Experimental study of pulsed corona discharge in air

We report on the results of experimental investigation of a pulsed corona discharge in electric fields with different degrees of nonuniformity under the action of a standard thunderstorm pulse in a wide range of voltages from the origination threshold to the breakdown. A high-sensitivity video camera makes it possible to record microdischarge and streamer processes in air long before the spark breakdown. It is known that the size of the corona discharge sheath increases with the supplied voltage, and the shape of the corona sheath depends on the polarity of the active electrode [1, 2]. It was demonstrated for the first time by Peek [3] that the range of voltages from the initiation of a corona discharge to the spark breakdown increases with the degree of nonuniformity of the electric field. We show that an analogous pattern is observed for a pulsed corona also. Our results show that the form of a pulsed corona discharge considerably depends on the pulse polarity, and a spark breakdown becomes possible when most branches in the streamer corona cover the electrode gap.     

The impulse dielectric behavior of N2 gas in sphere-plane gap

The focus of this work has been on the pre-breakdown phenomena and the breakdown characteristics of N₂ gas in a sphere-plane gap under various impulse voltages. Both electrical and optical experimental investigation methods were used. Following parameters were considered: gas pressure range from 0.2 to 0.6 MPa, electric field utilization factor of the electrode configuration 71%, positive and negative impulse waveforms with the rise time of 500 ns, 1.2 μs and 180 μs. The observed discharge processes before the breakdown through the light emission images by the ICCD camera are in good agreement with the streamer mechanism. Under both polarity stresses, discharges are initially concentrated around the tip of the sphere and later pointing towards the earth electrode. However, negative streamers are thinner and more diffuse. As expected, the breakdown voltages for negative polarity are lower than those for positive polarity regardless of the gas pressure and shape of the applied impulse voltage. The breakdown voltage is increased with shortening the rise time of pulse waveforms. As a substitute for SF₆, N₂ gas under pressures above 0.3 MPa can reach the standard rated withstand voltage for 24 kV C-GIS.     

针-板空气间隙流注放电起始过程的三维PIC/MCC仿真研究

流注放电作为自然界中闪电传播的预电离机制、高压输变线路间长空间间隙放电的重要初始阶段,在工业领域存在诸多潜在应用,近年来引起人们越来越多的关注.流注放电具有典型的多尺度、非线性的放电特征,实验观测中多呈现出分叉等不规则结构.为了研究其微观结构特性和行为特征,本文采用三维粒子仿真模型(PIC/MCC),着重研究了流注从针型正电极的起始和发展过程.模型采用了可变自适应网格、可变粒子权重以及并行计算等技术,有效地降低了三维粒子仿真的计算时间.通过调节针型电极上的施加电压幅值、改变气体组分及调整电极形状尺寸等,研究了放电参数变化对流注放电的分叉结构、半径等行为的影响.模拟结果表明:随着电压的升高,流注的半径及分叉数目增加;对比不同气体组分(纯氧以及不同比例氮氧混合气体),发现其对流注的分叉数目影响较为显著;针型电极结构直接影响了流注的起始时间和形貌.     

The lifetime of positive streamers in a pulsed point-to-plane gap in atmospheric air

A determination has been made of the lifetime of positive streamers produced in atmospheric air in a positive point-to-plane gap to which was applied a 40 nsec, 40 kV voltage pulse. The streamer tips have been detected at a given plane in the gap by means of a photomultiplier and high speed oscilloscope. It was found that the streamer tips continued to propagate in the gap, long (up to 35 nsec) after the voltage pulse was removed, in accordance with the predictions of a recently presented model of streamer propagation. Association of this long lifetime with the properties of an isolated tip was made possible by the detection of a new phenomenon at the anode. At the end of the voltage pulse a second luminosity was observed to leave the anode and to extend along the paths of the primary streamers for about one third of their length. This is shown to be the maximum extent of a more highly conducting trunk, behind and separated from the streamer tip, affecting both the propagation of the tip and the eventual transition of a streamer to a breakdown spark.     

