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。     

含水树枝XLPE电缆的超低频谐波响应电流特性实验研究

本文对含水树枝XLPE电缆在0．1Hz超低频正弦高电压和在50Hz工频电压下的非线性漏导电流特性进行了对比实验研究,发现含水树枝电缆样品的漏导电流波形畸变出现的电位与电压对时间的积分有关。分析认为,在电场激励下水分进入连接水树枝微孔的微裂纹是造成电漏导电流含有谐波电流的原因,这种超低频正弦电压激励下的漏导电流检测诃用来XLPE电缆绝缘的水树枝老化状态进行了现场离线检测。     

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.     

500kV XLPE电缆绝缘中树枝化现象的述评

介绍日本开发 5 0 0 k V XL PE电力电缆时研究树枝化现象的形成 ,评述 XL PE电缆形成电树和水树与场强的相关性、树枝引发场强和长期寿命特性的研究结果 ,说明由此确定 5 0 0 k V XL PE电力电缆的绝缘设计。     

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     

交联聚乙烯电缆中水树研究的现状

在14篇文献的基础上综述了交联聚乙烯(XLPE)电缆中水树研究现状,介绍了水树的定义、分类、特征及在水树产生和发展过程中的一些影响因素及抑制方法等,并且对水树研究中提出的新机理等做了简要的概括。     

检测XLPE电缆绝缘水树老化状态的超低频方波-工频叠加法研究

本文提出了能有效检测ＸＬＰＥ电缆绝缘水树老化状态的超低频方波－工频电压叠加法新技术,对含水树ＸＬＰＥ电缆分别放加单极性方波和正负对称方波电压叠加工频电压所产生的响应电流特性进行了实验研究,找出了最佳叠加工频电压值,最后对产生超低频方波－工频电压叠加电汉响应的机理及其检测的影响因素进行了探讨。     

A study of treeing phenomena in the development of insulation for500 kV XLPE cables

This review summarizes research on treeing phenomena, i.e. the formation of electrical trees and water trees, that has been undertaken in Japan for the development of 500 kV XLPE cable. Section 1 presents the results of factors affecting XLPE cable insulation breakdown under commercial ac and lightning impulse voltages. Section 2 verifies the phenomena of electrical tree formation in XLPE cable insulation using block samples and model cables, and gives the results of studies to determine the level electrical field stress initiation for such trees. Section 3 summarizes the results of studies on long-term aging characteristics, which is a particular problem under commercial ac voltages, while Section 4 explains how this research influenced the design of 500 kV XLPE cable insulation. All authors were members of `The investigation committee of fundamental process of treeing degradation' under IEEJ     

Thermoluminescence in XLPE cable insulation

Crosslinked polyethylene (XLPE) has been employed in underground transmission and distribution cables because of its excellent electrical and mechanical properties, such as low permittivity and dielectric loss, high degree of toughness, and good flexibility. An underground power cable operates at temperatures above ambient and the polymeric insulation is usually crosslinked to provide mechanical strength to withstand the high temperatures. Chemical crosslinking is commonly employed; however, chemical crosslinking creates byproducts such as acetophenone, α-methylene styrene, and cumyl alcohol. The general practice is to decrease the concentration of the volatile crosslinking byproducts from the newly manufactured transmission class cables before they are commissioned into service. The concentration of the byproducts is decreased by treating the cables at a high temperature in a vacuum oven. At present, to determine the residual concentration of the byproducts, the treatment has to be stopped, a sample of the polymer has to be cut from the treated cable and the byproducts have to be extracted for several hours from the polymer before they can be analyzed. This paper describes a novel, non-destructive optical method for determining the concentration of the byproducts in XLPE prior to cable installation. The method involves in situ detection and measurement of thermoluminescence emitted by the crosslinking byproducts during the pretreatment of the cable, It is shown that the measurement of the intensity of thermoluminescence provides a direct indication of the concentration of the byproducts and that it is more sensitive than mass spectrometry     

