Study On The Space Charge Characteristics Of XLPE Cable Based On Electrical-heating Aging

With the development of city construction,the overhead line is replaced by the cable line as an inevitable trend.Cross-linked polyethylene cables have excellent electrical properties and heat-resistant quality,its manufacturing process is mature and simple,so it is widely used in urban distribution network.Under the influence of electric field and temperature and other factors,there will be space charge appear in insulating layer of the cable in the process of long-term operation.The presence of space charge is an important factor to produce aging of cable's insulation.Space charge accumulation in the insulation layer can lead to distortion of local electric field,Destroy the material microstructure of cable's insulation.As the material microstructure are destroyed,there can be more space charge accumulate in cable's insulation,ultimately results in an dielectric breakdown.In this paper,in order to find out the distribution law of space charge in cross-linking polyethylene cable and the law of space charge characteristics development with aging,complete the aging properties of cable's insulation.The cross-linking polyethylene cable's space charge characteristics under electric-heating aging were studied.It is of great significance for the actual operation of the cables.Electrical and thermal accelerated aging test was implemented to XLPE cable.Apply alternating voltage 26.1kV and select 103?,114? and 135? three temperatures to sampling and testing in various stages of aging.We test the cable's body space charge,slice's DC space charge and slice,s AC space charge by using Pulsed Electro-Acoustic method.In order to corresponding to the result of space charge measurement,test measurements include X-ray diffraction(XRD),infrared spectra,frequency domain dielectric spectroscopy and gel content are experiment on slices from cable's different part.The experimental results show that there are some space charge which don't disappear immediately along with the remove of the electric field after aging.The space charge's maximum accumulation position of the all these aging cables are both in the position that covered by cable stress relief cone and in the middle level of insulating layer.With the increase of degree of aging,the amount of charge in the maximum accumulation position increases.The specular and axial width in the area of charge accumulation broaden.In the dielectric spectrum test,peak dielectric loss of this area is higher than other areas.In the initial stage of DC polarization of the cable section,charge is rapidly injected.The saturation value of the space charge accumulation continuously raises after an initial period of lo?vering down.Space charge is accumulated in the AC polarization of the cable sections.The analysis using the full phase average method found that the growth rate of the space charge accumulation continued to decline with the pressure time prolonged.The saturated value of space charge accumulation in the sample and space charge amount of direct slicing presents a consistent characteristic.The decay time constant of the charge decay curve in the short circuit dissipation process continuously raises after an initial period of lowering down.in the different insulation location of cable in different aging degree,there is some laws that crystallinity of the middle layer is smaller than the inner and outer layer,and grain size of the middle layer is larger than the inner and outer layer.The gel content of cable,s insulation layer is gradually reduced from the inner layer to the outer layer,and the gel content of the position where are covered by the stress cone is larger than the other positions.