High-voltage Sheath Research Articles

Deep learning-based corrosion-like defect localization technique for high-voltage cable aluminum sheaths using guided waves

High-voltage cable accessories are important parts of a power system and are needed to ensure the reliability of electrical connections. However, their aluminum sheaths are prone to corrosion under complex working conditions, which has a detrimental effect on the normal operation of the electric system. Ultrasonic guided wave detection is a promising non-destructive testing method suited to detecting aluminum sheath corrosion in complex high-voltage cable accessory structures. However, current ultrasonic guided wave detection methods still require manual extraction of the signal features and have a high reliance on professional knowledge. This paper proposes a deep learning-based corrosion-like defect localization technique for high-voltage cable aluminum sheaths using guided waves. First, the original ultrasonic guided wave signals of corrosion defects at different locations are obtained using an ultrasonic guided wave detection platform. Then, the original signals are input into a variable auto-encoder (VAE) network to obtain a low-dimensional representation for automatic feature extraction. Finally, the low-dimensional representation is input into a gated recurrent unit (GRU) based recurrent network for corrosion defect localization. In the feature extraction stage, the VAE can automatically extract the effective features and avoid the interference of noisy signals. In the defect localization stage, the GRU can accurately identify the location of corrosion defects. The experimental results indicate that the VAE-GRU method is capable of accurately identifying corrosion defects based on the original signals.

A Study on the Influence of End-Sheath Aging and Moisture Absorption on Abnormal Heating of Composite Insulators

Abnormal heating of composite insulators of high-voltage transmission lines concentrate at its end, especially in a high-humidity environment. In order to study the influence of end-sheath aging and moisture absorption on abnormal heating of composite insulators, in this paper, we first discuss the appearance test, temperature rise test, and dielectric characteristic test conducted on 110 kV decommissioned composite insulators. Test results indicated the temperature rise in composite insulators increased with ambient humidity, but temperature rise was not severely affected by surface contamination of its shed and sheath; in dry environments, the dielectric constant and dielectric loss factor of high-voltage end sheaths are higher than of those of medium- and low-voltage end sheaths, and the loss effect becomes more severe after moisture absorption in a high-humidity environment. After the tests, the authors established a COMSOL simulation model of composite insulators, to analyze changes in the electric field and thermal field of the end sheath of composite insulators due to the coupling of electric and thermal fields. It was concluded that the dielectric constant of a high-voltage end sheath of the composite insulator increased after moisture absorption, distorting the partial electric field on the surface; meanwhile, the dielectric loss factor increased significantly after water molecules intruded into the aging layer of the sheath as polar molecules. Therefore, the dielectric loss (leakage conductance loss and lossy polarization loss) caused by aging and moisture absorption of the sheath surface under partially high field strength in the high-humidity environment was the leading cause of abnormal heating at the high-voltage end of composite insulators. The conclusion of this paper serves as an important reference for revealing the causes of abnormal heating of composite insulators in high-humidity environments and the influence mechanism of external factors on abnormal heating.

Research on defect type diagnosis of composite insulator based on infrared detection technology

Composite insulator is widely used in Hainan area of China because of its lightweight, small volume, good anti pollution flashover performance and many other advantages. Aiming at the problem of how to correctly judge the aging state of composite insulators under live conditions in Hainan’s high humidity environment, this paper analyzes the main causes of composite insulator heating and the heating position and temperature rise of composite insulator under different defect types by building equivalent circuit, and puts forward several criteria for distinguishing the defect types of composite insulator by infrared detection technology; taking the abnormal temperature rise of 220kV returned composite insulator in Hainan as an example, the defect type of the composite insulator is analyzed by infrared detection technology, and the correctness of the infrared results is verified by the appearance inspection and anatomical analysis of the test object. The results show that the defect that the temperature rise is between 1-5°C and only appears among the fittings and the first umbrella is caused by the aging, moisture absorption or surface pollution of the umbrella cover; the defect that the temperature rise is above 5°C and the range extends from the high-voltage end sheath to several umbrella skirts is caused by the internal deterioration of the insulator. In conclusion, the conclusion provides a guarantee for the safe operation of power grid and has important reference value for the study of on-line monitoring of composite insulators.

