| High frequency power transformer(HFPT)is the core device of the power router that facilitates the flexible interconnection of electrical energy,the insulation performance of which under high frequency circumstance relates directly to and determines the development of high-voltage,compact-size,high power density,and large-capacity power routers.Owing to the magnetic flux leakage loss of core materials and skin effect of metal conductors,the long-term operation temperature of HFPTs is much higher than that of ordinary power transformers,putting forward higher requirements for the heat resistance characteristics of the insulation system.To ensure that the equipment is compact,the classic insulation system of HFPT includes windings'coating polyimide insulation film and the outer layer of pouring epoxy insulation.The insulation issues have gradually become one of the bottlenecks that restrict the wide application and development of HFPTs.Particularly,the polyimide film,which is directly contacted with the windings,bears high temperature,high frequency,and high voltage impact.The polyimide film is,therefore,considered as the insulation weakness of the equipment.The high frequency insulation performances determine the operation state of the whole insulation system in HFPTs.Under the high frequency and high voltage electric stress,the polyimide film faces the strong impact of dielectric loss,heating and micro discharge,leading to the early failure of insulation.Therefore,it is necessary to modify the insulation polyimide for lower high-frequency dielectric loss and greater micro discharge endurance,with the excellent high-temperature resistance,which are the main needs of insulations of HFPTs.Based on the modifications,aiming at the key issues such as the action mechanism of modified parameters on its excellent high-frequency characteristics,the high-frequency damage mechanism of modified materials and so on.A systematic research is carried out and a comprehensive control mechanism is proposed to balance the insulation performances.Based on the molecular simulations,including quantum mechanics(QM)and molecular dynamics(MD),the reactivity of several kinds of sulfur-containing diamine monomers is analysed.It is found that when at least one amino group is set in the opposite position of S-atom in the diaminodiphenyl sulfide(SDA)monomer,its reactivity is high enough to be introduced into the macromolecular as the modifying medium.In addition,the influence mechanism of molecular structures on the long-chain configuration of polyimide is clarified.When 4,4'-SDA is selected as the modification monomer,the long-chain regularity of polyimide is highest,which is determined by the stronger electron loss of S atom and molecular symmetry.The basic physical and chemical properties of the modified polyimides are predicted.It is found that the thermodynamic properties of the polyimide would not be affected by the introduction of phenyl sulfide group into the traditional Kapton(?)-type polyimide(PMDA-ODA)with 4,4-SDA as the media.On the basis,a molecular-design modification scheme of polyimide is proposed,which is random copolymerization of diamine monomers(4,4-SDA and 4,4-ODA)and dianhydride monomer(PMDA)by some means.And then,regulate the synthesis parameters on the basis of high frequency insulation experiments.The modified polyimide films with 0%,20%,40%,60%,80%and 100%phenyl sulfide in diamine are successfully synthesized based on the two-step method of thermal imidization,respectively,whose structures are characterized by FT-IR,as well as the completion of imidization and the effectiveness of the modification.The basic physical and chemical properties of various polyimide film specimens,such as heat,and crystallization,are studied respectively.The influence of molecular structure modification on the excellent thermal properties of polyimide is identified.The tendency of the micro crystallization characteristics of the specimens with the content of phenyl sulfide in diamine structure is obtained.The dielectric properties of the modified polyimide film specimens are studied employing high frequency dielectric spectroscopy.It is found that the variation of dielectric loss is similar to that of the crystallinity and optical absorption,and the dielectric loss factor of the 40%-SDA specimen is only about 1/3 of that of 0%one.The mechanism of the influence of phenyl sulfide group in diamine structure on the dielectric loss factor of polyimides at high frequency is explained,that is to destroy the balance of charge transfer complexation between adjacent polyimides'macromolecules,increase the dispersion of the lossy polarization dipole,disperse the relaxation time and reduce the dielectric loss factor.Based on the theoretical derivation and the first principle simulation,the quantitative analysis model of the dielectric loss factor of polyimide is proposed with the charge density of the core atom of the electronic-loss structure as the variable.Furthermore,the regulation mechanism of dielectric loss characteristics of polyimide,based on molecular design is explained.The triggering behavior of polyimide films with different content of the phenyl sulfide is studied by using the high frequency creepage discharge platform with needle-bar electrode configuration.Based on the results of physical-chemical and dielectric properties,the influence mechanism of the modification parameters on the inception behavior of creepage discharge is explained,that the creepage discharge inception voltage is independent of the modification in a constant environment.Combining with the 2-D surface charge dynamic distribution test,the development processes of high frequency creepage discharge on the modified polyimide films after the start of discharge are studied,and the influence mechanism of modification on the initial,development and flashover stages of surface discharge is clarified.The interaction mechanism of high frequency creepage discharge and surface charge of the modified films is further revealed.It is found that a large number of positive half cycle surface discharge is mainly caused by the negative surface charge induced electric field,and the impact of discharge on the material will increase the accumulation of negative surface charge,which promotes and interacts with each other.Based on the high frequency creepage discharge platform,the creepage discharge tolerance characteristics of polyimide films with different modifications are studied.Considering the physicochemical properties,the effect of modification parameters on the high frequency creepage discharge endurance of polyimide is determined.It is found that PSI40 polyimide film with the lowest dielectric loss heating and photon absorption radiated by discharge has the best endurance performance.Combined the microscopic SEM morphologies of specimens before and after discharges and the reactive force field molecular dynamics simulations of polyimide models under the action of discharge,the micro mechanism of the discharge damage of polyimide is clarified.It is found that the polyimide molecular chain breaks from C-N bond in the imide ring and then breaks down gradually under the strong stresses of discharge,which makes the compact stacking structure loose and produces a large number of voids inside,while the acetylene and other small-molecular gases products are released to increase the void pressure.The material is broken down and completely losses its insulation performances.Therefore,it is important to achieve the balance between reducing the direct impact of the strength on the material and increasing the resistance of the material to the strong field,for the purpose of improving the high frequency creepage discharge endurance of polyimide.In this study,modified polyimide films were prepared to face the insulation requirements of the high frequency power transformer.The high frequency dielectric loss factor of the best type specimens is about 1/3 of the traditional Kapton(?)type,whose high frequency creepage discharge withstand time is about 3 times.Based on the experimental and simulation results,the molecular-design regulation mechanism of polyimide on high frequency insulation performance is proposed. |
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