Development Of Electrical Trees Research Articles

The Effect of Mechanical Strain on Electrical Tree Development in XLPE

The Effect of Mechanical Strain on Electrical Tree Development in XLPE

Role of a 3D Nanoskeleton in Hindering Electrical Tree Growth in Nanocomposites: Insights from in Situ Self-Fluorescence Imaging.

Despite the proposal of nanodielectrics in 1994, the impact of nano- and microstructures on composite performance is still not completely understood. A key reason for this knowledge gap is the lack of in situ characterization of micro- and nanoscale structures within materials. In this study, we observed the self-excited fluorescence of a microscale-damaged microchannel inside a composite under the influence of an electric field. Furthermore, we conducted in situ imaging of the internal microstructures and discharge channels in the composite utilizing external laser excitation. The imaging results reveal that the electrical treelike damage in the composites grows with a single channel under the guidance of the nanoskeleton embedded in the matrix, which demonstrates that the three-dimensional (3D) nano-order skeleton hinders the development of electrical trees. Furthermore, we analyzed the nanoskeleton intervention's enhancement mechanism on the insulation properties of the composites. This work aids in the precision imaging-guided structural design of nanodielectrics.

Influence of Electric Field and Fillers on Electrical Tree Development in Epoxy Resin

Influence of Electric Field and Fillers on Electrical Tree Development in Epoxy Resin

Scalable in-situ surface-coated polymer dielectrics with significantly enhanced high-temperature breakdown strength

Wide bandgap inorganic layers exhibit the effect to improve the insulating properties as charge barrier layers between the electrode and polymer dielectrics. However, their influence mechanisms on the high-temperature breakdown behavior still need to be further revealed. Herein, the in-situ grown strontium oxide (SrO) layers were coated on the surfaces of polyimide (PI) film to conduct this research. Compared with neat PI, the breakdown strength and discharge energy density (efficiency >90%) of the coated film was significantly elevated up 45% and 1000% at 200 °C, respectively. The experimental results show that SrO greatly reduces the entropy of surface molecular conformations and increases the energy level of localized states of PI film. The SrO coating effectively improves the injection barrier and interface trap levels simultaneously, undertakes a higher electric field, and hinders the destruction of PI film's surface molecules by high-energy electrons, thus reducing the conduction loss and constraining the generation and development of electrical trees at high fields. This work not only provides a comprehensive insight into the in-situ oxide coating on the insulating properties in polymer dielectrics but also offers a paradigm for developing scalable dielectric materials with high breakdown strength used in harsh-condition electrification.

Characteristics Analysis of Water Needle Induced Electrical Trees in XLPE

In this paper, electrical trees initiated from water and metal needle are investigated, and the inception and morphological characteristics of water needle electrical trees are analyzed. The existing literature shows that electrical trees can be initiated from the metal needle or transformed from the water trees. But in this study, it is found that the electrical tree can be directly induced by water needle in addition to the above cases. Moreover, the comparative results also indicate that the electrical trees initiated from water needle and metal needle have different inception features and morphological characteristics. To obtain the water needle electrical trees initiation and development mechanism, the water needle tips are observed before and after the sample degas process. Besides, the electric field distribution of the metal needle tip, the water needle tip before and after pretreatment, and of tree branch tip during the electrical tree development are calculated. The observations and calculations results show that before the degas pretreatment, a micro-gas cavity is formed at the water needle tip due to the tension of the water surface. The cavity at the needle tip may cause and support partial discharge (PD), causing accumulative damage and material defects. Then the density of defects becomes sufficiently large, and a tree branch can be formed. After the branch formation, the water flows into the tree branch, and the micro-gas remains at the branch tip, which may facilitate the PD, and trees propagation, resulting in the diversified morphological characteristics of the water needle electrical trees.

Electrical Tree Characteristics of Epoxy Resin/Aln Nanocomposites in LN2 With Repetitive Nanosecond Pulse Voltage

Under the action of nanosecond pulse voltage, the insulation material is prone to partial discharge, which induces the generation of electrical trees and insulation failure. The epoxy resin/aluminum nitride (AlN) nanocomposites are developed to suppress the development of electrical treeing for cryogenic insulation of high-temperature superconducting power equipment. The effects of the content of aluminum nitride material on the relative permittivity and dielectric loss of composites were studied, as well as the growth characteristics of electrical treeing at liquid nitrogen temperature. The findings reveal that the samples with the AlN content of 3 wt% have the highest relative permittivity, rising by 15%, but nanoparticle doping has no significant effect on the dielectric loss of composites. Furthermore, the right amount of AlN nanoparticle doping will improve the electrical treeing resistance of epoxy resin and the insulation properties of composites.

