Space Charge Generation Research Articles

A method for calculating space charge distribution in ultra-high voltage direct current transmission lines based on the transport of diluted species theory

The occurrence of corona discharge in ultra-high-voltage direct current transmission lines results in the generation of space charge, which significantly affects the nearby electromagnetic environment. To investigate the micro-physical mechanisms of discharge and provide better guidance for the calculation of the ground ion current density and total electric field, a method based on the theory of the transport of diluted species is proposed. This method enables the visualization and analysis of the movement process of space charge in air medium, as well as the calculation of the total electric field below the transmission lines. To validate the effectiveness of this algorithm, experimental tests of the total electric field are conducted on actual transmission lines. The experimental results demonstrate that the algorithm not only highlights the evident differences in the transient microscopic processes between positive and negative corona discharges but also accurately calculates the total electric field below the transmission lines. In addition, considering the weather conditions in the Chongqing region, relevant recommendations are provided under the influence of wind speed, aiming to offer protection guidelines for individuals engaged in activities near transmission lines.

Numerical analysis of electro-thermo-convection in a differentially heated square cavity with electric conduction

Natural convection with an electric field in the classic differentially heated square cavity is numerically studied. The electric conduction model for the generation of free space charges, which applies to weak and moderate electric field with weakly conducting liquids, is specially considered. The whole set of governing equations is implemented in the open-source finite-volume framework of OpenFOAM. Thorough investigation has been undertaken to analyze the thermal and flow characteristics of electro-thermo convection. The results reveal that the introduction of an electric field leads to a suppressive influence on flow motion across all considered Rayleigh numbers (Ra), aligning with recent experimental findings. This effect becomes more pronounced with increasing conduction number (C 0), resulting in the reduction of flow intensity and a thicker thermal boundary layer. Consequently, heat transfer is subdued due to the electric field, causing a decrease in the Nusselt number (Nu) as C 0 increases. To elucidate the mechanism how the electric field impacts natural convection, the torques induced by the electric and buoyancy forces are computed. Higher C 0 will lead to a lower buoyancy torque and stronger electric torque, where the electric torque is opposite to the buoyancy torque, thus the weaker flow strength is shown at higher C 0. Finally, to quantify the reduction in heat loss, the relative Nusselt number (Rnu) is introduced. It is found that there is a critical Ra corresponding to minimal Rnu, and for the parameters considered in this study, the minimum Rnu of 0.563 is observed at C 0 = 0.2 and Ra = 1.2 × 104. For large Ra, the Rnu almost keeps constant with increasing Ra.

Nonlinear evolution of space synthetic electric field in positive DC corona mode transition

Positive corona discharge in the air exists under several distinctive forms depending on DC voltage amplitude, including pulses or pulseless glow. Previous studies obtained the current and optical image characteristics of corona discharge through experiments. However, the synthesized electric field characteristics of the different corona modes have not been deeply studied by experiments, and lack of sufficient understanding of the effect of space charges. In this paper, electro-optical modulation technology is used to investigate the characteristics of the space synthetic electric field (SSE) for positive DC corona modes transition. The experimental results show the significant nonlinear evolution characteristics of the SSE and discharge modes transition with the increase of applied voltage. In the pulsed mode, the SSE first changes slowly and then increases sharply. As the applied voltage increases, the discharge mode transition from pulse mode to pulseless glow mode, while the SSE changes slowly. The electric field change rate in the sharp increase is 2–3 times that in the slow change. Dynamic accumulation behavior of space charges is the main reason for the nonlinear evolution of the SSE. When the pulse current starts, the discharge amplitude and frequency are low, and the ions are far away from the ionization region, which has little effect on the migration process of the high field strength region and subsequent ions. With the increase of the applied voltage, the continuous accumulation of positive ions enhances the migration of subsequent positive ions, thereby rapidly increasing the SSE. During the glow discharge mode, the generation and dissipation of space charges are close to dynamic equilibrium, and the fusion of multiple positive ion clouds will increase the SSE.

