Uneven Electric Field Research Articles

Theory and Experimental Study of Corona Initial E-Field Strength of HVDC Large Fittings

The electrode were used to imitate the valve tower shield plate, grading ring, grading ball and grading shield for UHVDC valve hall. Uneven electric field of the irregular conductor can not be calculated theoretically or measured accurately. The variation of typical electrode corona inception electric field strength is analyzed and threshold value of the corona inception electric field of the valve hall fittings is acquired. With the adequate modification, limited electric field magnitude for the valve hall is proposed. With the gray system MGM (1, M) model fitting and predicting technology, the E-field strength single-point high-precision measurement and the the wide-area time interval corona initial E-field fitting and predicting operation can be realized. The experimental results show that the maximum error is 3.25% within the measured concentration range, which increases with applied voltage. The maximum RSD% is less than 2.7%, showing the good stability. This system can provide the novel method for the study of corona detection of large fittings in UHVDC valve hall.

The sensitivity of C4F7N to electric field and its influence to environment-friendly insulating gas mixture C4F7N/CO2

C4F7N has excellent insulation performance. By mixing with CO2, it is the highest potential alternative gas to the most greenhouse effect gas insulating medium SF6, which is widely used in gas insulated high voltage electrical equipment. It is necessary to explore its insulation stability under different electrical fields before application. In this paper, the influence of electric field on C4F7N/CO2 mixture with different gas pressures and mixing ratios is tested, analyzed and compared with SF6 by experiment and calculation. It was found that the electric field sensitivity of C4F7N/CO2 gas mixture is much higher than that of SF6 and CO2, where it is easy for partial discharge to emerge under non-uniform electric field. The insulation performance of C4F7N/CO2 gas mixture is extremely susceptible to the internal electric field of electrical equipment, and optimization of the equipment internal structure to avoid uneven electric fields is needed.

Numerical simulation of streamer discharge development processes with multi-component SF6 mixed gas

In recent years, more stringent environmental protection requirements have led to increasingly higher voltage power equipment that adopt SF6 mixed gas as an insulation medium. In this paper, the streamer development in SF6 mixed gas and the distribution of each gas component were studied over time. Thus, a discharge model of the needle plate electrode with SF6 mixed gas was established. The hydrodynamic model for SF6 mixed gas discharge in uneven electric fields was created by coupling the Boltzmann drift-diffusion equation and Poisson's equation. The influence of different factors, such as the applied voltage, gas mixture ratio, and electrode spacing, on the streamer discharge of SF6/N2 mixed gas, and the influence of the type of mixed gas (air and CO2) combined with SF6 on the streamer development process were studied. The simulation results showed that the higher SF6 content in the mixed gas made breakdown more difficult, and a higher pressure led to a more concentrated streamer. Comparing and analyzing the discharge processes under different SF6 mixed gases (SF6/CO2, SF6/air, and SF6/N2) showed that the streamer for the SF6/CO2 mixture took longer to develop toward the plate electrode with a more difficult breakdown. The presented simulation results provide a microscopic interpretation for the needle-plate discharge defects that appear in power equipment that utilize SF6 mixed gas.

Research on the Failure Process of Oil Paper Insulation of Surface Partial Discharge under Fluctuating DC voltage

Partial discharge along the surface seriously damages the oil-paper insulation performance and ultimately leads to insulation failure, which greatly reduces the safety and reliability of the converter transformer operation. It is important to understand the failure process of oil-paper insulation under partial discharge along the surface to avoid fault occurrence. In this paper, the failure process of oil-paper insulation under partial discharge of pulsating DC and AC voltage is studied. A partial discharge test platform for oil-paper insulation under pulsating DC voltage was built. The partial discharge test of oil-paper insulation along the extremely uneven electric field was simulated by using the needle plate and column plate discharge model. The oilpaper insulation under two kinds of voltages under the extremely uneven electric field was analyzed in depth. The similarities and differences of the failure process and their causes, the insulation design and operation and maintenance recommendations for preventing flashover under the extremely uneven electric field are proposed.

