Linear Dynamic Viscoelastic Response of Melts for Crosslinkable Polyethylene Insulation Cable Granules During Cross-linking

Two commercial crosslinkable polyethylene insulation cable granules, designed for 110 and 35 kV voltage insulation, with similar crosslinking temperature but different melting temperature, were chosen as experimental samples for examining their linear dynamic responses during cross-linking. It has been found that the gel contents of cable compounds for 110 and 35 kV insulation are almost the same after they have been cross-linked at the same temperature, pressure and time. And the sample melts show the similar dependence of the dynamic storage modulus, G; on strain. On the other hand, the dynamic temperature ramp test and the dynamic time sweep test indicate that the samples exhibit different dynamic viscoelastic responses during their crosslinking. An expression for describing their erosslinking process was proposed via probing rheokinetics of crosslinking for the two samples.

Nonlinear Response Analysis for Insulation Diagnosis of Water-tree Aged 10 kV XLPE Cable Using Pulse Voltage

The diagnosis of water trees of cable insulation is of great importance as the water-treeing is a primary cause of aging breakdown for the middle voltage cables. In this paper, it is described how the water-tree-aged 10 kV XLPE cables were diagnosed. The cables were subjected to electrical stress of 5.9 kV/mm and a thermal load cycle in a curved water-filled tube for 3, 6 and 12 months of aging in accor- dance with the accelerated water-tree test method. The aged cables were used as the samples for water-tree diagnosis. First, the water-tree degraded cable, was charged by a DC voltage, and then the cable was grounded while a pulse voltage was applied to it for releasing the space charge trapped in the water trees. The amount of the space charge, which corresponds to the deterioration degree of the water trees, was calculated. The effects of DC voltage amplitude, pulse voltage repetition rate and aging conditions on the amount of the space charge were studied. Obtained results show that the amount of the space charge has a positive correlation with the applied DC voltage and the ag- ing time of the cables, and that a peak value of space charge appears with the increase of the pulse voltage repetition rate. An optimum pulse voltage repetition rate under which the space charge can be released rapidly is obtained. Furthermore, the releasing mechanism of space charge by the pulse voltage is discussed. Accumulated results show that the presented method has a high resolution for the diagnosis of water tree degradation degree and is expected to be applied in practice in future.

Insulation Diagnosis of Service Aged XLPE Power Cables Using Statistical Analysis and Fuzzy Inference

Cables that have been in service for over 20 years in Shanghai, a city with abundant surface water, failed more frequently and induced different cable accidents. This necessitates researches on the insulation aging state of cables working in special circumstances. We performed multi-parameter tests with samples from about 300 cable lines in Shanghai. The tests included water tree investigation, tensile test, dielectric spectroscopy test, thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), and electrical aging test. Then, we carried out regression analysis between every two test parameters. Moreover, through two-sample t-Test and analysis of va- riance (ANOVA) of each test parameter, we analyzed the influences of cable-laying method and sampling section on the degradation of cable insulation respectively. Furthermore, the test parameters which have strong correlation in the regression analysis or significant differ- ences in the t-Test or ANOVA analysis were determined to be the ones identifying the XLPE cable insulation aging state. The thresholds for distinguishing insulation aging states had been also obtained with the aid of statistical analysis and fuzzy clustering. Based on the fuzzy in- ference, we established a cable insulation aging diagnosis model using the intensity transfer method. The results of regression analysis indicate that the degradation of cable insulation accelerates as the degree of in-service aging increases. This validates the rule that the in- crease of microscopic imperfections in solid material enhances the dielectric breakdown strength. The results of the two-sample t-Test and the ANOVA indicate that the direct-buried cables are more sensitive to insulation degradation than duct cables. This confirms that the tensile strength and breakdown strength are reliable functional parameters in cable insulation evaluations. A case study further indicates that the proposed diagnosis model based on the fuzzy inference can reflect the comprehensive aging state of cable insulation well, and that the cable service time has no correlation with the insulation aging state.

