Effect of Voltage Stabilizer Grafting on Electrical Properties of 500 kV DC XLPE Cable Insulation Materials
Chen Xiangrong1,2, Huang Xiaofan1, Wang Qilong1, Zhu Hanshan1, Hong Zelin1
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China; 2. Zhejiang Provincial Key Laboratory of Power Semiconductor Materials and Devices Hangzhou Global Scientific and Technological Innovation Center Zhejiang University Hangzhou 311200 China
Abstract：High-voltage direct current (HVDC) cables, serving as vital equipment for flexible DC power transmission, possess advantages such as extended transmission reach, substantial delivery capacity, and minimal power transmission losses. Currently, the most extensively utilized and mature insulation material for HVDC cables, both domestically and internationally, is cross-linked polyethylene (XLPE). Grafting organic substances is preferred to be used to enhance XLPE insulation performance than improving purity of XLPE base material or incorporating nano-fillers due to lower cost, higher stability, and better compatibility. Current domestic and international research on voltage stabilizers grafted onto XLPE is extensive, but there is scarce direct application in industrial insulating materials for 500 kV HVDC cables. Therefore, unsaturated aromatic small molecules are grafted onto the molecular chains of commercially available 500 kV HVDC XLPE insulation material in this work. Firstly, pure XLPE samples are prepared with 1%, 3%, and 5% of 4-acetoxy styrene (AOS) grafted onto XLPE to acquire AOS-grafted XLPE (XLPE-g-AOS). Secondly, the samples are characterized using microscopic morphology, Fourier-transform infrared spectroscopy, differential scanning calorimetry, DC conductivity, space charge, DC breakdown, and thermally stimulated depolarization current tests. Thirdly, the effects of AOS grafting on the microstructure, crystallization characteristics, insulation properties, and micro-scale charge dynamics of the XLPE material are investigated, and the role of AOS grafting in charge carrier trap characteristics and charge transport is analyzed. Finally, quantum chemical calculations elucidate the physical mechanisms by which AOS grafting enhances the electrical properties of XLPE materials at the molecular level, and the relationship between micro-scale molecular structure, trap characteristics, charge behavior, and macroscopic electrical performance is established. The results show AOS grafting results in a rougher cross-sectional morphology of XLPE-g-AOS. At an AOS weight fraction of 5%, AOS self-polymerizes to form nanoscale spherical particles. With increasing AOS content, the melting temperature and crystallinity of XLPE-g-AOS initially increase and then decrease. Among them, XLPE-g-AOS with a 3% weight fraction exhibits optimal crystallization characteristics. Moreover, AOS grafting reduces the DC conductivity of XLPE under high-temperature and high-electric-field conditions, decreases the accumulation of space charges, reduces electric field distortion, enhances the DC breakdown strength, and increases the shallow trap energy levels and quantities. Among them, XLPE-g-AOS with a 3% AOS weight fraction demonstrates the lowest DC conductivity, the least accumulation of space charges, the smallest electric field distortion, the highest DC breakdown field strength, and the largest shallow trap energy levels and quantities under high-temperature and high-electric-field conditions. When the AOS weight fraction increases to 5%, both the shallow trap energy levels and quantities of XLPE-g-AOS decrease, resulting in a decrease in breakdown strength and an increase in DC conductivity and electric field distortion. The following conclusion can be drawn from the experimental and simulation analysis: The grafting of AOS induces a shift in the trap distribution of XLPE, introducing a greater number and denser arrangement of shallow traps. These traps manifest as both hole traps and electron traps, exhibiting high electrostatic potential. Consequently, a uniform and compact lattice of shallow traps is established within the XLPE matrix, facilitating the dissipation of energy during the frequent trapping and de-trapping processes of high-energy charges. This impedes the migration of charge carriers, ultimately enhancing the electrical performance of the grafted XLPE.
陈向荣, 黄小凡, 王启隆, 朱汉山, 洪泽林. 电压稳定剂接枝改性对500 kV直流XLPE电缆材料电气性能的影响[J]. 电工技术学报, 2024, 39(1): 23-33.
Chen Xiangrong, Huang Xiaofan, Wang Qilong, Zhu Hanshan, Hong Zelin. Effect of Voltage Stabilizer Grafting on Electrical Properties of 500 kV DC XLPE Cable Insulation Materials. Transactions of China Electrotechnical Society, 2024, 39(1): 23-33.
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