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     

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.     

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.     

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。     

交联聚乙烯电缆绝缘材料中电树枝的导电特性研究

利用光学显微观察、局部放电测量和共聚焦Raman光谱分析相结合的方法, 研究了交联聚乙烯(XLPE)电缆绝缘材料中两种典型电树枝的导电特性.尽管具有相似的培养条件, 两种电树枝却呈现出完全不同的形态,其中9 kV下典型电树枝为枝-松枝状, 11 kV下为枝状, 而且电树枝生长及局部放电规律呈现出明显的差异.枝-松枝状电树枝主干通道内存在无序石墨碳的沉积, 根据石墨碳G带与D带的相对强度,估算碳层厚度约为8 nm,树枝通道单位长度电阻小于 10 Ω· μm^-1,足以抑制电树枝内局部放电的发展,电树枝呈现出导电型电树枝特征. 枝状电树枝通道内观察到荧光背景,存在材料劣化的产物,但不存在无序石墨碳的聚集, 通道具有明显的非导电特性而不足以抑制电树枝内局部放电的连续作用. 最后提出了XLPE电缆绝缘材料中导电型和非导电型电树枝的单通道生长模型, 利用等效电路理论对XLPE电缆绝缘材料中两种不同导电特性电树枝的生长机理进行了探讨.     

介质窗横向电磁场分布下的次级电子倍增效应

本文利用自编P3D3V PIC程序, 数值研究了BJ32矩波导传输TE₁₀模式高功率微波在介质窗内、 外表面引发的次级电子倍增过程, 给出了次级电子3维空间位置分布特征、介质窗表面法向静电场分布规律以及电子数密度分布特性. 模拟结果表明: 对于介质窗内侧, 微波强场区域率先进入次级电子倍增过程; 而对于介质窗外侧, 则是微波弱场区域优先进入次级电子倍增过程. 形成机理可以解释为: 微波坡印廷矢量方向与介质窗外表面法向相同而与内表面法向相反, 内侧漂移运动导致强场区域电子易于被推回表面, 有利于次级电子倍增优先形成; 外侧漂移运动导致强场区域电子易于被推离表面, 不利于次级电子倍增形成. 准3维模型相对1维模型: 介质窗内侧次级电子倍增过程中, 次级电子倍增进入饱和时间长、饱和次级电子数目少、平均电子能量高、 入射微波功率低、沉积功率低; 介质窗外侧次级电子倍增过程中, 次级电子倍增进入饱和时间短、饱和次级电子数目少、平均电子能量低、 入射微波功率低、沉积功率低. 沉积功率与入射微波功率比值与微波模式、强度及介质窗内外侧表面关系不大, 准3维和1维模型计算结果均在1%–2%左右水平. 关键词： 高功率微波 介质表面次级电子倍增 粒子模拟 横向电磁场分布     

硅橡胶电树枝老化过程的实验及模拟研究

在交流高电场下研究了硅橡胶的老化过程,通过显微镜观察到了硅橡胶老化后产生的具有分形特征的电树枝结构,并对不同电压下获得的电树枝进行了形态以及分形维数方面的描述.基于修正的DLA模型和球状生长模型分别构建了相应的模拟算法,并对电树枝结构的生长过程进行了数值模拟,获得了较好的电树枝模拟结果.同时,也分析了不同模型下的模拟结果与实验结果在形态以及分形维数上的差异.     

光波段柔性基超材料制备及光学性质

采用化学电沉积方法,通过控制沉积电压,电沉积时间等实验参量,确定出最优实验条件,在冰浴条件下,实验温度范围为0~2 ℃之间,在氧化铟锡导电薄膜表面沉积二维结构随机排列的纳米银树枝,并在银树枝表面涂覆一定厚度的绝缘薄膜聚乙烯醇后,与另外一层银树枝组装制作成银树枝/聚乙烯醇/银树枝复合结构.实验表明,参比样品在可见光波段400～750 nm处没有透射峰的出现,而银树枝/聚乙烯醇/银树枝复合结构在此波段有多个透射通带峰,且在相应波长表现出了聚焦效应.     

光波段柔性基超材料制备及光学性质

采用化学电沉积方法,通过控制沉积电压,电沉积时间等实验参量,确定出最优实验条件,在冰浴条件下,实验温度范围为0～2℃之间,在氧化铟锡导电薄膜表面沉积二维结构随机排列的纳米银树枝,并在银树枝表面涂覆一定厚度的绝缘薄膜聚乙烯醇后,与另外一层银树枝组装制作成银树枝/聚乙烯醇/银树枝复合结构.实验表明,参比样品在可见光波段400～750nm处没有透射峰的出现,而银树枝/聚乙烯醇/银树枝复合结构在此波段有多个透射通带峰,且在相应波长表现出了聚焦效应.     

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.     

Electrical tree initiation in silicone rubber under DC and polarity reversal voltages

With the fast development of high voltage DC (HVDC) cable, cable insulation under DC conditions has got more attention. In this paper, tests were conducted to study the electrical tree initiation in silicone rubber (SIR) under DC and polarity reversal voltages. It is found that electrical tree initiation has significant polarity effects under both DC and polarity reversal voltages. There are only single-branch-like trees and branch-like trees under DC and polarity reversal voltages. As for pre-stressing effects under polarity reversal, the pre-stressing voltage has positive effects to electrical tree initiation, while the pre-stressing time has little influence. Space charge distribution of SIR under high electric field was studied with flat plate pulsed electro-acoustic (PEA) system, and their characteristics were discussed to explain this phenomenon. Moreover, different treeing breakdown phenomenon is found under polarity reversal voltage which differs from that under DC voltage. The existence of fast charges and slow charges gives reasonable explanation to it. Special attention should be paid to the transient situation like polarity reversal which would result in irreversible effects more easily, and affect true length of life for HVDC cables.     

CSNS/RCS引出脉冲电源传输电缆终端绝缘技术

对脉冲电源系统72 h老练实验过程中高压电缆座打火并被击穿情况进行分析,建立高压电缆终端电场分布等效电路,推导出从电缆终端到金属屏蔽层的电势差公式,从理论上分析高压座被击穿的原因是电缆终端绝缘层未做倒角,空间电荷在此处集聚、温度升高导致的。通过定位被击穿高压座击穿孔的位置验证理论分析的正确性,最后提出高压电缆端部绝缘层处理方法,并做72 h老练实验验证了其正确性。     

Morphology of electrical trees in silicon rubber

The main cause of degradation and breakdown in silicon rubber (SIR) is electrical treeing. Based on a series of experiments, this paper discusses the morphology of the electrical trees. The types of morphology of electrical trees in SIR are concluded. The effective factors of the tree initial type are explored. And the propagation characteristics are also studied through long-term electrical tree ageing experiments. These results are also compared with the electrical trees occurred in on-site cable accessories and those in PE which are more familiar to researchers. Based on those experiment results, an explanatory mechanism is proposed.     

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.     

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.