Impulse breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF6

This paper presents the impulse pre-breakdown and breakdown characteristics of the plane-to-plane electrode system with a needle-shaped protrusion in SF₆ gas. The breakdown voltage–time (V–t) characteristics and the breakdown voltage–gas pressure (V–p) characteristics of a highly non-uniform SF₆ gas gap under positive and negative lightning impulse voltages are investigated in the pressure range between 0.1 and 0.5 MPa. The pre-breakdown developments are examined by the corona current and light emission measurements with high time resolution. As a result, the dielectric strengths versus time-to-breakdown of SF₆ gas gap under positive lightning impulse voltages were nearly independent of the gas pressure. The first streamer corona was initiated at the tip of the needle electrode, and the streamer corona pulses developed with a stepwise propagation. The discharge paths were zigzag, and the branches of the discharge channel for positive polarity were created. On the other hand, the leader channel in the negative polarity was thicker and brighter than that in the positive polarity.     

Pulsed corona discharge driven by Marx generator: Diagnostics and optimization for NOx treatment

A compact, repetitive Marx generator with an external trigger is constructed and coupled with a wire-to-plate corona reactor for a positive pulsed corona discharge studies. The reactor resistance and capacitance behavior during the pulse was observed. It was found that the reactor's capacitance increases three times during the pulse due to the streamer propagation from anode to grounded electrode. Using the time development of the capacitance and resistance during the pulse and the reactor inter-electrode distance, the streamer velocity has been calculated to be 1 × 10⁶ m/s, for system arrangement presented in this work. As an indicator of chemical activity of pulsed corona, ozone production was measured. Emission spectroscopy measurements in the UV region were performed to detect species that appear in the discharge and to determine vibrational and rotational temperatures, which are found to be 3200 K and 340 K respectively. As a measure of pollution control potential of the constructed pulsed corona system, NO oxidation efficiency was investigated and compared with results presented in literature. It was shown that pulsed corona systems with significantly longer pulse durations are competitive with several times shorter pulse duration systems, which implies that chemical efficiency of secondary streamers is comparable with efficiency of primary streamers.     

Theory of positive corona in SF6 due to a voltageimpulse

A theoretical examination is made of the mechanism of corona formation for a positive point-plane gap in SF₆ at 100 kPa. The impulse voltage applied has a rise time of 15 ns and peak value of 200 kV. Seed electrons are released 1 ns after the start of the voltage rise. For a 0.5-cm diameter positive sphere located 6.5 cm from a negative plane, the calculated circuit current initially consists of subnanosecond corona onset pulses, and then the current steadily rises to a maximum, as the voltage reaches a maximum, followed by a rapid fall in current. During the current rise a streamer moves out into the gap along a 100-μm channel, with the electric field in the streamer trail E>E*, where E* is the critical field where ionization equals attachment. The light output during the discharge is predicted to be a maximum at the anode with only a minor pulse of light at the streamer head, making it hard to detect. After the current maximum, recombination rapidly reduces the numbers of positive ions, negative ions, and electrons, but the net charge density remains constant and thus so does the electric field. The electric field is E~E* in the streamer trail, but has a sharp maximum, E≫E* at the head of the streamer trail. The origin of mid-gap precursors, observed when the streamer channel reilluminates after some 100 ns, is attributed to this field maximum in the remnant electric field. The evolution of positive ions, negative ions, and electrons is described by one-dimensional continuity equations, with the space-charge electric fields determined by the disk method. The effects of ionization, attachment, recombination, electron diffusion, and photoionization are all included. New numerical methods allow resolution of the streamer head and the anode fall region to be obtained with a 1-μm mesh, while following the streamer propagation for ~2 cm     

Impulsive-discharge formation in water

This paper reports on research on an impulsive electrical discharge in deionized water (=2 × 10⁴ · m) in the voltage range 17–50 kV. The research showed that the discharge proceeds through the development of subsonic (U=17–22 kV) or supersonic (U=25–50 kV) streamers, depending on the electric-field strength at the surface of the tip. When voltage impulses having an amplitude of 20–25 kV are unleashed, the development of both types of streamers is observed in the discharge gap. The experiments carried out by the authors do not confirm the hydrodynamic origin of the disturbances at the initiating electrode that give rise to the discharge's development. The authors hypothesize that streamer development is due to the ionization of water molecules. The energy expenditures to ionize the molecules in the streamer channel are estimated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 88–92, January, 1989.     

