Tree Growth In Epoxy Resin Research Articles

Morphological Feature Analysis of Electrical Tree Growth in Epoxy Resin Under Tensile and Compressive Stress

In this article, electrical tree morphological features in epoxy resin under tensile and compressive stress are investigated. The applied tensile and compressive stress varies from 0 to 30 MPa, whose direction is perpendicular to the needle electrode. The obtained results show that the tensile stress accelerates the tree growth, especially in the direction parallel to the tensile stress. With the increasing compressive stress, both the tree length and the corresponding accumulated damage decrease first and subsequently increase. By comparing the tree distribution in parallel and vertical directions, it is found that the tree mainly propagates to the ground electrode under the compressive stress. A tree growth curve is proposed to explain the experimental results, which reveals the defect generation and the tree propagation driven by both electrical and mechanical stresses. The growth curve can predict the tree-induced dielectric breakdown in epoxy resin under mechanical stress.

Effects of Gradient Magnetic Field on Charge Behavior and Electrical Tree Growth in Epoxy Resin

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.

Effect of Bipolar Square Wave Voltage with Varied Frequencies on Electrical Tree Growth in Epoxy Resin

An electrical tree in epoxy resin induced by a bipolar square wave voltage at varied frequencies has been investigated. The morphology, growing characteristics, fractal dimension, and initiation proportion of anelectrical tree at 10 kV for a frequency ranging from 50 Hz to 20 kHz was studied. The results show that electrical trees are all branch-like in epoxy resin. It was revealed that there are three regions in the propagation of an electrical tree: 50 Hz~2 kHz (region A), 2~4 kHz (region B), and 4~20 kHz (region C). When imposed frequencies are increased from 50 Hz to 20 kHz, the number of tree branches, growing rate, and initiation proportion increase in region A; the number of tree branches and initiation proportion increase, while the growing rate decreases, in region B; and the growing rate and initiation proportion increase while the number of tree branches decrease in region C. The electrical tree length and width increase linearly with treeing time when frequencies were lower than 2 kHz, while increasing exponentially with treeing time when frequencies are higher than 4 kHz. The dielectric power loss of epoxy resin under varied frequencies is computed by an expansion in Fourier series, and the results indicate that the dielectric power loss increases with increasing frequency. The characteristics of an electrical tree are ascribed to a synergistic effect of the dielectric power loss of epoxy resin, space charge, and partial discharge under a bipolar square wave field.

Application of the Gaussian Mixture Model to Classify Stages of Electrical Tree Growth in Epoxy Resin.

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.

The importance of particle dispersion in electrical treeing and breakdown in nano-filled epoxy resin

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.

3D XCT imaging of electrical tree growth in epoxy resin

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.

Chemical analysis of tree growth in epoxy resin using AFM-IR spectroscopy

Recent advances have allowed submicron features of electrical trees to be imaged. Until recently it has not been possible to analyze chemical aging on the same scale. Here, the use of atomic force microscopy — infrared spectroscopy (AFM-IR) to characterize chemical degradation around a needle tip, and in the vicinity of electrical tree branches grown in epoxy resin is explored. The spectral signatures associated with the degradation of epoxies during electrical treeing are successfully identified with 50 nm resolution. This allows identification of chemical degradation in unprecedented detail. The method of polishing and grinding to prepare the samples was found to remove degraded material from exposed channels. This issue was overcome by not fully exposing the tree, but bringing it close enough to the surface that the AFM-IR was able to see within the channel. A short tree examined immediately after initiation showed no chemical degradation, indicating that its growth was not a chemically active process, suggesting electromechanical fracturing is more likely. Mature channels showed no singular chemical signature. Carbonyl groups were identified in all mature channels and were particularly concentrated at the branching points studied, suggesting higher levels of degradation during branching or accumulation at points of bifurcation. It is believed that these reactions are precipitated by hot electron impacts during discharges. AFM-IR has provided unprecedented detail concerning chemical damage incurred during the treeing process and can provide vital information to inform models of aging.

DC electrical tree growth in epoxy resin and the influence of the size of inceptive AC trees

Electrical tree propagation is a precursor to dielectric failure in high voltage polymeric insulation. Tree growth has been widely studied under AC conditions, but its behavior under DC is not well understood. The aim of this work was to examine the importance of polarity and initiating defect size on DC tree propagation. Electrical tree propagation in epoxy resin under constant DC voltages of +60 kV and −60 kV was measured in samples with classical needle-plane electrodes, but with small initial trees (< 100 μm) incepted under lower AC voltages before the DC tests. Experimental results showed strong polarity dependence. In either polarity, the length of the initial AC tree had a major influence on the inception of the subsequent DC tree. The effect was attributed to the defect being influential if it is larger than the space charge injection region. As a consequence, there is a critical defect size that will accelerate failure of DC insulation dependent on space charge injection behavior of the polymer/electrode system in question — a critical finding for high voltage asset management.

