Simulation And Experimental Study On Space Charge And Charge Transport In HVDC Cable Insulation

Extruded insulated HVDC cable is the core equipment for constructing DC power grid.The space charge in plastic insulation will cause local electric field distortion,accelerate insulation aging and even cause breakdown.Many scholars think that the deep traps introduced by nano-modification can effectively inhibit the space charge in polymer.However,there are few studies on charge transport and the correlation between space charge suppression and variation of trap characteristics in nanocomposite dielectrics of HVDC cables.The space charge regulation and controlling methods in the insulation of HVDC cables under electrothermal coupling field need to be further studied.Two groups of LDPE/ZnO nanocomposites with different dopant particle sizes were prepared by melt blending.There are 5 mass fractions in each group.Studying on the electrical conductivity shows that the conduction currents of LDPE/ZnO nanocomposites with the particle size of 100nm are lower than those of pure LDPE under 50 kV/mm electric field.Among them,the conduction current of the composites with mass fraction of 3wt%is the smallest,and the conduction current of the composites with this mass fraction is always smaller than that of the pure LDPE in the range of 20-80? under 10kV/mm.The group with the ZnO particle size of 50nm:The conduction currents of the composite materials with 5wt%and 7wt%mass fraction are slightly lower than those of the pure LDPE under 50kV/mm electric field,while those of the materials with other mass fraction are higher.The results of space charge measurements show that there are a lot of homopolar and heteropolar space charge in pure LDPE under 50kV/mm.LDPE/ZnO composites with particle size of 100 nm and mass fraction of 5wt%have some inhibitory effect on homopolar space charge.The BCT model was used to simulate the space charge distribution of polyethylene-based nanocomposites with different trap characteristics under 50kV/mm electric field.It was found that:There are a lot of space charge accumulate in pure polyethylene.The effect of increasing trap density on space charge suppression is better in plane samples with constant temperature.Two trap characteristics are found to effectively suppress space charge:One is to increase trap depth and density a lot;the other is to reduce trap depth and density a lot.Based on this discovery,a trap regulated space charge suppression model is established on the basis of BCT model:The first trap characteristic causes the charge to be trapped around the electrode immediately after injection,then form a thin trapped charge layer.The trapped charge layer lowers the interface electric field and increases the charge injection barrier,so the injection current is greatly reduced.Finally,bulk conduction current and injection current reach a balance at a lower value.The second one leads to high conductivity,and then suppresses space charge by fast relaxation of charge.However,the second characteristic increases the leakage current.Through the simulation of space charge in HVDC cable insulation with different trap characteristics and ampacity by BCT model,it is found that the trap regulated space charge suppression model can be effectively applied to cable insulation under electrothermal coupling field.When ampacity is 1800A,temperature difference between insulation is 20? after working 4h,there is a large amount of charge accumulation in pure polyethylene,and both charge accumulation and electric field distribution are reversed(positive charge density:?in=2.5×10-3C/m3<?out=8×10-3C/m3,elecreic field:Ein=12.5kV/mm<Eout=15kV/mm).There is a little homopolar charges moving away from conductor about 12.5mm,and no phenomenon of space charge and electric field reversal occurs in the material with Nr=80×1019m-3,Et.20eV.While in the insulation of Nt=1000×1019m-3,Et=0.94eV,some postive space charge migrate away from conductor about 22.5mm,and the electric field reverses(Ein=6kV/mm<Eout=12.5kV/mm,maximum electric field is about 15kV/mm),indicating that increasing the trap depth suppress space charge better in insulation under electric-thermal coupling field.With the temperature difference increases caused by improvement of ampacity(1800A,2100A,2600A),the phenomenon of space charge accumulation and electric field reversal are more serious in pure polyethylene:when the temperature difference is 20,25 and 40?,the outside electric field reaches 15 kV/mm,17 kV/mm and 21 kV/mm.However,the nanocomposites introduced lots of deep trap(Nt=1000×1019m-3,Et=1.20eV)can always effectively inhibit space charge and electric field reversal.