Computation of the Electric Field in Cable Insulation in the Presence of Water Trees and Space Charge

The presence of space charge changes locally the electric field distribution in power cable insulation and may play an important role in tree development, thus accelerating the dielectric breakdown. This paper is concerned with the computation of the electric field in polyethylene-insulated power cables affected by water trees which grow from the following: 1) the inner semiconducting layer; 2) the outer semiconducting layer; and 3) the inner and outer semiconducting layers, taking into account the space charge corresponding to the ions present in the treeing area. Space charge in plane samples where trees have been developed in an accelerated manner was estimated using the thermal step method. Average charge values given by space charge measurements were then used for the electric field computation in cable insulation with continuous or/and individual water trees. For the calculation of the electric field, an analytical and a numerical method have been used. This paper shows that the space charge changes the electric field distribution inside and outside the trees (the field increases in some areas and decreases in others) and that the field variations depend on the magnitude and on the polarity of the space charge, as well as on the dimensions of the water trees developed in the cable insulation. The obtained results show that, in the presence of water trees and space charge, the initiation of electric trees is more probable in the case of individual water trees than in the case of continuous water trees.     

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。     

Development of New Diagnostic Method for Hot-Line XLPE Cables with Water Trees

A new insulation diagnostic method for XLPE cables containing water trees is presented. A dc component in the ac charging current of these cables was found to be a significant sign of the existence of water trees. The dc component has good correlations with such insulation characteristics of aged XLPE cables as ac breakdown voltage and dc leakage current. Criteria for insulation diagnosis of 6.6kV XLPE cables have been established. An automatic insulation diagnostic device has been developed. It is now being applied to hot-line XLPF cables in distribution systems of TEPCO (The Tokyo Electric Power Co., Inc.).     

Observation of water tree growth in polyethylene in real time

The growth of water trees that lead to breakdown in polyethylene insulation of underground power cables is studied in real time, using a special observation cell for polyethylene that can be placed on the stage of a conventional microscope. The growth of the water tree was followed by time-lapse photography. Sample results are given. The water tree is shown to grow from a single streamer to a bushlike structure. This structure acts as an electrode, and the cycle repeats, with another streamer being sent out     

交联聚乙烯电缆中的水树

水树对交联聚乙烯电缆运行寿命有害。本文通过在交联聚乙烯绝缘料试片和原尺寸交联聚乙烯电缆上进行的加速水树引发试验,观察研究水树枝的生长特性,比较二种不同交联方法对水树生长的影响。最后探讨了形成水树的可能机理。     

Effect of testing parameters on the outcome of the acceleratedcable life test

The effects of various testing parameters on the accelerated life test results of 15 kV underground residential distribution cables prepared from thermoplastic and crosslinked polyethylene insulation are considered. Testing parameters included dry versus water in the strand, voltage stresses 4X(34.6 kV) and 3X(26.0 kV), and hot spot temperatures of 90 and 75°C. The effect of shelf aging on the life of crosslinked polyethylene insulation is reported. Microscopic examinations were made for a number of the tested samples, and tree observations are discussed. The data presented provide insight to a number of the factors which reduce the life of polyethylene cable insulation used in the URD applications     

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.     

在线注入有机硅修复液对交联聚乙烯电缆中水树生长的影响

提出了水树老化电缆的在线修复方法,讨论了在交变电场下修复液对水树的抑制作用及其绝缘修复机理。采用高频高压水针电极法对新样本、预修复样本和在线修复样本进行加速水树老化。老化一个月后,使用显微镜观察样本中水树形态并测量其水树长度。通过差示扫描量热法分析样本绝缘层的劣化程度,同时利用扫描电镜和能谱分析仪对比水树区域的微观形貌及化学结构变化。研究表明,在线注入有机硅修复液能有效地抑制水树的生长。在电场的作用下,修复液分子和水分子同时向强电场区域(如微孔、水树区域等)进行扩散并发生反应,消耗水分并且生成凝胶颗粒填充微孔,一定程度上缓解了绝缘的劣化。     

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。     

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.     

