Electrical Tree Research Articles

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.

Effect of Crystalline Morphology on Electrical Tree Morphology and Growth Characteristics of PP Insulation: From Mesoscopic to Macroscopic

This paper aims to investigate the effects of crystalline morphology on electrical tree morphology and growth characteristics of isotactic polypropylene (iPP). The crystalline morphology of iPP is adjusted by altering the crystallized temperature, and the mesoscopic electrical treeing in these samples with a thickness of 280 μm is studied. The polarizing microscope images show that an electrical tree is easier to grow along the spherulites boundary and form a bush tree within spherulites. Compared with samples crystallized at a cooling rate of 2.5 °C/min and the isothermal temperature of 132 °C, the spherulite size of the sample crystallized at a cooling rate of 10 °C/min significantly decreases, resulting in an obvious decrease of the accumulated damage area and length of the mesoscopic electrical tree. The macroscopic electrical treeing is also conducted in the iPP samples with different crystalline morphologies with a thickness of 2 mm. There exists a relation between the mesoscopic and macroscopic electrical growth characteristics by using the Pearson correlation coefficient. It is concluded that the reduction of the spherulite size and the spherulite boundary gap limits the partial discharge and the development of an electrical tree in the amorphous region at the spherulite interface, making it easier to form the bush tree and leading to improved resistance to electrical treeing.

Enhancing Inhibition of Electrical Treeing in XLPE Cables Based on the Targeted Movement of Rejuvenation Fluid Induced by Electric Field

In this study, the inhibition effect of electrical treeing is investigated in a cross-linked polyethylene (XLPE) cable under the targeted movement of rejuvenation fluid induced by an electric field. First, the fluid concentration around the defects is characterized by Fourier transform infrared spectroscopy (FTIR) under different voltages, application durations, and application temperatures. These results show that with increasing applied voltage, application duration and temperature, the fluid concentration around the defects increases nonlinearly. Next, the electrical treeing features of samples with and without electric field induction show that the tree initiation voltage increases and the tree accumulated damage decreases for the samples with electric field induction. By analyzing the changes in the molecular chain structure of the XLPE under different influencing factors, it is found that with applied voltage and duration increase and application temperature decreases, the methyl groups increase and the methylene groups decrease, the diffusion channel of fluid is expanded. Finally, it is proposed that the dielectrophoretic (DEP) force, changes in molecular chain structure, and molecular thermal motion determine the behaviors of the targeted movement of fluid in insulation. The fluid concentration around the defects increases after the targeted movement of fluid, and the electrical treeing inhibition effect is enhanced.

Stability and Population of Radicals in Electrical Trees in Epoxy Resins Based on Electron Paramagnetic Resonance Spectroscopy

The stability and population of radicals in aged epoxy resins with different electrical trees are investigated by electron paramagnetic resonance. Branch trees and bush trees of various sizes are produced in three types of epoxy resins which are cured by different curing agents. By examining the lifetime of radicals in three epoxies, the mobility of polymer chains is found to act as a vital factor for stability. The recombination of radicals is faster in polymers with higher mobility, leading to a shorter lifetime of radical signals. A distinguishable singlet appears in specimens without visible tree channels, proving that radicals are a precursor of electrical trees. Among spectra from a series of branch trees, two species of radicals are found to be generated successively as tree size increases. By comparing the signal intensity of branch trees and bush trees of similar sizes, bush trees show a much smaller radical population, which is attributed to relatively low electrical stress at branch tips and slow degradation of polymer chains.

Heterogeneous mesoscopic structure modulation enhancing dielectric strength of polypropylene insulation

This paper focuses on the heterogeneous mesoscopic structure modulation enhancing dielectric strength of polypropylene insulation. α-nucleating agents are used to modulate the heterogeneous mesoscopic structure of polypropylene, and Voronoi Network models with similar structures are established to simulate the electric field distribution at the mesoscopic scale. The experimental results show that the addition of an α-nucleating agent effectively diminishes the spherocrystal size and increases its density. The high continuity and wide range electric field distortion caused by the too-large spherocrystal size and too-loose arrangement is thus suppressed. The improvement of electric field uniformity effectively inhibits the frequency and intensity of partial discharge. Compared with pure polypropylene, the partial discharge in polypropylene composite with α-nucleating agents at 0.1 wt% decreased by 2414 times, and its maximum amplitude decreased by 58.3%. The increase in the spherocrystal density also enhances its blocking effect on the electrical tree growth, and the length and width of it are reduced by 88.6% and 89.1%, respectively. The dispersive development of the electrical tree reduces the incidence of penetration channels along the electric field direction. The breakdown strength is therefore increased by approximately 20%. The simulation results demonstrate that the dielectric property difference between the crystal and amorphous regions within polypropylene at the mesoscopic scale is the main reason that triggers the electric field distortion at its interface and further leads to insulating degradation or even failure. In this study, heterogeneous mesoscopic structure modulation is proved to be an efficient means to enhance the dielectric strength of polypropylene insulation.

