Corona discharge from a grounded needle electrode inserted into a charged droplet cloud

The discharge occurring in a space-charge cloud was investigated by using a cloud of charged droplets. To produce a charged cloud with a definite space charge density and to cause a reproducible discharge, droplets ejected from an airless nozzle were induction charged and transported by air flow. The transportation of charged droplets by air flow was found to be quite effective in increasing the charge on the droplets and the charge density. Corona discharge was observed at a grounded needle electrode inserted into a charged droplet cloud with a charge density not exceeding 13 μC/m³. In negatively charged droplets, positive streamer coronas occurred. As the charge density of the cloud increased, the time interval of streamer pulses decreased and the discharge shifted to a glow corona. In positively charged droplets, the height and frequency of negative corona pulses increased with the charge density and the velocity of the charged droplet cloud. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 1–7, 1998     

Measurement of Charge Transfer in a Capacitive Discharge

An experimental investigation has been undertaken to measure the charge transfer from a conducting object during a capacitive discharge. To establish the validity of the absolute measurement of charge using a Faraday cup, a sphere was charged in a uniform electric field. It was confirmed that the charge agreed with the theoretical predicted Felici value. The sphere was then charged in a uniform electric field via a spark and the charge collected agreed with the Pauthenier charging model. The initial attempts to measure the charge using the accepted oscilloscope technique with an RC network grossly underestimates the real value of charge. A composite electrode consisting of an isolated smaller electrode located at the point of discharge has been shown to be accurate to within 5 percent. An explanation of the electrostatic principles is given to describe the operation of the system.     

Streamer current in a three-electrode system

A study has been conducted on positive streamer discharges in air at atmospheric conditions for a three-electrode system, The electrode system consisted of two parallel planes (one grounded and one supplied with a negative dc voltage) and a small, insulated needle, sticking out from the center of the grounded plane. A triggering positive square impulse voltage of 5 μs duration was applied to the insulated needle and the currents associated with the streamer discharge were measured simultaneously on all three electrodes. During the streamer propagation, the current measured at the needle was the conduction current while the other two were the displacement (or capacitive) currents generated by the movement of charge in the electrode gap. The objective of this study is to identify the three currents and to investigate if simple representations of the streamer can reproduce the displacement currents measured at the plane electrodes. Two models for the streamer were applied: (1) a charged sphere moving in the background field and (2) a channel with a constant voltage gradient extending in the gap. In both models it was assumed that the streamer propagated with a constant velocity, which was estimated from the measurements. The motion of the streamer was simulated by a series of electrostatic calculations, using a field calculation program. Comparison of the measurements with the simulations indicates that the charge of the streamer is confined to a spherical region (i.e. streamer head) and it is increasing continuously during its advancement in the electrode gap. A discussion on advantages and disadvantages with the two investigated models (sphere vs. channel with potential gradient) is conducted, and a possible hybrid model is suggested. In the proposed model, features from both considered streamer representations are included     

Observation of propagating brush discharge on insulating film with grounded antistatic materials

A propagating brush discharge (PBD) usually occurs on a highly charged thin film with a grounded metal backing plate and often becomes energetic enough to ignite a flammable gas/vapor and even a dust cloud. In an actual situation, a PBD is seen on the surface of a large-sized flexible intermediate bulk container (FIBC) during its filling and/or emptying processes in an industrial site when the fabric of the FIBC is highly insulating. Anticipating that a PBD could occur even on an antistatic FIBC if it were used with an insulating inner bag, various types of antistatic fabrics for FIBCs in contact with an insulating polyethylene terephthalate (PET) sheet were put to the test. An antistatic fabric was grounded, and the PET sheet on the fabric was charged with a corona charger. A PBD was then generated by approaching a grounded spherical electrode. The discharge patterns were observed, and the currents and charge amounts were measured. A thin metallic fiber woven on an insulating fabric was found to drastically enhance the transferring of charge during a PBD, probably because of the corona discharges emanating from the metallic fiber. The resistivity of the fabric also affected the charge transference. The incendiary nature of each mode of PBD was discussed.     