Electrical treeing in mechanically prestressed insulation

For several years it has been apparent that the mechanical characteristics of insulating materials have a pronounced influence on their electrical breakdown by treeing. As a consequence of this discovery, it is possible to look again from a different perspective at the treeing behavior in materials containing fillers and barriers. The effect of temperature on treeing can be interpreted in terms of the effect of temperature on the various mechanical properties of the material, and of any particulate or solid barrier inclusions. The internal mechanical stresses produced in resin samples containing cast-in metallic electrodes can affect treeing behavior both positively and negatively, depending on whether this stress in the material is tensile or compressive. In the light of this experience it has been found possible to enhance the treeing resistance of electrical insulation by using these mechanical effects in the most advantageous way, viz. by mechanically prestressing the material by the inclusion of fibers in tension; a method which is akin to that employed in the prestressing of concrete     

外施电压频率对XLPE电缆绝缘中电树枝生长特性的影响

本文研究了半结晶交联聚乙烯(XLPE)电缆绝缘试样在50～2000Hz正弦电压下电树枝的结构特征与生长特性,发现由于XLPE的半结晶聚集态物理结构,在小于250Hz施压频率下会生成枝状、枝状与丛林混合状及纯丛林状三类电树枝,在500Hz以上高频下则只能生成稠密枝状电树枝,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,而高频下的电树枝生长特性与材料的聚集态关系不大.高频电树枝与环氧树脂、有机玻璃等高聚物材料中的生长规律相同.半结晶高聚物在低频下的电树枝生长特性主要取决于晶界与无定形界面的微孔、杂质集中情况以及针尖电极与晶块或无定形区所处的相对位置,而在高频下电极向介质中注入与抽出电荷的过程较低频下猛烈,会形成较均匀的介质弱区,因此高频电树枝引发与生长规律较为单一.     

Effect of defects and moisture on insulation characteristics of XLPE insulated cable

Sample cables having a few types of defects (such as voids, contaminants, and protrusions) with more volume than usual, were prepared intentionally to clarify how these defects affect separately insulation characteristics under the presence/nonpresence of moisture. In the case of void inclusion, insulation characteristics are even more susceptible to ac voltage than impulse voltage, and the distribution of lifetimes is classified into a wornout failure type due to discharge deterioration. While protrusions and some other contaminants can greatly deteriorate original dielectric strength, these defects have a relatively slight effect on long-term endurance in the atmosphere. Therefore, the distribution of lifetimes is classified into an early-failure type. With reference to the influence of moisture, in a strong electric field (exceeding 15 kV/mm) and in a short-term region, there is an increase in harmfulness around defects with water absorbed, thereby inducing electrical trees and resulting in a breakdown. In the light electric field (no more than 15 kV/mm) and in a long-term region, harmfulness of defects will not deepen to a considerable extent, and the growth of water trees very possibly is influential.     

Electrical stress enhancement of contaminants in XLPE insulation used for power cables

The use of XLPE as the insulation for power cables has grown steadily since it first introduction more than 30 years ago. Today XLPE is rapidly becoming the preferred insulation system for even the highest transmission voltages. This preference is due to the high reliability, low dielectric losses, and low environmental impact that can be achieved with XLPE. The positive effects of high quality insulation materials on improved cable performance have been well known since the start of cable making. The purpose of this paper is to investigate the technical background for the cleanliness levels and to quantify the level of performance required from clean materials. The advantages of clean insulation materials are seen at all voltages. However, this work focuses on the technical basis for the benefits for HV and EHV cables, which typically are designed with a water impervious layer to ensure that the cable remains dry throughout its entire lifetime. The presence of metallic contaminants in MV cable is known to enhance the growth of trees by raising the electric stress level locally. The singular impact of cleanliness on the performance of MV cables is somewhat more complicated as it is influenced both by the cleanliness of the insulation and the ability of the insulation material to resist the growth of water trees.     

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.     

