Modification Of HVDC Semi-Conductive Shielding Compound And Its Effect On Space Charge Injection Of Insulating Layer

In HVDC cable,the inner semi-conductive shielding layer can eliminate the air gap between the metal conductor core and the insulating layer,it can make the connection between the two more closely and prevent the local electric field distortion caused by the long-term HVDC field of the cable.In addition,the inner semi-conductive shield also plays an important role in preventing the temperature rise of the insulation layer and the injection of space charge.The accumulation of space charge has been an important reason to prevent the progress and development of HVDC cable.The positive temperature coefficient effect of the semi-conductive shield will lead to the high resistance of the shield material at high temperature,which limits the increase of the HVDC cable operating temperature and ultimately leads to the decline of the transmission capacity of the HVDC cable.Therefore,in this paper,a variety of different substances are selected to study the semi-conductive shielding layer from different angles,which improves the ability of the semi-conductive layer to inhibit the injection of space charge into the insulating layer and significantly reduces the inherent positive temperature coefficient effect of the semi-conductive shielding layer.The main work contents are as follows:1.LiCoO₂ particles were prepared by sol-gel method.By adjusting the amount of citric acid and XRD test,the optimum amount of citric acid was obtained for the preparation of Li Co O₂.The particle size was reduced by ball milling.The morphology and size of the samples were characterized by scanning electron microscopy and transmission electron microscopy.The size of the samples after ball milling was uniform and the particle size was about several hundred nanometers.Li Co O₂/LDPE/EVA/CB semi-conductive shielding layer with doping ratio of 0 wt%,0.5 wt%,1 wt%,3 wt%,5wt%was obtained by blending the prepared Li Co O₂ with LDPE/EVA/CB and press forming with plate vulcanizing machine.By scanning the cross section,it is found that Li Co O₂ particles can be well dispersed in the polymer matrix.The tensile strength and elongation at break were measured,and it was found that the properties of the doped samples were reduced.The temperature-resistivity curve of the composite material was tested,and the resistivity of the sample decreased significantly after the doping of lithium cobalate.The PTC strength of different samples was calculated.When the content of doped lithium cobalate was 0.5 wt%,the PTC strength was the lowest,and compared with the undoped sample,the PTC strength decreased by nearly 20%.The space charge in insulating layer of different samples as semi-conductive layer was measured by thermal stimulation current method and electroacoustic pulse method.The test results show that the sample with 0.5 wt%doped Li Co O₂ has the best performance.Compared with the unmodified semi-conductive layer,the amount of space charge in the insulating layer decreases by 70%,indicating that lithium cobalate plays an important role in the semi-conductive shielding layer.2.β"-Al₂O₃ particles were synthesized by high temperature sintering method,and the particle size was about 10 microns.From XRD,we found that the crystallization ofβ"-Al₂O₃ is good,and there is a littleβ-Al₂O₃ phase.The preparedβ"-Al₂O₃ was mixed with the LDPE/EVA/CB matrix and cross-linked by moulding to obtain semi-conductive shield doped with different proportions ofβ"-Al₂O₃.The sample doped withβ"-Al₂O₃in this experiment was 0 wt%,2 wt%,4 wt%,6 wt%,8 wt%.The influence ofβ"-Al₂O₃on the PTC effect was studied,and the PTC strength was successfully reduced by 21%.The space charge of the insulating layer is measured when the field intensity is 10k V/mm and 20 k V/mm respectively.It is found that when the dopingβ"-Al₂O₃ ratio is4 wt%,the space charge in the insulating layer is the least.3.Use sandpaper with different mesh numbers to deal with the surface of the semi-conductive shield.Its surface was observed by scanning electron microscope and its space charge was measured.It is found that the hole on the surface of the semi-conductive shield reduces the injection of space charge into the insulating layer,which may be related to the contact area of the semi-conductive layer with the electrode and the insulating layer.It provides a new research angle for studying space charge injection into insulating layer.