Evaluation of jacket materials and configurations and effect ofmoisture absorbing materials on preventing water tree growth in powercables

Polymer insulated medium voltage cables have experienced premature failures in-service, due in large part to water treeing. Research has shown that the initiation sites of these water trees are often located where there are stress enhancements at the insulation/semiconducting shield interface and where water soluble contaminants permeate into the insulation. Material suppliers, cable manufacturers and users have introduced improved materials and cable designs as well as processing, manufacturing, transport, storage, and installation techniques that minimize contamination. Despite these advancements, gradual contamination can still occur due to the diffusion of ground water into the insulation. In fact, certain contaminants that are carried through the semiconductor/insulation shield interfaces tend to promote water tree growth. Manufacturers are now beginning to recognise this problem and have suggested the use of moisture resistant cables with jackets made of PVC, polyethylene or metal/polymer laminates along with underjacket water absorbing materials. This paper discusses the effectiveness of different jackets and materials and presents data on the effectiveness of water absorbing materials in delaying water permeation into the insulation. A simple and efficient cable-cell technique was developed which monitored the effectiveness of complete cable designs, by measuring the long-term water permeation resistance of short lengths of distribution class cables     

Long-term characteristics of XLPE cables

To study the long-term characteristics of XLPE cables installed in free air and in water, aging tests were conducted under various testing conditions using XLPE cables with both 3.5 mm and 6 mm insulation. From the Weibull plots of lifetime distribution under the voltage stress E_L as the minimum breakdown strength, the minimum value of time to breakdown t_L under the constant voltage was estimated. The results of accelerated aging tests of XLPE cables installed in free air demonstrated that the V-t characteristics of XLPE cables could not be described by the conventional inverse power law (t ∝ V⁻ⁿ) with a single constant life exponent n. Based on the microscopic observation of a sliced insulation removed from XLPE cables, it was concluded that bow-tie trees with longer tree length observed in cables tested in water were caused by the moisture from outside, whereas the trees in cables tested in free air were caused by the residual moisture originally existing in the insulation. The breakdown strength of the aged cables tested in water increases through cable drying. However, it does not recover to the original values.     

Water treeing in binary linear polyethylene blends: the mechanicalaspect

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions (M_w=2500 and 76000), which were either quenched in air from the melt or isothermally crystallised at 123°C. Although the morphology and initial mechanical properties of the materials tested were significantly different, the vented tree growth characteristics were similar for all of them. This is in disagreement with the electromechanical models of water treeing, which correlate water tree growth with the fracture toughness of the material. Time to breakdown distributions were also similar for both LDPE and the binary LPE blends, which indicates that, regardless of the initial material morphology and the actual structure of water trees, the length of water trees is one of the controlling factors in insulation failure. The visible light image of water trees in LPE blends did not disappear upon drying as it usually does in LDPE and crosslinked polyethylene insulation     

The chemical composition of water trees in EPR cable insulation

The earlier observation of carboxylate ions in vented water trees in service-aged XLPE cable insulation is extended to bow-tie trees in lab-aged XLPE cable insulation, and to bow-tie and vented trees in EPR cable and slab specimens. Carboxylate anions were the major carbonyl-containing functionality in the treed regions, except for cases where ester functionalities (from antioxidants or shield materials) or aromatic carbonyl species (curative residues) had accumulated. Ketonic oxidation products were detected, but at a lower level than carboxylates. The experimental difficulty of detecting carboxylate ions in the presence of interfering absorptions from methylene blue dye and clay filler was circumvented by the use of a derivatization technique involving sulfur tetrafluoride and with hydrogen chloride, which converted carboxylate ions to acid fluorides and carboxylic acids respectively. Some examples of initiation sites for water tree formation in EPR are provided. When a particulate initiation site of bow-tie trees in EPR could be identified, a common feature was the presence of transition metal ions     

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     

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     

The Protection of Underground Distribution Lines Against Pocket Gopher Damage by Chemical Methods

