高频电压下交联聚乙烯电缆绝缘中电树枝生长的动力学模型

针对高压交联聚乙烯电缆绝缘试样,在1000—2000Hz 10kV峰值正弦电压下,采用计算机实时显微数字摄像技术进行了电树枝培养实验.基于半结晶绝缘材料中电树枝生长机理和电树枝结构的分形特征,提出了一个在高频范围定量预测电应力驱动下交联聚乙烯电缆绝缘中电树枝生长特性的动力学模型,获得了电树枝生长率方程和从电树枝生长到击穿过程的寿命公式.将该模型预测值与实验中获得的电树枝生长规律实验数据进行比较,其结果有较好的一致性,表明提出的模型化方法可以应用到交联聚乙烯电缆绝缘中电树枝老化规律的定量分析研究中. 关键词： 交联聚乙烯电缆绝缘 电树枝 施压频率 动力学模型     

高压直流电缆聚丙烯绝缘电场调控

高压电缆是决定电力输送质量和容量的关键要素.聚丙烯由于自身优良的耐热性、绝缘性和绿色环保性被广泛关注,并应用于电缆绝缘材料开发.高压电缆聚丙烯绝缘材料需要承受脉冲电压和直流额定电压,容易引起电场畸变从而引发空间电荷积累.此外,电缆运行过程中,温度会急剧升高,严重影响电缆的绝缘性能,导致电树枝的引发和生长.因此需要对高压电缆进行电场调控从而抑制电场畸变、局部放电、电树枝化等劣化现象的出现.本文重点介绍了高压直流电缆聚丙烯绝缘材料电场调控的理论与方法,分析了当前电场调控的重点,最后展望了聚丙烯电缆绝缘的应用前景.     

发展在XLPE电缆绝缘内外侧的电树枝

以XLPE高压电力电缆内外侧绝缘中的电树枝特性为研究对象，通过分析电树枝引发与生长的统计实验规律和采用扫描电子显微镜分析发现，由于不同结晶状态的影响，电缆绝缘内外侧的电树枝特性存在很大的差异.引发于绝缘内侧电树枝引发时间短、生长速度快、电树枝形状具有多样性；起始于绝缘外侧的电树枝不仅引发时间长、生长速度极慢，而且电树枝形状(结构)比较单一.并对这两个位置电树枝的引发和生长机理进行了探讨. 关键词： 电树枝 结晶状态 统计规律 内侧和外侧绝缘层     

110kV冷绝缘高温超导电缆本体绝缘设计

对国内第一根基于YBCO涂层导体的110kV冷绝缘高温超导(CD HTS)电缆本体绝缘进行了设计。根据冷绝缘HTS电缆的结构特点,通过分析不同绝缘材料的介电特性,应用电场有限元数值分析模型和理论模型,计算了超导电缆本体电场分布,研究了超导电缆主绝缘厚度与局部放电起始场强的定量化关系,最后给出了110kV冷绝缘HTS电缆主绝缘材料与厚度的设计方案。     

聚丙烯中电树枝生长机理研究

耐电树枝老化特性是表征聚合物绝缘材料介电性能的重要参数之一.聚丙烯(PP)是典型半结晶聚合物,其复杂的非均匀聚集态结构影响电树枝的生长.本文对PP及加入成核剂的PP试样进行了耐电树枝化性能实验,通过偏光显微镜(PLM)及差示扫描量热法(DSC)分析加入成核剂前后PP的结晶形态、结晶度以及结晶结构对电树枝生长特征的影响.以相界面自由能的热驱动作用以及放电雪崩理论为基础,对电树枝生长的热力学和动力学机理进行分析,阐明电场分布对电树枝生长的重要作用.根据半结晶材料的结晶相和非晶相的物理性能,建立材料内部电场分布计算模型,模拟针-板电极条件下聚合物材料内部的局域电场分布情况,分析了电树枝通道的动力学生长特征,探讨了成核剂改变PP的结晶结构抑制电树枝沿电场生长的作用.     

重复频率脉冲下环氧树脂电树枝引发特性

脉冲功率设备中绝缘材料的电老化特性影响它运行的可靠性、安全性及寿命。在最大幅值约34 kV、脉冲上升时间约40 ns、半高宽约70 ns的纳秒脉冲下,进行了环氧树脂在不同脉冲重复频率下(1,25,50,100,300和500 Hz)的电树枝引发特性研究。实验得出环氧树脂电树枝老化的引发电压随频率变化的规律:随着频率的升高,环氧树脂材料老化产生电树枝的形态有所变化,高频下丛林状电树枝明显增多;材料的引发电压基本随频率升高而降低,正脉冲作用下E44环氧树脂的引发电压高于负脉冲作用下的;负脉冲下E44环氧树脂的引发电压高于有机玻璃材料的。讨论分析了各种因素随频率变化对环氧树脂材料电树枝引发特性的影响,高频时空间电荷的作用明显增强。     

