High Frequency Insulation Characteristics And Enhancement Design Of Functionalized Modified Polyimide Films

In recent years,high frequency power electronic transformer is widely used in energy Internet to solve the problem of grid connection of distributed energy.As the interturn cladding insulation of high-frequency power electronic transformer,polyimide(PI)works under the impact of high-frequency electrical stress for a long time,accelerating its aging,cracking and even insulation failure process,resulting in its short-term breakdown performance and long-term voltage endurance life unable to meet the insulation requirements in actual operating conditions,so it is urgent to carry out modification optimization design work.At present,the research on insulation properties of polyimide is limited to the field of DC voltage and power frequency voltage,and there is a lack of relevant experimental data and theoretical analysis under high frequency electrical stress.As a result,the breakdown mechanism and corona damage mechanism of polyimide at high frequency are still unclear.Therefore,the research on the regulation mechanism of high frequency insulation properties of polyimide by functional modification cannot be carried out.To this end,this paper has carried out a large number of experimental exploration and theoretical analysis,and the main research results are as follows:The optimized design of polyimide was completed by the combination of bulk phase modification and surface modification.In situ polymerization,silica(SiO2)and cage polysiloxane(POSS)were introduced into PI for nano-composite modification,and disiloxane(GAPD)was embedded into PI molecular main chain for molecular structure modification by blending method.The mass fraction of modified fillers were 1 wt%,5wt%and 10wt%.The surface modification of the bulk phase modified film was realized.The composite film was hydrolyzed in a strong alkali solution and placed in aluminum chloride solution for 4h for ion exchange.The hydrolysis time was 15min,30min,60min,90min and 120min.Finally,polyimide composite films with Al2O3-siloxo-Al2O3 "sandwich"structure were obtained.The basic physical and chemical properties of the composite films under different modification methods,such as infrared spectrum,surface and section morphology,crystallinity and mechanical strength,were characterized,and the rationality of the modification scheme was verified.When the content of SiO2 is too high,there will be local agglomeration,which affects the arrangement of molecular chains and decreases the crystallinity.The dispersion of POSS is good,and its cage hollow structure increases the conjugated system of PI to the greatest extent.The molecular structure modification changed the aggregation state of PI matrix.After the introduction of GAPD,the intermolecular force was enhanced,and the crystallinity and tensile strength were improved.The infrared spectrum showed that the surface modification retained the bulk phase modification structure,and Al2O3 was formed after secondary imiination,which increased the roughness of the surface.Moreover,with the longer hydrolysis time,the more Al2O3 content,the crystallinity and tensile strength decreased.A high frequency insulation test platform was set up to study the short time breakdown and long time voltage resistance of PI films before and after modification.The breakdown voltage of PI at high frequency increased nonlinearly with the increase of thickness.When the frequency increases from 5kHz to 20kHz,the breakdown field decreases by 28.96%35.13%.The bulk phase modification has an excellent effect on the improvement of the breakdown strength when the doping content is 5wt%.The order of breakdown field intensity at the same frequency is POSS/PI>GAPD/PI>SiO2/PI>PI.Based on the electrothermal coupling breakdown mechanism,the surface ablation phenomenon of the thin film at high frequency was analyzed,and the expression of the critical breakdown field intensity with respect to frequency and temperature was derived based on the thermal imbalance theory.When hydrolyzed for 60min,the corona resistance time of the modified film was 56.5min,4.04 times higher than that of PI(11.3min).The corona damage mechanism at high frequency was described from the aspects of ionization process,temperature effect and oxidative corrosion.The regulation mechanism of different modification methods on insulation properties of polyimide was revealed,which provided a reference for the subsequent regulation and modification scheme.Based on the correlation between the microcosmic properties of thin films and the high frequency insulation properties,the effects of different modification methods on the microcosmic properties were compared and analyzed.Dielectric constant and loss are used to characterize the polarization response ability,and trap characteristics and resistivity reflect the carrier motion characteristics.The dielectric,trap and resistivity test results show that the interface polarization is enhanced after the introduction of SiO2,GAPD mainly affects the orientation polarization and the deep-sink meridian density increases,POSS is dominated by space charge polarization.The nano composite modification mainly improves the energy level of the deep trap,while the molecular structure modification mainly increases the density of the deep trap,which can trap more carriers,reduce the mobility of carriers,increase the resistivity,promote the accumulation of more charges in the trap and weaken the external electric field,thus improving the insulation performance of the modified film at high frequency.