Studies on electrostatic surface discharges on corona-chargedpolymer surfaces

Electrostatic surface discharge phenomena on corona-charged thin polymer films such as FEP or PTFE Teflon, with surface potentials from 1 to 5 kV in both polarities, were studied. A grounded sphere electrode, 5 mm in diameter, approached the charged surface by a computer-controlled step motor until the surface discharge occurred. Discharge-current waveforms, with a very short risetime (a few nanoseconds), were observed to be related to the length (5-30 cm) or type (single wire or plain mesh) of the earth line. The discharge gap length between the film surface and the grounded electrode was also calculated. The results were strongly dependent on the polarity of the surface charge and were widely divergent, indicating the complicated surface discharge mechanism     

Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

Formation of a negative impulse discharge induced by an XeCl excimer laser in a long rod‐to‐rod electrode gap in air

Negative discharge induction experiments were performed with a rod‐to‐rod electrode configuration having a gap length of 0.8 m using a high‐power XeCl excimer laser of maximum output energy of 2 J/pulse and an impulse generator of maximum voltage of 1.2 MV. Development of a leader in the laser‐ionized plasma channel was observed by using an image converter camera. When an XeCl excimer laser is applied at a carefully controlled instant, the guiding effect for an impulse discharge is optimized. Three main results were obtained. First, the discharge started near the focal point and then developed bidirectionally toward the high‐voltage electrode and the grounded electrode. Midgap leaders were frequently observed. Second, it was observed that the lifetime of the laser‐ionized plasma channel was about 1 μs. Third, the discharge processes were classified into six stages. The mechanism of the streamer and leader propagation is discussed. © 2000 Scripta Technica, Electr Eng Jpn, 134(2): 11–18, 2001     

Charging of insulating spheres on the surface of an electrodeaffected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pauthenier's formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed     

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.     

Analysis of the behavior of ions produced by pulsed coronadischarge

The motion of an ion cloud which is produced by a corona discharge with the high-voltage pulse of a short duration and is introduced into parallel plate electrodes was calculated numerically. The ion cloud initially placed at the tip of a needle electrode was simulated by a number of ring charges and the trajectories of individual ring charges were calculated by using electric field strength obtained with a charge simulation method. Not only movement of the ion cloud but also the waveform of the induced current flowing through a ring electrode located at the center of the parallel plate electrode were simulated and analyzed. The ion cloud with an initial diameter of 0.5 mm expands to 5 mm by electrostatic repulsion within 10 μs after the beginning of drift. During drifting toward the counter electrode, the ion cloud extends wider to a diameter of around 20-25 mm. The size and velocity of ion clouds agree with those estimated by experiments and the waveform of induced current obtained by experiments was reproduced by this simulation     

Minimum Potential of Charged Insulator to Cause Incendiary Discharges

Incendiary discharges can occur when a grounded metal sphere approaches a charged insulator, but there are few studies on its electrostatic potential causing ignition. The aim of this research is to determine the minimum potential required for ignition of flammable gases and vapors mixed with air. Discharges between a charged plastic film and grounded metal spheres are experimentally investigated, and an equation for estimating the discharge energy is presented. Then the minimum potential to produce the incendiary discharge is considered from this equation and compared with the experiments. It becomes clear that the minimum potential is determined by the diameter of the grounded metal sphere, the potential of the charged plastic film, and its apparent capacitance. The results obtained with hydrogen-air and propane-air mixture are reported.     

