Study on the contamination rate of mineral oil in natural-ester retrofilled transformers

The replacement of mineral oil (MO) by ester (retrofilling) is a practice used in power transformers to reduce fire and environmental risk. Although measures are taken to remove traces of oil on the paper, a certain amount of MO remains, which migrates over time. This article presents a study to evaluate the migration of MO from paper to ester using techniques such as the iodine value or the measurement of permittivity and dielectric losses.This work was supported in part by the Spanish Research Agency under Project PID2019-107126RB-C21/AEI/10 and Project 13039/501100011033, and in part by the European Union's Horizon 2020 Research and Innovation Program under Grant 823969

Application of biodegradable fluids as liquid insulation for distribution and power transformers

In the last years the use of biodegradable fluids as liquid insulation for distribution and power transformers is becoming more common. The main biodegradable fluids are the natural and synthetic esters, although biodegradable hydrocarbons have been recently proposed as well. Biodegradable fluids have a much lower environmental impact than mineral oil, limiting the risk of soil contamination in case of leaks what makes them a suitable solution for some applications, as off-shore transformers or railway transformers. Additionally, these fluids have higher flash point than conventional mineral oils what reduces dramatically the risk of fire and collateral damage derived from explosion and fire. Despite of these advantageous factors, there are still some aspects that hinders the spread of their use, such as the difference on thermal properties, the lack of accepted maintenance procedures and the price. This paper presents the current situation of biodegradable insulating fluids, analyzing some of their properties and discussing the aspects that are still to be investigated to make them a real alternative to petroleum-based fluids.This work was supported in part by the European Union's Horizon 2020 Research and Innovation Programme through the Marie Sklodowska-Curie under Grant 823969, in part by the Ministry of Universities and Innovation Economy through Grants DPI2015-71219-C2 and PID2019-107126RB-C21

Influence of Nanoparticles on the Degradation Processes of Ester-Based Transformer Insulation Systems

The use of nanofluids as dielectric liquids for transformer insulation has been widely investigated during the last decade. A number of authors have performed extensive studies on liquids produced from different types of nanoparticles and base fluids, providing evidence of their good dielectric properties. Nevertheless, nanodielectric fluids are still at the research stage, and they are far from being a solution that can be applied to real transformers. One of the aspects that might be clarified is their compatibility with the rest of the materials present in the transformer and their behavior throughout the life of the equipment. This paper studies the aging process of cellulose impregnated with an ester-based nanofluid and compares it with the process that takes place when the impregnation liquid is a natural ester. Accelerated aging experiments were performed, and physical and chemical characterization of the process with several analytical techniques was carried out. The mechanical degradation of the cellulose was studied in terms of tensile strength, and the evolution of moisture in the paper and oil was monitored throughout the aging process. The study was completed with FTIR and XPS tests aimed at studying the chemical changes of the materials during the aging process. The experimental results suggest that the degradation rate of the cellulose is not significantly affected by the presence of nanoparticles. However, the XPS study revealed that the chemical reactions involved in the degradation processes of both types of insulation might differ. Several mechanisms are proposed in this work.This work was supported by the Spanish State Research Agency under grant PID2019- 107126RB-C21/ AEI/10.13039/501100011033 and by the Spanish Ministry of Economy and Competitiveness under grant DPI2015-71219-C2-2-R

Application of biodegradable fluids as liquid insulation for distribution and power transformers

In the last years the use of biodegradable fluids as liquid insulation for distribution and power transformers is becoming more common. The main biodegradable fluids are the natural and synthetic esters, although biodegradable hydrocarbons have been recently proposed as well. Biodegradable fluids have a much lower environmental impact than mineral oil, limiting the risk of soil contamination in case of leaks what makes them a suitable solution for some applications, as off-shore transformers or railway transformers. Additionally, these fluids have higher flash point than conventional mineral oils what reduces dramatically the risk of fire and collateral damage derived from explosion and fire. Despite of these advantageous factors, there are still some aspects that hinders the spread of their use, such as the difference on thermal properties, the lack of accepted maintenance procedures and the price. This paper presents the current situation of biodegradable insulating fluids, analyzing some of their properties and discussing the aspects that are still to be investigated to make them a real alternative to petroleum-based fluidsThis work was supported in part by the European Union’s Horizon 2020 Research and Innovation Programme through the Marie Sklodowska-Curie under Grant 823969, in part by the Ministry of Universities and Innovation Economy through Grants DPI2015-71219-C2 and PID2019-107126RB-C21

A review of moisture diffusion coefficients in transformer solid insulation - Part 2: experimental validation of the coefficients

