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

Effect of Voltage Stabilizer Grafting on Electrical Properties of 500 kV DC XLPE Cable Insulation Materials

为了研究4-乙酰氧基苯乙烯(AOS)电压稳定剂接枝改性对500kV高压直流交联聚乙烯(XLPE)电缆材料在不同温度、电场强度和接枝含量下的电气性能的影响,该文通过熔融接枝法制备了不同含量的AOS接枝XLPE(XLPE-g-AOS)试样,通过扫描电子显微镜、傅里叶红外光谱和差示扫描量热实验分别表征试样的微观形貌、理化结构和结晶特性,对试样进行了直流电导电流、空间电荷、直流击穿、热刺激去极化电流测量,得到试样在不同温度和电场强度下的电气性能参数,结合量子化学计算分析了AOS接枝对XLPE复合材料陷阱特性和电荷输运的影响。结果表明:AOS接枝使试样断面形貌更加粗糙,而较高含量接枝会出现纳米球形分散相;XLPE-g-AOS的熔融温度和结晶度比纯XLPE更高;AOS质量分数为3%的XLPE-g-AOS具有最大的浅陷阱能级和密度,且其在高温高电场下具有最低的直流电导率、最少的空间电荷积聚量、最小的电场畸变率和最高的直流击穿强度。接枝AOS引入了更深且更多的浅陷阱,形成了均匀致密的浅陷阱点阵,阻碍了载流子迁移。

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.