Research On The Influence Mechanism Of Composition And Structure On The Dielectric Properties Of Insulating Pressboard

With the development of society,the demand for electricity is increasing,and the demand for oil immersed transformers,one of the core equipment for power transmission,is higher.The lower the dielectric constant of the pressboard in oil-paper composite insulation under AC voltage,the higher the overall withstand voltage level.Therefore,how to reduce the dielectric constant of the oil-immersed pressboard while ensuring the breakdown field strength is the key to improving the insulation ability of oil-paper.A large number of studies have found that various process parameters,fiber components,and pore size parameters of pressboard can affect the dielectric constant and breakdown field strength of insulating pressboard.However,currently,there is a lack of coordinated optimization between preparation parameters such as beating,hot pressing,doping,and the relationship between pore size parameters and dielectric and breakdown properties of pressboard.Therefore,it is impossible to obtain a combination of preparation processes for low dielectric constant pressboard and accurate adjustment methods for insulating pressboard through pore size parameters,Therefore,this article is divided into three parts to study it.Firstly,a study is conducted on the process level and combination,taking into account the five parameters of beating pressure,time,and hot pressing temperature,pressure,and time.The influence relationship and weight of the process level and combination on the performance of 1mm high-density coniferous wood insulated paperboard are studied.The goal of reducing the dielectric constant of oil-immersed paperboard is achieved by adjusting the combination of process parameters according to the weight.The internal space charge of oil-immersed paperboard is measured using the electroacoustic pulse method The surface potential attenuation method is used to measure the surface trap parameters of oil-immersed paperboard,and microscopic observation is used to observe the fiber morphology,analyzing the impact of the process on the fiber/paperboard.The optimal process combination for high-density insulating paperboard is obtained as follows:the beating parameters are set at 3 kg and 60 min,and the hot pressing parameters are set at 160℃,50 min,and 15 MPa.The mechanism of the influence of process and surface structure on performance is that high beating pressure can increase the degree of fiber splitting and brooming,limit the transfer of charges between fibers,reduce the size of oil gaps,suppress the formation of discharge channels,and improve the breakdown field strength of oil-immersed paperboard;Appropriate hot pressing temperature,pressure,and surface structure can reduce the density and depth of interface and surface traps,resulting in more uniform charge distribution and lower interface polarization and dielectric constant.Subsequently,based on the optimal process combination,a study was conducted on the effects of different component doping on the properties of pressboard.The types and proportions of doped fibers were selected,and the effects of doping on the properties of pressboard were explored through methods such as electroacoustic pulse method.The results showed that CNF,Tencel fiber,and needle wood fiber had the best effect when the doping ratio was 3%and 10%,respectively.At this time,the dielectric constants of oil-immersed pressboard were 3.247 and 3.379,respectively.On this basis,three fiber doping attempts were conducted,and the optimal doping ratio was obtained as follows:coniferous wood fiber:CNF:Tencel fiber=90:3:7,and the dielectric constant of oil-immersed pressboard was further reduced to 3.211.Doping CNF can reduce the oil gap in the pressboard,reduce the free travel of electrons,suppress the formation of discharge channels,and improve the breakdown field strength of the pressboard;Doped fibers can reduce interfacial polarization and replace high dielectric constant fibers to reduce the dielectric constant of oil-immersed paperboard by reducing the trap energy level inside the paperboard.Finally,based on the preparation adjustment,the pore size parameters of the pressboard were measured using the mercury intrusion method to explore the relationship between pore size parameters and the dielectric constant of the prepared and oil-immersed pressboard.The correlation adjustment mechanism of preparation pore size dielectric constant was obtained through ridge regression analysis in multivariate statistical analysis.Found that under the optimal preparation method,there were many small holes in the paperboard and the degree of polarization of the internal interface of the paperboard was low.The correlation between porosity(p1),average pore diameter(p2),median pore diameter(p4),total pore volume(p5),total pore area(p6)and the dielectric constant(εr)was calculated as:εr=5.155-0.009p1+0.003p2-0.201p4+1.965p5-0.055p6,The Goodness of fit of the model was 0.924,and the deviation between sample data and validation data and the predicted value was within 3%,The model has good predictive ability.After comprehensive preparation,pore size parameters,and dielectric constant analysis of oil-immersed paperboard,it was found that the method to reduce the dielectric constant of oil-immersed paperboard is to increase the degree of fiber splitting and brooming or add small fibers to increase the internal pores of the paperboard as much as possible during the preparation process.