Electrical Tree Propagation Research Articles

Analysis of electrical tree propagation through glass fiber barrier

In this paper, two sets of samples are examined, one for plain polyester resin and the other for glass fiber composite. In both sets, a pin to plane configuration is used to grow electrical trees around the barrier area. Several patterns of tree propagation are investigated. Due to the effect of barrier to hinder the growth, tree shape and growth rate are influenced at both sides of the barrier. The structure of barrier and its adhesion to the surrounding resin are the main factors to be considered for selection of the barrier to prolong the lifetime of composite. Horizontal growth relationship with penetration delay is studied. This delay is aggravated by the lack of energy of horizontally grown leading branches. A mechanism, explaining the delay of a leading branch to penetrate the barrier, is introduced. The mechanical stresses determined by de-bonding, deformation and carbonization in the interfacial area are studied. Finally, the results of this work are very important for breakdown analysis, life assessment and improvement of composite insulation and power cables.

Investigation of electrical treeing in cable grade crosslinked polyethylene (XLPE) insulations

Using PC-based on-line monitoring system, extensive measurements were performed in insulation blocks cut from the crosslinked polyethylene (XLPE) insulation of 36, 69 and 132 kV rated cables produced in Saudi Arabia. A needle-plane electrode system was used with a needle radius of 10 mm. The role of 'applied voltage step-time duration' and associated time resolved initiation and growth of different types of tree structures were evaluated. Results show that a step of 3 kV/10 min duration provides good comparison to evaluate service aged and new XLPE insulation of different voltage ratings. Velocity of propagation of electrical trees is fast at initiation, but declines exponentially to a constant rate within 20 s of its initiation. Fractal analysis of such trees was also carried out using different types of technical grade XLPE insulations. This paper presents discussion on these results and sheds light on the electrical treeing phenomenon. Key words: Electrical treeing, crosslinked polyethylene (XLPE) insulation, on-line monitoring, bush-type tree, branch-type tree, new and aged cable insulation, fractal analysis.

Effects of Oil Palm Empty Fruit Bunch Filler on the Electrical Tree Propagation in Silicone Rubber

This paper presents the study on the effects of oil palm empty fruit bunch (OPEFB) micro-filler on electrical tree propagation in silicone rubber composite material. In this study, the oil palm empty fruit bunch (OPEFB) filler with mean size of 32μm was employed as an electrical tree inhibitor. The sample for this electrical tree experiment was in the form of leaf-like specimen which was characterized as filled sample and non-filled sample. The filled sample contained 1 wt% of OPEFB filler while the non-filled specimen contained no fillers. Obtained results show that the lengths of electrical trees in filled silicone rubber were shorter than tree length of non-filled silicone rubber for 60minutes propagation time. This tree length difference shows that the OPEFB micro-filler helped to retard the growth of electrical tree. Therefore, this new organic filler is found to be useful for electrical tree inhibitor in high voltage insulation.

N-tridecane as a model system for polyethylene: comparison of pre-breakdown phenomena in liquid and solid phase stressed by a fast transient

Pre-breakdown phenomena in pure n-tridecane in liquid and solid state have been investigated experimentally in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Pre-inception currents in liquid and solid state were recorded and compared to finite element calculations to find plausible high-field conduction models for n-tridecane and cyclohexane. n-tridecane was chosen as a model system for polyethylene based on the known similarities in energy bands for polyethylene and alkanes. The results obtained in pure n-tridecane were compared to similar experiments on cyclohexane and polymer systems. High-field conduction in liquid n-tridecane was found to have a similar field dependence as previously reported for cross linked polyethylene (XLPE). Conduction currents and inception probabilities were found to be almost independent of polarity in frozen and liquid n-tridecane, which is also in line with results obtained for XLPE. Conduction currents were observed at lower voltages in n-tridecane than in cyclohexane, reflecting the lower space charge limited field (SCLF) in n-tridecane. Inception of streamers in liquid cyclohexane occurs at lower voltages than in liquid n-tridecane, and a stronger polarity dependence for all phenomena was observed in cyclohexane compared with n-tridecane. A possible explanation for the differences between n-tridecane and cyclohexane is given based on the field needed for electron avalanches and the SCLF in the two liquids. Inception voltages, light emission and charge injection were found to be similar in liquid and solid n-tridecane. This indicates that the same processes are responsible for inception and propagation of electrical trees in solids and streamers in liquids when the material is stressed by a fast transient. The similarity between the measured properties in solid and liquid phases leads to the conclusion that electrical treeing mainly takes place in the amorphous regions of the solid phase.

