Influences of probe geometry and experimental errors on spatial resolution of surface charge measurement with electrostatic probe

When an electrostatic probe is used for surface charge measurement on an insulating plate with constant thickness or a long insulating pipe, the configuration of the measuring system is regarded as shift-invariant and the relation between the surface charge density and the probe output can be treated in the spatial frequency domain through Fourier analysis. In the inverse calculation from the probe output to the charge distribution, a Wiener-filter technique is effectively used to suppress the excessive amplification of the noise. Influences of the probe geometry and experimental errors on the spatial resolution of the total measuring system, including the restoration process with the Wiener filter, are quantitatively discussed through the analysis of transfer characteristics in the spatial frequency domain.     

Measurement of surface charge on opposite sides of a planar insulator using an electrostatic probe

A digital image processing technique using the Fourier transformation in the spatial domain and the Wiener filter has been developed. This method is applied to the measurement of the accumulated charge on the opposite sides of a planar insulator using an electrostatic probe. In the spatial frequency domain, the relational equations between the probe outputs and charge distribution are given by the first order simultaneous equations of two unknowns. In the course of the estimation of the charge distribution from the probe outputs, the Wiener filter is used to reduce the influence of the experimental noise. The spatial resolution of this method is also quantitatively discussed from an analysis in the spatial frequency domain. Using this technique, the distribution of the surface charge density on both surfaces of an insulating plate is measured immediately after the occurrence of a surface discharge. When the probe scans both sides of the plate, the charge distribution is estimated with a spatial resolution of 1.6 mm. In the case that the probe scans from only one side of the plate, the two-layered charge distribution is estimated with a limited resolution.     

Charge distribution measurement on a truncated cone spacer under DC voltage

A system for measuring charge distribution on an insulating spacer is developed. An electrostatic probe is set close to the spacer, and it moves along its surface maintaining a small gap. The accumulated charge is inversely calculated from the measured data by utilizing the relationship that is obtained through the numerical electric field computation. Using this system, the surface charge distribution on a truncated cone spacer of 80 mm diameter and 15 mm height is measured. The number of measured points is 3402, and the spatial resolution is 3.9 mm. After the application of DC 10 kV for 19 hours, the spacer surface is charged with a spotted pattern. The charge density reaches 60 pC/mm/sup 2/ at its maximum. In addition, the residual charge distribution of partial discharge from metallic particles on the spacer is observed.     

基于数字图像处理技术的绝缘子表面电荷测量标度问题的研究

如何实现对绝缘子表面电荷分布进行准确的测量,是高压绝缘领域一个重要的课题。针对这一问题,采用了一种基于数字图像处理的标度方法;在采用静电容探头法测量厚度一定的绝缘子表面电荷时可近似把整个测量系统按平移不平系统进行处理;通过作二维傅里叶变换,把传统的表面电荷测量标度过程转化到频域中完成。同时,为了提高标度的精度,结合数字图像处理中维纳滤波复原技术抑制测量中噪声项的影响。文中给出的算例表明,基于频域的标度方法可以大大地缩短标度计算耗时,并且在一定程度上改善了标度计算的精度。     

Charge-Decay Characteristics of Granular Materials Forming Monolayers at the Surface of Grounded Electrodes

Laboratory studies and in-field observations have shown that the charge-decay characteristics of the granular materials at the surface of the grounded roll electrode significantly influence the outcome of the electrostatic separation process. This paper validates an indirect method of charge-decay characterization, based on the measurement of the electrical potential at the surface of a monolayer of granular insulating material. The study was performed on three materials–-polyvinyl chloride, polyethylene, and rubber–-extracted from chopped electric wire wastes. The granules (characteristic size in the range 1–4 mm) were disposed on the surface of a grounded plate electrode (layer area: 100 mm $ times$ 100 mm; electrode area: 200 mm $ times$ 200 mm). A wire-type corona electrode, energized from a dc high-voltage supply, was employed for charging the granules. The potential due to the charge at the surface of the granular layer was measured with the capacitive probe of an electrostatic voltmeter connected to a personal computer. Data acquisition and processing were done using the LabView environment. The influence of particles characteristics and of ambient factors was studied. The findings enabled a more accurate modeling of discharging phenomena that affect the performances of electrostatic separators. The method can be easily adopted in electrostatic discharge studies for material characterization.      