The objective of this article is to experimentally verify the diffusion coefficients proposed by the various researchers. To validate these coefficients, drying experiments were carried out on impregnated and nonimpregnated paper and on pressboard samples. Taking into account the characteristics of the tested samples, two different experiments were performed: For nonimpregnated insulation, thermo-gravimetric experiments were performed determining the weight of a sample while being dried. . For impregnated paper, drying experiments were carried out in hot oil in which samples were periodically extracted and analyzed by the Karl Fischer method. The drying experiments were simulated using a diffusion model, solved by finite element analysis. The simulations were carried out using the diffusion coefficients proposed by the various researchers. Finally, the experimental results were compared with the simulated ones, and the accuracy and the range of application of the various coefficients were determined.This work was financed by the Spanish Ministry of Science and Technology under the project DPI2009-07093Publicad

Statistical study on the reference values of furanic compounds in power transformers

This article presents a statistical study devoted to obtaining the reference values of 2FAL in power transformers.This work was supported by CEIS and by the Ministry of Economy and Competitiveness of Spain through the project DPI2012-35819.Publicad

Assessing the use of natural esters for transformer field drying

In recent years, great attention has been paid to ester fluids as an alternative to mineral oil. Since the present use of these liquids is becoming a common practice in distribution transformers, even some experiences have been published reporting their application to power transformers. One of the main differences between ester fluid and mineral oil is the much greater capability of absorbing water by esters. In this paper, the possibility of using this kind of liquid in transformer field drying is assessed. Hot oil (HO) drying with mineral oil is one of the most widely used methods to dry transformers in the field, since it is a relatively simple and well-known process and it is less aggressive for the insulation than other drying methods. Moreover, drying the oil, while it is preferably hot, is the only method available to dry transformers online. However, the water extraction rate of the process is very poor because of the highly hydrophobic character of mineral oil and, in consequence, large drying times are needed to achieve a significant reduction in the water content of the insulation. A first theoretical analysis seems to indicate that the use of a less hydrophobic liquid would significantly reduce the drying times involved in the process. This paper aims to quantify the improvement of the HO drying process that is achieved by using ester fluids instead of mineral oil. Both drying agents were compared by means of theoretical simulations as well as laboratory tests.This work has been supported by the Spanish Government by means of the projects DPI2009-07093 and DPI2012-35819

Investigating the influence of moisture on the 2FAL generation rate of transformers: A new model to estimate the DP of cellulosic insulation

ABSTRACT: The analysis of the concentration of furanic compounds in oil has been accepted as one of the most reliable methods to obtain information about the condition of the transformer's cellulosic insulation. In this paper, the relation between the moisture content of the paper and the rate of generation of 2-furfuraldehidyde (2FAL) is studied. A statistical study is carried out to determine if transformers with higher moisture contents in oil tend to present higher contents of 2FAL, and, to this end, a database with more than 20,000 historical records of transformers is utilized. This study is completed with a set of accelerated ageing tests of transformer cellulosic insulation pieces, conditioned with different moisture contents. The impact of the moisture content of paper on the relation between the degree of polymerization (DP) and the 2FAL concentration is determined, and a correction factor has been proposed to include this variable in a model.This work was supported by the Ministry of Economy and Competitiveness of Spain through the projects DPI2012-35819 and DPI2015-71219-C2-2-R, and by CEIS.Publicad

Dielectric Strength of Nanofluid-Impregnated Transformer Solid Insulation

The interest in developing new fluids that can be used as dielectric liquids for transformers has driven the research on dielectric nanofluids in the last years. A number of authors have reported promising results on the electrical and thermal properties of dielectric nanofluids. Less attention has been paid to the interaction of these fluids with the cellulose materials that constitute the solid insulation of the transformers. In the present study, the dielectric strength of cellulose insulation is investigated, comparing its behavior when it is impregnated with transformer mineral oil and when it is impregnated with a dielectric nanofluid. The study includes the analysis of the AC breakdown voltage and the impulse breakdown voltage of the samples. Large improvements were observed on the AC breakdown voltages of the specimens impregnated with nanofluids, while the enhancements were lower in the case of the impulse tests. The reasons for the increase in AC breakdown voltage were investigated, considering the dielectric properties of the nanofluids used to impregnate the samples of cellulose. The analysis was completed with a finite element study that revealed the effect of the nanoparticles on the electric field distribution within the test cell, and its role in the observed enhancement.This work was supported by the Spanish State Research Agency under grant PID2019- 107126RB-C21/ AEI/10.13039/501100011033 and by the Spanish Ministry of Economy and Competitiveness under grant DPI2015-71219-C2-2-