Study On The Dielectric And Electrical Characteristics Of The Main Insulation For High Temperature Superconducting Amorphous Alloy Transformers

High-temperature superconducting amorphous alloy transformer（HTSAM）is a new type of power transformer with high-efficiency transmission,energy saving,noise reduction,safety and reliable,and it has a very good energy saving and emission reduction effect.However,HTSAM insulation materials run locate in liquid nitrogen（LN）,and the low temperature environment puts forward higher requirements on its physical,chemical and electrical properties,the traditional cellulose oil paper has been unable to be competent for the main insulation of HTSAM,and polyimide（PI）film as a low temperature resistant insulating material with excellent electrical and mechanical properties can be used as the main insulating material for of HTSAM.In this thesis,PI films modified by nano particles are used as the main insulating material.PI films with excellent dielectric and electrical properties are prepared by nano-modification methods.A test platform is constructed to investigate the effects of LN environment,nanoparticle type and mass fraction on the physicochemical,dielectric and electrical properties of the composite PI films.The main findings are as follows:（1）Modified PI films were prepared,which showed a flat surface with few defects by scanning electron micrographs.The Fourier infrared spectrograms of the films met the classical absorption peaks of PI films indicating that the prepared films met international standards.The water content of the films gradually decreased with the increasing of nano mass fraction,the breakdown field strength of the water-containing films is smaller than that of the unabsorbed ones,the breakdown field strength of the nano-SiO₂/PI films is larger than that of the Al₂O₃/PI films,and the breakdown field strength of the 25μm films is larger than that of the 50μm ones.The tensile strength of the films gradually decreased with the increasing of nano mass fraction.The decreasing of the tensile strength of the 25μm films is smaller than that of the 50μm ones.（2）The surface charge density of nanocomposite PI films decreases with the increasing of nano mass fraction,and the surface charge density in liquid nitrogen（LN）environment is smaller than that of the ambient temperature.The surface charge density of nano-SiO₂/PI films in both environments is smaller than that of nano-Al₂O₃/PI ones,the surface resistivity of films increases proportionally with the nano mass fraction and then decreases.The surface resistivity of the films showed an increasing trend and then decreases with the increasing of nano mass fraction,with the surface resistivity of 25μm ones is greater than that of 50μm ones,and the surface resistivity in LN is greater than that of the ambient temperature.（3）The effect of LN and nano-modification on the corona aging for PI films is investigated by a corona aging pretreatment system.The surface of corona-aged pretreated films showed defects formed by organic degradation,the tangent of dielectric loss angle（tanδ）of pretreated films decreased and then increased proportionally with the nano mass fraction,and the tanδof pretreated films in LN is smaller than that of pretreated films at ambient temperature,and the tanδof pretreated films is larger than that of unprepared ones.（4）The PDIV and AC breakdown field strengths of the films follow the same trend,both of them increasing and then decreasing with increasing of nano mass fraction.25μm ones have a greater PDIV than that of 50μm ones,and the PDIV of PI films in LN is greater than that in ambient environment.The interaction between the nanoparticles and the polymer matrix is tighter due to the condensation reaction in LN.In addition,the low temperature reduces the thermal effect on the film surface to decrease the thermal damage and increases the breakdown field strength.The AC breakdown field of 25μm PI films in LN is greater than that of 50μm ones for the same mass fraction with the nanoparticles adding;...