Study On Interface Charge And Breakdown Characteristics Of Oil-immersed Nano Modified Insulating Pressboard

UHVDC transmission is widely used due to its large transmission capacity,long transmission distance and good economy.Converter transformer functions as the hub connecting the direct current（DC）and alternating current（AC）power system.The internal insulation of converter transformer which is subjected to DC,combined AC-DC and polarity reversal electrical field besides AC electrical.What’s more,the oil-paper interface is prone to accumulate charge and distorts the electric field under the action of an electric field,which can easily cause insulation damage.This thesis focused on the oil-immersed insulating paper inside the converter transformer.The modified cellulose paperboard with different nano-Ti O₂ particle size（5 nm,10 nm,20 nm,30 nm）and nano content（1 wt%,3 wt%,5 wt%and 7 wt%）were prepared in the laboratory to explore the influence of nanoparticles on the electrical properties of the cellulose insulating paperboard and revealed the mechanism of nano-particle on the interface charge and trap energy level.The main research work of this paper is as follows:（1）A test platform for dielectric constant and electrical conductivity was built to explore the dielectric properties and electrical conductivity of modified and unmodified paperboard.The research results show that the dielectric constant and electrical conductivity of the modified insulating paperboard are reduced,and the dielectric constant and electrical conductivity of the modified insulating paperboard first decrease and then increase with the increasing of nano content and particle size.It can be concluded that the optimal dielectric properties is when the nanometer particle size is 10nm and the nanometer content is 5wt%.（2）By adding nanoparticles to modify the cellulose paperboard,it is found that the accumulation of charge at the interface of the oil-impregnated paperboard is significantly reduced,and the trap energy level density is reduced.In addition,it is found that the decay rate of the charge is also significantly reduced;the addition of nanoparticles can fill the cellulose defects,reduce the trap density,and reduce the amount of charge accumulation.At the same time,the strong interaction between nanoparticles and cellulose will inhibit the transfer of charges,so the charge transfer rate of the samples after modification will be reduced;the content and particle size of the added nanoparticles will also have a greater effect on charge decay and medium trap distribution.The impact is mainly because the agglomeration of Ti O₂ affects the charge and trap distribution characteristics.Under the polarity reversal,the dynamic trend of the interface charge is basically the same as that under DC,but there are different charge decay rates under different reversal times,The longer the polarity reversal time,the lower the dissipation rate.（3）The AC and DC compression strength of modified oil-immersed paperboard with different nano content and particle size was tested.The results showed that the AC and DC breakdown field strength of the modified paperboard first increased and then decreased with the increasing of nano content.The content of 5wt%is the maximum value.The AC/DC breakdown field strength also first increases and then decreases with the increase of the nanometer particle size,and the maximum is at the particle size of 10 nm.The modified insulating paperboard has the best electrical performance when the nanometer content is 5 wt%and the particle size is 10 nm.In this paper,the cellulose paperboard with low dielectric constant,low conductivity,high compressive strength and capable of inhibiting the accumulation of interface charge has been obtained by adding nanoparticles.There are some important reference to improve the electrical performance of the internal insulation structure of the converter transformer.