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Failure of underground cables due to high degradation-rate phenomena, as partial discharges followed by electrical treeing, is one of the major causes of cable outages. Breakdowns occur mostly in correspondence of joints and terminations, where the electric stress can exceed the design electric field due to bad workmanship or manufacturing. Researches are being carried out to improve the resistance of insulation materials to such harmful electrical-aging phenomena. The use of nanocomposite cross linked polyethylene, XLPE, is investigated in this paper. Experiments of electrical tree growth on silica-nanostructured XLPE specimens, during which the time behaviour of partial discharges is recorded, are presented and discussed. Data are processed through clustering analysis, showing that addition of silica nano fillers with 3 and 5 %wt concentration improves noticeably PD endurance and breakdown time of XLPE material. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of nano-doping on partial discharge (PD) and electrical tree growth in cross-linked polyethylene (XLPE), samples of 0.5 wt% magnesium oxide MgO/XLPE were prepared and an AC experiment was carried out. A color-coded diagram for PD signal was developed to describe the PD activity during the whole process of treeing. The results show that the PD intensity in positive half cycle of voltage (PHC) is larger than that in negative half cycle of voltage (NHC), and the PD magnitude in MgO/XLPE is lower than that in XLPE. Moreover, the tree length and morphology in MgO/XLPE are similar to those in XLPE. Furthermore, a finite element simulation based on the bipolar charge transport model was applied to investigate the effect of space charge on PD under AC voltage. Simulation results suggest that the characteristics of PD and tree growth may be associated with the accumulated charge around the needle tip. The distribution of negative charges in NHC would be wider than that of positive charges in PHC, which would weaken the electric field in NHC and lead to the less PD in NHC. Moreover, the range of charge shield in MgO/XLPE may be larger and is equivalent to increasing the effective radius of curvature of needle tip, which would lead to smaller electric field and lower PD magnitude in MgO/XLPE. However, the addition of nano-MgO reduces the crystallinity of material based on the differential scanning calorimetry (DSC) result. Hence, although the PD magnitude in MgO/XLPE is lower, the MgO/XLPE researched in this paper does not show significant resistance to AC tree growth. [ABSTRACT FROM AUTHOR]

Electrical treeing is a main cause of long term degradation of polymeric insulation and inorganic nanofillers have been added to improve the treeing resistance of polymers. In this work, a phase field model for the propagation of electrical tree in nanocomposites is presented, in which the damage status of insulation is described with a spatially and time dependent continuous variable. The evolution of damage phase is described with a modified Allen-Cahn equation, which is driven by the free energies originated from the phase separation, interface and electric field. The electric field distribution during growth of electrical tree is obtained by solving the iterative form of Poisson's equation with the spectral iterative perturbation method. Then the phase field model is applied to investigate the influence of nanofillers shape and distribution on propagation process of electrical tree. The results indicate that the phase field model can reproduce the dendritic shape of electrical tree, and the parallel nanosheet presents superior performance in hindering electrical tree than nanoparticle and random nanosheet. [ABSTRACT FROM AUTHOR]

The air pollution level of particulate matters in subway tunnel environments is severe. The present study conducted experimental measurements and analyses on dust-collection efficiencies and ozone emission rates in a lab-scale wind tunnel to determine a high-voltage electrode shape suitable for an electrostatic precipitator to remove airborne particles in subway tunnels. Six high-voltage electrode shapes (saw-type, few-saw-type, short-saw-type, tree-type, wave-type, and spike-type) were tested. The results indicate that while breakdown voltage of the spike-type high-voltage electrode was 66 % of other electrode shapes, its dust-collection efficiency per unit voltage was similar to that of other electrode shapes. However, the dust-collection efficiency per unit of power in the spike-type high-voltage electrode was significantly low. Moreover, the wave-type high-voltage electrode shape had 20 % lower dust-collection efficiency than the saw-type and tree-type high-voltage electrode shapes under conditions of low voltage and high flow velocities. Ozone emission rates for all shapes increased proportionally to the electric current. However, ozone emission rates for each high-voltage electrode shape are varied as 12–23 μg/s per mA. The saw-type and tree-type high-voltage electrode shapes were superior in terms of dust collection, 90 % collection efficiency was shown at about 1.5 W/(m/s)2. The saw-type had a lower ozone emission rate (16.4 μg/s per mA) than the tree-type (19.6 μg/s per mA). Thus, the saw-type shape was the most suitable as a high-voltage electrode in the electrostatic precipitator for removing airborne particles in a subway. The results also indicate that both dust-collection efficiencies and ozone emission rates should be evaluated concurrently when evaluating electrostatic precipitator high-voltage electrodes. [ABSTRACT FROM AUTHOR]

There is still a lack of effective means of observation for electrical trees in opaque materials, which hampers experimental and theoretical research on the growth of such trees in the oil-impregnated pressboards of power transformers. This presents a great obstacle to the accurate diagnosis and prediction of partial discharge (PD) defects. This paper describes a method to monitor the growth process of electrical trees in pressboards based on solution of the inverse problem for the electrostatic field. The three-dimensional structure of the carbonized branches is reconstructed from the positions of PDs. Each PD is located by the charge remaining in the material. This charge is calculated from the electric field intensity or potential at some points in the field using the mirror image method and solution of the inverse problem for the electrostatic field. The electric field intensity is measured using an array of metal strips near the zero-potential border while PDs are occurring. High-voltage PD experiments are performed on oil-impregnated pressboard strips to verify the feasibility of the method. From the results of these experiments, the accuracy of PD site location is found to be as good as 0.35 mm in the χ direction (the horizontal coordinate), which is sufficient to describe the general structure of electrical trees in oil-impregnated pressboard. The movement of the PD positions indicates the growth process of tree branches. The calculated positions and lengths of the branches are consistent with observations of carbonized tracks after the pressboard has been torn open. The results obtained here will be of benefit to establishing a real-time monitoring method for three-dimensional reconstruction of electrical trees to reveal their internal growth mechanism in opaque materials such as oil-impregnated pressboards. [ABSTRACT FROM AUTHOR]

