Moisture dynamics in transformers insulated with natural esters

• Autores: Rafael David Villarroel Rodríguez
• Directores de la Tesis: Belén García de Burgos (dir. tes.), Diego F. García Gómez (dir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2015
• Idioma: inglés
• Tribunal Calificador de la Tesis: Alfredo Ortiz Fernández (presid.), Carlos González García (secret.), Iliana Portugués Peters (voc.)
• Materias:
  • Ciencias tecnológicas
    • Tecnología de la instrumentación
      • Instrumentos eléctricos
• Enlaces
  • Tesis en acceso abierto en: e-Archivo
• Resumen

  • Power transformers are one of the most important components in an electrical system. Knowing their condition is essential to meeting the goals of maximizing the return of the investment and reducing the total cost associated with transformer operation. As is well known, moisture has a strong influence on the performance of celluloseoil systems in power and distribution transformers. An excessive water content accelerates the paper ageing rates, increases the presence of partial discharges (PDs) and decreases the dielectric strength of the insulation. Traditionally the insulation system of a power transformer is composed of oil impregnated paper and pressboard as well as mineral oil acting as dielectric fluid and coolant. In recent years, the use of natural esters as an alternative to mineral oil has increased considerably in distribution transformers and, although less usual, some experiences are starting to be reported on its use in power transformers. Natural esters are synthesized from a vegetable base, as the seeds of soya, sunflower, rapeseed, etc. They have greater affinity for water than mineral oils due to the fact of hydrogen bonds existing on molecules of natural esters. The behaviour of moisture inside the transformer insulation is a key aspect in loading studies. If the insulation operates drier the ageing of the paper rate is lower, and thus higher operating temperatures would be acceptable for solid insulation. Cellulose and oil have a very different behaviour with regard to moisture; cellulosic materials are hydrophilic while oil is highly hydrophobic. In consequence water in transformers is mainly contained in cellulosic insulation. However, the distribution of moisture between paper and oil is not static, but depends on the transformer operation condition, and specially on the temperature reached by the different materials. Moisture migration inside cellulosic insulation is a complex process involving heat and mass transfer phenomena. However, as the thermal time constant is much smaller than the diffusion time constant, moisture migration can be modeled as a diffusion process, using Fick’s second law. The diffusion coefficient of cellulosic materials depends on moisture concentration, and thus Fick’s equation becomes non-linear and the application of a numerical method is required to solve it. In this work, the moisture dynamics inside transformers insulated with natural esters have been studied. Different experiments have been developed to obtain solubility curves of natural esters and drying curves of cellulosic materials. In addition, theoretical models based in finite elements, and an optimization process were used to obtain the moisture diffusion coefficients for different materials. As a final result of the thesis, a multi-physical model is proposed that allows studing the dynamic behavior of moisture inside a transformer, insulated with mineral oil or with natural esters, under real operation.