Electrical Treeing and Partial Discharge Behavior in Epoxy Nanocomposites With In Situ Synthesized SiO2

The electrical treeing resistance of epoxy-SiO2 nanocomposites prepared by synthesizing functionalized SiO2 nanoparticles directly in epoxy was investigated and compared to pure epoxy [diglycidyl ether of bisphenol-A (DGEBA)]. Partial discharge (PD) measurements indicate a transition of initially nonconducting trees to conducting trees in both the pure epoxy and the nanocomposites. A correlation between the synthesis precursors and the PD behavior over time indicates that the presence of ions and remnant precursors hasten the transition to conducting trees and lower the voltage of tree inception. The resistance to tree growth was found to be dependent on the synthesis conditions of the nanocomposites. Slower tree growth was observed in nanocomposites prepared under neutral conditions, with a homogenous dispersion of SiO2 nanoparticles (30–50 nm, 5 wt%). The presence of ions from alkaline conditions and/or poor dispersion of SiO2 resulted in faster and more linear tree growth and lower initiation voltages than for pure epoxy.

Identification of Electrical Tree Aging State in Epoxy Resin Using Partial Discharge Waveforms Compared to Traditional Analysis.

Electrical treeing is one of the main degradation mechanisms in high-voltage polymeric insulation. Epoxy resin is used as insulating material in power equipment such as rotating machines, power transformers, gas-insulated switchgears, and insulators, among others. Electrical trees grow under the effect of partial discharges (PDs) that progressively degrade the polymer until the tree crosses the bulk insulation, then causing the failure of power equipment and the outage of the energy supply. This work studies electrical trees in epoxy resin through different PD analysis techniques, evaluating and comparing their ability to identify tree bulk-insulation crossing, the precursor of failure. Two PD measurement systems were used simultaneously-one to capture the sequence of PD pulses and another to acquire PD pulse waveforms-and four PD analysis techniques were deployed. Phase-resolved PD (PRPD) and pulse sequence analysis (PSA) identified tree crossing; however, they were more sensible to the AC excitation voltage amplitude and frequency. Nonlinear time series analysis (NLTSA) characteristics were evaluated through the correlation dimension, showing a reduction from pre- to post-crossing, and thus representing a change to a less complex dynamical system. The PD pulse waveform parameters had the best performance; they could identify tree crossing in epoxy resin material independently of the applied AC voltage amplitude and frequency, making them more robust for a broader range of situations, and thus, they can be exploited as a diagnostic tool for the asset management of high-voltage polymeric insulation.

Suppressing charge injection and preventing the extension of electrical trees of polymer-based composites through two-dimensional metal–organic frameworks nanosheets

Electrostatic capacitors have become an essential enabling technology in electronics and electrical power systems. The utilization of composite dielectric has significantly improved the discharged energy density (Ud). Nevertheless, optimizing the compatibility between fillers and polymers to further achieve miniaturization, lightweight, and integration remains a significant challenge. In this work, a novel composite film composed of two-dimensional (2D) metal–organic framework [Ni3(OH)2(1,4-benzenedicarboxylate)2-(H2O)4]⋅2H2O (2D Ni-MOF) nanosheets and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) have been prepared. 2D Ni-MOF nanosheets possess superior interface area and dispersibility, which could interact well with the polymer matrix to form high-quality composite films. Meanwhile, Ni-MOF nanosheets act as ordered scattering centers to prevent the extension of electrical trees and introduce deep trap energy levels to trap the generated carriers. Remarkably, the ultralow content of 2D Ni-MOF nanosheets (∼0.25 wt%) synergistically improves dielectric constant and breakdown strength, thus achieving highly Ud of 21.16 J/cm3 at 600 MV/m. Such a simple, environmentally friendly, and mass-producible preparation process explores a promising new paradigm for high-performance electrostatic capacitors.

