Space Charge Density Research Articles

Revealing the flashover mechanism of EP/GF composite insulation under DC combined harmonic voltage

Abstract In ultra-high voltage converter stations, the phenomenon of flashover along the outer insulation of dry hollow reactors made of epoxy/glass fiber composites poses a potential threat to the stability of the power system. To enhance its flashover performance under varying complex conditions, it is necessary to conduct in-depth research on the flashover mechanism of this material under different voltage forms. This study conducted tests on the flashover voltage, surface space charge distribution, and partial discharge parameters under varying AC-DC ratios and AC frequencies, and exploring their relationships. The results show that the DC content in the AC-DC ratio decreases, the flashover voltage decreases, the apparent total discharge of partial discharges increases, and the maximum surface space charge density decreases. The main influencing factor is the increase in the number of seed charges involved in gas ionization. Increasing the AC frequency, the flashover voltage decreases, the apparent total discharge increases significantly, and the surface charge density remains basically unchanged. The main reason is that the change in the number of alternating cycles further increases the number of seed charges. This study reveals the flashover mechanism of epoxy/glass fiber composites under different voltage forms, which provides theoretical support for subsequent material modification.

Effects of anode evaporation process on the anode sheath characteristics in vacuum arc plasma

Abstract The anode sheath of vacuum arc plasma plays a key role in the generation of anode plasma, but the effects of anode evaporation on the anode sheath remains unclear. In this paper, a theoretical model of a collisional sheath for multi-component plasma coupled with anode evaporation is developed, and the spatial evolution of the anode sheath at different anode surface temperatures is investigated. The results indicate that the distribution of charged particles density and potential in the anode sheath monotonically decreases in the absence or reduction of anode evaporation. When the anode surface temperature exceeds 1900K, a potential hump appears within the sheath. This is due to enhanced anode evaporation increasing the metal vapor density, which intensifies electron impact ionization and charge exchange collisions, resulting in a higher net space charge density. Finally, the effects of various collision reactions and electron temperatures on the potential hump are analyzed. These findings are meaningful for understanding the anode plasma generation mechanism and regulating the anode plasma parameters.

Analysis of the Double Head Streamer Discharge Simulation under Different Pressures

In this research, the dynamics of double-head streamer discharge initiation, propagation, interaction and breakdown in the air under different pressure values were presented. The double-head streamer discharge dynamics were analysed within a plane-to-plane electrode configuration. That was done through many aspects proposed, such as electron density, electric field, space charge density and streamer propagation speed. The simulation performed using ‘COMSOL Multiphysics’ is based on the finite element method and was carried out with the fluid model. The fluid model describes the movement of particle concentrations using partial differential equations (PDEs) together with Poisson’s equation; Poisson's equation and charge concentrations determine the electric field distribution in space. According to the results, as the pressure increased from (1, 2 and 3atm), the evolution time of the streamer increased from (0.563 to16.29ns) with the same breakdown voltage of 19kV. This means that the double-head streamer discharge developed faster with the decrease in pressure.

Machine learning assisted mechanism modeling for gas phase electrohydrodynamic system

In this paper, a hybrid physics-data driven model for electrohydrodynamic gas system (EHDGS) was developed by combining artificial neural network (ANN) with mechanism modeling method. ANN was used to correlate the relationship between the variables (electrode distance, diameter of grounding cylinder, applied voltage, electric field gradient, etc.) in a needle-cylinder EHDGS and the initial space charge density. The results showed that the ANN model of nine neurons can well predict the initial space charge density. The coefficient of determination (R2) reaches 0.9874, and the mean absolute error is as low as 0.0067. Subsequently, a hybrid mechanism model where the initial space charge density was predicted from the ANN model was constructed to simulate the needle-cylinder EHDGS. The experiment with the needle-cylinder EHDGS was carried out. The simulation results were in good agreement with the experimental data, demonstrating the reliability of the proposed hybrid model. The electric field distribution, space charge distribution, and flow field distribution behavior of the EHDGS were then analyzed in detail. The effects of key parameters on the flow characteristics of EHDGS were systematically studied, showing that higher voltage and shorter distance give higher flow rate up to 2.5 m/s. The diameter of the cylinder also significantly influences the breakdown voltage. Three dimensionless groups were defined and their effects on spatial charge density distribution were investigated. This study provides both insights and an efficient tool for the design and optimization of EHDGS.

