Water-tree characteristics in low-density PE under simulatedinverter voltages

Water tree characteristics have been investigated for low-density polyethylene by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. If we compare the water-tree length among single-frequency voltages, the water tree grows faster if the frequency is higher. However, if we superpose a power-frequency voltage on the high-frequency voltage, tree growth is suppressed. The tree length becomes shorter if the superposed power-frequency voltage is higher. This surprising result is explainable by considering that the zero-crossing of the applied voltage plays an important role in tree propagation     

Growth properties of sulfide trees in cables under dc stress

Investigations on the growth properties of sulfide tree in cable insulation, subjected to dc electric fields, are described in this paper. Measurements of electric strength and conductivity are correlated to tree growth, observed by optical and electron microscopy, during accelerated aging tests performed in sea water with hydrogen sulfide. It is shown that both electric strength and dc conductivity are very sensitive to the length and concentration of sulfide trees, and that the magnitude of the electric field affects tree shape and growth rate significantly     

A novel physical model for partial discharge in narrow channels

In order to reveal the mechanism of partial discharge (PD) in electrical trees, an artificial single channel is studied as model for tree channels. In a long-narrow channel, the behaviour of PD is similar to that in an actual tree. When the length of the channel is decreased, the φ-q-n pattern of PD in the channel changes from wing to turtle or rabbit like, and the range of light emission in the channel increases when the applied voltage is increased. Considering the influence of a PD on the total field distribution and thus on a later PD, a physical model is put forward to describe the PD behaviour. It is assumed that PD is induced by the high field at the needle tip, then propagates along the channel, and stops propagating when the field in the channel becomes lower than a critical value due to field divergence. Mathematical analysis on an ideal needle-plane electrode system clearly explains the physical meaning of the model. And the computer simulation by using the superposition method agrees with the experimental results very well. This model is also valid to describe the behaviour of PD in electrical trees     

Dielectric breakdown of low-density polyethylene under simulatedinverter voltages

Breakdown characteristics have been investigated for low-density polyethylene immersed in silicone oil by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. It was found that the breakdown voltage decreases as the high-frequency component increases. However, the number of zero-crossings of the applied voltage, which is very influential in water-tree growth, does not seem to play an important role in the dielectric breakdown process     

The ac superposition method,a new hot‐line diagnostic method for 6‐kV XLPE power cables

Water trees are the most hazardous factor affecting the life of XLPE distribution cables and the major cause of insulation failure. It is well known that insulation failure causes stoppages in electrical service and requires extensive repair work. Up to now, deterioration of cable insulation has been diagnosed mainly by the dc leakage current method. However, application of this method for diagnostic measurements requires interruption of electrical service. Several types of hot‐line diagnostic methods (including the dc component current and dc superposition methods) were developed to detect water tree deterioration. However, these methods have some shortcomings, such as being subject to effects of stray currents and the accuracy of measurements not being sufficiently high. Therefore, we have attempted to develop a new hot‐line diagnostic method. We investigated whether the signals produced by superposition of voltages of various frequencies to the cover layer of energized cables were correlated with deterioration by the water treeing. As a result, we found that a deterioration signal of 1 Hz was observed when we superposed an ac voltage (commercial frequency × 2 +1 Hz) on the cover layer of cable where the water tree had occurred in the insulation. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 49–58, 2000     

Fractal analysis of the treeing process from luminous dischargeimage and measurement of discharge magnitude

The purpose of this study is to analyze the growth of trees generated in PMMA from the apex of a void created by a needle tip. The analysis was based on fractal theory. In the measurement system, PMMA blocks were used with electrodes consisting of a needle and a plate. One cycle of an applied ac voltage was divided into twenty successive phase angle sections, labeled φ1, φ2, ..., φ20, from one negative peak to the next negative peak of the applied voltage. The quantitative evaluation of the fractal dimension D of the luminous discharge pattern was performed for each section, particularly, for the sections φ7, 8, 9, 17, 18 and 19 where the discharge luminescence reached around the tip of the tree. It was noted that some samples revealed substantially different tree patterns even under the same measurement conditions. The reasons for the different trees have been investigated by determining D and attempting to correlate tree growth and the change of tree length with aging. When D of the discharge luminescence at φ7, 8, 9, 17, 18 and 19 were large, luminescence with complex branches were generated. The present results indicate that tree growth and branching depend on the discharge at φ7, 8, 9,17, 18 and 19     

