Effect Of Magnetic Composite Electrode On Space Charge Injection In Polyethylene

The semiconductive shielding layer which acts as an important part of the high voltage direct current?HVDC?cables,can evenly distribute electric field during the high-voltage direct current transmission process and reduce the injection of space charge into the cable insulation layer.In order to further reduce the injection of space charge into the insulating layer,magnetic composite material electrode were successfully prepared.The experiment simulates the structure of HVDC cables and explores the effect of magnetic composite electrode on space charge injection in polyethylene.1.Fabricate strontium ferrite(SrFe₁₂O₁₉)were fabricated by sol-gel method and sol-gel combustion method.In this section,the detailed synthesis conditions were also investigated,including precalcination temperature,calcination temperature and the amount of iron and antimony compounds.In addition,the magnetic properties of strontium ferrite prepared,which were influenced by different methods under different ratios?Fe/Sr?,were also detected.These results show that precalcination temperature and calcination temperature are 600? and 900?,respectively.And the suitable ratio of Fe/Sr is less than 10.Comparing the hysteresis loops of strontium ferrite prepared by the two methods,the coercive force and residual magnetization demonstrate different trends.It is found that the strontium ferrite,which was prepared by sol-gel combustion synthesis method under fitting ratios?Fe/Sr=9,10?,shows saturated hysteresis loop and enhanced the magnetic performance.2.Carbon black?CB?,ethylene-vinyl acetate copolymer?EVA?,low density polyethylene?LDPE?and crosslinker are used as raw materials.The semiconductive shielding layer of CB/EVA/LDPE was prepared by melt blending and hot pressing cross-linking method.The dispersion of carbon black and the resistivity under different temperature were investigated.Subsequently,the CB/EVA/LDPE semiconductive shielding layer was used as the electrode,and the effect of different electric field at different temperature on the space charge injection of polyethylene was tested by electroacoustic pulse method.The results show that the carbon black in the CB/EVA/LDPE semi-conductive shielding layer is uniformly dispersed,and the resistivity exhibits a positive coefficient effect with temperature.At the same temperature,the space charge injected into the polyethylene by the CB/EVA/LDPE semiconducting electrode increases,with the increment of electric field.However,the phenomenon of space charge limiting current appears under super higher electric field,making it difficult for the space charge to be further injected.The injection of space charge in polyethylene at different temperatures of the same electric field shows different trends.3.The SrFe₁₂O₁₉/CB/EVA/LDPE semiconductive shielding layer with different mass fractions of strontium ferrite was prepared by melt blending and hot pressing cross-linking.The magnetic properties,mechanical properties and resistivity of the semiconductive layer were investigated.Then,the effects of the space charge injection on the polyethylene were investigated,under different electric field before and after magnetization,through electroacoustic pulse and thermal stimulation current methods.The results show that the strontium ferrite is successfully mixed into the CB/EVA/LDPE matrix through mixing,thin.A small amount of strontium ferrite exhibits an inhibitory influence on the positive temperature coefficient effect of the semiconductive shielding layer,which in turn exacerbates the positive temperature coefficient effect.With the increase of strontium ferrite mass fraction,the residual magnetization of SrFe₁₂O₁₉/CB/EVA/LDPE semi-conductive shielding layer becomes larger,the coercive force is not changed.However,both tensile strength and elongation at break show a trend of increasing first and then decreasing.Compared with the unmagnetized SrFe₁₂O₁₉/CB/EVA/LDPE composite electrode,the magnetized electrode shows enhanced suppression of space charge.And with the increase of strontium ferrite content,the space charge suppression effect is more obvious.