Electrical degradation dynamics of glass fiber-reinforced epoxy composites considering different fiber orientation

Glass fiber-reinforced epoxy composites (GFREC) have been widely used in the mechanical components of electrical and electronic devices because of its lightweight, high mechanical strength, excellent electrical insulation properties and corrosion resistance. The research on the electrical degradation dynamics of GFREC is of great significance to avoid the failure of electrical insulation system. Herein the composite samples were prepared using vacuum impregnation technique. The electrical degradation morphology and statistical characteristics (electrical tree length, width, expansion coefficient, breakdown probability and breakdown time) were discussed. The results show that the electrical tree will develop towards the fiber side and grow rapidly along the interface in GFREC. The interfacial orientation will affect the growth characteristics of electrical tree. When the angle between the glass fiber and the needle electrode is 45°, the composite material has relatively superior tree propagation resistance performance, lower breakdown probability and higher breakdown time. The effects of electric field distribution on the tree initiation and electrical degradation are discussed, which reveals the relationship between the electrical degradation and the fiber orientations. The electric field-driven tree growth (FDTG) model is used to explain the growth of trees along interfaces with different orientations from the perspective of dynamic electric field distribution.

Evaluation of space charge distribution under a divergent electric field by combining experiment and simulation

Under divergent electric fields, space charge is locally accumulated and controls the initiation and evolution of insulation degradation phenomena, such as electrical trees and partial discharge. The space charge profiles provide crucial information for evaluating the insulation performance. However, the existing space charge measurement methods are not directly applicable under divergent electric fields. In this paper, a method combining experiment and simulation is proposed to evaluate the space charge distribution around a needle electrode. An experimental system is established, and the acoustic waves are measured with a hydrophone placed on the symmetric axis. In order to obtain the space charge distribution, the space charge injection process is simulated using the bipolar charge transport model to reduce the freedom of the distribution. Then, calculations are made for the acoustic waves produced by the simulated space charge distributions. Using the signals without the contribution of internal charge, a correspondence between the experiment and simulation is established. For each experimental signal, the matched calculated signal is obtained using the zero-crossing time as a reference. A perfect match between them is presented after the amplitude compensation, which indicates that the space charge distribution in the experiment is identical to that in the simulation model.

Layer-Controlled Perovskite 2D Nanosheet Interlayer for the Energy Storage Performance of Nanocomposites.

Polymer-based nanocomposites are desirable materials for next-generation dielectric capacitors. 2D dielectric nanosheets have received significant attention as a filler. However, randomly spreading the 2D filler causes residual stresses and agglomerated defect sites in the polymer matrix, which leads to the growth of an electric tree, resulting in a more premature breakdown than expected. Therefore, realizing a well-aligned 2D nanosheet layer with a small amount is a key challenge; it can inhibit the growth of conduction paths without degrading the performance of the material. Here, an ultrathin Sr1.8 Bi0.2 Nb3 O10 (SBNO) nanosheet filler is added as a layer into poly(vinylidene fluoride) (PVDF) films via the Langmuir-Blodgett method. The structural properties, breakdown strength, and energy storage capacity of a PVDF and multilayer PVDF/SBNO/PVDF composites as a function of the thickness-controlled SBNO layer are examined. The seven-layered (only 14nm) SBNO nanosheets thin film can sufficiently prevent the electrical path in the PVDF/SBNO/PVDF composite and shows a high energy density of 12.8 J cm-3 at 508 MV m-1 , which is significantly higher than that of the bare PVDF film (9.2 J cm-3 at 439 MV m-1 ). At present, this composite has the highest energy density among the polymer-based nanocomposites under the filler of thin thickness.

Effects of Gradient Magnetic Field on Electrical Tree Growth Characteristics in Silicone Rubber

In this article, the positive and inverse gradient symmetric magnetic field and the asymmetric magnetic field with different angles are constructed. The influence of the magnetic field distribution on the electrical tree characteristics is investigated. The experimental results show that both the positive and the inverse gradient magnetic fields can promote the deterioration of the electrical trees. Under the symmetrical <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta 1.5$ </tex-math></inline-formula> T gradient magnetic field, the tree initiation probability increases from 55% to 100%, and the breakdown time decreases by 67.5%. Under the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta $ </tex-math></inline-formula> -1.5 T, the tree initiation probability increases from 55% to 75%, and the breakdown time declines by 57.5%. The morphology of the electrical trees is largely affected by the distribution of the magnetic field. The dispersion of the degradation channels increases with the enhancement of the magnetic field. Under the asymmetric magnetic field, the deterioration process is also accelerated, with the increase of the magnetic field. More importantly, the growth direction of the electric trees shifts to the side with the higher magnetic flux density. The magnetic field enhances the local electric field and promotes the occurrence of partial discharge by exerting a deflection force on the charges. Consequently, the growth rate and morphology of electrical trees are affected.