Transient superimposed voltage, which is accompanied by a temperature gradient caused by Joule heat, can threaten the insulation safety of high-voltage direct current (HVDC) cable terminations. In this study, an electrical treeing experiment was conducted on epoxy resin with temperature gradient under harmonic superimposed DC voltage. The temperatures in the high voltage side and ground side were controlled by heaters within that maintained the range of 30 to 120 °C, while the DC voltage was ±12 kV. An isothermal surface potential decay (ISPD) experiment was used to obtain the trap level distribution. The effects of the temperature gradient and voltage polarity on the tree growth were depicted by the tree morphology, tree length and accumulated damage. The results show that the temperature gradient affects the tree structure and accelerates its growth. The tree growth rate has a nonlinear trend under an increased temperature in the high voltage side, and shows a steady increase under an increased temperature in the ground side. The positive voltage has a more obvious acceleration effect on the tree growth than the negative voltage does. A charge transport model is proposed to describe the tree behavior under a temperature gradient. The nature of the charge carrier accounts for the trap distribution and polarity effect. The local temperature rise and superimposed voltage pose a threat to the insulation safety, and more attention should be given to the operation of the electrical apparatus. [ABSTRACT FROM AUTHOR]

This paper investigates the external circuit effect in corona discharge by means of numerical simulation, while the effect is usually disregarded in previous plasma simulations. A general numerical solution method of serial external circuit, which can compute surface charges on any shape of electrodes adaptively and solve electrode potential and circuit current simultaneously, is developed to support the investigation, due to lack of theoretical solution in irregular electrode systems. Brief description and validation of the scheme are presented in section 2. In general, the accuracy of numerical solution improves with mesh refinement in orthogonal mesh, due to stair step approximation. Simulations are performed in our self-developed 3D PIC-MCC (particle-in-cell, Monte Carlo collision) code with photoionization and external circuit. The simulation model including recently developed photoionization model is described briefly in section 3. Then the simulation results in a needle-plate electrode in air, such as spatiotemporal evolution of corona discharge and waveforms of electrode voltage and circuit current, are presented. Physical mechanisms of formation of branched corona discharge and self-consistent adjustment of electrode voltage and circuit current are discussed. The waveforms of voltage and current in this simulation are generally consistent with experimental result. The external circuit has negative feedback effect on the discharge, and can adjusts electrode voltage self-consistently. The application of external circuit improves the applicability of particle simulation method to the simulation of discharge phenomenon in actual devices. [ABSTRACT FROM AUTHOR]

This paper reports on the effects of four voltage stabilizers, m-aminophenylboric acid, 2-methoxy-5-pyridineboric acid, m-aminobenzoic acid and 4-dimethylaminobenzoic acid, on the insulation properties of polyethylene blends. 1 wt% voltage stabilizers were added into low density polyethylene (LDPE) blend containing 10 wt% high density polyethylene (HDPE) by a solution method. Physicochemical and electrical properties of the blends were analyzed by infrared spectroscopy, thermo-gravimetric analysis, differential scanning calorimetry, electric tree initiation tests, space charge and surface potential decay measurements. The results showed that the addition of the voltage stabilizers had a little effect on the thermal stability and melting properties of the LDPE/HDPE blend. When the voltage stabilizers were included into the blends, the tree initiation voltage (TIV) increased, in particular the blend with m-aminobenzoic acid showed the highest increase of 41% compared to the reference LDPE/HDPE. It was found that space charge accumulations of the blends were apparently suppressed by adding voltage stabilizers. Moreover, the blend with the m-aminobenzoic acid had the least space charge accumulation. The surface potential decay rate of the blends was significantly increased after adding voltage stabilizers. Quantum chemical calculations illustrated that the HOMO-LUMO energy gap was closely related to the TIV of the blends, whereas the m-aminobenzoic acid has the lowest energy gap level and the best effect of restraining the electrical tree initiation. The trap level calculation showed that the trap depth in the blends decreased with the addition of the voltage stabilizers, which was beneficial to suppress the accumulation of space charge in the materials. [ABSTRACT FROM AUTHOR]

In the present study, the growth and partial discharges properties of DC electrical trees in cross-linked polyethylene (XLPE) were investigated under positive and negative DC voltages. The obtained results revealed that the voltage rise exerted significant influences on growth properties of DC trees. In either polarity, DC electrical trees grew rapidly during rising voltage and maintained the rapid growth for a certain time after DC voltage reached a constant value. The growth rate of positive DC electrical trees was quite faster than that of negative ones, and channel breakdown mainly occurred in positive ones. Although the growth rates of positive and negative DC electrical trees were different, their shapes were similar (both were branch-like electrical trees with one or two main branches). Self-healing usually occurred in negative DC electrical trees, thus electrical tree branches gradually disappeared under the application of steady DC voltage or when the voltage was off. Partial discharges mainly occurred at the beginning of electrical tree growth; however, the amount of partial discharge was relatively small, and it gradually reduced even during the rapid growth of electrical trees, consequently, almost no partial discharges were found when the voltage was constant. Finally, a discharge-avalanche theory was proposed to explain the growth properties of DC electrical trees. [ABSTRACT FROM AUTHOR]

