An experimental, analytical and numerical study of the mechanical failure of the cellulosic insulation of continuously transposed conductors in power transformers under the influence of short circuits and thermal ageing

Abstract

The correct operation of power transformers is essential for the electrical system. Paper and board are the most common materials used as solid insulation between parts of the transformer at different potentials, due to their high dielectric strength, good cooling properties and cost-efficiency. Although it is generally agreed that the condition of the solid insulation is one of the factors that has a more relevant impact on the life expectancy of transformers, there is a lack of understanding about how its electrical and mechanical properties are affected by the ageing derived from high temperatures produced by electrical faults and chemical reactions in the insulating oil. The deformations caused by electromagnetic forces can result in breakage of the aged paper and that can cause a subsequent electrical failure, if the insulation stops fulfilling its dielectric purpose. This PhD thesis reviews the characteristics of the main materials used in power transformers, with special emphasis on insulating materials, the phenomena and factors which contribute to their ageing, and different experimental methods for ageing quantification. The different kinds of mechanical solicitations and possible failure modes of power transformers are reviewed, as well as several models for the mechanical behaviour and the prediction of failure of paper materials. The generic mechanical models for paper and board lack some essential features for the analysis of the cellulosic insulation of a transformer (influence of the dielectric oil-impregnation, degradation of material properties due to ageing, an approximate realistic geometry, and mechanical solicitations similar to those occurring in-service). The performance of a commercial insulated continuously transposed conductor (CTC), wrapped in four layers of dielectric paper (the two internal made of plain Kraft paper and the two external of crepe paper) was experimentally studied when subjected to deformations (eventually produced by short circuits or other mechanical loads during operation) at different ageing levels, quantified through the degree of polymerisation (DP), during the whole operating life of a power transformer. The mechanical properties of the paper in each ageing state were obtained under tensile and compressive testing, and the shear properties were estimated from tensile tests, as suggested by Yoshihara and Masahiro (2017), providing the material properties for a failure model. Then, insulated CTC samples were subjected to three-point bending tests producing deformations which could be caused by a short circuit in the windings of a power transformer, and the fractures produced in the paper were macroscopically and microscopically analysed. That bending test over a CTC sample was numerically simulated by means of ANSYS AIM Static Structural, in order to estimate the strains that the paper insulation suffers, needed as input in a failure model. Two failure analytical models applicable to a lamina, the maximum strain and the Norris failure criteria, were adapted to the plain Kraft and crepe paper insulations, and the numerical analysis was used to represent their predictions in each of the nodes of the CTC sample. A better understanding of the ageing kinetics and the evolution of the mechanical and electrical properties of the cellulosic insulation of power transformers can contribute to improving the technical and economic management and planning in electrical systems, and even to enhancing manufacturing techniques, in order to produce more reliable components. The modified Norris failure criterion, which considers the interactions among tension, compression and shear strains, made it possible to predict the areas of the insulation of the CTC in which the first cracks appeared, for different deformation levels and ageing states. A simple, reliable and conservative mechanical failure criterion, in combination with other engineering measurements, could constitute a useful tool for the estimation of the condition of power transformers and a more accurate prediction of their end-of-life.