Three-Dimensional Imaging and Analysis of Electrical Trees
Three-Dimensional Imaging and Analysis of Electrical Trees
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电树的三维成像与分析
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发表时间：
2014
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通讯作者：
R. Brandt
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作者：
R. Brandt
电气树枝是高压聚合物绝缘中微米级管状降解通道，是发电厂故障的前兆。因此，电气树枝对电力系统的可靠性和新绝缘设计的性能有着至关重要的影响。对实验室培育的电树进行成像是研究树木发育过程的重要工具。通常，用透明或半透明聚合物制备的电树是用传统的光学方法成像的。因此，大多数分析都是基于树木的二维(2D)图像，因此，有价值的信息可能会丢失。然而，电气树是复杂的相互连接的结构，需要树维(3D)方法来进行更完整的分析。本文研究了一种电树的成像和分析方法，以刻画其三维结构，并为进一步建模提供平台。实验室制造的电树使用X射线计算机断层扫描(XCT)和序列块面扫描电子显微镜(SBFSEM)进行成像，这两种三维成像技术提供亚微米的空间分辨率。生成了代表真实电气树的三维几何模型--电气树的虚拟复制品，并开发了新的指标来表征电气树的三维结构。这些参数指示了树木生长的差异，因此，它们可以用于研究电树的模式和分类结构。使用与生长垂直的树木横截面来分析树木的进程：测量树木通道的数量和它们所覆盖的面积。根据3D模型和2D投影计算了树木的分维，后者对于所研究的所有树型结构来说都较低。测量了树的骨架参数，如节点数、节段长度、弯曲度和分枝角度。大多数节段的平均长度为6~13�m，与不同树木生长模型提出的10�m基本一致。利用开发的可视化和表征综合工具，对灌木和树枝，包括早期生长的电树(长度为20-40�m)进行了分析和比较。通过两阶段树生长试验，用XCT分析了树枝的生长发育过程：第二生长阶段后的树干通道比第一生长阶段后的宽，而分维保持不变。XCT和SBFSEM的能力被测试用于成像光学不透明材料中的电树，例如微米和纳米填充的环氧化合物。使用这两种技术观察了填充高达20wt%微硅胶的环氧树脂中树木的一般结构。初步探索了利用虚拟复制品作为三维几何模型，用有限元方法模拟电场分布的方法。为了更完整地分析树木的结构，提出了一种成像技术的组合。相信本文所开发的三维技术平台将对理解电气树产生重大影响。
Electrical trees are micrometre-size tubular channels of degradation in high voltage polymeric insulation, a precursor to failure of electrical power plant. Hence, electrical trees critically affect the reliability of power systems and the performance of new insulation designs. Imaging laboratory-grown electrical trees has been an important tool for studying how trees develop. Commonly, electrical trees prepared in transparent or translucent polymers are imaged using traditional optical methods. Consequently, most of the analysis has been based on two-dimensional (2D) images of trees, thus, valuable information may be lost. However, electrical trees are complex interconnected structures that require a tree-dimensional (3D) approach for more complete analysis. This thesis investigates a method for imaging and analysis of electrical trees to characterise their 3D structure and provide a platform for further modelling. Laboratory created electrical trees were imaged using X-ray Computed Tomography (XCT) and Serial Block-Face Scanning Electron Microscopy (SBFSEM), 3D imaging techniques that provide sub-micrometre spatial resolution. Virtual replicas of the trees, which are the 3D geometrical models representing the real electrical trees, were generated and new indices to characterise the 3D structure of electrical trees were developed. These parameters were indicative of differences in tree growth and thus, they can be used to investigate patterns and classify the structure of electrical trees. The progression of the tree was analysed using cross-sections of the tree that are orthogonal to the growth: the number of tree channels and area covered by them were measured. The fractal dimension of the tree was calculated from the 3D model and from the 2D projections, the latter being lower for all the tree-type structures studied. Parameters from the skeleton of the tree such as number of nodes, segment length, tortuosity and branch angle were measured. Most of the mean segment lengths ranged 6-13 �m, which is in accordance to the 10�m proposed by various tree-growth models.The capabilities of XCT and SBFSEM imaging techniques were evaluated in their application to electrical trees. Bush and branch trees, including early-growth electrical trees (of length 20-40 �m), were analysed and compared using the comprehensive tool of visualisation and characterisation developed. A two-stage tree-growth experiment was conducted to analyse the progression and development of tree branches using XCT: tree channels after the second stage of growth were wider than after the first, while the fractal dimension remained the same. The capabilities of XCT and SBFSEM were tested for imaging electrical trees in optically-opaque materials such as micro and nano-filled epoxy compounds. The general structure of trees in epoxy filled up to 20 wt% micro-silica was observed using both techniques. The use of a virtual replica as the 3D geometrical model for the simulation of the electric field distribution using Finite Element Analysis (FEA) was preliminary explored. A combination of the imaging techniques is proposed for a more complete structural analysis of trees. It is believed that a great impact towards understanding electrical treeing will be achieved using the 3D technical platform developed in this thesis.