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Epoxy insulators in gas-insulated power apparatus are subjected to the combined effects of electrical and mechanical loads. In this work, a simulation model is built based on the energy theory to explore the electrical tree growth of epoxy resin under tensile and compressive stresses. With increasing AC voltage, the electrical tree growth is promoted, exhibiting a morphology with more branches. Tensile stress accelerates the electrical tree growth, while proper compressive stress has the opposite effect. However, when the compressive stress exceeds a certain value, electrical tree growth is promoted again. When the mechanical stress is vertical to the needle electrode, these effects primarily impact the length of the trees. Conversely, in parallel cases, mechanical stress mainly affects the width of the electrical trees. Filler doping play the role of obstacles as well as enhancing the electric field concentration, the electrical tree growth is firstly inhibited and then promoted as the doping content increases. The electrical tree morphologies of simulation and experiment are in good consistency, proving the reasonability of the simulation model. [ABSTRACT FROM AUTHOR]

Dielectric materials are foundational to our modern-day communications, defense, and commerce needs. Although dielectric breakdown is a primary cause of failure of these systems, we do not fully understand this process. We analyzed the dielectric breakdown channel propagation dynamics of two distinct types of electrical trees. One type of these electrical trees has not been formally classified. We observed the propagation speed of this electrical tree type to exceed 10 million meters per second. These results identify substantial gaps in the understanding of dielectric breakdown, and filling these gaps is paramount to the design and engineering of dielectric materials that are less susceptible to electrostatic discharge failure. [ABSTRACT FROM AUTHOR]

The partial discharge (PD) analysis method has remained one of the key technologies for cable condition assessment to diagnose cable operation state. In this paper, two power cable status evaluation methods are proposed based on electrical tree growth and data association rules, respectively. First, in the field of molecular thermodynamics, based on the law of energy conservation of electrical tree growth, this paper studies the relationship between PD signals in cable insulation and the length of the electrical tree. By using the length of electrical tree growth to calculate the probability of PD failure of cables, a cable state assessment method based on electrical tree growth is proposed. Second, for power cable status evaluation, the strong concealment characteristics of cables result in a long power supply recovery time and a serious threat to power supply reliability. Therefore, a group of comprehensive cable status parameters are set by analyzing the status parameters based on data association rules. The comprehensive state of power cables is divided into four elements, and the weight coefficients of cable states are calculated based on data association rule methods. Finally, the simulation results verify the effectiveness and accuracy for the two different power cable status evaluation methods. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Cross-linked polyethylene (XLPE), the most commonly used insulation system in underground cables, and gas-insulated substations are often subjected to high-voltage stresses of time-varying nature. Lightning overvoltages, fast transients, very fast transients, and AC voltages are the most common overvoltages striking the insulation. These stresses cause degradation of insulation, which is called treeing, and are considered as the major cause of the reduction in life of the insulation. The purpose of our work is the development of a stochastic model to inspect the spread of electrical trees under lightning impulse voltage. A numerical solution of Poisson's equation in the material is obtained first. The electric field distributions obtained on the surface of the material are used to compute the path of tree propagation by developing a program based on a probabilistic approach. The key parameters of the damage are evaluated by plotting histograms of the results obtained from many iterations. A statistical analysis of the obtained results has also been done, and the model has been found efficient. [ABSTRACT FROM AUTHOR]

The article titled "Power Cable Status Evaluation Method Based on Electrical Tree Growth and Data Association Rules" has been retracted by the journal Processes. The authors of the study raised concerns about major scientific limitations and errors in their work. The Editorial Board and the authors agreed that these limitations and errors compromise the validity of the findings, leading to the decision to retract the publication. The retraction was approved by the Editor-in-Chief of the journal. [Extracted from the article]

Partial discharge (PD) induces degradation in filament-like channels of electrical trees (ETs), leading to a significant reduction in material strength and lifespan. To enhance the assessment of insulation status in polyethylene (PE) based on PD characteristics during ET growth, a dynamic health monitoring evaluation model is developed. This model enables early critical warnings for insulation states. Findings reveal that during the rapid growth phase of ETs, the Dynamic Health Index of PE fluctuates within the range of [62%, 11%], with a failure rate reaching 54.30%. The transition point from the retardation stage to the rapid growth stage is identified as the critical warning threshold for insulation failure based on PD characteristics during ET growth. Utilizing linear fitting slopes of apparent discharge magnitude and ultra-high frequency amplitude in the time domain, the frequency bandwidth of ultra-high frequency signals, and average energy distribution facilitates effective differentiation between the retardation and rapid growth stages of ETs. This work establishes a robust foundation for developing an intelligent insulation state evaluation system, thereby enhancing the reliable operation of insulation systems. [ABSTRACT FROM AUTHOR]

