Insulation Property Modification Methods Of Polypropylene For HVDC Cable

High voltage DC(HVDC)cable is an important equipment of DC transmission system.Developing polypropylene insulation material for HVDC cable is of great significance to the development of large capacity and eco-friendly cable insulation.It’s easy to accumulate space charge in HVDC cable insulation material under the long-term DC electrical field,which causes local electric field distortion and accelerate insulation aging.In the process of cable operation,the insulation is also affected by high temperature,DC and pulse composite voltage,which can lead to insulation thermal aging and electrical tree degradation,threatening the safe operation of the cable transmission system.This thesis investigates the influence and mechanism of blending vinyl elastomer,nano graphene and radical scavengers on the electrical properties of polypropylene composite.This dissertation provides the theory and experiment evidence for the development of non-crosslinked HVDC cable insulation.The main works and conclusions are as follows:(1)The mechanical properties of PP were modified by blending vinyl elastomer ULDPE.It was found that when the content of elastomer was low,its dispersion and particle size in PP matrix were more uniform,which was conducive to the compatibility of two phases.At this time,the mechanical properties and thermal properties of PP blends were better.Blending elastomer introduced interface of PP/ULDPE into the blends,and the deep trap became larger in the blends.For the PP/ULDPE blends,the space charge accumulated a little,the conductivity and the breakdown strength decreased.(2)PP/ULDPE/graphene nanocomposites were prepared by melt blending.It was found that with the increase of temperature,homopolar charge injection increased in PP,heteropolar charge accumulation occurred in the blends.The breakdown strength of PP and its blends decreased and conductivity increased.The results of quantum chemical calculation show that the surface of graphene nanoparticles with planar structure has a strong negative potential and strong attraction to charge.A little nano graphene doping can introduce a large number of deep traps into the composite,making the space charge accumulation and conductivity of the composite smaller and the breakdown strength larger.(3)PP/ULDPE/radical scavenger composites were prepared by melt blending method.Based on quantum chemical calculation,it was found that the carbonyl group,the triazine group in the radical scavenger molecules constituted chemical traps.The trap energy level and trap density of the modified composites increased,which inhibited the accumulation of space charge.At the same time,the radical scavenger could capture the active radicals and improve the electrical breakdown resistance of the composites.It was found that the composite,modified by polymer type radical scavenger with long branched molecular chain and main chain will not break under cyclic action,show the best electrical properties.(4)It was found that the change of the microstructure and the introduction of chemical defects in the process of thermal aging introduced deep traps in polypropylene and its modified composites,which inhibit the accumulation of space charge.After aging,the trap energy level,density and breakdown strength of the modified composites are still higher than that of pure polypropylene and PP/ULDPE blends.It was found that the electrical tree growth resistance of the modified composites was better than that of PP and its blends under DC and pulse combined voltage,and the deep traps introduced by nano graphene and radical scavenger,as well as capturing the free radicals produced in the process of electrical tree degradation by the radical scavenger,could inhibit the growth of electrical tree.It is an effective way to improve the electrical properties of polypropylene for HVDC cable by using nano graphene and radical scavenger.