Decay products and contaminants in transformer insulation: Mechanisms, dielectric impacts, and sustainable regeneration strategies

The long-term reliability of oil-filled power transformers is strongly governed by the condition of their insulating liquid, whose degradation directly affects dielectric strength, thermal performance, and the integrity of the oil–paper insulation system. Conventional maintenance practices rely heavily on periodic testing and full oil replacement, approaches that are costly, environmentally burdensome, and often unable to address the underlying chemical and interfacial ageing mechanisms occurring within the oil–paper insulation system. This review addresses the critical need for a unified understanding of degradation pathways and sustainable restoration strategies for aged transformer liquids. It examines the coupled thermo-oxidative, moisture-driven, and metal-catalyzed reactions responsible for acidity increase, sludge formation, dielectric loss, and CuₓS deposition associated with sulphur–copper interactions. Diagnostic indicators and characterization techniques are analyzed alongside modern regeneration approaches, particularly adsorbent-based reclamation, solvent-assisted purification, and hybrid treatment processes. Beyond technical restoration, techno-economic and environmental assessments highlight regeneration as a viable alternative to disposal-driven practices, supporting circular utilization of insulating liquids and valorization of waste sludge. The review identifies emerging high-reactivity sorbents, nano-engineered materials, and advanced monitoring tools as key directions for transitioning transformer oil management from reactive replacement to predictive, sustainable lifecycle control. • Review clarifies key mechanisms of transformer oil degradation and sludge formation. • Study examines sulphur corrosion and CuₓS deposition in insulation failure. • Study shows reclamation restores insulating liquid performance more sustainably than replacement. • Study highlights advanced adsorbents and diagnostics for future transformer reliability.