Research On The Effect Of Antioxidant MAE Grafting On DC Dielectric And Thermal Stability Of Crosslinked Polyethylene

High-voltage direct current（HVDC）transmission has presented a great many of advantages and has developed rapidly in pace with power electronics technology as well as modern industry for some times.At the same time,HVDC cables have received widespread attention as an indispensable part of the DC transmission system.Therefore,the requirements for HVDC cable insulation materials are also increasing,and it is extremely significant to research high-voltage insulation materials with better dielectric properties and thermal stability.In the development and application of insulating materials under HVDC electric fields,Space charge accumulation is an extremely serious problem.Under the action of space charge,the local electric field of insulating materials is often distorted,and even the insulating layer of insulating materials is broken down.on the other hand,the cable insulation materials are prone to aging problems such as thermal oxidative aging and photo-oxidation aging during operation,which leads to a substantial decline in their physical,chemical,thermal,electrical,and mechanical properties,which in turn affects its stability.Therefore,in order to suppress the accumulation of space charge and reduce the damage of the insulating materials during the thermal oxidative aging process,in this thesis,a reactive antioxidant MAE grafted modified cross-linked polyethylene（XLPE）insulating materials containing polar groups were prepared,and their DC dielectric properties and thermal stability at different test temperatures were studied.In this thesis,dicumyl peroxide（DCP）is used as a free radical initiator and crosslinking agent at the same time.The antioxidant MAE molecule is grafted onto the XLPE molecular chain by free radical addition and cross-linking reaction.Thus,MAE grafted modified XLPE insulating materials containing different contents of antioxidants were prepared.Infrared spectroscopy tests show that the antioxidant MAE has been successfully grafted onto the XLPE molecular chain.The gel extraction and thermal extension experiments tests show that the grafting of the antioxidant MAE has little effect on the cross-linking reaction of polyethylene（PE）.MAE grafted modified XLPE insulating materials still have a higher degree of cross-linking.The space charge accumulation characteristics,DC conductance current density and DC breakdown strength of the pure XLPE and XLPE-g-MAE insulation materials containing different contents of antioxidant MAE at different temperatures were tested.The space charge tests show that,compared with pure XLPE,the antioxidant MAE graft modification can greatly inhibit the space charge accumulation and electric field distortion in the XLPE insulation materials,and as the test temperature rises,the suppression effect is more obvious.In addition,the space charge distribution characteristics and electric field distribution of XLPE insulation materials added with0.3 phr antioxidant 300,antioxidant 1010,antioxidant 4010 NA or antioxidant 4020 at room temperature were compared,and in terms of suppressing space charge accumulation or suppressing electric field distortion,it was found that MAE-grafted XLPE insulation materials are superior to commonly used antioxidant 300,1010,4010NA/XLPE insulation materials.The test results of DC conductance current density and DC breakdown strength show that XLPE-g-MAE insulation materials containing different contents of antioxidant MAE,compared with pure XLPE,reduce the DC conductivity at different temperatures and increase its breakdown strength.The polar functional groups of the antioxidant MAE fixed on the XLPE macromolecular chain will generate deep traps at the grafted MAE site,which can capture space charges and scatter charge carriers,reduce the number of charge carriers and the mobility of carriers,which achieves the effects of restraining space charge,reducing the density of DC conductivity and improving the breakdown strength.The oxidation induction time（OIT）of the antioxidant MAE grafted modified XLPE insulation materials were tested and an accelerated thermal aging experiment was carried out at 135℃ and 168 h.The results show that the OIT of the XLPE insulating materials grafted with the antioxidant MAE are significantly greater than that of the antioxidant 300,1010,and 4020/XLPE insulating materials when the same contents of antioxidants are contained.The tensile tests show that the antioxidant MAE grafted modified XLPE insulation materials still maintain good mechanical properties after accelerated thermal aging treatment.Antioxidant MAE grafted modified XLPE insulation materials have excellent thermal stability.