Droplet Electrophoresis with Internal Free Ions: Effect of Permittivity Changes in the Electric Double Layer.

Droplet electrophoresis (EP) is of interest in biological systems, microfluidics, and separation techniques. We investigate EP of an oil droplet that contains free ions and is stabilized in an electrolyte solution through an amphoteric surfactant. The presence of mobile ions within the droplet leads to the creation of a distinct nonzero space charge density inside the droplet and consequently, formation of an inner EDL inside the droplet in addition to the traditionally considered outside EDL. While we assume the permittivity inside the inner EDL to remain constant, we consider both the case of constant and variable permittivity in the outer EDL. Our findings demonstrate a change in the droplet direction of motion in the electric field when transitioning from acidic to alkaline pH, regardless of permittivity and ionic strength in both oil and electrolyte. We further find a significant reduction in the magnitude of droplet velocity in the case of a variable permittivity due to reduction of the local space charge density within the EDL surrounding the droplet. When decreasing the viscosity ratio of the oil to the electrolyte, in all cases we find a reduction in droplet velocity. This decline is attributed mostly to the formation and strength of a vortex around the droplet. We finally demonstrate that with constant permittivity in the outer EDL, the variation in κaouter has a more significant effect on the droplet's EP velocity than altering κainner. However, in cases where the body forces inside of the droplet dominate, minor changes in the outer electrolyte concentration have no influence on the droplet motion, which is relevant for biological colloids that can contain significant free internal charges. Our results are important for the manipulation of biological colloids, water and waste treatment such as lubricant removal from processing streams.

Sedimentation of a Charged Soft Sphere within a Charged Spherical Cavity.

The sedimentation of a soft particle composed of an uncharged hard sphere core and a charged porous surface layer inside a concentric charged spherical cavity full of a symmetric electrolyte solution is analyzed in a quasi-steady state. By using a regular perturbation method with small fixed charge densities of the soft sphere and cavity wall, a set of linearized electrokinetic equations relevant to the fluid velocity field, electrical potential profile, and ionic electrochemical potential energy distributions are solved. A closed-form formula for the sedimentation velocity of the soft sphere is obtained as a function of the ratios of core-to-particle radii, particle-to-cavity radii, particle radius-to-Debye screening length, and particle radius-to-porous layer permeation length. The existence of the surface charge on the cavity wall increases the settling velocity of the charged soft sphere, principally because of the electroosmotic enhancement of fluid recirculation within the cavity induced by the sedimentation potential gradient. When the porous layer space charge and cavity wall surface charge have the same sign, the particle velocity is generally enhanced by the presence of the cavity. When these fixed charges have opposite signs, the particle velocity will be enhanced/reduced by the presence of the cavity if the wall surface charge density is sufficiently large/small relative to the porous layer space charge density in magnitude. The effect of the wall surface charge on the sedimentation of the soft sphere increases with decreases in the ratios of core-to-particle radii, particle-to-cavity radii, and particle radius-to-porous layer permeation length but is not a monotonic function of the ratio of particle radius-to-Debye length.

Measurement of space charge density distributions and dielectric resonance enhancement of beam deflection properties of KTN crystal

In recent years, potassium tantalum niobate (KTN) electro-optical deflection devices have gained considerable attention because of their notable advantages, such as large deflection angles, low operational voltage requirements, and compact dimensions. This study uses the phase-shifted interferometric optical path to characterize the influence of direct current (DC) voltage on charge density. An interferogram is acquired using the four-step phase-shifting technique, enabling the calculation of phase delays and deducing the variation in charge density. Experimental results demonstrate that the charge density near the cathode increases with an increase in DC voltage. Subsequently, we utilize the frequency dependence of the dielectric constant of the KTN crystal on the electric field. The dielectric constant can be enhanced when the characteristic frequency of the motion of the polar nanoscale region matches the frequency of the electric field. This field-induced enhancement effect improves the beam deflection performance of the KTN crystal. Application requirements in the field of high-speed random scanning can be realized through the mechanism of the KTN crystal co-acting with DC and alternating current electric fields.

Self-Healing Properties of Water Tree with Microcapsule/Cross-Linked Polyethylene Composite Material Based on Three-Layer Core-Shell Structure.