Thermal aging causes electrochemical changes in a polymeric dielectric, instigating the formation of microvoids and oxidized carbonaceous layers. These voids increase in number and size with aging time. This and other chemical changes result in weight reduction, change in dielectric constant of the sample, and affect electrical tree growth inside the polymer. The current work aims to understand the electrochemical changes in epoxy resin due to thermal aging and their effect on electrical tree growth. Physicochemical analysis is performed using field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). The formation of carboxyl groups and voids encourages space charge accumulation resulting in local field enhancement. Inception of trees occurs in the regions of high field stress. Carbonization of tree tubules due to aging makes the walls conducting and promotes tree growth. Furthermore, thermal aging leads to gap formation and carbonization at the metal–dielectric interface, which also affects the nature of electrical tree growth in aged insulation. [ABSTRACT FROM AUTHOR]

The effect of adding electron‐buffering voltage stabilizer (1 wt% m‐aminobenzoic acid) on the electrical tree‐induced degradation of cross‐linked polyethylene (XLPE) was studied at high temperatures. The electrical tree inception, growth, and partial discharge characteristics were simultaneously analyzed at 30, 50, and 70°C, respectively. The results showed that the growth rate of electrical tree and partial discharge activity as well as the discharge repetition rate, significantly increased with the temperature for pure XLPE and XLPE stabilizer blend. However, the XLPE stabilizer blend exhibited higher tree inception voltage, lower tree growth rate, and partial discharge activity than pure XLPE. The addition of the voltage stabilizer reduced the carbon layer thickness in the tree channel and offered higher resistance to the electrical tree growth. Surface potential decay demonstrated that the voltage stabilizer addition decreased the trap energy level, but increased the shallow trap density of the XLPE. Quantum chemical calculations showed that the voltage stabilizer enhanced the ability to buffer the high‐energy electrons in the XLPE. A high‐energy electron buffering mechanism was also proposed to explain the effect of voltage stabilizer on the electrical treeing and partial discharge activity in the XLPE at high temperatures. [ABSTRACT FROM AUTHOR]

of this work proposed a mathematical model and three types of solutions for conductive electrical tree growth in dielectrics. In this section, experiments are presented using a cone-plane electrode and nanosecond voltage pulses to investigate the electrical tree growth in polymethyl methacrylate (PMMA) samples. The results show that the tree length ${l}$ is proportional to the tangent of the pulse number ${N}$ under high-voltage pulses but ${l}$ approaches a limiting length under relatively low voltage pulses. These results agree with those predicted from the model. Therefore, the mathematical model for conductive electrical tree growth in dielectrics is verified. [ABSTRACT FROM AUTHOR]

By postulating that the electrical tree growth rate ($\textit {dl} / \textit {dt}$) is proportional to the rate of destruction due to partial discharges (PDs), a mathematical model for conductive electrical tree growth in dielectrics is put forward. Three types of solutions are presented for this mathematical model, which predict three cases of electrical tree growth in dielectrics; namely, the development case, the stagnation case, and the transition case. The physical nature of the three cases is presented, which is the comparison of the local field (${E}_{\text {loc}}$) and the critical field (${E}_{c}$) caused by the space charges due to a PD. When ${E}_{\text {loc}} > {E}_{c}$ , $\textit {dl}/\textit {dt} > {0}$ and the electrical tree develops. When ${E}_{\text {loc}} < {E}_{c}$ , $\textit {dl}/\textit {dt} < {0}$ and the electrical tree stagnates. When ${E}_{\text {loc}} = {E}_{c}$ , $\textit {dl}/\textit {dt} = {0}$ and the electrical tree stays at a transition state. [ABSTRACT FROM AUTHOR]

The growth and partial discharge (PD) characteristics of electrical tree in crosslinked polyethylene (XLPE) needle-plate electrode samples are investigated under constant dc voltage at different temperatures. The results reveal that under the experimental conditions, the electrical trees are all branch trees, and the discharge intensity fluctuates with time. The increase in temperature promotes the growth of electrical tree as well as the intensity of PD. Five texture features of PD ${q}$ - ${t}$ histogram are extracted and used as the elements to evaluate the insulation status of XLPE needle-plate electrode under dc voltage based on the gray correlation degree. The area ratio and lateral tree ratio of electrical tree is extracted based on the gray histogram. The lateral tree ratio increases with the increase in temperature, which is closely related to the intensity of PD. Gray correlation theory is used to evaluate the status of XLPE insulation. The results reveal that the evaluated insulation statuses based on the gray correlation degree are the same as those based on experience, which indicates that the proposed diagnosis method of XLPE insulation status based on the gray correlation degree with ${q}$ - ${t}$ histogram texture features as elements has a high accuracy. [ABSTRACT FROM AUTHOR]