Studies on the Effect of Accelerated Thermal Aging on Electrical and Physicochemical Properties of XLPE Cable

The degradation of crosslinked polyethylene (XLPE) cable insulation during service, such as thermo-oxidation and water treeing may lead to a premature electrical breakdown of the XLPE insulation cables. Therefore, it is necessary to optimize the period of replacement to evenly distribute the replacement cost by ascertaining the deterioration degree. Estimation of the aging degree is at present the most important task for diagnosis of the residual lifetime of the power cable insulation. This paper presents a study on the changes in the dielectric properties of the thermally aged XLPE cables in the frequency range from 0.07～10 MHz. Based on electrical and physicochemical characterization, some new "dactylograms" for the thermally aged XLPE cable insulation have been proposed.

Electrical Aging Phenomena of Power Cables Aged by Switching Impulses

Due to the insufficient information regarding the aging phenomenon of cables caused by switching impulses, we aged 15 kV XLPE and EPR cable samples by 10 000 switching impulses in experiments and tested them. Plus in order to compare the aging phenome- non under multi-stress conditions, additional EPR cable samples were aged by rated AC voltage and current with switching impulses superimposed. We used measurements of partial discharge parameters to monitor the cables’ conditions during their aging process, and the AC breakdown voltages measurement to evaluate the cables after aging. Moreover, the Fourier transform infrared (FTIR) spectroscopy measurements revealed the changes of insulation materials after aging. The measurement results confirm that the accelerated aging of cable samples had taken place. The impacts of each individual aging factor are shown through the selected measurements and comparison. The study also helps to assess the reliability of the XLPE and EPR cables under similar condition while serving in power systems.

基于电应力损伤特征值的XLPE绝缘电压耐受指数评估方法研究

XLPE绝缘的电压耐受指数n表征绝缘电寿命特性,对加速寿命试验中获取的失效数据进行分析可以获取n值,然而,加速寿命试验中的外施场强与试验所获失效数据的分析方法影响n值的准确性。本文研究电压耐受指数n的评估方法,对XLPE绝缘试样分别进行恒定应力试验与步进应力试验,提出基于损伤特征值的试验数据分析方法,该方法通过损伤矩阵建立试验中累积损伤阈值(Dc)与n的映射关系,以矩阵秩的变化作为n值的选取依据。结果表明:本文所提方法在恒定应力试验中获取的XLPE绝缘电压耐受指数n2=12.43,在步进应力试验中获取的XLPE绝缘电压耐受指数n3=12.04,较线性拟合获取的XLPE绝缘电压耐受指数n1=12.83仅相差3%与6%,且本文所提方法能够节省75%以上的试验时间,验证了该方法的有效性。

RECENT DEVELOPMENTS IN PHOTOINITIATED CROSSLINKING OF POLYETHYLENE AND ITS INDUSTRIAL APPLICATIONS

The recent developments in the photoinitiated cross-linking of polyethylene and the significant breakthrough of its industrial application are reviewed. The enhanced photo-initiation system, the dynamics of photoinitiated crosslinking, the optimum conditions, the crystal morphological structures and related properties, and the photo- and thermo-oxidation stability of photocrosslinked polyethylene (XLPE) materials have been elucidated systematically. A new technique for producing photocrosslinked XLPE-insulated wire and cable is described in detail. It can be expected that the future applications of photocrosslinking technique of polyolefins will be very promising.

