DC breakdown voltage characteristics of saturated liquid helium in the presence of metallic particles

This paper deals with the breakdown voltage characteristics of saturated liquid helium in the presence of a needle-shaped or spherical metallic particle to obtain insulation design data for pool-cooled, low-temperature superconducting coils and to find the predominant factor affecting the breakdown voltage of liquid helium at a given state. The results show the following. (1) The generation of bubbles at the instant of collision of a particle with an electrode is caused mainly by the kinetic energy released from the moving particle to the liquid helium, while the electrostatic energy accompanying the microdischarge between the particle and the electrode has a lesser effect on it. (2) The lowest breakdown voltage at different particle conditions appears in the case of a free needle particle. The low breakdown voltage is caused by the bubble triggered by the particle collision, the high electric field at the tip of the needle particle, and the rich initial electrons supplied by the microdischarge.     

Proposal for new particle deactivation methods in GIS

This paper deals with consideration of particle motion control and its trapping in the deactivation of foreign contaminating moving particle in simulated GIS. The particle motion behavior in nonuniform field gap without/with different shaped spacers was investigated experimentally under DC and 60 Hz AC voltages. The obtained results were confirmed experimentally as well as by calculating the electrostatic force acting on the particle in the nonuniform field gap and are reported elsewhere. Whereas, in this paper, the observed particle motion behavior around simple shaped spacer is simulated by solving the particle motion equations analytically in the presence of electrical gradient and image forces. The simulation agreed fairly well with the experimental results. Furthermore, in the nonuniform electric field, the effect of dielectric coating on the grounded electrode on the particle motion and particle deactivation is also briefly discussed. On the basis of the obtained results of particle motion behavior without/with different shaped spacers and the effect of dielectric coating on the ground electrode, different methods of particle deactivation are suggested as well as confirmed experimentally     

GIS中不均匀直流电场下球状自由导电微粒运动分析

针对气体绝缘系统(GIS)中自由导电微粒无害化的问题,研究导电微粒在GIS中不均匀电场作用下的运动规律,建立球状自由导电微粒在楔形不均匀电极系统中的受力模型,采用Runge-Kutta方法对微粒运动方程进行求解,并对球状自由导电微粒在不均匀直流电场中的运动轨迹进行仿真,同时考虑电压波动和电极表面粗糙等随机因素对微粒运动轨迹的影响。另外,研究了自由导电微粒的运动轨迹与施加电压、微粒初始位置、微粒和电极材料的性质及电极表面状况的关系。研究结果表明:在一定的电场条件下,微粒运动会处于一种谐振状态,而施加电压的幅值、波形以及电极表面的反射系数对微粒运动轨迹有显著影响     

Generation, growth and collapse of bubbles on collision of particle with electrode in DC electrically stressed liquid helium

The paper deals with the bubble formation process in the presence of a free conducting particle in liquid helium to understand the electrical insulation environment in pool cooled superconducting devices under particle contaminated conditions. Experiments were conducted with DC stressed parallel plane electrodes containing a free spherical metallic particle of 1 mm radius in saturated normal and saturated superfluid helium. Experimental results show that a single bubble is always generated at the moment of particle collision with the electrode and the bubble behavior depends strongly on the state of the liquid helium. The bubble expanding and shrinking processes were analyzed on the basis of energy balance among released electrostatic and kinetic energies of the particle at the moment of particle collision with the electrode, kinetic energy of liquid driven by bubble growth, work done on environmental pressure and internal energy of the bubble gas. The analysis shows a fairly good agreement with experiments.     