One of the main phenomena that contributes to the non-success of cable insulation made of cross-linked polyethylene (XLPE) is electrical treeing. To improve the XPLE cable insulation, the use of nanofiller has been introduced. Adding the nanofiller in the based composite offers better cable lifetime and resistance to deal with the cable failure. One of the potential nanofillers that can increase the insulation performance of XLPE cable is silica nanofiller. To this extent, the studies on silica nanofiller in XLPE are focusing on the impulse breakdown strength, dielectric loss, permittivity, space charge, alternating current (AC), and partial discharge. The studies reveal that the dielectric properties of the XLPE nanocomposite have significant improvement. Therefore, this work investigates the effect of various concentrations of silica nanofiller in XLPE composite as electrical tree inhibitor. The concentrations of silica nanofiller in XLPE were 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, 1.5 wt%, and 1.75 wt%. The silica nanofillers have 96%-99% purity, 20-30 nm sizes and the shapes are spherical. As a result, the XLPE composite containing 1.5 wt% silica nanofiller demonstrate higher tree inception voltage and detaining the tree propagation speed, which could be considered as an inhibitor medium of electrical tree growth. [ABSTRACT FROM AUTHOR]

Polymer nanocomposites have been widely exploited in different engineering applications like high‐voltage engineering application. This is mainly associated with the ease at which they can be tailored toward specific applications. In the present work, alumina‐ and organoclay‐based nanofillers were separately incorporated in cross‐linked polyethylene (XLPE). Different formulations of the XLPE nanocomposites were fabricated with the help of an internal mixer after which they were molded to produce test specimens. The effect of nanofiller variation (0.25 and 0.5 wt% loading) on the electrical treeing phenomenon is reported herein. It was observed that the incorporation of 0.5 wt% alumina significantly enhances the tree inception voltage. In addition, compared to 0.25 wt% alumina‐, 0.25 and 0.5 wt% organoclay‐filled XLPE nanocomposites, 0.5 wt% alumina was found to lower the length and growth of the electrical tree, as well as the expansion coefficient of the XLPE nanocomposite. [ABSTRACT FROM AUTHOR]

Despite the importance of the electric treeing phenomenon, a detailed understanding of the mechanisms is still lacking. Therefore, in this study, we analyze electrical trees as composites of very thin 3D tubes between a treeing needle and a plate electrode and calculate the energy-release rate to estimate the prerequisite force required for crack propagation. This study implements a new method using an analogy between concepts of fracture mechanics and Maxwell stress to calculate the energy-release rate of line-shape elements. We obtain the energy-release rate around a volume near the treeing needle and electrical trees and compare it before and after tree growth. To verify the results, path independence is tested, and it is found to be conserved only when the path does not include the inside surface of the treeing needle. In addition, the results show that the 1D-growth of the tree has little effect on the value of the energy-release rate, and therefore, it is necessary to predetermine the order and an adequate calculation area. Moreover, the value of the energy-release rate is found to be proportional to the square of the applied voltage. These results imply that calculating the energy-release rate exhibits a potential to mathematically evaluate the growth of electrical trees. [ABSTRACT FROM AUTHOR]

In this paper, the electrical tree growth characteristics of silicone rubber (SiR) are studied under impulse superimposed on DC voltage and a temperature gradient (TG) field. The temperature difference between high voltage (HV) and ground (GD) electrodes varies from -90 to 90 °C resulting in a TG field in the samples. The electrical tree morphology, length and accumulated damage are discussed. Experimental results show that with the increase of absolute value of TG, the electrical tree length and accumulated damage continue to increase, and the tree morphology turns from branch-like tree to bush-branch hybrid tree. The positive TG has a more obvious acceleration effect on the tree growth than does the negative TG. Under the TG, the temperature dependent carrier transport results in space charge accumulation and electric field distortion at low temperature side, which is the main reason for the different growth characteristics of electrical tree. Besides, the accumulated damage of DC superimposed with heteropolar impulse voltage is larger than that under DC superimposed with homopolar impulse voltage. It is concluded that the temperature gradient and impulse superimposed on DC voltage promote the electrical tree growth of HVDC cable accessories insulation. [ABSTRACT FROM AUTHOR]

Initiation and growth of electrical trees can be linked to the development of partial discharge (PD) activity within an insulation system. We present in this paper changes in PD characteristics that appear during the electrical tree growth in a crosslinked polyethylene (XLPE) based insulation at different stages of its degradation. A DC voltage with superimposed high frequency component is applied to the material specimens for the detection and characterization of PD traces observed during the degradation. The method is flexible and offers a possibility to vary DC voltage level independently of the superimposed disturbance, its frequency and rise time, including rapid voltage steps. It also enables to create a phase-locked phase-resolved PD pattern (PRPD) facilitating various analyses of the characteristics. It is postulated that measurements of the changes in PD characteristics found under such excitations can be used for performing diagnostics and prognostics of material condition. This approach also implies new possibilities to evaluate different material concepts. [ABSTRACT FROM AUTHOR]

