The Effect Of Thermal Aging On Properties Of Nano-SiO₂ Modified High Voltage Direct Current Cable XLPE Insulation Materials

Compared with traditional high voltage AC power transmission,high voltage DC power transmission has such advantages as small line corridors,low failure rate,large transmission capacity and transmission distance,as well as accessing the asynchronous grid.As the main insulation materials of DC power transmission cable,crosslinked polyethylene?XLPE?insulation materials have more excellent mechanical properties and thermal stability on the basis of maintaining the excellent electrical properties of polyethylene?PE?,so they are widely used in electrical power grid.With the constant rise of voltage grade,higher and more comprehensive requirements are being proposed to the performance of the cable insulation materials in transmission and distribution lines.Currently,one of the main methods is to dope nanoparticles into the XLPE matrix to improve its electrical performance.In the actual operation process of cables,the load current and short circuit current can heat the conductor of the cables.Degradation will occur when insulation materials suffer long-term impact of heat and oxygen,leading to complex physical changes and chemical reactions.Thermal stress combined with other aging factors seriously accelerates the aging and deterioration of cable insulation materials,which threatens the safe operation of cables.Doping nanoparticles can improve the performance of insulation materials,but the researches concerning the aging-resistant performance of the modified insulation materials are not many enough.Therefore,the research on the thermal aging performance of Si O₂/XLPE insulation materials has great significance on developing nano-modified high voltage DC XLPE cable insulation materials.In this paper,the accelerated thermal aging experiments were conducted on unmodified XLPE and Si O₂/XLPE,and the influence of thermal aging on the microscopic structure,dielectric properties and mechanical performance was investigated.Microscopic structure of the samples experienced different thermal aging periods was characterized by using scanning electron microscope?SEM?,infrared spectroscopy?FTIR?,differential scanning calorimetry?DSC?and gel content testing.Electrical properties such as DC breakdown strength,electrical conductivity,space charge distribution,relative permittivity and loss factor,and the mechanical performance of the two kinds of insulation materials before and after thermal aging were tested at the same time.Experimental results showed that the long time thermal aging could cause the electrical conductivity increase,the space charge accumulate,DC breakdown strength and the mechanical strength decrease for unmodified XLPE and Si O₂/XLPE insulation materials.Macroscopic performance of insulation materials was closely connected with the changes of the microscopic structure.The breakage of molecular chain,damage of crystallization and production of the chemical groups are the primary causes of the macroscopic performance degradation for insulation materials.By comparison,Si O₂ nanoparticles endowed XLPE insulation materials with better ageing resistance properties in some respects.