Estudio experimental, analítico y numérico del fallo mecánico del aislamiento celulósico de conductores continuamente transpuestos en transformadores de potencia bajo la influencia de cortocircuitos y envejecimiento térmico

RESUMEN: El papel dieléctrico de un transformador de potencia está sometido a un proceso de envejecimiento, producido por altas temperaturas y reacciones químicas en el aceite dieléctrico. Las fuerzas electromagnéticas que afectan a los conductores pueden producir la rotura del papel, llegando a causar el fin de la vida útil del transformador. Para mejorar la comprensión de estos fenómenos, un conductor continuamente transpuesto (CTC) con cuatro capas de dieléctrico (papel Kraft ordinario y tipo crepe) fue impregnado en aceite nafténico y envejecido durante diferentes periodos, representando toda su vida útil. Muestras del CTC fueron sometidas a deformaciones de flexión compatibles con un cortocircuito, analizándose las grietas generadas en el papel. El papel dieléctrico envejecido fue caracterizado mecánicamente (a tracción, compresión y tangencialmente), y el ensayo a flexión se simuló numéricamente en ANSYS Static Structural, para estimar las deformaciones en el papel. Dos criterios analíticos de fallo (máxima deformación y criterio de Norris) fueron adaptados para predecir las regiones del papel dieléctrico susceptibles de fallar para un estado de envejecimiento y deformación impuesta determinados, obteniendo predicciones coherentes con los resultados experimentales y validando así el enfoque de Norris.ABSTRACT: The dielectric paper of power transformers is subjected to ageing caused by high temperatures and chemical reactions in the dielectric oil. The electromagnetic forces the conductor is subjected to can cause the breakage of the paper and even the end of the useful life of the transformer. To improve the understanding of those phenomena, a continuously transposed conductor (CTC) with four layers of dielectric paper (plain Kraft and crepe papers) was impregnated in naphthenic oil and aged during different periods, representative of its whole lifespan. CTC samples were subjected to bending deformations compatible with a short circuit, and the cracks generated in the paper were analysed. The aged paper was mechanically characterised (in tensile, compressive and shear modes) and the bending tests were simulated in ANSYS Static Structural, to estimate the strains in the paper. Two analytical failure models (maximum strain and Norris criterion) were modified to predict the regions in the dielectric paper susceptible to fail for a particular ageing state and deformation level, obtaining a good agreement with the experimental results and thus validating the Norris approach

Mechanical behaviour of the cellulosic dielectric materials of windings in power transformers in operation

Power transformers are crucial elements in electrical systems, and the end of their useful life is commonly conditioned by the degradation of the cellulosic insulation materials inside them. These materials are subjected to elevated temperatures and mechanical stresses, generated by electrical solicitations which deform the copper conductors and subsequently the paper, and also to the chemical reactions which take place in the dielectric oil in which the paper is impregnated. In order to better understand the behaviour of cellulosic insulation, we have studied the previous bibliography analysing the mechanical behaviour of cellulosic materials. At present, there are no experimental results analysing how the paper responds to the deformation suffered by the copper conductor in a realistic situation. We have developed a simulation model describing the mechanical behaviour of a standardised copper conductor wrapped with four layers of dielectric paper, using ANSYS Workbench Static Structural, which will be compared with experimental results afterwards

Degradation of the compression strength of spacers made of high-density pressboard used in power transformers under the influence of thermal ageing

The structural components inside power transformers are commonly made of high-density pressboard, due to its suitable mechanical and dielectric properties. Among these components are the spacers used in the windings of transformers, which are subjected to compressive loading during operation. The spacers are immersed in dielectric liquid and subjected to high temperatures and chemical reactions during the lifespan of the transformer, which result in the degradation of their dielectric and mechanical properties. The performance and reliability of the power transformer greatly depends on its mechanical integrity, so it is necessary to understand how ageing degrades the mechanical response of the high-density pressboard. In this study, spacers made of high-density pressboard and pieces of copper conductor were immersed in uninhibited paraffinic oil and aged at 150 °C for different periods of time, trying to realistically represent the process suffered by a power transformer during its whole lifespan. The evolution caused by the thermal ageing over some chemical parameters (acidity and moisture content) and dielectric properties (AC breakdown voltage, dielectric dissipation factor, resistivity and degree of polymerisation) of the oil and the pressboard was studied experimentally. Compressive mechanical tests were performed on samples of the aged high-density pressboard, and the compressive stiffness during the ageing process was related with other chemical and dielectric parameters

Experimental dataset on the tensile and compressive mechanical properties of plain Kraft and crepe papers used as insulation in power transformers after ageing in mineral oil

