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

Abstract

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.

Research on the Influence of Compression and Offset of Cushion Blocks on the Axial Strength of Transformers

“…Reference [18] investigated the impact of cushion blocks on the axial winding strength and of bracing ribs on the radial winding strength, with specific consideration of factors such as the number and width of cushion blocks. References [19,20] indicated that, over the transformer's lifespan, cushion blocks were immersed in dielectric fluid and exposed to high temperatures, thermal aging, and chemical reactions, resulting in the degradation of their dielectric and mechanical properties and weakening the transformer short-circuit resistance. In references [21,22], a two-dimensional simulation was established and the leakage flux and stress distribution of high-and low-voltage winding were analyzed; short-circuit tests were carried out and it was pointed out that the electromagnetic force generated by the fault current might lead to instability in and the collapse of the winding.…”