| With the increase of voltage level,the effect of space charge on the insulation performance of oil-impregnated pressboard in converter transformer is significantly enhanced.How to suppress the accumulation of space charge is an important issue of high voltage.Doping modification of TiO2 nanoparticles is one of the effective methods to solve the space charge problem.The two-phase interface introduced by TiO2 nanoparticles can accelerate the dissipation of space charge.The formation of the two-phase interface is closely related to its scale and surface modification.Therefore,it is necessary to study the effect of TiO2 nanoparticles' size and surface modifier on the modification of creeping flashover in order to further explore the modification mechanism.Fistly,the creeping flashover voltage of oil-impregnated paperboard modified by TiO2 nanoparticles with different scales(5nm,10nm and 15nm)and different modifiers(none,proper and excess)was tested.Secondly,the surface potential decay curves of all kinds of oil-impregnated paperboards were tested by the isothermal surface potential decay device and the trap parametes were calculated by the T.J.Sonnostine model.Finally,combined with the streamer images captured in its' development process,the bubble model was used to illustrate the effect of shallow traps on the development of streamer at oil-impregnated pressboard induced by TiO2 nanoparticles.Due to permittivities are also changed by TiO2 nanoparticles,tests to verify the better permittivity match being good to insulation performer were designed to carried out.The results of flashover under lightning impulse show that when the scale of TiO2 nanoparticles is reduced from 15nm to 5nm,the improvement proportion of flashover voltage under positive lightning impulse increases from 1.1%to 17.43%,and that of negative flashover voltage increases from 5.86%.to 32.48%.Oleic acid indirectly affects the modification effect of TiO2 by changing the distribution of nanoparticles in transformer oil.The modification effect of TiO2 without oleic acid modification is the worst,and that of nanoparticles modified with appropriate amount of oleic acid is the best.Modification is reduced after the addition of excess oleic acid.Results of isothermal surface potential decay experiments show that TiO2 nanoparticles can accelerate the dissipation rate of space charge deposited on the paperboard surface and reduce the space charge accumulation.And the smaller the TiO2 nanoparticles' scale is,the faster the space charge dissipate.It takes approximately 2*105 seconds to decay from the initial potential to near zero potential for transformer oil-impregnated paperboard,while that for oil-impregnated paperboard modified by 5nm,10nm and 15nm TiO2 nanoparticles just lasts 40,80 and 7000 seconds,which is 2?4 orders of magnitude faster.Trap parameters of oil-impregnated pressboards indicate that TiO2 nanoparticles can reduce the trap depth of oil-impregnated pressboard and increase the density of shallow traps.The shallow traps are conducive to the dissipation of space charge.TiO2 nanoparticles can reduce the trap depth of oil-impregnated pressboard and increase the density of shallow traps.The shallow trap can speed up the electrons speed on the surface of the pressboard,so that the space charge of the same polarity with the streamer channel is generated in the streamer tip,thus an electric field opposite to the direction of the field strength is suppressed to suppress the streamer development,thereby improving the creeping flashover voltage.Since TiO2 nanoparticles can not only increase the shallow trap density,but also improve the dielectric constant matching of oil and paperboard to reduce the influence of polarization charge.Therefore,TiO2 nanoparticles increase the flashover voltage of the oil-impregnated paperboard by increasing the shallow trap density and the degree of dielectric matching between transformer oil and paperboard. |
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