Positive corona in air using a point/cup electrode system

The authors have previously published some measurements of negative corona currents in air using a spherically-symmetric point/cup electrode system, and also the related simulations. In this paper the corresponding current/voltage measurements and simulations for positive-point corona, again using the highly symmetric electrode system, are described. Although the corona onset voltages are again predicted with reasonable accuracy for different point radii and air pressures, the currents are very much lower, presumably due to the effect of positive space charge accumulating close to the point electrode (since the simulations assume the absence of space charges). In agreement with this assumption, the streamer pulses are only seen at or just above the onset voltage. Only avalanches occur from about 1 kV above the onset voltage.     

Control of corona characteristics with third electrode and biasvoltage

A novel corona control system consisting of a point-to-plate air gap with a cylindrical third electrode has been proposed. Negative and positive coronas from a point-to-plate air gap with a cylindrical third electrode set up close to the corona point of the needle have been investigated with emphasis on the role of the cylindrical third electrode, the bias voltage amplitude and polarity. It was found that the point-to-plate air gap with the cylindrical third electrode had a negative resistance characteristic on its I-V characteristic for negative polarity discharge, which is very different from the same electrode system without the third electrode. Discharge characteristics of the needle point, such as the corona onset voltage and the corona activity, could be controlled by adjusting the field intensity of the corona point by changing the corona point height from the third electrode and varying the bias voltage amplitude and polarity of the third electrode     

Influence of ground and corona currents on dielectric behavior of small air gaps

This paper investigates the influence of grounding on the field distribution and on the dielectric behavior of small rod-plate and rod-rod air gaps. This effect can be attributed to the grounding of one electrode. In the different arrangements with one electrode grounded or with the electrodes symmetrically charged the experimental results of the corona and breakdown are recorded and compared to the simulation results of the field distribution. It is resulted that the grounding influences significantly the electric field distribution, the dc corona and the dc breakdown. The field is less inhomogeneous in grounded rod - plate air gaps and in rod-rod air gaps with symmetrically charged electrodes, and consequently the values of the corona onset and the breakdown voltage are higher. In longer air gaps the corona current influences the field distribution and consequently the dc breakdown. The effect of grounding on the breakdown is weakened when the dc polarity of the voltage is negative, otherwise it is enhanced. In air gaps with lengths > 3 cm (negative dc polarity) or >8 cm (positive dc polarity) it is overlapped by the effect of the corona current. Relations between the field strength, the corona current, the corona onset and the breakdown voltage arise. The principle of action-reaction is valid.     

低气压下直流正极性冰电极电晕特性研究

覆冰将改变绝缘介质的放电过程，导致电力系统绝缘性能下降，为深入研究覆冰绝缘子串中空气间隙的电晕放电特性，基于冰棱-冰板电极系统，通过脉冲电流传感器和紫外线成像仪CoroCAM IV+揭示了气压对起晕电压、放电模式、放电波形及其幅值等的影响。通过数学分析提出表征不同大气压下不同间隙长度时起晕电压的修正公式，分析和讨论气压对放电脉冲及其幅值的影响。理论分析表明，随着气压的降低，覆冰空气间隙起晕电压呈幂函数或指数函数降低，特征指数同间隙长度相关。试验结果与理论分析一致，进一步揭示了覆冰环境下电晕放电的特性。     

多高压电极系统电晕放电的研究

研究了多高压电极的放电特性现象。结果表明，由于各高压电极间受电场相互屏蔽作用的影响，使电晕放电通道的光学图形发生了向电极外侧偏移，同时各高压电极间距处在一定范围内可提高电晕放电起始电压。还利用分割电极的方法测量了平面电极上的电流密度，其极大值点可相应向外侧偏移，在负极性放电时在一定条件下能产生正流注放电。     