介质窗口材料对高功率微波传输特性的影响

运用光学方法,分析平面波通过无限大介质平板的传输特性,进而分析单层介质窗口的引入对微波传输性能的影响。在S波段微波源上对聚四氟乙烯材料进行了真空环境下的微波放电击穿实验,通过长时间的表面放电,在聚四氟乙烯材料表面出现电痕和电树枝。在100 ns脉冲宽度下,未发生大面积表面放电前,通过光学照相,在介质表面观测到局部放电现象,认为局部放电仍能导致材料表面破坏。作为对比,进行了聚苯乙烯材料的电老化实验,实验结果表明聚苯乙烯材料具有良好的耐电痕、耐电树枝老化特性。     

静电场中多孔PZT95/5铁电陶瓷的通道电-机械击穿模型及分析

作为爆电电源的多孔PZT95/5铁电陶瓷具有极为重要的工程应用背景,但它在强电场作用下易发生电击穿失效,从而影响其放电效率,甚至造成电源失效。基于多孔PZT95/5铁电陶瓷材料在外电场作用下内部形成导电通道以致电失效的机制,通过通道内部局部放电及通道电-机械击穿机理,建立了导电通道诱导的多孔铁电陶瓷的电击穿模型并进行了相关的理论分析。基于本模型,给出了不同孔隙率下铁电陶瓷的电击穿临界电场强度,预测结果与实验测试结果吻合良好,且材料孔隙率越大,内部电击穿通道的特征尺寸越大,导致铁电陶瓷材料的电击穿临界场强显著降低。     

外电场下交联聚乙烯电介质材料分子结构变化及其电老化微观机理研究

为了从微观角度分析交联聚乙烯(XLPE)材料的电树枝老化,本文采用分子模拟方法计算并优化得到了XLPE分子结构.沿着聚乙烯链施加不同大小电场强度,分析交联聚乙烯分子的几何结构、偶极矩、极化率、电荷分布、前线轨道能量和红外光谱变化规律.计算结果表明,随着外电场的增大,交联聚乙烯分子红外光谱发生较大变化;当外施电场达到0.026a.u.后,红外光谱图中出现虚频,表明分子空间结构不再稳定,易发生断键;另外从前线轨道图的变化可以看出断键现象最先发生在交联聚乙烯链端部;沿着电场方向,原子所带电荷量由交联处向端部转移,当外施电场达到0.029a.u.后,链端部的C-H和C-C键断裂产生H·和CH_3·自由基.游离的自由基会形成空间电荷并发生积聚,产生局部较大场强,从而进一步影响交联聚乙烯链的空间结构.而电介质内部微观特性的变化必定会导致交联聚乙烯材料绝缘性能的下降,这些变化对揭示交联聚乙烯电缆电树枝形成的微观规律具有重要研究意义.     

冷绝缘超导电缆绝缘材料测试综述

绝缘材料在液氮中的各种性能将直接影响到冷绝缘高温超导(HTS)超导电缆的质量。对四种绝缘材料:电缆纸、聚丙烯层压纸、聚酰亚胺薄膜和低密度聚乙烯薄膜在常温和低温下的电阻率、介电常数、介质损耗角正切、低温介电强度和局部放电进行实验研究和对比,得到了有价值的数据。     

Analysis of electrical tree propagation in XLPE power cable insulation

Electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stress. In this paper, the characteristics of electrical tree growth in XLPE samples have been investigated. XLPE samples are obtained from a commercial XLPE power cable, in which electrical trees have been grown from pin to plane in the frequency range of 4000-10,000 Hz, voltage range of 4-10 kV, and the distances between electrodes of 1 and 2 mm. Images of trees and their growing processes were taken by a CCD camera. The fractal dimensions of electric trees were obtained by using a simple box-counting technique. The results show that the tree growth rate and fractal dimension was bigger when the frequency or voltage was higher, or the distance between electrodes was smaller. Contrary to our expectation, it has been found that when the distance between electrodes changed from 1 to 2 mm, the required voltage of the similar electrical trees decreased only 1or 2 kV. In order to evaluate the difficulties of electrical tree propagation in different conditions, a simple energy threshold analysis method has been proposed. The threshold energy, which presents the minimum energy that a charge carrier in the well at the top of the tree should have to make the tree grow, has been computed considering the length of electrical tree, the fractal dimension, and the growth time. The computed results indicate that when one of the three parameters of voltage, frequency, and local electric field increase, the trends of energy threshold can be split into 3 regions.     

