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.     

Sliding Discharge Optical Emission Characteristics

In this work, several optical studies in an atmospheric pressure sliding plasma sheet have been performed. This discharge is generated using two electrodes flush mounted on an insulating flat plate (upper electrodes), and a third electrode flush placed on the opposite side of the plate facing the upper inter electrode gap (lower electrode). A DC negative voltage is applied to one of the two upper electrodes and to the lower electrode, while the other upper electrode is biased with an AC voltage. In this configuration a sliding discharge is produced on the flat plate within the upper electrodes gap. The sliding discharge optical emission of the spectral bands corresponding to the 0-0 transition of the second positive system of N₂ (lambda = 337.1 nm) and the first negative system of N₂ ⁺ (lambda = 391.4 nm) have been measured. Also the light spatial distribution in the plasma sheet has been studied using a CCD camera coupled to interferential filters corresponding to the wavelengths investigated. The reduced electric field in the plasma sheet has been derived from the measurement of the intensity ratio of the nitrogen lines. This study has been realized varying the amplitude of the DC voltage and the amplitude and frequency of the AC voltage. The reduced electric field strength is found to be almost constant for all the experimental conditions, with a value of 500 plusmn 100 Td (1 Td = 1.10^-17 V cm²).     

The impact of electrode dielectric coating on the insulationintegrity of GIS/GITL with metallic particle contaminants

Coating the inside surface of gas insulated switchgear (GIS) enclosures with a dielectric film reduces the deleterious effect of electrode surface roughness, impedes the development of metallic particle initiated microdischarges, increases the field required to lift particles, and reduces the charge acquired by particles, all of which help alleviate the adverse effect of contaminating metallic particles on insulation withstand. The performance of particle-contaminated compressed gas systems with dielectric coated electrodes is analyzed. Two mechanisms for the transfer of charge from electrodes to contaminating particles are considered, namely, conduction through the coating layer and microdischarges in the surrounding gas. The paper presents an electrostatic study of the particle lifting fields with dielectric covered electrodes. The overall breakdown strength of the system is evaluated and the results are discussed in the light of experimental findings     

Dynamic motion of a conductive particle in viscous fluid under DCelectric field

When plastic waste is liquefied for recycling, the impurities present could influence the quality of the reprocessed products. In order to explore the possible method for removing solid impurities using electrostatic forces, the motion of spherical conductive particles under a uniform electric field has been carefully investigated. In this paper, a solid impurity and liquefied plastics are simulated by a conductive spherical particle and viscous insulating liquid, silicone oil, respectively. Experimental results indicate that the particle undergoes repeated motion between the parallel electrodes. The motion of the particle can be divided into four modes: settling on the lower electrode, moving upward, settling on the upper electrode, and moving downward. The higher the applied voltage, the faster the average particle velocity and the shorter the resting time of the particle. The particle accelerates after leaving the electrode and decelerates before reaching the other electrode. This deceleration could be explained by the viscous effect of the liquid layer between the particle and the electrode. The settling mode could be explained by the fact that there is a liquid flow induced by the particle motion and it pushes the particle against the electrode until the flow decreases. It is, therefore, suggested that the hydrodynamic effect is dominant in our case, as well as the electrostatic force     

Dynamics of metallic particle contaminants in GIS withdielectric-coated electrodes

Electrical insulation performance of compressed gas insulated switchgear (GIS) and gas insulated transmission line (GITL) systems is adversely affected by metallic particle contaminants. Dielectric coatings applied to the inside surface of the outer enclosure of a coaxial GIS/GITL system improve the insulation performance in several ways. Coating has the effect of smoothing the electrode surface and reducing the prebreakdown current in the gas gap. Also, the electrostatic charge acquired by a particle is reduced and hence the range of its motion under an applied power frequency field is inhibited. The movement of such particles is complex and dependent on several parameters. In this paper the dynamics of wire particles in a coaxial system under AC voltage is studied when the inside surface of the outer enclosure of a coaxial GIS/GITL system is coated with a high resistance material. Suggestions for reducing the particle excursion in GIS/GITL systems are discussed     

Motion of conducting particles causing inadvertent outages in GIS

Inadvertent failure of a HV gas-insulated system (GIS) is traced to a seemingly innocuous mechanism in particle-induced breakdown. Impending dangers from inconspicuous conducting particles in a large physical system are quite intriguing. The dynamic behavior of the particle due to electric field/particle/fluid medium interaction is a complex phenomenon. To understand this interaction a credible database has been generated for the field-induced motion of the particles on a physical prototype model. Induced surface charge acquired by the particle is calculated based on the proposed model (field calculations) and by considering it to be a prolate ellipsoid. Methods for deactivating potentially dynamic particles using dielectric coated active parts and grounded enclosures, and a novel technique called reverse polarity charging have been suggested. Performance of GIS in the presence of metallic particles has been studied with bare electrodes and with dielectric coatings of different conductivities with a view to obtain an optimal value of the conductivity     