The Influence of Physical Parameters on Space Charge Characteristics in Cable Insulation Under AC Electric Field

The space charge dynamic behavior and accumulation are important contributions to the formation of electrical trees, as recognized by crosslinked polyethylene (XLPE) cable insulation under alternating current electric stress. In this paper, in order to investigate the reasons for the charge accumulation phenomenon revealed by experiments under ac stress, a bipolar space charge transport model with symmetrical and asymmetrical parameters is built under AC stress. The charge accumulation mechanism is analyzed based on the variation of space charge density with different voltage application time and voltage phase. The effects of each major physical parameter are simulated and analyzed, respectively. The results show that there is no obvious space charge accumulation with the transport model under symmetrical physical parameters. However, asymmetrical parameters in the transport model result in different motions between positive and negative charges, which greatly enhances the charge accumulation. Each main parameter has an influence on charge accumulation in different perspectives and extents. The applicability of the simulation model with asymmetrical parameters is evaluated by comparing experiment and simulation results. This study contributes to the understanding of space charge generation mechanism under AC voltage and serves as a basis for improving the stability of XLPE electric power cables.

Measurement and Numerical Simulation of Discharge Characteristics in Air at Medium Frequency Voltages

Due to the increasing use of power electronic converters in electrical grids, high voltage insulation systems are stressed by medium frequency voltages. The breakdown voltage of air decreases with increasing frequency especially for inhomogeneous electric fields. Here, breakdown voltages for different homogeneous and inhomogeneous electric fields are measured. Finite element method plasma simulations of space charge generation and their influence on the electric field distribution before the breakdown are done to support the explanation of the decreasing breakdown voltage in the frequency range from 50 Hz up to 30 kHz.

Systems Theory: Simulation Analysis of Space Charge Generation Mechanisms in Transformer Oil under High Electric Field

Simulation analysis plays an important role in Systems Theory nowadays. In order to reveal the mechanism of space charge injection and generation in the process of liquid dielectric breakdown, the discharge of transformer oil between needle-plane electrodes under high electric field is studied. Based on the 2-D hydrodynamic model and Poisson equation of electric field, the system simulation model of oil discharge by different charge generation mechanism is established, based on which the inception and propagation process of discharge is simulated by COMSOL. By system simulation, the temporal and spatial distribution of the electric field intensity, space charge density, electric potential and temperature of transformer oil is obtained and deeply analyzed. The simulation results prove that the space charge generated by metal field emission and ionic disassociation are neither of the major factor for streamer formation in transformer oil, while the Zener molecular ionization and impact ionization are the major factors. Our research improves the understanding of the inception, propagation and breakdown process for discharge in transformer oil, and also the ionization mechanism in the liquid dielectric

The Difference in Molecular Orientation and Interphase Structure of SiO2/Shape Memory Polyurethane in Original, Programmed and Recovered States during Shape Memory Process.

In order to further understand the shape memory mechanism of a silicon dioxide/shape memory polyurethane (SiO2/SMPU) composite, the thermodynamic properties and shape memory behaviors of prepared SiO2/SMPU were characterized. Dynamic changes in the molecular orientation and interphase structures of SiO2/SMPU during a shape memory cycle were then discussed according to the small angle X-ray scattering theory, Guinier’s law, Porod approximation, and fractal dimension theorem. In this paper, a dynamic mechanical analyzer (DMA) helped to determine the glass transition start temperature (Tg) by taking the onset point of the sigmoidal change in the storage modulus, while transition temperature (Ttrans) was defined by the peak of tan δ, then the test and the calculated results indicated that the Tg of SiO2/SMPU was 50.4 °C, and the Ttrans of SiO2/SMPU was 72.18 °C. SiO2/SMPU showed good shape memory performance. The programmed SiO2/SMPU showed quite obvious microphase separation and molecular orientation. Large-size sheets and long-period structures were formed in the programmed SiO2/SMPU, which increases the electron density difference. Furthermore, some hard segments had been rearranged, and their gyration radii decreased. In addition, several defects formed at the interfaces of SiO2/SMPU, which caused the generation of space charges, thus leading to local electron density fluctuations. The blurred interphase structure and the intermediate layer formed in the programmed SiO2/SMPU and there was evident crystal damage and chemical bond breakage in the recovered SiO2/SMPU. Finally, the original and recovered SiO2/SMPU samples belong to the surface fractal system, but the programmed sample belongs to the mass fractal and reforms two-phase structures. This study provides an insight into the shape memory mechanism of the SiO2/SMPU composite.

