Growth Mechanism And Suppression Methods Of Electrical Tree In EPDM For HVDC Cable Accessory

The high-voltage cable accessory is the key component of high voltage direct current(HVDC)cable transmission.In insulation materials of cable accessories,electrical tree plays a key role in the safe and reliable operation of HVDC cable system.This work researches into the insulation materials of cable accessories which are made of EPDM.The rules and mechanisms of charge transportation and electrical tree are investigated in EPDM based on various mechanical stress and temperature conditions of the cable accessories.The suppression mechanisms of electrical tree are researched in EPDM by employing octavinyl-polyhedral oligomeric silsesquioxane(OVPOSS)and antioxidants.This research provides experimental basis and theoretical support for insulation design of HVDC cable accessories.The main works and conclusions are as follows:(1)According to the growth characteristics of electrical tree in EPDM of cable accessories with mechanical stress,the electrical tree was promoted by the tensile strain of cable accessories.The inception probability and growth rate increased with the increase in tensile strain of EPDM.The growth characteristics of electrical tree showed an opposite trend with the compression strain,and the electrical tree growth was delayed.Based on the molecular dynamics simulation,it was found that the free volume and cohesive energy of EPDM were the key factors during the electrical tree growth.The physical model of electrical tree was established based on charge transportation.(2)According to the growth characteristics of electrical tree in EPDM with different temperatures,the inception process of electrical tree was promoted by the constant high temperature,and the deterioration area of electrical tree was also increased.The occurrence probability of bush tree increased with the temperature increasing.The physical structure and charge transportation of EPDM were in an unbalanced state during the temperature rising,which promoted the electrical tree growth and affected their morphological distribution.Higher temperature rising would lead to more serious degradation by electrical tree in EPDM.Based on local segment relaxation,free volume and inter-molecular potential energy characteristics of EPDM,a physical model of trap distribution,charge transportation and electrical tree was established with the temperature taken into account.(3)EPDM/OVPOSS composites were prepared by physical blending to suppress electrical tree growth.According to the physical and chemical properties of EPDM/OVPOSS composites,there was cross-linking effect between OVPOSS and EPDM when the OVPOSS content was low.The deep trap level and trap density became larger and the charge-capturing ability was improved.Thus,electrical tree suppression ability was enhanced in EPDM/OVPOSS composites.When the OVPOSS content was high,the cross-linking reaction of OVPOSS generated micro-scale physical defects.The deep trap level and trap density became smaller and the charge-capturing ability was weakened.Therefore,the electrical tree suppression ability was decreased in EPDM/OVPOSS composites.(4)Based on the trap distribution via isothermal surface potential decay technique and quantum chemical calculation,the chemical traps that were formed by the conjugated benzene ring,polar hydroxyl group of antioxidants caused the trap level and trap density of EPDM to increase.Then the charge transportation regulation ability was enhanced to suppress electrical tree growth.The composite antioxidant modified EPDM was with larger trap energy level and trap density than the single antioxidant.Thus,the charge transportation regulation ability became stronger and the electrical tree suppression ability was more remarkable.Employing the composite antioxidant to modify EPDM was an effective method for improving the electrical tree suppression in the cable accessories.