To overcome the degradation of insulating properties caused by the water tree aging of cross-linked polyethylene (XLPE), a self-repairing material for XLPE based on a microcapsule system is proposed. Three-layer shell nucleus microcapsules/XLPE composites with different microcapsule doping content are prepared. The water tree aging experiments are carried out using the water-needle electrode method to analyze the ability of microcapsules to repair the damaged areas of water trees. The results show that, compared with the XLPE material without microcapsules, the electrical properties of composites decline significantly when the doping concentration of three-layer shell nucleus microcapsules is large. When the doping concentration is 1.0 wt%, the microcapsule/XLPE composite breakdown strength has no noticeable change, and the dielectric loss factor does not change significantly, the space charge density decreases, and the space charge properties have been improved considerably. When the water tree branch develops to the position where the microcapsules are located, the microcapsules will rupture and release their internal repair materials and catalysts and react with water to produce an organic silicone resin to fill the water tree cavity, which can achieve an excellent self-healing effect. In addition, the nano-SiO2 on the surface microcapsules can make the microcapsules and matrix better integrated, which avoids the microcapsule accumulation that tends to occur when incorporating microcapsules, thus improving the repair rate.

Propagation of nanosecond discharge in an air gap containing a water droplet: modelling and comparison with time-resolved images

The plasma-water interface is a complex medium characterized by interesting physical and chemical phenomena useful for many applications such as water processing or material synthesis. In this context, optimizing the transport of reactive species from plasma to water is crucial, and it may be achieved by increasing the surface-to-volume ratio of the processed object. Herein, we study the characteristics of a streamer produced by nanosecond discharge in air gap with a droplet of deionized water. The discharge is characterized experimentally by electrical measurements as well as by 1 ns-intergated ICCD images. To report plasma properties that are not accessible through experiment, such as the spatio-temporal evolution of electron density, electric field, and space charge density, a 2D fluid model is developed and adapted to the experimental geometry. Due to the fast propagation of the ionization front, the droplet is considered as a solid dielectric. The model solves Poisson’s equation as well as the drift-diffusion equation for electrons, positive ions, and negative ions. The utilized transport coefficients are tabulated as a function of the reduced electric field. Helmholtz equations are also included in the model to account for photoionization. The electron impact ionization source obtained from the model is compared to experimental 1 ns-integrated ICCD images, and a good agreement is observed. Finally, the model is used to investigate the influence of droplet dielectric permittivity and wetting angle (the angle between a liquid surface and a solid surface) on the properties of the discharge. Overall, the data reported herein demonstrate that the model can be used to investigate plasma properties under different conditions.

Modeling and Bulk Characterization of 4HSIC Radiation Detector in Sentaurus TCAD Simulation Environment

This paper gives an insight into the need for radiation detection and the most commonly flexible and efficient radiation detector. It also examines bulk characteristics of 4H-SiC semiconductor radiation detector with Ni and Ti as metals for the contact. Bulk characterization of the device, including: doping concentration, electrons and holes behaviors, space charge and current densities were carried out. The modeling is conducted using Sentaurus Technology Computer Aided Design (TCAD) to examine charge transport in bulk 4HSiC material. Data obtained were further analyzed through Sentaurus visual, sentaurus Techplot and Excel to clearly determine the characteristics of the device. It is observed that when the semiconductor and metal are in contact, the Fermi-level is established where the doping concentration varied with either magnitude of the doping concentration or nature of the dopant. Similarly, Schottky and ohmic contacts and temperature effect were observed from the device characteristics which demonstrate that, the detector can withstand a temperature from the range of 100K to 700K in no fluctuating state.

Ultrafast oscillation in a field emission-driven miniaturized gaseous diode

We report an ultrafast oscillation (up to ∼104 GHz) in the early stage of field emission-driven microdischarges. Spatiotemporal behaviors of electron density, space charge density, and electric field, exhibiting high-frequency oscillations, are demonstrated based on first-principle particle-in-cell/Monte Carlo collision simulations. Intermittent electron emission fluxes are identified from the electron phase space distributions whereas the ions are rather non-oscillatory in the transient timescale. The mechanisms of oscillation with growing amplitude are found to be related to the rapid modification of the field emission current affected by the space charge electric field, which is also accompanied by the fast response of the electron transport and ionization in a dynamic double-layer sheath. Further, a transport equation for emission current is solved to estimate the oscillation frequency, which agrees well with the simulation results. This study provides a more precise understanding of the formation of the field emission-driven microdischarge, which informs the design and optimization of miniaturized gaseous electronic devices.

