Space charge accumulation in power cable XLPE insulation

The accumulation of space charge in the crosslinked polyethylene (XLPE) insulation of a power cable sample under dc stress was investigated. The sample was held at 82°C and atmospheric pressure, and at 82°C and 0.1 Pa, for periods up to several days. Such conditioning removed pre-existing space charge, When an external dc field was applied after conditioning at atmospheric pressure and 82°C, space charge accumulated at a rate considerably faster than that observed before conditioning. However the rate of accumulation following conditioning at 82°C under reduced pressure was much slower than that observed before conditioning. The faster rate could be recovered by reconditioning the sample at atmospheric pressure after conditioning under reduced pressure. Inversion of the equilibrium space charge profile, following reversal of the applied field, was observed. This observation is consistent with a model incorporating a spatially-inhomogeneous polarization in the amorphous volume of the XLPE, and electron injection/extraction at the electrodes. The electron transfer between electrode and XLPE in either direction involves the same narrow window of combined donor and acceptor states in the insulator, centered on the Fermi level. The spatially-inhomogeneous polarization in the XLPE originates in a spatially-inhomogeneous distribution of dipole complexes. It is suggested that the dipoles are formed by a process equivalent to nonpermanent oxidation of the XLPE, requiring water as a catalyst     

Space charge accumulation in two power cable grades of XLPE

The accumulation of space charge in XLPE (cross-linked polyethylene), cross-linked using DCP (dicumyl peroxide) or a silane-based grafting process, was studied via the LIPP (laser-induced-pressure-pulse) technique. Planar samples 0.5 mm thick were obtained from the XLPE insulation of power distribution cables. DC fields to 10 kV/mm were applied at temperatures in the range 20 to 90°C. Usually the DCP samples developed heterocharge, and the silane samples homocharge. However, the observed space charge densities were not very different. Nearly all of the charge accumulated within 100 μm of each electrode, with very little in the remainder of the volume. Both sample types showed a near-perfect inversion of the equilibrium space charge profiles on reversing the applied voltage polarity. This observation is explained in terms of charge injection at the electrodes, electron transfer between electrode and XLPE in either direction involving the same narrow `window' of combined donor and acceptor states in the insulator, centered on the Fermi level     

Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cmdc fields

This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model     

Space charge development and breakdown in XLPE under DC field

The correlation between space charge accumulation and insulation failure has been investigated in four, polyethylene-based materials subjected to a DC field of 50 kV/mm. Two of the materials contained tree-retardant additives and all four materials were aged for extended periods up to 19,000 h. The specimens were removed periodically from the external field and subjected to space charge density measurements under no-voltage conditions using the thermal step (TS) technique. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge measurements provided valuable insight into a possible link between the dynamics of space charge development and the time of DC aging. In particular, a consistent surge in the amount of accumulated space charge shortly before insulation breakdown was observed in practically all samples. Also, a consistent relationship between time to breakdown and the position of the internal space charge density peaks in the sample was observed. Samples that had space charge density peaks further apart consistently survived longer than specimens for which the peaks were closer together. These phenomena, if confirmed, could be used to screen new materials for DC voltage applications.     

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps     

交联聚乙烯绝缘电力电缆封头工艺的改进

介绍交联聚乙烯绝缘电力电缆封头工艺改进及其使用效果     

Factors of hetero space charge generation in XLPE under dc electric field of 20 kV/mm

This report deals with the mechanism of space charge accumulation in cross‐linked polyethylene (XLPE) under dc electric field. Space charge was measured by the pulsed‐electroacoustic method with applying dc stress of 20 kV/mm. A large amount of hetero space charge accumulated in fresh XLPE samples. Factors influencing the space charge accumulation were analyzed in regard to cross‐linking by‐products and antioxidant. No space charge was seen when the fresh sample was degassed to remove cross‐linking by‐products. Introducing acetophenone, one of the cross‐linking by‐products, in a degassed sample produces no space charge, suggesting that acetophenone itself could not be the direct factor of space charge formation. However, heating this sample up to 150 °C results in formation of hetero space charges as in virgin samples. Hence, it is concluded that hetero space charges may be formed when impurities, such as an antioxidant, dissociate thermally with the help of acetophenone and that the dissociated products are attracted toward both electrodes under a dc field to form the hetero space charges. © 1999 Scripta Technica, Electr Eng Jpn, 129(2): 13–21, 1999     

