Effect Of Oxygen Content On Thermo-oxygen Aging Characteristics Of XLPE Cable Insulation At High Temperature

With the advancement of the urban power transmission and distribution network into the ground and the rapid development of modern industry,the cable consumption and load demand are increasing,which puts forward higher requirements for the insulation performance and reliability of power cables.Cross-linked polyethylene（XLPE）power cable has become the first choice for modern power transmission cable due to its excellent electrical and physio-chemical properties,light structure,convenient installation,and not limited by height drop.At present,in-service cables in some cities have facing aging problems,which may lead to sudden failures in the process of power transmission and consumption.Therefore,it is of great significance to explore the aging characteristics of XLPE cable insulation according to actual operating conditions for the prevention of sudden failures of transmission cables.In this paper,accelerated thermal oxygen aging under different conditions was carried out on 110 k V XLPE power cable.The micro-structure and performance of XLPE samples were characterized by Fourier infrared spectroscopy,scanning electron microscopy,broadband dielectric spectroscopy and power frequency breakdown test.Then the thermal oxygen aging mechanism at elevated temperature considering the oxygen concentration and the ion catalysis and other factors was studied.Firstly,the XLPE samples were thermally aged at 160℃ in air.At the initial stage of aging,a slight increase of the carbonyl（C=O）index,a slight decrease of the number of spherulites and the changeless crystallinity of XLPE samples can be observed.The structure of spherulites is still complete and there is no substantial damage.After 96 hours,the carbonyl index begin to rise rapidly,and a rapid decrease of the crystallinity can be found.It can be also observed that the integrity of the spherulite structure has been destroyed and some defects on spherulites appear gradually.At this time,the breakdown field strength at 50Hz（E_b）of samples decreased significantly.After aging for 240 hours,E_b is about 58.26k V/mm,which is about 22.95%lower than that of unaged sample.Then,in order to study the effect of oxidation reaction rate on thermal oxygen aging,thermal oxygen aging test at 160℃ was carried out at three different pressures of 0.06MPa,0.04MPa and 0.02MPa.The results show that the structural and performance parameters of XLPE samples show a change trend of basically unchanged at first and then rapidly deteriorates with an obvious inflection point.When the air pressure is 0.02MPa,the oxygen content in the air is least,and the inflection point of structural and performance parameters appears in 240 hours of aging;At a higher air pressure of 0.06MPa,the inflection point is advanced to 144 hours of aging,that is,the higher the oxygen concentration in the aging atmosphere,the earlier the inflection point appears.Therefore,the time of inflection point is closely related to the speed of oxidation reaction.In order to confirm this inference,the oxidation induction period（OIT）of different aging samples was measured.It can be found that the time of inflection point corresponds to the time required for the antioxidant in the sample to be completely consumed.In the early state of aging,because the oxidation reaction is inhibited by antioxidants,the micro-structure and the macro-performance of XLPE are basically changeless.After the total consumption of antioxidants,the oxidation reaction proceeds rapidly and the structure and properties deteriorate rapidly.Finally,several cables with a length of about 8cm were aged at 160℃ in air.During the aging,these cables show non-uniform deterioration along the radial direction.Specifically,the aging speed of outer insulation is the fastest,and that of middle insulation is the slowest.In the aging process,the invasion of oxygen is from outside to inside.It is easier for oxygen to invade the outer insulation than the inner and middle insulation,so the oxidation reaction rate is higher and the aging speed of the outer insulation is the fastest.The inner insulation is closer to the copper core,and the copper in the cable core can diffuse into the inner insulation and act as a catalyst in the process of oxidation reaction,which improves the rate of oxidation reaction.Therefore,the aging rate of inner insulation is also higher than that of middle insulation.