Electrical Treeing Phenomena in Two-Layer Silicone Gel with Different Crosslinking Degrees and its Dielectric Strength

Silicone gel is widely used to encapsulate power modules, and improvement of its dielectric strength has been required. So, the purpose of our research is to improve dielectric strength of silicone gel encapsulant, and we focus on crosslinking degree of silicone gel. Previous studies have shown that growth mechanism of electrical tree changes with crosslinking degree of silicone gel. This suggests the possibility that the presence of the interface by different crosslinking degrees inhibits the tree growth. In this paper, we have investigated the tree growth and breakdown characteristics in silicone gel - crosslinking degrees graded layer materials. The interfaces in our study are arrange as being vertical to the line of electric force. Consequently, it was clarified that barrier effect of interface by different crosslinking degrees and the relaxation of electric field in low crosslinking degree region retards on tree growth, which improves the dielectric strength.

Mitigation of electrical treeing at high temperature in nano‐SiO2 doped epoxy resin

AbstractIn this work, electrical tree inception and ageing experiments were conducted to investigate the initiation, growth characteristics and structural characteristics of electrical trees of nano‐SiO2 doped epoxy resin at different temperatures. Electrical tree inception and ageing experiments were conducted using power frequency voltage of 15 kV at 20, 40, 60, 80 and 100°C. Electrical breakdown properties of epoxy resin were investigated by applying AC and DC voltages at 20, 40, 80 and 120°C. There are two major observations based on the experimental results. With the increase of temperature, the morphological structure of the electrical trees changes from dendritic to plexiform, the tree inception voltage decreases and the growth rate of the electrical trees increases obviously. Either under DC or AC voltage, the breakdown strength of the nano‐SiO2 doped epoxy resin decreases as the temperature increases. Finally, based on the space charge and the trap level measurement, the mechanism of the electrical tree propagation and breakdown in nano‐SiO2 doped epoxy resin was analysed.

Effect of charge transport on electrical degradation in polypropylene/organic molecular semiconductor composites for HVDC cable insulation

Polypropylene is considered to be the material of choice for environmentally friendly high voltage direct current cable insulation. The high power transmission of electrical energy exposes insulating materials to high temperatures and electric fields, resulting in the degradation of material properties. This paper reports that organic molecular semiconductors with strong electron affinity can effectively modulate electrical properties of polypropylene. The charge injection and transport process are analyzed by considering a combination of relations describing various conduction models in dielectrics, including the Richardson–Schottky (RS) emission and the hopping conduction. Based on the performed experiments, a modified Wiesmann–Zeller (WZ) model is proposed to simulate the electrical treeing process of polypropylene. The electrical treeing results are well verified with the simulation results, which offer a valuable tool for further analysis of the effect of intrinsic barrier height, hopping distance, and activation energy on the electrical degradation in the material. This work provides an insightful analysis of multiple charge transport mechanisms affecting the electrical degradation of the polymer, which is crucially essential for the rational design of high-performance insulation materials.

Direct Observation of Local Dielectric Conductive Paths and Their Dynamics in the Degraded Multilayer Ceramic Capacitors

Scanning thermal microscopy is used to perform direct imaging local conductive path and dynamic behaviors in the degraded multilayer ceramic capacitors (MLCCs) due to the thermal conductivity difference between the dielectric layers and the conductive regions. For local conductive path, its electrical tree dynamic growth behaviors under the dc bias on and off state are clearly obtained in the thermal image. Such phenomena reveal that space–charge‐limited current mechanism dominates in local conductive path region of MLCCs. The results give a strong demonstration of scanning thermal microscopy as a powerful tool for imaging local conductive behavior in MLCCs, which provides us a direct, unique view to clarify local breakdown mechanism and enriches our insights in the insulation resistance degradation and the reliability of MLCCs.