针尖曲率半径对硅橡胶电树枝老化特性的影响

针对硅橡胶材料电树枝老化特性展开了试验研究。采用针板电极结构,测量了不同针尖曲率半径的起树电压和电树枝形态。结果表明,硅橡胶的起树电压较低,抗电树枝老化性能较差,导致了硅橡胶预制式电缆附件故障频发;硅橡胶中的起始电树枝多以单枝状的通道形成为标志,随后发展为4种不同的电树枝形态。进而提出了硅橡胶电树枝形态发展模型,并以此讨论了不同形态的电树枝老化特性。研究还发现,现场实际运行的硅橡胶预制式电缆附件故障解体后发现了单枝通道起始电树枝,其在运行电压作用下容易发展为细枝型形态,但同样会引发电缆附件的击穿。试验结果为现场故障分析提供了参考依据。     

几种因素对水树生长影响的研究

通过在实验室采用水针法对交联聚乙烯绝缘块进行水树枝的培养,重点研究了施加的交流电压、试验时间、频率以及针尖曲率半径对水树枝生长的影响。并通过数学分析验证试验结果,结果表明：这4种因素与水树枝长度均近似成幂函势关系;电压和针尖曲率半径对水树生长的促进作用较时间和频率更为明显;而且所施电压越高、针尖曲率半径越小、频率越高、时间越长,则水树生长越快。     

高频下电树老化引起的高密度聚乙烯击穿现象

为了分析高频下电树老化引起的高密度聚乙烯击穿现象,实验研究了冰水淬火高密度聚乙烯(HDPE)薄膜在频率50 Hz~90 kHz范围内的交流电压作用下电树老化引起的破坏现象及电树生长过程中的电树形态特性的变化。观测高频下树老化引起击穿过程中电弧运动发现:不同频率下HDPE薄膜中电树生长过程中存在树枝、树干、丛状和击穿型4种基本电树形态;在高频下不同电树形态存在形态转换的关系,且随着频率和电压的升高,相同条件下电树老化引起的击穿破坏概率大大增加。试验结果表明,在较高频率下树的形态易向击穿型发展。最后从高频电压对材料极化过程的影响及高频所引起的材料疲劳效应、热效应加剧角度出发,对在高频下电树老化易于引发击穿的特性展开了深入的讨论,解释了高频或中频电气设备易发生绝缘老化破坏的现象。     

In-service methods for distinguishing the dc component due to water treeing from noise currents in xlpe cable

This paper describes the water tree diagnoses of 6.6-kV class XLPE cables by the dc component method. The dc current component is measured by energizing the cable with ac high voltage, therefore, it would be suitable for on-line diagnoses. The dc component is considered to be caused by the rectifying effect of water trees inside the insulation. In laboratory tests, the dc component showed a good correlation with the degradation. However, almost no correlation was seen in the on-site measurement. The authors found that the cause of this bad correlation is the galvanic effect inside the jacket. This generates the noise current through the grounding circuit when the jacket resistivity is low. Based on this finding, the equivalent circuit of on-site measurement was proposed. Considering the equivalent circuit, this paper proposes nine methods for discriminating the dc component due to water tree from noise current. Also described are the analytical results of each method. Furthermore, the on-site or laboratory tests show that some of them are effective in separating the dc component due to water tree from noise current.     

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

Dry‐cured and extruded three‐layer (E‐E type) 6.6‐kV cross‐linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the ac breakdown voltages of dry‐cured and E‐E type 6.6‐kV XLPE cables removed after 18 to 25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.     

