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.

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 ageing. 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 ageing 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 characterised through the degree of polymerisation and tensile testing. Failure initiation and propagation in the insulation of the CTC is analysed 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 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].

Purpose: Transformers are indispensable for bulk power transfer and safe distribution, operating for decades and representing major capital outlays. Recent surges in material prices, ageing fleets and stricter EU efficiency rules compel operators to plan replacements years ahead and weigh competing designs. Although life‑cycle assessment (LCA) could steer these choices, it remains little used in the sector. By reviewing fifteen transformer LCAs (2007–2026), we propose a methodological roadmap that supports decision‑ready analyses and comparative studies. Methods: The reviewed transformer LCAs were benchmarked against ISO 14,040/44. For each study we examined: goal and scope definition, including functional‑unit choice and boundary completeness; life‑cycle inventory practice; impact‑assessment method and category set; quantification of operational losses, recycling and end‑of‑life; sensitivity and uncertainty analysis; linkage to eco‑design or cost optimization; and transparency of interpretation and reporting. Recurring strengths, omissions and inconsistencies were synthesized into guidance that addresses functional‑unit alignment, data‑quality documentation, allocation and cut‑off rules, loss modeling, and the need for critical review. Results and discussion: Across the reviewed studies, goals are often vague and functional units inconsistent (e.g., kg CO₂-eq/MVA vs. kg CO₂-eq/MWh, among others), undermining comparability and sometimes biasing results. Data gathering is hampered by the many actors spanning extraction, manufacture, long operation, and disposal, yet data-quality descriptors are seldom provided. System boundaries differ, with cut-off criteria and allocation rules rarely justified; capital equipment and shared utilities are not explicitly allocated, and end-of-life is often deferred. Sensitivity analysis appears in just nine papers and full uncertainty propagation in only two. This methodological dispersion limits the usefulness of current transformer LCAs for planners, designers, and regulators. Among the studies that include the use phase, several report that operational no-load and load losses contribute about 90–99% of total life-cycle impacts, while manufacturing, transport, and disposal are generally secondary contributors. Conclusions and recommendations: Operational no‑load and load losses remain the dominant environmental hotspot, so loss‑reduction technologies are the foremost mitigation lever. However, many current transformer LCAs are hampered by vague functional units, truncated system boundaries, minimal sensitivity testing and the lack of ISO‑required critical review. Progress hinges on richer public databases, fully harmonized cradle‑to‑grave scopes, voltage‑appropriate functional units, dynamic loss modeling that reflects evolving grid mixes, transparent allocation of shared utilities and capital equipment, clear data‑quality criteria and routine uncertainty propagation. Implementing these improvements will turn transformer LCAs into reliable tools for eco‑design, procurement and carbon‑policy decisions.

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.