工频电压下片状自由金属微粒带电特性与运动规律研究

GIS/GIL因安装、运输等环节会不可避免地产生多样性的金属微粒,而片状金属微粒因其"潜伏性"较难检出,从而威胁GIS/GIL的绝缘性能。采用数值计算结合实验方式开展了工频电压下片状金属微粒起跳与运动规律研究。片状金属微粒采用旋转椭球体模型进行等效,并通过理论推导获得片状金属微粒的带电量与临界起跳场强。搭建工频电压下片状金属微粒运动观测平台。采用高速相机观测其起跳和运动行为,并与数值计算结果进行对比分析。结果表明:片状金属微粒起跳场强与厚度的1/2次方近似呈正比关系,而随微粒半径增大呈现出微弱的上升趋势。密度较大的片状金属钢微粒竖起后,存在原地旋转与沿地电极表面来回滚动两种运动模式,并在微粒底部与电极间隙伴有微弱的局部放电;密度较小的片状金属铝微粒竖立后,在电极间隙内发生剧烈地上下跳动。     

DC corona discharge from a wire particle floated with a microgap in parallel plate electrodes

Conductive particles existing in a high-voltage system could deteriorate the system performance owing to an electric discharge via the particles. In case when the particles exist in a dc high-voltage electrode system, the location of the particle would inevitably affect the discharge aspect. A corona-onset and breakdown voltage from a metallic wire particle were measured in detail. The wire particle has a length ranging from 6 to 12 mm, and a diameter of 0.25 mm. It was fixed between a parallel plate electrode with a spacing of 20 or 30 mm. When the particle was close to the negative electrode, a corona discharge occurred stably, and the corona-onset voltage was increased as the gap length between the negative electrode, and the positive end of the particle increased up to 0.8 mm. In contrast, when the particle was close to the positive electrode, the breakdown always occurred without preceding the corona discharge. At the middle region between the electrodes, the corona onset occurred at almost constant voltage.     

Charging and behavior of a spherically conducting particle on a dielectrically coated electrode in the presence of electrical gradient force in atmospheric air

This paper discusses the charging and behavior of a spherically conducting particle on a dielectrically coated electrode in non-uniform dc and ac electric fields between non-parallel plane electrodes in atmospheric air. The charging mechanism of a particle resting on a dielectrically coated electrode is investigated by observing light emissions from partial discharges and by utilizing the dust figure technique. The charge acquired by a particle is estimated from the results of particle motion onset voltage measurement and direct measurement with a Faraday cage. The basic particle movement as well as particle-triggered breakdown characteristics under non-uniform electric field distribution along the electrode surface is also investigated. The results suggest that the effect of the electrical gradient force/dielectro-phoretic force on particle motion on a dielectrically coated electrode should be adequately considered in the design of gas insulated systems (GIS). This is because a particle can move laterally on the coated electrode toward increasing electric field regions by the action of the electrical gradient force which is independent of the charge on the particle. Also the possibility of the particle charging and lifting at high field regions or reaching and adhering onto an insulating spacer further increases the risk of breakdown.     

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。     

Bipolar DC corona discharge from a floating filamentary metalparticle

The motion of a filamentary particle and also the discharge from a particle have been extensively investigated for the development of a prevention and protection method against breakdown in gas-insulated switchgear. The particle showed several different patterns of motion, e.g., "standing," precession motion, oscillating up and down, etc. Each of these types of motion depends on corona discharge from the particle itself. Since this motion cannot be explained only by monopolar discharge from one end of a particle, bipolar discharge from both ends of a particle occurring simultaneously has been investigated by applying the floating particle method. The particle length and the gap between the particle and electrode were chosen as parameters. From experimental results, several interesting features were obtained. The curve of bipolar corona. current lies between negative and positive corona currents of monopolar curves. There is a sudden current increase at the threshold voltage that seems to be a new feature of the corona current from a filamentary particle. So far, the experimental results indicate that this threshold voltage is not a function of particle length, but it could depend slightly on length. Flashover voltage is a function of particle length and the gap. It decreases with length and increases with the gap     

Discharges from moving particles in GIS [gas-insulated switchgear]

