Study Of Physicochemical And Electrical Properties Of Insulating Oil Paper Modified By Nano-TiO₂ Under Thermal Aging

As an important part of electrical equipment in the AC-DC transmission system,the insulation of converter transformers has become a key concern in their design,manufacture,operation and maintenance.The main insulation on the valve side of the converter transformer is subject to hydrolysis,oxidation and cracking at higher operating temperatures,resulting in degradation of the oil paper and affecting its insulation performance.Charge injection,accumulation and migration will occur within the oil paper insulation under complex operating conditions such as AC-DC compound electric fields and polarity reversal,increasing the risk of flashover and insulation breakdown along the surface and affecting the safe and stable operation of the converter transformer.Therefore,this paper aims to improve the thermal aging and breakdown resistance of the main insulation on the valve side of the converter transformer.Combined with the group’s previous research on nano-insulation modification technology,it reveals the changes in the physical and chemical properties and their mechanisms of the modified oil paper at different thermal aging stages,and analyses the charge accumulation and dissipation characteristics on the surface of different types of oil paper to comprehensively evaluates the effect of nanoparticles addition on the breakdown performance.The main research contents and results are as follows.（1）In this paper,5wt%mass fraction,10 nm particle size of oil paper（B5）modified by nano-Ti O₂ was prepared after pretreatment with silane coupling agent,and unmodified oil paper（P0）was used as a control group.A thermal aging experimental platform was set up.P0 and B5 samples were subjected to an accelerated heat treatment at 130°C.The physical,chemical and electrical properties of the oil paper were tested at different thermal aging stages.（2）A platform for testing the physical and chemical properties of oil paper has been set up to reveal the deterioration of the internal structure of oil paper due to thermal aging and the improvement mechanism of the physical and chemical properties of oil paper by nanoparticles.The results of the study show that thermal aging reduces the activation energy of the oil paper and the cellulose macromolecule chains are easily broken,resulting in the reduction of various physicochemical properties.Nano-Ti O₂ can combine with cellulose molecules to form hydrogen bonds,making the glycosidic bonds in cellulose molecules less likely to be broken and the internal structure is more stable.The physicochemical properties of the modified oil paper are improved at different stages of thermal aging.（3）The charge accumulation and dissipation characteristics of the oil paper surface were investigated by corona charging method,and it is found that the doping of nanoparticles reduces the charge accumulation effect whether the paper is charged with a positive or negative voltage,and the surface of oil paper is more likely to accumulate negative charges.The study of the trap energy level distribution of oil paper shows that the addition of nanoparticles reduces the trap density,with shallow traps having a higher density than deep traps,resulting in a faster decay of charge in the early stages and a slower decay in the later stages.The increased aging leads to an increase in charge accumulation inside and on the surface of the pressboard,and the rate of decay of positive charges is higher than that of negative charges at different stages of thermal aging.（4）The oil paper was pre-applied with DC voltage to build up space charge at the oil paper interface and nanoparticles are found to attenuate the effect of space charge distortion fields.The results of the AC/DC and polarity reversal breakdown tests carried out on the oil paper show that the modified oil paper composite system has superior breakdown performance at different stages of thermal aging compared to ordinary oil paper.