From Mineral Oil to Dielectric Nanofluids: Review on Breakthroughs, Bottlenecks, and the Road to Commercialization

Power transformers are critical components of electric power infrastructure, and their liquid insulation systems are decisive for operational safety, reliability, and longevity. Conventional insulating fluids, particularly mineral oil, face increasing scrutiny due to low biodegradability, poor thermal performance, and non-renewable origin. Insulating oil-based nanofluids, engineered by dispersing nanoparticles (1–100 nm) into base oils, have emerged as transformative candidates for next-generation transformer liquid insulation. This review provides a comprehensive and critically integrated analysis of insulating oil-based nanofluids, systematically covering historical development, synthesis methodologies, colloidal stabilization strategies, and multi-technique characterization approaches. Dielectric performance metrics, including AC, DC, lightning-impulse breakdown voltages, partial-discharge inception voltage, and dielectric loss, are critically reviewed alongside thermal-conductivity enhancements and the thermo-viscous trade-off. Experimental evidence demonstrates that optimally formulated nanofluids enhance AC breakdown voltage by 20–60%, improve thermal conductivity by 10–40%, and significantly elevate partial discharge inception voltage depending upon various factors such as doping concentration, dispersion quality, moisture content, particle size/morphology, nanoparticle-oil system compatibility, etc. However, long-term colloidal instability, nanoparticle migration, compatibility with ageing products, and absence of standardized testing protocols continue to impede industrial deployment. This review identifies key research gaps and outlines a roadmap toward reliable, sustainable, and commercially viable insulating nanofluids for power transformer applications.