Erosion of focus rings in capacitively coupled plasma etching reactors

In plasma etching reactors, the structure surrounding the wafer, often called a focus ring (FR), plays an important role in maintaining uniform fluxes of reactants across the wafer. The FR is typically made of dielectric materials. During the plasma etching process, the sheath that forms over the wafer to accelerate ions anisotropically into the surface extends over the FR. Electrical charging of the FR modifies the sheath relative to that over the wafer. On the one hand, one wants the sheath to be uniform across the wafer-FR boundary to enable uniform fluxes to the edge of the wafer. On the other hand, maintaining a high voltage sheath over the FR will erode the FR, which is undesirable as the FR is a consumable component that must be periodically replaced in high volume manufacturing. In this work, we computationally investigated the consequences of dielectric constant ɛr of the FR materials on erosion of the FR. The series capacitance of the FR and its underlying structure is typically smaller than that of the wafer and its underlying structure. As a result, the FR charges quickly relative to the wafer, which then reduces the voltage across the sheath on top of the FR. The ion energy and angular distributions (IEADs) striking the FR are, therefore, generally lower in energy with a broader angular distribution. With ɛr = 2, the ion energies striking the middle to the outer edge of the FR are 30–180 eV, whereas for ɛr = 100, the ion energies are 120–380 eV. At the transition between the wafer and the FR, there is a skew in the IEAD as large as 15° that results from the difference in sheath thickness above the wafer and the FR. This skew and the erosion rate across the FR are functions of the dielectric constant of the FR material. With low ɛr, the FR charges quickly, less plasma is produced above the FR, and there is less voltage across the sheath that results in less FR erosion. Increasing ɛr of the FR produces a higher sheath voltage as well as higher ion fluxes over the FR, which increases erosion, while the skew at the edge of the wafer is less severe. The material of the subsurface portion of the FR, which dominates its capacitance, is an important consideration in the design of the substrate assembly.

Charged particle dynamics and distribution functions in low pressure dual-frequency capacitively coupled plasmas operated at low frequencies and high voltages

In high aspect ratio (HAR) dielectric plasma etching, dual-frequency capacitively coupled radio-frequency plasmas operated at low pressures of 1 Pa or less are used. Such plasma sources are often driven by a voltage waveform that includes a low-frequency component in the range of hundreds of kHz with a voltage amplitude of 10 kV and more to generate highly energetic vertical ion bombardment at the wafer. In such discharges, the energetic positive ions can overcome the repelling potential created by positive wall charges inside the etch features, which allows high aspect ratios to be reached. In order to increase the plasma density a high-frequency driving component at several 10 MHz is typically applied simultaneously. Under such discharge conditions, the boundary surfaces are bombarded by extremely energetic particles, of which the consequences are poorly understood. We investigate the charged particle dynamics and distribution functions in this strongly non-local regime in argon discharges by particle-in-cell simulations. By including a complex implementation of plasma-surface interactions, electron induced secondary electron emission (δ-electrons) is found to have a strong effect on the ionization dynamics and the plasma density. Due to the high ion energies at the electrodes, very high yields of the ion induced secondary electron emission (γ-electrons) are found. However, unlike in classical capacitive plasmas, these γ-electrons do not cause significant ionization directly, since upon acceleration in the high voltage sheaths, these electrons are too energetic to ionize the neutral gas efficiently. These γ- and δ-electrons as well as electrons created in the plasma bulk and accelerated towards the electrodes to high energies by reversed electric fields during the local sheath collapse are found to induce the emission of a high number of δ-electrons, when they hit boundary surfaces. This regime is understood fundamentally based on the following approach: first, dual-frequency discharges with identical electrode materials are studied at different pressures and high-frequency driving voltages. Second, the effects of using electrodes made of different materials and characterized by different secondary electron emission coefficients are studied. The electron dynamics and charged particle distribution functions at boundary surfaces are determined including discharge asymmetries generated by using different materials at the powered and grounded electrodes.

