Research On Deterioration Mechanism Of Insulation Performance Of XLPE In Long-term Service

Power cable lines are widely used in urban distribution networks with the advantages of high-power supply reliability,simple structure,easy installation and beautiful laying.In recent years,the frequent occurrence of cable accidents seriously affects the reliability of urban power supply.The high-voltage XLPE cable will be affected by various factors during the long-term actual operation,and its degradation process is extremely complicated,which lays hidden dangers for the occurrence of cable accidents.The thesis selects the insulation materials of 110 kV XLPE cables with long-term service for 15 years and 30 years as the research object,and takes the same type of high-voltage spare cables as the comparison group,and carries out corresponding research on the degradation law of the actual operating cables with the service life.In combination with micro,mesoscopic and macro,the degradation process of cables with different service life was analyzed and studied respectively,so as to provide theoretical reference for the degradation research of XLPE cable insulation in long-term service.Compared with the spare cable,the crystallinity of the long-term service cable sample gradually decreases with the service life,and the crystallization area is gradually destroyed.The large XLPE chain breaks,generating more small molecular chains of different sizes and other free radicals,making the thermal weight loss the first The proportion of the phase increases,and the thermal stability decreases;the long-term electro-thermal coupling effect gradually volatilizes the cross-linking impurities in the cable,and the carbonyl index of the material drops rapidly;the cable suffers multiple short-circuit faults during service,and the high temperature caused by the short-circuit current makes the cable insulation The crystallization behavior changes,and the failure temperature cooling process is accompanied by recrystallization,which generates crystal grains of different sizes,which reduces the melting temperature and widens the melting range.According to the space charge distribution characteristics,as the service life increases,the crystallization zone destruction and the occurrence of thermal oxygen aging gradually enlarge the internal defects of the cable,the accumulation of space charge packets is more obvious,and the density of deep traps inside the sample increases.The further destruction of the crystalline region provides more free space for the movement of the macromolecular chain.The local operation of the main chain and the movement of the entire chain become the main source of dielectric loss.Because the cable recrystallizes during service operation,smaller crystal grains will cause the interface to increase,which reduces the free path of electrons,makes breakdown more likely,and causes the power frequency short-term breakdown field strength to gradually decrease.This paper also explores the correlation between the speed of sound of the medium and the degradation of the cable.The study found that the sound velocity of the medium has a primary relationship with temperature.As the temperature increases,the sound velocity of the medium decreases in direct proportion.It may be that the increase in temperature makes the irregular movement of small molecules inside the sample more violent,hindering the transmission of sound waves in the medium.With the increase in service years,the sound velocity of the medium at the same temperature has increased.This may be due to the rupture of XLPE macromolecular chains,and the increase of short chains and small molecular groups provides a large number of resonance free radicals for the transmission of sound waves,speeding up the medium.The transmission of the speed of sound.The research results are expected to provide a new test method and theoretical basis for the assessment of the aging state of power cable insulation.