Study On Preparation And Properties Of Nano KH550-TiO₂ Modified Binary Mixed Natural Ester Insulating Oil

With the comprehensive promotion of the rural revitalization strategy and the modernization of agriculture and rural areas,the electricity consumption conditions in rural areas have significantly improved,and the electrification of agricultural equipment has been more widely used,and the demand for electricity in rural areas continues to increase.The above puts forward higher requirements for the safe and stable operation of distribution networks（rural power grids）and their equipment.Oil-immersed transformers are the core power equipment for grid operation,and oil-paper insulation is the main insulation material inside the transformers.Among them,traditional mineral insulating oils have poor degradability and low flash and ignition points.In the event of transformer overheating or internal short circuit faults,fire or explosion accidents may occur,threatening the safe and stable operation of the power grid.Natural ester is a kind of liquid dielectric with high burning point and renewable energy.Under the background of"carbon peak,carbon neutralization",natural ester may replace traditional mineral insulating oil as an ideal insulating medium for power equipment.However,the inherent defects of natural ester make it unable to be directly used as insulating oil.Therefore,developing a natural ester insulating oil with good flow performance,high thermal conductivity and high insulation strength is the current trend.It is also of great significance to improve the voltage level of the power grid and maintain the safe and stable operation of the power grid.In this study,the main components and mixing ratio of binary mixed natural ester insulating oil were determined firstly,and the nano-TiO₂ modified natural ester insulating oil was prepared with binary mixed natural ester insulating oil as the base fluid.Then,The surface of nano-TiO₂ was modified by silane coupling agent KH550.Nano KH550-TiO₂ modified natural ester insulating oil,which can be dispersed and stabilized for a long time,was prepared by using the ultrasonic dispersion instrument through the"two-step method".The physical and chemical parameters of nano KH550-TiO₂modified natural ester insulating oil at different concentrations were tested,such as thermal conductivity,kinematic viscosity,flash point,etc.The influence of nano KH550-TiO₂ on the electrical properties and thermal aging characteristics of natural ester insulating oil was investigated,and the differences in acid value,dielectric loss factor and breakdown voltage of insulating oil under different aging degrees were analyzed and compared.The micro-mechanism of nano KH550-TiO₂ interface to enhance the breakdown voltage of insulating oil was analyzed by first principles.At the same time,molecular dynamics was used to simulate the diffusion behavior and distribution of polar molecules in the oil-paper system during the aging process,and the influence of nano KH550-TiO₂ on the thermal aging characteristics of oil-paper insulation was further explored.The main conclusions of this paper are as follows:（1）Nano-TiO₂ modified by silane coupling agent KH550 has excellent long-term dispersion stability in natural ester insulating oil.The organic groups on the surface of nano KH550-TiO₂ improve the dispersion of nanoparticles in oil and reduce the aggregation of nanoparticles.After 30 days of standing,the transmission coefficient of nano KH550-TiO₂ modified natural ester insulating oil has little change,and the change rate remains at 5%.The addition of nano KH550-TiO₂ with a concentration of0.09 g/L significantly increased the thermal conductivity of natural ester,and the thermal conductivity increased by 6.37%.The kinematic viscosity,flash point and ignition point of nanofluids with different concentrations have little change.Therefore,the effect of the addition of nanoparticles on the flow and combustion properties of natural esters can be ignored.（2）Nano KH550-TiO₂ can effectively improve the breakdown voltage and volume resistivity of natural ester insulating oil and reduce its dielectric loss factor.Compared with pure oil,the breakdown voltage of nano KH550-TiO₂ modified insulating oil with a concentration of 0.03 g/L increased by 37.9%,the volume resistivity increased by an order of magnitude,and the dielectric loss factor decreased by 59.8%.The improvement effect of nano KH550-TiO₂ on the electrical performance of insulating oil is generally better than nano-TiO₂.The influence mechanism of the interface of nanoparticles on the breakdown performance of natural esters was investigated.The results showed that the surface amino groups and silicon hydroxyl groups of nano-TiO₂ modified by KH550 had strong positive electrostatic potential and could adsorb polar molecules and charged particles in oil.Moreover,the grafted KH550 increases the interface between nanoparticles and natural esters,introduces more electronic traps,and slows down the development of electronic streamers in oil.The interface formed by nano KH550-TiO₂ and natural ester molecules has stronger binding energy,which is more conducive to improving the dispersion of nanoparticles in natural ester insulating oil.（3）Nano KH550-TiO₂ can effectively improve the thermal aging resistance of natural ester oil-paper insulation system,thus delaying the aging of oil-paper insulation system.The results show that compared with pure natural ester insulating oil,nano KH550-TiO₂ natural ester insulating oil has lower acid value and micro-water content at the end of aging,and the dielectric loss factor rises more slowly.The breakdown voltage of nano KH550-TiO₂ insulating oil decreased more slowly,only 7.27%.This shows that the oil-paper insulation system added with nano KH550-TiO₂ has fewer aging products and lower degradation degree during the accelerated thermal aging process.From the distribution of polar molecules in oil-paper insulation system,most polar molecules in nano KH550-TiO₂ oil-paper system are concentrated in insulating oil.Nano KH550-TiO₂ forms a hydrogen bond structure with polar molecules,which can adsorb some polar molecules and weaken the diffusion trend of polar molecules to cellulose.The addition of nano KH550-TiO₂ can effectively protect the internal hydrogen bond structure of cellulose,thus delaying the thermal aging process of oil-paper insulation system.