Induction of Waves on a Horizontal Water Film by an Impinging Corona Wind

The electric wind produced by corona discharge of a high-voltage electrode in air is employed for destabilizing a horizontal water film. In wire-to-plane geometry, the phenomenon is characterised by current versus voltage curves and visual observations of the onset of free-surface oscillations. The effect of the following parameters is examined for both positive and negative coronas: distance between the wire and the film (S), film thickness (h), wire diameter (phi) and composition, applied voltage (HV), and relative humidity (RH). The free-surface destabilisation is retarded by increasing d and phi and is insensitive to h in the tested range. The onset of corona discharge is predicted by Peek's law and compared with the experimentally observed threshold. In negative corona discharge, the current values are higher and the film is destabilised at lower HV than in positive polarity. Humidity tends to decrease the corona current at a given HV. Correlations are proposed for the current-voltage curves, in terms of the mean electric field in the inter-electrode gap and of RH, satisfactorily agreeing with the experimental data. Both positive and negative corona currents turn out to be stable for days of operation. The power loss by corona discharge is in any case lower than 12 W. Wave induction on the liquid-gas interface can effectively enhance heat and mass exchange between the two phases.     

Effect of Discharge Electrode Parameters on the Flow Velocity Profile of the Wire?rod Type Electrohydrodynamic Gas Pump Exit

Experimental investigation has been conducted to study the effects of corona wire diameter, pipe length, and corona polarity on outlet flow velocity distribution profile of a wire-rod type electrohydrodynamic (EHD) gas pump. Upon applying negative or positive dc high voltage between a wire electrode (outer diameter (o.d.) 60 μm, 200 μm, or 300 μm) and a rod electrode (o.d. 3 mm) in atmospheric air, corona discharge occurs and EHD gas flow is generated in the direction from the wire electrode to the rod electrode through a cylindrical pipe (inner diameter (i.d.) 20 mm). For both polarities, the discharge current and average flow velocity increase monotonically on increasing the applied voltage before the onset of spark discharge. Using wire electrodes with a smaller diameter, stable corona discharge between corona onset and spark onset is generated in a wider voltage range, and the discharge current becomes larger, resulting in a higher flow velocity. The maximum average flow velocity of 2.0 m/s, corresponding to a flow rate of 38 l/min, was achieved with a wire of diameter 60 μm by applying a voltage of ?16 kV.     

Comparisons of direct-current breakdown characteristics in dry air and SF6 gaps using a parallel-plane arrangement with variable height protrusion

Breakdown voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to nonuniform gap in dry air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge crossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the dc voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset. The experimental observations have been explained qualitatively on the basis of a streamer model and precise electric field calculations of gap.     

Characterization of Dual Corona-Electrostatic Electrodes for Electrostatic Processes Applications

The dual corona-electrostatic electrodes consist of one or several ionizing elements (wires, needles, and blades) attached to a metallic support. This paper analyzes the characteristic features of a particular type of such electrodes and formulates recommendations on their design and utilization. The experiments focused on several models of dual corona-electrostatic electrodes, the ionizing element being the edge of a thin metallic blade. The tested devices had similar cylindrical metallic supports but blades of different sizes. Thus, it was possible to investigate the influence of electrode geometry on corona onset voltage and spark-over threshold in configurations that are specific to industrial electrostatic processes. Most of the experiments were performed by using roll-type electrostatic separators. A special experimental setup was employed for the study of the distribution of corona current density at the surface of a plate collecting electrode. For the model having the blade edge closer to the metallic support, the corona onset voltage was higher, and the corona discharge affected a smaller area at the surface of the collector. These experiments are discussed in relation with the results of the numerical analysis of the electric field generated by the different electrode configurations. At a given voltage and distance between the ionizing element and the collector, the presence of the cylindrical support diminishes the electric field at the edge of the blade and distorts the field lines. These effects were exploited in the design of the electrode system of an electrostatic separator for the recycling industry.     