Bilayer-structured nanocomposites with ultrahigh energy density and large discharge efficiency at high temperature via balancing energy storage and thermal conductive layers

Dielectric polymers are good candidates for electrostatic energy storage due to their large breakdown strength (Eb) and high reliability, but they cannot be capable of working efficiently at high temperature. Here, we have designed and developed (Pb0.97La0.02)(Zr0.93Sn0.03Ti0.04)O3 (PLZST) antiferroelectrics (AFEs)@dopamine (DA)/polyetherimide (PEI)-Al2O3/PEI bilayer nanocomposites. Bilayer-structured configurations combine advantages of orthorhombic PLZST AFEs with large maximum electric displacement (Dmax) and low remnant electric displacement (Dr) at elevated temperature, and Al2O3 with high thermal conductivity, which can make nanocomposites possess narrow high-temperature electric displacement-electric field loops. Large dielectric difference of PLZST@DA/PEI and Al2O3/PEI layers causes the reapportionment of the applied electric field at the interfaces of adjacent layers, which inhibits growth of electrical trees and reduces electrical conduction loss, thus resulting in improved Eb. Consequently, benefited from high Eb of 4570 kV/cm, low Dr of 0.89 μC/cm2, and large Dmax-Dr of 4.75 μC/cm2, the bilayer nanocomposite exhibits not only a large charge–discharge efficiency of 73% but also an ultrahigh discharged energy density of 10.27 J/cm3 at 150 °C, which far better than those of recently reported polymer composites. Finite element simulations on the evolution behavior of electrical trees and dissipation of Joule heat further confirm the rationality of the designed bilayer structure.

Microcapsule-Based Autonomous Self-Healing of Electrical Damage in Dielectric Polymers Induced by In Situ Generated Radicals.

Dielectric polymers are playing important roles in electrical and electronic industries. However, aging under high electric stress is a main threat to the reliability of polymers. In this work, we demonstrate a self-healing method for electrical tree damage based on radical chain polymerization, which is initiated by in situ radicals that are generated during electrical aging. Acrylate monomers contained in microcapsules will be released and flow into hollow channels after the capsules are punctured by electrical trees. Autonomous radical polymerization of the monomers will heal the damaged regions, which is triggered by radicals resulting from polymer chain scissions. After optimizing the healing agent compositions by evaluating their polymerization rate and dielectric properties, the fabricated self-healing epoxy resins showed effective recovery from treeing in multiple aging-healing cycles. We also expect the great potential of this method to heal tree defects autonomously without the need to switch off operating voltages. This novel self-healing strategy will shed light on building smart dielectric polymers with its broad applicability and online healing competence.

A computational modelling of carbon layer formation on treeing branches

Polymeric insulation employed in electrical power industry undergo irreversible and unpredictable ageing due to partial discharges (PDs). In particular, the dielectric properties of polyethylene (PE) used in high voltage cables are often compromised by the formation of electrical trees. In this work we assume that the propagation of treeing channels involves the injection of carbonic material into the gas interacting with the surface of the defect. Experimental characterizations proved that, in certain conditions, disordered graphitic carbon can form in some areas of the electrical trees, thus increasing surface conductivity and inhibiting PDs. The chemical mechanism involved in this process is yet to be clarified. Here we propose a model for this process. By means of a series of molecular dynamics simulations, we show how the chemisorption of gaseous molecules on a PE surface can lead to a bidimensional carbonic structure. The characterization of the density of states of such systems suggests that the presence of pure carbon adsorbed on the polymer causes an increase in surface conductivity.

Electrical treeing and luminescence in a polymer p-n junction under extreme reverse bias

A polymer p-n junction was subjected to an extreme reverse bias, up to 1000 V, to investigate its reverse bias characteristics and possible breakdown. A full breakdown did not take place even after more than 2 h of continuous stress. Electrical treeing, a pre-breakdown phenomenon commonly encountered in insulating polymers, was instead observed. The electrical treeing was accompanied by intense light emission, and incurred significant material loss in the polymer film, the latter which was attributed to hot electron bombardment. The impact by hot electrons also caused light emission from the luminescent conjugated polymer. The polymer junction was realized via in situ redox doping reactions in a planar solid-state light-emitting electrochemical cell made from a polymer mixed conductor. The cell was activated with a forward bias and subsequently cooled to freeze the p-n junction. The reverse bias of a doped polymer junction brings interesting opportunities to study the electrical treeing phenomenon, the luminescent conjugated polymer and the still lacking reverse-bias characteristics of an organic p-n junction.