| The charges injected from the interface between semiconductive screen and insulation layer are the main source of space charge accumulation in crosslinked polyethylene(XLPE).DC XLPE cable failure often occurs near the interface,so the semiconductive screen directly influences the performance of high voltage direct current(HVDC)XLPE cable.The semiconductive materials made by Borealis are expensive and only match with XLPE applied in DC cables,which makes the cost higher in domestic HVDC transmission lines.So the preparation of semiconductive materials which are with the advantages of reliablity and powerful adaptability becomes so urgent.In this thesis,the semiconductive materials were added with the new nanoscale layered modified agents and the two-layer films were the different semiconductive screens heat-pressed to DC XLPE(XLPE applied in direct current cable)and AC XLPE(XLPE applied in alternating current cable).The semiconductive screen affecting charge transport at the interface,indirectly influencing charge transport and accumulation in XLPE,and the role of the semiconductive screen in charging progress were researched.The charge transport mechanism of two-layer film was preliminarily revealed.The average roughnesses and max roughnesses of the modified semiconductive screens which were measured by AFM were reduced.The modified agents changed the surface morphology of semiconductive screen and the interfaces between the semiconductive screens and DC and AC XLPE were more smooth,which were observed by SEM.The thermal diffusivity and the specific heat of semiconductive screens which were tested by laserflash thermal analyzer changed largely at 90?.The thermal diffusivity variation tendency of the modified semiconductive screen was the same with that of the unmodified semiconductive screen,which implied that the thermal characteristics were almost not affected by the modifed agents.The unmodified and modified semiconductive screens which would affect the charge injecting progress were comprehensively studied.When the modified semiconductive screens added with the same quality fraction modified agents were used as the electric contacts,the shapes of DC and AC XLPE charging currents were almost the same under reverse-polarity voltage.So the semiconductvie screen was more important than the insulation layer on the charge transport at the interface.Breakdown field strength of charging two-layer films reduced.The breakdown field strengths of DC XLPE with the unmodified semiconductive screen,and semiconductive screen added 2wt% and 5wt% modified agents declined by up to41.7%,27.3% and 26.0%.The breakdown fields of AC XLPE with the unmodified semiconductive screen,and the semiconductive screen added 2wt% and 5wt%modified agents declined by up to 31.7%,35.3% and 24.2%.The descent rate of the breakdown fileds were mainly lower when the modified semiconductive screens were as the electrical contact.So the reducing of DC and AC XLPE breakdown field at reverse-polarity voltage was suppressed by the modified semiconductive screen.At the tests of two-layer film conductive current under different temperature,the slopes of log curve of I-E were irregular and ln curve I-d were far away from-3.The charge transport of XLPE with semiconductive screen did not obey the MottGurney function.The conductive current tests were measured after different time charging at room temperature and 70?.The semiconductive screens added 2wt% modified agents were as the long-time injecting electrodes,and the transtion electric fields of DC XLPE and AC XLPE were 20 k V/mm.The transtion electric fields almost did not change with the increasing of injecting time and temperature.After long-time high field and high temperature injecting,the two segment slopes of I-E curves approached 1 and 2 when the unmodified semiconductive screens were as the electrical contacts.And the slopes almost did not change with the charging time and temperature.When the modified semiconductive screens were as the electrical contacts,the conductive current slopes ranged from 1.9 to 4.5 at high electric fields,which decreased with the injecting time and temperature.The charge transport of XLPE with semiconductive screen after long-time high field and high temperature injecting was qualitatively judged as the space charge limited current mechanism.The semiconductive screen played a dominant role in metal-semiconductvie screen-insulation-metal system when the charges injected,compared the conductive current values at different temperatures.The role of ionic conductivity was reassess in metal-insulation-metal system.The view that space charge limited current did not present in XLPE with Al electrode was proposed... |
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