Trees In Polyethylene Research Articles

Correlation of optical and dielectric properties of water trees in polyethylene

Many experimental works report that the presence of water trees lowers the voltage needed for the initiation of dielectric breakdown in polyethylene. The influence of water trees on the dielectric breakdown is generally related to their length. Here, in addition to this spatial property, we consider the extinction of the light by the water trees due mainly to scattering of the incident light by water microcavities. This effect is described by means of the water tree absorbance. The absorbance itself is related to another main variable: the water content. We also take into account the existence of a structure in the water tree, i.e., of a heterogeneous distribution of water microcavities inside the trees. Water content and spatial water distribution allows us to define a local permittivity for each region in the tree. These elements are the basis of a model for the computation of the disturbance induced by the tree in the spatial distribution of the electric field (reduction of the field inside the tree, enhancement outside). The field enhancement effect is computed as a function of the water concentration. Measurements of the light transmitted by given water trees are performed; they allow us to propose an estimate of the water contained by different water trees. From these concentrations, different field enhancement coefficients are computed from the model. They are then compared with measurements of electrical treeing inception voltage performed with and without water trees. Theoretical values obtained from the model are found in good agreement with experimental data.

Charge transfer and tree initiation in polyethylene subjected to AC voltage stress

An electrical tree is grown in the region of a dielectric subjected to an intensified local electric field. It can be artificially generated in a specimen by using a needle-to-plane electrode configuration. It is characterized by an incubation period which usually tends to decrease with applied voltage. During this period, charge carriers (electrons and positive holes) move back and forth repeatedly between the electrode and the dielectric. This repeated charge transfer is believed to be mainly responsible for tree initiation. Two main contributions can be conceived: the space charge buildup and the attack on the polymer chains by high-energy charge carriers. A space-charge distortion model is proposed for the initiation of an electric tree in polyethylene. This model can also include material aging processes initiated by injected carriers. It can explain ultraviolet light emission and morphological effects which are experimentally observed in association with tree initiation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of space charge on the initial growth of electrical trees in polyethylene

This paper describes the influence of space charge on the growth of an electrical tree which is an important factor of the dielectric breakdown in polyethylene-insulated power cables. Polyethylene and ethylene vinyl-acetate copolymer with a semi-conducting needle were used as the treeing specimens. The statistical analysis showed that the growth rate and the unbalance of positive and negative partial discharges increased as the vinyl-acetate content increased, and that the growth per cycle decreased with the increase in frequency. A model of the tree growth accompanied by the space charge behavior was proposed considering the above experimental results. The tree growth is considered to be caused by the dielectric breakdown due to the field enhancement at the tree tip during the positive half cycle of the applied voltage, the field enhancement being due to residual negative space charge injected during the previous negative half cycle. The negative space charge in polyethylene would be mainly supplied through the partial discharge inside the tree.

ポリエチレンの高次構造と初期トリー

ポリエチレンの高次構造と初期トリー

ポリエチレン中の初期電気トリーの成長に及ぼす空間電荷の影響

ポリエチレン中の初期電気トリーの成長に及ぼす空間電荷の影響

Three-dimensional optical microscopy of water trees in polyethylene

Polyethylene is used as high-voltage electrical insulation in cable, and fails when subjected to the combination of water environment and electrical field. The three-dimensional microstructure of water trees, which are precursors to dielectric breakdown, has been revealed with laser scanning confocal optical microscopy. Two-dimensional images which represented optical serial sections were recorded in digital form, and reconstructed in a computer to produce three-dimensional views of the microstructure. Both stereo pairs and computed tomographic reconstructions were made. The material was stained with a fluorescent dye and the fluorescent image was compared to the standard unstained image. The dye was found to increase the signal-to-noise ratio and improve the quality of the image significantly.