频率对高密度聚乙烯电树老化特性的影响

为了解频率对高密度聚乙烯电树老化特性的影响,在50Hz~90kHz较宽频率范围的交流电压作用下,研究了冰水淬火高密度聚乙烯(HDPE)薄膜的电树老化特性。结果表明,频率对电树起始形态具有重要的影响,随着电压频率的升高,树枝型电树的起始几率逐渐降低,丛状型电树的起始几率逐渐升高,电树逐渐由树枝型起始为主向丛状型起始为主转变,树干型和直击型为高频下所特有的电树起始形态。随着电树的生长,电树形态存在转换的可能,低频下,起始占主导的树枝型电树向丛状和树干型转变;高频下,起始占主导的丛状型电树则极易转变为树干和击穿型,导致绝缘的破坏。电树的发展可分为起始、滞长、生长和击穿期4个阶段。频率的提高加快了电树的发展速度且减少了电树的发展阶段,使发生击穿的几率大为增加。     

一种基于分形的XLPE电缆在线绝缘诊断方法

通过建立水树精确数字仿真模型,提出一种新的在线绝缘诊断方法:即高频信号叠加法.在电缆屏蔽层叠加高频低幅电压信号,以凸现损耗电流波形,利用分形方法对采集到的损耗电流信号进行分析.XLPE电缆的水树的劣化使得损耗电流中谐波分量增加.谐波分量主要由于水树的非线性电压-电流特性引起.谐波分量的出现使损耗电流信号的分形维数增加.分别计算了容量、信息、关联三种维数,分析了各种维数和水树劣化之间的内在联系.分形维数很好的反映了电缆中水树劣化的程度,可以作为在线绝缘水树劣化的诊断依据.     

高温下110kV交联聚乙烯电缆电树枝生长及局部放电特性

利用实时显微数字摄像与局部放电连续测量系统,采用典型针-板电极结构,研究了高温下不同外施工频电压作用时110kV级交联聚乙烯(XLPE)电缆绝缘中典型电树枝的形态特征、引发、生长规律及其局部放电特性。实验结果表明,温度对XLPE电缆绝缘中典型电树枝的形态、引发与生长时间具有非常重要的影响。在高温下,不同外施工频电压作用时电树枝的形态呈现出多样性的特点,50°C下典型电树枝形态为枝状、枝-松枝状和丛状,70°C下为枝状,90°C下为滞长型和枝状。高温下电树枝引发时间随外施电压升高而减小,而且在同一外施电压下,电树枝引发时间随温度升高而减小,这是由于在高温下XLPE电缆绝缘中片晶熔化,无定形相增加,介质中自由体积扩大,从而更有利于电树枝引发。研究发现在低电压(9kV)下,电树枝生长过程中由于通道电导率增加而抑制了通道内局部放电的发展,局部放电作用减小,电树枝生长速度减慢,分形维数较高;而11kV以上电压作用时,电树枝在局部放电的连续作用下呈枝状向对面电极快速生长,同时高温下XLPE弹性模量下降,击穿场强降低,局部放电作用加剧,电树枝生长明显加速,电树枝分形维数较低。     

交联聚乙烯电缆水树缺陷的修复方法研究

采用硅氧烷修复液修复交联聚乙烯电缆老化试样中的水树,进而分析修复效果及机理。将介质损耗因数为4%～6%,绝缘电阻7 500～10 000 MΩ的短电缆在7.5 kV 450 Hz交流电压下老化至介质损耗因数达到20%左右,绝缘电阻3 500～5 000 MΩ。然后用压力注入式修复装置把修复液注入缆芯对水树缺陷进行修复。以介质损耗因数、绝缘电阻和击穿电压为指标对修复效果进行评判;通过显微镜切片观察修复前后水树微观形态;通过仿真修复前后水树附近电场分布来分析和验证水树的修复机理。实验结果证明,修复液可以充分与电缆水树中的水发生反应生成胶状聚合物填充水树通道;修复后电缆介质损耗因数、绝缘电阻和击穿电压恢复到新电缆水平;改善了绝缘层电场分布;有效地抑制了水树生长。实验表明,该修复液可有效修复电缆中的水树缺陷,提高电缆绝缘水平。     