On the Performance and Structural Stability of Cathodic Electrocatalysts with Complex Nanoscale Morphology

Increasing complexity of nanosynthesis has produced a variety of intricate nanoparticle shapes which have attracted significant attention for applications in electrocatalysis. These complex nanoelectrocatalyst shapes not only introduce high surface areas and high energy active sites, but also improve electrocatalytic performance by altering electric field at the surface thereby concentrating reagents and accelerating electrocatalysis. However, long-term structural stability of complex nanoelectrocatalysts necessary for their industrial implementation often remains unknown and difficult to predict. Here, we study structural stability in cathodic reactions (CO2 and water electrolysis) of noble metal nanoparticles with star- and core-cage structures that collectively cover major structural nanoscale features including extrusions, holes, voids, and nanogaps. Based on structural and surface analysis, density functional theory and finite element method calculations, we demonstrate that uneven electric field and current density distributions in these morphologies influence metal mobility resulting in their structural degradation. Our results revealed that extrusions and holes facing the anode form electric field hot spots for electrolysis, that also facilitate surface reconstruction. The holes and voids perpendicular to the direction facing the anode form current crowding spots within the metal that facilitate electromigration via local Joule heating, while being blind spots for electrolysis due to low local electric field in these areas. Finally, structural features within nanocages are electrically shielded resulting in them being blind spots for both electrolysis and electromigration. This study provides guidelines for the structural design of electrocatalysts, highlighting that it should not only be dictated by the surface atomic organization and the generation of electric field hotspots, but also by current distribution within the electrode itself, to ensure not only high material performance but also improved stability.

Polarity effect of PD in oil-pressboard insulation under highly uneven electric field

The polarity effect of the partial discharge (PD) in oil-pressboard (oil-PB) insulation under a highly non-uniform electric field is remarkable. For intensive analysis of the characteristics and mechanisms of this polarity effect, a non-uniform electric field was simulated by a needle-plate model with oil-PB composite insulation. The PD statistical features, PD pattern and PD pulse were studied via pulse current method, the charge distribution was simulated on the basis of hydrodynamic drift-diffusion model and bipolar charge transport model, and the cause of the polarity effect was analysed based on the PD mechanism which is quantificationally explained via a PD circuit model. It was found that the average apparent discharge amplitude and PD repetition rate of the neg-PD are higher; the average equivalent frequency and equivalent time of neg-PD are higher and lower respectively; the pos-PD pulse is unimodal and the neg-PD pulse is oscillating with some secondary discharges at AC voltage or a back discharge at DC voltage; and the neg-PD pulse has a narrower width owning to the fast rising edge and falling edge. The PD current pulse in the coupling loop is essentially caused by the charging process of the streamer-shorted defect. The cause of the polarity effect is that the positive streamer evolves slowly and can puncture the entire oil gap with an energy-concentrated channel, whereas the negative streamer with a diffused channel evolves faster but cannot puncture the entire oil gap.

A Liquid-Metal-Based Dielectrophoretic Microdroplet Generator.

This paper proposes a novel microdroplet generator based on the dielectrophoretic (DEP) force. Unlike the conventional continuous microfluidic droplet generator, this droplet generator is more like “invisible electric scissors”. It can cut the droplet off from the fluid matrix and modify droplets’ length precisely by controlling the electrodes’ length and position. These electrodes are made of liquid metal by injection. By applying a certain voltage on the liquid-metal electrodes, the electrodes generate an uneven electric field inside the main microfluidic channel. Then, the uneven electric field generates DEP force inside the fluid. The DEP force shears off part from the main matrix, in order to generate droplets. To reveal the mechanism, numerical simulations were performed to analyze the DEP force. A detailed experimental parametric study was also performed. Unlike the traditional droplet generators, the main separating force of this work is DEP force only, which can produce one droplet at a time in a more precise way.

Eliminating Tip Dendrite Growth by Lorentz Force for Stable Lithium Metal Anodes

AbstractLithium metal anodes are deemed as the “Holy Grail” for next generation high energy density batteries, due to the reported highest specific capacity (3860 mAh g−1) and the lowest negative electrochemical potential (−3.04 V vs the standard hydrogen electrode). However, the notorious tip‐induced dendrite growth leads to low Coulombic efficiency, restricted lifespan, and even catastrophic short‐circuits, blocking the roadmap of their commercialization. Here, a magnetic field is introduced into the lithium plating process. The Li+ concentrated around the tips by the uneven electric field distribution can be taken off the hotpots by the Lorentz force and the tip dendrite growth can be eliminated. The relationship between current density and magnetic flux intensity is established by monitoring the deposited lithium morphology as well as the electrochemical performance, which is confirmed by mathematic modeling and COMSOL Multiphysics simulation. It is also demonstrated that the Lorentz force–induced tip dendrite elimination can be utilized practically by assembling permanent magnet‐containing prototype coin cell. It is anticipated that this physical approach can be applied to other high energy density systems as well.

