Experimental Study on the Cyclic Ampacity and Its Factor of 10 kV XLPE Cable

The load varies periodically, but the peak current of power cable is controlled by its continuous ampacity in China, resulting in the highest conductor temperature is much lower than90℃, the permitted long-term working temperature of XLPE. If the cable load is controlled by its cyclic ampacity, the cable transmission capacity could be used sufficiently. To study the 10 kV XLPE cable cyclic ampacity and its factor, a three-core cable cyclic ampacity calculation software is developed and the cyclic ampacity experiments of direct buried cable are undertaken in this paper. Experiments and research shows that the software calculation is correct and the circuit numbers and daily load factor have an important impact on the cyclic ampacity factor. The cyclic ampacity factor of 0.7 daily load factor is 1.20, which means the peak current is the 1.2 times of continuous ampacity. If the continuous ampacity is instead by the cyclic ampacity to control the cable load, the transmission capacity of the cable can be improved greatly without additional investment.

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.

Aging Characteristics of XLPE Insulation of 110 kV Cables in the Initial Stage of Operation

The aging characteristics of the XLPE insulation of 110 kV power cables in the initial stage of operation are studied in this paper.The XLPE insulation from cables with different running time was tested by using electrical and physicochemical methods,from which the degradation state of XLPE under multiple stresses during operation was derived.The broadband dielectric spectrum test taken from 10–1 to 10^(6) Hz showed that the dielectric constant and dielectric loss of the XLPE insulation first decrease and then increase with the increase of cable running time,which show obvious variation until 10 years of operation.The AC breakdown strengths of the XLPE insulation of operated cables with the service year no longer than 10 years were higher than that of new cables.The mechanical properties of the XLPE insulation showed no dependence with the cable running time.The crystallinity and perfection of the crystallization area were found to be enhanced during operation and the polar functional groups inside the XLPE insulation were found to be determined not only by the cross-linking by-products but also by the oxidation process during cable operation.The experimental results in this paper are important for improving the operation reliability and reducing the maintenance costs of the cables.

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.

电、热、力仿真研究平滑铝护套高压XLPE电缆缓冲层设计

平滑铝护套电缆可有效规避皱纹铝护套电缆缓冲层缺陷,但目前国内对其缓冲层结构设计最优参数尚不明确。通过对绝缘膨胀进行计算、试验验证及公式修正,研究平滑铝护套电缆绝缘膨胀及间隙设计;建立平滑铝护套电缆仿真模型,分析有无空气间隙对电场分布的影响,以及不同缓冲层厚度对平滑铝护套电缆温度、热应力分布的影响;建立平滑铝护套电缆简化结构四点弯曲仿真模型,研究在重力作用下其机械应力分布。结果表明:综合重力、绝缘膨胀等因素,220 kV 1200 mm^(2)平滑铝护套电缆缓冲层和铝套之间的负间隙大小建议设计为1.3~1.6 mm。在平滑铝护套电缆缓冲层设计时,应在满足绝缘热膨胀的同时尽可能减薄缓冲层的厚度,提高电缆载流量,避免铝护套承受过大的热应力。

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。

基于宽频阻抗谱BIS的XLPE电缆绝缘电树枝缺陷定位方法

提出了一种改进的宽频阻抗谱BIS方法来定位XLPE电缆中的电树枝缺陷。进行了不同温度下的电树枝实验,以启动不同类型的电树枝。利用矢量网络分析仪测试了电树枝生长过程中的高频电容特性。采用改进的定位算法计算了含有不同类型电树枝缺陷的20m长电缆的定位谱,结果表明切片样品的电容随着电树枝的生长而减小。丛林状电树对电容参数的影响大于松枝状电树枝。在丛林状电树枝的定位谱中发现了一个典型的双峰,在松枝状电树枝的光谱中发现了单个峰。

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.

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.

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。

国产与进口XLPE绝缘高压交流电缆短时与长时击穿特性

为探究国内外高压电缆XLPE绝缘料的性能差异,制备了220 kV国产与进口电缆绝缘切片试样,对比了两种绝缘料的分子量、熔融和结晶特性、交联副产物含量、短时击穿特性与长时击穿特性。实验结果表明:国产220 kV XLPE电缆绝缘的分子结构、结晶度、熔融温度以及副产物含量均与进口绝缘料无明显差异。分析了两种绝缘料短时击穿场强的Weibull分布特性,发现30~50℃下国产XLPE绝缘特征击穿场强高于进口绝缘料,而当温度超过70℃时国产XLPE绝缘特征击穿场强显著下降且低于进口绝缘料,并且国产XLPE绝缘料击穿场强分散性更高。分析了70℃下两种绝缘料在5个电场下的击穿时间,采用反幂模型分析了击穿电场和时间直接的关系,结果表明国产与进口220 kV绝缘料寿命指数值分别为19.3和19.5。研究结果对国产XLPE绝缘的研发与应用提供了理论与实验支撑。

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。

Statistical Characteristics of Partial Discharge Caused by Typical Defects in Cable Joint under Oscillating Voltage

The oscillating voltage test is a nondestructive detection method for partial discharge of XLPE （cross linked polyethylene） cable and has been applied recently. This paper made three kinds of varying severity artificial defect models of cable joints in 10 kV XLPE cable. Oscillating voltage is applied to the model, by use of pulse current method to detect partial discharge signals. In order to study the statistical characteristics of partial discharge of cable joint under the oscillating voltage, three-dimensional statistical map has been made. The results show that for the same kind of defects, with the increases of the defect severity, the discharge interval extended, the magnitude and the number of partial discharge increase, for different kinds of defects, obvious differences exist among the maps, this may established a foundation for the further study of the partial discharge pattern recognition of XLPE cable under oscillating voltage.

Line Differential Protection Scheme Modelling for Underground 420 kV Cable Systems

Based on the analysis of a specific relay model and an HVAC （high voltage alternating current） cable system, a detailed approach to EMTDC/PSCAD modelling of protective relays is presented. Such approach allows to create complex and accurate relay models derived from the original algorithms. Relay models can be applied with various systems, allowing to obtain the most optimal configuration of the protective relaying. The present paper describes modelling methodology on the basis of Siemens SIPROTEC 4 7SD522/610. Relay model was verified experimentally with its real equivalent by both EMTP-simulated and real world generated current signals connected to the relay.

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。