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.     

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     

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.     

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.     

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.     

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     

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。     

A radar backscatter model for forest stands

A Monte Carlo method for calculating the radar backscatter cross-sections from a forest stand is presented in this study. The model for the forest stand consists of two layers: the top layer contains a mixture of randomly orientated dielectric circular disks or small needles representing the leaves of the deciduous trees or coniferous trees respectively, as well as cylindrical-shaped scatterers, representing the branches of the foliage, while the bottom layer consists of randomly distributed vertical cylinders modelling the tree trunks. The phase matrices of the disks and needles are formulated using the generalized Rayleigh-Gans approximation whereas those of the branches and tree trunks are obtained from the infinite-cylinder approximation. The ground is modelled as a Kirchhoff rough surface based on the stationary-phase approximation. The relative contributions from the tree trunks, branches and trunks, leaves and trunks, and the entire forest stand are studied separately. It is found that leaves seem to dominate at higher frequencies while branches at lower frequencies. Effects of various physical and geometrical parameters such as dielectric constants, volume fractions, sizes and orientation distributions of the scatterers in a forested canopy are also studied. Finally, the results from the model are compared with the measured angular distributions of the radar backscattering cross-sections from some field measurements reported in the literature.     

小兴安岭主要树种冠层光谱季相变化研究

森林每年随季节变化而出现形态和生理机能的规律性变化,该变化在一定程度通过其光谱特征有规律地展现。准确地掌握森林冠层光谱特征随季节变化的规律不仅是遥感解译的关键,也为树种识别、动态监测和生化参数反演提供理论基础。利用地物光谱仪对研究区9个主要树种的冠层光谱数据进行长期观测,获取了春夏秋冬四个季节的反射光谱曲线并生成一阶导数曲线,同时还计算了常用的植被特征参数,进而分析不同树种在相同季相和不同季相的光谱特征,对比不同树种在可见光和近红外波段的季相变化特征和差异,探索遥感手段用于树种分类识别的最佳波段、最佳时相。结果表明：不同树种在各生长季光谱具有独特的特征,落叶树种光谱特征因季节的改变而呈现有规律的变化, 而常绿树种光谱特征年变化不明显。光谱特征的变化有效地反应了物候的变化,应用多季相的数据进行分类可以取得最好的效果,常绿树种和落叶树种光谱特征在冬季差异明显,而夏季是采用单季相遥感数据进行树种识别的最佳时节。     

扫描探针针尖与试样间的静电力分析与建模

针对主体为圆锥、尖端为球冠的扫描探针针尖物理模型,提出了针尖与试样之间等效电场分布假设,基于该假设导出了二者之间静电力的解析模型,分析了针尖结构参数与静电力间的关系.模拟计算结果和数值计算结果一致,并与相关的实验结果吻合,证实了电场分布假设的合理性.     

侧柏衰弱木和蛀干害虫受害木的热红外成像检测

植物的水分和能量代谢在其生命活动中起着至关重要的作用。尽管用热红外成像技术研究此类功能已有报道,直接用于林木生长状态检测的研究为数不多。本研究通过创建温差指数缩小了观测间的热温波动,提高了热像温度观测的精度,增加了被测对象间的可比性。在此基础上,用热红外成像技术检测了侧柏[Platycladus orientalis (L.) Franco]伐根面以及生长锥木芯之边材与心材的差异和变化,研究了侧柏树干和鳞叶的水分和能量代谢的关系。结果发现随活力状态的变化侧柏林木边材与心材树液(sap)比值不同,而且不同活力状态的侧柏其鳞叶同样表现出热温的差异。相对于叶面积与边材面积的比值而言,热红外光谱更多地反映了树液的状态而不是木材的比例。边材和心材的树液比反映在热红外成像设备中的则是边材与心材的热温比值,该指标有潜力早期反映侧柏树木健康。柏肤小蠹(Phloeosinus aubei Perris)和双条杉天牛(Semanotus bifasciatus Motschulsky)等蛀干害虫是常见的侧柏衰弱木、濒死木、枯立木、伐倒木和新栽幼树上寄生的害虫。经过大量的热红外成像观测,侧柏衰弱木和蛀干害虫受害木边材和心材热温比明显地增大。因此,该技术有潜力成为检测侧柏林木活力和确定受害状况的重要指标,成为侧柏生态公益林抚育和经营管理的重要依据。     

