Modified Preparation And High-frequency Creeping Discharge Characteristics Of Silicon-oxygen Functionalized Polyimide Films

With the development of the Energy Internet,the instability of distributed energy has brought great challenges to the grid,and the high-frequency power transformer are an effective solution to this problem.As a common material for coating insulation of high-frequency power transformers,polyimide needs to withstand complex stress environments such as high-frequency electrical stress and thermal stress.Under the same electric field strength,compared to the breakdown of the body,the film is more prone to creeping discharge,which is one of the key factors restricting the development of high-frequency power transformers to high voltage,large capacity and high reliability.Studies have shown that modification helps to improve the insulation performance of materials and the introduction of silicon-oxygen structure into the polymer helps to improve its comprehensive performance.Therefore,different kinds of silicon-oxygen structures are used to modify and synthesize polyimide.The systematic study of the creeping discharge characteristics of modified polyimide under high-frequency alternating current stress is of great significance to the subsequent study of film modification.The main research results obtained in this paper are as follows:A modified preparation platform for polyimide film was built and the preparation process was optimized.The molecular weight of polyimide is increased,the introduction of impurities is reduced and the uniformity of the film is ensured.Two silicon-oxygen structure compounds of nano-silica(SiO2)and disiloxane(GAPD)were selected to modify the polyimide by nano-composite modification and molecular structure modification respectively.Name the pure polyimide P0,introduce 1%,2%,5%molar content of GAPD and name them G1,G2,G5 according to the content,and introduce equimolar content of SiO2 nanoparticles and name them as Sil,Si2,Si5 according to the content.For the seven polyimide films prepared by modification,the basic physical and chemical properties of the films were obtained by infrared spectrometer,ultraviolet-visible spectrophotometer,scanning electron microscope and other basic instruments.The research results show that the modified silicon oxide functionalized film can meet practical applications in terms of basic physical and chemical properties,but its physical and chemical properties are different.After adding SiO2,bright nanoparticles appeared in the polyimide matrix,but slight agglomeration appeared in Si5.The ultraviolet spectrum was slightly red shifted,and the crystallinity decreased.The surface resistivity and volume resistivity decreased,and the more SiO2 content,the faster it decreased.But the glass transition temperature and mechanical properties were improved.After the introduction of GAPD,the aggregation state of the polyimide matrix changed.The more GAPD content,the higher the crystallinity,the more the red shift of the ultraviolet spectrum,the stronger the film absorbs in the ultraviolet region,the stronger the charge transfer complexation,and the greater the surface resistance and volume resistivity,the better the glass transition temperature and mechanical properties.By building a high-frequency creeping discharge experiment platform in a constant temperature and humidity environment,the high-frequency creeping discharge characteristics of the silicon-oxygen functionalized polyimide film are obtained.The test results show that the initial voltage of the film's creeping discharge was in the range of 3.5-3,7 kV,and the P0 creeping voltage was the lowest.The introduction of two types of silicon-oxygen structures can help to improve the surface discharge life of the films.Among them,Si5 of Si group has the longest life,3.40 times that of P0,and G5 of G group has the longest life,which is 4.77 times that of P0.Further studied the long-term electrical aging process,collected and analyzed the surface discharge signals of Si5,P0,and G5.Found that the three films all exhibited obvious polarity effects,which were manifested in the positive half-cycle discharge amplitude and the number of discharges was far greater than the negative half-cycle.In addition,the discharge amplitude of the three films fluctuated in the early and mid-term and increased in the later period.The number of discharges continued to increase.The discharge form was dominated by dense discharge.The difference was that the G5 had the lowest surface discharge amplitude and the number of discharges in the whole process,and the least and the slowest growth rate.The characteristics of creeping discharge in the early stage of Si5 discharge were similar to that of P0,but in the later stage,the discharge of P0 developed rapidly and the intensity of discharge increased greatly.The mechanism of the influence of silicon-oxygen functional modification on the high-frequency creeping discharge characteristics of polyimide was further studied,and the internal correlation characteristics between the characteristic parameters of the high-frequency creeping discharge were established.It was revealed that the changes in the influence mechanism of physical and chemical properties on the characteristics of high-frequency creeping discharge of the modified polyimide.The research results show that the electrical resistivity mainly affects the development of corona at the film-air interface and the accumulation and dissipation of surface charges.The crystallinity mainly affects the de-trapping and sinking process of surface charges,and the charge transfer complexation and light absorption mainly affect the compactness of materials and spatial photoionization and other processes.The material structure mainly affects the scattering of electrons and other processes.In addition,in the process of long-term electrical aging,different physical and chemical properties have different effects on the discharge process at different periods and there is a synergistic effect.The two modification methods change the above-mentioned physical and chemical properties to different degrees,resulting in different characteristics of high-frequency creeping discharge,and provide a reference for the subsequent further adjustment of the modification plan.