Silicone gel is widely used to encapsulate power electronic modules. The weakness of the electrical insulation is surface discharges initiated at the gel-substrate-electrode triple junction and the subsequent formation of cavities; so called electrical trees. The propagation characteristics of cavities under high frequency and fast rise time voltages, which are typical waveforms formed in power electronic modules, have not been fully understood. Thus, in this research, the influence of frequency, rise time, and polarity on electrical treeing under repetitive voltage impulses are investigated. The results show that discharge tracks under positive impulses have filamentary shapes, and discharge tracks under negative impulses have spindle-shapes. The cavity length increases with the frequency and is especially long when the rise time is short under high frequency. This indicates the importance of understanding the cavity propagation characteristics under actual voltage waveforms in power electronics modules. [ABSTRACT FROM AUTHOR]

Insulating fillers are often mixed with polymer dielectrics in order to improve dielectric breakdown strength. However, filler-loaded dielectric films show higher variability, which limits their large-scale utility and application. In this paper, we present a computational framework for predicting the spatial distribution of electrical tree formation in a polymer dielectric in the presence of insulating fillers. The framework is based on the numerical solution of Poisson's equation inside the dielectric film and the application of the Monte Carlo model for finding the electric breakdown path. The simulation results explain several puzzles associated with the breakdown of polymer-filler dielectrics, such as (1) the improved breakdown strength with insulating filler mixing, (2) high degree of variability in breakdown measurements, and (3) effects of filler size, shape and position on the breakdown. In addition, the proposed framework not only provides the physical understanding of the breakdown process but also inspires new design concepts to improve the breakdown strength of the polymer composite. [ABSTRACT FROM AUTHOR]

The increasing demand for tree and forest health monitoring due to ongoing climate change requires new future-oriented and nondestructive measurement techniques. Electrical resistivity (ER) tomography represents a promising and innovative approach, as it allows insights into living trees based on ER levels and ER cross-sectional distribution patterns of stems. However, it is poorly understood how external factors, such as temperature, tree water status, and electrode installation affect ER tomograms. In this study, ER measurements were carried out on three angiosperms (Betula pendula , Fagus sylvatica , Populus nigra) and three conifers (Larix decidua , Picea abies , Pinus cembra) exposed to temperatures between −10 and 30°C and to continuous dehydration down to −6.3 MPa in a laboratory experiment. Additionally, effects of removal of peripheral tissues (periderm, phloem, cambium) and electrode installation were tested. Temperature changes above the freezing point did not affect ER distribution patterns but average ER levels, which increased exponentially and about 2.5-fold from 30 to 0°C in all species. In contrast, freezing of stems caused a pronounced raise of ER, especially in peripheral areas. With progressive tree dehydration, average ER increased in all species except in B. pendula , and measured resistivities in the peripheral stem areas of both angiosperms and conifers were clearly linearly related to the tree water status. Removal of the periderm resulted in a slight decrease of high ER peaks. Installation of electrodes for a short period of 32–72 h before conducting the tomography caused small distortions in tomograms. Distortions became serious after long-term installation for several months, while mean ER was only slightly affected. The present study confirms that ER tomography of tree stems is sensitive to temperature and water status. Results help to improve ER tomogram interpretation and suggest that ER analyses may be suitable to nondestructively determinate the hydraulic status of trees. They thus provide a solid basis for further technological developments to enable presymptomatic detection of physiological stress in standing trees. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Minor physical damage can reduce the insulation performance of epoxy resin, which seriously threatens the reliability of electrical equipment. In this paper, the epoxy resin insulating composite was prepared by a microcapsule system to achieve its self-healing goal. The repair performance to physical damage was analyzed by the tests of scratch, cross-section damage, electric tree, and breakdown strength. The results show that compared with pure epoxy resin, the composite has the obvious self-healing performance. For mechanical damage, the maximum repair rate of physical structure is 100%, and the breakdown strength can be restored to 83% of the original state. For electrical damage, microcapsule can not only attract the electrical tree and inhibit its propagation process, but also repair the tubules of electrical tree effectively. Moreover, the repair rate is fast, which meets the application requirements of epoxy resin insulating material. In addition, the repair behavior is dominated by capillarity and molecular diffusion on the defect surface. Furthermore, the electrical properties of repaired part are greatly affected by the characteristics of damage interface and repair product. In a word, the composite shows better repair performance to physical damage, which is conducive to improving the reliability of electrical insulating materials. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, the electrical tree mechanisms during a growth period from tens of microns to hundreds of microns were investigated for cross-linked polyethylene under a stepped DC voltage. The results show that the electrical tree grows along the direction of the needle tip pointing to the ground, and the tree growth follows a typical single branch type. The growth of the electrical tree is mainly detected during the voltage rising process and is rarely observed during the voltage maintenance step. Moreover, the length increment of the tree decreases with the increase in the length of the initial tree. The simulation results for the charge transport in the needle–plane electrode suggest that these growth characteristics are associated with the energy released near the needle tip during the charge trapping and detrapping processes. [ABSTRACT FROM AUTHOR]