10 kV电缆中间接头受潮XLPE-SiR绝缘界面放电演化分析

10 kV电缆中间接头交联聚乙烯硅橡胶(XLPE-SiR)绝缘界面在受潮条件下的放电行为严重影响配电网的可靠性,鉴于对绝缘界面放电行为的发展和特征研究较少,分析在受潮条件下10 kV电缆中间接头XLPE-SiR绝缘界面的放电行为,从仿真和试验2个方面验证XLPE-SiR绝缘界面的击穿过程。结果表明:在水分的作用下,XLPE-SiR绝缘界面会出现局部放电,并逐步演变为电弧放电。放电会产生2个影响:一方面引起绝缘材料的热解,形成电痕;另一方面,放电产生的热量导致水蒸发,有效地阻止了绝缘界面处放电的继续。随后,在存在电痕的情况下,进一步研究XLPE-SiR绝缘界面的放电行为。结果表明,当XLPE-SiR绝缘界面存在电痕时,水分的存在会迅速导致绝缘界面击穿。10 kV电缆中间接头绝缘界面受潮直接决定了其绝缘界面放电—短路故障的发展过程,因此在实际电缆中间接头的生产及施工工艺中需要加强防水管控。

MODIFICATION OF POLYETHYLENE BY in situ FORMED SODIUM ACRYLATE

To improve the water tree resistance of PE, linear low-density polyethylene （LLDPE） was compounded with sodium acrylate （NaAA） for in situ polymerization, in which NaAA was fbrmed through the neutralization of acrylic acid （AA） with sodium hydroxide （NaOH） before adding dicumyl peroxide （DCP） to initiate the in situ graft polymerization and homo polymerization. A series of LLDPE/NaAA compounds were investigated for their water absorption ratio （WAR） measurement, water treeing, mechanical and dielectric properties. The results strongly suggest that NaAA can improve the water tree resistance of LLDPE. In addition, the LLDPE/NaAA compounds possess satisfactory mechanical properties and dielectric properties. Characterization of LLDPE/NaAA compounds by using Fourier transform infrared spectrometry （FTIR） suggests that the neutralization and polymerization reaction could be achieved effectively. Using adequate DCP content is the key factor for controlling the polymerization of NaAA with precise conversion ratio.

油纸及XLPE绝缘高压直流电缆的电场分布特性研究

直流电缆主要采用油纸及交联聚乙烯(XLPE)作为绝缘,两类材料的电导率特性不同,对电缆中电场分布的影响不同。本文基于500 kV典型直流电缆结构建立有限元模型,对油纸及XLPE绝缘电缆开展热-电耦合仿真分析,研究了两种绝缘在不同温度及温差下的场强分布特性,分析了绝缘电导率温度系数及电场系数对场强分布的影响,对比了相同温度及场强范围内油纸及XLPE绝缘的电导率变化特性。结果表明:在电场强度为10~25 kV/mm和温度为20~90℃的范围内,油纸绝缘中的电场分布受绝缘温差的影响,在确定温差下几乎不随绝缘温度而变化;而XLPE绝缘中的电场分布同时受绝缘温度和温差的影响。此外,油纸绝缘的电导率整体比XLPE绝缘高,随温度和场强的变化幅度大,这可能是油纸电缆绝缘中电场反转发生在更低温差下的原因。对于高压直流电缆中应用的绝缘材料,应控制电导率温度系数即减小温度敏感性。

基于多维介电参数雷达谱图的XLPE电缆老化状态综合评估

极化-去极化电流(PDC)法被广泛用于评估交联聚乙烯(XLPE)电缆的绝缘状态,但现有研究倾向于采用单一介电特征参数反映绝缘的老化情况,未充分利用PDC结果中丰富的介电响应信息,且单一介电参数难以适用于不同的老化类型。为解决上述问题,本文提出了一种改进的雷达谱图评估方法,以综合利用多维介电参数,全面评估XLPE电缆的绝缘状态。为验证所提方法的有效性,对人工加速热老化与水树老化的XLPE短电缆试样进行了PDC测试,并利用提出的多维介电参数雷达谱图方法对PDC测试结果进行分析。结果表明:雷达谱图中多边形面积参数能够较好地反映两种老化XLPE电缆的老化程度。