Time and space-resolved experimental investigation of the electron energy distribution function of a helium capacitive discharge at atmospheric pressure

Using a collisional radiative model coupled with optical emission spectroscopy (OES) of helium n = 3 levels, the electron temperature (Te) of an atmospheric-pressure capacitively coupled radiofrequency (AP-CCRF) discharge in helium is determined with space and time resolution. When the AP-CCRF discharge is sustained in the mode, Te varies from 0.2 to 7.2 eV. In this case, high values of Te (>5 eV) occur only during a brief instant (<10 ns) in the high-voltage sheath. When the AP-CCRF discharge is sustained in the mode, Te varies from 0.3 to 0.4 eV during the complete cycle. The physical meaning of these electron temperatures are then analyzed by considering possible departure from the Maxwellian electron energy distribution function (EEDF). As a first approximation to non-Maxwellian distribution functions, a two-parameter EEDF was used to fit the OES data. This approach yields an overpopulation of high energy electrons with respect to the Maxwellian form in the mode and the opposite trend in the mode.

Computational analysis of the electron beam source with hollow anode

The computational analysis of the plasma source with the hollow anode is performed using the two-dimensional Particle-in-Cell Monte Carlo collisions model. This source operates in two molecular gases (CO2 and N2) in the abnormal glow mode, i.e., it is supported by the secondary electron emission from the cathode. It is obtained that this plasma source can be used for the generation of high-energy electrons (energy &amp;gt; 100 eV) only if the cathode potential exceeds 500 V. Otherwise, the high-energy electrons generated in the high-voltage cathode sheath dissipate almost all their energy in the plasma column and get thermalized.

影响高压电缆护层环流的因素分析

影响高压电缆护层环流的因素分析

Overcoming sheaths overlapping in a small diameter metallic tube with one end closed and using a high density plasma from a high power pulsed hollow cathode discharge

High voltage sheaths are formed when plasmas are produced by application of high negative voltage pulses to conductive supports or components, as in Plasma Immersion Ion Implantation (PIII) treatments of materials surfaces. For parts with concave shape, as inside metal tubes, these sheaths behave quite differently according to the tube configuration and size, as well as, PIII treatment pressure of operation and pulsing parameters. In this work, an SS304 tube of 1.1 cm internal diameter and 20 cm length was pulsed typically at -0.5 to -2.6 kV, 20 μs pulse length, 500 Hz repetition rate, nitrogen pressure of 5x10-2 mbar and with one side closed configuration. Different currents (between 10 and 30 A) were used to produce plasmas with sheaths that overlapped or not, depending on the currents used. To study these sheath behaviors, a simple plasma diagnostic technique based on a bi-dimensional mapping of the deposition of sputtered materials and by etching via the plasma on a Si wafer target surface, both coming out from the tube, was used. This mapping showed clearly the border line situation between overlapping and non-overlapping sheaths in that small tube which allowed to estimate the plasma density to be around 1011 cm-3 at such a sheath condition, as previously anticipated by Sheridan. Above that border condition, nitrogen PIII was successfully obtained in such a small tube of SS304, producing TiN and Ti2N in samples of Ti6Al4V placed inside the tube, when temperatures higher than 800°C were reached there. Below the border, no significant uptake of nitrogen was possible. Using this type of experimental set-up, it is now possible to explore different hollow cathode behaviors, efficient or high temperature (above 800°C) PIII conditions and also new utilizations of the plasma ejected from the tube.

