Accuracy of furan analysis in estimating the degree of polymerization in power transformers

MSc (Eng), School of Electrical and Information Engineering, Faculty of Engineering and the Built Environmen

High Voltage Insulating Materials-Current State and Prospects

Studies on new solutions in the field of high-voltage insulating materials are presented in this book. Most of these works concern liquid insulation, especially biodegradable ester fluids; however, in a few cases, gaseous and solid insulation are also considered. Both fundamental research as well as research related to industrial applications are described. In addition, experimental techniques aimed at possibly finding new ways of analysing the experimental data are proposed to test dielectrics

Influence of Ester Liquids on Dielectric Strength of Cellulose Kraft Paper

Generally, impregnation of solid insulation is performed to increase the dielectric strength and reduce the dielectric losses of solid insulants. This increase in dielectric strength depends on the oil’s diffusion and dielectric properties. This paper investigates the diffusion behavior of mineral oil and ester fluids (synthetic, natural, and mixed) to understand the influence of oil diffusion on paper breakdown voltage. To better understand this phenomenal influence, cellulose insulation paper of different thicknesses has been considered. Wetting characteristics of various oil-paper insulation systems were investigated with and without thermal stressing. Thermal aging was carried out as per modified ASTM D1934 at 110 °C, 140 °C, 160 °C, and 185 °C respectively for 2 weeks. The wetting characteristics and influence of different oils on paper breakdown voltage were explicitly reported. It is inferred that paper wetting characteristics are attributable to the type of oil, the thickness of paper, and the aging factor of oil-paper insulation. Importantly, the increase in paper dielectric strength and diffusion behavior for ester fluids is found to be superior to that of the mineral insulating oils

A Novel Oil-immersed Medium Frequency Transformer for Offshore HVDC Wind Farms

In this project, a design of an oil type medium frequency transformer for offshore wind farm applications is proposed. The design is intended for applications when series coupling of the output of the DC/DC converters of the wind turbine on their secondary side is done to achieve a cost-effective high voltage solution for collecting energy from offshore wind parks. The focus of the work is on the insulation design of the high voltage side of a medium frequency transformer where the magnetic design constraints should also be satisfied.Above all, a proof of concept is made demonstrating a possible solution for the design of the transformer for such a DC/DC converter unit. The transformer suggested is using oil/paper as insulation medium. Furthermore, characterisation of an eco-friendly biodegradable transformer oil for this type of HVDC transformer application is made. Moreover, an introduction of reliable high frequency characterisation test methods to medium frequency transformer designers is made. In addition, the Non-Linear Maxwell Wagner (NLMW) relations are further developed to form a method for the development of an HVDC MFT transformer. All in all, the DC series concept has been further developed one step closer to pre-commercialization, i.e. from TRL 1 to about 2

Moisture dynamics in transformers insulated with natural esters

Mención Internacional en el título de doctorPower 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.Los transformadores de potencia son los componentes más importantes de un sistema eléctrico. Conocer su condición de funcionamiento es fundamental para maximizar el retorno de la inversión y reducir el coste total asociado a la operación y el mantenimiento del transformador. Como es bien sabido, la humedad tiene una fuerte influencia en el rendimiento del sistema celulosa-aceite en los transformadores de distribución y potencia, el contenido excesivo de agua acelera el envejecimiento del papel, aumenta la presencia de descargas parciales (PDs) y disminuye la resistencia dieléctrica del aislamiento. Tradicionalmente el sistema de aislamiento de un transformador de potencia se construye a partir de papel y cartón prensado impregnado en aceite mineral, que actúa como fluido dieléctrico y refrigerante. En los últimos años, el uso de ésteres naturales como una alternativa al aceite mineral ha aumentado considerablemente en transformadores de distribución y aunque menos habitual, se está comenzando a implementar su uso en transformadores de potencia. Los aceites o ésteres naturales se sintetizan a partir de una base vegetal, como es semillas de soja, girasol, colza, etc. Estos fluidos tienen mayor afinidad por el agua que los aceites minerales aislantes debido al hecho de presentar enlaces de hidrógeno en sus moléculas El comportamiento de la humedad en el interior del aislamiento del transformador es un aspecto clave en los estudios de capacidad de carga. Si el aislamiento opera seco la tasa de envejecimiento del papel es menor y por lo tanto aceptaría una mayor temperatura de funcionamiento. La celulosa y el aceite tienen un comportamiento muy diferente con respecto a la humedad; los materiales celulósicos son hidrófilos mientras que el aceite es altamente hidrofóbo. En consecuencia la mayor humedad en un transformador está contenido en su aislamiento celulósico, sin embargo la distribución de la humedad entre el papel y el aceite no es estática, sino que depende de la condición de funcionamiento del transformador y principalmente de la temperatura alcanzada por los diferentes materiales. La migración de humedad en el interior del aislamiento celulósico es un proceso complejo que implica la transferencia de calor y de difusión. Sin embargo, como la constante de tiempo de transferencia de calor es mucho menor que la constante de tiempo de difusión, la migración de humedad puede ser modelada como un proceso de difusión, utilizando la segunda ley de Fick. El coeficiente de difusión de materiales celulósicos depende de la concentración de humedad y por lo tanto la ecuación se convierte en no lineal y se necesita implementar un método numérico para resolverlo. En este trabajo, se ha estudiado la dinámica de humedad dentro de transformadores aislados con ésteres naturales. Diferentes experimentos han sido desarrollados para obtener las curvas de solubilidad de los ésteres naturales y curvas de secado de materiales celulósicos. Adicionalmente, se utilizaron modelos teóricos basados en elementos finitos y un proceso de optimización para calcular los diferentes coeficientes de difusión de humedad para diferentes materiales. Como resultado final de la tesis se propone un modelo multifísico que permite estudiar el comportamiento dinámico de la humedad en el interior del transformador, aislado con aceite mineral o con ester natural, en condiciones de funcionamiento reales.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Alfredo Ortiz Fernández.- Secretario: Carlos González García.- Vocal: Iliana Portugué