In this paper, a high-field magnet experimental system is setup to investigate the dielectric properties of epoxy resin in the presence of high magnetic field. An asymmetric ferromagnetic electrode system is used to build gradient magnetic field and investigate its effect on tree characteristics of epoxy resin. Influence mechanisms of gradient magnetic field on dielectric properties and tree characteristics of epoxy resin are discussed through combining experimental results and simulation calculation. Experimental results show that electrical trees are more easily to be initiated and spread more rapidly in gradient magnetic field. The conductivity of the epoxy resin increases with the increase of magnetic field strength due to energy level splitting, which is analyzed through electrodynamics calculation. The relative permittivity decreases, and the dielectric loss tangents increase when high magnetic field is applied. Changes of conductivity and relative permittivity affect the charge behavior and electric field distribution in epoxy resin, resulting in the particular electrical tree. [ABSTRACT FROM AUTHOR]

This article identifies the threat to high-voltage direct current (HVDC) polyethylene insulation posed by the growth of fine electrical trees on the removal of high dc voltages. Such fine trees have been identified previously growing under ac voltages, especially in glassy epoxy resins. However, fine tree growth is distinguished as not being associated with measurable partial discharges (PDs) and so presents a particular threat to high-voltage (HV) system integrity. In the case reported, fine trees are seen to grow in low-density polyethylene (LDPE) from smaller traditional dark trees, on the removal of a negative dc voltage. The growth is seen even in the case of a relatively slow ramp down over many seconds. The growth of a fine tree leaves the system potentially vulnerable to rapid aging when re-energizing. For cable systems, the threat is that such a tree will grow and is essentially invisible to asset managers using the traditional method of PD measurement. [ABSTRACT FROM AUTHOR]

The gradient textured plate-like Na 0.5 Bi 4.5 Ti 4 O 15 (P-NBT4)/PVDF composites have been realized through tape casting process of layer by layer, which loadings are gradually decreased from bottom layer to top layer. The micro-capacitance formed by P-NBT4 particles and interfacial polarization between layers enables the composites to obtain a high permittivity. In addition, the different electric field of gradient composites will weaken the growth of electric tree between layers, as well as the P-NBT4 ceramic particles in composite films can extend the breakdown path, so that the composites will undergo premature breakdown. By comparing, the large energy storage density (16.41 J/cm3) at the relative low electric field of 350 kV/mm is obtained in the gradient textured composites. [ABSTRACT FROM AUTHOR]

In this paper, the effects of harmonic superimposed DC voltage on electrical tree growth characteristics in ethylene propylene diene monomer (EPDM) for HVDC cable accessories insulation are studied. Harmonic component with a frequency from 50 to 450 Hz is superimposed on DC voltage to analyze the mechanism of harmonic amplitude, order and voltage polarity on electrical tree growth. The results show that the electrical treeing process is promoted with an increase in harmonic amplitude and order, which brings a longer tree length and more accumulated damage to the insulation. Especially when the amplitude of harmonic component exceeds that of DC voltage, the electrical treeing processes are significantly deteriorated, resulting from the large amount of energy released by the recombination of hetero charges. The growth of electrical tree has an obvious polarity effect which tends to be branch-like tree under positive voltage and bush-like tree under negative voltage. The polarity effect may be caused by different propagation processes of electron avalanche under different voltage polarities. As harmonic order increases, the main channel of positive electrical tree is broadened, while the negative tree becomes denser with a lot of bifurcation. These results indicate that harmonic component accelerates the initiation and propagation of electrical tree by affecting the charge transport behavior under the superimposed electric field. [ABSTRACT FROM AUTHOR]

This paper presents the growth law of electrical tree in glass/epoxy resin composite (GER). Two influencing factors on electrical tree growth, the relative permittivity of the glass and the adhesion type of the interface, are investigated. An improved WZ model is built to further explore the mechanism of electrical tree growth in GER. The electric field between the needle electrode and the glass is enhanced when the relative permittivity of glass is higher than that of epoxy resin, which is responsible for the attractive effect of the glass. The breakdown strength of the interface determines the growth mode of electrical tree. When the breakdown strength of the interface is as low as the air gap, the electrical tree showed ‘linear growth’ at the interface. High breakdown strength of the interface lead to ‘step growth’ of electrical tree and prolong the growth time compared to ‘linear growth’. [ABSTRACT FROM AUTHOR]