Electrode influence on the propagation of water trees in polyethylene

The influence of the nature of various solid electrodes in contact with the liquid on the propagation of water trees was studied in laboratory experiments, using the water-needle method. Several hypotheses put forward to account for the observed electrode effect are examined. It is shown that the role played by the electrode is essentially due to ions entering into solution in the liquid through electrode corrosion.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

TEM Observation of Electrical Tree Paths and Microstructures in Polyethylene

The starting point of AC electrical trees in polyethylene and the surrounding microstructures were observed by a transmission electron microscope (TEM) for the first time. To minimize the size of tree paths, the authors developed a new apparatus for preparing specimens in which the tree growth was suppressed to less than 10 µm in length. The tree was started from a needle electrode made of carbon-filled polyethylene. The AC tree was a hollow pipe of about 100 nm diameter immediately after its generation. There were constrictions inside the tree, resulting in a corrugated surface. The starting point of the AC tree was a microscopic convex of several tens to one hundred nanometers, which was based on the cohesion of carbon particles of the electrode. The AC tree path was virtually parallel to the lamellae.

Water treeing using the water needle method: the influence of the magnitude of the electric field at the needle tip

The initial growth kinetics of water trees in polyethylene is measured for different values of the radius of curvature of the needle tip and for different applied voltages. The initial propagation rate is shown to be determined by the magnitude of the electric field at the needle tip, except at its highest values. The local electric field at the solution/polymer interface or in its vicinity plays an essential role in the growth mechanisms of water trees; it determines the initial propagation rate up to values of about 100 to 300 V/ mu m. Above these field values the water tree growth is found to be controlled by slower processes, e.g. the migration of the liquid through the polymer. These results are used in discussing some experimental conditions which are required to get reproducible results in water needle tests.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An Investigation Into the Initiation Mechanisms of Water Trees in Polyethylene from Attempts to Detect an Emission of Light

A very sensitive device has been used to try to detect the emission of light during the initial phase of the growth of water trees. No emission was detected, either before the onset of growth of the trees or during the propagation, contrary to what is observed in electrical treeing. This experiment eliminates light emission as a tool to examine initiation and propagation mechanisms in water treeing. It is suggested that there is no basic difference in the very nature of the mechanisms which govern the initiation and propagation phases of the growth of water trees, contrary to what is accepted for electrical treeing.

Electrical treeing in polyethylene under hydrostatic pressures

The internal discharge magnitude, the percentage of samples with electrical trees, and the tree penetration length in polyethylene for various a.c. fields have been found to be strongly dependent upon the applied hydrostatic pressure and the time of electrical stressing. The strong pressure-dependence of electrical treeing properties is in support of the model in which the creation, by carrier injection from electrical contacts, of low-density domains or channels in the bulk, followed by dissociative trapping and recombination, is a necessary prelude to the occurrence of impact ionization which leads to the formation of trees. Hydrostatic pressure suppresses the development of low-density domains or channels and thus improves the treeing properties.

Water Absorption during Water Treeing in Polyethylene

The amount of water in water trees has been measured as a function of tree length. It is roughly proportional to the volume of the water tree and the fraction of the tree volume filled with water is of the order of several percent. This water content in water trees is approximately proportional to the applied electric stress.

Effect of Frequency on the Growth of Water Trees in Polyethylene

The effect of frequency on the growth of water trees in polyethylene has been investigated using the water needle test. An accelerating effect was observed up to a maximum between 4 and 8 kHz with both NaCl solution and deionized water. This phenomenon cannot be ascribed to a frequency dependence of the electrical behavior of the trees. It results from the mechanical properties of the polymer itself and can be interpreted by analogy with environmental stress cracking. The frequency dependence of the growth rate appears to be a consequence of the existence of a mechanical absorption peak in polyethylene located at about 30 kHz at room temperature.

Electrical Breakdown Due to Discharges in Different Types of Insulation

By comparing the propagation of electrical trees in polyethylene with the evolution of artificial air-filled cavities in epoxy, it is possible to envisage the dielectric breakdown as a treeing process. Experiments were carried out on epoxy resin specimens with voids of 0.250 mm thick and 3 mm in diameter. The propagation of electrical trees was studied in low density polyethylene in which a needle was inserted, a metallic plane in close contact with the polymer being the grounded electrode. For a cavity, the delocalization of discharges was observed by recording the corresponding pulses against time and was confirmed by optical observations. On the other hand, the study of the partial discharge characteristics of electrical trees has shown the existence of several distinct phases. They give a good idea of the nature and the localization of trees. To take into account the decentralization of the discharges, the classic equivalent circuit of a cavity has been modified. The model explains the behavior of the discharge number but does not take into account the permanent change in the dimensions and the electrical characteristics of the medium, particularly in the case of treeing.

