XLPE电缆绝缘中的电树枝结构及其生长特性

Structures and Propagation Characteristics of Electrical Trees in XLPE Cable Insulation

为研究不同频率下半结晶XLPE电缆绝缘材料中的电树枝引发、生长及结构特征,系统的归纳了XLPE电缆绝缘中可能出现的电树枝特征及其与材料聚集态和残存应力的关系;采用变频高电压发生器、实时显微数字摄像技术及专用试样加工工艺,进行了大量的电树枝培养实验。实验研究发现,由于半结晶高聚物的不均匀结晶和电缆生产过程中在绝缘层中产生的残存应力的影响,使得50Hz施压频率下XLPE电缆绝缘试样中会生成枝状、枝状与丛林混合状及纯丛林状3类电树枝,而>500Hz高频下则只能生成稠密枝状电树枝,它们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,而高频下则关系不大。分形分析这些电树枝的结构后发现,电树枝的生长特性与其分形维数及其分形维数的变化有对应关系,故可用分形维数分类和定量描述这些电树枝.最后探讨了几种不同结构电树枝的生长机理。

The initiation, propagation and structures feature of electrical trees under sine high voltage of 50-2000Hz in semi-crystalline XLPE cable insulation were studied, and the structures feature of electrical trees and the relation between the structures feature of electrical trees and the aggregation state and residual mechanical stress of material in XLPE cable insulation samples were summarized systematically. The main experiment setups included a high voltage generator with adjustable frequency, a living digital camera and a microscope computer system. A special needle electrode and sample processing technology were used in the experiment. It is experimentally found that there are three kinds of electrical trees, the branch-like tree, mixed tree combining branch with jungle, and single jungle-like trees, which will appear in the sample at the frequency of 50Hz due to non-uniform crystallization and micro-defects concentrated in XLPE cable insulation. Only one kind of electrical tree, the dense branch-like tree, will develop at the frequency equal to or higher than 500Hz. The propagation mechanisms of these kinds of electrical trees mainly depend on crystalline state of the dielectric and the frequency of applying voltage. The structures and propagation characteristics of electrical trees are not related to the aggregation state of material at high frequency （≥500Hz） but closely related to aggregation state of material in low frequency （50Hz）. The propagation characteristics of electrical trees not only depend upon the crystal-boundary and microspore in amorphous interface, but also depend 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 initiation charge from and to dielectric by electrode are more violent at high frequency than in low frequency and 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 are similar to each other. Based on the analyses of the fractal properties of these electrical trees, it is found that the propagation characteristics of electrical trees can be characterized by the growth time dependence of fractal dimension, and that these electrical trees can be classified and quantized by means of fractal dimension. Finally the explanation for these experimental resuits is given.