High-current-density gas ion ribbon beam formation

Experimental results are presented to demonstrate the possibility of forming extended repetitively pulsed low-energy gaseous ion beams with a notably high current density. The symbiosis of plasma-immersion extraction of ions and their subsequent ballistic focusing in the semi-cylindrical geometry of the focusing system was first used to form a high-density nitrogen and argon ion ribbon beam. The gas-discharge plasma was formed using a hot-cathode-arc discharge-based modified extended source. A nitrogen and argon ion beam was steadily formed at bias potentials of an amplitude up to 1000 V and a pulse duration of 2–80μs at a pulse frequency of 103-104 pulses/s. In the experiments, an inverse change in current amplitude of the focused ion beam was observed with respect to the initial nitrogen and argon plasma densities. The change in the current ratio formed by the nitrogen and argon ion beams in comparison with the initial plasma density is associated with the effect of the ion atomic mass on a high-voltage sheath formation near the grid electrode and the ratio of the sheath width and grid cell dimensions. An argon ion beam with a current of 0.35 A and a nitrogen ion beam with a current of 0.6 A at a focused ribbon beam length of 23 cm were obtained by installing a grid focusing system in the form of a partial cylindrical surface (radius 7.5 cm) at 35 cm from the gas-discharge plasma generator output. A decrease in distance to 20 cm ensured an increase in argon ion current to 0.8 A and nitrogen ion current to 1.3 A. The maximum ion current density at a distance that corresponded to the cylindrical grid radius for nitrogen and argon ions exceeded 0.08 A/cm2 and 0.05 A/cm2, respectively.

High intensity, macroparticle-free, aluminum ion beam formation

We describe a vacuum arc based system for the generation of repetitively pulsed metal ion beams with very high current density, and the results of experimental investigations of the performance of this new system, as well as the results of numerical simulations. Our approach uses a DC vacuum arc as a metal plasma source, and the beam-forming technique is a hybrid method using features of conventional gridded ion extraction together with some plasma immersion attributes. A hemispherical grid is located some distance from the plasma source so as to intercept part of the metal plasma flow and repetitively pulse-biased to the requisite ion beam energy. Ions are accelerated in the high voltage sheath that forms in front of the biased grid and focused into a converging beam that can be of a very high current density. Space charge neutralization is provided by cold plasma that streams through the grid during the bias-off part of the cycle. At the same time, macroparticles in the vacuum arc plasma are blocked from viewing the target by a metal disk positioned centrally on the hemispherical grid. In the work outlined here, we formed 5 keV aluminum ion beams with a current density up to 470 mA/cm2 at a negative bias amplitude of 3 kV and a pulse repetition rate of 105 pulse per second. The experiments and numerical simulations demonstrate the formation of high-intensity macroparticle-free aluminum ion beams for material surface modification and other possible applications.

Thermal degradation of flame-retarded high-voltage cable sheath and insulation via TG-FTIR

In energy application system, electrical energy is economic, practical, clean and easy to control and transfer. Electrical cables, especially high-voltage cables, have been extensively and increasingly applied in industries, and however their fire safety deserves more attention. Thermal behaviours of flame-retarded high-voltage Cable Sheath and Insulation were studied using SDT Q600 thermal analyser. The cable sheath pyrolysis profiles show three peaks while the insulation has only one peak. Possible reaction pathways and activation energy evolution with various conversion rates were analysed using both model-free and model-fitting methods. The zero order model and the 6th order model are responsible in two cable sheath pyrolysis stages, respectively. The one order model plays an important role in the pyrolysis of cable insulation. Evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) shows that the amount of six main components produced in the order of most to least is CO2 > CH stretching > CH bending > CH aliphatic bending > CH2 deformation > CCl stretching for the cable sheath while the relationship of C–H stretching > CH scissor vibration > CO2 > CO & CC stretching is determined for the cable insulation.

Influence of negative bias pulse parameters on accumulation of macroparticles on the substrate immersed in titanium vacuum arc plasma

Abstract The paper presents the results of an experimental study of the influence of a substrate negative bias with various pulse lengths and pulse repetition rates ranging from several pulses per second to 10 5 pulses per second on the macroparticle accumulation on the substrate immersed in DC titanium vacuum arc plasma. It was found that the increase of the bias pulse frequency from 10 up to 10 5 pulses per second at pulse length 7 μs, as well as bias pulse lengths from 5 μs to 25 μs at fixed frequency of 30 pulses per second at bias pulse amplitude ˗2 kV and ion-plasma substrate treatment time 30 s led to gradual almost linear up to 3-fold decrease of the surface number density of MPs on the substrate immersed in titanium vacuum arc plasma at plasma ion saturation current density of 10 mA/cm 2 . The decrease of ion current density down to 3.5 mA/cm 2 leads to the reduction of the macroparticle suppression efficiency. The influence of multiple recharging of the macroparticles in plasma and the sheath on the reflection of these macroparticles in the sheath electric field is discussed. It was experimentally found that the shape of MPs and their adhesion to the substrate surface strongly depends on bias pulse, plasma and substrate parameters. It was shown that cooled vacuum arc MPs can be melted in high-voltage plasma sheath.

