Modification And Space Charge Investigation Of Polypropylene Composites For HVDC Cables

With the disadvantages of traditional XLPE in HVDC cables,such as cross-linking byproducts,complicated processing technology and non-recyclable,it is necessary to develop environment-friendly cable insulation materials to achieve the breakthrough in cable insulating materials and promote the development of HVDC transmission technology.The insualting materials of HVDC power transmission cable are severly restricted by the accumulation of space charge,which causes electric field distortion and has a great effect on ageing and breakdown characteristics.This thesis takes popropylene(PP)as the study object,and carries out the mechanical and electrical properties optimization by elastomer blending and nanoparticles filling.This work also investigates the effects of mechanical stretching on space charge characteristes of PP composites,as well as the corresponding mechanisms.This dissertation provides the experiment and theory evidence for the development of environment-friendly HVDC cable insulation materials.The main research work and achievements are as follows:(1)The elastomers of EPDM,POE and EVA were blended with PP to optimize its mechanical property,and the potential of different blends for HVDC cable application were compared.It was found that: the blending had little effect on the melting temperature,crystallization temperature and crystal type of PP,and the mechanical flexibility of PP was greatly improved by blending;due to the interfaces between PP and elastomer,more space charge accumulated in the blends and the detrapping process was inhibited;with the increasing of elastomer content,the volume resistivity of PP was increased,while the breakdown strength was decreased;the electrical properties of PP/EPDM blend and the mechanical properties of PP/EVA blend were poor,while the PP/POE blend showed excellent performance in both properties,which had much potential for HVDC cable application.(2)The ZnO nanoparticles were filled with PP/POE blend to adjust its electrical properties.It was found that the PP/POE/ZnO composites showed high melting points and excellent mechanical flexibility;with the increase of ZnO content,the amount of accumulated space charge was firstly decreased and the enhanced,while the volume resistivity and breakdown strength exhibits increase first and then decrease;the ZnO nanoparticles can introduce deep traps in PP/POE/ZnO composite,which can help to form a shielding layer beside the electrodes to suppress charge injection and decrease carrier mobility;it was concluded that the modification of PP by combination of elastomer blending and nanoparticle filling is an effective way to develop environment-friendly cable insulation materials.(3)The space charge characteristics of PP composites under different mechanical stretching ratios were investigated.It was found that: the space charge accumulation was enhanced after stretching;with increasing the stretching eleongatin ratio,the apparent carrier mobility exhibits increased first and then decreased,while the trap depth showed an opposite tendency;for PP/POE blend with higher POE content,the highest value of apparent carrier mobility appeared at higher elongation ratio;under mechanical stretching condition,the PP/POE/ZnO composite still maintained the space charge suppression effect,and the amount of accumulated space charge was lower than that of PP/POE blend without nanoparticles filling.