Fabrication Of Micro-nano Al₂O₃/PTFE Functional Film On The Cellulose Insulation Pressboard Surface And Its Space Charge Supression Mechanism

The converter transformer is one of the most important apparatus in the high voltage direct current(HVDC)transmission system.The oil-paper insulation system of the outlet device in valve side withstands the complex voltages press such as alternating current(AC),direct current(DC),and AC superposed DC during operation.Therefore,it is the frequent occurrence of insulation faults in valve side.Under DC electric field,space charge injection and accumulation in the oil-paper insulation system will distort the electric field distribution,induce insulation aging and lead to insulation breakdown.At present,domestic and foreign researchers have adopted the method of bulk doping different nanoparticles to achieve the suppression of space charge injection of cellulose insulation paper.However,the inhibition effect is limited by the ununiform dispersion of nanoparticles.In view of the fact that the micro-nano structure functional layer could not only impart the novel characteristics to the material interface,but also maintain the material bulk property,research on exploring a new way of constructing the micro-nano functional layer on the surface of the cellulose insulating paper instead of the traditional doping of nanoparticles to realize the space charge suppression will provide a valuable reference for ensuring the stability of the oil-paper composite insulation performance of the valve side outlet device.In this paper,RF reactive magnetron sputtering technology was used firstly to fabricate a single micro-nano Al₂O₃ or PTFE functional layer on the surface of insulating pressboard.The micro-morphology and the composition of the single micro-nano layer were characterized.The basic electrical properties especially the space charge injection and accumulation characteristics of the functional layer of insulating pressboard were analyzed.Secondly,the experimental parameters were optimized based on the test results above to construct a micro-nano Al₂O₃/PTFE composite functional layer on the surface of the insulating paper.The space charge characteristics and DC breakdown characteristics of the insulating pressboard with micro-nano Al₂O₃/PTFE functional layer were tested and analyzed.Finally,the space charge supression mechanism of the micro-nano Al₂O₃/PTFE functional layer was discussed.The main results and conclusions obtained in this paper could be drawn as follows:(1)The single micro-nano Al₂O₃ or PTFE functional layer on the surface of the insulating pressboard was succussfully fabricated.The layer composition and surface morphology was characterized.It was confirmed that the micro-nano functional layer obtained by sputtering Al target by oxygen reaction was composed of Al₂O₃ in the amorphous region.The morphology characterization showed that the nanoparticles grew on the surface of the cellulose molecule and formed a fusion zone with the cellulose molecules.The fluorocarbon film could be obtained by RF magnetron sputtering of PTFE target.The PTFE molecules were also present on the surface of the cellulose molecule in an amorphous form of nanoparticles.As the sputtering time increased,the film thickness and the particle size increased.(2)The influence of single micro-nano Al₂O₃ and PTFE functional layer on the basic electrical properties,space charge and breakdown characteristics of oil-impregnated insulating paper was obtained.The single micro-nano Al₂O₃ functional layer lifted the electrical resistivity of the insulating paperboard,and showed a significant inhibition effect on space charge injection and accumulation.The volume resistivity of the pressboard with reactive sputtering sputtering Al target for 60 min is about 100 times higher than that of the untreated pressboard.The space charge injection amount is reduced by about 34%,and the DC pre-voltage breakdown voltage is increased by 13%.The micro-nano PTFE layer increases the volume resistivity of the insulating pressboard by about 20 times,the AC breakdown voltage by 8%,and the DC breakdown voltage by 3%.(3)The space charge suppression mechanism of the micro-nano Al₂O₃/PTFE composite functional layer was discussed.The construction of the micro-nano Al₂O₃/PTFE composite functional layer reduced the space charge injection amount of the insulating paperboard by 40%and lifted the DC pre-press breakdown voltage by15%.The mechanism of suppressing space charge injection could be attributed to two reasons:one is the increment of trap density and another is the increment of electrode/medium contact barrier.