature on the capillarity of the different insulating liquids studied.Grado en Ingeniería en Tecnologías Industriale