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 $\delta$), 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.