高压直流海缆XLPE绝缘空间电荷与电场分布特性研究进展及展望

高压直流海缆是大容量、低损耗、远距离跨海输电的核心装备,是实现规模化海上风电并网、电网跨海互联的关键基础。当前快速增长的海上风电发展需求,要求直流海缆具有更高运行电压、更大容量、更大长度,因此对交联聚乙烯(crosslinked polyethylene,XLPE)电缆绝缘材料提出了更高要求。文中首先综述了国内外高压直流海缆的发展历程,对比分析了XLPE绝缘与绕包绝缘的优势与不足。针对直流海缆绝缘面临的大长度连续挤出过程中的焦烧问题和工厂模塑接头绝缘界面问题,论述了XLPE绝缘焦烧产物和工厂接头绝缘界面缺陷对空间电荷与电场分布的影响规律及机理,介绍了全尺寸直流海缆空间电荷与电场分布特性的有限元仿真和数值计算方法,分析了稳、暂态工况下直流海缆XLPE绝缘空间电荷与电场分布的演变规律,阐明了直流海缆绝缘层厚度对空间电荷与电场分布的影响机理。最后,对直流海缆XLPE绝缘未来的发展进行了展望,指明了高压海缆绝缘国产化亟需解决的关键问题。

热老化对XLPE电缆本体中典型缺陷所引发的电场畸变的影响

由缺陷所引发的局部电场畸变是电缆故障的重要诱因之一。现有的关于缺陷的仿真研究主要针对于未服役的电力电缆开展,缺少老化过程对缺陷所引发的电场畸变程度的影响研究。该文通过COMSOL有限元仿真软件研究热老化对电缆本体中外护套破损、受潮水珠、杂质颗粒、半导电层凸起和水树枝5种典型缺陷所引发的电场畸变的影响。结果表明,5种典型缺陷均会造成不同程度的电场畸变,其中水树枝所造成的电场畸变率最高,为456.2%;外护套破损缺陷引发的电场畸变率最低,为51.6%。随着热老化时间的增加,靠近外半导电层的缺陷所引发的电场畸变率逐渐升高,引发放电故障的风险提高;靠近内半导电层的缺陷所引发的电场畸变率逐渐降低,呈现出自愈的趋势;而位于绝缘中部的缺陷引发的电场畸变对热老化并不敏感。上述结果的产生源自于热老化交联聚乙烯(crosslinked polyethylene,XLPE)电缆绝缘的相对介电常数沿电缆径向的非线性分布。所构建的相对介电常数沿电缆径向的分布函数及推导该函数的方法有助于更为准确地研究交变电场下老化、服役电缆绝缘的电场分布特性。

220 kV电缆XLPE绝缘材料热氧老化性能对比

以进口同级材料为对比,对国产220 kV交联聚乙烯(XLPE)交流电缆绝缘料开展不同温度下的热氧老化实验,采用红外光谱、差示扫描量热法、凝胶含量、机械性能、电气性能等测试对材料热氧老化前后性能进行系统研究。结果表明,材料化学成分、结晶性能以及交联度受老化温度影响较小,而机械和电气性能变化明显。随着老化温度升高,进口料羰基指数增大更明显,2种材料凝胶含量先增大后减小,其中进口料老化前后凝胶含量均相对较低,更容易结晶。机械性能测试表明,进口料热氧老化性能相对更优,且老化前后均表现出更高的拉伸强度和断裂伸长率。2种材料热氧老化后介质损耗因数均上升,但二者差异以及由此导致的绝缘层温升不大。国产料的交流击穿场强和电树枝起始电压更高,电树枝生长更为缓慢,且无论老化与否,国产料在耐电性能上均保持明显优势。

