Study On Conductance Nonlinearity And Breakdown Characteristics Of EPDM Based Composites

The insulation problem is one of the key issues in high voltage direct current(HVDC)transmission system,and the higher the system voltage level,the more important and difficult the insulation problem.As an important part of the HVDC cable transmission system,cable accessories are also the weak link of the transmission system due to local electric field concentration.The application of nonlinear conductivity materials is an effective solution to solve the problem of electric field concentration inside cable accessories,but ethylene propylene diene monomer(EPDM)as a cable accessory reinforced insulation material has weak field-dependent conductivity.In order to solve the local electric field distribution concentration problem in the accessory,in this paper,the EPDM is modified to intelligently change its electrical conductivity with the change of the electric field,and to form a good match with the electrical characteristics of the main insulation,while ensuring the breakdown field strength of the composite.In this paper,the conductive silver particles(Ag)and hexagonal boron nitride(BN)with excellent insulating properties is used as the fillers,and the single-doped Ag particles and Ag-BN co-doped EPDM-based composites areas research target.The effects of doping content and doping method on the microstructure,DC conductance and breakdown performance of composites were systematically studied.Although the single-doped Ag particles make the composites have better nonlinear conductivity,the breakdown strength of the composites is seriously degraded at the same time.The composites obtained by co-doping Ag-BN can guarantee a certain field-dependent conductivity,simultaneously the DC breakdown field strength of the composites is greatly improved.It is proved that the composites can have better comprehensive electrical properties by introducing the conductor Ag and the insulator BN together.In order to avoid the agglomeration of the nano-silver particles(AgNPs)and further improve their dispersion in the matrix,the AgNPs were grown directly on the flake BN by liquid-phase chemical reduction method to obtain AgNPs/BN powder.This not only avoids the reunion of AgNPs,but also has the combined characteristics of AgNPs and BN.The nonlinear conductivity,DC breakdown strength and thermal conductivity of AgNPs/BN/EPDM composites at different temperatures of 30 ?,50 ? and 70 ? were studied systematically.The results show that: with the increase of the content of AgNPs grown on the BN,the nonlinear conductivity of the composites is more obvious,accompanied by the gradual decrease of the threshold field strength and the increase of the nonlinear coefficient.The breakdown field strength of the composites is gradually reduced and has a weak temperature dependence.Compared with pure EPDM,the breakdown field strength of the modified composites is less affected by temperature,thus avoiding the severe degradation of the composites at high temperature,which is related to the high thermal conductivity of the composites by filling with AgNPs/BN.Finally,the simulation results show that the electric field concentration at the root of the stress cone is obviously improved when the modified composites is selected as the reinforced insulation of the cable joint,and the electric field distribution inside the joint is more uniform.