Study On Electrical Properties Of EPDM Composite Dielectric Based On Intrinsic Conductivity Behavior Of Inorganic Filler

With the gradual increase in the scale of power grids,the role of high-voltage direct current（HVDC）technology in the field of power transmission has become more and more significant.Among them,the cable accessory is one of the important components of the HVDC transmission system.Because of the complex design structure and the large difference in conductivity between the insulating materials of each part,the electric field distribution inside the cable accessory will be uneven.At the same time,the cable core usually works in At higher temperatures,the internal temperature of the accessory decreases radially,resulting in a temperature gradient,which in turn affects the conductance distribution inside the accessory,leading to local electric field concentration and affecting the insulation strength of the accessory.Therefore,in this thesis,EPDM-based composite media was prepared by doping and modifying EPDM with three inorganic fillers with different properties.Firstly,the prepared EPDM-based composite media was characterized by microscopic morphology and structure,and then the DC conductivity and DC breakdown strength properties of the prepared composite media were studied.Finally,the intermediate joint model was established by COMSOL Multiphysics simulation software to study the use of composite media.When used as reinforced insulation materials,the internal electric field distribution of cable accessories under the conditions of electrical and thermal coupling is studied to study the effect of non-linear composite media to homogenize the internal electric field.This article first selects silicon carbide（Si C）with excellent nonlinear conductivity as inorganic filler,prepares Si C/EPDM composite media by adjusting the doping content of Si C,and studies its conductivity and breakdown characteristics at different temperatures.The results show that the increase of silicon carbide doping content will increase the conductivity of the composite medium,and the nonlinear conductivity characteristics will become more significant,but it will reduce the DC breakdown field strength of the composite medium;temperature changes will cause the DC conductance of the composite medium and The DC breakdown characteristics have changed significantly.An increase in temperature will increase the conductivity of the composite medium,but will reduce the nonlinear conductivity and DC breakdown field strength.The excellent I-V nonlinear characteristics of copper calcium titanate（CCTO）make the prepared CCTO/EPDM composite media have higher nonlinear conductivity.When the doping content is 30 wt.%,the nonlinear coefficient can reach 5.71,and the temperature The increase of the CCTO/EPDM composite medium’s conductivity increases,the threshold field strength and the DC breakdown field strength decrease,but it still has a higher nonlinear conductivity coefficient.After VO₂ with metal-insulator phase change（MIT）characteristics is doped into EPDM,the conductivity of VO₂/EPDM composite medium with increasing doping content and temperature changes the same as that of Si C/EPDM composite medium,but when the temperature increases At70°C,the filler VO₂ in the VO₂/EPDM composite medium is transformed into a metallic state,resulting in a higher conductivity of the VO₂/EPDM composite medium,but this is accompanied by a serious deterioration of the breakdown strength.The simulation results show that the composite medium prepared by doping inorganic fillers with nonlinear conductivity characteristics can significantly reduce the electric field concentration phenomenon at the root of the stress cone of the intermediate joint,and as the doping content of inorganic filler increases,the effect of reducing the electric field at the root of the stress cone is more effective.