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。     

An improved partial SPIHT with classified weighted rate-distortion optimization for interferential multispectral image compression

Based on the property analysis of interferential multispectral images, a novel compression algorithm of partial set partitioning in hierarchical trees (SPIHT) with classified weighted rate-distortion optimization is presented.After wavelet decomposition, partial SPIHT is applied to each zero tree independently by adaptively selecting one of three coding modes according to the probability of the significant coefficients in each bitplane.Meanwhile the interferential multispectral image is partitioned into two kinds of regions in terms of luminous intensity, and the rate-distortion slopes of zero trees are then lifted with classified weights according to their distortion contribution to the constructed spectrum.Finally a global ratedistortion optimization truncation is performed.Compared with the conventional methods, the proposed algorithm not only improves the performance in spatial domain but also reduces the distortion in spectral domain.     

古树管理监测系统设计

古树是城市绿化、美化的一个重要的组成部分,具有重要的科学、历史和观赏价值。传统的树木管理方式采用人工管理,费时、费力,又容易出错,利用计算机网络可以把各方面的人力、物力和财力充分联合起来,然后通过分工协作,充分发挥联机优势,把各种信息资源收集后存储到数据库再在网络上共享,使传统的信息处理手段与先进的计算机网络有机地结合起来。在此理念的基础上,借助VS2010和SQL Server 2008工具设计出便于操作的古树管理监测系统,对现有的古树基本信息与养护信息录入管理方式进行了研究,提出了基于B/S架构的智能化管理,借助浏览器实现局域网内多平台访问,在节省时间提高效率的同时,还可以通过系统的监测预警等功能有效的协助工作人员对古树进行养护护理,更加高效的保护我们的古树资源。     

Entropy-Based Greedy Algorithm for Decision Trees Using Hypotheses

In this paper, we consider decision trees that use both conventional queries based on one attribute each and queries based on hypotheses of values of all attributes. Such decision trees are similar to those studied in exact learning, where membership and equivalence queries are allowed. We present greedy algorithm based on entropy for the construction of the above decision trees and discuss the results of computer experiments on various data sets and randomly generated Boolean functions.     

Potential for assessing long-term dynamics in soil nitrogen availability from variations in delta15N of tree rings

Numerous researchers have used the isotopic signatures of C, H, and O in tree rings to provide a long-term record of changes in the physiological status, climate, or water-source use of trees. The frequently limiting element N is also found in tree rings, and variation in its isotopic signature may provide insight into long-term changes in soil N availability of a site. However, research has suggested that N is readily translocated among tree ring of different years; such infidelity between the isotopic compositions of the N taken up from the soil and the N contained in the ring of that growth year would obscure the long-term N isotopic record. We used a 15-year 15N-tracer study to assess the degree of N translocation among tree rings in ponderosa pine (Pinus ponderosa) trees growing in a young, mixed-conifer plantation. We also measured delta13C and delta15N values in unlabeled trees to assess the degree of their covariance in wood tissue, and to explore the potential for a biological linkage between them. We found that the maximum delta15N values in rings from the labeled trees occurred in the ring formed one-year after the 15N was applied to the roots. The delta15N value of rings from labeled trees declined exponentially and bidirectionally from this maximum peak, toward younger and older rings. The unlabeled trees showed considerable interannual variation in the delta15N values of their rings (up to 3 and 5 per thousand), but these values correlated poorly between trees over time and differed by as much as 6 per thousand. Removal of extractives from the wood reduced their delta15N value, but the change was fairly small and consistent among unlabeled trees. The delta13C and delta15N values of tree rings were correlated over time in only one of the unlabeled trees. Across all trees, both delta13C values of tree rings and annual stem wood production were well correlated with annual precipitation, suggesting that soil water balance is an important environmental factor controlling both net C gain and transpirational water loss at this site. Our results suggest that interannual translocation of N among tree rings is substantial, but may be predictable enough to remove this source of variation from the tree-ring record, potentially allowing the assessment of long-term changes in soil N availability of a site.     

