I. Introduction
Mineral oil has been used since the first commercial oil-filled transformer was introduced, while dielectric fluids such as synthetic ester and natural ester are seen as a modern alternative. However, mineral-oil still plays an important role in the insulation of high voltage apparatus, especially power transformers and instrument transformers [1]–[2]. Focusing on the advancement of nanotechnologies and their application, nanoparticles hold promise that they can be used with mineral-oils to enhance their properties, which may lead to more compact designs and the reduction of manufacturing costs [3]–[4].