The impact of harmonic 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.

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.

This experimental study sought to investigate the influence of interfaces and ventilated channels on electrical tree growth in epoxy resin. The electrical trees were developed in point-plane geometry samples and tested within the voltage range of 6 - 15 kV rms. Vented channels of diameters in the range 0.16 - 0.30 mm were employed. The distance to the grounded plane from both the epoxy resin interface and the ventilated channels varied between 0.5 - 1.5 mm. The time to breakdown is increased when the location of the interface and channels is centred near the insulation gap or is closer to the plane surface, but decreases when the interface is closer to the needle. A narrower breakdown path is observed when the interface layer is present.

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.

This work investigates electrical tree propagation in a glassy epoxy resin under constant DC voltages of +60 kV and −60 kV, using samples with classical needle-plane electrodes but having small AC trees (henceforth called ‘initial trees’) incepted prior to the DC tests. The characteristics of DC trees propagating from an initial tree is analysed via a sequence of 2-D projections of trees captured using a CCD camera during the test. The experimental results suggest that DC tree growth rate is polarity dependent even when initiated by the presence of an initial AC tree. The shape of the initial AC tree does not have a profound influence on the shape of the DC tree. The length of the initial tree, however, may have a determinant influence on the growth of DC tree. DC treeing exhibits a runway propagation and statistical analysis suggests that tree growth starts to accelerate after reaching a certain distance across the insulation.

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.

Electrical treeing is one of the principal factors affecting the insulating capability of solid insulation. Epoxy resin with mica sheets is used in machine insulation. In the present paper, a point-plane electrode arrangement was used in order to simulate the electrical tree propagation in such an insulating system. The mica sheets investigated were of comparable thickness with the epoxy resin. The simulation was performed with the aid of Cellular Automata (CA). Laplace equation has been solved setting appropriate boundary conditions at the interfaces between epoxy resin - mica sheet and epoxy resin - electrodes. The simulations were based on the local field variation, the local dielectric strength and also on the variation (even slight) of the dielectric constant of the insulating materials. Such local variations of the dielectric constants render possible local variations of the electrical field and, consequently, of the tree propagation. Mica sheets as barriers inside the main insulating material preventing the electrical tree growth. The thickness of mica sheets plays an important role in the propagation of trees. The simulations indicate that even in the case of a partial penetration of the mica sheets, no complete breakdown results. The applied voltage level plays also a crucial role in the resulting type of electrical tree. Higher applied voltages result in a bush-type tree, whereas lower applied voltages result in more pointed trees. The simulation results agree with experimental data.

The electrical tree tests of epoxy resin were conducted by applying compressive stress and tensile stress orthogonal to each other. Results show that the compressive stress is within 15 MPa, and there is competition between compressive stress and tensile stress, which closes the defect and inhibits the electrical tree growth. When the compressive stress is larger than 20 MPa, the shear stress is dominant, there is a cooperative relationship between the tensile stress and shear stress, which opens defects and facilitates the electrical tree growth.

Electrical tree growth in epoxy resin under square waves superimposed with DC voltages is studied in this paper. Using the needle-plane configuration, a ±15 kV DC voltage superimposed on a 15 kV pk 50 Hz square wave was applied to grow electrical trees. A standard 50 Hz sinusoidal source was also used for comparison. A CCD camera imaged the electrical tree growth during testing and associated partial discharges were recorded. It was observed that the tree shape under 15 kV square-wave voltages combined with positive or negative 15 kV DC voltages are branch-like, whereas bush-branch trees grow under 15 kV bipolar square-wave voltages. Positive unipolar square-wave voltages resulted in faster growth than negative unipolar square waves. The different tree structures were also associated with distinct characteristic PD activity.

Electrical trees developed using point-plane samples have been investigated under three different voltage conditions: AC, AC with positive DC bias, and AC with negative DC bias. Visual observations mainly indicate two types of electrical tree progression from initiation to breakdown: "forward and backward" (FB) trees and "forward" (F) trees. FB trees can be observed in AC tests, while F trees occur in AC with DC bias tests. The difference between AC with negative DC bias and AC with positive DC bias is the growth of a rapid long branch prior to breakdown under negative DC bias conditions. Based on the pulse sequence analysis (PSA) technique applied to the PD data associated with electrical tree growth, the findings confirm that PSA curves under different voltage tests have different regions and PSA features can be indicators of tree growth.

Electrical tree is an electrical aging phenomenon which seriously destroys polymer insulation. The addition of nano particle can improve the dielectric properties of polymer, and it is also expected to play a role in inhibiting electrical tree. In the current paper, the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles uniformly dispersed into the epoxy resin, and the electrical tree in nanocomposite containing up to 5 wt% nano SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> was observed clearly. Under the voltage of 18kV, the effect of nano SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> fillers on the initiation and growth of electrical tree in epoxy nanocomposite were studied. It was found that the initiated rate of electrical tree could reduce with the increase of nano filler content, and the electrical tree shape would gradually become from branch in pure epoxy resin to jungle in 5 wt% SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /EP nanocomposite, the growth rate of electrical tree would also slowed sharply. This proved that nanoparticles' addition changes the growth mode of electrical tree. The observation results of partial discharge during electrical tree growth also corresponded with this phenomenon. A viewpoint was proposed that the increase of shallow trap and the change of electrical tree's conductivity are the primary cause of the emergence and development of electrical tree being inhibited. The results in this paper provide useful references for revealing the electrical tree initiation and growth mechanism, and for finding the material or method which can inhibit electrical tree.

During operation, the epoxy resin insulation is subjected to high temperature and mechanical stress. In this paper, in order to reveal the characteristics of the electrical tree under mechanical stress and high temperature, the treeing tests in epoxy resin are carried out with different tensile and compressive stress under room temperature and high temperature. The applied stresses are set as 10, 20, and 30 MPa with the temperature of 30 or 90°C. It is found that tensile stress can promote the growth of electrical tree along the direction of the stress, while the tree is inhibited first and then promoted with the direction perpendicular to the stress with the increasing of compressive stress. High temperature can promote the growth of electrical tree, but the effect of the mechanical stress on the tree structure is weakened under high temperature. The distribution of defects and molecular chain movement are discussed. Mechanical stress tends to be concentrated on the defects and can accelerate the molecular chains being breakdown. High temperature makes it easier for the molecular chains to be destroyed, while releases the stress, so that the trees grow in a random direction.

The growth of electrical trees in heterogeneous solid dielectrics was investigated by means of two-dimensional numerical simulations. In contrast to conventional approaches the introduced newly developed algorithm is based on a deterministic model together with a spatial distribution of the electrical breakdown strength. With standard needle plane geometries, two main types of treeing, bush-like and branch-like, were simulated correctly in respect to their shape and the general voltage dependency. The tree propagation in homogeneous dielectrics as well as the effect of barriers upon treeing was simulated. This tree formation was compared to tree growth in epoxy resin with an embedded glass-mica layer. The effect of electrically weak interfaces between the polymer and the barrier was investigated and the mean time to breakdown was compared for different set-ups. From this comparison it was concluded that only barriers with intact interface zones, whose breakdown strength is not weakened, lead to an optimum of tree resistance.

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.

Autonomous Self-Healing of Electrical Degradation in Dielectric Polymers Using In Situ Electroluminescence