Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.     

Behavioral characteristics of nanosecond pulsed discharge in coaxial electrodes

The environmental improvements by pulsed discharge plasma, a type of nonthermal plasma, have received much attention all over the world. The observation of discharge plasmas is beneficial for better understanding of the plasma physics of this growing field. Recently, nanosecond (ns) pulsed discharge with a short pulse duration of 5 ns achieved the higher energy efficiency on ozone generation and NO removal. However, the underlying mechanisms of these high efficiencies remain unclear. In the present study, the effects of electrode geometry on propagation process of ns pulsed discharge in coaxial electrodes were investigated using an ICCD camera. As the results, increasing wire diameters from 0.2 to 2.0 mm lead the different streamer discharge parameters; propagation velocity of streamer heads from 4.8 mm/ns to 12.5 mm/ns, peak of discharge current from 243 to 328 A, respectively. Therefore, it is concluded that the wire diameter is one of the paramount parameters to control the characteristics of the ns pulsed streamer discharge.     

基于分形理论的水中流注放电仿真

水中流注放电的特性及流注发展机理研究对于污水处理、杀菌消毒、水下声源和高压绝缘具有重要的意义。基于分形理论,构建了考虑气泡生长过程的水中流注放电仿真模型,并仿真研究了不同电压幅值和溶液电导率下的放电特性。结果表明,随着外施电压和溶液电导率的增大,通道电荷量和流注发展速度均增大,而放电预击穿延时减小。计算得到的流注发展速度、预击穿延迟时间随水溶液电导率的变化等与已报道的实验相符。     

水中脉冲放电活性粒子的研究

通过发射光谱和化学方法检测蒸馏水中脉冲放电产生的活性粒子，研究了氧气流量造成的放电形式变化(流注和火花放电)，活性粒子(包括羟基、激发态氢原子、氧原子和过氧化氢)变化。实验表明放电形式对各种活性粒子的产率有明显的影响，而气体流量的大小直接决定放电形式的转变，其中氢原子发射光谱强度的变化可以直接表示这种转变，这种转变也决定了OH和H发射光谱强度的差异。     

气体成分对水中脉冲放电产生过氧化氢的影响

通过化学方法检测了水中脉冲放电产生的H2O2,通过拍摄照片研究流注长度的变化。实验发现,在分别通入氮气和氧气的情况下,随着气体流量和放电时间的增加,H2O2的浓度逐渐增加;在电极间距为5cm,所加电压为35kV时,通氮气比通氧气生成的H2O2多,通氮气时流注长度较通氧气时要长。在电极间距为3cm,所加电压为21kV时,通氧气比通氮气生成的H2O2多,而流注长度差异不大。     

Positive streamer discharges along insulating surfaces

By measuring the currents associated with the streamer discharge along silicone rubber surfaces, parameters of streamer propagation such as the minimum field of streamer crossing, the field of stable streamer propagation, the mean velocity and the streamer charge distribution have been analyzed and compared to the streamer discharge in air alone. Clear differences were observed in the measured currents for the individual surfaces at low background fields (285 kV/m). For higher fields the streamer crosses the gap almost independently of the surface type. The minimum streamer field was found to be slightly increased compared to air. The field of stable streamer propagation also was higher than in air. It is ~ 570 kV/m, larger than that of the streamer discharge in air (~ 500), under the same conditions. The streamer speed was found slightly increased in the presence of the silicone rubber surface and the distinction between the individual surfaces was modest. A discussion on possible mechanisms for the observed differences in the streamer speed and currents with and without the insulator surfaces is presented. The net positive charge of the streamer along an insulating surface seems to be distributed along the streamer channel rather than localized in the front part of the channel as the case for the streamer in air     

Study of the Development of Positive Streamers Along an Ice Surface

This paper presents the results of fundamental investigations on the inception and propagation of corona discharge on an ice surface stressed with a standard lightning impulse voltage. High-speed photography and photomultiplier techniques were used to observe and record the propagation of the streamers. The effects of several experimental parameters such as freezing water conductivity and HV rod electrode radius on the streamer inception parameters were investigated. Moreover, time to first streamer, inception voltage and corresponding field, as well as streamer propagation velocity and charge deposited by a streamer on ice surface were measured. The results are discussed and emphases are laid on the main factors influencing the development of positive streamers on ice surface.     

