外施电压频率对XLPE电缆绝缘中电树枝生长特性的影响

本文研究了半结晶交联聚乙烯(XLPE)电缆绝缘试样在50～2000Hz正弦电压下电树枝的结构特征与生长特性,发现由于XLPE的半结晶聚集态物理结构,在小于250Hz施压频率下会生成枝状、枝状与丛林混合状及纯丛林状三类电树枝,在500Hz以上高频下则只能生成稠密枝状电树枝,分别对应于不同的生长机理.低频下电树枝生长特性和电树枝结构与材料的聚集态密切相关,而高频下的电树枝生长特性与材料的聚集态关系不大.高频电树枝与环氧树脂、有机玻璃等高聚物材料中的生长规律相同.半结晶高聚物在低频下的电树枝生长特性主要取决于晶界与无定形界面的微孔、杂质集中情况以及针尖电极与晶块或无定形区所处的相对位置,而在高频下电极向介质中注入与抽出电荷的过程较低频下猛烈,会形成较均匀的介质弱区,因此高频电树枝引发与生长规律较为单一.

Influence of frequency of voltage applied on the propagation characteristics of electrical trees in XLPE cable insulation

In this paper the structures and propagation characteristics of electrical trees under high voltage of 50-2000Hz in the cross-linked polyethylene(XLPE) cable insulation samples are studied.It is found that there are three kinds of electrical trees,the branch-like tree,mixed tree combining branch with jungle,and pure jungle-like trees,that will appeared in the sample when frequency is less than 250Hz due to non-uniform crystallization and micro-defects concentrated in XLPE cable insulation.Only one kind of electrical tree,the dense branch-like trees,will be developed when frequency is equal to or higher than 500Hz. The propagation mechanisms of these kinds of electrical trees are mainly decided by crystalline state of the dielectric and the frequency of voltage applied.The structures and propagation characteristics of electrical trees is closely related to aggregation state of material in low frequency,and is nothing to do with the aggregation state of material at high frequency.The propagation law of electrical trees in XLPE cable insulation at high frequency is similar as in epoxy resin and PMMA.The propagation characteristics of electrical trees not only depends upon the crystal-boundary and microspore in amorphous interface,but also depends upon the impurity concentration and the relative position between electrode tip and sphere-crystals or amorphous region in low frequency.Because the processes of injection and elicitation charge from and to dielectric by electrode are more violent at high frequency than at low frequency,it can form relatively uniform dielectric weak region at the front of needle electrode,so the initiation and propagation law of electrical trees at high frequency is similar each other.