The role of degassing in XLPE power cable manufacture

This article focuses on a range of elements critical to this process; form the fundamental chemistry, through computational and measurement techniques to the solutions that are in use today. Degassing contributes greatly to the quality of power cables by improving the certainty in electrical testing and improving the dielectric properties. To ensure that the degassing process delivers the expected benefits, it is important to measure and model the process by which the crosslinking byproducts are desorbed from the cable polymers. Weight loss and HPLC have shown themselves to be the most effective and practical measurement techniques     

Correlation between space charge distribution and water-treelocation in aged XLPE cable

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables     

直流XLPE电缆绝缘中空间电荷的抑制方法综述

交联聚乙烯(XLPE)因其优异的介电、理化性能而被广泛应用于电缆绝缘领域。在电缆的服役过程中,电缆绝缘内部会积聚空间电荷,严重时可引发电场畸变,导致电缆击穿事故发生。对于直流XLPE电缆,空间电荷的积聚及影响更加不容忽视。针对直流XLPE电缆绝缘中产生的空间电荷积聚效应,目前学界主要采用共混改性、聚合物链段接枝极性基团、纳米掺杂改性及制备高纯净绝缘料等方法来进行控制,改性后的直流XLPE电缆绝缘对空间电荷产生的抑制效果均有所提升。文中首先对上述直流XLPE电缆绝缘中空间电荷的抑制方法进行综述,介绍其抑制原理以及相应的抑制效果,然后对比总结不同抑制空间电荷方法的优缺点,最后对未来直流XLPE电缆绝缘中空间电荷抑制方法的研究发展作出展望。     

退役高压XLPE电缆绝缘空间电荷行为研究

文中对两回110 k V退役高压交联电缆进行180 a预鉴定试验,目的在于研究不同运行年限的退役高压交联电缆老化前后的空间电荷行为差异,并评估电缆重新投入实际运行的可靠性。通过电声脉冲(PEA)法测量试验前后绝缘层交联聚乙烯(XLPE)试样的空间电荷分布,结合傅里叶红外光谱(FTIR)实验、X射线衍射(XRD)实验揭示试样的微观结构、聚集态结构状态变化与空间电荷的迁移、积聚和消散过程的潜在关系。实验结果表明:实际运行16 a的电缆绝缘由于存在大量杂质使得空间电荷的积聚严重,老化试验后,杂质的减少和淬火反应使得空间电荷的积聚降低且消散过程加快;实际运行32 a的电缆绝缘空间电荷的积聚和消散过程缓和,老化试验后,由于绝缘降解作用和晶态结构的破坏使得空间电荷的积聚加剧且消散过程变得缓慢。     

XLPE 电缆绝缘中的电树枝种类及其影响因素

通过大量样品实验研究,本文发现,在同等实验条件下,由于材料聚集状态的差异,在同一XLPE电缆绝缘中引发和生长的电树枝可分为枝状、丛林状、藤枝状、松枝状和混合状等五类;不同结构的电树枝对高压XLPE电缆运行可靠性的影响不同;对产生这几种电树枝结构的内在材料聚集状态特征进行了研究。     

Understanding treeing phenomena and space charge effect in gamma-irradiated XLPE cable insulation

In the present work, electrical trees were experimentally generated in virgin and gamma-irradiated XLPE cable insulation, under AC voltages. Fibrillar type of electrical tree structure forms from the defect site in gamma-irradiated specimens. Failure times due to electrical trees were analysed by adopting Weibull distribution studies. The results of the study indicate that gamma-irradiated specimens, where the rate of tree propagation is high, causing early failure of insulation. Pulsed electro acoustic emission (PEAE) technique was adopted to understand the space charge formation in XLPE cable insulation. The results of the study indicate that homo and hetero charges form near to the electrodes especially in gamma-irradiated XLPE cable insulation. Variations in space charge density at different voltage magnitudes with time were analysed. It is observed that the electric field in the insulating material becomes highly non-uniform on occurrence of hetero charges. It is also realized that on removal of applied voltage, the charge decay is less with gamma-irradiated specimens. Based on the present study, it could be concluded that Space charge density in insulating material and life of insulating material due to electrical tree formation shows inverse relationship.     