Investigation of electrical treeing in perspex material

Perspex has been known for a long time as a polymeric material, and it has been used for a large number of electrical and non-electrical applications. The present work was carried out to investigates the ageing mechanism of perspex material under a high electric field. The electrical treeing phenomenon was studied using perspex samples with electrodes of a pin-to-plane configuration. The growth of an electrical tree in Perspex was measured and analysed with the aid of an advanced microscope, equipped with a high-resolution camera and connected to a personal computer. Several distinct stages were assigned to characterise the electrical tree development. The area occupied by the electrical tree channels was calculated using equal-area squares. This approach was employed to measure the growth rate of electrical trees under dry and wet conditions. The tree construction, shape and growth speed were studied and analysed to distinguish between treeing phenomenon under wet and dry conditions of fabricated perspex specimens. The absorption of water has increased the tree growth inside the samples, and ions with water have accelerated the breakdown process. The findings of this study are essential to improve the performance of perspex material, which is widely used in a variety of applications for both energy and non-energy purposes.

Influence of Repetitive Square Voltage Duty Cycle on the Electrical Tree Characteristics of Epoxy Resin.

The application of wide band-gap power electronic devices brings more challenges to insulating packaging technology. Knowing the influence of applied voltage parameters on insulation performance is helpful to evaluate the insulation condition of electric power equipment. In this paper, the effect of repetitive square wave voltage duty cycle on the growth characteristics of electrical trees in epoxy resin was studied. The experimental results show that the square wave voltage duty cycle has a significant influence on treeing features. The electrical tree proportion initiation has shown a decreasing trend, and the shape of the electrical tree changes from pine-like to branch-like by increasing the duty cycles. The length and damaged area of electrical tree increased with the increase in the duty cycle up to 10% and then decrease by increasing the duty cycle higher than 30%. It indicates that a low duty cycle will enhance the electron injection and accumulate space charges and thus accelerate electrical tree development. Under short duty cycles, the electric field due to the shielding effect near the needle tip suppresses the electrical tree growth, which results in treeing growth stagnation. The obtained results are helpful to keep these parameters in mind during the design of epoxy-based insulation such high-voltage rotating machines and power electronic device packaging.

Proposal of finite element analysis method for dielectric breakdown based on Maxwell’s equations

This paper proposes a new approach to the simulation of discharge phenomena in dielectric breakdown including field fluctuations based on Maxwell’s equations in the finite element framework without introducing the artificial stochastic disturbance. This method is based on the idea that field fluctuations caused by the development of electrical treeing propagates as electromagnetic wave governed by Maxwell’s equations, and triggers next dielectric breakdown in a chain reaction. The obtained model can be regarded as the equivalent circuit model in which the resistors representing local property of material are arranged. The key point of the proposed method is the ease of expressing dielectric breakdown as replacement of the arranged resistors. Dielectric breakdown expressed as the local change of conductivity leads to the inhomogeneity of the conductivity. In addition to this simplest model for dielectric breakdown, constraint on the discharge paths to the finite edges in the circuit is applied. This corresponds to the introduction of the implicit inhomogeneity. This implicit inhomogeneity plays a significant role in the process of formation of branching discharge patterns. The capability of introduction of the implicit inhomogeneity and the constraint on the possible candidates for propagation paths enable the proposed method to find out a bifurcated solution and make it easier to analyze formation of discharge patterns in dielectric breakdown. This paper explains these features of the proposed mathematical model with results of numerical simulation of several example problems.

Electrical Treeing in Power Cable Insulation under Harmonics Superimposed on Unfiltered HVDC Voltages

Insulation degradation is an irreversible phenomenon that can potentially lead to failure of power cable systems. This paper describes the results of an experimental investigation into the influence of direct current (DC) superimposed with harmonic voltages on both partial discharge (PD) activity and electrical tree (ET) phenomena within polymeric insulations. The test samples were prepared from a high voltage direct current (HVDC) cross linked polyethylene (XLPE) power cable. A double electrode arrangement was employed to produce divergent electric fields within the test samples that could possibly result in formation of electrical trees. The developed ETs were observed via an optical method and, at the same time, the emanating PD pulses were measured using conventional techniques. The results show a tenable relation between ETs, PD activities, and the level of harmonic voltages. An increase in harmonic levels has a marked effect on development of electrical trees as the firing angle increases, which also leads to higher activity of partial discharges. This study of the influencing operational parameters of HVDC converters on power cable insulation is predicted to contribute to enhancements in cable design and progressive advancement in condition monitoring and insulation diagnostic techniques that can lead to more effective asset management in HVDC systems.

