This review is focused on the research of dielectric fluids, especially commonly used power transformer oils enhanced by nanoparticles, i.e., nanofluids. There are differences between various combinations of base fluids and nanoparticles prepared in different ways. The main goal of this review was to present recent research in this field sorted by the used nanoparticles. Nanofluids based on mineral oils, natural, or synthetic esters were investigated in terms of the nature of nanoparticles, particularly Al2O3, TiO2, Fe2O3, Fe3O4, graphene, fullerene, and others. The combinations of environmentally friendly oils and nanoparticles were presented. Finally, the article focused on the description of current dielectric fluids usable in power transformers and the possibilities of improving new and existing fluids with nanoparticles, especially their physical, dielectric, and chemical properties, but with regard to environmental aspects.
Keywords: biodegradable fluids; insulation oils; nanofluid; nanoparticles; stability.