Electric field and space charge of spherical electrode at highvoltage concentric with a grounded conductive sphere

The solution of the space-charge Poisson's equation is presented for a spherical electrode at high voltage concentric with a grounded conductive sphere. The obtained electric field is given in terms of the ion current emitted by the electrode and an integration constant. Both parameters are functions of boundary conditions. Using measured values of the ion current for a range of boundary conditions and applying these boundary conditions to determine the integration constant suggest a minimum value for the ion mobility of 1.9×10^-4 m² /V.s. Approximate formulas for the ion current and the electric field in terms of the independent parameters were also developed. Also, it was shown that this spherical system with its solution can be used to make accurate evaluation of the ion mobility. The obtained solutions can be applied with good approximation to many practical electrostatic systems involving point electrode at high voltage generating ions. Although the subject matter is very general and could have a fundamental research aspect, in this paper, it is meant to be applied to the area of electrostatic powder paint coating. Therefore, all the examples of boundary conditions and illustrations are typical of the electrostatic powder paint coating     

稍不均匀电场中绝缘子附近导电微粒受力分析

建立稍不均匀电场中绝缘子附近的自由导电微粒所受作用力的2-D计算模型,通过对微粒表面Maxwell应力张量进行计算,得到微粒在电场中所受的静电力。改变电极倾斜角、绝缘子材料以及绝缘子的外形,对影响自由导电微粒运动的因素进行了分析,结果显示通过减小高压电极倾斜角度,减小绝缘子材料的介电常数,并采取合适的绝缘子外形, 可减小微粒受到的向上的静电力和微粒受到的指向绝缘子方向的水平静电力,从而可以减小微粒浮起和附着于绝缘子表面的概率。     

Dynamic analysis of motion of spherical metallic particles in non-uniform electric field

This paper presents a theoretical analysis of the dynamics of motion of spherical metallic particles under non-uniform fields for direct-current gas-insulated switchgear (dc GIS) and for electrostatic separators/sizers (ESS). The particle equations of motion between a pair of diverging conducting plates are numerically solved in three dimensions using a computational algorithm. The upper plate is energized by HV dc or HV ac of variable frequency, while the lower one is grounded. In the case of ESS, the lower electrode is also mounted horizontally on a vibratory conveyor. The results reveal that the particle exhibits several phenomena during motion depending on its initial position, radius and density, angle between the diverging plates, tilt angle of the electrode system, and frequency and amplitude of the applied voltage. The influence of vibratory-conveyor variables on the separation/sizing process is studied in the light of the particle trajectory in the third dimension. Moreover, the results are interpreted using dimensional analysis. The use of HV ac causes the separation/sizing process to be imprecise; the higher the applied-voltage frequency and the larger the particle radius, the higher is the risk of breakdown. Depending on the tilt angle and orientation of the earthed electrode relative to the divergent angle between the electrodes, particle trapping can be helped or hindered. Therefore, several factors should be taken into consideration in the design and installation of the particle drivers and traps in GIS. Finally, the effects of dielectric coating on the electrode and of gas pressure on the particle trajectory are also studied.     

Dielectric Constant Measurements Using Dielectrophoretic Levitation

A dielectrophoretic levitation scheme is used to measure the dielectric constant of insulating dieletric liquids and small solid spherical particles. The fixed ring-disk electrode system is first calibrated against a known standard dielectric liquid by levitating a small gas bubble in the liquid at a fixed location. The measurements are then performed by measuring the voltage required to levitate a gas bubble in the unknown liquid at the same location with respect to the electrode structure. A similar procedure is used for solid spherical particles. The system is unique because it requires no detailed knowledge of the electric field. Fabrication of the electrodes does not require precision machining, and the structure is relatively insensitive to leveling. The size of the gas bubble or solid particle is not critical as long as it is much smaller than the electrode structure. The measurement precision is limited by the accuracy of the voltage measurements and by the accuracy to which the dielectric constant (of the standard) and the specific gravities (of the unknown and the standard) are known. The measurements presented support the contention that this method is a simple and reliable method of measuring the dielectric constant of insulating dielectric liquids.     