A Tutorial on Theoretical and Computational Techniques for Gas Breakdown in Microscale Gaps

Paschen’s law (PL), derived based on the Townsend avalanche (TA) condition, is commonly used to predict gas breakdown. For microscale gaps near atmospheric pressure, TA is insufficient to drive breakdown and ion-enhanced field emission (FE) dominates. Accurately predicting breakdown voltages for these gaps is critical for numerous applications, including environmental remediation, medicine, combustion, and propulsion. This tutorial summarizes theoretical and computational approaches for predicting this behavior and demonstrating the transition between the TA and FE mechanisms. It focuses on the derivation of closed-form solutions from a theory that accounts for the generation of additional positive space charge at the cathode due to electrons generated by the strong FE-induced electric fields. Appropriate simplifications using a matched asymptotic analysis in terms of the total ionization in the gap agree well with simulations and experiments and show the transition from FE for small gaps to PL at larger gaps. Specifically, this theory shows that the breakdown voltage varies linearly with gap distance when FE dominates, agreeing with both the experimental and simulation results. The particle-in-cell Monte Carlo collision (PIC/MCC) simulations used to predict the ionization coefficient provide additional insight into the mechanisms involved. Future benefits of extended theoretical and computational research for examining microscale and nanoscale breakdown and electron emission, particularly assessing the impact of electrode surface structure and device design and coupling with additional emission mechanisms, will be discussed.

Space charge suppression of polyethylene induced by blending with ethylene-butyl acrylate copolymer

High-voltage direct current (HVDC) power cables have been paid more attention due to the big issue concerning the main insulation materials. The residues in the industrial production process will cause space charge generation and accumulation in the materials, which has become one bottleneck to limit the development of power cables up to higher voltage levels. In this paper, the LDPE matrix was modified by three types of ethylene-butyl acrylate (EBA) copolymers (1.0 wt%) with different polarities (BA content) via melt blending to optimize the space charge behavior of the LDPE insulation. The micromorphology and structure of the blends are examined by polarized light microscope and differential scanning calorimetry. The space charge distribution is tested by the pulsed electroacoustic method. The trap level is studied by the thermally stimulated current. The results show that EBA introduces more deep traps and decreases the shallow traps. The medium-polar EBA (16% BA content) can effectively suppress charge accumulation, and have the same suppressed effect on XLPE/EBA blends. This study will provide important insights into the design and development of advanced HVDC power cable applications.

Study on the ageing of insulators based on the surface charge accumulation process

In order to study the transient motion of the space charge around the air tower insulator and the charge distribution of the insulator, a two-dimensional mixed numerical model of the insulator string based on chemical reaction is proposed. The simulation is used to obtain the space charge generation mechanism and its distribution on the insulator surface. The chemical reaction of particles mainly involves ionisation and recombination. Based on the simulation model, it is found that the accumulation of electrons on the surface of the insulation is the main reason for the distortion of the surface of the insulator. With the increase of the discharge time, the electron temperature decreases and the reaction rate decreases. The ionisation reaction gradually reaches the dynamic equilibrium. The umbrella skirt under the surface of the ion to stabilise the moment to be earlier than the umbrella skirt surface ions to achieve the steady state of the moment. A numerical model of air discharge based on Monte Carlo collision is proposed, which can well simulate the distribution of charge around the insulator of atmospheric tower. It is very important to study the phenomenon of the insulator along the surface discharge.

Measuring the charge density along the radius in concentric cylinders configuration by sensing system

The measurement of space charge density caused by the corona discharge around the overhead transmission line is a tough problem till nowadays. For this problem, a system for measuring the one-dimensional (1D) distribution of space charge density in a coaxial cylinder model under negative corona discharge in ambient air is developed. This system is a scale model which can provide research basis for actual overhead line measurement. The system consists of acoustic emission, space charge generation, and signal-receiving systems. The sound wave of an ultrasonic transducer is used to stimulate space charge generated by a small corona cage, and the applied voltage ranges from −34 kV to −38 kV. Charge vibration produces an electrical field (E-field) that can be measured with an E-field antenna. After obtaining the E-field signal, the adaptive noise cancellation method is adopted to eliminate noise interference so that the accuracy of charge density can be improved. The Townsend's analysis solution is used to calculate the theoretical value of space charge density. Experimental results are consistent with the simulation value and thus confirm the validity of the proposed adaptive noise cancellation method and sensing system.