Dynamic Effect of Overscreening on Electrochemical and Thermal Properties of EDLC Electrodes during Charging/Discharging Cycles

Overscreening was commonly observed during charging/discharging cycles of electric double layer capacitors(EDLCs). This study presented a two-dimensional continuum electrochemical thermal model with a distribution function for concentration considering overscreening to investigate how overscreening affected the dynamic formation of electric double layer (EDL) near electrodes in macroscale. The results indicated that ion distribution was co-affected by EDL formation and overscreening near the electrode. Correspondingly, layered space charge density distribution resulted in a thicker diffuse layer, whose thickness was determined by the sum of co-ion and counter-ion diameters. Additionally, the diffuse layer capacitance was proportional to the reciprocal of counter-ion diameter, while the total capacitance was proportional to a linear combination of counter-ion and the larger ion diameters. Moreover, the shift from endothermic to exothermic for the total heat generation rate was dominated by heat of mixing, and was caused by the local ion mixing due to overscreening, resulting in local entropy increase. The results of this study can be used to further investigate the effect of overscreening on ion transient transport dynamic and heat generation rates.

A NUMERICAL ANALYSIS ON THE SUBMICRON- AND MICRON-SIZED PARTICLE SEDIMENTATION IN A WIRE-TO-PLATE ELECTROSTATIC PRECIPITATOR

Electrostatic precipitators (ESPs) are frequently utilized in collecting fine organic and inorganic materials from continuous liquid with few moving parts and high efficiency using electrically charging the particles. In this study, cross-sectional 2D geometry of a wire-to-plate electrostatic precipitator the parametric data of which originally published elsewhere was numerically modeled and validated to investigate submicron-micron particle charging in terms of diffusion and field charging mechanisms and precipitation behavior of particles with detailed electric field properties. Electric field, gas flow, and particle trajectory equations are coupled and solved in a multiphysics solver. Particle tracking is realized with the Lagrangian approach. Results indicate variations in electric field strength and space charge density between corona electrodes, with space charge present in the entire precipitation channel. Between two different charging mechanisms, diffusion charging prevails for charge accumulated on submicron particles, whereas field charging becomes dominant for particles larger than 1μm diameter. However, for the ESP configuration considered in this study, particles reach a charge saturation in less than 0.7 seconds, regardless of their size. Although calculated precipitation efficiencies for micron-sized particles can reach to 100%, efficiencies for submicron particle range drop with increasing particle size, as diffusion charging rapidly loses its effectiveness, in 50-250nm range.

Numerical Study of the Influence of Using Semicircular Corrugated Collecting Plate and Inverted Semicircular Corrugated on the Electrostatic Precipitator

Electrostatic precipitators (ESPs) play a crucial role in removing harmful particles in various industries. Ongoing studies continuously strive to improve their performance and efficiency, leveraging the inherent advantages of ESPs. Researchers are exploring various types of collecting and discharge electrodes to achieve this. This study specifically focuses on comparing two types of collecting electrodes, Semicircular Corrugated Plates (SCPs) and inverted Semicircular Corrugated Plates (InvSCPs), against Flat Plates (FPs) as a reference case. Three distinct forms of ESPs were simulated, and the SCP and InvSCP designs were evaluated against the FPs, specifically assessing the impact of modifying the collecting electrode design on key electrostatic precipitator characteristics, such as electrical field distribution, space charge density distribution, particle trajectories, and particle collection efficiency. Notably, the study found that using SCP resulted in the highest particle collection efficiency compared to InvSCP and FP for the same range of particles.