The aim of this work is the investigation of the performances of nanocomposite materials in the presence of electrical treeing. The nanocomposite materials based on polyolefin matrices were prepared by extrusion and then molded into slabs from which it was possible to prepare apposite specimens to perform electrical treeing tests. The polymeric employed matrices are low density polyethylene (LDPE) and ethylene vinyl acetate (EVA). The considered fillers were commercial modified montmorillonite having a lamellar structure and unmodified silica presenting a spherical shape. The behavior of the different materials were compared and correlated with the material properties and structure. The treeing inception voltage (TIV) and the time to breakdown (TBD) as the final event of the electrical tree growth were considered. Furthermore, in order to identify the presence of defects inside the specimens due to the preparation process the partial discharge (PD) activity was monitored. The PD patterns were acquired by a phase resolved partial discharge analyzer (PRPDA). [ABSTRACT FROM AUTHOR]

Leakage power consumption of clock distribution networks (CDNs) is an important challenge in modern synchronous integrated circuits with billions of deeply scaled transistors. Multithreshold CMOS technology is commonly used to provide power reduction in standby mode while maintaining high performance in active mode. In this paper, a novel dual-threshold-voltage repeater circuit with split inputs–outputs (SPLIT-IOs) is employed for suppressing leakage currents in gated CDNs. Three floor planning strategies are considered for clock distribution across the chip with signal transition times of less than or equal to 50 ps at the leaves. Depending on the power supply voltage and floor plan, the standby leakage power consumption is reduced by 50.36%–78.43% with the proposed clock tree with SPLIT-IO repeaters as compared to the conventional three-level H-tree in a 45-nm CMOS technology. The spread of standby leakage power due to process variations is compressed by 36.72%–73.77% with the proposed clock tree as compared to the standard network. The proposed circuit technique significantly lowers the total energy consumption of partially active networks with local clock gating as well. The energy savings provided by the SPLIT-IO buffers are enhanced with the scaling of power supply voltage and frequency in synchronous systems-on-chip. [ABSTRACT FROM AUTHOR]

Copyright of Computing & System Journal (C&S) / Revista de Sistemas e Computação (RSC) is the property of FACS Servicos Educacionais S.A. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Atmospheric-pressure plasma jet (APPJ) array could effectively enlarge the treatment area for the requirement of application efficiency. The downstream uniformity of jet array is the determinant for the even processing effect. In this paper, a three-jet 1-D APPJ array with the electrode structure of needle–ring is constructed, and the position of needle electrode (high-voltage electrode) is adjusted to investigate the influence of electrode arrangement on the downstream uniformity of the 1-D APPJ array. The results show that the relative difference of the positions of the middle and side needle electrodes ${d}$ has great influence on the downstream uniformity. When the distances of the middle and side needle electrodes to the tube nozzles ${a}$ and ${b}$ are both fixed at 40 mm ($d= 0$ mm), the middle plume is suppressed. With increasing the distance of the side needle electrodes to the tube nozzles ${b}$ to make $d=1$ , 2, and 3 mm, the middle plume is prolonged and the lengths of all plumes are almost equal at $d=2$ mm. With the intensified charge-coupled device (ICCD) image measurement, it is found that the desirable synchronicity of bullets propagation is obtained at $d = 2$ mm. The electrical field distributions of different d are simulated, which can be seen that the high electric fields of the side jets can suppress the propagation of middle jet and at $d= 2$ mm, the three jets have similar maximum values of the electrical field strength along the direction of needle electrode resulting in similar plume lengths in the APPJ array. [ABSTRACT FROM AUTHOR]

Featured Application: From a practical point of view, the phenomenon of electrical treeing can be an exploitation problem, especially in the elements or places with locally increasing of electric field intensity, because it is an irreversible process. Such places are e.g., connecting points of the power network elements or parts of electrical devices in which there is constant insulation. The results of the carried out analyzes may complement the knowledge of identifying the type of PD occurring based on selected signal parameters. Electrical treeing is one of the effects of partial discharges in the solid insulation of high-voltage electrical insulating systems. The process involves the formation of conductive channels inside the dielectric. Acoustic emission (AE) is a method of partial discharge detection and measurement, which belongs to the group of non-destructive methods. If electrical treeing is detected, the measurement, recording, and analysis of signals, which accompany the phenomenon, become difficult due to the low signal-to-noise ratio and possible multiple signal reflections from the boundaries of the object. That is why only selected signal parameters are used for the detection and analysis of the phenomenon. A detailed analysis of various acoustic emission signals is a complex and time-consuming process. It has inspired the search for new methods of identifying the symptoms related to partial discharge in the recorded signal. Bearing in mind that a similar signal is searched, denoting a signal with similar characteristics, the use of artificial neural networks seems pertinent. The paper presents an effort to automate the process of insulation material condition identification based on neural classifiers. An attempt was made to develop a neural classifier that enables the detection of the symptoms in the recorded acoustic emission signals, which are evidence of treeing. The performed studies assessed the efficiency with which different artificial neural networks (ANN) are able to detect treeing-related signals and the appropriate selection of such input parameters as statistical indicators or analysis windows. The feedforward network revealed the highest classification efficiency among all analyzed networks. Moreover, the use of primary component analysis helps to reduce the teaching data to one variable at a classification efficiency of up to 1%. [ABSTRACT FROM AUTHOR]

To understand the non-breakdown phenomenon during the accelerating electrical tree ageing in cross-linked polyethylene (XLPE) cable insulation, the non-breakdown mechanism of electrical tree is investigated in this study. The needle electrodes were inserted into the cable insulation samples to accelerate electrical tree ageing. The electrical trees which were not broken down had a larger expansion range and branch density. Referring to the actual cable slice for the internal structure of the sample, the electrical tree model with different number of tree channels was built. The correspondence between electric distortion and the number of tree channels was discussed. According to the calculation of the electric field distribution by the finite element method, the results showed that the electric field strength at the tip of electrical tree was reduced when the number of channels was increased. Compared with the single electrical tree channel, when the insulation was completely carbonised in the electrical tree region the electric field strength at the tip of electrical tree was reduced by 20.8%. The electric field shielding which was formed by the tips of electric trees could reduce the electric field strength at the tip of the tree and it could result in the formation of the non-breakdown electric tree. [ABSTRACT FROM AUTHOR]