气体介质对多间隙气体开关电晕均压与自击穿特性的影响

快脉冲直线变压器型驱动源(FLTD)是近年来快速发展的新型脉冲功率源技术,多采用多间隙气体开关作为开关器件。电晕均压措施有利于提升开关击穿性能,但不同气体中电晕放电有显著区别。本文首先研究了空气中针电极对单间隙电晕放电特性的影响,确定了电晕针电极的尺寸,之后研究了N2,CO2,SF6/N2混合气体、C4F7N/N2混合气体中的电晕放电特性,研究了电晕均压6间隙气体开关击穿电压及其稳定性随气体种类和气压的变化规律。实验结果表明,N2中电晕电流较大且不稳定,空气中电晕电流比N2中低,且电晕放电较为稳定,微量强电负性气体加入会极大降低电晕放电电流。当采用空气和N2作绝缘介质时,气体开关击穿电压随气压升高线性增加,但存在低值击穿,微量强电负性气体混合N2可显著提升击穿电压的稳定性。1%SF6/99%N2混合气体在0.18 MPa时,击穿电压约为197.33 kV,标准偏差占击穿电压比例为1.50%,1%C4F7N/99%N2混合气体在0.15 MPa时,击穿电压约为190.42 kV,标准偏差为0.55%。这表明,微量环保替代气体C4F7N与N2的混合气体对于提升多间隙气体开关击穿电压稳定性有显著作用。     

Formation of ball streamers at a subnanosecond breakdown of gases at a high pressure in a nonuniform electric field

The formation of a diffuse discharge plasma at a subnanosecond breakdown of a “cone–plane” gap filled with air, nitrogen, methane, hydrogen, argon, neon, and helium at various pressures has been studied. Nanosecond negative and positive voltage pulses have been applied to the conical electrode. The experimental data on the dynamics of plasma glow at the stage of formation and propagation of a streamer have been obtained with intensified charge-coupled device and streak cameras. It has been found that the formation of ball streamers is observed in all gases and at both polarities. A supershort avalanche electron beam has been detected behind the flat foil electrode in a wide range of pressures in the case of a negatively charged conical electrode. A mechanism of the formation of streamers at breakdown of various gases at high overvoltages has been discussed.     

Structure of the glow of a nanosecond diffuse discharge in a strongly nonuniform electric field

The structure of the glow of a diffuse discharge in air under atmospheric pressure is studied in detail in the “rod (cathode)-plane” geometry for an electrode gap of 10 cm and a cathode tip radius from 3 cm to 2.4 μm. The amplitude of voltage across the gap was ～ 220 kV for voltage growth rate of ～10¹³ V/s and a pulse duration of 180 ns. It is found that the shape of the discharge glow strongly depends on the radius of the cathode tip. For large values of the radius, the multichannel form of the glow prevails, which is statistically transformed into a volume glow as the radius decreases from 5 mm to 60 μm. This transition is accompanied by a decrease in the amplitude of the discharge current from 440 to 140 A on the average. For ultrasmall radii of the cathode (2.4–7.7 μm), the multichannel form of the glow prevails again and the amplitude of the discharge current increases up to ～300 A.     

大气压氦气多路低温等离子体射流

为了研究大气压低温等离子体多路射流阵列的放电特性,设计一个实现7路低温等离子体射流的放电装置,采用单电极放电结构,在开放的大气环境下通入氦气。采用高压窄脉冲重复频率电源激励驱动该放电装置,电源脉冲宽度约230 ns,脉冲上升沿约为120 ns。在重复频率为500 Hz的条件下,通过高速摄影初步发现放电电流脉宽约为110 ns,且无反向放电。试验结果表明:平均射流长度随电压幅值增加而增加,在一定电压幅值时射流长度有达到饱和的趋势,这是由于射流通道尾部有空气进入,电压幅值已不再是主要原因;只有在合适的气体流量值时,才能够获得较长的平均射流长度,这是由于气体流量过大或过小时射流均不足以维持形成的放电通道;此外,中心电极放电射流长度受气体流量影响较大,气体流量在一定值时可以观察到中心电极有较长的射流,射流放电强度较弱,气体流量过大或过小时中心电极几乎无放电,这是由于四周电极更易形成放电射流,削弱了中心电极放电。 ,     

