Electrical Tree Characteristics and Mechanism of Grafted Polypropylene Cable Insulation
Zhang Wenjia1, Wang Wei1, Yuan Hao2, Li Qi3, He Jinliang3
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China; 2. SINOPEC (Beijing) Research Institute of Chemical Industry Co. Ltd Beijing 100013 China; 3. State Key Laboratory of Control and Simulation of Power System and Generation Equipment Department of Electrical Engineering Tsinghua University Beijing 100084 China
Abstract：To meet the requirements of increasing the transmission capacity of the power grid, the development of recyclable polypropylene (PP) cable insulation material has become a hotspot in recent years. The electrical tree is a serious electrical ageing phenomenon of insulation. When the cable runs in a high voltage environment for a long time, the ageing deterioration process will introduce a large number of defects in the insulating layer, and these defects will produce a local high electric field, which may initiate an electrical tree and result in insulation failure. A large number of research results show that graft modification can stably improve the dielectric properties of PP materials, but it is not clear whether graft modification can improve the electrical tree properties of PP cable insulation materials under long-term operation and the related mechanism. In this paper, the methyl methacrylate (MMA) monomer is grafted by the aqueous suspension method. A needle-needle electrode structure is applied in the electrical tree test. The needles are pre-embedded to avoid mechanical damage. The results show that compared with pure PP, the electrical tree initiation time of grafted PP is significantly improved, and the deterioration area and growth rate of the electrical tree are significantly reduced. With the increase of the MMA content, the electrical tree performance improves monotonously. Although the morphology of the trees is branch-like, the number of branches decreases as the grafting content increases. The trap distribution characteristics are studied, and it is found that after the MMA is grafted onto the PP matrix, the level of deep traps increases. As MMA content increases, the density of the deep traps increases gradually. The crystallization characteristics are studied in this paper. The results of differential scanning calorimetry show that the crystallinity drops when MMA is grafted, for the steric hindrance effect of the large side group volume of MMA monomers that hinders the orderly folding of PP chains. The spherulite morphology is studied through polarized microscopy, it is found that the size of the spherulites decreases monotonously with the increase of the grafting content. The analysis shows that the polar functional group carbonyl in the grafted MMA monomer can introduce a large number of deep traps into the PP matrix, which is conducive to inhibiting the carrier trapping, detrapping, recombining and transporting behaviours. As is known, the dielectric properties of the crystal region are better than that of the amorphous region. And the heterogeneous nucleation effect of MMA monomer on the PP molecular chain increases the spherulite number and reduces the spherulite size, significantly compressing the spatial scale of the amorphous channel. The changes MMA brings to trap and crystallization characteristics together make the electrical tree initiation characteristics of grafted PP improved. In addition, the decrease in spherulite size of grafted PP makes the amorphous channel more tortuous and narrower, which significantly increases the resistance of electric tree growth along the direction of the electric field, and then improves the electric tree growth characteristics of grafted PP. With the increase of MMA monomer grafting content, the electrical tree initiation and growth characteristics of grafted PP gradually improve.
张雯嘉, 王伟, 袁浩, 李琦, 何金良. 接枝聚丙烯电缆绝缘材料的电树枝特性及机理[J]. 电工技术学报, 2024, 39(1): 88-98.
Zhang Wenjia, Wang Wei, Yuan Hao, Li Qi, He Jinliang. Electrical Tree Characteristics and Mechanism of Grafted Polypropylene Cable Insulation. Transactions of China Electrotechnical Society, 2024, 39(1): 88-98.
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