Insulation Modifications And Electrical Properties Of EPDM Used For High Voltage DC Cable Accessories

High voltage DC cable accessories are essential equipment for the DC transmission system.Under DC voltage,the electric field distribution in accessories mainly depends on the conductivity of the accessories insulation and the cable body insulation(XLPE),which are closely related to temperature.This paper researches the insulation materials of cable accessories that are made of EPDM.The DC dielectric properties of EPDM are improved by filling with inorganic powders and grafting with organic compounds,respectively.The conductivity of the modified EPDM is reasonably matched with XLPE to achieve a uniform distribution of the electric field in accessories,ensuring that the accessory insulation has more significant insulation margin,thereby improving the long-term work stability of the cable accessories.The sol-gel and high-temperature calcination are used to prepare copper calcium titanate(CCTO)powder.After melt blending and hot pressing,high dispersion and low-filling CCTO/EPDM composites are prepared.The conductivity,dielectric properties,and breakdown strength of CCTO/EPDM composites are investigated.The results show that the conductivity of CCTO/EPDM composites increases significantly with the increase of CCTO content.Moreover,its conductivity exhibits obviously nonlinear characteristics with the increase of the electric field strength.Thermal stimulation current(TSC)results show that the decrease of the charge trap energy level and density with doping CCTO,which effectively improves the carrier mobility.The relative dielectric permittivity and dielectric loss of CCTO/EPDM composites increase significantly with the increase of CCTO content,while breakdown strength decreases slowly.The finite element electric field simulation results show that the conductivity of 8vol.%CCTO/EPDM is reasonably matched with XLPE,which can effectively alleviate the local concentration of electric field in accessories.It is the first attempt to apply small polar molecules and auxiliary cross-linker agents for the graft modification of EPDM,respectively.The dielectric properties of EPDM can be improved by adjusting the local electronic state distribution.The electronic energy state density and functional group dissociation are calculated by the first-principles method to select the most influential small polar molecule and auxiliary cross-linker agents,and combined with results of the thermal stimulation current(TSC)test to analyze the conductivity characteristics and carrier transport mechanism of EPDM modified with two organic molecules.The results indicate that the dissociation energy of the hydroxyl group in graftable small molecule glycerol monooleate(GMO)is low,and the ionic carriers generated by the dissociation under a high electric field can increase the conductivity of EPDM.The benzene ring and maleimide structure in auxiliary crosslinking agent N,N’-1,3-Phenylene bismaleimide(N,N’)form a large electron delocalization space,which can effectively improve the carrier mobility.EPDM grafted with two polar molecules can introduce shallow traps,which can alleviate a wide margin decrease of breakdown strength caused by the increase of conductivity.The element electric field simulation results show that the effect of homogenizing electric field distribution of EPDM modified by two kinds of organic molecules is similar to that of 8vol.% CCTO/EPDM.The amount of organic molecular grafting is small,which can not affect the process performance of the material,and has a limited negative impact on the electric strength.Aromatic ketone voltage stabilizers,VPE and AOHBP,are selected as other types of organic molecules.The graft ability of VPE and AOHBP is characterized by infrared spectroscopy,and the influences of VPE and AOHBP grafting on the DC breakdown strength,conductivity,and electrical tree resistance of EPDM are systematically investigated.The results indicate that VPE and AOHBP can be grafted onto EPDM molecular chains via free radical addition reaction in cross-linking process.And the voltage stabilizer molecule can effectively inhibit the initiation and growth of the electrical tree in the amorphous polymer EPDM and increase the DC breakdown field strength of EPDM at different test temperatures.The mechanism is that VPE and AOHBP possess both a high electron affinity energy and a narrow energy,which can absorb hot-energy electron energy in a non-ionized way and release it in a non-Auger manner way.Meanwhile,the benzene ring groups of VPE and AOHBP molecules possess both a high atomic vibrational frequency and carrier capture cross-section,which can absorb the kinetic energy of hot-energy electron.VPE grafting can increase the trap depth and inhibit carrier transport,resulting in the decrease of conductivity in EPDM,so it is called a conductivity-inhibiting voltage stabilizer.AOHBP grafting can reduce the deep trap density and increase the shallow trap density,resulting in improving the conductivity and conductivity nonlinearity of EPDM,so it is called a conductivity-enhancing voltage stabilizer,which can improve the electric field distribution of the accessories significantly.In particular,the graftable AOHBP has excellent resistance to emigration.Its effect of improving the conductivity and breakdown field strength of EPDM is maintained efficiently for a long time.