Space charge behavior in full-size 250 kV DC XLPE cables

The space charge in full size 250 kV HVDC power cables was measured using the pulse-electroacoustic method. Measurements of two types of newly developed DC XLPE cables with 20 mm insulation were taken under a DC voltage of 500 kV with the conductor temperature at room temperature and at 85°C. A qualitative analysis of the space-charge distribution and a quantitative analysis of the electric fields in the vicinity of semicon interfaces were conducted. It was shown that the field in the vicinity of the inner semicon tends to increase by 10-40% when the polarity of the applied voltage was reversed, in the case when the conductor was kept at 85°C. However, the distortion of the electric field was significantly less than that expected in conventional XLPE cables. As the result, the subjected DC cables are considered to have stable DC characteristics from the viewpoint of space charge-behavior     

工频电压下110kV级XLPE电缆绝缘中电树枝生长阶段特性

基于实时显微数字摄像技术和局部放电连续测量相结合的实验方法,研究了不同外施工频电压下110kV级XLPE电缆绝缘中电树枝的结构特征及其生长特性。结合大量实验结果,研究发现在不同外施电压的作用下,电树枝结构呈现出多样性的特点,生长过程及其局部放电具有鲜明的阶段特性,而且每个阶段存在不同生长机理。     

Study of electrical treeing phenomena in XLPE cable samples using acoustic techniques

Electrical trees were generated experimentally in the actual 33 kV underground XLPE cable insulation material under the AC voltages. A tree like structure and a bush type of tree structure can form from the point of defect site under the AC voltages. Acoustic emission technique was adopted to identify the point of inception, propagation and termination of electrical trees. A variation in the dominant frequency content of the acoustic signal was observed as and when the tree propagates in the insulation structure. The characteristic variation in the magnitude of the acoustic emission signal with time, indicates that tree propagation as an intermittent growth process. The energy content of the acoustic signal characterizes that the energy released due to partial discharges, at every step growth of the tree structure is not the same. The partial power measurement in the present study provides an indication to the growth process of electrical trees and to the near point of failure of polymer insulation material due to electrical treeing.     

220kVXLPE绝缘电力电缆的设计及试制

通过 2 2 0 k V交联聚乙烯绝缘电力电缆的设计和试制 ,对以下几个问题进行分析和说明 :(1 )电缆的导体结构 ;(2 )绝缘厚度的设计 ;(3)导体屏蔽及绝缘屏蔽的屏蔽方式及厚度选取和材料的选用 ;(4)金属护套的功能及其厚度的选定、轧纹形状及阻水性能试验验证 ;(5)纵向阻水层结构的设计 ;(6)试制过程的工艺要点。文中还指出了通过产品的型式试验 ,证明产品的技术指标达到或超过设计要求     

Space change behavior in full scale ±250 kV dc XLPE cables

Space charge in full size ±250 kV dc cables was measured using a pulse-electroacoustic method. Two newly developed types of dc XLPE cables with 20 mm thick insulation were subjected to measurements at a dc voltage of 500 kV, keeping the conductor temperature either at room temperature or at 85°C. Qualitative analyses of the space charge distribution and the quantitative analysis of the electric fields in the vicinity of semicon interfaces were made. It was shown that the field in the vicinity of the inner semiconductor tends to increase by 10 to 40% when the polarity of the applied voltage was switched, in the case when the conductor was kept at 85°C. However, the distortion of the electric field was significantly smaller than that expected with conventional XLPE cables. As a result, dc cables are considered to have stable dc characteristics from the viewpoint of space charge behavior. © 1998 Scripta Technica, Electr Eng Jpn, 124(2): 16–28, 1998     

雷电波侵入XLPE电缆绕组的暂态特性

利用雷电波侵入XLPE电缆绕组的外端口特性得出其暂态等值电路,并进行分析求解.发现避雷器的放电电压和残压越高,电缆绕组上的电压越高;但是侵入波越陡、入口电容越大,反而可能使得电缆绕组上不会出现高于避雷器残压的过电压,有利于电缆绕组的保护.这与传统变压器绕组出现具有余弦振荡性质的过电压有很大不同.对绕组输入模拟雷电波进行实验并使用EMTDC仿真,对比表明电缆绕组在雷电冲击电压作用时外端口特性可用R、C串联支路来近似等效.为这种新型变压器的设计、保护技术提供了参考依据.