Basic mechanisms of electrification of weakly conductive multicomponent media

Four different physical mechanisms of electrification (generation of an uncompensated electric space charge) in electrohydrodynamic flows of weakly conductive multicomponent mixtures of liquids and gases are analyzed.

Modeling of UHV and Double-Circuit EHV Transmission-Line Exposure to Direct Lightning Strikes

This paper comprises the first systematic investigation into the factors that render ultra-high voltage and double-circuit extremely high voltage transmission lines more vulnerable to direct lightning strikes by shielding failure. Such lines are typically characterized by one or more of the following features: massive conductor bundles, extreme conductor heights, and exceptionally high operating voltage. The model addresses the effects of the upward connecting leader onset of conductor proximity, ground field due to cloud charges prior to negative leader descent, positive space-charge generation at ground wires, as well as the operating voltage during the positive half cycle. It has been found that such factors have significant effects on the lateral attractive distance of both conductor bundles and ground wires. An agreement between model findings and field experience is found to be generally satisfactory.

Effect of Dispersion of Platinum Nanoparticles on the Proton-Conducting Properties of Strontium Cerate and Zirconate

Introduction Ionic properties of metal oxides possibly changes in the vicinity of the interface with a hetero-phase material due to the formation of space charge layer and/or the strain originated from different coefficients of thermal expansion between the two materials, followed by the rearrangement of charge carriers [1]. In particular, the former case can be examined in a material with well dispersed second phase particles as an intriguing approach to alter the macroscopic properties. [2, 3]. This so-called nanoionics composite materials can be applied in energy conversion devices such as solid oxide fuel cells that operate over low to intermediate temperature ranges if the effect enhances the ionic conductivity. In this work, we have examined and clarified the effects of homogeneously dispersed platinum nanoparticles on the proton conductivity of strontium cerate and zirconate perovskites. The work demonstrates that interfaces engineering can either enhance or reduce the conductivity of proton perovskites type oxides. 2. Experimental SrCe0.95Yb0.05O3 − δ (SCYb), SrZr0.9Y0.1O3 − δ (SZY) and their composites with platinum were prepared by combustion synthesis using EDTA, citric acid, and ammonium nitrate as polymerization agents [2, 3]. The amount of platinum was adjusted so that the amount of platinum metal in the composite oxide was 0.5 vol % to the original metal oxide. Phase purity of the samples were confirmed by X-ray diffraction (XRD). The conductivity was measured using a four-probe AC impedance method with changing the surrounding atmospheres in turn, wet Air, wet Ar, wet 1%-H2- wet Ar, wet Air, which are all saturated with water vapor at 17°C: p(H2O)=1.9 kPa. 3. Results and discussion Single phase perovskite type structures were confirmed for both platinum doped and undoped SZY (Pt-SZY) and SCYb (Pt-SCYb) sintered at 1350 and 1100 °C, respectively. These temperatures were optimal for complete solid dissolution of platinum, and no metallic platinum phases were detected. The highest relative density obtained for both specimens were 83.7 and 70.2 % respectively. The electrical conductivity of Pt-SZY and Pt-SCYb measured at 800°C under reducing and oxidizing atmospheres revealed a reversible nonionic phenomenon as a result of precipitation and dissolution of platinum nanoparticles. Fig.1 shows the electrical conductivity of Pt-SZY and Pt-SCYb as a function of time in the respective atmospheres. As observed, the electrical conductivity of both specimens are higher in air mainly due to hole conduction within this range. The electrical conductivity then decreases significantly when the atmosphere is change to hydrogen as shown in Fig. 1(a) for Pt-SZY. In the case of Pt-SCYb an opposite change takes place, i.e. an increase in the conductivity can be seen for the same changes of the atmosphere (Fig. 1(b)). Comparing these electrical conductivities with the un-doped electrolytes, the introduction of platinum is understood to have an effect that lead to the decrease in conductivity for Pt-SZY. This change in electrical conductivity has been explained in terms of a percolation model [2, 3] in which precipitated platinum nanoparticles are assumed to form a resistive skin in the oxide phase due to band bending, thereby blocking the electrical conduction pathways in SZY. The electrical conductivity behavior of Pt-SCYb in 1% H2 seems to suggest an accumulation of positive charges at the vicinity of the interface between Pt-SCYb with respect to the band bending phenomenon. Band bending usually occurs at the hetero-interface of proton conductors with metal due to the differences in the work function and the corresponding generation of space charges [2, 3]. This work function mismatch can either lead to an increase in the concentration of protons that are in equilibrium with the holes and electrons. However, the conductivity of Pt-SCYb, even though indicated an initial increase in 1% H2, the conductivity value is similar to the conductivity of SCYb in the same atmosphere. Thus the conductivity of Pt-SCYb decreases in air relative to the conductivity of SCYb. References 1) J. Maier, Nature Materials, 4 (2005), p. 805 2) Matsumoto, H., Tanji, T., Amezawa, K., Kawada, T., Uchihisa, Y., Sakai, T., Ishihara, T., Solid State Ionics, 182 (2011), p. 13 3) Matsumoto, H., Furuya, Y., Okada, S., Tanji, T., Ishihara T., Science and Technology of Advanced Materials, 8 (2007), p. 531 Figure 1