Electrical treeing is one of the main reasons for long-term degradation of high voltage insulation especially in the cable accessory which commonly made from silicone rubber due to non-uniformly structures of the cable accessories. Recently, the combination of nanofillers with the silicone rubber matrix can reduce the possibility of the electrical treeing to grow further by changing its patterns and slow-down its propagation. However, the influences of nanofillers on the tree hindrance and its patterns are not well understood. This paper explores the influence of nanofiller on tree pattern in silicon rubber. The electrical tree patterns were characterized using fractal analysis. The box-counting method was used to measure the fractal dimension and lacunarity to obtain the structure of the tree pattern during the electrical tree growth. The structure of the electrical tree in silicone rubber nanocomposites has higher fractal dimension and lacunarity. Sample with nanofiller possesses dominant fractal dimension of tree growth compared to the sample without nanofiller. [ABSTRACT FROM AUTHOR]

Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ∼10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments. [ABSTRACT FROM AUTHOR]

This article presents a study on electrical treeing performances with its associated partial discharge (PD) and the influence of filler concentration in silicone rubber (SiR) samples which are filled with silicon dioxide (SiO2) and silicon nitride (Si3N4) as nanofillers for electrical tree growth suppression. There are many researches on electrical treeing in SiR with SiO2 nanofillers but none of the publication have reported on Si3N4 nanofillers for suppression of the electrical tree growth. In this study, the treeing experiments were conducted by applying a fixed AC voltage of 10 kV and 12 kV at power frequency of 50 Hz on unfilled SiR, SiR/SiO2, and SiR/Si3N4 nanocomposites with different filler concentrations by 1, 3, and 5 weight percentage (wt%) and the treeing parameters were observed with its correlated PD patterns. The outcome from this study found that the SiR/Si3N4 nanocomposites were able to withstand the electrical treeing better than the pure SiR or SiR/SiO2 nanocomposites. Furthermore, the increase in filler concentration improved the electrical tree performances of the nanocomposites. This finding suggests the Si3N4 can be used as filler in polymeric insulating materials for electrical tree inhibition. Meanwhile, the PD activity shows increment when the tree progresses thereby indicating correlation in both parameters which can be as key parameter for monitoring unseen treeing in non-transparent samples. [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]

This paper reports observations of electrical tree growth by X-ray computed tomography (XCT) in multiple steps. Growth between times of observation has been obtained by subtraction of an image from the subsequent one. Partial discharge (PD) signals were also recorded during tree growth. It is determined that not all PDs support tree extension. By comparing the energy released by PD and the vaporization energy of newly grown tree channels, it is found that only the PDs that reach the tree tips can contribute to the growth of tree length. In such cases, the efficiency of energy conversion was very high, whereas PD resulting in increases in tree channel diameter had a much lower conversion efficiency. Nano XCT images reveal many finer branches along the trunk of the main tree and in the front of the main channel tree tips. It seems that those finer branches led to tree growth, but were not directly associated with local PD. The new imaging techniques have shown great value in resolving the relationship between partial discharge activity and electrical tree growth, providing a platform for deeper understanding of the tree growth process, and interpretation of partial discharge measurements. [ABSTRACT FROM AUTHOR]

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 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]

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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]

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]

• The tree structures, inception probability, growth rates at 77 K were studied. • The Raman spectra of the degradation products within the tree channels at room temperature and 77 K were analyzed. • The effects of oxygen and graphitic carbon on the tree characteristics of epoxy resin were analyzed. The insulation structure which is mainly composed of epoxy resin is a key part of the superconducting magnet system. Due to the special operating environments, superconducting coils must address the challenges of cryogenic liquids and various kinds of high voltages. In this paper, the electrical tree characteristics of epoxy resin at 77 K were investigated. An experimental setup with a partial discharge (PD) device and a cryostat filled with liquid nitrogen (LN 2) was set up. Optical microscopy and confocal Raman spectrometer were applied to investigate the accumulation of degradation products within the tree channels. Each group of samples was tested at a range of AC voltages (50 Hz) from 8 kV rms (root mean square) to 20 kV rms and the PD experiments were carried out at both room temperature and 77 K. The results indicated that the color of electrical trees was darker and the tree structures were more complicated at 77 K compared with those at room temperature. The weak reaction between oxygen and the degradation products within the tree channels was responsible for the darker tree channels at 77 K. The density of electrical trees increased and the damage to materials caused by electrical trees was more serious at 77 K. Meanwhile, the tree inception probability decreased and the tree growth was inhibited obviously at 77 K. It's also revealed that the degradation products within the tree channels were confirmed to be graphitic carbon based on the corresponding Raman spectra. The size of graphitic domains within the tree channels at 77 K was more than 2 times of that at room temperature, which contributed to the tree growth acceleration in the later growth stage at 77 K. [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]