Effects of N,N-dimethylaniline and trichloroethene on prebreakdown phenomena in liquid and solid N-tridecane

Prebreakdown phenomena in 0.1 M N,N-dimethylaniline (DMA) and 0.1 M trichloroethene (TCE) in n-tridecane have been investigated experimentally in liquid and solid phases in a needle-plane geometry with impulse voltage. Light emission and charge injection from the high-field electrode were measured. Results have been compared with results obtained in neat n-tridecane and in cyclohexane with the same additives. Adding TCE to n-tridecane increases the p re-inception current and decreases the inception voltage for negative and positive polarity. The increase in pre-inception current is probably caused by trap assisted conduction, and may be responsible for the decrease in inception voltage. TCE had no effect on propagation of streamers in n-tridecane. The main effect of DMA in n-tridecane is to enhance the propagation of streamers and electrical trees with positive polarity. This is in line with what has been reported for cyclohexane, and is explained by the low ionization potential of DMA. Charge injection for negative streamers is reduced by the addition of DMA in n-tridecane, while the emitted light is increased. This indicates that some of the energy otherwise used for propagation is emitted as light. A low electronic excitation energy for DMA at 3.8 eV supports this hypothesis. The additives have the same effect on prebreakdown phenomena in solid and liquid phases with positive polarity, and thus the same mechanism is suggested to be responsible for electrical treeing and streamer inception and propagation. The scatter is generally found to increase when going from liquid to solid phase, which is explained by the inhomogeneity in the solid phase. The transition to solid phase with negative polarity typically results in an even larger scatter than for positive polarity. The effect of additives seems to be secondary to the morphology at the point electrode with negative polarity.

Influence of homocharges and nanoparticles in electrical tree propagation under DC voltage application

The influence of nanoparticles and homocharges on the propagation of electrical treeing in polymer insulation is examined for a needle-plane electrode arrangement. A simulation is carried out using a model based on Cellular Automata (CA). A DC voltage application on the needle electrode is assumed. Nanoparticles are introduced in the polymer matrix in the vicinity of the needle electrode, and simulations with different homocharge densities are performed. It is confirmed that the propagation of electrical trees is hindered by the presence of nanoparticles and homocharges. A larger quantity of homocharges forms a barrier to the injection of charge carriers in the nanocomposite sample. Electrical trees seem to go around and/or stop at nanoparticles and thus, their propagation becomes more difficult. In other words, the proposed simulations show that electrical trees follow a tortuous path, avoiding the nanoparticles.

A MODEL FOR ELECTRICAL TREEING IN EPOXY NANOCOMPOSITE INSULATING MATERIALS

Electrical treeing is a pre-breakdown mechanism, and is the responsiblefor the long-term degradation of polymer insulation. This paper has beenenhanced both the life-time of tree propagation to breakdown and thetree growth rate of electrical tree propagation by adding variousamounts of nanoparticles as Clay, Silica, ZnO, and Al2O3 to variousindustrial materials CT200 Epoxy Resin, and LY/HY-5052 Epoxy Resin.This research has focused on the development of nanocompositematerials with electrical tree growth resistance superior to the unfilledmatrix, and has stressed particularly the effect of filler volume fractionon the breakdown resistance. Finally, this research aims to present asystematic and comparative study about the effect of addingnanoparticles to dielectric insulation, and to understand the role of thesenanoparticles, Clay, Silica, ZnO, and Al2O3 particles in the CT200Epoxy Resin and LY/HY-5052 Epoxy Resin which can made significantimprovement in breakdown resistance, life-time of insulation anddecreasing the tree growth rate of electrical tree propagation.