Computation of corona generated ionic current environment of unipolar UHVDC transmission lines using gas bimolecular collision theory

Corona on UHVDC transmission line conductors initiates corona power loss and the inevitable ionic current flow environment at ground level. To limit this substantial flow of ionic current at ground levels, it is necessary to establish an appropriate line design configuration, including the diameter of the line conductor and the height level above the ground. It is a tedious and expensive process to establish this appropriate line design solely based on experimental findings. Hence, the computational based numerical analysis with the support of final experimental validation aids in arriving at a safe and optimal transmission line design dimension configuration. Thus, the ionic current flow environment of UHVDC transmission lines may be characterized by ionized space charge density, ionic current flow, and electrical field distribution. To estimate this ionic current flow, it is necessary to estimate the ionized space charge density in the ionization region, which incorporates the solution of the Poisson's equation. It is intricate to solve Poisson's equation with known boundary conditions: as ionization process depends on the atmospheric parameters also. Multiple computational techniques are developed to estimate the corona generated ionic current environment under the HVDC lines by waiving these atmospheric parameter effects and claims the lack of computational accuracy. In this paper, the unipolar corona equation, usually called Poisson's equation was solved by estimating the space charge ion density using the concept of gas bimolecular collision theory. The gas bimolecular collision theory inculcates the procedure to estimate the collisional reaction rate constant between the gas molecules at given atmospheric temperature and pressure parameters. Therefore, this paper developed a unique equation solution for the estimation of transition of the ionic current magnitudes at known atmospheric temperature and pressure ambient conditions. This unique equation solution prevents the multiple actual testing procedures offers to obtain safe and optimal line design dimension configurations. Multiple experimental studies are carried out up to ± 900 kV to compare the computed results in the outdoor and indoor climatic conditions at the ultra-high voltage laboratory, CPRI. Also, the computed results are compared with the results published in the literature. Based on the comparison of results, the developed computational technique exactly matches the experimental results and claims improved accuracy compared to other computational techniques.

Parameters Influencing Space Charge Density in Vessels by Spraying Water

Spraying water in vessels generates electrostatically charged clouds of droplets. If the resulting space charge density is sufficiently high, brush discharges to conductive earthed parts may occur. In vessels where potentially explosive atmospheres might occur, the prevention of hazardous space charge density is required. This work gives an overview of all the relevant parameters that influence the space charge density. For this purpose, own measurements are presented and compared with knowledge from the literature. The measurements have been carried out with earthed vessels and nozzles built of conductive material. To take the state of the art into account, pump pressures of up to 2500 bar have been used. The result of this work provides support for dimensioning vessels and water spraying techniques to minimize the risk of ignition hazards due to brush discharges of the electrostatically charged clouds of droplets.

Impact of electric body force on lifted flame displacement speed: Numerical insights

Recent advancements in experimental capabilities have unveiled that externally applied electric fields primarily affect flames through flow modification induced by an electric body force. Specifically, the displacement speed of a flame could be significantly increased (decreased) by a parallel (orthogonal) electric field to the flame's propagation direction, while the flame's propagation speed remained negligibly influenced by an electric field. Here we present numerical findings attributing a modified upstream velocity to an electric body force created by redistributed space charges and a resulted electric field. Our investigation involved both simulation and experiment on propagating edge-flames from a stationary lifted flame exposed to the parallel external electric field. Nitrogen diluted methane jet in a coflow air was employed, and seven charged species (electron, H3O+, O2–, O–, OH–, CHO3–, and CO3–) were included in a reduced methane kinetic mechanism. Both the experiment and simulation consistently demonstrated that enhanced flame displacement speeds resulted from a reverse flow in the upstream, created by the electric body force. A stronger reverse flow, and thus more enhanced flame displacement, was achieved when the electric field direction was aligned with the flame propagation direction compared to the opposite direction. This difference in the reverse flow was attributed to variations in the electric field, caused by the different electric field screening zone within the flame. Surprisingly, space charge densities in the upstream were similar in both polarities, challenging previous hypothesis. The rapid transformation of electrons into O2–, CHO3–, and CO3– imposed a substantial number of charges in the upstream. This insight enhanced our understanding of how flame displacement can be controlled by applying electric fields. Importantly, we suggested that the use of a simple ionic kinetic model may fail to predict the flow field and, consequently, cannot adequately investigate modified flame characteristics.