The trend to underground distribution construction provides relief from some causes of service interruption, but in certain areas service interruptions are then possible due to pocket gopher damage. The protection of underground distribution lines against pocket gophers is accomplished by the use of a chemical, termed R-55? Repellent. Water solutions of this chemical are applied to an earth envelope about the installation which provides a repellent barrier to pocket gophers. Applications of the repellent solution are not complicated, especially when a recently developed device is used for plow-in installations. This chemical has been proven effective by controlled field tests, cooperative field trials, and commercial use over a period of years and is approved for use in this application. No significant resistivity or physical changes were noted in tests where water solutions of the repellent were applied on thermoplastic semiconducting insulation shielding. It is recommended as a low-cost effective method for the reduction of service interruptions due to pocket gophers.     

The Performance of Strippable and Bonded Screened Medium-Voltage XLPE-Insulated Cables Under Long-Term Accelerated Aging

XLPE high-voltage underground cables are being extensively used in Arabian Gulf countries. Most of these cables are being manufactured locally. Although XLPE has excellent dielectric characteristics, extensive premature failures have been reported by the local power utilities. The main causes of such premature failures are related to manufacturing defects and insulation degradation mainly caused by water treeing phenomena. In this research work, five types of 15-kV, XLPE-insulated cables were subjected to accelerated aging under simultaneous electrical, thermal, and environmental stresses for periods of 60, 120, and 180 days. After each aging period, partial-discharge (PD), mechanical properties, ac dielectric strength and the extent of degradation due to water treeing of these cables were measured. The results show that PD and mechanical properties show just little variations whereas the ac-retained dielectric strength decreases rapidly in the first 60 days after which the rate of its decrease retards. Correlations are established between the retained dielectric strength and the lengths of the longest water trees. Pros and cons of the insulating materials that are used as well as the role of strippable and bonded semiconducting screens are also discussed     

硅氧化焼注入后水树老化交联聚乙烯电缆电气特性和微观结构（英文）

To evaluate the effect of using siloxane liquid to rejuvenate water tree defects in cross-linked polyethylene(XLPE)cables,we investigated the electrical properties and micro-structures of water-tree aged XLPE cables after siloxane liquid injection treatment.The water-tree aged samples were prepared by performing accelerated aging experiment using water-needle electrodes,and the siloxane liquid is injected into the aged cable through a pressurized injection system.Dielectric loss factors of the samples before and after the rejuvenation were compared.The water trees and the internal filler were observed using scanning electron microscope(SEM).Electrical properties of the reactants are measured.Electric field simulation is conducted to verify the rejuvenation effect by finite element method.The results show that the siloxane liquid diffused into the insulation layer in a short time and reacted with water in the water trees.The electrical properties of the formed organic filler are in accord with that of XLPE.Therefore,the action between siloxane and water can inhibit the growth of water trees and reduce electric field distortion of the water tree areas.As a result,insulation performance of the cable is enhanced.A 70 m long cable was aged and rejuvenated in laboratory and an on-site rejuvenation experiment was conducted,and in both cases the dielectric loss factor and leakage current halved after rejuvenation.     

Severe degradation of the conductor screen of service and laboratory aged medium voltage XLPE insulated cables

The main purpose of this paper is to show the strong correlation between corrosion of the metallic aluminum conductor and the formation of interconnected cracks / voids in the conductor screen, creating initiation sites for vented water trees in service aged medium voltage XLPE cables. The results show that porous structures in the conductor screen previously reported for laboratory aged insulation systems, also develop in the conductor screen in service aged medium voltage XLPE cables. These structures can bridge the screen and serve as path for contaminants and corrosion products from the aluminum conductor and initiate water trees. A prerequisite for the formation of such structures is the presence of liquid water at the interface between the conductor and conductor screen causing corrosion. The initiation site of such structures has been identified, and is likely caused by environmental stress cracking (ESC). Initiation sites were determined in all cables, but porous structures in the conductor screen were only observed in the cable suffered from service failure, where liquid water had entered the cable conductor between the strands. Severe degradation of the XLPE insulation was observed at the initiation sites for water trees growing from these structures.