Distribution of electric field strength around a large-scalecharged particle cloud

To investigate an electrical discharge occurring from or in a space-charge cloud, a large-scale charged particle cloud was formed by using a cloud generator consisting of a blower and corona charger. The distribution of the electric field strength around a charged particle cloud has been investigated to determine the behavior of charged particles. The soil-conditioning particles were charged by corona charging and blown by high-speed air flow in a test room, 5 m wide, 10 m long, and 3 m high. The average charge-to-mass ratio of the particles blown by this method was 170 μC/kg. The space-charge density of the cloud was calculated at the order of 10 μC/m³ from the electric field strength outside of the cloud. While the electric field strength at the outside of the cloud increased up to 52 kV/m within 2 m downstream from the cloud generator, it decreased below 25 kV/m farther than 2 m away from the cloud generator due to dispersion of charged particles. The change in the electric field strength due to dispersion of charged particles can be qualitatively explained by a simple cloud model. The velocity of charged particles transported by air flow and mobility of charged particles are found to be effective factors increasing the electric field strength around the large-scale charged particle cloud     

Fast electrostatic discharges between a charged thin dielectricfilm and a spherical electrode with or without dielectric coating

Short-gap (less than 1 mm) electrostatic discharge (ESD) between a charged thin dielectric film on an earthed-plate electrode and a spherical electrode, connected to the plate, with or without an uncharged thin dielectric film coating was observed. Discharge-current waveform, discharge gap length, current rise time, discharging photograph, and current peak value were recorded by the authors' measuring system. The initial charge supply effects by irradiation of β-ray or ultraviolet light were found to be not highly apparent; that is, the reproducibility of ESD experiments was not greatly improved. Surface-charge-polarity dependence of surface ESD between charged and uncharged dielectric films decreases with increase of the film thickness. The discharge gap length of ESD with a 1.5 μm uncharged film coating on the spherical electrode decrease to 50 or 70% compared with that between a charged film and a bare spherical metal electrode. Peak current and transferred charge value of ESD also decrease to roughly one-third of that between the film and the spherical electrode     

Accurate modelling of field stresses in an unshielded three-core power cable

This paper uses the charge simulation technique to calculate the potential and electric field stress distribution at the surfaces of the conductors and the dielectric of an unshielded three-core power cable. The conductors and the sheath are simulated by a suitable finite number of complex line charges. The unshielded three-core cable is placed on a grounded plane and at varying heights above it to calculate the electric field as a function of the height of the insulated cable from the ground. The maximum value of the electric field stress at the grounded plane due to the presence of the insulated three-core cable is detected to check the discharge activities at the grounded plane surface. Comparison is made with field stresses arising from a single-core unshielded insulated cable which are given in the literature. The charge simulation technique is found to be satisfactory to model and calculate accurately the field stresses in a three-core unshielded power cable.     

大气压氦气介质阻挡放电的二维演化过程

为了研究大气压氦气的介质阻挡放电机理,利用像增强高速相机(ICCD),以20ns曝光时间,拍摄了多组时序发展的时空分辨放电图像,揭示了放电形式由汤森放电向辉光放电的演化。所测阴极位降区厚度0.4mm,应属亚辉光放电类型,而底面拍摄的图像则揭示了放电径向发展的过程。将放电图像转化为可视性更强的三维图像后,发现放电由覆盖整个电极的微弱汤森放电起始,在场强略高的中心处率先发展,并迅速向外扩散。     

纳秒级脉冲电压作用下聚丙烯膜反极性充电现象的研究

采用针-板电极系统，对无纺聚丙烯过滤膜纳秒级脉冲电压作用下电介质膜空间电荷形成过程进行研究。发现了聚丙烯膜积累的空间电荷与所加脉冲电压极性相反的聚合物膜反极性充电现象。正极性脉冲峰值越高越容易引起聚合物膜反极性充电。峰值足够高且脉宽足够窄的负极性脉冲也导致反极性充电。脉宽大于200ns的负极性脉冲作用下，聚合物膜充电过程为同极性充电，并且电荷积累量与脉冲峰值无关，随着脉宽的增大均趋于同一个稳定值。据此，文中提出了基于强场注入和反向注入两种充电过程并存的新理论模型，强调了针-板空气间隙固有放电特性对聚合物膜充电过程的影响。     