Porous equipotential body with heterogeneous nucleation sites: A novel 3D composite current collector for lithium metal anode

Lithium metal is an ideal anode material for lithium battery thanks to its ultra-high specific capacity and lowest redox potential. However, uncontrollable growth of lithium dendrites hinders its application due to local aggregation of lithium ions and anisotropic growth of lithium metal driven by uneven electric field distribution. Here a strategy is proposed to address this issue via utilizing porous equipotential body decorated with heterogeneous nucleation sites as advanced three-dimensional current collector. The conductive network of carbon nanofibers is recognized as a porous equipotential body, where the interior electric filed is zero due to the well-defined electric field shielding effect. In addition, copper nanoparticles as heterogeneous nucleation sites are uniformly anchored on carbon nanofibers, which guide lithium deposition evenly. As a result, lower impedance and higher Coulombic efficiency (∼97%) are achieved in modified lithium-copper cell. In particular, the cycling lifetime of modified symmetric lithium-lithium cell is expanded up to 500 cycles at extremely large current density of 50 mA cm−2.

Critical breakdown path under low-pressure and slightly uneven electric field gap

The determination of the critical breakdown path in slightly uneven electric field has played a significant role in gas discharge starting process and electrode surface erosion. In order to study the law of the critical path position in the low-pressure breakdown case, a new algorithm based on the Monte-Carlo collision model and the postulation of " forward-back trajectory for electrons” is established, namely the determination of the critical path(DCP) model. In the DCP model, some electric field lines among the electrodes are regarded as the potential breakdown paths, and the probability of the excitation and ionization collisions between the electrons and the neutrals can be obtained by the Monte-Carlo model. The most probable path to trigger the breakdown will be selected from among all the potential paths, namely the critical breakdown path, and the corresponding breakdown voltage of the critical path will be calculated. A breakdown test with two different electrode devices is performed to examine the accuracy of the DCP model: the critical path and breakdown voltage obtained by the DCP could be examined respectively by capturing the surface traces of negative electrode and measuring the breakdown voltage. According to the test results, the critical breakdown path can transit at different gap pressures or flow rates, and this observation is qualitatively consistent with the calculation results. Meanwhile, the relative error maximum of the breakdown voltage obtained by DCP is less than 7.9%. The accuracy of the DCP model partly depends on the background pressure, and the background pressure in the application case should be less than 103 Pa. Based on the DCP model, the numerical analyses of another four electrode devices are conducted to acquire the common law about the critical breakdown path. According to the calculation results, the transition zone has both a high frequency of critical path transition and a " fluctuant and similarly straight” breakdown voltage curve with the gap pressure or flow rate increasing, and the critical path transition direction follows the rule of " from longer paths to shorter paths”. Lastly, the inherent laws of those regulations about the critical path are revealed by the DCP model.

Alternating Current Discharge Characteristics and Simulation Analysis of Rod-Plane Short Air Gaps under Salt Fog Conditions

In this paper, smog meteorological conditions in the natural environment is simulated by the salt fog method. The study of the alternating current (AC) discharge characteristics of rod-plane short air gaps in salt fog environments has important guiding significance for how to strengthen the external insulation strength of ultra-high voltage (UHV) transmission lines and electrical equipment in smog environments. The rod-plane short air gap is selected as the model to simulate the extremely uneven electric field. The AC discharge test is carried out in the salt fog environment with different conductivity, and the finite element method (FEM) is used to simulate the distribution of electric field in air gap under salt fog environment conditions. The results show that under clean fog conditions the AC discharge voltage in the air gap increased by 15.1% to 35.5% compared to that under dry conditions. With the increased conductivity of salt fog, the AC discharge voltage in air gap decreased by 4.1% to 9.2% compared to that under clean fog conditions, and the reduction is within 10%. The distortion of the electric field and the adsorption of free electrons in the gap by droplets lead to the decrease of the electric field intensity in the air gap. With the increase of the conductivity, the electric field intensity in the air gap increases slightly. Meanwhile, the influence of salt fog and its conductivity on the AC discharge voltage of rod-plane short air gap is examined, becoming saturated with the increase of the gap distance and the conductivity of salt fog.