Phase resolved PD patterns in treeing in the presence of voids

One of the important aspects of insulation condition monitoring through partial discharge (pd) detection is the need to establish a trend in the discharge pattern for a given insulating system. Treeing is one of the principal routes to failure in insulating materials, and its growth is accompanied by pd activity. In this work, a study of the evolution of the phase resolved pd patterns during the various stages of tree growth in the presence of void is undertaken. Air-filled artificial voids are injected into the polymer, and their effect on the growth of a pre-existing tree observed. The pd patterns were seen to change drastically on tree channels reaching the void. The tree channel reaching the void was seen to undergo intense pd activity followed by widening of the channel. Electrical pd detection is performed simultaneously with online visual observation of tree growth with a high resolution high speed camera.     

Structure characteristics of electrical treeing in XLPE insulation under high frequencies

Electrical tree structure is one of the most important influencing factors for electrical treeing characteristics in polymers. In this paper, we focused on the structure characteristics of electrical treeing in cross-linked polyethylene (XLPE) insulation under high-frequency voltages. The tree structure characteristics include structure distribution characteristics and structure conversion characteristics. The influences of voltage, frequency, and pin-plane spacing on tree structure characteristics were analyzed based on the experimental results. It can be concluded that tree structures regularly change with the local electric field and frequency. The electric field in a very small zone near the needle tip is an important influencing factor for the formation of bush-like trees, and the lowest frequencies for the observed pure-vine-like trees increased with voltage. For double-structure trees, the local electric field at the transition location of the two structures remained almost unchanged with voltage and pin-plane spacing, but obviously increased with frequency. In order to investigate the relations of the growth rate and fractal dimension with tree structure characteristics, a new parameter, the energy threshold W_t, has been introduced and calculated for different tree structures.     

Effect of annealing on conductivity in XLPE mid-voltage cable insulation

A new study of the electrical conductivity of crosslinked polyethylene (XLPE) mid-voltage (MV) cable insulation is presented. Its main objective is to show the effect of annealing treatments on MV cables under actual service conditions. Complementary time domain (absorption/resorption currents) and frequency domain (dynamic electrical analysis) techniques are applied on different laboratory samples containing XLPE insulations: sections of XLPE insulated cable (with and without semiconducting screens) in the case of absorption/resorption currents and, in the case of dynamic electrical analysis, a thin ribbon obtained from the cable insulation by mechanical procedures. For annealing temperatures below a certain critical temperature, conductivity decreases both for XLPE cylinders (cable sections from which inner and outer semiconducting screens have been removed) and for real cables (sections of cable with semiconducting screens) but its value is smaller in the case of cables. If the annealing temperature is higher than the critical temperature, the behaviour of conductivity is more complex. In XLPE, cylinders conductivity initially decreases with the annealing time but after some annealing time it begins to increase, it passes over a maximum and eventually it decreases monotonically. In the case of real cable sections, conductivity grows, tending to a saturation value, which is noticeably higher than the corresponding value of the maximum obtained for XLPE cylinders. The experimental results are explained satisfactorily by means of the Mott equation that takes into account hopping conduction assisted both by temperature and electric field.     

冷绝缘超导电缆绝缘设计及测试方法的简介

超导电缆主绝缘的设计在整个超导电缆的设计中占有十分重要的地位。文中介绍了冷绝缘HTS电缆主绝缘的设计方法和绝缘性能的检测方法;对于超导电缆耐受电压、设计场强和绝缘厚度的确定以及超导电缆主绝缘的性能检测都做了详细的总结;同时也介绍了由于失超引起的局放对绝缘设计的影响。该文所介绍的设计方法和性能检测方法能够为超导电缆主绝缘的设计和绝缘性能检测提供一些参考。     

Electrically conducting ion tracks in diamond-like carbon films for field emission

Electrically conducting channels in an insulating carbon matrix were produced by 140-MeV Xe ion irradiation. The high local energy deposition of the individual ions along their pathes causes a rearrangement of the carbon atoms and leads to a transformation of the insulating, diamond-like (sp³-bonding) form of carbon into the conducting, graphitic (sp²-bonding) configuration. The conducting ion tracks are clearly seen in the current mapping performed with an atomic force microscope (AFM). These conducting tracks are of possible use in field emission applications. Received: 4 May 1999 / Accepted: 5 May 1999 / Published online: 1 July 1999     

100kV直流高温超导电缆终端应力锥设计

本文提出了一种新的单直线应力锥设计方法,根据PPLP材料的绝缘特性,结合超导电缆的本体尺寸和设计电压,给出了增饶绝缘厚度和应力锥轴向长度的设计数据,其中增饶绝缘厚度为3.5 mm,应力锥轴向长度为38.5 mm,并对单直线应力锥进行电场数值分析,结果表明,所设计的单直线应力锥满足100 kV直流高温超导电缆运行要求。