| HVDC transmission systems are widely used because of their many advantages.Higher voltage DC cables have a very wide range of market demands.Space charge and steady state electric field distribution inversion are the bottleneck restricting the development of HVDC cables.Nano-modification of XLPE by doping a small amount of carbon black is a possible technical approach to overcome technical bottlenecks,that is,to effectively mitigate the electric field distortion in DC cable insulation.During the operation of cable,heat will be generated to cause insulation aging,which in turn will cause changes in the dielectric properties of the insulating material.The steady-state electric field distribution of HVDC cables depends on the spatial distribution of the electric conductivity of the insulating material,and the transient electric field distribution depends on the dielectric constant of the insulating material and the spatial distribution of the space charge.Therefore,it is of great academic value and engineering significance to study the effect of thermal aging of nano-CB/XLPE composites on the conductivity and dielectric parameters of the composites.Firstly,nano-CB/XLPE composites were prepared by mechanical blending technology,and the thermal aging test conditions were determined.The aging test temperature is 140℃,which is tested every two weeks,and total aging time of the test is 14 weeks.Finally,8 groups of test samples with different aging time were obtained,including the unaged samples.For the samples with different aging time,the conductivity at different temperatures and different field strengths was measured.The non-linear function fitting technique was used to obtain the non-linear function of conductivity and electric field strength and temperature.The experimental results show that the whole thermal aging process is divided into two stages: at the initial stage of aging,the conductivity decreases with the increase of the aging time,and the activation energy and the electric field dependence coefficient increase slightly with the increase of the aging time,but the change is not significant;at the later stage of aging,the conductivity increases rapidly with the increase of aging time,and the activation energy of the conductance and the dependence coefficient of the electric field decrease with the increase of aging time.For the samples with different aging time,the broadband dielectric spectrum of the samples was measured at the same time.The results show that the whole thermal aging process is divided into two stages: at the initial stage of aging,the dielectric constant and dielectric loss factor are small,and the change with aging time is not obvious;at the later stage of aging,the dielectric constant and dielectric loss factor increase rapidly with the increase of aging time,and the dielectric spectrum shows a significant relaxation polarization behavior.In order to explore the internal mechanism of the change in the dielectric properties of composites,this paper uses Fourier Transform Infrared Spectroscopy(FTIR),Scanning Electron Microscopy(SEM),Differential Scanning Calorimetry(DSC),and Isothermal Relaxation Current(IRC)test to analyze the composite materials with different aging time.The analysis was carried out to study the change of the microstructure and chemical composition of the materials with aging time,and the observation results well confirmed the variation of dielectric properties. |
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