Understanding Electrical Treeing Phenomena in XLPE Cable Insulation Adopting UHF Technique

Understanding Electrical Treeing Phenomena in XLPE Cable Insulation Adopting UHF TechniqueA major cause for failure of underground cables is due to formation of electrical trees in the cable insulation. A variety of tree structure can form from a defect site in cable insulationvizbush-type trees, tree-like trees, fibrillar type trees, intrinsic type, depending on the applied voltage. Weibull studies indicate that a higher applied voltage enhances the rate of tree propagation thereby reducing the life of cable insulation. Measurements of injected current during tree propagation indicates that the rise time and fall time of the signal is of few nano seconds. In the present study, an attempt has been made to identify the partial discharges caused due to inception and propagation of electrical trees adopting UHF technique. It is realized that UHF signal generated during tree growth have signal bandwidth in the range of 0.5-2.0 GHz. The formation of streamer type discharge and Townsend type discharges during tree inception and propagation alters the shape of the tree formed. The UHF signal generated due to partial discharges formed during tree growth were analyzed adopting Ternary plot, which can allow one to classify the shape of tree structure formed.

Analysis of electrical tree propagation in XLPE power cable insulation

Electrical treeing is one of the major breakdown mechanisms for solid dielectrics subjected to high electrical stress. In this paper, the characteristics of electrical tree growth in XLPE samples have been investigated. XLPE samples are obtained from a commercial XLPE power cable, in which electrical trees have been grown from pin to plane in the frequency range of 4000–10,000 Hz, voltage range of 4–10 kV, and the distances between electrodes of 1 and 2 mm. Images of trees and their growing processes were taken by a CCD camera. The fractal dimensions of electric trees were obtained by using a simple box-counting technique. The results show that the tree growth rate and fractal dimension was bigger when the frequency or voltage was higher, or the distance between electrodes was smaller. Contrary to our expectation, it has been found that when the distance between electrodes changed from 1 to 2 mm, the required voltage of the similar electrical trees decreased only 1or 2 kV. In order to evaluate the difficulties of electrical tree propagation in different conditions, a simple energy threshold analysis method has been proposed. The threshold energy, which presents the minimum energy that a charge carrier in the well at the top of the tree should have to make the tree grow, has been computed considering the length of electrical tree, the fractal dimension, and the growth time. The computed results indicate that when one of the three parameters of voltage, frequency, and local electric field increase, the trends of energy threshold can be split into 3 regions.

Investigations of electrical trees in the inner layer of XLPE cable insulation using computer-aided image recording monitoring

Using a computer-aided image recording monitoring system, extensive measurements have been performed in the inner layer of 66 kV cross-linked polyethylene (XLPE) cables. It has been found that there are three kinds of electrical trees in the samples, the branch-like tree, the bush-like tree and the mixed tree that is a mixture of the above two kinds. When the applied voltage frequency is less than or equal to 250 Hz, only the mixed tree appears in XLPE samples, when the frequency is greater than or equal to 500 Hz, only the dense branch-like tree develops, both of which are attributed to the coexistence of non-uniform crystallization and internal residual stress in semicrystalline XLPE cables during the process of manufacturing. Through the fractal analyses of these electrical trees, it has been found that both the propagation and structure characteristics can be described by fractal dimension directly or indirectly. It is suggested that the propagation and structural characteristics of electrical trees are closely related to the morphology and the residual stress in material at low frequency, i.e., the propagation characteristics of electrical trees depends upon not only the boundaries between big spherulites and amorphous region, but also the impurity, micropore concentration and the relative position of needle electrode tip with respect to spherulites or amorphous region in the low frequency range. However, at high frequency, it has nothing to do with the morphology of material. It is suggested that the injection and extraction process of charge from and to dielectrics via the needle electrode are more intense at high frequency than in low frequency. Thus, it can form relatively uniform dielectric weak region in front of needle electrode, which leads to similar initiation and propagation characteristics of electrical trees at high frequency.