Influence of ion-neutral collision parameters on dynamic structure of magnetized sheath during plasma immersion ion implantation

AbstractA cold magnetized plasma sheath is considered to examine the gas pressure effect on the sheath dynamics. A fluid model is used to describe the plasma sheath dynamic. The governing fluid equations in the plasma are solved from plasma center to the target using the finite difference method and some convenient initial and boundary conditions at the plasma center and target. It is found that, the ion-neutral collision has significant effect on the dynamic characteristics of the high-voltage sheath in the plasma immersion ion implantation (PIII). It means that, the temporal profile of the ion dose on the target and sheath width are decreased by increasing the gas pressure. Also, the gas pressure substantially diminishes the temporal psychograph of ion incident angle on the target.

Structure of the velocity distribution of sheath-accelerated secondary electrons in an asymmetric RF-dc discharge

Low-pressure capacitively-coupled discharges with additional dc bias applied to a separate electrode are utilized in plasma-assisted etching for semiconductor device manufacturing. Measurements of the electron velocity distribution function (EVDF) of the flux impinging on the wafer, as well as in the plasma bulk, show a thermal population and additional peaks within a broad range of energies. That range extends from the thermal level up to the value for the ‘ballistic’ peak, corresponding to the bias potential. The non-thermal electron flux has been correlated to alleviating the electron shading effect and providing etch-resistance properties to masking photoresist layers. ‘Middle-energy peak electrons’ at energies of several hundred eV may provide an additional sustaining mechanism for the discharge. These features in the electron velocity (or energy) distribution functions are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths: a stationary sheath at the dc electrode and an oscillating self-biased sheath at the powered electrode. Since at those energies the mean free path for large-angle scattering (momentum relaxation length) is comparable to, or exceeds the size of the discharge gap, these ‘ballistic’ electrons will not be fully scattered by the background gas as they traverse the inter-electrode space. We have performed test-particle simulations in which the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytical model has been developed to predict existence of peaked and step-like structures in the EVDF. Those features can be explained by analyzing the kinematics of electron trajectories in the discharge gap. Step-like structures in the EVDF near the powered electrode appear due to accumulation of electrons emitted from the dc electrode within a portion of the RF cycle, and their subsequent release. Trapping occurs when the RF sheath voltage exceeds the applied bias, and is decreasing. The secondary electrons originating from the dc-biased surface also form a peak near the energy equal to the bias potential. Additional peaks, at lower energies, are formed by the electrons emitted from the RF electrode and eventually escaping to it. The latter can be grouped according to the number of bounces between the sheaths during their residence time in the discharge. Each of such groups may give rise to an individual peak in the distribution. The trap-and-release theory developed in this paper provides a convincing explanation for the observations of the ballistic and ‘middle energy peak’ electrons detected in experiments.

Characterization of a radio frequency hollow electrode discharge at low gas pressures

A radio frequency (RF) hollow discharge configuration is presented, which makes use of a combination of RF plasma generation and the hollow cathode effect. The system was especially designed for the treatment of nanoparticles, plasma polymerization, and nanocomposite fabrication. The process gas streams through the plasma in the inner of the cylindrical electrode system. In the here presented measurements, pure argon and argon with oxygen admixtures are exemplarily used. The discharge is characterized by probe measurements in the effluent, electrical measurements of the discharge parameters, and visual observations of the plasma glow. It is found that the RF fluctuations of the plasma potential are weak. The plasma potential resembles the one of a DC hollow cathode discharge, the RF hollow electrode acts as a cathode due to the self-bias, and a high voltage sheath forms in its inner cylinder.