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. [Display omitted] • The dielectric properties difference between crystal and amorphous regions results in field distortions at its interface. • The α-nucleating agent refines crystal structure, and the high continuity electric field distortion is thus suppressed. • In the PP-α, uniform electric field distribution suppresses PD, and the resistance of ET is thus significantly improved. • The densely arranged spherocrystals limit the penetrating ET channel growth, and the breakdown strength is thus enhanced. [ABSTRACT FROM AUTHOR]

• Formation of the electrical tree structure due to PD in XLPE insulation under sinusoidal high voltage. • Analyses of the tree growth characteristics with presence of a void at different applied voltage. • Identify the degradation of XLPE insulation and corresponding images of PRPD pattern during electrical tree growth instances. • PRPD image feature extraction, PD detection, and classify the level of insulation degradation using CNN and pre-trained NN. • Further, analyze the electric field strength in XLPE insulation in the presence of a void using COMSOL multiphysics. Partial discharge (PD) activities due to void and crack in high-voltage cross-linked polyethylene (XLPE) cables deteriorate insulation properties. The existence of void amplifies the electric field stress caused by variations in the dielectric constant between air and the insulation that is all around it. PD activities on such XLPE insulation under high voltage stress are responsible for the growth of electrical- trees. Deep learning techniques have given opportunities for automated feature extraction and classification of the severity level of discharge activity during tree growth. In this work, the XLPE sample specimens are aged at different applied AC voltages, and the respective PD signals are continuously saved for every 500 cycles. PD signals are used for three different stages of pattern classification including fast-growth, slow-growth, and breakdown phases during the growth of an electrical tree with the help of a convolutional neural network (CNN) and pre-trained neural network (VGG, Inception, ResNet, Xeception, DenseNet, NasNet) architectures. For each stage, 500 phase-resolved PD (PRPD) pattern images are obtained for training the neural network (NN) model. The performance of CNN with Adam optimizer obtains 98.44% total accuracy, whereas Densenet201 with Nadam achieves 99.78%. [ABSTRACT FROM AUTHOR]

Cross-linked polyethylene (XLPE) may be operated under repetitive pulse voltage and low temperature environment. In recent years, many influencing factors to the treeing process, which is a main reason for the aging of XLPE have been researched. However, the tree behavior under repetitive pulse voltage and low temperature in XLPE is still not clear. In this paper, the tree behavior under repetitive pulse voltage and low temperature was investigated and analyzed. A positive pulse voltage with the amplitude of 12 kV and the frequency of 300 and 400 Hz was applied on the needle-plate electrodes. The experimental temperature was set to 30, -30, -90 and -196 ?C. Four typical morphologies of the electrical trees are observed, including bush, branch, branch-pine and stagnated tree. It is revealed that low temperature has an obvious effect on tree structures and it can reduce the tree growth rate and extend the time to breakdown. It is also revealed that the tree inception probability becomes lower with the decrease of temperature. With the increase of pulse frequency, the tree growth rate, the fractal dimension and the number of tree channels increases and the tree inception probability becomes higher, but the morphologies of electrical trees stay the same. [ABSTRACT FROM PUBLISHER]

Reliable power cable systems are necessary for efficient electrical power transmission over a long distance. However, electrical trees could grow in the cable systems, such as the cable insulation, joint and termination, eventually leading to insulation breakdown, therefore requiring serious attention. This paper concentrates on the effects of single-layer and double-layer barriers, which are improved techniques for suppressing electrical tree growth in silicone rubber with ethylene-vinyl acetate (EVA) as the barrier material. A simulation work using the finite element method was carried out on silicone composite with the barrier to study the influence of the barrier distance on the electrical tree propagation due to high electric field stress using a needle-plane electrode configuration. In addition, electric field distributions in silicone rubber with single-layer and double-layer barriers were investigated through experiment and simulation. The results illustrated that the electrical tree growth in the barrier region had been slowed down by the double-layer barrier, and the time to the breakdown had increased significantly. [ABSTRACT FROM AUTHOR]

This work investigates the electrical tree growth of epoxy resins. Repetitive bipolar square wave voltage is applied to the epoxy resins and different parameters rise time, frequency, and duty cycle are studied. The frequency range of repetitive square voltage ranges from 50 Hz to 20 kHz, the rise time ranges from 70 ns to 620 ns, and the duty cycle ranges from 5 to 90%. The results reveal that the initiation of the electrical tree is more rapid under the influence of high voltage frequency having a shorter rise time. Moreover, the electrical tree growth is faster under the impact of a shorter rise time with a duty cycle greater than 10%. The rise time mainly affects the initiation process of electrical trees. In comparison, the primary influence on the electrical tree initiation and its growth is attributed to the impact of the frequency applied. • Power electronics devices made up of Silicon carbide (SiC) can operate power equipment at high voltage and high frequency. Therefore, the insulation of these power equipment faces some serious stresses coming from these devices. • Epoxy resin is widely used insulating material for such power equipment and electrical tree is one of the main sources for aging of these materials. • Electrical tree initiation and growth is different and more severe at repetitive square wave voltage than sinusoidal or DC voltage. • Pin-plane epoxy resin is synthesized to investigate the electrical tree growth under different bipolar square voltage parameters such as rise time, frequency and duty cycle. • The mechanism of electrical tree initiation and tree growth is discussed in details. [ABSTRACT FROM AUTHOR]

