Structure Of Water Trees Research Articles

Study on water tree resistance and self‐healing properties of multi‐monomer melt‐grafted polyethylene

AbstractIn order to suppress the damage of cable insulation caused by water tree structure formed by micro‐droplets at insulation defects in low density polyethylene (LDPE) materials under AC electric field. In this paper, the siloxane group is grafted onto LDPE by melt grafting method, and different content of comonomer styrene (St) is added to improve the grafting rate of siloxane group grafted polyethylene. The effect of St on the microscopic characteristics of LDPE grafted KH570 is studied. In order to verify the accuracy and timeliness of water tree self‐healing of grafted polymer insulation materials, the microstructure characteristics of LDPE ungrafted and grafted polymer samples are analyzed by Fourier transform infrared spectroscopy, gel content test, and differential scanning calorimeter. Second, the AC dielectric properties of LDPE ungrafted and grafted polymer samples are studied by AC breakdown experiment and space charge test. Finally, the water tree length, micropore defects, and self‐healing of the samples are observed by optical microscopy and polarization and depolarization current method, and the changes of chemical composition in the water tree area before and after aging are analyzed by infrared spectroscopy. The principle and mechanism of repairing water tree defects with grafted monomers are explained. This paper not only combines the previous research results of self‐healing materials, but also provides a new method for the preparation process of LDPE grafted polymer, which has a good research prospect in practical application.

Diagnosis of Water Tree Aging in XLPE Cable by the Loss Current Harmonic Component Under Variable Frequency Power

Water treeing is one of the main aging in XLPE cable insulation, which has the direct effect on the reliability of cable long-term operation. The diagnosis of water tree aging is a premise to ensure the safe operation of cables. The water treed XLPE insulation shows a nonlinear conductivity characteristic, which leads to a harmonic component in the corresponding lose current under the standard sinusoidal voltage. So the loss current harmonic component method can be an effective diagnosis of water tree aging. The loss current measurement system based on the high voltage current comparator bridge is used in this paper, which is designed by Harbin University of Science and Technology and Jiangsu Electric Power Company Research Institute. The feasibility of the application of variable frequency series resonant power source in the loss current measurement, which is aimed to diagnose the water treeing in XLPE insulation, is studied in the laboratory and field test. The results show that the output voltage waveform of the variable frequency series resonant power source is quite close to the standard sinusoidal voltage, although there are some high frequency sharp peaks. So the variable frequency series resonant power source can be used as the high voltage test power supply in the loss current harmonic component measurement system. The loss current harmonic component has power frequency interference during the field test, but this can be solved by differential frequency principle. The microscopic observation of water tree structures during the anatomy of the cable not only indicates the effectiveness of the results and diagnosis during the field test, but also proves the feasibility of the application of variable frequency series resonant power source in the loss current measurement. The existence and degree of the water tree can be estimated according to the test results, which can be a strong basis of the standardization of water tree aging diagnosis. Furthermore, the expensive standard sinusoidal wave generator and linear power generator is replaced by the variable frequency series resonant power source, which reduces the cost and weight of the testing system.

Development of water tree structure in polyester resin

It is commonly accepted that water trees can be initiated and developed within insulation by the simultaneous action of moisture and alternating electric field. Despite the appreciable achievements and extensive works in the field of water treeing, there is no definite mechanism explaining its initiation and development. In this work, the results of a regular optical examination of the detailed structure of water trees are reported. Various patterns of water droplets are clearly illustrated before the initiation of water trees. The role of water absorption, as a main source of the tree, is not limited to the initial stage of tree formation but it is important for the creation of channels in the later stages. This idea is highlighted via a number of photographs, describing the main stages of the building process of water trees. New evidences on the mechanisms interpreting the development of water trees are introduced. Finally, the relationship between the obtained patterns of water filled voids and the subsequent shapes of water trees is discussed

Diagnostic dielectric spectroscopy methods applied to water-treed cable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10/sup -5/ to 10/sup 5/ Hz. Insertion loss measurements were also carried out in the frequency range of 3/spl times/10/sup 5/ to 3/spl times/10/sup 9/ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables.

Water treeing in binary linear polyethylene blends: the mechanical aspect

Water treeing tests were performed on low density polyethylene (LDPE) and four different binary blends of sharp linear polyethylene (LPE) fractions (M/sub w/=2500 and 76000), which were either quenched in air from the melt or isothermally crystallised at 123/spl deg/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 morphology and behavior of the water tree

The internal structure of water trees is examined in detail, particularly the nonuniformity of their internal structure. The quantitative space distribution of bouquets and microcavities is given. Based on the fact that the water tree is composed of a number of microcavities filled with liquid, the authors analyze the behavior of water trees, i.e., the dielectric properties of the zone of the water tree and its influence on the distribution of the dielectric field. An interpretation of the development of the water tree is proposed and a possible cause of water-tree-induced breakdown is suggested. >

The structural characteristics of water trees in power cables and laboratory specimens

The physical structure of water trees grown in power cables and in laboratory specimens made of crosslinked polyethylene are studied and compared. Detailed observations are made by fluorescence microscopy. This technique, used for the first time in this type of research provides images with good contrast and high resolution. It is completed by 3-D views acquired by confocal laser scanning microscopy. In both cases the tree structure is a network of continuous submicroscopic branched and zigzag tracks. The previous published observations are critically examined. Moreover, the self-luminescence of water trees and its neighborhood is studied, and is shown to be localized in the tree tracks.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Composition and growth of water trees in XLPE

On the basis of experimental results, a model describing the composition and the structure of water trees is derived. It is concluded that water trees are in fact salt trees, i.e., they consist of tracks of hydrophilic salt groups. These tracks most likely run through the amorphous phase of polyethylene. In water trees in cables, the salts are carboxylate salts and/or inorganic salts. In needle test objects the concentration of the inorganic salts is much higher. The differences in chemical composition appear to correlate with the aging conditions. The degradation products in water trees reflect the processes which took place during aging. In this way the occurrence of oxidation, ion diffusion, and chain scission is confirmed. A water tree growth hypothesis is formulated, which incorporates an electrochemical and an electrophysical effect. The first effect is very important in service-like aging, whereas the latter predominates in needle tests.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of water treeing in an amorphous and in a semicrystalline polymer

In this paper, we compare the growth and the structure of water trees in amorphous polystyrene and semicrystalline low-density polyethylene. The structure and the growth of the water trees observed in each of the polymers are very different. These different aspects are discussed in terms of mechanical fatigue, and the role played by the crystalline layers in the resistance of semicrystalline polymers to water treeing is pointed out.

Some Observations on the Structure of Water Trees

The results of an experimental study of gas permeation through water trees and the electrical field modifications produced by water trees in low density polyethylene are presented. The results show that water trees in low density polyethylene contain water in a dispersed form and that interconnecting channels are absent.