The solid insulation in the windings of power transformers, which generally consists of oil-impregnated thin paper, is one of the key elements for the performance and durability of these electrical machines. Insulation paper is subjected to static and dynamic forces of electromagnetic origin, in combination with high temperatures and chemical reactions, during the operating life of a power transformer. The mechanical properties of the cellulosic insulation are relevant parameters because its breakage could result in the electric failure of the transformer. Indeed, paper manufacturers usually provide values of the tensile strength and elongation at breakage of the insulating paper in its two principal material directions, the MD (machine direction) and CD (cross-direction). However, paper is a highly anisotropic material and its material properties evolve as the paper insulation ages. The paper insulation in an operating transformer is subjected to a multiaxial stress state field including compressive and shear stresses. This article reports experimental data on the tensile and compressive mechanical properties of two types of paper, plain Kraft and crepe paper, typically used as insulation in power transformers, under different ageing states (which were induced through accelerated thermal ageing and quantified by means of the degree of polymerisation). These data could be reused for several purposes. They can improve the current understanding of the mechanical response and degradation processes of the cellulosic insulation in power transformers, and give some reference values that can be compared with others obtained in the factory by manufacturers. In the field of engineering failure analysis, those values could be reused for the assessment of mechanical failure of paper materials used in power transformers, see [1]

A modification of the Norris failure criterion for the prediction of the mechanical failure of the aged paper insulation in the windings of a power transformer

The deterioration of the insulation in the windings of power transformers affects their lifespan. A commercial insulated Continuously Transposed Conductor (CTC) was studied experimentally, numerically and analytically. The purpose was to understand the mechanisms governing the mechanical failure of the insulating paper, and to achieve a criterion for predicting failure under different conditions. Samples of that insulated CTC were extracted from a coil and aged at 150∘C for different durations inside vessels filled with naphthenic oil. Then the degree of polymerisation and tensile, compressive and shear mechanical properties of the insulation were measured/estimated. Aged insulated CTC samples were subjected to three-point bending tests, producing deformations compatible with a short circuit, and the fractures in the insulation were analysed. The bending test over a CTC sample was simulated by means of a FEM Program. The Norris failure model, applicable to a lamina, was adapted to the studied insulation materials. The predictions of that failure criterion agreed with experimental observations

Experimental and numerical analysis of cellulosic insulation failures of continuously transposed conductors under short circuits and thermal ageing in power transformers

The integrity of the cellulosic insulation in power transformers is considered one of the most relevant parameters that affects their performance and reliability. Electric faults, such as short circuits, have thermal and mechanical effects that degrade the paper and can eventually produce the end-of-life of the transformer. The evolution of the properties of the paper insulation of a commercial continuously transposed conductor due to thermal ageing was characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the paper was analysed macroscopically and microscopically using scanning electron microscope. A finite element numerical mechanical model of the conductor was implemented to reproduce the experiments and to obtain the load level and strain state that produce failure at each ageing state, aiming at developing a failure model for the insulation. This model may contribute to an improvement in manufacturing processes and management of the electrical system

Analysis of laccase-like enzymes secreted by fungi isolated from a cave in northern Spain

[EN] Laccases belong to a family of multicopper enzymes able to oxidize a broad spectrum of organic compounds. Despite the well-known property of laccases to carry out bleaching and degradation of industrial dyes and polyphenolic compounds, their industrial use is often limited by the high cost, low efficiency, or instability of these enzymes. To look for new microorganisms which produce laccases that are potentially suitable for industrial applications, we have isolated several fungal strains from a cave in northern Spain. Their phenotypic analysis on agar plates supplemented with ABTS (2,2 '-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) disclosed two laccase-positive strains. Further genotyping revealed that they belonged to the Gliomastix murorum and Conidiobolus thromboides species. The secretion of G. murorum and C. thromboides laccase-like enzymes was then confirmed by zymography. Further identification of these polypeptides by mass-spectroscopy revealed the nature of the laccases and made it possible to predict their functional domains and other features. In addition, plate assays revealed that the laccases secreted by both G. murorum and C. thromboides were capable of degrading industrial dyes (Congo Red, Indigo, and Eriochrome Black T). Homology modeling and substrate docking predicted the putative structure of the currently uncrystallized G. murorum enzyme as well as its amino acid residues potentially involved in interactions with these dyes. In summary, new biochemical and structural insights into decolorization mediated by G. murorum laccase as well as identification of laccase-like oxidase in C. thromboides point to a promising future for these enzymes in biotechnology.AIOTEK, Grant/Award Number: SPE12UN84; Basque Foundation for Science; Basque Government, Grant/Award Number: PRE-2013-1-90

