The Influence Of Typical Defects On The Electric Field Distribution Of XLPE Cable And Joint Under The Thermal Aging Condition

With the rapid development of urban construction and the vigorous promotion of distribution network projects,the role of power cables in urban distribution networks has become more and more important.Cross-linked polyethylene(XLPE)power cables have become the preferred choice for modern power transmission cables due to their excellent electrical and physicochemical properties,lightweight construction and convenient installation methods,which are not limited by height drop.However,due to the influence of production process,transportation conditions,installation process and operation environment,it is difficult to avoid various defects in the body and intermediate joints of power cables.These defects may cause electric field distortion during operation,thus accelerating insulation aging and even resulting in breakdown accidents,which seriously affect the safe and stable operation of power lines.Therefore,it is necessary to conduct research on the defects in power cable bodies and connectors as well as the resulting electric field aberrations.First,the 35 k V XLPE cable body and its connectors are studied,among which the XLPE and silicone rubber(SIR)insulation materials are used for accelerated thermal aging tests.The samples with different aging times are tested using a broadband dielectric spectrum tester to obtain the relative permittivity and conductivity values of the two materials at different degrees of thermal aging.The material parameters are provided for subsequent simulation studies.Then,a three-dimensional simulation model of 35 k V XLPE power cable body is established by COMSOL finite element simulation software.The effects of moisture in the cable body,outer sheath damage,impurity particles and raised defects in the inner semi-conducting layer and the size of the defects on the electric field distribution of the cable body are investigated.The results show that the four typical defects cause different degrees of electric field distortion,with the most serious ones caused by impurity particles reaching.186.9%;The lowest electric field distortion rate is caused by the outer sheath damage which is 25.7%.Based on this,the temperature distribution function along radial direction is constructed by simulating the temperature field of the cable body.The distribution function of the relative dielectric constant of XLPE along the radial direction of the cable considering thermal aging conditions is established by combining the function of oxygen absorption of semi-crystalline polymer at different temperatures.Simulation of electric field distribution of the cable body with defects is performed.After considering the thermal aging conditions,the electric field distortion caused by the defects near the outer semiconducting layer gradually increases with aging time,and the electric field distortion caused by the defects near the inner semiconducting layer gradually decreases with aging time.Finally,a three-dimensional simulation model of 35 k V XLPE power cable intermediate connector is established by COMSOL finite element simulation software.The effects of different types and sizes of defects on the electric field distribution of cable intermediate connectors are investigated,including five typical defects: broken prefabricated parts,scratches on the surface of the main insulation,raised semi-conducting layer,impurities in the insulation layer and water branches.The results show that the five typical defects cause different degrees of electric field distortion,with the most serious ones caused by water branches reaching 440%;The lowest electric field distortion rate is caused by scratches on the main insulation surface which is 25.7%.Considering the thermal aging factor,the electric field aberration rate caused by impurities in the insulation layer and scratches on the surface of the main insulation increases gradually with aging time;The electric field aberration rate caused by two kinds of defects,namely,raised semi-conducting layer and water dendrite,decreases with aging time;The electric field aberration caused by broken prefabricated parts is not sensitive to thermal aging.