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

Abstract

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.

Experimental dataset on the compressive mechanical properties of high-density pressboard used in power transformers spacers

“…The ageing temperature of 150 °C was selected for this study based on prior publications by the same authors involving diverse cellulosic materials used as insulation in transformers [8] , [9] , [10] . These earlier investigations revealed that a lower ageing temperature would have required over seven months of accelerated thermal ageing to achieve an insulation state comparable to the entire operational lifespan of an actual transformer.…”

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

“…Those cracks were classified according to their size, after being studied macroscopically and microscopically, through Scanning Electron Microscopy. More details about the ageing process, the bending tests and their results can be found in [4].…”

“…The tensile strain 𝜀 𝑡 (%) and stress 𝜎 𝑡 (𝑀𝑃𝑎) were obtained as in ( 1) and ( 2), where 𝐿 0 and 𝐿 𝑓 (𝑚𝑚) are, respectively, the initial distance between jawns and recorded elongation; 𝐹 (𝑁) is the recorded force; 𝐴 (𝑚𝑚 2 ) is the cross-sectional area of the paper strip (product of the width, 𝑤, by the thickness, 𝑡). The detailed results of tensile tests in MD and CD can be found in [4].…”

“…Due to the adherence between the insulation paper and the copper, the strain field on both can be assimilated. The main characteristics of the FE model (geometry, supports and material properties, connections and mesh definitions) and how the results were obtained, are discussed in detail in [4]. The obtained strains were used as input in the mechanical failure model described in 2.3 (B).…”

Impact of the Use of Vegetable Oil on the Mechanical Failure of the Cellulosic Insulation of Continuously Transposed Conductors in Power Transformers

“…There are numerous recent studies on the influence of vegetable oils in the chemical and dielectric properties of the transformer insulation, such as [5][6][7][8][9]. Nevertheless, there are only a few which analyse their impact on the mechanical properties of the solid insulation, for instance [10,11]. Thus, it can be claimed that there is a lack of understanding about how the ageing process with vegetable oils affects the mechanical performance of the dielectric paper in transformer components such as Continuously Transposed Conductors (CTCs), especially for papers different from Kraft type, such as crepe.…”