• Nano fillers with silane treated surface improve dielectric breakdown times associated with electrical treeing. • Nano-filler dispersion critical to performance. • Partial discharge characteristics more bursty in better dispersed material, with quiescent periods. • Maximum partial discharge magnitude depends upon non-conductive length of tree. • Tree growth controlled by conversion to conductive state at needle tip. The addition of nano-fillers has been widely proposed as a method to enhance the dielectric properties of high voltage polymeric insulation, though there are mixed reports in the literature. Here the potential of silica nano-particles to extend the time to failure specifically through resistance to electrical tree growth in epoxy resin is determined. The benefit of silane treating the nano-particles before compounding is clearly established with regard to slowing tree growth and subsequent time to failure. The growth of trees in needle-plane samples is measured in the laboratory with loadings of 1, 3 and 5 wt% nano-filler. In all cases the average times to failure are extended, but silane treatment of the nano-particles prior to compounding yields much superior results. The emergence of a pronounced inception time before tree growth is also noted for the higher-filled, silane-treated cases. The average time to failure of silane-treated 5 wt% filled material was 28 times that of the unfilled resin. The improvement in performance between the nanocomposites with untreated and treated fillers is attributed to fewer agglomerations and improved dispersion of the filler in the treated cases. Measurements of Partial Discharge (PD) indicated significant differences in PD patterns during the growth of trees in the treated and untreated cases. This distinction may provide a quality control method for monitoring materials. In particular, long periods in which PDs were not measured were observed in the silane-treated cases. Visual imaging of tree growth in the unfilled material allowed the changing nature of the tree from fine to tree to dark tree to be observed as it grew. Corresponding PD measurements suggest the dark tree is gradually becoming conductive, and that growth of maximum PD measured is dependent on the relative rates of the growth of the tree and its carbonization. X-ray computer tomography identified significant differences in average tree channel diameters (a reduction from 2.8 µm to 2.0 µm for 1 wt% and 3 wt% cases). This implies that in addition to tree length changes, evaporated tree volumes also change and may explain the change in partial discharge characteristics observed. [ABSTRACT FROM AUTHOR]

In high-voltage (HV) insulation, electrical trees are an important degradation phenomenon strongly linked to partial discharge (PD) activity. Their initiation and development have attracted the attention of the research community and better understanding and characterization of the phenomenon are needed. They are very damaging and develop through the insulation material forming a discharge conduction path. Therefore, it is important to adequately measure and characterize tree growth before it can lead to complete failure of the system. In this paper, the Gaussian mixture model (GMM) has been applied to cluster and classify the different growth stages of electrical trees in epoxy resin insulation. First, tree growth experiments were conducted, and PD data captured from the initial to breakdown stage of the tree growth in epoxy resin insulation. Second, the GMM was applied to categorize the different electrical tree stages into clusters. The results show that PD dynamics vary with different stress voltages and tree growth stages. The electrical tree patterns with shorter breakdown times had identical clusters throughout the degradation stages. The breakdown time can be a key factor in determining the degradation levels of PD patterns emanating from trees in epoxy resin. This is important in order to determine the severity of electrical treeing degradation, and, therefore, to perform efficient asset management. The novelty of the work presented in this paper is that for the first time the GMM has been applied for electrical tree growth classification and the optimal values for the hyperparameters, i.e., the number of clusters and the appropriate covariance structure, have been determined for the different electrical tree clusters. [ABSTRACT FROM AUTHOR]

This paper aims at investigating the electrical tree characteristics of isotactic polypropylene (iPP)/syndiotactic polypropylene (sPP) blends for thermoplastic cable insulation. PP blended samples with sPP contents of 0, 5, 15, 30, and 45 wt% are prepared, and electrical treeing experiments are implemented under alternating current (AC) voltage at 50, 70, and 90 °C. Experimental results show that with the incorporation of sPP increasing to 15 wt%, the inception time of electrical tree increases by 8.2%. The addition of sPP by 15% distinguishes an excellent performance in inhibiting electrical treeing, which benefits from the ability to promote the fractal dimension and lateral growth of branches. Further increase in sPP loading has a negative effect on the electrical treeing resistance of blended insulation. It is proved by DSC and POM that the addition of sPP promotes the heterogeneous crystallization the of PP matrix, resulting in an increasing density of interfacial regions between crystalline regions, which contains charge carrier traps. Charges injected from an electrode into a polymer are captured by deep traps at the interfacial regions, thus inhibiting the propagation of electrical tree. It is concluded that the modification of crystalline morphology by 15 wt% sPP addition has a great advantage in electrical treeing resistance for PP-based cable insulation. [ABSTRACT FROM AUTHOR]

