| Cross-linked polyethylene(XLPE)insulated DC cable is one of the key equipment for constructing direct current power grid.In order to ensure the safety and stability of system operation,higher requirements are imposed on it in engineering applications.XLPE insulated high-voltage DC cables are prone to charge injection and space charge accumulation under a high-voltage DC electric field,which can cause distortion of the electric field,insulation aging,and dielectric breakdown and so on.Therefore,there is an urgent need to study modified cross-linked polyethylene materials with well suppression of space charge effects and conductivity characteristics.Scholars at home and abroad have reported that maleic anhydride(MAH)with polar groups can be successfully grafted onto XLPE to produce crosslinked polyethylene grafted maleic anhydride(XLPE-g-MAH)material,but there are still many problems need to be solved urgently.On the one hand,the research on space charge suppression characteristics of XLPE-g-MAH is not enough.With the development of space charge measurement technology at different temperatures,it provides an opportunity for further research on space charge suppression characteristics.On the other hand,it is difficult to graft MAH to PE macromolecular chains in industrial production.For example,if grafting process is performed in the cable insulation cross-linking process,MAH will have a large amount of thermal weight loss at the temperature of the cross-linking.The released gas will form micro-pores in the insulation and endanger the insulation performance.Aiming at the many problems of XLPE-grafted MAH in actual engineering,this paper adopts a new type of grafting process,namely ultraviolet(UV)radiation grafting technology.Using a radiation source composed of UV LED light source with the wavelength of 365 nm to irradiate the polyethylene in molten state.Underthe irradiation,the photo-grafting initiator mixed therein generates free radicals,and induces the grafting of small molecules of MAH to the polyethylene.Precisely controls the irradiation time and dose can obtain the graft masterbatch.By diluting the masterbatch and carrying out the cross-linking reaction,the XLPE-g-MAH material is finally prepared,and test the relevant physical and chemical properties and DC dielectric properties of the materials.The determination of the grafting rate of the material and the infrared spectrum show that MAH has been successfully grafted onto the XLPE macromolecular chain.The determination of the gel content shows that the method of UV-grafted MAH can be used to prepare low-gel or zero-gel materials.In terms of space charge suppression characteristics at different temperatures,the inhibition effect of photo-grafted materials is better than that of ordinary XLPE materials,and the UV-grafted material does not produce large electric field distortion even at 80°C.TSC tests show that UV-grafted MAH introduces deeper traps to polyethylene.In order to study the effect of trap characteristics,the first-principle method of all-electron numerical orbit was used to calculate the energy density of electrons.The results show that there are two kinds of electron bound states in the energy band gap of XLPE-g-MAH materials,and their trap energy levels are 1.4e V and 1.9e V.After analysis we think that the dense and uniformly distributed deep traps can capture the charge in the material to form a charged lattice,and the generated coulomb field can effectively block the injection of charge into the material.In addition,the XLPE-g-MAH material has a lower conductivity,and its dependence on temperature and electric field is significantly reduced.Besids the UV-grafted modified material has a higher dielectric breakdown strength(DBS).Even in a high temperature the DBS is also higher than that of XLPE.This study suggests a prospective strategic scheme in graft modification for realizing high performance insulating materials applied to HVDC cable productions. |
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