预鉴定试验对高压直流电缆绝缘电导特性的影响

Effects of Pre-qualification Tests on Conductivity Characteristics in HVDC Cable Insulation

对200 k V交联聚乙烯直流电缆进行了预鉴定试验,采用气相色谱-质谱法(GC-MS)和差示扫描量热法(DSC)分析了预鉴定试验前后交联聚乙烯主绝缘层内侧材料的理化特性,并在30~90℃下对主绝缘层内侧切片试样的极化电流以及去极化电流进行检测,分析了预鉴定试验对交联聚乙烯电导特性的影响。结果表明:交联聚乙烯电缆绝缘层内侧材料中的主要交联副产物为甲基苯乙烯、苯乙酮和苯甲醇,经预鉴定试验中的冷、热循环老化后,3种主要交联副产物含量均明显下降,表明预鉴定试验可促进交联聚乙烯电缆绝缘层内侧交联副产物的脱离,从而使得电缆绝缘层内侧交联副产物含量下降;交联聚乙烯的电导特性与交联副产物含量密切相关,预鉴定试验后主要交联副产物的含量下降,减少了由副产物等杂质高电场解离形成的离子含量,从而有效降低了交联聚乙烯的电导率;电缆交联聚乙烯绝缘的结晶度在预鉴定试验后略有上升,对应的陷阱深度变浅,松弛过程变快;对去极化电流曲线使用三阶指数模型进行拟合,发现经预鉴定试验电缆的松弛时间常数均小于未试验电缆。受无定形区与晶区的界面极化以及交联副产物影响的松弛过程所对应的时间常数随着温度的上升呈先增大后减小的趋势。

The pre-qualification test was conducted on the 200 k V XLPE DC cables, and the physical and chemical properties of the inner side materials of XLPE main insulation layer before and after pre-qualification test were analyzed by gas chromatography-mass spectrometry（GC-MS） and differential scanning calorimetry（DSC）. The polarization and depolarization currents of the inner side slices of main insulation layer were detected at 30-90 ℃, and the influence of pre-qualification test on the conductivity characteristics of XLPE was analyzed. The results show that the main crosslinking by-products in inner materials of insulation layer in XLPE cable are methyl styrene, acetophenone, and benzyl alcohol, and their contents decrease obviously after the cold and thermal cycling ageing in pre-qualification test, which indicates that the pre-qualification test can promote the detachment of cross-linking by-products in the inner side of XLPE cable insulation layer, leads to the decrease of cross-linking by-product contents. The conductivity characteristics of XLPE are closely related to the crosslinking by-products. After pre-qualification test, the contents of major crosslinking by-products decrease, and the ion content formed by high electric field dissociation of impurities reduces,so the conductivity of the crosslinking polyethylene reduc-es effectively.The degree of crystallinity of the XLPE increases slightly after the pre-qualification test,and the corresponding trap depth decreases and the relaxation process becomes faster.The depolarization current curves are fitted with third-order exponential model,and it is found that the relaxation time con-stant of the cable after pre-qualification test is smaller than that of the untested cable.The time constantcorresponding to the relaxation process,which is affected by the interface polarization between amor-phous region and crystalline region and the crosslinking by-products,increases at first and then decreaseswith the increase of temperature.