Electro-optical modulation with immunity to optical damage by bipolar operation in potassium lithium tantalate niobate.

A method for suppressing the formation of optical damage in quadratic electrooptic devices operated at short wavelengths is presented. Formation of optical damage is attributed to the generation of a trapped space charge induced by photoionization of impurity ions by the propagating beam. It is shown that in potassium lithium tantalate niobate where the electrooptic effect is quadratic, operating the electrooptic device by a bipolar driving voltage prevents the space charge from accumulating, which inhibits the formation of the optical damage. A 6 hours continuous operation of electrooptic modulator for a 30 W/cm(2) at λ = 445 nm input beam is demonstrated.

Improvement of polypyrrole nanowire devices by plasmonic space charge generation: high photocurrent and wide spectral response by Ag nanoparticle decoration.

In this study, improvement of the opto-electronic properties of non-single crystallized nanowire devices with space charges generated by localized surface plasmon resonance (LSPR) is demonstrated. The photocurrent and spectral response of single polypyrrole (PPy) nanowire (NW) devices are increased by electrostatically attached Ag nanoparticles (Ag NPs). To take advantage of plasmon-exciton coupling in the photocurrent of the device, 80 nm of Ag NPs (454 nm = λmax) were chosen for matching the maximum absorption with PPy NWs (442 nm = λmax). The photocurrent density is remarkably improved, up to 25.3 times (2530%), by the Ag NP decoration onto the PPy NW (PPyAgNPs NW) under blue light (λ = 425-475 nm) illumination. In addition, the PPyAgNPs NW shows a photocurrent decay time twice that of PPy NW, as well as an improved spectral response of the photocurrent. The improved photocurrent efficiency, decay time, and spectral response resulted from the space charges generated by the LSPR of Ag NPs. Furthermore, the increasing exponent (m) of the photocurrent (JPC ∼ V(m)) and finite-differential time domain (FDTD) simulation straightforwardly indicate relatively large plasmonic space charge generation under blue light illumination. These results prove that the performance of non-single crystallized polymer nanowire devices can also be improved by plasmonic enhancement.

Complex Electron Heating in Capacitive Multi-Frequency Plasmas

Complex spatio-temporal electron heating dynamics are observed in kinetic simulations of geometrically symmetric low-pressure capacitive argon plasmas driven by multiple consecutive harmonics of 13.56 MHz. These dynamics are caused by an electrically induced asymmetry that leads to the self-excitation of plasma series resonance oscillations of the current. Such oscillations cause a nonsinusoidal movement of the boundary sheath edges and multiple phases of fast sheath expansions. These expansion phases lead to the generation of negative space charges that propagate into the bulk, where they affect the heating rate significantly and relax quickly.