Electrical Property Test and Research of Insulated Operating Rods with Metal Joint for Live Working

Sectional structure with metal joint is commonly found in an insulated operating rod for live working in an extra- or ultra-high voltage power transmission line but the metal joint is inclined to discharge because uneven distribution of electric fields around the metal joint. In this paper, power frequency discharge tests are made on operating rods with metal joint and insulated joint, including even boost and discharge test and 300s power frequency voltage withstand test and test results are analyzed and studied. Results show: operating rods with two types of joints in the even boost test have basically the same discharge voltage values; the operating rod with insulated joint has significantly higher discharge voltage value than that with metal joint in the 300s voltage withstand test; the metal joint causes local electric field concentration and internode capacitance effect, leading to uneven electric field point position distribution of the insulated operating rod and reduced electrical property. However, the electrical property of the operating rod may be enhanced by mounting a grading ring at the end of the operating rod or coating functional coating outside the operating rod.

Effect of Atmospheric Conditions on Discharge Characteristics of Rod-plane Air Gap

The rod-rod air gap electric fields are perfectly symmetrical in all kinds of extremely inhomogeneous electric field while the maximum asymmetry exists in the rod-plane air gap electric field. Other types of uneven electric field air gap breakdown characteristics fit in between. The rod-plane air gap electric field is an important reference for the designing of insulation strength. In this paper, the impact tests of rod-plate air gap are carried out in the field environment of different altitudes. Discharge characteristic curves of switching and lightning impulse at different environmental factors were given. The increases of the 50% discharge voltage of the rod-plane air gap switching impulse and rod-plane air gap lightning impact are larger. The influence of altitude on rod-plane air gap lightning impact is tiny. Through the analysis of test results, the influence mechanism of environmental factors on the discharge characteristics is also discussed.

Electrohydrodynamic assisted droplet alignment for lens fabrication by droplet evaporation

Lens fabrication by droplet evaporation has attracted a lot of attention since the fabrication approach is simple and moldless. Droplet position accuracy is a critical parameter in this approach, and thus it is of great importance to use accurate methods to realize the droplet position alignment. In this paper, we propose an electrohydrodynamic (EHD) assisted droplet alignment method. An electrostatic force was induced at the interface between materials to overcome the surface tension and gravity. The deviation of droplet position from the center region was eliminated and alignment was successfully realized. We demonstrated the capability of the proposed method theoretically and experimentally. First, we built a simulation model coupled with the three-phase flow formulations and the EHD equations to study the three-phase flowing process in an electric field. Results show that it is the uneven electric field distribution that leads to the relative movement of the droplet. Then, we conducted experiments to verify the method. Experimental results are consistent with the numerical simulation results. Moreover, we successfully fabricated a crater lens after applying the proposed method. A light emitting diode module packaging with the fabricated crater lens shows a significant light intensity distribution adjustment compared with a spherical cap lens.

Investigation of flame structure in plasma-assisted turbulent premixed methane-air flame

The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes (pretreatment of reactants and direct in situ discharge). OH-planar laser-induced fluorescence (PLIF) technique is used to diagnose the turbulent structure methane-air flame, and the experimental apparatus consists of dump burner, plasma-generating system, gas supply system and OH-PLIF system. Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes: regime I for voltage lower than 6.6 kV; regime II for voltage between 6.6 and 11.1 kV; and regime III for voltage between 11.1 and 12.5 kV. In regime I, aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role, while in regime III, the temperature rising effect will probably superimpose on the chemical effect and amplify it. For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field, the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape. With regard to in situ plasma discharge in flames, the discharge pattern changes from streamer type to glow type. Compared with the case of reactants pretreatment, the flame propagates further in the upstream direction. In the discharge region, the OH intensity is highest for in situ plasma assisted combustion, indicating that the plasma energy is coupled into flame reaction zone.

Advanced biofuel news: Joule, Red Rock Biofuels announce merger

Most of the anodic aluminum oxide (AAO) templates are performed by potentiostatical method at 0–10 °C to inhibit the Joule's heat enhanced dissolution in aluminum oxide for well-ordered cell configuration. In this article, we propose the hybrid pulse anodization (HPA) method with effective suppression of Joule's heat generation to fabricate AAO at room temperature. Effects of purity of aluminum (Al) foils and pulse voltage on the evolution of pore characteristics have been investigated. The AAO morphology is captured by scanning electron microscope and analyzed via gray-scale imaging in order to identify the pore size distribution. The increased applied potential results in the widened pores and non-uniform cell arrangement due to the increased current density and variation. Moreover, low-purity Al foils lead to the reduced AAO distribution uniformity owing to the uneven electric field induced pits on the Al surface for inferior pore arrangement. Extending both the positive and negative pulse period from 1 s to 5 s during HPA can enhance the distribution uniformity of AAO from high-purity Al by up to about 95%. In addition, the relationship between AAO configuration and Al purity and pulse voltage is further discussed and established.