Temperature‐dependent effect of gas pressure on electrical tree in XLPE cable

Cross-linked polyethylene (XLPE), which is an important material in the cable insulation, faces the challenge of high temperature with high internal gas pressure during the process of overload. To clarify the impacting mechanism of temperature and gas pressure, this study presents the characteristics of electrical trees and partial discharge (PD) under different gas pressure and different temperature in XLPE cable. Experiments were conducted using XLPE cable insulation as samples. The results show that gas pressure has different effects on the initiation and propagation of electrical trees at different temperatures. XLPE is in the glassy state at low temperature, and the gas pressure suppresses the development of electrical trees during the growth period. When XLPE is in the elastomeric state, the high temperature accelerated the PD under high gas pressure. The analysis shows that more trapped charge and more branch channels will exist at high temperatures with high gas pressure, which is the primary cause behind the development of streamer, thus accelerating the progress of PD. XLPE cable in the case of high temperature and high gas pressure should cause more attention.

Study on the Relationship between Electrical Tree Development and Partial Discharge of XLPE Cables

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.

The investigation and improvement of the lifetime of high voltage ceramic capacitor under repetitive frequency operation

The lifetime tests for High Voltage Ceramic Capacitors (HVCC) were investigated under repetitive frequency operation at 25 Hz and three kinds of failure modes were summarized. The significant innovation of this paper is testing HVCC, made in the laboratory, under repetitive pulses and proposing several effective improvement methods. The results show that the lifetime of HVCC meets inverse power-law with the applied voltage and the voltage acceleration factor is about 5.8. Failure modes of HVCC are inner breakdown of ceramic dielectric, interface breakdown of ceramic-epoxy and abscission of brass terminals. The inner breakdown is in connection with abrupt destructive discharge under much higher stress while the interface breakdown is related to interface delamination and development of electrical trees in relatively low strength. The third failure mode is always caused by high current density. Based on the failure modes, several methods were taken to improve the performance of HVCC, which included coating semiconductor at the edge of silver layers, adding coupling agent to ceramic-epoxy interface and improving the structure of brass terminals. The lifetime of HVCC under repetitive frequency operation has been extended greatly from 103 shots to more than 105 shots after the investigation and improvement.

Effect of the Dielectric Inhomogenity Factor's Range on the Electrical Tree Evolution Solid Dielectrics

The main contribution of the presented paper is to investigate the influence of the Dielectric Inhomogeneity Factor on the electrical tree evolution in solid dielectrics using cellular automata. We have a sample of the XLPE which is located between needle-to-plane electrodes under DC voltage. The electrical tree emanates from the end of the needle in which the electric stress attains a dielectric strength of the material. At every time step, Laplace's equation is solved to calculate the potential distribution which changes according to electrical tree development. Dynamic simulations clearly demonstrate the influence of the range of the Dielectric Inhomogeneity Factor on the electrical tree growth. Simulation results confirm the published technical literature.

Effect of Magnesium Hydroxide on Voltage Lifetime of Epoxy Composites

SUMMARYIn general, a polymer insulator subjected to high voltage reaches breakdown as a result of the development of electrical trees. Magnesium hydroxide, a metal hydroxide, has the property of releasing water of crystallization at high temperatures. If the energy of a partial discharge is used by the decomposition of magnesium hydroxide, the growth of electrical trees will be inhibited and the lifetime of the polymer insulator can be expected to improve. Therefore, we have studied the effects of magnesium hydroxide on treeing. We confirmed that the electrical breakdown time of epoxy resin with 30 parts per hundred (resin) of magnesium hydroxide was 100 times longer than that of epoxy resin without it. Magnesia was detected through electron beam diffraction analysis of the filler particles located near the electrical tree, and it was found that a dehydration reaction took place in the magnesium hydroxide. In other words, as an explanation of the effect of inhibiting an electrical tree, the effect of the energy dispersion of a partial discharge is suggested along with the barrier effect of the fillers.