真空环境下介质表面电荷分布的测量方法

对比分析了真空中介质表面电荷分布测量中三类较实用的方法:表面电位法、静电容探头法、普克尔斯效应反射法,阐述了真空中介质表面电荷测量方法的研究新成果,最后提出了这一领域留待解决的新问题。     

High resolution measurements of surface charge densities on insulator surfaces

The uses and limitations of the electrostatic probe for the measurement of charge densities on insulating surfaces are discussed. A development of the technique is described in which two important limitations have together been overcome: (i) The effects on the probe signal of charges on all points of the surface have been taken into account by means of a matrix inversion procedure; (ii) A robotic control system has been developed which enables the probe to follow and scan a wide range of axi-symmetric insulator profiles. The degree of resolution achieved enables the probe system to display and measure charge densities in individual streamer channels of a corona discharge on a polytetrafluoroethylene (PTFE) surface. An example is given and comparison made with a dust figure of the same event.     

伽玛线辐射对环氧树脂表面电荷消散特性的影响

应用在核电站、宇宙航天器等辐射环境中的环氧树脂绝缘材料因表面电荷积聚而易于引发沿面放电事故,研究高能辐射对材料表面电荷动态特性的影响对保障电气设备的绝缘安全具有重要意义。为此,选取厚1.5mm的环氧树脂薄板为试样,采用60 Co伽玛射线源以10kGy/h的辐射率辐射试样,总辐射量为100和1 000kGy。通过直流电晕向试样表面注入电荷,采用静电电位计测量不同时刻的表面电荷分布,分析伽玛线辐射对电荷消散特性的影响。结果表明:表面电荷呈双指数规律消散;随着总辐射量从0增大至1 000kGy,表面电荷的消散速度加快。伽玛线辐射引发的化学反应使试样表层的羰基和羟基数量增加,表面陷阱能级变浅,因而提高了电荷的消散速度。     

Noncontact surface charge density profiles measurement on top andbottom of a charged film

Equivalent surface charge density profiles on both the top and bottom surfaces of corona-charged films were measured without any contact by using a surface potential voltmeter with a horizontal resolution of about 2 mm and a laser displacement meter with a vertical resolution of 10 μm. A personal computer controls the sample position with a resolution of a few micrometers, which depends on a digital X -Y stage and a backside ground electrode position (distance from the bottom of the charged film). Surface potential profiles at a few different distances between the ground electrode (back) and the sample film were recorded on a floppy disk, and equivalent surface charge density profiles on the top and the bottom of the film were calculated and plotted automatically by using a personal computer. Some interesting electrostatic phenomena related to their charge behavior are reported as are its applications     

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     

Pockels效应表面电荷测量中电荷反演算法的研究

Poekels效应表面电荷测量技术以其特有的技术优势逐步用于表面放电和等离子体的研究,然而如何准确获取表面电荷分布仍然是一个重要的研究课题.对Pockels效应表面电荷测量技术中的表面电荷反演算法进行研究,采用二维傅里叶变换将测量信号和表面电荷的复杂关系转换到频域中进行计算,并采用维纳滤波算法进行信号处理.计算结果和传统算法的计算结果进行了对比,结果表明,改进算法在数值计算过程中考虑材料厚度、介电特性及不同位置电荷影响等因素,计算方法更为合理.该方法可用于不同材料表面电荷的测量计算.另外,算法中维纳滤波器是计算算法中关键的组成部分,对降低系统噪声,提高数值计算的稳定性能够起到很重要的作用.     

Computational and Experimental Study of Ionic Space Charge Generated by Combined Corona–Electrostatic Electrode Systems

Numerical computation of the electric field intensity and space charge density in electrode systems consisting of ionizing and nonionizing elements, connected at the same direct current (dc) high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. The aim of the present paper is to demonstrate the effectiveness of an original method of field computation in the analysis of the factors that influence the distribution of the ionic space charge in such combined corona–electrostatic electrode systems. The computations and the experiments were carried out for an ionizing wire of diameter 0.3 mm, located at different distances$h$(10–30 mm) from a tubular support of diameter 25 mm. Several interelectrode distances (20–45 mm) were simulated. The extension of the zone at the surface of the grounded electrode, which is affected by the space charge, diminishes when reducing the intervals between these elements of the electrode system, and, at similar applied voltage, the density of the corresponding corona current increases. The experimental data were in good agreement with the computed results, validating the accuracy of the numerical method of space-charge calculation in this special electrode configuration.     

