Research On The Long-term Degradation Process And Failure Mechanism Of The Insulation At The SF₆-epoxy Interface Under Operating Conditions

The reliability of gas-solid interface insulation is directly related to the stability of DC equipment and the DC power network.The most widely used SF6-epoxy resin interface insulation structure has been studied for its flashover failure mechanism under operating stress characteristics,which is an important means to improve the reliability of DC power equipment.However,existing studies on the DC flashover characteristics of the SF6-epoxy gas-solid interface have focused on short-term failure mechanisms and on the correlation between short-term surface charge accumulation patterns and DC flashover characteristics.This approach is usually based on the assumption that the intrinsic state of the epoxy surface material remains stable over the full cycle of the power equipment.The explanatory power of the relevant theories is weakened for DC equipment with internal insulation flashover failures after long-term operation,so the existing theories cannot accurately reveal the causes of gas-solid interface failure under long-term operating conditions,and it is difficult to provide reasonable and accurate support for the optimal design of insulation materials and insulation structures.In practice,epoxy surfaces are subjected to the combined effects of DC electric fields and temperature gradient fields,and with the gradual miniaturisation of equipment and increasing transmission power,epoxy surface insulation is subjected to more intense electrothermal conditions,and the question of whether the microstructure of epoxy surface materials changes under the combined effects of long-term electric and thermal conditions and how this affects long-term surface charge build-up and flashover characteristics becomes inevitable.Therefore,this paper investigates the evolution of the intrinsic properties of epoxy surface materials under DC-temperature gradients and the evolution of the surface charge build-up properties to reveal the long-term failure mechanism of the SF6-epoxy solids interface insulation.The results of this paper are of great significance for further clarifying the direction of performance regulation of insulation materials inside DC equipment,ensuring the safe and stable operation of DC gas-insulated power equipment and improving the reliability of DC power networks.To address the problem that the long-term evolution characteristics of epoxy surface charges under DC-temperature gradient are unclear,which restricts the analysis of long-term DC electric field and insulation failure mechanism along the surface,the short-term and long-term accumulation characteristics of epoxy surface charges under DC-temperature gradient are measured,and the spatial and temporal distribution characteristics of surface charges considering the degradation of epoxy surface intrinsic materials are obtained.The short-term charge accumulation mechanism on the epoxy surface under DC-temperature gradient is proposed to be the enhancement of the normal field at the interface due to the temperature-dependent difference in material conductivity between the two sides of the gas-solid interface,and the large amount of charge provided by Schottky injection at the high-temperature electrode accumulates on the epoxy surface during the migration process under the influence of the distorted normal field and dominates the surface charge accumulation characteristics.Three stages of long-term surface charge accumulation under a DC-temperature gradient are delineated:a rapid phase of average charge density growth dominated by Schottky injection and migration along the distorted normal field,a phase of surface charge redistribution due to the differential influence of the induced electric field on the charge transport process in the high/low temperature region,and a phase of secondary surface charge growth due to degradation leading to a continuous increase in charge density in some regions of the epoxy surface,the latter two of which The latter two phases are to some extent dependent on high applied voltage and high hot spot temperature respectively.For the 10 mm high epoxy pillar,the three phases of the surface charge build-up characteristics at the temperature gradient inflection points occur around 48 h and 264 h of voltage application,respectively.To address the problem that the long-term evolution of the intrinsic physicochemical parameters and surface charge trap parameters of epoxy surfaces under the combined effect of DC-temperature gradients is unclear,and the intrinsic mechanism between the secondary growth phase of surface charge and the evolution of epoxy surface materials is unknown,28-day degradation experiments were carried out under different combinations of electrothermal stress,and the changes of chemical groups,elemental ratios and surface charge trap parameters of epoxy surfaces with degradation time were obtained.It was found that the degradation led to a decrease in the content of methyl,hypomethyl and methylene groups on the epoxy surface,with a maximum decrease of 17.7%,while the change in the content of ether and carbonyl groups was less than 5%,but it had a polar effect.The highest percentage of change in the oxygen element is found in the area of high temperature surface degradation and has a polar effect.The epoxy surface and cross-sectional morphology observations confirm that the material degradation leads to the formation of a large number of pores at the mesoscopic scale.The evolution of the material surface intrinsic parameters directly leads to a change in surface trap properties,with both the density and energy level of surface traps increasing after degradation.Simulations of molecular density of states and electrostatic potential reveal that the source of charge traps introduced by material degradation is the free radicals generated by weak bond breakage,and that the molecular structure changes due to molecular bond breakage increase the density and energy level of charge traps.At the same time,the significant charging characteristics of oxygen-containing radicals make them prone to become negatively charged and migrate under the influence of electric fields,leading to a polar effect in the trend of the ratio of oxygen elements on the surface.The microscopic causes of the significant deterioration of epoxy under DCtemperature gradients were investigated and revealed to be the dominant factors in the deterioration of epoxy surface insulation properties.It is found that the weakest bond energy of the epoxy backbone is more than 55.0%lower than that of other typical organic insulating materials,and that the charge trap energy level in the epoxy molecular structure is higher and the polar sites on the molecular surface have a stronger charge binding effect.It is revealed that the dominant factor of degradation in the high and low temperature degradation regions of the epoxy surface under the DCtemperature gradient is different,with the dominant factor of degradation in the high temperature region being the thermal cleavage of the molecule under the action of heat energy,and the dominant factor of degradation in the low temperature region being the increase in local electromechanical energy caused by the accumulation of surface charges.As the surface degradation will further lead to an increase in surface charge traps,the charge density will further increase,forming a positive feedback phenomenon of "degradation-increase in traps-increase in charge density-degradation".The long-term flashover characteristics of epoxy under DC-temperature gradients are investigated and the correlation mechanism between the long-term flashover probability and the long-term evolution of the surface charge induction electric field under different polarity voltages is revealed,pointing out that when a positive polarity voltage is applied to the low-temperature electrode,the negative polarity surface charge concentration region gradually moves away from the low-temperature triple bond with the redistribution process,which is the intrinsic reason for the reduction of the initial electron source and the gradual decrease of the flashover probability.The different evolution of the electric field at positive and negative polarity voltages is also an important reason for the polarity effect on the long-term flashover probability.Based on the above research results,a long-term failure mechanism for gas-solid interface insulation under DC thermal stress is proposed,taking into account the deterioration of the epoxy surface material,charge accumulation and electric field evolution,i.e.:high temperature thermal cracking and the electromechanical energy provided by the electric charge promote the breakage of weak bonds and the formation of free radicals on the epoxy surface molecules,the effective adsorption area of the molecular skeleton to the charge increases after bond breaking and the adsorption strength increases,and the free radicals introduce new electron traps in the forbidden band structure.The introduction of new electron traps in the forbidden band structure will lead to a continuous increase in surface charge density The migration and diffusion of charged radicals and hydrocarbon radicals lead to significant changes in the ratio of oxygen elements on the surface and the formation of numerous cracks.The distortion of the electric field by the surface charge leads to a long-term evolution of the surface charge,and the combination of the evolution of the surface electric field,the strong field region near the triple bond on the low temperature side and the high charge density region eventually leads to flashover.Finally,this paper finds a method for long-term insulation reliability enhancement under DC electrical heating conditions based on silicon nitride inorganic insulating ceramics.The results of this paper provide a theoretical basis for the longterm reliability enhancement of SF6-epoxy interfacial insulation under DC thermal operating conditions.