Characteristics of laser-triggered electric discharges in air

An experiment of laser-triggered electric discharges has been conducted in a uniform background electric field. By focusing the beam of a pulsed Nd-YAG laser (wavelength 532 nm, pulse duration 5 ns) in a narrow (15 mm) plane-parallel air gap exposed to a DC voltage, streamer discharges and disruptive discharges were triggered. The laser energy and the background electric field strength were varied in the ranges 0-180 mJ and 0.8-1.4 MV/m, respectively, and the laser plasma was induced either close to the anode, close to the cathode or mid-gap. From images of the electrode gap and from current measurements, the occurrence of and time to the streamer discharge and the disruptive discharge were determined. No distinct level was found of either the laser energy or the plasma energy for the transition from one discharge case to another, even if the general trend was that the energy required for a certain discharge case was reduced when increasing the background electric field. These and other observations make the method of laser triggering a less suitable method for studies of streamer initiation and streamer propagation, but revealed several interesting features of laser-triggered electric discharges.     

考虑载流子密度扰动的植物绝缘油中初始流注放电仿真

在现有泊松方程及电流连续性方程的放电模型基础上,通过引入载流子密度波动源建立具有多分支流注形貌的放电改进模型;通过COMSOL软件仿真分析了雷电冲击电压为120kV、160kV和200kV时的放电结果。结果表明:改进模型的流注分支形貌比原模型更接近试验结果;最大场强主要分布在流注头部,且均超过2×108 V/m,其中200kV时由二次流注引起的场强和电荷变化最为明显。此外,3种电压下的z轴流注发展长度和速度还表明流注分支间电荷的相互排斥对流注有抑制作用。而对比电压上升沿时间分别为5ns、10ns、50ns和120ns时的流注形貌还发现:电压上升沿时间越短,流注半径越大,分支越多。该文的研究成果有助于分析放电过程中载流子密度波动对流注分支的影响机理。     

Space charge effect for streamer initiation and propagation in water subjected to reciprocal traveling wave voltage pulse

This paper deals with initiation and propagation processes of streamer discharges in water subjected to high-speed repetitive voltage pulse. The 50%-duty repetitive pulses were produced by the reciprocal traveling wave in a pulse forming cable which was connected with a point-plane electrode gap in a water-cell. Laue plot of time lag distributions for the streamer initiation indicated that the initiation probability during the second pulse application became much higher than that during the first pulse application. We concluded that the change in space charges distribution due to internal field during the rest-time after the first pulse enhanced the streamer initiation probability at the second pulse application. Temporal development in the positive streamers during the repetitive pulse application was observed using a gated image intensifies. When the pulse was relatively high, the streamer discharges started the propagation at the first pulse and then the discharge channel became long step by step at the subsequent pulse applications. It was also found that the periodical emission due to excitation of OH radicals was detected for a long period after the development of the streamer discharge in a spectroscopic measurement.     

沿冰层表面流注传播场稳定性的实验研究与分析(英文)

The electric field required for stable propagation of a positive streamer along an ice surface was investigated by measuring the currents associated with the streamer discharge.The influence of the surface properties,namely the existence of a water film on the ice surface and surface contamination,was analyzed.Results showed that in the presence of an ice surface,except in case of low conductive surface,streamers propagate stably with an external field lower than that generally measured for propagation in air alone(approximately 5 kV/cm).For higher contamination levels,the stability field was found to be slightly influenced by the temperature,while for lower contamination levels,it decreased significantly with an increase in temperature.     