Electrical insulation characteristics of cold dielectric high temperature superconducting cable

For the optimization of electrical insulation design for high temperature superconducting (HTS) cable, evaluation of electrical insulation characteristics especially for butt gap of LN/sub 2/ impregnated cold dielectric (CD) which consists of the wrapped tape insulation impregnated with LN/sub 2/ plays an important role. This paper presents partial discharge (PD) inception and breakdown characteristics in LN/sub 2/ impregnated butt gap model which modeled a weak point of the wrapped tape insulation impregnated with LN/sub 2/ and cable model with short length with polypropylene laminated paper (PPLP/sup /spl reg//), Nomex/sup /spl reg// paper and cellulose paper. PD current pulse was found to have a steep rise time of /spl sim/ ns and amplitude of /spl sim/ tens /spl mu/A at PD inception voltage region. Little dependency of breakdown stress on the insulating material is found. PD inception stress is almost independent of insulation thickness of 1 to 3 mm. The requirement insulation thickness for 66 kV class HTS cable is estimated to be /spl sim/ 5 mm under PD-free condition from viewpoint of long-term reliability.     

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。     

Impulse breakdown superposed on ac voltage in XLPE cable insulation

This paper discusses tree inception and breakdown voltage characteristics of XLPE cable insulation subjected to impulse voltages superimposed on ac voltage. The tree initiation tests were performed on laboratory-molded specimens equipped with needle electrodes, whereas the breakdown tests were conducted on a full-sized cable. The impulse tree initiation stress was found to be dependent on the magnitude of the pre-applied ac stress and the relative polarities of the impulse and the ac peak at the instant of their superposition. Although the impulse polarity has an effect on the tree inception, the general behavior is that the tree inception stress always decreases with an increase of the pre-applied ac stress. This phenomenon is discussed in terms of the space charge effect and the influence of the impulse voltage application itself. The impulse strength of a full-sized cable insulation was found to be independent of the pre-applied ac stress as long as that stress did not exceed the operating stress of a 500 kV cable insulation. However, subjecting cable insulation to higher ac stresses before impulse application caused a reduction of its breakdown strength as compared with the insulation without ac prestressing     

Weld lines in power cable insulation

The quality of XLPE insulation of power cables has improved markedly in recent years, and insulation failures caused by contamination have diminished. As a result, focus has shifted on the effects of insulation morphology. There are concerns about the weld lines regarding the anisotropy and frozen-in mechanical stress. The weld line that is formed has different properties than the matrix and the region is considered to be a mechanically weak point. In a recent study, seams were observed in the XLPE insulation of power cable. Their type and intensity vary because of the melt temperature and homogeneity in the extrusion. The mass distribution also has a certain influence. Typically, the seams are quite harmless for the dielectric properties of the insulation. Occasionally, severe weld lines may be considered as possible weak regions in the insulation construction. By performing electrical needle tests on four cable samples, it was found that the weld line containing sector of the cable is measurably weaker than the insulation in general. The rotating mass distributor, or more specifically, the conical extrusion was able to modify, level out and reduce seam effect. While there is no guarantee that the extrusion will have high insulation quality, it is possible to reduce the seams. The traditional extrusion process, however, should be considered as a less robust solution than the rotating mass distributor with respect to the seams.     

Dependence of XLPE insulated power cable wave propagation characteristics on design parameters

Propagation losses occur in medium voltage power cables as pulses propagate through them. Since cables have many components, these can make different contributions to the propagation losses. The relative contributions of the conductors, insulation and semi-conducting screens to the propagation characteristics of the cable are analyzed. The propagation characteristics of the cables are studied by a developed "approximate" model providing analytical expressions that can quantify the contributions to the losses by the different parts of the conductor and dielectric system. The model is compared with an "exact" model and is tested on four cables     

工程用交联聚乙烯绝缘控制和仪表电缆的发展

本文阐明了发展工程用交联聚乙烯绝缘控制和仪表电缆的必要性 ,由于这类电缆的交联方式可以有多种选择 ,对于中、小型电线电缆厂的发展较为有利。本文简述了这类电缆的一般要求 :参数、规格和性能 ,并提供了部分规格波阻抗计算参考值。此外对硅烷交联聚乙烯的热寿命评定试验及其结论作了简要说明。最后建议宜尽快制订这类产品标准。