Study On Moisture Distribution And Electrical Conductivity In Transformer Oil-based TiO₂ Nanofluids

In recent years,the high voltage-grade transmission network with UHV AC-DC transmission as the strong skeleton puts forward higher requirements for the insulation performance of transformer oil,and practice proves that moisture is the main factor affecting the insulation strength of transformer oil,and the existing research finds that nanoparticles can significantly improve the moisture deterioration of transformer oil.The research in this field mainly focuses on the modification effect of nanoparticle type and concentration on the macro-breakdown performance of nanotransformer oil,but there is no further study on the change law and mechanism of the water distribution and its conductivity characteristics in nanotransformer oil.Therefore,in this paper,the transformer oil modified by titanium dioxide nanoparticles with different morphology,particle size and surface modification was used to study the influence of the microstructure and moisture content of the nanoparticles on the moisture distribution and conductivity characteristics in pure transformer oil and nano-transformer oil.The mechanism of the microscopic characteristics of nanoparticles on the moisture distribution and conductivity characteristics in the oil is discussed and analyzed.In this paper,TiO2 nanospheres with diameter of 5 nm,10 nm and 15 nm,TiO2 nanorods with length-to-diameter ratios of 3,4 and 5,and TiO2 nanoparticles with surface modification by acetic acid,caproic acid and oleic acid,are prepared by solvent-thermal method with tetrabutyl titanate as titanium source,oleic acid and oleylamine as modifiers.Then nano-modified transformer oil with good dispersion stability was prepared.By adjusting the moisture content of pure oil and nanotransformer oil within the range of 10-50 ppm,with the increase of moisture content,the content of low-polymer water molecules in pure oil decreased,and the content of water clusters increased.The content of water clusters in nano-transformer oil is significantly reduced,and the cluster of dimer/trimer water molecules increases,and with the decrease of particle size of spherical particles,the appearance of the shape from rod to spherical,the trend is more significant.According to the test results of the bonding state of the nanoparticle surface,it is found that it is precisely due to the increase in the content of polar Ti-O bonds on the surface of the nanoparticle that a large number of solid-liquid interfaces composed of the polar groups of the nanoparticle and the nonpolar oil molecules are formed in the oil medium,so that the water molecules in the oil are more easily adsorbed on the surface of the nanoparticles,and the existence of the dimer/trimer water molecules is mainly formed.Therefore,the adsorption of water molecules on the surface of the nanoparticles not only improves the uniformity of moisture distribution in the oil,but also greatly inhibits the formation of water clusters in the oil,which is the fundamental reason why the nanoparticles improve the water resistance of transformer oil into deterioration.By testing the electrical conductivity of transformer oil modified by TiO2 nanoparticles with different morphologies,sizes and surface modifications,it is found that the smaller the size of nanoparticles and the higher the conductivity of nanotransformer oil with spherical appearance.This is because the smaller the size of the nanoparticles,the shape from a rod to a spherical shape,are conducive to improve the nanoparticles and oil solid liquid interface,promote the nanoparticles in the oil hydrate impurity ion adsorption,improve the surface charge of nanoparticles,but also conducive to improve the transfer rate of nanoparticles,and thus improve the conductivity of nano-transformer oil.Titanium dioxide nanoparticles were modified by the surface of the carboxylic acid-based modifiers,and the conductivity of nanotransformer oil decreased significantly.This is due to the chemical adsorption of a bidentate coordination on the surface of the nanoparticles,which reduces the density of the surface defect state and the amount of charged particles absorbed on the surface,which reduces the conductivity.The study of the influence law and mechanism of water distribution and conductivity characteristics in transformer oil by nanoparticle microstructure,it will provide the theoretical and experimental basis for the development of nanoparticle modified transformer oil with excellent comprehensive performance.