| The inner semi-conductive shield layer（abbreviated:semi-conductive layer）,as an indispensable transition structure of High Voltage DC cables（HVDC）,plays the role of homogenizing the electric field and closely connecting the core and insulation layer.HVDC cable insulation layer with the growth of the operating age,the internal space charge accumulation more and more serious,which in turn leads to insulation material electrical performance degradation,threatening the safe and stable operation of the cable.At present,the more concerned about the High Voltage cable insulation material nanocomposite dielectric will reduce the purity of the insulation material,long-term operation there is a risk of breakdown characteristics degradation.Studies have shown that the performance of the semi-conductive shield layer as a mandatory path for charge injection from the metal core to the insulation layer can affect the charge accumulation characteristics of the insulation layer.Therefore,it is important to carry out research on the modification of semiconductive shielding materials to solve the problem of charge accumulation in the insulation layer of HVDC cables.In this paper,the effect of graphene（denoted as G）and its derivative nitrogen-doped graphene（denoted as NG）on the space charge accumulation characteristics of the insulation layer of HVDC is investigated from two perspectives:semi-conducting composite body modification and the"semi-conducting shield/insulation layer"interface.On the one hand,from the perspective of semi-conducting composite body modification,G and NG were used as the second conductive additives to study the effect of their doping on optimizing the surface roughness of the semi-conducting composite,suppressing the PTC effect and suppressing the space charge accumulation characteristics of the insulation layer under the effect of G.On the other hand,from the perspective of semi-conducting layer/insulation layer interface,highly conductive and flexible G was used to coat the interface,the effect of different interface coating states on the space charge accumulation characteristics of the insulating layer and the breakdown field strength of the composite specimen is investigated.Firstly,the G layer was prepared by redox method;the NG powder specimens were prepared by heat treatment method,seven kinds of semi-conducting composite specimens with different ratios were prepared by regulating the G and CB contents,and four kinds of semi-conducting composite specimens were prepared by regulating the NG content;the highly conductive and flexible G was modified by KH-570 to enhance its adhesion with the target specimen surface,coated on the surface of the semi-conductive or insulation layer specimens,and three kinds of composite structure specimens were prepared by hot fusion welding without coating layer,semi-conductive layer only and insulation layer only.Secondly,the microstructures of G and NG powders were characterized by transmission electron microscopy（TEM）,Raman spectroscopy（Raman）and X-ray diffractometer（XRD）,respectively.It was found that the layer number of G powder was7 layers with good reduction effect;the sheet diameter of NG was roughly 2.93～5.02μm with complete structure and less defects.G and NG were introduced into the semi-conductive shielding material as the second conductive additives,respectively,and the effects of different conductive particles and contents on their physicochemical properties were investigated by regulating the amount of conductive particles added using scanning electron microscopy（SEM）,atomic force microscopy（AFM）and infrared（FTIR）.Microstructure analysis showed that the number of conductive particles and conductive pathways increased significantly with the increase of conductive additive content;roughness analysis showed that with the increase of G content,the high doping amount of G decreased by 72.5%compared with undoped graphene specimens;with the increase of NG content,the roughness showed a trend of decreasing and then increasing,and the0.5phr NG specimens decreased by 32.13%compared with undoped NG specimens,and the comparison found that the modification effect of G on the surface is significantly greater than that of NG.Thirdly,the effects of the second additives G or NG on the high temperature stability and space charge accumulation characteristics of the composites were characterized by means of volume resistivity,electroacoustic pulse assay（PEA）and thermally stimulated depolarization current（TSDC）.The volume resistivity analysis shows that the volume resistivity tends to decrease with the increase of the conductive additive content,and the inhibitory effect of G on the PTC effect of the semiconducting composites is significantly better than that of NG.The analysis of the charge characteristics of the insulation layer shows that the amount of space charge accumulated inside the insulation layer is reduced by 44.7%for the same CB content with high doping G than with undoped G semiconducting composites.It was found that 3phr G instead of5phr CB could effectively reduce the space charge accumulation in the insulation layer.1phr NG semiconducting composite had the lowest internal space charge of 1.2×10-7C,with a reduction of 40.89%.It was found that the inhibitory effect of G on space charge accumulation inside the insulation layer was significantly better than that of NG.Finally,three composite structure specimens with different coating states were prepared by thermal fusion method and characterized by scanning electron microscopy,thermally stimulated depolarization current and universal breakdown tester for their microstructure,space charge accumulation characteristics inside the insulation layer in different interfacial contact states and breakdown characteristics of composite specimens with different interfacial states,respectively.The microstructure analysis shows that the semiconductive layer coated with graphene has a more uniform surface,and the interface contact of the semiconductive layer coated composite specimen is tight and without air gap.The analysis of space charge accumulation characteristics of the insulation layer shows that the composite specimen coated with only the semiconductive layer has the lowest space charge inside the insulation layer of 1.56×10-9C,which is51.55%lower than that of the uncoated specimen.The breakdown characteristics showed that the graphene-coated semiconductive layer/insulation layer composite specimen had the highest breakdown field strength of 184.75 k V/mm.The overall analysis showed that the best interface matching characteristics were achieved by coating the graphene on the semiconductive layer side only. |
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