Electrical Trees In Polyethylene Research Articles

Modeling the effect of uniform and nonuniform dispersion of nanofillers on electrical tree propagation in polyethylene dielectric

A phase-field model is developed in this paper based on the similarity between mechanical fracture and dielectric breakdown. Electrical treeing is associated with the dielectric breakdown in solid dielectrics by the application of high voltages. Instead of explicitly tracing the propagation of conductive channel, this model initializes a continuous phase field to characterize the extent of damage. So far, limited research has been conducted for simulating the effect of nanofiller dispersion on electrical treeing. No study has modeled the effect of uniform and nonuniform dispersion of nanofillers with varying filler concentration on treeing. Since electrical treeing tends to decrease the breakdown strength of solid dielectrics therefore, nanofillers are widely used to distract the tree from a straight channel to distribute its energy in multiple paths. Diverting a straight treeing channel into multiple paths reduces the chances of its propagation from live to dead-end hence, improving the breakdown strength. The physical and chemical nature of nanofillers has a crucial impact on increasing the resistance to treeing. In this paper, phase-field model is developed and used to simulate electrical treeing in polyethylene for varying concentrations of alumina nanofiller using COMSOL Multiphysics. Tree inception time, tree-growth patterns, and corresponding changes in dielectric strength is studied for both dispersions. Electrical treeing under different concentrations of alumina nanofillers with uniform and nonuniform dispersion is investigated in polyethylene as a base material. It is observed that fillers with uniform dispersion increases the resistance to treeing and tree inception time. Highest resistance to treeing is observed by adding 1% nanoalumina uniformly in raw polyethylene. Moreover, in uniform dispersion the tree deflects into multiple branches earlier than nonuniform dispersion impeding the damage speed as well.

The Effect of DC Electro-thermal Ageing on Electrical Treeing in Polyethylene

Electrical treeing is among the main mechanisms responsible for electric breakdown in polymeric high voltage cables. In this paper, electrical treeing in electro-thermally aged low density polyethylene (LDPE) was evaluated by means of a real-time microscopic detection system. A wire – plane electrode test object with semiconducting tab was used in the study. The ageing was performed at a constant temperature of 80°C and an applied DC voltage of 10 kV of both polarities and lasted from 100 up to 800 hours. For testing the resistances to electrical treeing, tests were performed by applying 50 Hz AC voltage at a ramping speed of 0.5 kV/s. The obtained experimental results showed that mainly the thermal component of the ageing had an influence on electrical treeing parameters, namely on the initiation voltage as well as the shape and occurrence frequency of the trees. With an increase of the ageing time, the tree inception voltage gradually decreased.

Concoction Component Analysis of Electrical Treeing in Polyethylene by Micro-Infrared Spectroscopy

Regular infrared spectroscopy is utilized to research the debasement locale after electrical treeing in polymers. Be that as it may, the measurement of the infrared light spot is too vast contrasted and the tree channel and the position of the light spot is uncertain. Thusly, the smaller scale infrared spectroscopy is proposed in this paper, which is plan to do synthetic investigation for electrical tree range in polyethylene at a micrometer scale. An inclined air conditioning voltage was connected for electrical tree initiation in polyethylene, using a wire-plane terminal structure. Cuts of tests with electrical trees were then examined through small scale infrared spectroscopy. The trademark bunch dispersion inside a chose treeing zone was given through smaller scale infrared mapping. The examination comes about demonstrated the presence of C=O, C-OH and C-O-C gatherings, and the substance of C-OH and C-O-C gatherings was more than C=O gathering. The presence of C=O gathering was went with the C-OH gathering, and the last absorbance is higher, so it is construed the C=O assemble have a tendency to debase additionally as per the Norrish sort II administration. What's more, not all the electrical tree region had the oxygen containing gatherings. The material corruption amid electrical treeing was examined with and without oxygen.

Chemical component analysis of electrical treeing in polyethylene by micro-infrared spectroscopy

Conventional infrared spectroscopy is used to investigate the degradation region after electrical treeing in polymers. However, the dimension of the infrared light spot is too large compared with the tree channel, and the position of the light spot is imprecise. Consequently, the micro-infrared spectroscopy is proposed in this paper, which is aim to do chemical analysis for electrical tree area in polyethylene at a micrometer scale. A ramped ac voltage was applied for electrical tree inception in polyethylene, utilizing a wire-plane electrode structure. Slices of samples with electrical trees were then analyzed through micro-infrared spectroscopy. The characteristic group distribution within a selected treeing area was provided through micro-infrared mapping. The experiment results indicated the existence of C=O, C-OH and C-O-C groups, and the content of C-OH and C-O-C groups was more than C=O group. The appearance of C=O group was accompanied with the C-OH group, and the latter absorbance is higher, so it is inferred the C=O group tend to degrade further according to the Norrish type II regime. In addition, not all the electrical tree area had the oxygen containing groups. The material degradation during electrical treeing was discussed with and without oxygen.

Raman microprobe analysis and ageing in dielectrics

Subsurface voids in samples of electrically stressed low density polyethylene (LDPE) were analysed using confocal Raman microprobe spectroscopy (CRMS). An optical depth profiling technique was used to probe a void along the optic axis whilst a burst void was analysed at various lateral positions. Spectra from the voided samples showed signatures with similar features to those found in previous studies of electrical trees in polyethylene, including the presence of the D and G bands of disordered sp2 carbon. Results and spectra were then compared to the depth profiles and spectra from block and thin film samples of polyethylene and polystyrene which indicated that the established theory behind CRMS is oversimplified and that the detected signal is largely influenced by the optical properties of the material in question. Overall the study showed that despite some spatial resolution limitations of the technique, depth profiling is a useful tool in the analysis of aged polymers and dielectrics as it can show the variations in chemical composition with respect to position along the lateral and optic axis, a property especially relevant to electrical trees.