Effect of Magnesium Hydroxide on Voltage Lifetime of Epoxy Composites

In general, a polymer insulator subjected to high voltage reaches breakdown as a result of the development of electrical trees. Magnesium hydroxide, a metal hydroxide, has the property of releasing water of crystallization at high temperatures. If the energy of a partial discharge is used by the decomposition of magnesium hydroxide, the growth of electrical trees will be inhibited and the lifetime of the polymer insulator can be expected to improve. Therefore, we have studied the effects of magnesium hydroxide on treeing. We confirmed that the electrical breakdown time of epoxy resin with 30 parts per hundred resin of magnesium hydroxide was 100 times longer than that of epoxy resin without it. Magnesia was detected through electron beam diffraction analysis of the filler particles located near the electrical tree, and it was found that a dehydration reaction took place in the magnesium hydroxide. In other words, as an explanation of the effect of inhibiting an electrical tree, the effect of the energy dispersion of a partial discharge is suggested along with the barrier effect of the fillers.

Evaluation of the performance of several object types for electrical treeing experiments

A comparison of the efficiency between various test object types for studying the development of electrical trees in polymers is presented. Two types of newly developed wire-plane objects are investigated and compared with two versions of the traditionally used needle-needle object. The two needle-needle objects are prepared according to ASTM standard and act as references, whereas the alternative objects rely on the use of an ultra-thin tungsten wire (10 and 20 μm) for providing the highly divergent electric stress. In one of these object types the wire extends from a semiconducting tab embedded in the tested material whereas the embedded wire is extended and connected externally by means of a copper tape in the other type. The comparison is made using cross linked polyethylene (XLPE) for the testing purposes. The wire type objects provide some promising advantages, including parallel formation of several electrical trees and an exposure of a larger material volume. Further advantages include the simplification of both the manufacturing and the measuring procedures. Among the newly developed test objects, the one without the semiconducting tab is considered most advantageous as it allows for more accurate treeing initiation measurements at a lower voltage level. A disadvantage with the wire electrode is its tendency to form kinks during object preparation, which may lead to difficulties in correctly estimating the field strength locally as well put strain on the surrounding material. However the multitude of electric trees formed in each object allows for trees growing at such imperfections to be discarded in following data analyses. Although finding a statistical method that makes good use of all the data poses a challenge, a suggested approach is presented.

Electrical Tree Simulation Based on the Self-Organization Criticality

So far much effort has been made to understand the development of electrical treeing. For the simulation based study of electrical treeing, the most common method is to apply DBM stochastic model to simulate the growing of electrical treeing patterns. Previous simulation results showed that this stochastic model is capable of simulating the real electrical treeing patterns in a point-to-plane electrode system. However, this model only allows the tree channels to propagate on equipotential lines proportional to local electrical field. Therefore, it is necessary to develop a novel stochastic model to simulate the electrical patterns in order to get a good agreement with experimental results.

The impact of electrical treeing on a cable-embedded fibre

The development of electrical trees near to a cable-embedded fibre is investigated using composite samples with a pin to plane configuration. The tree growth under high electric field is monitored using advanced optical system, equipped with a high-resolution camera and microscope. The slow stressing of polyester material around the fibre surface is assessed as a function of electrical tree development. The tree growth is evaluated by observing the expansion and enlargement of branches in the horizontal and vertical directions. The impact of electrical trees on the fibre optic cables is translated into a distortion of the fibre surface and into irregularities of the surrounding treed area. The shapes and areas of deformed fibre are used as criteria for characterizing cable conditions exposed to high field. A deformation factor is introduced to describe these irregularities in the embedded fibre. Although the focus of the present paper is on the treed region, located between the pin tip and the fibre surface, the conditions of the fibre-to-ground region are also examined.

エポキシ樹脂中のトリー伸展と放射電磁波

Detection of electro-magnetic waves is one of a good diagnosis tool that can be used for the condition assessment of insulating systems. Detection and frequency analysis of electro-magnetic wave caused by partial discharge in epoxy resin have been investigated. Electro-magnetic waves radiated by partial discharge could be detected by using simple system with a conventional discone antenna and a spectrum analyzer. Detected electro-magnetic wave spectra have two specific frequency regions: one is at lower than around 200MHz and another is higher than around 200MHz. From observation of electrical tree development and measurements of discharge current and partial discharge level, the former is associated with a void discharge in epoxy resin and the latter is identified as tree development. By using these specific frequency regions, it is applicable to detection of partial discharge and insulating assessment for power apparatus and their accessories including epoxy resin.