Corona onset as a design optimization criterion for high voltagehardware

Hardware for use on HV systems is designed to be corona free. Recent hardware design activity has been centered around attempts to increase the corona onset voltage by optimizing the electric field distribution on its surface. Corona onset, however, is not only a function of the surface electric field, but also its rate of decay away from the surface, and the temperature and pressure of the gas in which it is immersed. While corona onset is a better criterion, formulas for predicting it have been validated only for simple geometries. The first goal of this work, then, is to validate corona onset conditions for more practical electrode geometries. The second goal is to determine whether the use of corona onset rather than surface electric field as an optimization criterion can result in hardware with a measurably higher corona onset voltage. To test this idea, two electrodes were designed, one using electric field optimization and the second using corona onset optimization. The corona onset voltage of each electrode was then measured and the results compared with predicted values. It was found that the measured results compared favorably with the predicted values and that the use of corona onset optimization can result in a modest but measurable increase in corona onset voltage     

空气湿度对导线电晕起始电压的影响

我国特高压工程中,输电线路要经过山地、湖泊等高湿度地区,空气湿度对架空输电线路电晕放电有较大影响。为研究湿度对导线电晕放电的影响,利用同轴线-筒电极研究了不同大气湿度对交流以及直流正负极性电晕起始电压的影响规律。试验结果表明:湿度对直流正极性电晕起始电压影响较小,而对直流负极性以及交流电压电晕起始电压影响较大:在相对湿度达50%～60%时,直流负极性与交流起晕电压最高。分析认为,空气湿度对空间电荷分布的影响以及湿度对导线表面状态的影响是导致起晕电压变化的主要因素。     

Charge and voltage characteristics of corona discharges in acoaxial geometry

An engineering model is proposed to evaluate the variation of corona charge and corona current as a function of applied voltage for coaxial geometry (q-v curves). The two main assumptions of the model are the following: for streamer discharges to propagate continuously, the background field must be at, or just above, a critical level; and the streamers maintain a constant voltage gradient in the corona sheath. The model simulations are compared with the experimental q-v curves and a reasonable agreement is obtained between them     

Premises for the mathematical modeling of the combinedcorona-electrostatic field of roll-type separators

Corona and induction charging mechanisms are frequently associated in modern roll-type electrostatic separators. Various electrode configurations have been proposed and numerous attempts have been made in order to fully characterize them. This paper approaches this problem from a computational point of view. The boundary-element method is employed for analyzing the electrostatic field distribution generated by a typical arrangement, consisting of a wire-type corona electrode and an ellipse-profile nonionizing electrode. The computed results pointed out the effect of various parameters on the uniformity of the electric field near the ionizing element and at the surface of the grounded rotating roll electrode. These data can be used by the designer in order to improve the electrode configuration, produce a uniform field in the active zone of the separator, and reduce the corona inception voltage. At the same time, they validate two important premises for the mathematical modeling of the combined corona-electrostatic held of a roll-type separator: (1) Peek's law can be used for evaluating the electric field strength at the surface of the wire electrode at corona onset in any of the usual electrode configurations; and (2) the distribution of the electric field in the active zone of the separator is affected only by the geometry of the high-voltage electrode system     

基于UV检测的UHV输电线路起晕电压的试验研究

Corona discharge is being detected by UV imaging detection technology at home and abroad in recent years.This technology is used in the corona tests of conductor bundles in this paper.In order to further research the corona characteristic,optimize geometry parameters and diameter of sub-conductor,and increase corona onset voltage of transmission lines,corona tests of three model conductors which are placed inside the outdoor corona cage are conducted.Corona cage could be used to simulate the corona activities on transmission lines under a low voltage and different conditions in an effective and economical way.Photon which was created by UV light as a result of corona discharge on conductors is detected by the UV detection apparatus.The photon number within unit interval,namely photon counting rate is adopted as the parameter of quantifying the intensity of corona discharge.According to the apparent change of photon number,corona onset voltage can be judged.All tests are conducted under almost same atmosphere condition.Using the method,corona onset voltage is acquired.The results indicate that the tests have a good repeatability,in other words,repeating same test twice same result can be aquired.The corona onset voltage can be acquired exactly from the curve of applied voltage vs.photon counting rate.Therefore UV detection apparatus can not only used to find discharge point exactly,but also applied on corona discharge research and live detection for power equipments.The method using in this paper is proved that is a new available method.     