Motion characteristics of a charged metal particle in insulating oil under flow state

Problems due to discharge by charged metal particles in insulating oil under flow state are examined in this study. The motion characteristics of a charged metal particle in a horizontal transformer oil tract are investigated, and a mechanical model of a charged metal particle under flowing oil and an AC field is proposed. The particle's equations of motion are numerically solved using the fourth‐order Runge–Kutta computational algorithm. The trajectory of a spherical iron particle in typical motion state under flowing oil and AC field is simulated. The influence on the motion characteristics of a charged metal particle in terms of the oil flow rate, electric field strength of the oil tract, particle scale, initial state, and particle rotation is analyzed. Results reveal that when a charged metal particle is moving fast along the direction of oil tract flow, reciprocating oscillation simultaneously occurs in the vertical direction. Its trajectory is significantly affected by the oil flow rate, the electric field strength of the oil tract, particle scale, initial state, and particle rotation. Moreover, the degree of oscillation of the charged metal particle may be the main cause of degradation of the insulating oil under flow state. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

The study of the performance of an electrostatic valve used forbulk transport of particulate materials

In many mass transfer processes, it is necessary to accurately control the flow of particulate materials. Commonly used mechanical valves have serious drawbacks which can be overcome by the use of electric field, which can locally originate interparticle compressive forces throughout the bulk material as a result of the greatly enhanced electric field and charge flux densities occurring at the contact points between the particles or between the particles and the boundary. Such interparticle electroclamping forces can be established by applying an electric potential gradient between a separated pair of conductive electrode grids placed perpendicularly across the flow within the duct where the material flows. The flow control of particulate materials is, thus, achieved using no moving parts. When an electric field is applied to a packed bed of particulate solids, several types of electrical force (electrostatic attractive force, dielectrophoretic force, and electroclamping force) may be generated, depending on the bulk and surface resistivities of the particle, the geometry of the electrodes, as well as the nature of the applied field. The influence of the electrode geometry on flow control was investigated using computer modeling of the potential based on finite element techniques. Furthermore, the effect of the applied field with respect to the magnitude, frequency, pulsewidth, and pulse shape on flow controllability was experimentally investigated. The influence of the moisture content of turnip seeds on flow controllability and specific charge was investigated, and the results obtained are discussed in this paper     

Electrical characteristics of an electrostatic valve used for bulktransport of agricultural seeds

In an electrostatic valve used for bulk transport of agricultural seeds, the interparticulate electroclamping forces can be established by applying electric potential gradient between a separated pair of conducting electrode grids placed perpendicularly across the flow. The flow control of particulate material is thus achieved using no moving parts. When an electric field is applied, several types of electric field forces are generated, depending on the bulk and surface resistivities of the particles, the geometry of the electrodes, the applied field, and the geometry and the conductivity of the materials used for the transport channel. In this study the current-voltage characteristics of the valve were experimentally investigated for different flow control parameters. The triboelectrification of turnip seeds caused by the frictional contact on the channel walls was investigated and compared with the valve current. A range of wall liner materials with different electrical properties conductive to insulating were tested. The materials used for the wall liner did not significantly influence the current characteristics of the electrostatic valve. The effect of pulse duration of the applied potential on charge-to-mass ratio of the discharged material was studied. The results obtained show that the level of net charge acquired by the particles could be controlled by controlling the duty cycle of the electroclamping field     

Flow visualization and image analysis of gas-phase AC corona discharge induced electrohydrodynamic liquid flow in a stratified fluid

A phenomenon of AC gas-phase needle-plate corona discharge induced electrohydrodynamic (EHD) liquid flow in a stratified fluid has been investigated experimentally. The flow visualization of flow pattern is carried out by a fluorescent dielectric liquid tracer, where the steady EHD liquid motion induced by the corona discharge is presented in visual impressions. The results show that the EHD liquid flow direction on an electrode axis is not from needle to plate electrodes axis when the needle position was above the liquid layer. For the case of the needle electrode tip immersed into the dielectric liquid, it is well known that the dielectric liquid flows in anticlockwise rotation by contraries. The flow velocity distribution is experimentally obtained by a particle image velocimetry with image processing, where the measurement system takes sequential digital images of flow field illuminated by a laser light sheet for computed tomography. The mechanistic model based on interfacial momentum transfer effects on liquid-phase fluid motion is discussed in detail.     