• Treeing in HVDC insulation is dominated by AC ripples and less affected by DC fields. • With AC ripples, the time to tree initiation is shorter at the cathode than the anode. • Positive DC biasing promotes non-conducting tree growth, but not conducting trees. • Negative DC biasing results in a retardation on bush tree growth but not branch trees. • DC biasing influences the partial discharge characteristics, but not their magnitude. Electrical tree growth is a well-documented process leading to failure of high voltage polymeric insulation under AC stresses. However, tree growth in HVDC insulation failure is not well understood. This work considers electrical tree degradation in polymeric insulation subjected to combined AC and DC voltages. Tests in LDPE samples of needle-plane geometry yield three types of electrical trees, which grow depending on the magnitude of the AC components, irrespective of the DC voltages. In tests with an AC component of 10 kV (peak amplitude), 12 kV and 15 kV trees are distinguished by both the conductivity of tree channels and the tree shape, and are referred to as either conducting trees, non-conducting branch trees or non-conducting bush trees respectively. With 10 kV AC, tree initiation was significantly accelerated by superimposing −20 kV DC. With +20 kV DC, the incepted trees had more bifurcations, but there was no major change to tree initiation time. Space charge measurements on thin LDPE films provide a basis for understanding the difference between tree initiation with +DC and −DC voltages. The subsequent propagation of conducting trees were not influenced by biasing the 10 kV AC. With 12 kV AC and 15 kV AC, the growths of non-conducting branch and bush trees were both accelerated by positive biasing. A retardation was observed in bush tree growth with −15 kV DC. The effects of DC voltage polarity on tree morphology are different for branch and bush trees. Different relationships between PD magnitudes and tree growth were also found between branch and bush trees. The tree length determined PD magnitudes in branch trees and the evolution of PD in bush trees suggest that the additional DC stress has no impact to PD magnitudes. Nevertheless, DC bias effects on the symmetry between positive and negative discharges have been evidenced through PD analysis for bush trees. Moreover, the modifications on the shape of bush trees are believed to be associated with the changes in PD asymmetry. This work has illustrated the importance of AC ripples in the failure mechanisms in HVDC insulation. [ABSTRACT FROM AUTHOR]

Electrical tree is an important factor in the threat of the safety of cross-linked polyethylene (XLPE) insulation, eventually leading to the electrical failure of cables. Polycyclic compounds have the potential to suppress electrical treeing growth. In this paper, three types of polycyclic compounds, 2-hydroxy-2-phenylacetophenone, 4-phenylbenzophenone, and 4,4′-difluorobenzophenone are added into XLPE, denoted by A, B, and C. Electrical treeing characteristics are researched with DC-impulse voltage at 30, 60, and 90 °C, and the trap distribution and carrier mobility are characterized. It has been found that although three types of polycyclic compounds can all suppress the electrical tree propagation at different voltages and temperatures, the suppression effect of these polycyclic compounds with the same DC-impulse polarity is worse than with the opposite polarity. As the temperature increases, the suppression effect becomes weak. The energy level and deep trap density are the largest in XLPE-A composite, leading to a decrease in the charge transportation and resulting in the suppression of electrical treeing growth. Experimental results reveal that the polycyclic compound A has great application prospects in high voltage direct current (HVDC) cables. [ABSTRACT FROM AUTHOR]

In this paper, the inhibitory effect of preinjected rejuvenation liquid on trees in cross-linked polyethylene (XLPE) cables was investigated. Experimental samples were prepared by inserting needles into XLPE samples, and many equally-spaced holes existed in the outer semiconductive layer. All cable samples were divided into two groups. One sample group was treated with rejuvenation liquid, while the other group was the control group. A tree accelerated aging system was used to obtain trees in the XLPE cable samples. During the aging experiment, an impulse voltage was applied to the samples repeatedly. The micromorphologies of the two groups were observed. Based on the micromorphologies, two parameters were determined: the initiation rate of electrical trees and the average length of trees. Furthermore, the electric field distribution was simulated to analyze the initiation of electrical trees. The results indicate that an electrical tree is much harder to initiate in the pretreated XLPE cables than in the untreated cables. This phenomenon is likely attributed to the dielectrophoretic forces in the pretreated cables. Moreover, rejuvenation liquid deposited in XLPE causes a substantial reduction in the Maxwell stress of molecular chains. Rejuvenation liquid inhibits electrical tree initiation and water tree growth to a great extent. [ABSTRACT FROM AUTHOR]