Ejection of Electrode Molten Droplet and Its Effect on the Degradation of Insulator in Gas Spark Switches

Electrode erosion is a notable issue influencing the performance and restricting the lifetime of gas switches. In the erosion process, electrode materials heated by spark channel will melt or evaporate and be driven to depart from electrode surface, contaminating the gas switch insulator, and potentially inducing flashover accidents. In this paper, sputtering spots are observed near the erosion crater in a two-electrode gas switch, which indicates the existence of molten droplet ejection. In addition, a set of polymethyl methacrylate rings are inserted in the gas switch to detect the ejection of molten droplet as well as to simulate the switch insulator for investigating the degradation of surface insulation strength. The results show that the ejection of molten droplet is along the tangent surface of electrode with a small divergent angle. After repeated discharges, the insulator surface is bombarded by the droplet ejection and forms dense cracks and embedded metal particles in a narrow band. The spatial varied droplet ejection causes a decrease in both flashover electric field and surface resistance. It also leads to different surface resistances in different regions, which can result in an uneven electric field distribution. Thus, droplet ejection increases the probability of surface flashover accidents in gas switches.

Status assessment of polymeric materials in mineral oil under electro-thermal aging by frequency-domain dielectric spectroscopy

Transformer development is limited to smaller sizes and larger capacities due to two major shortcomings: first, the solid insulation that is used in an oil-immersed transformer, which is generally cellulosic paper, cannot resist high temperatures, and second, the significant difference in the dielectric constants between the cellulose paper and the transformer oil will lead to an uneven electric field. In light of this situation, in combination with the requirements of oil-immersed transformers for the dielectric, physicochemical, and mechanical properties of solid insulations, in this study two kinds of polymeric materials are chosen as the research objects - Polycarbonate (PC) and Polyester (PET) - that can resist high temperature, and then, a comparative analysis is made with cellulose paper. They are put in the transformer oil for electro-thermal aging for 300 days at 90, 110, and 130°C, respectively. The variation characteristics of permittivity and the dielectric dissipation factor with frequency were examined in stages (30 days, 140 days, 180 days, 250 days, and 300 days). This paper studies the characteristics of polymer materials so as to analyze whether they can be used in an oil-immersed transformer by considering the materials’ physical properties, electron microscope scanning (SEM), and degree of polymerization (DP) at different stages during aging. This valuable exploratory work has never been studied before. If these two new materials can be applied, it will be of great significance to the innovation of the insulation structure of a power transformer.

Negative Corona Onset Characteristic of the UHV Conductors Based on the Corona Cage

Negative corona onset characteristics of the stranded conductors based on the ultra-high voltage (UHV) corona cage were studied in the paper. Based on the corona self-sustaining criterion in extreme uneven electric fields and the secondary emission process of the photoelectrons, the corona onset calculation model in the UHV corona cage is established and the corona current tests of the single LGJ900-75, 6 bundle LGJ900-75, 8 bundle LGJ400-35 conductors in dry and rain conditions were done in the UHV corona cage, and the rain rates are 2.4 mm/h, 20 mm/h and 30 mm/h. Corona onset electric field strength is gained by E-I tangent method, and the onset electric field strength in dry condition proves that the calculation model can be used to calculate the corona onset characteristics of the bundle conductors in the UHV corona cage. A further analysis proves that: the negative corona onset voltage of the conductor increases with the bundle number and the diameter of the sub conductor, but decreases with the bundle space in the corona cage. The onset electric field strength is influenced little by bundle space and bundle number, but decreases with the increase of the diameter of the sub-conductor. The surface irregularity coefficient decreases with the rain rate.

Slightly uneven electric field trigatron employed in tens of microseconds charging time.

To solve the issue of operation instability for the trigatron switch in the application of tens of microseconds or even less charging time, a novel trigatron spark gap with slightly uneven electric field was presented. Compared with the conventional trigatron, the novel trigatron was constructed with an obvious field enhancement on the edge of the opposite electrode. The selection of the field enhancement was analyzed based on the theory introduced by Martin. A low voltage trigatron model was constructed and tested on the tens of microseconds charging time platform. The results show that the character of relative range was improved while the trigger character still held a high level. This slightly uneven electric field typed trigatron is willing to be employed in the Tesla transformer - pulse forming line system.