DC corona discharge of a metal filament particle withinparallel-plate electrodes

For the prevention of a particle-initiated breakdown within gas-insulated switchgear, fundamental characteristics of particle motion were investigated. It was found that the particle motion depends not only on Coulombic force, but on the corona discharge from the tip of a particle. Thus, the corona current from a fixed particle on the lower electrode was measured, for either positive or negative corona case. The measured current showed different characteristics of current depending on the polarity. The corona onset voltage of negative corona is somewhat lower than positive, but the breakdown voltage of negative corona is much higher. Although the current characteristics follow the square law, the applicable region of this law for positive corona is very small. Positive corona depends on the tip configuration contrary to negative corona     

Factors affecting corona on twin-point gaps under dc and ac HV

The factors affecting corona formation on twin-point/plane gaps are investigated both theoretically and experimentally. In the theoretical analysis, an electrostatic field computation program based on the charge simulation method was used to get the electric field profile on the plate for different gap lengths and interspacings between the two points. Also, the effect of voltage level, air pressure and gap length on the number of axial streamers occurring across the critical field line was introduced for a constant time duration. The variation of the integral of the ionization coefficient with the axial length from the point was studied. Finally, a study of the effect of air pressure and gap length on both the corona inception voltage and critical length was presented. Corona formation in single and twin-point/plane gaps also was investigated experimentally under both high direct and alternating voltages. There are many factors affecting such corona formation. The investigated factors were the interspacing between the two points, gap length between the point and the plane, ambient temperature, relative humidity, polarity or frequency of the applied voltage, and electrode material. In order to study and explain such phenomena, a photographic investigation together with a simultaneous measurement of both the applied voltage and the corona current were introduced. Curve fitting of the dc corona current vs. gap length and voltage for both single and twin-point/plane gaps gave the conventional relationship, but in case of twin point the power of the gap length varies between 1.2 and 1.3     

非均匀电场中气固混和体直流电晕特性研究

实验研究了非均匀电场中流化床状态气固混和体的直流电晕特性 ,观察了Al2 O3 气固混和体和聚丙烯气固混和体在不同极性、不同电压下的电晕波形。关于电压极性、占空比以及间隙距离对两种气固混和体起晕电压影响的实验发现 ,起晕电压随占空比增加而降低 ;正极性下Al2 O3 气固混和体的起晕电压比聚丙烯气固混和体低 ,而负极性下则相反     

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     

电晕笼中直流导线起晕电压的测量方法

电晕起始电压是反映导线电晕特性的重要参数。针对现有的电晕起始电压测量方法不能区分导线局部起晕电压和全面起晕电压的不足,在实验室使用的小电晕笼中测量了不同类型和表面状态的直流导线的起晕电压。通过分析导线电晕发展过程,提出使用电流系数曲线测量直流导线起晕电压的方法。与已有的测量方法进行对比的结果表明,结合使用电晕脉冲法和电流系数法测量电晕笼中直流导线的局部起晕电压和全面起晕电压是可取的。     

Positive Wire-to-Plane Coronas as Influenced by Atmospheric Humidity

A method is suggested for calculating the inception voltage and corona current in humid air. It is found that the photoelectrons distribution within the ionization layer, which depends upon the gap geometry, plays an important role in deciding whether the inception voltage increases or decreases with relative humidity. The inception voltage is one of the boundary conditions required in the theoretical analysis of the corona V-I characteristics also presented. The experimental results reported in the literature which differ widely can be explained in the light of the present findings.