Study On The Thermophysical Properties Of Transformer Oil Modified By Nano-SiO₂

With the rapid development of economy, voltage levels are growing faster in order to meet the demand for upgrading the power grid system. In order to guarantee long-term and safe operation of power system, transformer with high-performance serving as the link of power transmission and distribution systems is required. Nowadays it has become a research hotspot in improving the heat dispersion and insulating performance of the transformer without increasing its external dimension. Later in the20th century, some researchers have proved that the thermal conductivities of the liquids can be increased by dispersing nanoparticles into them. Meanwhile, recent studies have shown that the insulating performance of transformer oil is improved by adding a small amount of nanoparticles. Therefore, the existing problems in using the transformer oil can be solved by nanometer-scale modification.In the work, SiO2nanoparticles were employed to improve the thermal properties of transformer oil No.25. Some properties, such as the manufacture process、 stability、 viscosity and the thermal conductivity, of the nano-modified transformer are studied through different testing, so as to lay a foundation for the simulation process of the usage of modified transformer oil in transformer and the practical application.Firstly, the preparation method of nano-modified transformer oil and the dispersion stability mechanism of nanoparticles in base fluid are studied. A two-step method is selected to prepare nano-SiO2modified transformer oil. Meanwhile, ultrasonic vibration, mechanical stirring and surface modification were employed to ensure the stability of nano-transformer oil. Results indicate that there exists an optimum additive amount of oleic acid serving as surfactant by analyzing the stability of modified transformer oil prepared in the experiment, and the stability was obviously affected by the amount of the surfactant. Moreover, the stability become better at higher temperature within the temperature ranges from20℃to100℃.Then the viscosity property is researched when achieving stable nano-modified transformer oil. The experimental results show that the viscosity of the mixed oil can be increased by adding the nanoparticles, and the viscosity varies nonlinearly with increase in volume fraction of nanoparticles. A large deviation between the testing results and theoretical calculating values of viscosity appears as comparing with the classical formula calculating viscosity of suspension liquid. When modifying the classical formula by introducing the solvable layer model, the calculating values match well with the experimental results. In addition, experimental research is done aiming at getting the values of temperature dependence viscosity of modified transformer oil, and a fitted equation of viscosity and temperature is gained by selecting suitable formula based on experimental data.Finally, based on the theory of transient hot wire method, an experimental platform to measure liquid thermal conductivity is built, and the thermal conductivity was measured here. Results indicate that the thermal conductivity of nano-modified transformer oil was increased by the increase in volume fraction of nanoparticles. Meanwhile, it is found in experiment that temperature can also affect the thermal conductivity of nano-modified transformer oil significantly. With the same additive volume fraction, the value of the thermal conductivity is larger when the temperature is higher. Furthermore, a comparative study is done in the paper between the testing results and the values obtained from the models forecasting the thermal conductivity of nanofluids. Finally, according to the experimental data, a fitted linear curve was obtained illustrating the thermal conductivity of the nano-modified transformer oil changing with the temperature and volume fraction.