2-mm-thick large-area CdTe double-sided strip detectors for high-resolution spectroscopic imaging of X-ray and gamma-ray with depth-of-interaction sensing

We developed a 2-mm-thick CdTe double-sided strip detector (CdTe-DSD) with a 250μm strip pitch, which has high spatial resolution with a uniform large imaging area of 10 cm2 and high energy resolution with high detection efficiency in tens to hundreds keV. The detector can be employed in a wide variety of fields for quantitative observations of hard X-ray and soft gamma-ray with spectroscopic imaging, for example, space observation, nuclear medicine, and non-destructive elemental analysis. This detector is thicker than the 0.75-mm-thick one previously developed by a factor of ∼2.7, thus providing better detection efficiency for hard X-rays and soft gamma rays. The increased thickness could potentially enhance bias-induced polarization if we do not apply sufficient bias and if we do not operate at a low temperature, but the polarization is not evident in our detector when a high voltage of 500 V is applied to the CdTe diode and the temperature is maintained at −20 °C during one-day experiments. The “Depth Of Interaction” (DOI) dependence due to the CdTe diode’s poor carrier-transport property is also more significant, resulting in much DOI information while complicated detector responses such as charge sharings or low-energy tails that exacerbate the loss in the energy resolution.In this paper, we developed 2-mm-thick CdTe-DSDs, studied their response, and evaluated their energy resolution, spatial resolution, and uniformity. We also constructed a theoretical model to understand the detector response theoretically, resulting in reconstructing the DOI with an accuracy of 100 μm while estimating the carrier-transport property. First, we evaluated the energy resolution using 22Na, 133Ba, and 57Co reconstructing the DOI effect from the energy correlation on both the anode and cathode sides. We obtained an energy resolution of 4.3 keV (FWHM) at 356 keV. Second, we formulated a theoretical detector-response model that reproduced the experimental data. Using the model, we determined the mobility-lifetime products of carriers μτe,h=(4±1)×10−3,(1.15±0.05)×10−4 [cm2/V] with the space charge density of nsp=−6.0×1010 [1/cm−3] and then reconstructed the DOI with an accuracy of 100 μm. Finally, we evaluated the spatial resolution and demonstrated to resolve patterns of 250-μm-width slits with an X-ray test chart. We found no positional variation in the detector response. We realized the detector that has high energy resolution and high 3D spatial resolution with a uniform large imaging area.

The Calculation Method of the Floating Conductor Potential in the Ion Flow Field

For bipolar DC transmission projects, when one pole conductor is charged and running normally, and the other pole conductor is in a standby state (including cold standby and hot standby), if the standby pole conductor is not grounded, a high charging potential will appear on the surface. On the one hand, it will affect the insulation coordination and protection setting of the DC system. On the other hand, it will also affect the ion current and synthetic electric field on the ground. This article studies the calculation method of the potential of suspended conductors in the presence of floating conductors in the ion flow field, establishes a calculation model based on the minimum absolute value of the space charge density on the surface of the conductor, and calculates and analyzes the distribution of ionizing current and synthetic electric field below the ground of high-voltage DC lines in the presence of suspended conductors. The distribution laws of ground field strength and ion current in single-circuit bipolar DC transmission lines are studied when one pole is suspended, the other is operated, and when one circuit is operated. One circuit is shut down in double-circuit DC transmission lines on the same tower. For practical engineering, on the one hand, it is possible to predict the potential of suspended conductors in the ion flow field and propose adjustment plans for protection settings, insulation coordination, etc., in advance. On the other hand, when considering abnormal operating conditions, the impact of ion flow fields on the ground electromagnetic environment can be reduced by changing the geometric architecture and line selection.

Exploring effect of charge dissipation in charged powder space on electric field distribution in silo

The electric field distribution is important for understanding the underlying mechanism of electrostatic discharge in silos and for developing anti-static strategies. However, the charge dissipation of charged powder usually is not well modeled in the existing electric field simulation, resulting in inaccurate estimation of the electric field in silos. This paper studied the electric field distribution in a cylindrical silo based on the space charge density distribution created by the charged particles. The influences of the normalized filling time, gap distance, and the silo height-to-diameter ratio on the electric field were explored. The results indicate that the area with a larger space charge density shrinks from the heap bottom to the heap surface, leading to a decline in the electric field and suggesting a lower probability of electrostatic discharge at the larger normalized filling time. Continuous decrease of the gap distance when the heap surface is close to the silo ceiling will lead to a sharp increase in the electric field, regardless of the charge dissipation. The silos with a height-to-diameter ratio greater than 2 are demonstrated to have a lower probability of electrostatic discharge and a space higher utilization ratio.