Characteristics Of Periodic Grounded Dc Tree In High Voltage Direct Current Cable Insulation

In recent years,with the rapid development of high voltage direct current(HVDC)transmission technology,the application of polymer insulated HVDC cables has become increasingly widespread.The insulation of the polymer insulated power cable is prone to produce electrical trees under the effect of long-term electric field,which leads to the decrease of the dielectric strength and seriously threatens the safe and stable operation of the power cable.At present,although researchers around the world have conducted a lot of researches on electrical trees in polymers,most of them just focused on AC conditions,and those under DC conditions are rather limited.At the same time,it has been reported that adding a small amount of inorganic nanoparticles into the polymer matrix can improve the AC electrical treeing resistance of the material,but its influence on the DC electrical treeing still remains to be studied.This paper focused on the possible grounded DC tree in HVDC cable insulation.Considering that the electric treeing was closely related to the space charge behavior in the material,the effect of nanoparticle concentration on the space charge characteristics of the material was first investigated,and the concentration leading to the smallest space charge accumulation amount was determined.On this basis,a new HVDC cable insulation material was developed,which was based on cross-linked polyethylene(XLPE)and contained nano-MgO.After that,periodic grounded DC tree experiments were conducted on the MgO/XLPE material and a commercial grade XLPE HVDC cable insulation material at 20~80?.The effects of temperature and nanoparticle addition on the electrical tree characteristics were investigated by means of physicochemical analysis,space charge measurement and charge transport simulation.Results of the space charge measurement of MgO/low density polyethylene(LDPE)with different concentrations showed that the space charge accumulation amounts in MgO/LDPE were smaller than that in pure LDPE,and that in the 0.5wt% MgO/LDPE was the smallest.Results of the synchrotron radiation small-angle x-ray scattering(SAXS)analysis showed that there was interphase between nano-MgO and LDPE matrix,which regulated the charge transport in the material by acting as charge trap as well as charge recombination center.As concentration increased,the thickness of the interphase increased,and the depth of charge traps as well as the number of charge recombination centers introduced by the interphase increased accordingly.When the concentration was 0.5wt%,although the charge traps were deeper than those at 0.1wt%,they were still shallow and promoted the charge transport.Meanwhile,the number of charge recombination centers was larger at 0.5wt% than that at 0.1wt%,and promoted the charge recombination.Under the combined influence of the above two points,the 0.5wt% MgO/LDPE showed the smallest space charge accumulation amount.Results of the periodic grounded DC tree experiments of MgO/XLPE and XLPE at 20,40,60 and 80? showed that the tree initiation ratios,tree lengths and tree widths all increased with the increase in voltage.For the same material,when the temperature and the voltage amplitude stayed the same,the tree initiation ratios under negative voltages were all larger than those under positive voltages.As temperature increased,larger tree initiation ratios,tree lengths and tree widths,as well as denser tree shapes were observed in both materials,indicating that the temperature increase promoted tree initiation and growth.When the temperature and the voltage stayed the same,compared with XLPE,the MgO/XLPE showed smaller tree initiation ratio,tree length and tree width,indicating that the electrical treeing resistance of MgO/XLPE was stronger than that of XLPE to some extent.Besides,it was found that the trees in MgO/XLPE were denser than those in XLPE at higher temperatures,and the positive tree shapes were obviously different from those in XLPE,which were similar to the negative tree shapes.The effect of temperature on the periodic grounded DC tree characteristics was analyzed by taking XLPE as an example.Results of the differential scanning calorimetry(DSC)analysis,space charge measurement and charge transport simulation of XLPE showed that,with the increase in temperature,the mechanical strength of the material decreased,so that less charge trapping and detrapping might cause tree initiation and growth.At higher temperatures,the trapped charge density accumulated near the needle tip during DC prestressing decreased,but the charge trapping-detrapping became more frequent,resulting in more hot electrons.At the same time,the hot electron motion was intensified at higher temperatures,which was more likely to cause the molecular chain fracture.In addition,at the instant of grounding,although the trapped charges were fewer at higher temperatures,the charge detrapping became stronger,so the electrical mechanical energy released might be larger than that at lower temperatures,promoting tree initiation and growth.With the increase in temperature,the trapped charge distribution range near the needle tip increased significantly,resulting in larger tree length and tree width.Moreover,the trapped charge distribution became more uniform,so the branches were easier to form,and the tree shape became denser.Results of the DSC analysis,space charge measurement and charge transport simulation of MgO/XLPE showed that,although the mechanical strength of MgO/XLPE was slightly weaker and the charge trapping-detrapping during DC prestressing were more frequent than that of XLPE,the charge trap depth became shallower,so the energy released by a single charge trapping decreased,which inhibited the hot electron generation.Meanwhile,there was negative interfacial charge layer between the nano-MgO and XLPE matrix,where the hot electrons were prone to be scattered and lose energy,which inhibited the tree initiation and growth to some extent.At the instant of grounding,although the charge detrapping became stronger,the trapped charges were fewer,so the electrical mechanical energy released might be smaller.In addition,the injected charge distribution was changed due to the interfacial charge layer between the nano-MgO and XLPE matrix,so the tree length and width decreased.Moreover,the interfacial charge layer attracted holes and repelled electrons,so that positive tree branches grew around the nano-MgO,and negative tree branches grew through adjacent nanoparticles,both resulting in more branches and denser tree shape.