Adaptation of the impregnation conditions of cellulosic transformer solids to the use of natural esters

Transformers’ operation and end of life mainly depend on its solid insulation. It is subjected to different stresses that cause the cellulose degradation. The ageing process is highly dependent on the moisture content and the correct impregnation. In the impregnation process, the dielectric coolant fluid is absorbed by the rest of the transformer porous materials, especially the insulating cellulosic materials, conditioning their properties. Due to the transition to a more sustainable energy system, new lines of research that explore alternatives to traditional mineral oils, as esters, are being developed. However, it is necessary to study their behaviour in the different manufacturing processes and during the operating life of the transformers for their future application. In this paper, the changes to be made in the impregnation process to include a soya-based liquid, taking the place of mineral oil, and considering as solid dielectrics Crepe Paper, Diamond Dotted Paper (DDP), Kraft and Presspaper (PSP) are studied.Work supported in part by the Spanish Ministry of Science and Innovation (National Research Project Asset Management of Biodegradable-Fluid-Based Transformers under Grant PID2019-107126RB-C22/AEI/ 10.13039/501100011033), by the Universities and Research Council of the Government of Cantabria (Project ‘‘Biodegradable Fluids in Electrical Power Transformers: Solid Dielectric Impregnation and Thermal Modeling with Thermal Hydraulic Network Models (THNM)’’ under Grant VP32, 2019-2), and by the University of Cantabria (Industrial Doctoral Program 2016, Scholarship DI13)

Study of the impregnation process of cellulosic materials in different dielectric liquids

RESUMEN: Los transformadores de potencia utilizan aceites como aislantes y refrigerantes de sus partes activas. Tradicionalmente se han utilizado aceites de origen mineral, derivados del petróleo, como líquido dieléctrico en este tipo de máquinas. Sin embargo, a causa a su inflamabilidad y al impacto medioambiental que pueden producir en caso de fuga, que condicionan la seguridad tanto de la instalación como de su entorno, se han propuesto aceites alternativos que cumplan con la misma función refrigerante y aislante a los convencionales. Estos aceites, ésteres de origen vegetal o sintético, son un tipo de fluido que resuelven estas problemáticas encontradas con los de origen mineral. Sin embargo, es necesario estudiar su comportamiento en los diferentes procesos de fabricación y durante la vida útil de los transformadores para su futura integración en el mercado. En el proceso de impregnación se da lugar a la puesta en contacto del líquido dieléctrico con el resto de los elementos del transformador, especialmente con los devanados recubiertos de material aislante, como una de las fases del proceso de fabricación de la máquina eléctrica. Este estudio se centra en la influencia del fluido utilizado en dichos materiales aislantes, de naturaleza celulósica, encargados de aislar las partes activas del transformador, dotando al conjunto una elevada rigidez dieléctrica. Se pretenden identificar los cambios necesarios a realizar en el proceso de impregnación para incluir los ésteres, tomando el lugar del aceite mineral, de forma que se equiparen los tiempos y la calidad habitualmente lograda durante la impregnación con los aceites tradicionales. Para ello, se realizan ensayos experimentales que simulan el proceso de impregnación real llevado a cabo en la industria, variando las condiciones de temperatura. Se estudia, la influencia de la temperatura, tanto en las características de los líquidos dieléctricos, como la viscosidad y la tensión superficial, como en el propio proceso de impregnación con los distintos aceites, que son comparados con el mostrado por el aceite mineral. Se comprueba que el descenso del valor de la viscosidad, que se consigue incrementando la temperatura del proceso, tiene gran influencia en el proceso de impregnación, por encima de otras propiedades como la tensión superficial y el ángulo de contacto. Gracias a esto, se logran niveles de impregnación similares a los del aceite mineral a temperatura ambiente con ésteres calentados, logrando adaptar el proceso tradicional a estos fluidos.ABSTRACT: Power transformers use oils as insulators and coolants for their active parts. Oils of mineral origin that derived from petroleum have traditionally been used as dielectric liquid for these types of machines. However, due to their flammability and the environmental impact that they can produce in the event of leakage, which affects both the safety of the installation and its surroundings, alternative oils have been proposed. These alternative oils obtain the same cooling and insulation function as conventional oils. These oils, esters of vegetable or synthetic origin, are a type of fluid that avoid the problems that are encountered with those of mineral origin. However, it is necessary to study their behaviour in the different manufacturing processes and during the useful life of the transformers for their future integration into the market. In the impregnation process, the dielectric liquid comes into contact with the rest of the transformer elements, especially with the windings covered with insulating material, as one of the phases in the manufacturing process of the electrical machine. This study focuses on the influence of the fluid used in these insulating materials of cellulose nature, which are responsible for insulating the active parts of the transformer, giving the whole a high dielectric strength. The aim is to identify the necessary changes to be made in the impregnation process to include esters, taking the place of mineral oil, in order to match the times and quality normally achieved during impregnation with traditional oils. Experimental tests are accomplished to simulate the real impregnation process carried out in industry, varying the temperature conditions. The influence of temperature is studied, both on the characteristics of the dielectric liquids, viscosity and surface tension as well as on the impregnation process itself with the different oils. These are compared with those shown by the mineral oil. It is found that the decrease in viscosity value, which is achieved by increasing the process temperature, has a greater influence on the impregnation process than other properties such as surface tension and contact angle. In result to this, impregnation levels similar to those of mineral oil at room temperature are reached with heated esters, making it possible to adapt the traditional process to these fluids.Grado en Ingeniería Eléctric