Effect of pulse duration on electrical tree and breakdown process of epoxy resin in LN2

The international thermonuclear experimental reactor (ITER) project has attracted much attention in the world and its major part, namely the superconducting magnet system, uses epoxy resin as the basic insulation material. Epoxy resin needs to address the challenge of the liquid nitrogen temperature and the pulse voltage with changing duration due to the special operating environment. This paper investigates the effect of pulse duration on the characteristics of electrical tree growth in epoxy resin under low temperature. The tested samples were stressed with different pulse durations in a needle-plate geometry electrode system. The pulse duration was set to 50, 110 and 220 µs and the experimental temperature was −30, −90 and −196 °C. Fractal dimension, accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that the typical structures of electrical tree are obviously different with the variations of low temperature and pulse duration. It is revealed that the increase of pulse duration promotes the growth of electrical tree under the same low temperature. In addition, larger pulse duration may lead to a more complex tree structure, which indicates the larger value of fractal dimension and accumulated damage. Meanwhile, obtained results show that large pulse duration plays an important role in promoting the electrical tree's growth processes, including propagation and breakdown characteristics.

Characterization of tree growth in filled epoxy resin: the effect of filler and moisture contents

The effect of silica filler and moisture absorption on the structural characteristics of tree growth in epoxy resin has been investigated. The physical properties of the resin were varied by changing its filler concentration and immersed in distilled water at 50degC. The fractal dimension of the electrical tree and its relationship with filler and moisture contents were determined. The damaged area of tree in various concentrations of filler was also estimated. Furthermore, the lacunarity of tree with identical fractal dimension was calculated as a complement of fractal dimension characterization. In addition, moisture absorption and diffusion properties of the specimens used were also examined. It is considered that the filler would create such an obstruction to the tree growth both in moisture and non-moisture conditions. Under non-moisturized condition, the more filler concentration, the more obstruction would be generated, leading to the significant suppression of tree growth. Likewise, the introduction of filler brought a rise in fractal dimension due to the increase of branches. It is concluded that the existence of filler makes the tree structure more complicated by introducing obstacles to tree propagation, leading to the high fractal dimension of the tree. In addition, it was found that the fractal dimension of the tree was related to the fractal dimension of the composite material including filler particles. On the other hand, the lacunarity of "without-filler" specimen would be lower than that of in filled specimens. It is considered that the tree growth in filled specimen would have "localized-branching" at the interface between filler and resin leading to the heterogeneous inherent structures of the trees. As a complement of the complexity characterization by fractal dimension, the lacunarity can be a useful tool to quantify the heterogeneous structures of trees growth in composite materials. The moisture could hardly be absorbed into the filler compared to the resin. It is shown by the higher value of diffusion coefficient of without-filler specimens than with-filler. 20 h of immersing is appropriate to achieve 40% in moisture content at the needle tip

The Use of Fractal Dimension in Characterizing Tree Growth in Filled Epoxy Resin under Humid Condition

The structural response of tree growth in epoxy resin blended with silica filler has been investigated. The physical properties of the resin were varied by changing its filler content and exposing to humid air. The fractal dimension of the electrical tree and its relationship with filler content and humidity were determined. The damaged area of tree in various contents of filler was also estimated. It is considered that the filler would create such an obstruction to the tree growth both in humid and dry conditions. At the ambient condition, the more filler content, the more obstruction would be generated, leading to the significant suppression of tree growth. Likewise, the introduction of filler brought a rise in fractal dimension due to the increase of branches. It is concluded that the existence of filler makes the tree structure more complicated by introducing obstacles to tree propagation, leading to the high fractal dimension of the tree. In addition, it was found that the fractal dimension of the tree was very relational to the fractal dimension of the composite material including filler particles.

Tree growth in epoxy resin at the final stage of internal partial discharge degradation

Tree growth in epoxy resin at the final stage of internal partial discharge degradation

エポキシ樹脂の内部部分放電劣化末期におけるトリー進展現象

エポキシ樹脂の内部部分放電劣化末期におけるトリー進展現象