Investigation of buffer traps in AlGaN/GaN-on-Si devices by thermally stimulated current spectroscopy and back-gating measurement

Thermally stimulated current (TSC) spectroscopy and high-voltage back-gating measurement are utilized to study GaN buffer traps specific to AlGaN/GaN lateral heterojunction structures grown on a low-resistivity Si substrate. Three dominating deep-level traps in GaN buffer with activation energies of ΔET1 ∼ 0.54 eV, ΔET2 ∼ 0.65 eV, and ΔET3 ∼ 0.75 eV are extracted from TSC spectroscopy in a vertical GaN-on-Si structure. High back-gate bias applied to the Si substrate could influence the drain current in an AlGaN/GaN-on-Si high-electron-mobility transistor in a way that cannot be explained with a simple field-effect model. By correlating the trap states identified in TSC with the back-gating measurement results, it is proposed that the ionization/deionization of both donor and acceptor traps are responsible for the generation of buffer space charges, which impose additional modulation to the 2DEG channel.

Investigation of Space Charge Distribution of Low-Density Polyethylene/GO-GNF (Graphene Oxide from Graphite Nanofiber) Nanocomposite for HVDC Application

This paper reported a research on space charge distribution in low-density polyethylene (LDPE) nanocomposites with different types of graphene and graphene oxide (GO) at low filler content (0.05 wt%) under high DC electric field. Effect of addition of graphene oxide or graphene, its dispersion in LDPE polymer matrix on the ability to suppress space charge generation will be investigated and compared with MgO/LDPE nanocomposite at the same filler concentration. At an applied electric field of 80 kV/mm, a positive packet-like charge was observed in both neat LDPE, MgO/LDPE, and graphene/LDPE nanocomposites, whereas only little homogenous space charge was observed in GO/LDPE nanocomposites, especially with GO synthesized from graphite nano fiber (GNF) which is only -100 nm in diameter. Our research also suggests that dispersion of graphene oxide particles on the polymer matrix plays a significant role to the performance of nanocomposites on suppressing packet-like space charge. From these results, it is expected that nano-sized GO synthesized from GNF can be a promising filler material to LDPE composite for HVDC applications.

Complete suppression of reverse annealing of neutron radiation damage during active gamma irradiation in MCZ Si detectors

For the development of radiation-hard Si detectors for the SiD BeamCal (Si Detector Beam Calorimeter) program for International Linear Collider (ILC), n-type Magnetic Czochralski Si detectors have been irradiated first by fast neutrons to fluences of 1.5×1014 and 3×1014neq/cm2, and then by gamma up to 500Mrad. The motivation of this mixed radiation project is to test the radiation hardness of MCZ detectors that may utilize the gamma/electron radiation to compensate the negative effects caused by neutron irradiation, all of which exists in the ILC radiation environment. By using the positive space charge created by gamma radiation in MCZ Si detectors, one can cancel the negative space charge created by neutrons, thus reducing the overall net space charge density and therefore the full depletion voltage of the detector. It has been found that gamma radiation has suppressed the room temperature reverse annealing in neutron-irradiated detectors during the 5.5month of time needed to reach a radiation dose of 500Mrad. The room temperature annealing (RTA) was verified in control samples (irradiated to the same neutron fluences, but going through this 5.5month RTA without gamma radiation). This suppression is in agreement with our previous predictions, since negative space charge generated during the reverse annealing was suppressed by positive space charge induced by gamma radiation. The effect is that regardless of the received neutron fluence the reverse annealing is totally suppressed by the same dose of gamma rays (500Mrad). It has been found that the full depletion voltage for the two detectors irradiated to two different neutron fluences stays the same before and after gamma radiation. Meanwhile, for the control samples also irradiated to two different neutron fluences, full depletion voltages have gone up during this period. The increase in full depletion voltage in the control samples corresponds to the generation of negative space charge, and this increase in concentration of negative space charge goes up with the neutron fluence. If we assume the reverse annealing is also taking place for the two gamma-irradiated samples with similarly different concentrations of negative space charge generated, the observed effect of no changes in space charge (no changes in Vfd) in these two gamma-irradiated samples would imply that concentrations of positive space charge created in these two control samples are different at the same gamma dose, and gamma irradiation effectively “switched off”, the RT (room temperature) reverse annealing of neutron irradiation. It has also been found that as soon as the gamma irradiation stops, the RT reverse annealing of neutron irradiation-induced defects resumes with same rate as that of the control detectors. This behavior in mixed radiation samples (neutron plus gamma) would suggest some nonlinear effect (defects induced by mixed-radiations are not additive of those by individual radiation alone), or interaction of radiation induced acceptor-type and donor-type defects.