Influence of Bias Parameters and Plasma Density on Vacuum Arc Macroparticle Accumulation

Several regularities of the accumulation of vacuum arc metal macroparticles (MP) on the sample on repetitively pulsed biasing (105 Hz, 7 µs, −0.5 kV to −3.5 kV) have been investigated. It has been shown that the substrate temperature plays a very important role in controlling the MP amounts on the sample. A possibility to change the metal particle number density on the substrate in the range from 105/cm2 to 107/cm2, depending on bias pulse parameters, sample temperature, and ion plasma saturation current density, has been demonstrated. The shape of MPs and their adhesion to the substrate surface depends strongly on the MP energy balance in a high-voltage space charge sheath.

Ion energy distributions measured inside a high-voltage cathode in a BF3 pulsed dc plasma used for plasma doping: experiments and ab initio calculations

The purpose of this paper is the characterization of a plasma-doping system in BF3. Energy distributions (IEDs) of boron ions crossing a high-voltage sheath (up to 1 kV) are measured in a BF3 plasma under different experimental conditions. It is demonstrated that relative percentages of boron ions reaching the cathode and IED shapes are governed by the number of collisions inside the sheath. Heavy molecular ions , are dissociated inside the sheath and light boron ions (B+, BF+) are created. Based on experimental results and ab initio calculations of boron ion structures, the mechanisms occurring in the sheath are discussed. Furthermore, the charge exchange cross-section between and BF3 is estimated to be 8.8 × 10−19 m2.

Spatial evolution of the electron energy distribution function in a low-pressure capacitively coupled plasma containing argon and krypton

The spatial evolution of the electron energy distribution function (EEDF) in the axial direction of a capacitively coupled plasma with two parallel plate electrodes is investigated using an optical emission line-ratio method for Ar/Kr discharges. When the rf power is increased from 25 to 400 W at a pressure of 400 mTorr, we observe a transition from convex EEDFs to concave ones and a sharp increase in electron density, due to an α–γ mode transition, which is believed to be caused by the high-energy electrons originating in the high-voltage sheath. We also investigate the spatial evolution of the EEDF when the pressure is increased from 45 to 500 mTorr at a power of 100 W. The EEDF is uniform at pressures below 180 mTorr and becomes non-uniform at higher pressures, owing to the decrease in the energy relaxation length of the high-energy electrons.

From electron depletion to quasi-neutrality: the sheath–bulk transition in RF modulated discharges

The boundary sheaths of all plasmas are characterized by a gradual transition from unipolarity (electron depletion, ne ≪ ni) to ambipolarity (quasi-neutrality, ne ≈ ni). Capacitively driven sheaths exhibit a transition which is expanded by the RF modulation and smoothed by thermal effects, i.e. by the finiteness of the electron temperature Te and the Debye length . Sheath models which neglect thermal effects (‘step models’) are restricted to strongly modulated high voltage sheaths with VRF ≫ Te/e and fail when this condition is not met. This work presents an improved analysis of the sheath–bulk transition which takes both modulation and thermal effects into account. Based on a previously found asymptotic solution of the Boltzmann–Poisson equation (Brinkmann 2007 J. Appl. Phys. 102 093393), approximate algebraic (i.e. closed) expressions for the phase-resolved electrical field E and electron density ne in RF sheaths are derived. Under the assumption that the modulation is periodic (not necessarily harmonic) with ωRF ≫ ωpi, also the phase averages of the field and the electron density can be expressed in closed form. These results—together referred to as the advanced algebraic approximation (AAA)—make it possible to formulate efficient and accurate models for RF driven boundary sheaths for all ratios of VRF to Te/e. As an example, a harmonically RF modulated, collision-dominated single species sheath is studied. The outcome is compared both with the numerically constructed exact solution and with the well-known step model approach of Lieberman (1989 IEEE Trans. Plasma Sci. 17 338). It is found that the AAA can reproduce the exact numerical solution within a few per cent for all ratios of VRF to Te/e. The step model, in contrast, exhibits strong deviations even for large eVRF/Te and fails completely in the case of weak modulation.