A high-current rail-type gas switch with preionization by an additional corona discharge

The characteristics of a high-current rail-type gas switch with preionization of the gas (air) in a spark gap by an additional corona discharge are investigated. The experiments were performed in a voltage range of 10–45 kV using a two-electrode switch consisting of two cylindrical electrodes with a diameter of 22 mm and a length of 100 mm and a set of laterally located corona-discharge needles. The requirements for the position and size of the needles are defined for which a corona discharge is ignited before a breakdown of the main gap and does not change to a sparking form, and the entire length of the rail electrodes is efficiently used. The fulfillment of these requirements ensures stable operation of the switch with a small variation of the pulse breakdown voltage, which is not more than 1% for a fixed voltage-pulse rise time in the range from 150 ns to 3.5 μs. A short delay time of the switch breakdown makes it possible to control the two-electrode switch by an overvoltage pulse of nanosecond duration.     

Gas spark switches with increased operating life for Marx generator of lightning test complex

A new design of gas spark switches with an increased operating life and stable dynamic characteristics for the Marx generator of the lightning test complex has been developed. The switches are characterized by the following parameters in the mode of operation: voltage up to 80 kV, discharge current up to 50 kA, flowing charge up to 3.5 C/pulse. An increased operating life is achieved by using torus-shaped electrodes with increased working surface area and a trigger electrode in the form of a thick disk with a hole located between them. Low breakdown delay time and high stability of breakdown voltage under dynamic conditions are provided by gas preionization in the spark gap using UV radiation of an additional corona discharge in the axial region.     

不同条件下大气压脉冲弥散放电特性

利用上升沿约0.5 s、半高宽约6 s、幅值可达40 kV的微秒脉冲电源和上升沿约150 ns、半高宽约300 ns、幅值可达50 kV的纳秒脉冲电源激励大气压弥散放电,并分别采用刀型和锯齿电极放电。通过电压电流测量和发光图像拍摄,改变施加电压种类、脉冲重复频率、高压电极结构和气隙距离等参数,研究了不同条件下弥散放电特性。实验结果表明:纳秒脉冲电源和微秒脉冲电源均能在大气压空气中激励大面积的弥散放电,弥散放电面积最大达90 cm2;放电的均匀性受脉冲参数与电极形状影响显著,其中刀型电极条件下纳秒脉冲激励的弥散放电均匀性最佳;相同条件下纳秒脉冲弥散放电的瞬时功率大于微秒脉冲弥散放电,最高可达275 kW,而纳秒脉冲弥散放电的能量小于微秒脉冲弥散放电;保持其他条件不变,弥散放电传导电流幅值随着气隙距离的增加而降低,放电强度随着脉冲重复频率的增加而增强,弥散放电的工作电压范围随着脉冲重复频率的增加显著降低。因此在低频、刀型电极结构中易于获得均匀与较大工作电压范围的大气压弥散放电。     

Neutralization of charged insulating granular materials using AC corona discharge

The control of the residual electric charge carried by granular materials processed in various electrostatic installations is a prerequisite for the safe and efficient operation of the respective industrial equipment. The aim of the present work was to evaluate the neutralization efficiency of polyethylene granules exposed to an AC corona discharge from a wire-type electrode. The control variables and their domains of variations were the following: the amplitude and the frequency of the applied high voltage (16–18 kV, 20–400 Hz), the position of the corona electrode (3–7 cm above the ground plate that carries the sample) and the duration of the exposure to the corona discharge (4–10 s). The surface potential decay method was used for monitoring the charge carried by granular samples of PE before and after neutralization. Design of experiment methodology was employed to evaluate the influence of each of the above mentioned control variables and determine the optimum operation conditions. The efficiency of the neutralization was characterized by the ratio between the values of the surface potential before and after AC corona discharge exposure. The obtained results show that neutralization efficiency may be improved by increasing the frequency of the high voltage as well as by adequately correlating its amplitude with the inter-electrode spacing.     