Conducting particle motion and particle-initiated breakdown in dcelectric field between diverging conducting plates in atmospheric air

This paper deals with the analysis of spherical conducting particle motion as well as particle initiated breakdown in electric fields between diverging conducting plates with dc voltage in atmospheric air. Motion of spherical particle was estimated by solving the motion equation numerically, and the results agreed well with the experimental ones. It was found that when the particle is placed on the horizontal electrode surface where the electric field is nearly equal to the lifting field, the particle progresses towards a higher electric field region by the effect of Coulomb force and electrical gradient force. This is true for a nonspherical conducting particle. The obtained results for particle motion suggest that much attention should be paid in the design of GIS particle traps to forces acting on the particle directly towards the higher electric field region. Moreover, the particle-initiated breakdown is discussed on the basis of the experimental and theoretical results. It was found that one of the reasons for reduction of the breakdown voltage is the effect of microdischarge between the particle and an oppositely charged electrode     

Electrostatic actuation of liquid droplets for micro-reactorapplications

A device for the electrostatic transport and manipulation of liquid droplets on a solid surface is described. Arrays of microelectrodes are fabricated on a substrate, which is covered by a hydrophobic layer. Water droplets, typically a microliter in volume, take spherical shape on the surface and are caused to move by switching the voltage applied to the electrode array. The transport of droplets, deflection of a droplet in either of the bifurcating paths, and the mixing of two droplets by coalescence are experimentally demonstrated. The device can be used for microchemical reactors where transport, sorting and mixing of reagents constitute basic unit operation     

Modeling of conductive particle behavior in insulating fluidsaffected by DC electric fields

Several electrostatic technologies, such as separation of granular mixtures, flocking, printing, or biological cell manipulation, are based on the accurate control of conductive particle motion in insulating gases or liquids by means of relatively high DC electric fields. This paper is aimed at characterizing the behavior of such particles by numerical modeling of two aspects: (1) particle motion under the action of electric field forces and (2) insulation breakdown triggered by mobile particles. The equations of particle motion were written by taking into account both gravitational and drag forces, as well as the rebound at particle impact with the electrodes. If the particles move in ionized air, their charge varies in time. In that case, the equation of particle charge should be added to the mathematical model. The output data of the programs for numerical simulation of particle behavior are in good agreement with the available experimental results. Particle movements were shown to be influenced by the intensity of the electric field, by the density of the space charge, by size and mass density of the particles, as well as by their coefficient of restitution at impact with the electrodes. The conclusions regarding the behavior of conductive particles in insulating fluids are useful for the development of improved electrostatic separation technologies; they are of particular interest to all manufacturers of high-voltage equipment     

离子风对静电除尘器中颗粒物脱除的影响机制研究

静电除尘器中电晕放电产生的离子风是影响颗粒迁移和沉积的重要因素,通过建立多过程耦合的静电除尘器数值模型,研究了不同极配型式静电除尘器的电场分布特性及离子风对空间流场和颗粒物脱除的影响规律。结果表明,针刺电极电场强度和离子电荷密度的最高值均略高于芒刺电极,但是芒刺电极的静电场分布更加均匀。在外加电压为60k V时,针刺电极和芒刺电极的离子风最高流速分别可达7.91m/s和4.62m/s,离子风会促进荷电颗粒向收尘极板运动,同时也会形成近壁高速区造成堆积颗粒的不均匀分布并导致二次扬尘。由此,针对不同形式静电除尘器提出了合理调整放电电极结构方法,进行离子风调控以强化颗粒物脱除。     

Motion behavior and deactivation method of free-conducting particle around spacer between diverging conducting plates under DC voltage in atmospheric air

