Investigation Of Nonlinear Electrical Conductivity Of EPDM Based Electric Field Grading Composites In High Temperature And Electric Field

According to the cross-region,high-capacity and high-voltage construction layout of power grids all around the world,the application and development demand of high voltage direct current(HVDC)transmission technology has been increasing.Considering that cable accessory is one of the important part of HVDC cable system,within which the electric field grading material is one of the essential part for keeping the stability of cable accessory.But the research about temperature influence is still insufficient.Solutions to improve the nonlinear conductivity as well as the performance stability at high temperature are restricted to using metal or carbon based conducting material.In this work,ethylene propylene diene monomer(EPDM)is used as the matrix,which is one of the commonly used insulating matrix materials in electric field grading composites.Zinc oxide(ZnO)is one of the most commonly used nonlinear conductive filler.Tungsten disulfide(WS2)is semiconductor with flake multilayer structure.Different content of these two semiconductors were used as nonlinear filler to faberacate ZnO/EPDM composite and WS2/EPDM composite respectively.Researches about those composites were made,including nonlinear electric conducitivity at different temperature and some other related properties.Details are shown as follow:In this work,ZnO/EPDM and WS2/EPDM composites were fabricated and their properties relating to nonlinear conductivity were investigated.Basing on those results,by using small amount of multi-wall carbon nanotubes(MWCNTs)or ILs,MWCNTs-WS2/EPDM,ILs-ZnO/EPDM and ILs-WS2/EPDM composites were fabricated.Properties of nonlinear conductivity at different temperature,DC breakdown strength,and thermal conductivity were investigated,respectively.The details are shown as follow:(1)ZnO/EPDM composites with different ZnO content were fabricated through a twin roller mixer.Researches were focused on the influence of temperature on the nonlinear conductivity as well as the influence of ZnO content on the thermal conductivity of composites.Results indicated that the nonlinear conductivity of ZnO/EPDM composite were stable at 25 ℃ and 50 ℃,but the nonlinear conductivity of 60phr-ZnO/EPDM composite met some decrease at 80℃.The nonlinear conductivity of ZnO/EPDM composites at 50℃ began to be weaker than the value at 25℃ when ZnO content is higher than 30phr.(2)Exfoliation were used to homogenize the nano WS2,and the morphology difference between original-WS2 and exfoliated-WS2 were compared.Original-WS2/EPDM and exfoliated-WS2/EPDM composites were fabricated and the nonlinear conductivity of them were studied.It indicated that the influence of exfoliation of WS2 on the nonlinear conducticity of WS2/EPDM composites were obvious,which also proved that the WS2 could be used as nonlinear filler.The influence of temperature on nonlinear conductivity was investigated,so did the DC breakdown strength and thermal conductivity.Obvious nonlinear conductivity was shown when WS2 content was higher than 30phr.The nonlinear conductivity of WS2/EPDM composites with 50phr and 60phr WS2 content met some decrease at 50℃ and 80℃.(3)Basing on above researches,very small amount of multi-wall carbon nanotubes(MWCNTs)were mixed with 30 phr exfoliated-WS2 to get the MWCNTs-WS2 mixture,which were further dispersed into EPDM to fabricate MWCNTs-WS2/EPDM composite.The influence of MWCNTs on thermal conductivity and nonlinear conductivity of MWCNYs-WS2/EPDM composite were inveetigated.It indicated that the moderate amount of MWCNTs contributed to the improvements of thermal conductivity.(4)ILs-ZnO/EPDM and ILs-WS2/EPDM composites were fabricated by using small amount of ILs mixing with ZnO and WS2 in advance,respectively.The experiment results shown that nonlinear conductivity of 0.75phr-ILs-ZnO/EPDM and 0.50phr-ILs-WS2/EPDM shown the best thermal stability.The addition of ILs had no influence on the thermal conductivity of composites.(5)Basing on the experiment results of ILs-ZnO/EPDM composites,the ILs content of 0.75 phr were chosen to fabricate 0.75phr-ILs-30phr-ZnO/EPDM composites to explore the influence of other 8 different ILs on the nonlinear conducticity.It indicated that there was not much difference of nonliear conducticity between them at 25 ℃ and 50 ℃.The difference turned to be obvious at 80 ℃.