Nanosecond pulsed power generator for a voltage amplitude up to 300 kV and a repetition rate up to 16 Hz for fine disintegration of quartz

A generator of high-power high-voltage nanosecond pulses is intended for electrical discharge disintegration of mineral quartz and other nonconducting minerals. It includes a 320 kV Marx pulsed voltage generator, a high-voltage glycerin-insulated coaxial peaking capacitor, and an output gas spark switch followed by a load, an electric discharge disintegration chamber. The main parameters of the generator are as follows: a voltage pulse amplitude of up to 300 kV, an output impedance of ≈10 Ω, a discharge current amplitude of up to 25 kA for a half-period of 80–90 ns, and a pulse repetition rate of up to 16 Hz.     

120 kV下常压空气纳秒脉冲电晕放电特性

使用上升沿15 ns、脉宽30~40 ns的重复频率纳秒脉冲电源对120 kV下大气压空气中管-板电极结构电晕放电进行了实验研究,通过电压电流测量、放电图像拍摄和X射线探测分析了纳秒脉冲电晕放电特性。结果表明:纳秒脉冲电晕放电中存在X射线辐射,但辐射强度较弱,X射线辐射计数随着气隙距离的增大而减少,随着脉冲重复频率的增大而增多;放电空间的残余电荷加强了下一个脉冲到来时的局部电场,从而导致高重复频率下易于出现分散的电晕通道。     

Time-resolved processes in a pulsed electrical discharge in argon bubbles in water

A phenomenological picture of a pulsed electrical discharge in gas bubbles in water is produced by combining electrical, spectroscopic, and imaging characterization methods. The discharge is generated by applying 1 m\mu s pulses of 5 to 20 kV between a needle and a disk electrode submerged in water. An Ar gas bubble surrounds the tip of the needle electrode. Imaging, electrical characteristics, and time-resolved optical emission spectroscopic data suggest a fast streamer propagation mechanism and the formation of a plasma channel in the bubble. Comparing the electrical and imaging data for consecutive pulses applied to the bubble at a frequency of 1 Hz indicates that each discharge proceeds as an entirely new process with no memory of the previous discharge aside from the presence of long-lived chemical species, such as ozone and oxygen. Imaging and electrical data show the presence of two discharge events during each applied voltage pulse, a forward discharge near the beginning of the applied pulse depositing charge on the surface of the bubble and a reverse discharge removing the accumulated charge from the water/gas interface when the applied voltage is turned off. The pd value of ~ 300–500 torr cm, the 1 μs long pulse duration, low repetition rate, and unidirectional character of the applied voltage pulses make the discharge process here unique compared to the traditional corona or dielectric barrier discharges.     

X-ray radiation due to nanosecond volume discharges in air under atmospheric pressure

The formation of nanosecond discharges in atmospheric-pressure air versus the applied pulse polarity and discharge gap geometry is studied. It is shown that the polarity of high-voltage nanosecond pulses and the electrode configuration have a minor effect on the volume discharges under a variety of experimental conditions. When the spacing between needle-like electrodes is large, the discharge is asymmetric and its glow is weakly dependent on the sign of the potential applied to the electrode. Negative voltage pulses applied to the potential electrode generate X-ray radiation from both the surface and volume. For a subnanosecond rise time of the voltage pulse and diffusion character of the discharge, the X-ray radiation comes from the brightly glowing region of a corona discharge. The average values of the fast electron velocity and energy in nitrogen are calculated. At field strengths E/p < 170 kV/cm atm, the average velocity of a fast electron bunch is constant because of central collisions. At field strengths E/p > 170 kV/cm atm, fast electrons run away. Central collisions are the reason for X-ray radiation from the volume.