Water Treeing in Polyethylene - A Review of Mechanisms

A literature survey on the mechanisms of water-treeing electrical cables is presented. The survey provides an introduction, some background, and examples of current research on water-treeing mechanisms. The mechanisms of water-treeing have been broken into three categories: chemical, electrical, and mechanical. Along with discussion on the types of mechanisms, methods are discussed for reducing tree growth by fillers, additives, or structural changes in polyethylene. Some interesting new theories have been proposed which may help to explain the appearance and growth of water trees in electric cables.

Analysis of the Propagation of Electrical Treeing Using Optical and Electrical Methods

This paper describes an investigation into the growth of electrical trees in polyethylene by the simultaneous measurement of light intensity and partial discharges. The emission of light has been studied using an image intensifier. A comparison of the results obtained for different types of trees has been made. The detection of discharges shows that each category of tree is characterized by a particular discharge rate.

Effects of Charge Injection and Extraction on Tree Initiation in Polyethylene

Initiation of electrical treeing in polyethylene is discussed with the emphasis of a charge injection and extraction process, which is considered to feature the incubation period during which there are no apparently visual change and no detectable partial discharges. Some evidences for the process are shown by electrical, thermal and optical liberation of charge injected from a metal electrode into polyethylene. Effects of metal work functions on tree formation are demonstrated. On the basis of the above find- ings, a field emission and equi-energy theory is proposed for tree initiation and a derived V-t characteristic explains experimental results better than the common power law (t ?V-n) for lifetime.

ESR Study of Free Radicals in Electrical Trees in Polyethylene

The electron spin resonance (ESR) spectrum of polyethylene samples containing electrical trees (or, where appropriate, breakdown channels) is found to be rich in structure. Much of this structure is correlated with the conditions under which the tree was produced, thus providing valuable information on the treeing phenomenon. However, in no case thus far has the paramagnetic center responsible for a given spectral component been identified.

A Study of Inception and Growth of Water Trees and Electrochemical Trees in Polyethylene and Cross Linked Polyethylene Insulations

This paper presents an experimental investigation into the effect of different factors, such as contaminants, humidity content, temperature, field strength and frequency on water treeing in polyethylene (PE) and cross linked polyethylene (XLPE). For this investigation, model test specimens have been used to supplement cables for testing. Colored trees and their points of initiation have been analyzed for their content of impurity elements and always are found to contain sulfur and some metal. The results show that the relative humidity in the insulation must exceed approximately 70% to produce a significant number of water trees. The average size of the trees shows a very small dependence upon time and temperature and a slight decrease with increasing field strength. An increase in frequency from 50 to 5000 Hz approximately doubles the average size of the trees. The trees appear during the first 300 hours of voltage application, but since no growth rate can be observed at either frequency, it is not justified to establish a frequency acceleration factor in general.

Growth of Water Trees in Polyethylene and Silicone Rubber by Water Electrodes

In this paper, various experimental results are described of water trees obtained in polyethylene and silicone rubber, using the water electrode method. The mechanisms of initiation and growth of water trees are discussed. The growth of water trees depends upon the applied voltage, the frequency, and the water resistivity below 10 k?·cm. No differences in the growth of water trees were found among NaCl, KC1 and BaC12 solutions having equal resistivity. It is concluded that Maxwell stress, exerted on polyethylene or silicone rubber by applying an alternating voltage, forms micro-cracks in the material. Water permeates into the polymer, because the dielectric constant of water is higher than that of polyethylene or silicone rubber. With a high electric field and a high frequency, vaporization of water due to dielectric heating may occur: vapors may permeate into the amorphous region, and micro-cracks are formed by the vapor pressure.