基于有限元弱解式的棒-板长空气间隙先导放电空间电场仿真研究

对棒-板长空气间隙先导放电过程的空间电场分布以及带电离子浓度等特征参数进行仿真计算研究。建立棒-板长空气间隙放电的二维模型,导出流注-先导放电的二阶偏微分方程,通过有限元弱解形式(weak form)数值计算方法求解先导放电过程中产生的电子、正、负离子浓度与空间电场的大小。仿真结果显示：气体放电所产生的空间电荷对空间电场分布影响显著;间隙距离1.5 m的棒-板长空气间隙下,外加500 kV、250/2 500 ms正极性操作电压时的先导放电起始条件为,流注头部带电离子浓度达到4′1013 cm-3数量级,空间电场最低达到10 kV/cm;先导放电形成后,先导通道内电场约为1~2 kV/cm,先导起始时间在400 ms左右且以3′104 m/s速度传播;有限元弱解形式能有效消除计算离子流中的数值振荡,使偏微分方程求解迅速收敛。     

Characteristics of the effect of an impulse supply voltage on the structure and characteristics of a surface discharge

Results of an experimental investigation of surface barrier discharge in a three-electrode system are presented. An analysis is done of the flow intensity of charged species extracted from the plasma layer of the surface discharge by the field of high dc potential of the third electrode. A high unipolar periodic impulse voltage of 15-kHz frequency, impulse duration from 0.5 to 45 ms, and 150-ns impulse-front duration is used to form the surface discharge. A It is revealed that the voltage-impulse duration has a significant effect on the intensity of charged species flow (extracted current), the character of the influence being dependent on the voltage-impulse polarity. The influence of voltage-impulse duration on the surface-discharge current is revealed as well. Pictures of the structure of surface discharge at the rise and the fall of the voltage impulse made by means of a high-speed camera are presented. The difference of the discharge structure for different polarities of voltage impulse is revealed. It is suggested that the surface-discharge characteristics are dependent on a combination of a number of factors, such as the parameters of the impulse voltage, volume charge in the gas gap, and the charge on the barrier surface, as well as the material and surface structure of the barrier.     

A finite-element analysis of the electrostatic force on a uniformlycharged dielectric sphere resting on a dielectric-coated electrode in adetaching electric field

In the electrophotographic process, charged toner particles are transferred from one surface to another with an electric field. To enable electric field transfer of toner, the externally applied field strength must be greater than a threshold value, so that the Coulomb force can overcome the toner adhesion force at the supporting surface. In this paper, the threshold field strength to detach a charged dielectric particle is determined efficiently by using the Galerkin finite-element method to simultaneously solve the Laplace equation for the field distribution and an overall constraint equation for the force balance. This computational method also enables calculation of the electrostatic adhesion force and is applicable to various particle-electrode configurations. For illustrative purposes, however, we consider the axisymmetric problem of electric field detachment of a dielectric sphere with uniform surface charge resting on a planar dielectric-coated electrode. The analysis is particularly focused on the dependence of the electrostatic force upon the dielectric overcoating thickness and spacing between parallel plate electrodes. The electrostatic force on a uniformly charged particle in contact with a surface is found to be influenced significantly by the thickness of dielectric overcoating and the spacing between electrodes when either becomes less than five times the particle radius     

Influence of charged droplet cloud on flashover of air gaps for lightning impulse voltage

Transmission lines passing through a mountainous region often are struck by winter lightning. As a result, numerous double-circuit faults occur. Space charge might be responsible for this phenomena. Several investigations on flashover characteristics have been performed focusing on reduction of flashover voltage due to ionic space charge formed by corona discharge. In this paper, flashover characteristics of an air gap within a charged droplet cloud ejected from an airless nozzle using an induction charging method are discussed. The droplets are charged in the range -200 to 200 μC/kg, forming the space-charge density of approximately 13 μC/m³. The flashover voltage and the time lag to flashover were measured when 1.2 × 50 μs lightning impulse voltage applied to a rod-sphere gap is placed within the charged cloud. Flashover voltage for a positive rod increased markedly within a positive charged cloud by increasing the charge-to-mass ratio. The increase of flashover voltage was up to 80 percent in comparison with uncharged droplets. Flashover voltage for the negative rod decreased about 20 percent in the cloud of either polarity. The results show that the presence of charged droplets in the air gaps affects the discharge process significantly.