Molecular Profiling of Circulating Tumour Cells Identifies Notch1 as a Principal Regulator in Advanced Non-Small Cell Lung Cancer

Knowledge on the molecular mechanisms underlying metastasis colonization in Non-Small Cell Lung Cancer (NSCLC) remains incomplete. A complete overview integrating driver mutations, primary tumour heterogeneity and overt metastasis lacks the dynamic contribution of disseminating metastatic cells due to the inaccessibility to the molecular profiling of Circulating Tumour Cells (CTCs). By combining immunoisolation and whole genome amplification, we performed a global gene expression analysis of EpCAM positive CTCs from advanced NSCLC patients. We identified an EpCAM+ CTC-specific expression profile in NSCLC patients mostly associated with cellular movement, cell adhesion and cell-to-cell signalling mediated by PI3K/AKT, ERK1/2 and NF-kB pathways. NOTCH1 emerged as a driver connecting active signalling pathways, with a reduced number of related candidate genes (NOTCH1, PTP4A3, LGALS3 and ITGB3) being further validated by RT-qPCR on an independent cohort of NSCLC patients. In addition, these markers demonstrated high prognostic value for Progression-Free Survival (PFS). In conclusion, molecular characterization of EpCAM+ CTCs from advanced NSCLC patients provided with highly specific biomarkers with potential applicability as a “liquid biopsy” for monitoring of NSCLC patients and confirmed NOTCH1 as a potential therapeutic target to block lung cancer dissemination.This work was funded by InveNNta (Innovation in Nanomedicine); Operational Programme for Cross-border Cooperation: Spain-Portugal (POCTEP) and European Regional Development Fund (ERDF). Javier Mariscal is recipient of a fellowship from Escola de Doutoramento Internacional Campus Vida of the University of Santiago de Compostela. Laura Muinelo-Romay is supported by ISCIII as Responsible of the Liquid Biopsy Analysis UnitS

EVIDENT smartphone app, a new method for the dietary record: comparison with a food frequency questionnaire

Background: More alternatives are needed for recording people’s normal diet in different populations, especially adults or the elderly, as part of the investigation into the effects of nutrition on health. Objective: The aim of this study was to compare the estimated values of energy intake, macro- and micronutrient, and alcohol consumption gathered using the EVIDENT II smartphone app against the data estimated with a food frequency questionnaire (FFQ) in an adult population aged 18 to 70 years. Methods: We included 362 individuals (mean age 52 years, SD 12; 214/362, 59.1% women) who were part of the EVIDENT II study. The participants registered their food intake using the EVIDENT app during a period of 3 months and through an FFQ. Both methods estimate the average nutritional composition, including energy intake, macro- and micronutrients, and alcohol. Through the app, the values of the first week of food recording, the first month, and the entire 3-month period were estimated. The FFQ gathers data regarding the food intake of the year before the moment of interview. Results: The intraclass correlation for the estimation of energy intake with the FFQ and the app shows significant results, with the highest values returned when analyzing the app’s data for the full 3-month period (.304, 95% CI 0.144-0.434; P<.001). For this period, the correlation coefficient for energy intake is .233 (P<.001). The highest value corresponds to alcohol consumption and the lowest to the intake of polyunsaturated fatty acids (r=.676 and r=.155; P<.001), respectively. The estimation of daily intake of energy, macronutrients, and alcohol presents higher values in the FFQ compared with the EVIDENT app data. Considering the values recorded during the 3-month period, the FFQ for energy intake estimation (Kcal) was higher than that of the app (a difference of 408.7, 95% CI 322.7-494.8; P<.001). The same is true for the other macronutrients, with the exception g/day of saturated fatty acids (.4, 95% CI −1.2 to 2.0; P=.62). Conclusions: The EVIDENT app is significantly correlated to FFQ in the estimation of energy intake, macro- and micronutrients, and alcohol consumption. This correlation increases with longer app recording periods. The EVIDENT app can be a good alternative for recording food intake in the context of longitudinal or intervention studies. Trial Registration: ClinicalTrials.gov NCT02016014; http://clinicaltrials.gov/ct2/show/NCT02016014 (Archived by WebCite at http://www.webcitation.org/760i8EL8Q).Spanish Ministry of Science and Innovation and Carlos III Health Institute/European Regional Development Fund (FIS: PI13/00618, PI13/01526, PI13/00058, PI13/01635, PI13/02528, PI12/01474; RETICS: RD12/0005, RD16/0007), Regional Health Management of Castilla and León (GRS 1191/B/15, GRS 909/B/14, GRS 770/B/13), and the Infosalud Foundatio