Based on the slice materials of 35 kV and 110 kV XLPE cables, an experimental platform is built to study the relationship between electrical tree and PDs in XLPE with different voltage levels. There are three significant statistical characteristics of the PDs during the growth of electrical trees. The analysis of the results shows that each growth stage has certain characteristics. Different features existed between the growth of the electrical trees and the PD in the insulation of the 35 and 110 kV cables. Evident characteristics such as large spans of time and frequency were present as the electrical trees grew violently in the equivalent time-frequency diagram at every stage. These results could provide criteria for the identification of the deterioration using PD to monitor cables in service at rated voltages. The results are important for the identification of defects in cable insulation in order to provide an early warning of insulation breakdown in the cables. [ABSTRACT FROM AUTHOR]

A new planar antenna is proposed for multiple applications in ground‐penetrating radar. The proposed antenna offers ultra‐wide band characteristics ranging from 100 MHz to 6 GHz. The antenna composed of two elements: an electrical tree‐shaped portion and a magnetic loop antenna, made by joining an electrical antenna with a ground plane via 50 Ohm resistor. The loop antenna portion provides a low‐frequency operation without increasing physical size of the antenna. Proposed antenna has relatively small dimensions 20 × 22.5 cm2 as compared with similar antennas covering the same bandwidth. Time domain analysis of this antenna is done to check its performance for applications in impulse‐based radar systems. The antenna has been manufactured and experimental results are added to validate simulation results. [ABSTRACT FROM AUTHOR]

This paper focuses on the electrical tree growth characteristics in epoxy resin with dc-impulse voltage caused by line fault and switching surges in liquid nitrogen (LN2). Electrical tree initiation tests were conducted on molded epoxy resin with a needle–plate system. The test samples were stressed with a dc voltage amplitude of 10 kV and an impulse voltage amplitude of 24 kV at 77 K. With a wide range of voltage frequencies and different impulse polarities, four different waveforms of dc-impulse voltage were used as applied voltage. Result shows that the electrical tree tends to grow longer when the dc and the impulse are of the same polarity. The impulse frequency plays a crucial role in tree initiation and growth process. The tree initiation probability increases with the frequency rising from 50 to 400 Hz in epoxy resin. Different charge transportation processes are discussed based on four different voltage waveforms. The collision ionization and charge recombination have a decisive effect on electrical tree initiation. The impulse voltage is worth regarding in the operation of a superconducting cable system. [ABSTRACT FROM AUTHOR]

To investigate liquid nitrogen (LN2) effects on insulation materials in high temperature superconducting (HTS) systems, the electrical tree was studied in ethylene propylene diene monomer (EPDM)/antioxidant specimens by using a needle–plane electrode system. The pulse amplitude and frequency were 25 kV and 200 Hz, respectively. In LN2, as the free volume was reduced and the nonband energy got larger, the charge carriers were difficult to migrate in the samples. The charge trapping center was decreased from 0.925 to 0.195 eV as the temperature was reduced from 293 to 77 K, there were less charges to collide with molecular chains in LN2. The electrical tree growth was restrained in EPDM specimens under LN2. When the antioxidant was filled into EPDM, the charge trapping center was increased from 0.195 to 0.21 eV, the nonband energy was improved to reach a more stable state. Thus, the antioxidant could be served as the degradation inhibitor in EPDM/antioxidant specimens by regulating the charge transportation to restrain the electrical treeing process. This research has substantial commercial significance for insulation design and preshipment inspections in HTS insulation areas. [ABSTRACT FROM AUTHOR]

Cross-linked polyethylene (XLPE) is widely used in a warm dielectric (WD) superconducting cable. As other insulating materials, electrical tree is a threat to XLPE. When XLPE is used in cable termination connected to a high-temperature superconducting cable and a WD superconducting cable, there will be a steep temperature gradient. With the growth of the tree, the temperature gradient will be influenced by the change of temperature. In the experiment, nine temperature gradients were prepared and applied on the samples. An ac voltage was employed to initiate electrical tree in XLPE samples. Three tree structures have been observed, and they change with the gradient. In the same tree, the branch will also change in respect to its growth. Both the tree length and width are suppressed by the decrease of temperature. With the change of temperature gradient, the decrease of temperature influences the fractal dimension and accumulated damage obviously. In the gradient 1–5, the speed of tree growth in the first 20 min is much higher, while trees in the other gradients grow slowly. [ABSTRACT FROM AUTHOR]

The present paper deals with the phenomenon of electrical treeing in solid insulation, under the influence of mechanical stresses. In this short review, it is indicated that mechanical stressing can affect the propagation of electrical trees and -depending on whether it is tensile or compressive- it can facilitate (or render more difficult) the breakdown. In aged insulating materials, electrical trees can appear very quickly and can lead to breakdown. [ABSTRACT FROM AUTHOR]