基于改进XGBoost算法的XLPE电缆接头故障自动化诊断与测量研究

该文研究基于改进XGBoost算法的XLPE电缆接头故障自动化诊断方法。以35 kV XLPE电缆接头为例,设计局放模拟实验,测量4种绝缘故障局放信号,生成二维局放图谱。从中提取描述投影形状和正负半周轮廓差异的故障特征,构建一维向量输入XGBoost模型,实现故障自动化诊断。应用哈里斯鹰算法优化模型参数,提高诊断分类性能。实验结果表明,该方法能有效测量不同故障类型的局放图谱,并以其特征实现高精度的XLPE电缆接头故障自动化诊断,确保了电缆长期稳定运行,更好地保障了电力安全。

XLPE电缆绝缘热老化特性规律实验研究

目前石化企业高压电缆“老龄化”日益严重、运行风险逐年增大,热老化是影响电缆整体绝缘状态的重要因素。开展XLPE电缆绝缘材料制备实验和模拟热老化实验,获取了0~480 h的XLPE薄片老化试样;并分别开展介质损耗因数、极化指数和局部放电测量实验,得到关键绝缘特征量随绝缘热老化程度的特性规律。研究结果表明,介质损耗因数、极化指数随XLPE电缆绝缘老化过程分别近似呈指数规律增长、均匀增长,局部放电量随老化过程逐渐增大且放电趋于集中。研究结果对于探明XLPE电缆绝缘劣化特性规律及验证XLPE绝缘材料热老化机理具有指导意义。

高压及超高压XLPE电缆附件施工时电缆绝缘处理工艺

在XLPE电缆附件安装施工过程中,需将电缆接头与其他附件进行连接,在现场施工时,经常出现电缆绝缘处理不到位的情况,导致电缆接头与其他附件无法良好连接,影响电缆使用寿命。因此,在现场施工前对电缆绝缘进行处理十分必要。本文主要对高压及超高压XLPE电缆附件施工时电缆绝缘处理工艺进行分析。通过对高压及超高压XLPE电缆附件施工中电缆绝缘处理工艺的分析,总结现场施工经验,可为现场施工提供一定的参考。

加速水树老化对XLPE电力电缆绝缘性能的影响

交联聚乙烯(XLPE)因具有稳定的理化性能和良好的电气性能而被广泛应用于中高压电缆绝缘中,而电缆长期运行会导致其内部绝缘的化学成分和物理形态的变化。为此,通过加速水树老化实验研究了水树老化对10kV交联聚乙烯电缆绝缘材料理化性能和介电性能的影响。随着老化时间的延长,发现电缆的外层绝缘首先发生化学结构变化,结晶度和密度减小,试样内外层热失重温差增大,熔融温度点向低温偏移,并且发现在介电损耗谱的低频段出现了新的损耗峰。理化和介电两个方面的分析结果表明,老化首先发生在外层绝缘,在电应力和机械应力的作用下,绝缘内部出现微观缺陷,表现为微观结构的变化。提出加速水树老化的物理和化学过程,阐明了水树老化过程对电缆绝缘材料性能的影响机制。

XLPE电缆绝缘中的电树枝结构及其生长特性

为研究不同频率下半结晶XLPE电缆绝缘材料中的电树枝引发、生长及结构特征,系统的归纳了XLPE电缆绝缘中可能出现的电树枝特征及其与材料聚集态和残存应力的关系;采用变频高电压发生器、实时显微数字摄像技术及专用试样加工工艺,进行了大量的电树枝培养实验。实验研究发现,由于半结晶高聚物的不均匀结晶和电缆生产过程中在绝缘层中产生的残存应力的影响,使得50Hz施压频率下XLPE电缆绝缘试样中会生成枝状、枝状与丛林混合状及纯丛林状3类电树枝,而>500Hz高频下则只能生成稠密枝状电树枝,它们分别对应于不同的生长机理。低频下电树枝生长特性和电树枝结构与材料的聚集态密切相关,而高频下则关系不大。分形分析这些电树枝的结构后发现,电树枝的生长特性与其分形维数及其分形维数的变化有对应关系,故可用分形维数分类和定量描述这些电树枝.最后探讨了几种不同结构电树枝的生长机理。