This paper deals with free conducting particle motion around different shaped spacers between diverging conducting plates under DC voltage in atmospheric air. Spherical particle motion was observed experimentally and the results were discussed on the basis of the electrostatic force acting on the particle with and without spacer. The results show that a particle around a spacer tends to move laterally towards or away from the spacer towards higher field regions, depending on the spacer configuration. It lifts at higher field positions and sometimes adheres to the spacer. The effects of spacer angle and spacer material (permittivity) on free conducting particle motion are also discussed. For comparing the results obtained with a sphere, a wire particle motion around different shaped spacers is also investigated. The obtained results suggest one of the methods to prevent/suppress the conducting particle from approaching or adhering to spacers, is to reduce the field strength in the vicinity of the triple junction of gas, solid dielectric and electrode in gas insulated system (GIS).     

Effect of Moving Speed of Charged Metallic Spherical Electrode on Electrostatic Discharge

In this investigation, the nature of the electrostatic discharge (ESD) that occurs when a charged object moves toward a stationary grounded object is experimentally clarified. The spark lengths, discharge currents, and induced voltages in a magnetic probe were measured when a charged metallic spherical electrode connected to a 422 pF capacitor approached a stationary grounded object, which was the current target, for different moving speeds of the charged metallic spherical electrode in a range of 1 mm/s to 100 mm/s. The charge voltages of the capacitor were +6.5 kV and +10 kV. Based on the results, the average gap length shortened with the speed of the spherical electrode. The average peak values of the discharge current and the induced voltage were likely to increase with the speed of the spherical electrode. The average rise times of the discharge current and the induced voltage were likely to drop with the speed of the spherical electrode. The relation between the spark length and the discharge current due to the ESD can be explained qualitatively by using an equation derived from the spark resistance formula proposed by Rompe and Weizel.     

Numerical Modeling of Conductive Particle Trajectories in Roll-Type Corona-Electrostatic Separators

Several attempts have already been made to simulate particle trajectories in roll-type electrostatic separators. However, the predictive value of the results is limited by an excessive number of simplifying assumptions regarding the electric field distribution, as well as particle charging and discharging mechanisms. The present work is aimed at improving the existing models by taking into account: (1) the non-uniformity of the electric field in the active zone of the separator and (2) the effect of spark discharges occurring between the electrodes. Based on previous observations, the conductive particles were assumed to lift-off when no longer exposed to corona discharge. The numerical simulations were performed for particles of various sizes. The electric field was computed in each point of the trajectory using a finite element program. It was found that: (1) some of the smaller particles impact the static electrode and are deviated to the middling compartment of the collector and (2) field annealing which accompanies spark discharges significantly affects the trajectories of conductive particles. The results of this study could guide the design of new electrostatic separation applications.     

Electrical breakdown triggered by a free conducting spherical particle in saturated liquid He I and He II under uniform dc field

This paper is concerned with the pre-breakdown phenomena and the breakdown voltage characteristics, in the presence of a free moving conducting spherical particle, of saturated normal liquid helium (He I) and saturated superfluid liquid helium (He II) under uniform dc field. Experiments show that the particle lifts off around the theoretical value of the lift-off electric field, and the particle oscillates between the electrodes at higher applied voltages. In that case, the microdischarge appears just before the charged particle collides with the oppositely charged electrode, and the bubble is generated at the moment of every collision of the particle with the electrode. It is confirmed theoretically as well as experimentally that the maximum bubble radius in He II is nearly proportional to the E/sub in//sup 1/3/, which is the released energy from the particle for the bubble generation. The insulation environment before the electrical breakdown suddenly changes at the /spl lambda/-point since the bubble behavior in He I and He II greatly differs. In the characteristics of the breakdown voltage vs. liquid pressure, a clear discontinuity appears at the /spl lambda/-point. Furthermore, the breakdown voltage in the parallel plane gap contaminated by a particle is lower than that in the rod-plane gap without a particle. It is found that the breakdown voltage characteristics are closely related to the trigger effect of the microdischarge and the bubble generation.