Electrical trees are one of the main mechanisms of degradation in solid polymeric insulation leading to the failure of high voltage equipment. They are interconnected networks of hollow tubules typically characterized from two-dimensional (2D) projections of their physical manifestation. It is shown that complete characterization requires a three-dimensional (3D) imaging technique such as X-ray computed tomography (XCT). We present a comprehensive set of parameters, quantitatively characterizing two types of tree topology, conventionally known as bush- and branchtype. Fractal dimensions are determined from 3D models and from 2D projections, and a simple quantitative relationship is established between the two for all but dense bush trees. Parameters such as number of nodes, segment length, tortuosity and branch angle are determined from tree skeletons. The parameters most strongly indicative of the differences in the structure are the number of branches, individual channel size, channel tortuosity, nodes per unit length and fractal dimension. Studying two stages of a bush tree's development showed that channels grew in width, while macroscopic parameters such as the fractal dimension and tortuosity were unchanged. These parameters provide a basis for tree growth models, and can shed light on growth mechanisms. [ABSTRACT FROM AUTHOR]

Electrical treeing is the dominating failure phenomena in solid insulating material. Treeing occurs due to voids, dust and impurities present in the insulation material, change in the crystalline structure, stress occurring in the semi-crystalline structure and environmental factors. The growth characteristics of electrical tree is analyzed by injecting high voltage continuously using transformer in XLPE sample. [ABSTRACT FROM AUTHOR]

Copyright of Journal of the Society of Electrical Materials Engineering / Denki-Zairyo-Gijutsu-Zasshi is the property of Society of Electrical Materials Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

An original design for an electrochemical chamber and the construction of a high-voltage AC generator for electrochemical etching (ECE) of nuclear tracks in CR-39 polycarbonates sheets are presented. The electrochemical chamber was designed and manufactured in an engineering plastic in a vertical machining center. The high-voltage AC generator was built by using different electronic circuits: a signal oscillator, a preamplifier, a power amplifier, two power supplies and a voltage converter transformer. The experimental tests showed that the design of the electrochemical chamber allowed that ECE be developed efficiently, achieving a dielectric breakdown in the polycarbonate, and the generator supplied the electric current in a stable manner and with an excellent performance throughout the ECE. The conditions of chemical etching and electrochemical etching, as well as the electrical treeing generated in the CR-39 are also presented. [ABSTRACT FROM AUTHOR]

Grounded DC tree is a serious threat for the safe operation of XLPE cable. In this paper, growth properties of grounded DC electrical trees in XLPE under ±40 kV DC voltage were investigated and analyzed. The results showed that DC voltage polarity had a remarkable impact on the growth characteristics of grounded electrical tree. Sparse-branch and dense-branch electrical trees were more likely to form under positive and negative voltage, respectively. Meanwhile, the time of typical grounded DC tree samples growing to 1mm was also calculated. The results indicated that the average growing time under positive voltage could be a bit shorter than that under negative voltage. The results in this paper can help in the understanding of DC tree mechanism and provide a support for the later experiments. [ABSTRACT FROM AUTHOR]

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service. [ABSTRACT FROM AUTHOR]

Electrical tree initiation behavior was investigated in XLPE insulation samples made from a HVDC cable by applying constant DC and grounded DC voltages under different temperatures. No trees appeared when a constant DC voltage of −60 kV was applied for 12 days at room temperature or 20 days at 90°C, and the tree initiation time was estimated to be more than 200 days by Bayes statistical processing of zero-failure data. In the grounded DC voltage test, −15 kV prestress could trigger a tree, and under −35 kV, the tree initiation rate reached 100 %. With the increase of DC prestress, both the initiation rate and tree length increased. Grounded DC trees were branch trees with dark-colored channels, and a similar material damage could be caused by the same DC prestress. The connection of a grounding resistor could reduce the damage of sample caused by the grounded DC voltage, and the tree initiation rate declined with the increase of the resistance. According to the experimental results, the tree initiation time under constant DC voltage was not significantly shortened at 90°C, whereas tree initiation under grounded DC voltage was promoted by the increase of temperature. From 25 to 70°C, the tree branches became longer and denser, and extended in more varied directions. The observations are explained by the injection, transfer, trapping and detrapping of space charges, and charge detrapping is believed to be the cause of material damage. [ABSTRACT FROM AUTHOR]

Electrical lifetime of alumina-mesogenic epoxy composite sheet is improved by the addition of mica without reducing thermal conductivity. We developed an electrical tree progression simulation to optimize the composition ratio of four species of fillers (mica and three types of alumina fillers). This simulation is based on the electric field analysis with the use of a two-dimensional finite-difference method. The simulation revealed that the electrical lifetime of the composite sheet increases depending on the amount of mica filler although it is saturated at 1.3 vol. %. The calculated electrical lifetime of the sheet with mica filler is 1.5–2.0 times longer than that of the sheet without mica. This result agrees with the experimental at high applied voltage of 7 kVrms. A low mica ratio is suitable in the view of the thermal conductivity of the composite sheet because thermal conductivity of mica filler is much lower than that of alumina filler. The effective amount of mica filler was determined at 1.3 vol. % to possess high thermal conductivity and long electrical lifetime. As a result, a composite sheet with thermal conductivity of 8.7 W m−1 K−1 and the electrical lifetime 13 times longer than the sheet without mica at 4 kVrms was obtained. [ABSTRACT FROM AUTHOR]