Principle of surface charge measurement for thick insulatingspecimens

We discuss the method for measuring surface charge density accumulated on the surface of a solid dielectric (insulating specimen) such as a supporting spacer in gas and vacuum insulated equipment. For such thick specimens, the probe response does not correspond to the charge density directly below the probe, so the measurement necessitates multipoint data together with the aid of numerical field calculations. The probe gives either induced charge or floating potential in response to the surface charge. We compare various previously proposed techniques and give a reasonable procedure for analyzing the data     

Propagation characteristics of a positive surface streamer

The potential distribution of a developing positive surface streamer was measured by the Pockels effect. The transient potential distribution along a linear path was measured at minimum resolutions of 20 µm and 2 ns. Additionally, velocity of a streamer, average electric field along a streamer, and the developing length were measured while changing the insulator thickness from 0.8 to 5.0 mm. The potential along a positive surface streamer decreases linearly with the distance from the high‐potential electrode, and its gradient is about 500–650 V/mm, which is independent of the inception voltage and the insulator thickness. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Dynamic observation of needle-plane surface-discharge using theelectro-optical Pockels effect

Dynamic observation of surface charge distribution is the main advantage of the electro-optical Pockels effect technique over the dust figure technique, the photographic Lichtenberg figure technique, and the static potential probe scanning method. This technique is demonstrated here to observe the surface charge distribution deposited by partial discharge during application of one period of an 8 kV sinusoidal voltage to a needle-dielectric plane electrode system with zero gap spacing. Polarity effect and backdischarge are prominent in ac surface discharges. The observed polarity effect in the charge pattern shows that if the dielectric surface is initially free of surface charge, positive `streamer' channels burst out intermittently and radially from the needle tip resulting in a spoke-like surface charge distribution, while negative streamers expand almost uniformly in all radial directions resulting in a nearly circular surface charge distribution. This behavior results in quasi-permanent positive surface charges. Residual negative surface charge from prior discharges has a considerable influence on the trajectory of subsequent positive streamer discharges, i.e. causing deviation of positive streamer channels from the radial direction; whereas residual positive surface charge has little influence on the radial development of surface charge from subsequent negative discharges. This measurement technique has a potential for widespread application in investigating the dynamics of surface charging phenomena     

Electrification of polymer surface caused by sliding ultrapurewater

It has been thought that the conductivity of pure water is high enough to neglect the streaming electrification. However, with the recent advancement of semiconductor industries, ultrapure water appeared to cause electrification problems at the washing and the rinsing processes of semiconductor wafers. The investigation on the electrification phenomenon of polymer (PTFE) surface with ultrapure water is described in this article. Water droplets are always charged positively after sliding on the PTFE surface, and the surface potential on PTFE is mostly negative and varies along the droplet path becoming positive downstream. As the conductivity of water becomes closer to that of ideal water, the amount of droplet charge increases drastically. This electrification phenomenon is also enhanced by decreasing the thickness of the PTFE plate. It suggests that the Coulombic force between the charge on the PTFE surface and the mirror image charge is an important factor. We have proposed a model which is based on the electric double layer similar to conventional streaming electrification for insulating liquid, assuming that the charged droplet leaves the excess positive charge on the polymer surface     

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     

Advanced measurement of surface charge distribution with high potential

A measuring technique with a multilayer insulator is proposed to enable charge distribution measurement on an insulator of high potential. By using this technique, the residual charge distribution of the positive surface discharge was successfully measured with an electrostatic probe; nevertheless its potential is 10 times higher than the measurable range of an electrostatic probe. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Portable space charge measurement system using the pulsed electrostatic method

The pulsed electroacoustic (PEA) method is used to measure space charge distributions in insulating materials. A high-resolution PEA measurement system and signal procedure has been discussed in previous papers. Since the signal procedure is necessary for high-resolution measurement, the system must include a computer. In this paper, a new PEA system without signal processing is introduced. In order to eliminate the inevitable signal distortion of the conventional PEA method, the new system employs a new voltage waveform generator, one that is different from the very narrow pulse generator used in conventional PEA systems. Using the new waveform, designed based on the impulse response of the measurement system; the output signal can show the space charge distribution directly, without the need to conduct deconvolution. Thus, space charge distributions can be measured on site, such as in a factory, with a portable oscilloscope. The spatial resolution of the space charge measurement of the system is 10 /spl mu/m, which is the same as that of conventional PEA systems. Such a portable PEA system can contribute to various industrial applications.