Suppressing Interface Charge Between LDPE And EPDM For HVDC Cable Accessory Insulation

HVDC extruded cable is the key equipment of flexible DC transmission technology.However,the interfacial charge between different dielectrics is easy to accumulate in the HVDC cable accessories due to the diccontinuty of conductivity and permittivity,which will cause electric field distortion and insulation failures.In this dissertation,new EPDM composites for HVDC cable accessories were explored and the effects of surface molecular structure design,nonlinear conductivity and doping of nanoparticles on the interface charge behaviors of EPDM/LDPE bi-layer insulation were researched by experiments and numerical simulation,in order to provide experimental and theoretical support for HVDC cable accessory insulating materials with high performance.The main research work and conclusions are as follows:(1)The interface charge behaviors and trap distribution of surface molecular structure modified EPDM/LDPE bi-dielectrics was investigated with the improved PEA method.It is found that the formation of C-F layer on the EPDM surface can reduce the dielectric constant and the interface charge accumulation.The suppression effects owe to the weakening of surface state in trapping interface charge and the shielding effect in preventing space charge injection.(2)EPDM/SiC composites were prepared by physical blending method.The effects of SiC volume fraction on the nonlinear conductivity and carrier mobility of the composites were analyzed.Results show that the threshold field of nonlinear conductivity is decreased and the nonlinear coefficient increases with the SiC volume fraction.The change of EPDM trap level decreases the effects of interface barrier on blocking the space charge migration.In addition,the nonlinear conductivity characteristics can uniform the electrical field between the electrode and the dielectric and thus reduce the charge injection at the same time.The introduction of the shallow traps promotes the detrapping process and accelerates the charge dissipation.(3)The effects of nano-sized carbon black doping on the dielectric properties,conductivity,carrier mobility and trap energy level distribution of EPDM were tested and analyzed.Small amount of CB nanoparticle doping can reduce the conductivity and carrier mobility,thus improving the matching degree of EPDM/LDPE interface and decreasing the interface charge accumulation.Besides,nanoparticle doping introduces the deep traps and increases the Schottky barrier for charge injection from electrodes,which then improves the space charge accumulation and the electric field distortion.(4)Numerical simulation of interface charge behaviors was performed by using a bipolar charge transport model with different interface characteristics including the surface states,interface barrierand carrier mobility.Furthurmore,the influence of interface charge accumulation on the electric field distribution in HVDC cable accessory was analyzed.The model provides a more accurate numerical simulation for the regulation mechanism of the interface charge,which offers theoretical support to suppress interface charge accumulation in the HVDC cable accessory.