The understanding of electrical trees plays a crucial role in the development of reliable high voltage power systems. The AC electrical trees are different from DC ones, however, there are only limited studies dealing with this topic. For the first time, the differences between AC and DC trees are carefully compared in a systematic way. A photomultiplier tube (PMT) was used to collect the electroluminescence (EL) during the AC and DC tree initiation. Optical microscope, CCD camera and scanning electron microscope (SEM) were employed to image the micro-morphology during the electrical tree initiation and propagation. Ageing characteristics were acquired by the confocal Raman spectrometer and infrared thermal imaging. The experimental results indicated that the initiation and propagation of AC and DC trees are totally different. Based on these, the growth mechanisms of AC and DC trees have been proposed, which provides a better understanding of the electrical treeing processes. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) characteristics are inherently linked to associated electrical tree growth characteristics. This paper employs both phase-resolved patterns and pulse-sequence analysis (PSA) to study PDs in a short branch-tree grown from a metallic needle tip. It is found that the shape of PSA dV-dV patterns vary with applied voltage; in particular the values of characteristic voltage difference increase with applied voltage. At 10 kV the value of characteristic voltage difference decreases with tree growth, the average number of PDs per cycle increases, the PD magnitudes decrease, and the characteristic wing-like phase-resolved pattern evolves into a turtle-like pattern. This paper proposes a physical model relating PD activity to tree development, considering space charge accumulation inside the tree channel. All key features of experimental results are explained by the model. It is shown that the characteristic voltage differences are determined by the local PD inception and extinction electric fields. These key parameters determining the PD events can be calculated giving a clear physical interpretation of PD measurements, which is invaluable to those using PD as a condition monitoring or asset management tool. [ABSTRACT FROM AUTHOR]

In this paper, initiation behaviors of DC electrical tree in silicone rubber were studied under different voltage polarities and temperature gradients (temperature of needle and ground electrodes ranged from 20 to 120 °C). Electrical tree initiation probability, tree shape, and tree length were obtained via the statistics. The results show that polarity effects, where the positive DC voltage is easier for trees to initiate, appear evidently under various temperature situations. As for the temperature gradient, tree initiation voltage decreases first and then returns to a larger value with the increasing needle temperature when the ground electrode temperature is fixed at 20 °C. Both the probability of multiple-branch tree and corresponding tree length increase and subsequently decrease. However, tree initiation voltage decreases alone and tree length increases with the rise of ground electrode temperature when the needle temperature is fixed at 20 °C. Homo charge injection becomes easier with increasing needle temperature following the Schottky-Richardson approximation under ramped DC voltages. In addition, electric field strength at the tip decreases when the needle temperature is higher than ground temperature, since the conductivity is sensitive to the local temperature and electric field. It is believed that the needle temperature and the temperature gradient dominate tree initiation behaviors, where the former affects the collisional ionization and partial relaxation of silicone rubber chains, and the latter decides electric field distribution in the bulk. [ABSTRACT FROM AUTHOR]

Tree ageing is a common failure pattern of polymers in high voltage devices. The cumulative breakdown process of PMMA under microsecond (μs) pulses was observed with an on-line microscope, a microsecond pulse generator, a needle-plane electrode and organic glass (PMMA) samples. Aside from the common phenomena, some abnormal phenomena were observed, which include: (1) sub-trees grow from the tip of firstly-initiated tree rather than the needle; (2) dark region emerges in the vicinity of the needle and successive bubbles are ejected; (3) ‘growth-ring-shape’ mechanical crack increases in front of the needle pulse by pulse. These phenomena were compared with those under a nanosecond (ns) pulse and those under an AC voltage. Similarities and differences between these phenomena were generalized. By comparison, it is concluded that the local strong field around the tips of the trees and huge energy released in the cumulative breakdown process are the main dynamics for the tree to grow under μs pulses. The local strong field corresponds to the electron breakdown mechanism and is responsible for the sub-tree initiation phenomenon. The huge energy corresponds to the thermal and the mechanical breakdown mechanisms, and is responsible for the gas release phenomenon and the ‘growth-ring’ phenomenon. [ABSTRACT FROM AUTHOR]

This paper deals with polymer nanocomposites and their related breakdown mechanisms. Polymer nanocomposites seem to be a very promising alternative to conventional polymers regarding high voltage applications. Some developed breakdown models are discussed as well as the mechanism of treeing in such materials. Treeing simulation results are presented. [ABSTRACT FROM AUTHOR]

SUMMARY: Reliability in the insulating materials is necessary in order to supply stable power. However, the nonuniform electric field is caused by foreign matters or voids in the insulating material. The deterioration of insulating material is accelerated due to the nonuniform electric field and it finally leads to a dielectric breakdown. It is likely that the space charge distribution is related to the deterioration of the insulating material. The pulsed electroacoustic method can make it possible to detect the space charge distribution and to get the valuable information for the deterioration diagnosis. In this study, a line‐plate electrodes system was used to bring nonuniform electric field and the epoxy resin was inserted as the insulating material between the line electrode and the plate electrode. The changes of space charge distribution were observed under nonuniform electric field before the dielectric breakdown. As a result, this study indicates that the charge is injected from the line electrode and the space charge distribution changes due to the deterioration of the epoxy resin. [ABSTRACT FROM AUTHOR]

One of the problems encounters in implementing vegetable oils (natural esters) as a transformer insulating oil is their viscosity which is high. To address this problem, monoester derived from vegetable oil was introduced. This paper deals with partial discharge (PD) behaviors of dry and moist samples of a monoester type insulating oil, palm fatty acid ester (PFAE), under AC high voltage utilizing a needle plane electrode configuration. Variations of average PD charge and PD number against the variation of voltage application, as well as their polarity discrimination under both dry and moist sample conditions are analyzed. PD pulses phase distributions of both dry and moist sample oils are elaborated. Finally, a mechanism of the water molecules effect on PD behavior is proposed based on experimental results and shown schematically. [ABSTRACT FROM AUTHOR]

