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

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 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]

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

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

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

Study on the mechanical failure of the cellulosic insulation of continuously transposed conductors in power transformers under the influence of short circuits and thermal ageing

The dielectric paper of power transformers is subjected to ageing caused by high temperatures and chemical reactions in the dielectric oil. The electromagnetic forces that affect the conductor 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 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. Samples of the conductor were subjected to bending deformations compatible with a short circuit, and the cracks generated in the paper were analysed. The aged dielectric paper was mechanically characterised (in tensile, compressive and shear modes) and the bending tests of the conductors were simulated in ANSYS Static Structural, to estimate the strains in the paper. The Norris failure criterion was 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 this new version of the Norris approach

Impact of the use of vegetable oil on the mechanical failure of the cellulosic insulation of continuously transposed conductors in power transformers

Natural esters have become of interest for the industry in recent years as dielectric liquids for power transformers, and many studies are focused on their dielectric and chemical properties and on their influence in the degradation of the solid insulation due to aging. However, very little is known about their impact on the evolution of the mechanical properties of the paper insulation, which are acknowledged to have a considerable influence in their overall performance and reliability during the operating life of power transformers. This work studies the effects of thermal aging with vegetable oil in some commercial components, which are commonly used in power transformers, such as an insulated continuously transposed conductor (CTC) and samples of thermally upgraded crepe insulation. The changes in the properties of the crepe paper insulation are characterized through the degree of polymerization and tensile testing. Failure initiation and propagation in the insulation of the CTC is analyzed macroscopically. The results are compared with those obtained when using mineral oil, showing that the use of vegetable oil has a protective effect over mechanical properties of the studied types of paper insulation

The impact of thermal ageing on the compression strength of radial spacers in power transformers

Radial spacers made of high-density pressboard play a critical role in power transformers, providing support and maintaining distance between winding disks for efficient dielectric oil flow through cooling ducts. However, prolonged exposure to high temperatures leads to chemical reactions and degradation of mechanical and dielectric properties, potentially compromising transformer integrity. In this study, high-density pressboard spacers immersed in paraffinic oil underwent thermal aging to simulate the transformer lifespan. Various chemical, dielectric, and mechanical parameters were measured, including AC breakdown voltage, dielectric dissipation factor (tan δ), permittivity, resistivity, moisture content, and oil acidity. Moreover, the degree of polymerisation and compressive stiffness of the pressboard were determined. By establishing mathematical relationships, it was possible to estimate the compressive stiffness indirectly and non-destructively, from the degree of polymerisation and oil acidity measurements. These findings offer valuable insights for evaluating the condition of radial spacers and ensuring the reliable operation of power transformers