Damage Mechanism Of Epoxy Resin Under Arc Ablation And Strategies To Enhance Its Arc Ablation Resistance

The surface electrical performance and mechanical strength of epoxy resin(EP)insulation in the break of HVDC circuit breaker is one of the critical factors that determine the reliability of the equipment.However,plagued by the relatively poor thermal conductivity,the EP insulation in the break is quite likely to be damaged under arc ablation during the operation process,which is the important bottleneck that restrict the development of HVDC circuit breaker.Focused on these issues,in this work,we studied the damage mechanism of EP insulation under arc ablation in depth and explored the corresponding strategies to enhance its arc ablation resistance.The thermal damage properties and surface electrical performance of EP under arc ablation were obtained firstly,and the thermal damage process,along with the surface insulation failure mechanism were revealed by virtue of the self-built pyrolysis kinetic model afterward.Based on the results,a kind of novel hierarchically oriented 3D boron nitride framework reinforced by the epoxy group grafted glass fibers was successfully built in EP matrix,thus synergistically enhancing the arc ablation resistance and mechanical strength of EP insulation.The main research contents and corresponding conclusions are summarized as follows,(1)An integrated experimental platform of EP insulation under arc ablation in SF₆atmosphere was successfully built up,and the thermal damage properties and surface electrical performance of EP were accordingly explored.Based on the channel model and energy balance principle of the burning arc,the calculation model of arc energy was obtained,and the parameters of the arc ablation experiment were determined according to the energy equivalence principle.After that,the thermal damage properties of EP under arc ablation were studied,along with the effects of arc ablation on surface morphology,roughness,surface conductivity and DC flashover voltage.The results indicate that,when the local arc is applied,the surface temperature of EP insulation begins to increase rapidly,finally leading to the penetrating damage of the insulation.With the increase in arc energy,the surface roughness and surface conductivity of EP increase remarkably,while its DC flashover voltage decreases distinctively,until the surface electrical performance of EP insulation is thoroughly destroyed.(2)The decomposition products and thermal response properties of SF₆/EP insulation under arc ablation were studied,and the corresponding thermodynamic mechanism was revealed.The results demonstrate that the SF₆ molecules in the test cavity are degraded into two kinds of carbides and four kinds of sulfides under arc ablation.And a large number of irregular fluctuations and pores appear on EP surface at the same time,generating regional amorphous carbides with sp³ hybridization.With the increase in arc energy,the relative contents of gaseous and solid products will tend to be in dynamic equilibrium and maintain synchronous growth after that.Based on the experimental results,a pyrolysis kinetic model of EP insulation under unilateral arc heat flow was built to study its thermodynamic properties under arc ablation,such as the evolution laws of phase transformation,pyrolysis degree,temperature,pore pressure and physical parameters,et al.Combined with the simulation and test results,the degradation process and thermal damage mechanism of EP insulation under arc ablation in SF₆ atmosphere were elucidated comprehensively.(3)The surface charge evolution law of EP insulation and surface streamer propagation characteristics of EP/SF₆ interface after suffering from arc ablation were determined.Based on the results,the effects of arc ablation on surface electrical performance of EP were analyzed.The amorphous carbides generated after suffering from arc ablation can decrease the dielectric properties of the insulation and form crystalline-amorphous interface defects on EP surface.This causes the thermions emitted from the HV metal electrode to migrate to the ablated region and accumulate there,accordingly forming local insulation failure region on EP surface.On the other hand,the generated carbides can decrease the propagation distance of the surface streamer and enhance the absorption of photons radiated from the head of the streamer when an external electrical field is applied.This is conducive to the propagation of surface streamer on EP/SF₆ interface and giving rise to the avalanche development of surface discharge,thus greatly decreasing the surface electrical performance of EP/SF₆composite insulation.(4)A kind of reinforced 3D skeleton was successfully fabricated via ice-templated assembly strategy,among which the exfoliated h-BN micro-platelets(BNNS)and the epoxy group grafted glass fibers(e GF)are served as assembly components,and the corresponding novel EP composite was obtained with the assistance of vacuum-assisted impregnation,thereby synergistically enhancing the arc ablation resistance and mechanical strength of EP insulation(when the BNNS concentration is 11 wt.%and e GF:BNNS=1:1,the arc ablation resistance of the 3D-e GF&BNNS/EP composite increases by 3.46 times,the thermal conductivity increases by 19.04 times,and the tensile strength increases by 85.5%,compared with those of the pure EP).The long-range aligned 3D BNNS based skeleton built in the polymer matrix can effectively improve the thermal management ability of EP,accordingly decreasing the carbonization degree of the insulation under arc ablation,while the incorporation of e GF can eliminate the interfacial defects between the 3D skeleton and EP matrix by virtue of the crosslinking reactions occur on e GF surface during the curing process of the resin,thus alleviating the localized strain concentration of EP composite.As an inspiring result,the contradiction between the enhancement of arc ablation resistance and mechanical properties of EP insulation was fundamentally solved.