This paper discusses the effect of doping of electro-insulating liquids with nanoparticle materials on the thermal properties of the obtained nanoliquids and heat transport in the transformer. Mineral oil, synthetic ester, and natural ester were used as base liquids. The effectiveness of doping base liquids with nanoparticles was supported by ultraviolet-visible (UV/VIS) measurements. In turn, Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) confirmed the absence of intermolecular interactions (i.e., hydrogen bonding). The influence of modification of electro-insulating liquids with fullerene C₆₀ and titanium dioxide TiO₂ nanoparticles on such thermal properties as thermal conductivity, specific heat, kinematic viscosity, density, and thermal expansion was investigated. Based on these properties and the theory of similarity, the cooling efficiency of the transformer filled with the analyzed nanofluids was determined. Nanofluids’ cooling effectiveness was compared with the cooling effectiveness of the base liquids. This comparison was supported by an analysis of Grashof, Prandtl, and Nusselt numbers. It has been shown that the modification of electro-insulating liquids with nanoparticles widely used in order to improve their dielectric properties, such as C₆₀ and TiO₂, does not have a significant influence on their thermal properties. The addition of fullerene C₆₀ caused an increase in kinematic viscosity, which was compensated by the increase in specific heat. In the case of TiO₂, the addition of this nanoparticle resulted in an increase in kinematic viscosity and a decrease in specific heat, which were balanced out by the increase in thermal conductivity. In summary, the heat exchange-capacity of liquids did not change due to doping with nanoparticles.

This paper presents the effect of the impact of moisture in paper insulation used as insulation of transformer windings on its thermal conductivity. Various types of paper (cellulose and aramid) and impregnated (mineral oil, synthetic ester, and natural ester) were tested. The impact of paper and impregnated types on the changes in thermal conductivity of paper insulation caused by an increase in moisture were analyzed. A linear equation, describing the changes in thermal conductivity due to moisture, for various types of paper and impregnated, was developed. The results of measuring the thermal conductivity of paper insulation depending on the temperature are presented. The aim of the study is to develop an experimental database to better understand the heat transport inside transformers to assess aging and optimize their performance.

The article discusses thermal properties of synthetic ester admixed with nanoparticles. The analyzed thermal properties were: thermal conductivity λ, kinematic viscosity υ, density ρ, specific heat c_p, and the thermal expansion factor β- all obtained by means of measurements. On the basis of these, the authors calculated the heat transfer factor α, which determines the ability of the liquid to heat transport. The authors used nanoparticles of fullerene C₆₀ and titanium oxide TiO₂. The analysis of the thermal properties was done for the temperatures of 25, 40, 60 and 80 °C. The authors analyzed the impact of nanoparticles C₆₀ and TiO₂ on thermal properties of synthetic ester. They proved that fullerene C₆₀ in principle had no influence on heat transfer factor α of the ester, while titanium oxide TiO₂ had some positive influence on the factor, the value of which increased about 1–3%.

The article presents an analysis of thermal properties of a mixture of mineral oil and synthetic ester used in transformers. The oil/ester rate was as follows: 100%/0%, 95%/5%, 80%/20%, 50%/50%, 20%/80%, 5%/95%, and 0%/100% The measured thermal properties were: thermal conductivity, specific heat, viscosity, density, and the thermal expansion factor. On the basis of measurement results, the authors calculated the heat transfer factor α. This factor determines the ability of a liquid and it has the key influence in transformer cooling. The measurements were done for a wide temperature range from 25 to 80°C.

The article describes research results of thermal properties of mineral oil and natural ester. Percentage proportions of both the liquids were as follows: 100/0, 95/5, 80/20, 50/50, 20/80, 0/100. The authors present measurement results of thermal conductivity, viscosity, specific heat, density, and thermal expansion of the created mixtures. The measurements were taken in a relatively wide temperature range: 25 °C, 40 °C, 60 °C, and 80 °C. On the basis of the measurement results, convection heat transfer coefficient α was calculated and the most advantageous proportion of both the components of the mixture was pointed in terms of cooling effectiveness of the transformer.