Initiation and growth of electrical trees in LDPE generated byimpulse voltage

The initiation and growth of electrical trees generated by impulse voltages were studied in a molded low-density polyethylene (LDPE) specimen with a metal foil electrode. The Weibull plots of the number of impulse voltage shots to the tree initiation and the puncture of the specimen were analyzed. The difference between the plots of positive and negative impulses demonstrates the difference between the initiation and growth of positive and negative trees. Taking account of the estimation of the electric field strength in the vicinity of the electrode, the tree initiation process was examined. Through the examination of the Weibull plots of the number of impulse voltage shots up to the puncture of the specimen the deterioration of LDPE caused by the impulse tree was analyzed     

Space charge behavior under ac voltage in water-treed PE observedby the PEA method

The pulsed electroacoustic method (PEA) has been applied to observe space charge formation under ac application (7 kV_pk, 50, 0.1 and 0.001 Hz) in water-treed polyethylene samples in order to understand the degradation mechanism of water trees. A system with phase-resolving capability has been developed to measure the space charge distribution at any phase angle of the applied ac voltage. By comparing the observed charge distributions with theoretically predicted ones, it has been shown that the interfacial polarization due to the difference in conductivity between the treed and the regions without trees dominate the space charge behavior. It has become clear that the PEA method can be a powerful nondestructive method for evaluating water tree length     

低密度聚乙烯/纳米蒙脱土复合材料的水树枝生长特性

低密度聚乙烯是高压电力电缆的主要绝缘材料,水树枝生长特性与聚乙烯高压电力电缆绝缘击穿具有紧密联系。采用熔融插层复合法制备了一种低密度聚乙烯/蒙脱土纳米复合材料;设计制作了纳米复合材料的水树枝老化试样及试验装置,在试验中观测了试样的水树枝生长长度,并对试样的水树枝引发率进行了统计,分析了低密度聚乙烯/蒙脱土纳米复合材料的吸水率对水树枝生长的影响;采用差示扫描热法分析了试样的结晶度和晶粒尺寸均匀性,通过分析低密度聚乙烯/蒙脱土纳米复合材料的结晶行为,说明了纳米蒙脱土对纳米复合材料中水树枝的抑制机理。试验与分析结果表明:掺杂质量分数为3%的纳米蒙脱土粒子能够有效地提高低密度聚乙烯的结晶度,使晶粒尺寸分布均匀,吸水率减小,延缓水树枝在低密度聚乙烯中的引发与生长。     

复合控制单相直流侧串联型有源电力滤波器

单相直流侧串联型有源电力滤波器由分别工作在高、低频率的全控型功率器件以及二极管构成的混合全桥逆变器组成,串联在整流桥的直流输出端和直流负载之间,只需处理部分功率,可实现降压的功能,是一种新型的单相整流桥谐波治理方案。将平均电流控制应用于直流侧串联型有源电力滤波器的控制将有助于其实现数字控制。文中首先分析了直接应用平均电流控制时控制效果差的原因:由于在低频开关的切换点前后,高频开关的占空比需要实现0、1的跳变,而当采用单一的调制方式时,调制信号不能产生与占空比相对应的波形变化,引起了输入电流的波形畸变。文中提出了一种在不同的工作区间分别采用前沿调制和后沿调制相结合的复合控制策略,避免了对调制信号的突变要求,但可实现占空比的跳变,成功地将平均电流控制应用于单相直流侧串联型有源电力滤波器的控制,可大幅提高输入电流波形质量。实验结果对文中的分析结论和提出的复合控制方式的正确性进行了验证。     

不同电极系统下XLPE电缆电树枝生长特性的研究

The electrical tree discharge channel will be formed at concentrate spot of electric field in solid insulation dielectric,in order to study the difference of electrical tree under different electrical field,the short-cable electrode system with actual XLPE cable was designed,experiments were performed under 12 kV,15 kV,18 kV,21 kV compare to the needle-plate electrode system.Experiment results show that the electrical tree of short-cable electrode system have the same growth trend with the needle-plate electrode system in the growing characteristic,the dense of electrical tree increase with the increase of voltage level,electrical tree of short-cable electrode system growth is slower than the needle-plate electrode system at the same voltage;To get the same shape of electrical tree,the voltage of short-cable electrode system must be higher than needle-plate electrode system,the results show that the semiconductor layer and the copper shield layer outside of XLPE cable have very important affection on the electrical trees degradation.