Nanofluids for Power Transformer Insulation: A Critical Review of Dielectric Performance, Ageing, and Oil–Paper System Interactions

Nanofluids have emerged as promising candidates for enhancing the dielectric and thermal performance of insulating liquids used in power transformers. While numerous studies report significant improvements in breakdown voltage (up to +10–40%) and thermal conductivity, the underlying mechanisms remain only partially understood and often contradictory, particularly with respect to long-term stability and ageing behavior. This paper presents a comprehensive and critical review of nanofluids applied to transformer insulation, adopting a system-level approach focused on the oil–paper insulation system. The analysis reveals that the reported performance strongly depends on key parameters such as nanoparticle concentration, dispersion quality, and experimental conditions, leading to significant inter-study variability. Dielectric improvements are shown to be maximized within narrow concentration ranges and may deteriorate due to nanoparticle aggregation, while thermal enhancements are often accompanied by increased viscosity, resulting in a thermo-hydraulic trade-off. Furthermore, this review highlights major contradictions in the literature, including the paradoxical relationship between electrical conductivity and dielectric strength, as well as the unclear impact of nanofluids on cellulose ageing. The findings demonstrate that performance observed at the fluid level cannot be directly extrapolated to real transformer conditions without considering the complex interactions between nanoparticles, oil, cellulose, and moisture. To address these limitations, a conceptual framework termed Nano-Modified Composite Insulation (NMCI) is proposed. This model provides a unified description of multiphase interactions and offers a basis for a more realistic evaluation of nanofluids under operational conditions. This work emphasizes the need for standardized experimental methodologies and long-term studies and provides clear research directions toward the development of reliable and industrially applicable nanofluid-based insulation systems.