The insulation property at high voltage frequencies has become a tough challenge with the rapid development of high-voltage and high-frequency power electronics. In this paper, the electrical treeing behavior of silicone rubber (SIR) is examined and determined at various voltage frequencies, ranging from 50 Hz to 130 kHz. The results show that the initiation voltage of electrical trees decreased by 27.9% monotonically, and they became denser when the voltage frequency increased. A bubble-shaped deterioration phenomenon was observed when the voltage frequency exceeded 100 kHz. We analyze the typical treeing growth pattern at 50 Hz (including pine-like treeing growth and bush-like treeing growth) and the bubble-growing pattern at 130 kHz. Bubbles grew exponentially within several seconds. Moreover, bubble cavities were detected in electrical tree channels at 50 Hz. Combined with the bubble-growing characteristics at 130 kHz, a potential growing model for electrical trees and bubbles in SIR is proposed to explain the growing patterns at various voltage frequencies. [ABSTRACT FROM AUTHOR]

The diagnosis of insulation performance in the high-voltage alternating current (HVAC) and high-voltage direct current (HVDC) systems has always attracted many researchers in the field of electrical engineering. New diagnostic technology and condition monitoring apparatus are of prime importance for HVAC and HVDC developments. In this study, we introduce a valuable technique called direct current integrated charge (DCIC-Q(t)) for determining the degradation of insulating materials by means of charge accumulation. This work demonstrates the typical results of full-size cross-linked polyethylene (XLPE) cables by using this effective method. Firstly, we describe the basic measurement principle of DCIC-Q(t) technique and the evaluation details of the degradation. Then, the validity of this technique is confirmed by comparing with the pulsed electroacoustic (PEA) results. The similar charge behaviors of polystyrene and low-density polyethylene are presented via the DCIC-Q(t) and PEA methods, which indicates the good availability of this technique. Three underground, full-size, 6.6-kV XLPE cables were diagnosed using the DCIC-Q(t) method. Two cables were degraded by water and electric stress for a different time under controlled conditions. The degradation extent can be evaluated easily by the charge accumulation from the DCIC-Q(t) results. Furthermore, based on the results of charge dynamics related to the applied voltage, the properties of the degraded cable by water tree are discussed. [ABSTRACT FROM AUTHOR]

Effects of mechanical stretching, produced after the expansion of cable accessories, on the electrical tree growth characteristics of EPDM (Ethylene Propylene Diene Monomer) are reported in the present work. FFV (fractional free volume) and CED (cohesion energy density), which were respectively related to free path of electrons and molecular chains splitting during electrical tree propagation process, were calculated through molecular dynamics simulations under different stretching ratios. A needle-plane electrode system with the stretching ratio ranging from 0 to 30% was used to investigate the electrical treeing characteristics in the experimental study. The simulation results show that the FFV becomes larger at higher stretching ratio, which enhances the collision between the hot electron and the molecular chain during the treeing process by extending the free path of the hot electron. While CED tends to become smaller at higher stretching ratio, reducing the cohesion strength for the tree channel crack to grow. Moreover, FFV and CED are investigated during the stretching recovery process, where FFV tends to decrease and CED turns to be larger. The experimental results indicate that mechanical stretching affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes shorter as stretching ratio increases from 0 to 30%. Tree length and accumulated damage become larger with increasing the stretching ratio. Electrical tree propagation mechanism considering chemical and physical traps, which are generated under mechanical stretching, are also discussed and analyzed. [ABSTRACT FROM AUTHOR]

To ensure a certain interface electrical strength resistance, the cable accessories are densely coated onto the main cable insulation. Compressive stress is generated in the normal direction of high voltage cable by the spring clamp device or the banding force after the expansion of EPDM (Ethylene Propylene Diene Monomer) stress cone. This paper presented a compressive stress experimental study considering electrical treeing propagation in EPDM by employing a needle-plane electrode system. The results indicate that compressive stress affects the electrical tree inception and propagation characteristics. The inception time of electrical tree becomes longer as compressive rate increases from 0 to 30%. Tree length and accumulated damage become smaller with increasing the compressive rate. Electrical tree structure is also depended on compressive stress, and branch tree structure becomes the main electrical tree morphology at higher compressive rate. For better understanding the effects of compressive stress on electrical tree propagation characteristics, FFV (fractional free volume) and CED (cohesion energy density) were calculated and analyzed through molecular dynamics simulations under different compressive rates. Results show that the FFV becomes smaller at higher compressive rate, which shortens the free path of the hot electron and weakens the collision with the molecular chain during the treeing process. While CED tends to be lager under higher compressive rate, which improves the cohesion strength for the crack to grow in the tree channel tips and restrains the propagation of electrical tree. [ABSTRACT FROM AUTHOR]

This work assesses the initial and crucial part of electrical treeing degradation, the inception stage, focusing on its dependence on applied voltage waveform and frequency. Tests have been performed on needle-plane configuration samples in solids and gels. A physical model has been formulated through an adaptation of an established theory for solids in which electrical tree inception is related to damage-producing injection currents. The voltage rise time appeared to be the most important parameter influencing the tree inception in the gel, while in the solid material the frequency is more relevant. The analysis leads to the conclusion that tree inception in gels is due to a single high-energy event, in contrast to what is commonly known for solids where damage accumulation takes place. A tree inception model is proposed for the gel, in which initiation is driven by a pressure wave generated by the electric field and the space charge injected into the sample. The model fits the experimental data and may be used to predict the tree initiation for different waveforms and voltage values. [ABSTRACT FROM AUTHOR]