Investigation On Effects Of TiO₂ Nanoparticles On Surface Flashover Characteristics Of Transformer Oil-Pressboard Insulation System

With the rapid development of UHV power grid,the insulation level of power transformer has been put forward higher requirements.Oil-pressboard is the main insulation medium in oil-immersed transformer,and its insulation performance is directly related to the safe and stable operation of transformer.In recent years,the research results at home and abroad show that the addition of nanoparticles can improve the insulation performance of transformer oil-pressboard insulation under lightning impulse voltage.However,there is no systematic study on the effect of nanoparticles on the modification of transformer oil-pressboard.At the same time,the mechanism of surface flashover modification of nano-modified oil-pressboard insulation has not been reported.Therefore,the surface flashover characteristics,streamer development characteristics and electron migration characteristics of transformer oil-pressboard insulation modified by different size spherical(5nm,10nm,15nm)and rod-shaped(aspect ratio 3,aspect ratio 4,aspect ratio 5)nanoparticles were tested in this paper.Furthermore,the mechanism of surface flashover modification of transformer oil-pressboard insulation by TiO2 nanoparticles is obtainedUnder standard positive and negative lightning impulse voltage,the influence of TiO2 nanoparticles on the flashover characteristics of transformer oil-pressboard was tested.The experimental results show that the addition of TiO2 nanoparticles can increase the flashover voltage and prolong the cut-off time of oil-pressboard.The smaller the size of spherical nanoparticles,the better the modification effect of transformer oil-pressboard;the smaller the aspect ratio of rod nanoparticles,the better the modification effect of transformer oil-pressboard.At the same time,the modification effect of spherical nanoparticles on transformer oil-pressboard is higher than that of rod-like ones.Moreover,the positive and negative flashover voltage of 5 nm spherical TiO2oil-pressboard with the best modification effect were 35.82%and 27.33%higher than those of pure oil-impregnated paperboard.Based on the supplementary exposure method and schlieren method,a streamer-shooting platform was built to observe the influence of TiO2 nanoparticles on the development characteristics of positive and negative streamer at the oil-pressboard interface before breakdown.The results show that the addition of TiO2 nanoparticles inhibits the streamer development rate and accelerates the streamer dissipation rate.The smaller the size or aspect ratio of nanoparticles,the shorter the cut-off length of streamer.For positive streamer,the surface streamer develops along the surface of pressboard and extends to the surrounding liquid.The addition of nanoparticles makes the branch of streamer channel more.For negative streamer,the surface streamer of oil-pressboard is fully adhered to the surface of pressboard and develops forward.The addition of nanoparticles makes the branch of streamer channel less.Based on the breakdown time delay method,an electronic mobility test platform for oil-in-oil and oil-pressboard interface was built.The effects of nanoparticles and pressboard addition on the electronic mobility in flashover were studied.The test results show that the electronic mobility of transformer oil increases by 58.1%with the addition of TiO2 nanoparticles.While the electronic mobility of transformer oil-pressboard interface increases by 15.6%with the addition of TiO2 nanoparticles.In other words,the addition of pressboard weakens the influence of nanoparticles on the electron migration process.Finally,the modification mechanism of the nanoparticles on the flashover characteristics is revealed.On the one hand,the addition of TiO2 nanoparticles will introduce a large number of shallow traps,so that the charge is transported in the shallow trap zone,which accelerates the dissipation of charge and reduces the distortion effect of space charge on the electric field.On the other hand,the addition of TiO2 nanoparticles increases the dielectric constant matching coefficient,resulting in more uniform electric field distribution in the oil-paper medium,and lowering the electric field strength in the oil gap on the cardboard surface,thereby improving the interfacial insulation properties of the transformer oil-pressboard.