Towards sustainable transformer insulation design: Advances in thermally upgraded kraft and Nomex paper under high thermal stress

The growing demand for renewable energy integration and compact transformer designs has intensified thermal and electrical stresses on insulation systems, raising urgent concerns about reliability and sustainability. Thermally upgraded kraft (TUK) paper, though cost-effective, exhibits rapid performance loss under elevated temperature and moisture conditions, which could limit its suitability for long-term service. Aramid-based papers, such as Nomex® 410 and the cellulose–aramid hybrid Nomex® 910, offer superior thermal stability, low moisture uptake, and improved dielectric strength, which position them as sustainable alternatives for high-stress applications. Following this, this review synthesizes recent progress in paper insulation design, which includes hybrid cellulose–aramid, plasma fluorination, and composite engineering, aimed at extending service life while reducing lifecycle costs. Furthermore, comparative analyses of ageing studies demonstrate that aramid-based paper retains greater mechanical integrity, breakdown voltage, and oxidation resistance than TUK. Beyond technical performance, emerging strategies such as recycling aged aramid insulation and adopting circular economy practices underscore the potential for greener transformer manufacturing. Therefore, by integrating advanced material engineering with sustainability principles, this review highlights future directions for insulation design that can ensure reliable, efficient, and environmentally responsible operation of power transformers under high thermal stress. • Comparative review of TUK and aramid insulation under thermal stress. • Nomex papers show superior thermal, dielectric, and mechanical performance. • Surface modification and hybrid structures enhance insulation reliability. • Recycling and nanotechnology open pathways for sustainable insulation. • Review outlines future insulation design for high-demand transformers.