基于叶片反射光谱特征的银杏健康量化评价技术

准确地诊断树木健康状况是城市森林树木管理工作的基础,也是目前生产中急需的技术。通过土壤和植物养分分析诊断树木健康可靠性差,通过形态特征调查诊断树木健康费时、费力,如何快速、准确、无损地诊断树木健康已经成为城市树木健康管理的重要技术瓶颈。以北京市银杏为研究对象,对基于叶片反射光谱特征的树木健康诊断技术进行了研究。通过13个外貌形态特征聚类将树木健康划分成健康木、亚健康木、一般健康木和不健康木4个等级,不同健康等级树木叶片色素含量差异极其显著(p<0.001) ,因叶绿素含量与光谱反射率之间存在相关关系,所以采用叶片反射光谱特征判断树木健康状况是可行的。采用因子分析法,通过15个叶片反射光谱指标构建了能够综合反映叶片反射光谱特征的绿度指数、色素指数、三边指数。不同健康等级间叶片反射光谱指标以及三个反射光谱指数均有极显著差异(p<0.001)。所以,采用三个反射光谱指数构建了银杏健康评价的多元二次模型,经检验模型预测精度达到79%,可以作为银杏树木健康快速诊断。选取的光谱指标较为全面,方法简洁,并通过综合分析,确定了不同健康等级树木核心形态指标以及叶片的绿度指数、色素指数、三边指数等综合得分以及得分范围,为生产中直接使用该方法诊断银杏健康状况提供了标准。     

On the spectrum of the Laplacian on regular metric trees

A metric tree Γ is a tree whose edges are viewed as non-degenerate line segments. The Laplacian Δ on such a tree is the operator of second order differentiation on each edge, complemented by the Kirchhoff matching conditions at the vertices. The spectrum of Δ can be quite varied, reflecting the geometry of a tree.

We consider a special class of trees, namely the so-called regular metric trees. Any such tree Γ possesses a rich group of symmetries. As a result, the space L ²(Γ) decomposes into the orthogonal sum of subspaces reducing the operator Δ. This leads to detailed spectral analysis of Δ. We survey recent results on this subject.     

卫星干涉多光谱图像压缩新算法

根据卫星干涉多光谱图像的成像特性,提出一种基于分类权值率失真优化截取和自适应编码深度控制的部分SPIHT光谱图像压缩算法.首先根据干涉区域类型和编码平面的重要性,对各棵零树各个编码过程赋予不同的重要性权值,然后采用部分SPIHT算法对每棵零树独立编码,编码时根据比特平面层中重要系数的统计概率自适应地进行3种编码模式的选择,同时依据重要性权值和深度控制因子自适应地控制每棵零树的编码深度,最后在编码深度内,根据不同干涉区域的零树对恢复光谱的失真贡献,采用分类权值率失真方法对码流进行优化截取,使码流分配与失真达成最优.实验结果表明,本算法比传统算法更好地保护了光谱信息.     

PSO嵌入SVM算法的活立木材积预报研究

为了建立立木材积模型,每年有数十万棵优质活立木被伐倒,这是一种破坏性较大的实验。应用光电经纬仪自动量测与手工量测活立木地径、胸径相结合, 通过活立木材积计算软件批量计算,获得中林系107杨(Zhonglin aspens No.107)的胸径、树高、树干材积值400组数据集。采用粒子群算法嵌入支持向量机(PSO-SVM)建立了非线性智能活立木材积预报模型,并以400组实验数据集作为研究资料,随机抽选300组数据的胸径和树高作为输入值,材积为输出值,用MATLAB软件运行PSO-SVM工具箱,训练得到PSO-SVM模型,用100组数据进行检验预测。研究表明,PSO-SVM算法模型预测值与实测值间复相关系数r为0.91,r值比Spurr二元材积模型计算值高出2%,平均绝对误差率提高0.44%。引用经典Spurr二元材积模型计算值和PSO-SVM模型预报值进行对比,认为将PSO算法引入到SVM参数优化中, 使活立木材积预报具有自学习能力和自适应能力,PSO-SVM模型对样本数量要求较小、预报准确率高、学习速度快,具有很好的推广价值与应用前景。