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

In this paper, an unevenly distributed gradient magnetic field is constructed. The electrical tree characteristics in silicone rubber (SiR) under a gradient magnetic field are investigated. The cumulative damage is increased by 365% under Δ1.5 T and by 411% under Δ-1.5 T gradient magnetic field. A modification method based on graphene nanosheet doping is proposed to improve the insulation performance of the SiR under a gradient magnetic field. With the introduction of an appropriate amount of graphene nanoparticles, the damage area of the SiR/Graphene nanocomposites is reduced to 19.9% in the nonmagnetic field, and is reduced to 17.4% and 14.0% under Δ±1.5 T gradient magnetic field respectively. The time to breakdown is extended by 40-84%. The experimental results show that the magnetic field can affect the charge behavior and enhance the electric field near the electrode. The growth of the electrical tree is accelerated due to the enhanced partial discharges. The graphene nanosheets reduce the interface electric field and inhibit the growth of electrical trees by capturing the injected charges.

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.

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

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.

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.

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.

In this paper, Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> particles with different diameters are added to epoxy resin. Dielectric properties and the electrical tree growth characteristics of the specimens are tested in magnetic field. Experimental results show that magnetic field leads to higher charge carrier mobility and more serious electrical tree degradation. The addition of ferromagnetism particles with a diameter of 20 nm generates more deep traps and causes the decrease of conductivity. Moreover, nanocomposites show great inhibition effect to electrical tree whether high magnetic field is applied. Particles with a diameter of 500 nm play a role as scattering center of electrical tree and cause larger damage area. Ferromagnetism particles with a diameter of 30 μm induce local magnetic field and electric field concentration, accelerating the growth of electrical tree. Quantum chemical and electrodynamics calculation are combined to analyze the energy level distribution and the charge carrier dynamic behavior in high magnetic field. Changes in electrical tree growth result from the size effects of Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> particles, including changing relative permeability, as well as acting as exciton killer, scattering center and weak points.

Surface flashover taken place along the epoxy resin at the terminal of high-field magnets is a serious defect which threatens the safety operation of the whole device. Flashover behavior is different from that in other traditional electric equipment due to the presence of high magnetic field. In this paper, a superconducting magnet system was set up to investigate the surface flashover behavior of epoxy resin in presence of different strengths and directions of external magnetic field. Experimental results show that flashover voltage decreases first and then shows reverse trend with the increase of magnetic field strength when Lorentz force drifts the electrons away from the surface. And the flashover voltage increases when Lorentz force drifts the electrons along or into the surface. High magnetic field restricts the polarization of epoxy resin and the initial electron emission at the triple junction is reduced. Lorentz force not only changes the collision ionization probability on the gas side but also affects the flashover path. This work will be helpful to the terminal insulation design of high-field magnets.

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 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.

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 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.

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

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.

Electrical treeing in solid polymeric insulation is physical evidence of internal damage and ultimately leads to equipment failure. This study considers the effect of frequency on treeing breakdown times in epoxy resin between 50 Hz to 450 Hz. The tree structures generated from 50 to ~250 Hz were filamentary, had narrow channels and spread throughout the insulation. Tree structures generated from ~250 Hz to 450 Hz were darker, having wider channels and less branching. The rate of tree initiation and propagation increases with increasing frequency, resulting in reduced time to breakdown. This has implications that harmonics in AC and DC networks require careful management to ensure insulation reliability is not compromised.