On the structure and chemistry of electrical trees in polyethylene

The structure and chemistry of two electrical trees (designated Tree A and Tree B) grown in low density polyethylene have been studied by a combination of confocal Raman microprobe spectroscopy, optical microscopy and scanning electron microscopy. Despite being grown under similar conditions (A, 30 °C and 13.5 kV; B, 20 °C and 13.5 kV), these two trees exhibit very different structures. Tree A exhibits a branched structure while Tree B is more bush-like. In Tree A, the very tips of the structure are made up of hollow tubules, which exhibit just the Raman signature of polyethylene. On moving towards the high voltage needle electrode, fluorescent decomposition products are first detected which, subsequently, are replaced by disordered graphitic carbon. From the relative intensity of the graphitic sp2 G and D Raman bands, the constituent graphitic domains are estimated to be ∼4 nm in size, which leads to a local tree channel resistance per unit length of 1–10 Ω µm−1. These structures are therefore sufficiently conducting to prevent local electrical discharge activity. In Tree B, the observed fluorescence increases continuously from the growth tips to the needle. Here, the tree channels are not sufficiently conducting to prevent electrical discharge activity within the body of the tree. These results are discussed in terms of mechanisms of tree growth and, in particular, the chemical processes involved.

A Raman microprobe study of electrical treeing in polyethylene

The chemical nature of electrical tree growth in a blend of high and low density polyethylene has been studied by confocal Raman microprobe spectroscopy. The observed spectra, which are easily perturbed by the Raman probe beam, can be described in terms of three components, the relative intensities of which vary from place-to-place on the sample. Throughout the body of the tree, the usual Raman bands of polyethylene are seen, but superimposed upon a pronounced fluorescent background. This suggests a degree of material degradation throughout the structure, even where there is no visible evidence of electrical discharge damage or ageing. The individual channels that make up the fractal structure of the tree can then be divided into two distinct categories. Within the core of the tree, their Raman spectra are made up of two elements; fluorescence, plus the G and D bands of sp2 hybridized carbon. Here, the tree channels are best thought of as hollow tubules surrounded by a carbonaceous shell. At the tips, the channels are characterized simply by the Raman bands of polyethylene superimposed upon a reduced fluorescent background, irrespective of their size. That is, in this region, the tree channels are simply hollow tubules within the dielectric. The transition between these two structures occurs relatively abruptly. These observations are related to the treeing process.

Three-Dimensional Fractal Analysis of Real Electrical Trees in Polyethylene

In this paper, real electrical trees in polymeric insulating materials were discussed from the viewpoint of their fractal characteristics. Experiments on real electrical trees were carried out in the needle to plane electrode geometry under ac voltage application. Samples in this investigation were made of cross-linked polyethylene (XLPE). Two methods of reconstructing three-dimensional (3D) patterns of real electrical trees were developed. One is the computerized tomography method (CTM) and the other is the serial sectioning method (SSM). The relationship between tree characteristics and 3D fractal dimensions of tree patterns was studied. The experimental results showed that the spatial tree patterns have a fractal characteristics and that the tree characteristics can be characterized by fractal dimensions.

Ultrasonic visualization method of electrical trees formed in organic insulating materials

The only practical method to visualize electrical trees formed in organic insulating materials has been to section a specimen with a microtome or a diamond cutter, especially in translucent or opaque materials. In our experiment, a 25 MHz ultrasonic transducer was used to scan a specimen within which an electrical tree had been formed. Ultrasonic echo signals reflected from the treeing region were detected by the same transducer and fed to a electronic measuring system. By determining the horizontal coordinates at which each echo signal from the tree was detected, two-dimensional ultrasonic images were obtained for various types of electrical trees in polyethylene. Furthermore, three-dimensional ultrasonic images of bush-type trees were obtained by means of a new detection or measurement method developed by the authors. These images agree well with those obtained microscopically by sectioning the specimen with a microtome.

Pulse-sequence analysis: a new method for investigating the physics of PD-induced ageing

A new method is presented of partial discharge data analysis for the investigation of space charge and degradation phenomena in high-voltage insulation systems. The basic principles of the pulse-sequence-analysis (PSA) are illustrated and the characteristic differences with regard to standard procedures that have been established during the past few years are outlined. A description is given of an electronic partial discharge acquisition system, which is designed on a plug-in interface board for a standard PC to perform long-term measurements of all the discharge parameters required for the succeeding analysis. Electrical treeing in polyethylene is used to demonstrate the usefulness of the method. It is shown that information on the physics of treeing phenomena especially with regard to the influence of local space charges and their build-up and decrease over time as well as the development of the local degradation can be obtained by considering correlations between consecutive discharge pulses. Some experimental results from different stages of the electrical tree growth are analysed and their meaning with regard to the local microscopic phenomena inside the tree or in the vicinity of the tree tips discussed. This paper is based on an oral presentation at a conference on Partial discharge, September 1993, Canterbury, titled 'Pulse-sequence analysis, a way to get a better insight into the physics of discharges'.

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.

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.

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.

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.

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.