| The deterioration of insulation performance of GIS basin insulator is one of the important factors that lead to GIS equipment failure.The external defects represented by metal particles and the internal defects represented by bubbles/cracks can cause electric field distortion and partial discharges in the micro area forming complex thermal stress.It leads to the disintegration of condensed microstructure of epoxy resin insulating dielectric(EP),which is the main reason for the performance deterioration of basin insulator.However,due to the unclear understanding of the damage mechanism of dielectric molecular chain and the evolution law of the deterioration process,the current suppression methods of basin insulator deterioration still have great limitations.To deeply and systematically study the influence law and deterioration mechanism of local defects on the insulation performance of EP,achieving active suppression and self-healing of dielectric damages induced by defects by adjusting the micro/nano structure of insulating materials and introducing self-healing components,which is of great academic significance and engineering application value to ensure the safe operation of GIS equipment.In this paper,the evolution of micro/nano structure and the degradation of insulation properties of EP induced by linear aluminum particles attached on the surface and internal bubbles/cracks were studied.And the molecular dynamics model of cross-linked epoxy resin system was established to analyze the damage process of EP molecular chain caused by thermal stress induced by partial discharges at the end of attached particles,and the micro mechanism of the influence of free volume distribution and charge trap level of the system on bubbles/cracks induced electrical tree growth characteristics at different ambient temperatures was studied.On this basis,it was proposed to comprehensively suppress the particle lifting,surface insulation degradation of EP induced by the attached particle,and electric tree damage of EP induced by bubbles/cracks by adjusting the micro/nano structure of the insulating material and the using the self-healing composites.The main research contents and corresponding conclusions are summarized as follows.(1)The dynamic behavior observation platform of aluminum particles in GIS scaled experimental chamber was built.And the deposition characteristics of aluminum particles on the surface of insulator with different sizes were obtained.On this basis,aluminum particles were attached to EP surface,and the evolution law of micro/nano structure and the degradation of insulation performance along the surface were studied.Besides,the growth characteristics of EP electrical branches were obtained by the micro image acquisition system of the electrical tree channel.And the three-dimensional morphologies of the electrical branches were reconstructed by laser(excitation wavelength 488 nm)induced auto-fluorescence in situ.The results show that aluminum particles are deposited in the area near the metal shell on the insulator surface,and the electric field stress concentration at the end of the attached particles induces partial discharge,which leads to the deterioration of EP along the surface.With the increase of degradation time,the leakage current,roughness and C–Fn bonds content of EP surface gradually increase,while C=C and C–O bonds content decrease.When the ambient temperature increases,the voltage of EP tree decreases and the volume of electrical branch increases per unit length,and its morphology changed from dendrite(40°C)to cluster(60°C and 90°C).(2)The 1:1 ring opening crosslinking reaction between bisphenol A diglycidyl ether and Methyltetrahydrophthalic anhydride was simulated by Perl script,and the molecular dynamics model of crosslinked epoxy resin system was automatically constructed.Then,Reax FF molecular dynamics simulation was used to simulate the pyrolysis process of crosslinked system under thermal stress induced by partial discharge at the end of aluminum particles.The evolution law and formation path of gas pyrolysis products and carbon were obtained,and the micro mechanism of EP surface degradation induced by attached particles was revealed.Besides,molecular dynamics simulation was used to calculate the free volume ratio of the crosslinking system at different ambient temperatures,and the micro mechanism of the influence of ambient temperature on the bubbles/cracks induced EP electrical tree growth was revealed.Under the effect of thermal stress,the fracture of C–O bond in the main chain leads to the decrease of cross-linking degree of EP.And the rapid release of gas pyrolysis products(mainly CO2 formed by decarboxylation reaction),which leads to the increase of surface roughness and structural relaxation evolution of EP.And the carbon(sp3)in the?position of phenyl ether and the carbon in the meta position of phenyl ether fall off from the main chain,resulting in the stacking of carbon element C2 and the formation of carbonization short-circuit channel.The increase of ambient temperature will significantly increase the free volume of EP crosslinking system and reduce the trap energy level,which leads to the enhancement of carrier damage to the molecular chain,thus promoting the growth and density of electrical treeing in EP.(3)Zn O microvaristors were synthesized by ball milling,spray granulation and high temperature calcination.The Zn O microvaristor/EP nonlinear conductive composite films were prepared by physical blending method.The effects of doping concentration of Zn O microvaristors on the partial discharge and the particle lifting field were studied,and the optimum doping concentration was obtained.Furthermore,the mechanism of the composite film suppressing PD and particle charging was revealed by COMSOL simulation.The results show that when the doping concentration is 70wt%,the nonlinear conductivity composite film can effectively suppress the partial discharges at the end of the particles and increase the lifting field to more than 2.8 times that without coating.The reason is that the conductivity adaptive regulation of the composite film can effectively relieve the concentrated electric field stress at the end of the particles,thus preventing the particles from charging from the partial discharge source,However,if the doping concentration continues to increase,the charging rate of the particles caused by the conduction of the film will be accelerated,and the enhancement of the lifting fields of the particles will be weakened to a certain extent.(4)Graphene fluoride sheets(GFS)obtained by liquid phase exfoliation method were used as thermal management materials,and hydroxyapatite nanowires(HAPNWs)synthesized by solvothermal method were used as toughening auxiliary materials,and the composite film was fabricated by vacuum assisted self-assembly and impregnation.Then,the inhibition mechanism of GFS content on the surface degradation induced by attached aluminum particles was studied,and the optimal doping content was obtained.The results show that when the quality of HAPNWs:GFS=2:3,the planar thermal conductivity of the composite film is the highest(6.2 W·m-1·K-1,about 30 times higher than that of EP),and the decrease of flashover voltage along the surface is reduced from24.5%of EP to 9.6%(taking the deterioration to 10 days as an example).The reason is that the directionally overlapped GFS in the composite film provides an efficient way for phonon transmission and greatly improves the thermal stress dissipation capacity of the dielectric.With the increase of GFS content,the inhibition effect of the composite film on the thermal stress-induced degradation of the particles increases.But if the GFS content is further increased,the composite film will be easily broken during the preparation process.(5)Silicon coated?-Fe2O3(?-Fe2O3@Si O2)nanoparticles were synthesized by improved St(?)ber method,and the magnetic controlled microcapsules with?-Fe2O3@Si O2/polyurea formaldehyde resin composite shell and UV photosensitive liquid core are prepared by Pickering emulsion polymerization.And?-Fe2O3@Si O2nanoparticles incorporated into the shell serve as magnetic targets and help the two microcapsules to navigate to the region of high electrical stress forming magnetically gradient-distributed microcapsule/epoxy self-healing composites.On this basis,the electrical properties and self-healing characteristics of the self-healing composite were studied,and the mechanism of the magnetic controlled microcapsule on the repair of electrical tree damage was revealed.The results show that the breakdown voltage of the composite after secondary self-healing is about 90%of that without electrical tree damage,indicating the continuous photo induced self-healing of bubbles/cracks induced electrical tree damage is realized at low capsule content.The microcapsules are magnetically distributed in the area prone to electrical damage,which can greatly reduce the doping concentration of microcapsules and improve the repair efficiency,and fundamentally solve the contradiction between low self-healing component content and high self-healing performance of traditional composites. |
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