Electrical Treeing Propagation in Nanocomposites and the Role of Nanofillers: Simulationwith the Aid of Cellular Automata

Electrical Treeing Propagation in Nanocomposites and the Role of Nanofillers: Simulationwith the Aid of Cellular AutomataIn this paper the propagation of electrical treeing in nanodielectrics using the DIMET (Dielectric Inhomogeneity Model for Electrical Treeing) is studied. The DIMET is a model which simulates the growth of electrical treeing based on theory of Cellular Automata. Epoxy/glass nanocomposites are used as samples between a needle-plane electrode arrangement. The diameter of nanofillers is 100 nm. The electric treeing, which starts from the needle electrode, is examined. The treeing growth seems to be stopped by the nanofillers. The latter act as elementary barriers to the treeing propagation.

Propagation mechanism of electrical tree in XLPE cable insulation by investigating a double electrical tree structure

This paper presents our experiments and analysis of the electrical tree growing characteristics. The relationship between electrical tree propagation and the material morphology in XLPE cable insulation has been studied by researching the structure and growth characteristics of a double structure electrical tree. It has been found that, due to the influence of uneven congregating state, difference in crystalline structure, and the existence of residual stress in semi-crystalline polymer, five types of electrical tree structures (branch, bush, bine-branch, pine-branch, and mixed configurations) would propagate in XLPE cable insulation. Three basic treeing propagation phases (initiation, stagnation, and rapid propagating phases) are presented in electrical tree propagating process. If initiation phase is very active, the single branch tree will propagate while if this phase is weak then the bush tree will occur more easily. There would be a clear double structure of electrical tree when it grows at submicroscopic structure uneven region of the material. A new parameter, the expansion coefficient is introduced to describe the electrical tree propagation characteristics. In addition, two other coefficients being used to describe our experimental results are dynamic fractal dimension and growth rate of electrical tree.

Dynamics model for electrical tree propagation in cross-linked polyethylene cable insulation under high frequency voltage

In this paper we investigated the structures and propagation characteristics of electrical trees observed in the high volatage cross-linked polyethylene (XLPE) cable insulation samples under high volatage of 10kV at frequency of 1000—2000Hz. Based on the specific tree growth mechanism and the fractal nature of tree structures, a dynamic model has been developed to qualitatively predict the electrical tree growth characteristics in XLPE cable insulation when subjected to an electrical stress under high frequency voltage, and then the tree growth rate equation and life formula of tree growth to breakdown are derived.We performed electrical tree growth tests in XLPE cable insulation samples at high frequency and compared the results, which showed that the proposed model was in good agreement with the expreriment.So,it seems that the proposeed model can be applied to the qualitative analysis of aging law of electrical tree in XLPE cable insulation.

Retardation of electrical tree propagation by resin degradation

AbstractIn order to improve insight into the effect of delaminations between the conductor and the groundwall insulation on the ageing of high voltage rotating machinery insulation, the influence of the polymer degradation on tree growth has been studied. Simultaneous partial discharge measurements and photograhic observations have been conducted in order to investigate the effect of the degradation of the resin on electrical tree propagation. The photographic observation indicates that the degradation of the resin retards the tree propagation after the tree having failed to penetrate a mica barrier. As the resin is degraded and in part rendered conducting, the partial discharges change both in magnitude and in terms of phase resolved partial discharge patterns. These partial discharge measurements are promising to improve insight into the processes involved in winding insulation ageing, when photographic observations are unattainable. Moreover the differences found between various resins offer an explanation why various types of winding insulation have been found to differ in their capability to withstand thermal cyclic ageing. Copyright © 2006 John Wiley & Sons, Ltd.

Electrical breakdown in high-voltage winding insulations of different manufacturing qualities

Due to an increasing demand for new, or the refurbishment of power stations, high-voltage rotating machines continue to play a significant role in generating electrical energy. An important component of these machines is their electrical insulation. Although much research has been done to improve the material, about a quarter of all failures still are related to insulation problems. Research to improve winding insulation is very complex because a large number of factors influence insulation life, among them the raw materials, size of the mica tapes, geometrical characteristics of the bar and taping- or manufacturing techniques. The results show that electrical tree propagation is the main electrical degradation mechanism that leads to breakdown of the main wall insulation and that poor impregnation of the insulation and type of taping can reduce time to breakdown significantly.