The study of the motion of moving metallic particles in GIS requires a good knowledge of the carried charge of the particle. In this paper, measurements of the charging process of a moving particle, performed in a realistic single-phase GIS, are presented. By measuring the charge impulses and the voltage phase at consecutive impacts, it is possible to estimate the carried charge and then compare the results with the existing models of the particle charge. Also, besides the discharges from in-flight corona reported by several authors, it is shown that, for higher voltage levels, several discharges between the particle and the enclosure (outer electrode) can occur as the particle impacts with the enclosure     

交流电压下绝缘子附近导电微粒运动特性

在施加交流电压条件下,对楔形布置的电极间聚四氟乙烯(PTFE)绝缘子附近自由导电微粒运动进行了实验观察.通过对微粒运动过程进行摄像和记录微粒起始运动电压,讨论球形导电微粒和线形导电微粒的运动特性.对不同外形绝缘子附近导电微粒运动规律进行了分析,认为绝缘子的存在能够降低微粒运动起始电压,并且能够吸引部分球形导电微粒沿电极表面滚动到达绝缘子表面;当绝缘子表面与接地电极夹角为锐角时绝缘子附近的微粒运动起始电压会显著降低.对绝缘子存在情况下的楔形电极间电场进行计算,并对微粒运动规律做出了合理的解释.     

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).     

Motion of metallic particles in gas insulated systems

The presence of metallic contamination can seriously reduce the insulation performance of a gas insulated system (GIS). This presentation is focused on the motion of elongated particles, since these particles are considered the ones most frequently found in GIS. The aim of the work has been to learn more about the motion and thereby to suggest improvements to the models that traditionally have been used. With a good model, a more accurate calculation of the motion of a particle and, in turn, a better estimation about its severity in a system can be made     

Motion and size estimation of a free moving metallic particle in GIS based on propagation properties of acoustic waves

We have investigated the propagation properties of Lamb waves and motion characteristic of free moving metallic particle in a gas‐insulated switchgear (GIS) tank to diagnose the insulation performance of GIS. We have proposed a new identification technique for defect location in GIS based on the time–frequency analysis of the Lamb waves using the Wavelet Transform. In this paper, we measured the acoustic signals excited by a metallic particle colliding with the tank sheath. Furthermore, we measured the moving particles under a high electric field in GIS using a high‐speed video camera. As a result, we examined the relationship between maximum lifting height and the flight time during subsequent impacts of a moving particle. Moreover, we estimated particle size based on the propagation properties of Lamb waves. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(1): 26–33, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20023     

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     

燃煤可吸入颗粒物在驻波声场中动力学特性的研究

为研究可吸入颗粒物PM 2.5在声波场中的动力学特性, 利用可视化技术建立研究颗粒物动力学特性的实验装置。采用电厂飞灰作为颗粒源，利用高速CCD摄像机动态地显微拍摄颗粒在水平声场中的运动轨迹，并结合数值方法理论模拟颗粒的运动轨迹。结果显示颗粒在驻波场的波节点处不振动、不飘移；波腹处只振动、不飘移；其余各点处的颗粒既振动、也向波节方向漂移，并与竖直方向的运动叠加，形成在声场中的运动轨迹。颗粒振动频率与声波频率相同，但因颗粒存在惯性，振动速度幅值小于声波速度幅值，且相位滞后。动力学特性的结果显示：振动使得颗粒在单位时间内扫过的面积大大增加，有利于颗粒间的碰撞团聚；漂移特性有利于颗粒间的碰撞和气流中颗粒的分离。     

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     

Dielectrophoretic manipulation of particles

We have demonstrated that, in general, the dielectrophoretic (DEP) force experienced by a particle has two components. The first depends upon inhomogeneities in the applied electrical field strength and upon the in-phase part of the resultant dipole moment induced in the particle and can be identified with the conventional dielectrophoretic (cDEP) force. The second relates to nonuniformities in the phase distribution of the applied field and to the out-of-phase part of the induced dipole moment. A nonuniform phase distribution corresponds to a field that travels through space. This second force component gives rise to traveling-wave dielectrophoresis (twDEP). In this paper, we describe several electrode configurations designed to produce electric fields capable of inducing cDEP and twDEP forces for the purpose of manipulating particles. Using DS19 Friend murine erythroleukemia cells as test particles having well-characterized dielectric properties, we investigated the electrokinetic behaviors for these electrodes as a function of frequency and electrode excitation mode. Several characteristic cell electrokinetic behaviors were identified, including trapping, linear motion, levitation, and circulation of the cells, depending on the excitation characteristics of the electrodes and the cell dielectric properties. We describe these findings and rationalize them in terms of the field distributions produced by the electrodes, the particle dielectric properties, and generalized dielectrophoresis theory. The biotechnological applications of dielectrophoretic manipulation are then discussed