The effect of barriers on electrical tree propagation in composite insulation materials

The propagation of electrical trees is of a particular interest for the power engineering industry as it is one of the major causes for breakdown in high voltage equipments. Composites, materials with barriers and a surrounding polymer matrix are used to extend time to breakdown of the insulations. Since the material, structure or processing of the barriers are various and can determine time to breakdown significantly, the major influences of barriers on the propagation of electrical trees is described in this paper in experiments and numerical simulations. Although it was the motivation, to determine tree growth in the composite structure of mica-epoxy winding insulations, the results can also be used for other composite materials. The trees grew in needle-plane samples and their growth characteristic was optically analysed. The results show that tree propagation can be slowed down when introducing a barrier between the needle and the plane electrode and may cause significantly increased time to breakdown values. The increase depends on the barrier materials used, their thickness, the dielectric strength of the interface to the surrounding epoxy resin and the width of the barrier. When choosing multiple barriers, their arrangement influences time to breakdown significantly. Overlapped barriers show much higher time to breakdown values than impinged ones. From the results, it can be concluded that the selection of the barrier materials, their processing and their arrangement play a major role towards time to breakdown of composite insulation materials.

Detection of electrical tree propagation by partial discharge measurements

AbstractThe partial discharge (PD) activity during propagation of electrical treeing in epoxy resin is described. The electrical trees grew in needle‐plane samples without and with an internal barrier up to the final breakdown. The simultaneous tree growth and discharge activity show a correlation between the propagation state of the tree and the thereby measured PD. Especially, the changes in the tree structure can clearly be detected. Based on these characteristics a new model describing tree growth is presented. It appears that measurements from machine insulation can be interpreted in terms of the PD pattern. Copyright © 2005 John Wiley & Sons, Ltd.

Thermodynamic model for electrical tree propagation kinetics in combined electrical and mechanical stresses

In this paper we develop a kinetic model for the growth of electrical tree structures in solid polymeric insulation that allows for combined electrical and mechanical stresses. We present an energy balance analysis during the tree growth process and show the total tree extension driving force is not a material constant due to the existence of a damage process zone around the tree tip and the dissipated heat and the trapped energy remaining in the damage process zone. We derive both the tree growth rate equation and life formula of tree propagation to breakdown, based on the specific tree growth mechanism and the fractal nature of tree structures. We perform electrical tree growth tests in epoxy resin samples with and without mechanical residual stresses, and present results which show that the proposed model can give predicted tree propagation times to failure in good agreement with the experimental data of the tree growth subjected to a combined electrical and mechanical stress

Study of electrical treeing phenomena in XLPE cable samples using acoustic techniques

Electrical trees were generated experimentally in the actual 33 kV underground XLPE cable insulation material under the AC voltages. A tree like structure and a bush type of tree structure can form from the point of defect site under the AC voltages. Acoustic emission technique was adopted to identify the point of inception, propagation and termination of electrical trees. A variation in the dominant frequency content of the acoustic signal was observed as and when the tree propagates in the insulation structure. The characteristic variation in the magnitude of the acoustic emission signal with time, indicates that tree propagation as an intermittent growth process. The energy content of the acoustic signal characterizes that the energy released due to partial discharges, at every step growth of the tree structure is not the same. The partial power measurement in the present study provides an indication to the growth process of electrical trees and to the near point of failure of polymer insulation material due to electrical treeing.

Simulation of electrical tree propagation using cellular automata: the case of conducting particle included in a dielectric in point-plane electrode arrangement

The electrical tree propagation is simulated in this paper using Cellular Automata. The polymethylmethacrylate (PMMA) sample has given dimensions and inside this volume a small spherical conducting particle is considered. Point-plane electrode arrangement is used and a DC voltage is applied. The electric strength is calculated over the insulating volume and the electrical tree propagation is shown as the result of the electrical field and the inhomogeneity factor that incorporates the dielectric constant fluctuations of the insulating material.