Propagation characteristics of impulse creepage discharge in perfluorocarbon liquid

In this paper, the propagation characteristics of a creepage discharge on the surface of a solid insulator with a back electrode in perfluorocarbon liquid under an impulse voltage application are investigated. The propagation process is observed in detail by means of a high‐speed schlieren optical technique, and simultaneously waveforms of a current and a charge are also measured. Consequently, the polarity effect of the streamer propagation and the positive streamer which propagates stepwise can be seen. The latter is concurrent with the sparse current pulse which corresponds to the charge step‐variation. These results are compared with ones obtained in transformer oil. © 2000 Scripta Technica, Electr Eng Jpn, 131(4): 19–28, 2000     

短空气间隙流注放电的实验观测技术综述

流注放电是气体间隙放电的重要阶段,流注放电的机理、仿真及实验研究是高压放电等离子体领域研究的重点之一,其中流注放电的实验研究是流注放电机理及仿真研究的基础。然而流注放电具有多时空尺度、多粒子碰撞、多物理场耦合等复杂特点,这对流注放电的实验观测提出了巨大的挑战。该文针对短空气间隙流注放电的实验观测,分别从短空气间隙流注放电实验设置和短空气间隙流注放电过程观测技术2个方面综述了国内外相关实验方法、平台及取得的研究进展。在此基础上,该文对目前短空气间隙流注放电研究所需要解决的关键问题和未来的发展趋势进行了探讨,认为未来短空气间隙流注放电实验研究进一步发展的关键在于建立更高精度与更高时空分辨率的多物理量同步观测系统,观测并分析单个流注发生发展的完整过程;探索新的实验手段和测量技术,获取电子平均能量等关键特征参数;深入研究数字图像处理技术,挖掘放电光学图像蕴含的更深层次的特征信息,进而完善对流注放电机理的研究。     

棒-板长间隙正极性流注生长概率模型及应用

为研究长空间隙放电分散性和放电路径随机性,建立了结合传统流注放电理论和分形生长理论的正极性流注生长概率模型。首先基于经典的流注起始判据,计算了棒-板间隙流注起始电压;选择空间电位和电子崩形成时间作为流注生长控制变量,结合泊松方程、电荷连续性方程和欧姆定律描述流注通道内部的电荷转移,仿真流注的持续发展过程;当流注到达板电极后,选择放电树枝中电位梯度最大的通道作为最后的主放电通道。分析表明,该模型可描述流注区空间电荷随时间的积累过程,棒-板间隙击穿时间的分散性以及放电路径的随机性;由其计算得的棒-板间隙主放电通道分形维数与试验结果相吻合,同时基于该模型对棒-导线-板间隙放电选择概率分布的计算结果也与试验所得规律一致。     

大气中短空气间隙流注放电过程数值仿真

介绍了气体放电过程的流体数值模型,并将有限元(finite element,FE)和通量校正传输(flux-corrected-transport, FCT)法结合求解该模型。针对求解时间过长的问题,引入网格自适应剖分法和并行计算方法,提高了计算精度和效率。采用所提出方法仿真了棒板间隙流注放电过程,与实验结果吻合较好。基于仿真方法研究了电压大小、电极形状对放电的影响。基于气体放电现象的物理本质,从微观上研究了气体放电过程,得到了流注过程中空间电荷、电场分布等难以直接测量的微观参量。     

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.     

Variation of the Dynamics of Positive Streamer with Pressure and Humidity in Air

The dynamics of positive streamer in air under different pressure and humidity conditions was studied. The widely used three-electrode system was used to obtain the streamer propagation probability and average propagation velocity as a function of background field. A simulation was performed based on the 1.5D fluid model. Simulated intrinsic stability field for streamer propagation and corresponding average propagation velocity were in agreement with those deriving from experimental results. The intrinsic stability fields obtained with the empirical equations proposed by the author and other investigators were compared. It was analyzed that the effect of the amplitude and duration of voltage pulse applied on the needle. The statistical distribution of propagation field was explained by the dispersion of discharge time delay. The decrease of intrinsic stability field with the reduction of pressure and/or humidity was due to the increase of the conductivity of the streamer channel as a result of the weakening of attachment and/or electron-ion recombination effect.     

低气压下流注放电特性的研究

流注的产生和传播是空气间隙击穿中一个重要的物理过程,研究低气压下流注的产生和传播过程有助于理解空气间隙的击穿放电,对高海拔地区输电线路及输变电设备的设计和运行有着重要的意义。在可调节气压湿度的有机玻璃罐中,利用“三电极”结构,应用光电倍增管测量了不同气压湿度下,流注传播概率和平均传播速度随平板间电场强度的变化。通过数据拟合得到了流注稳态传播电场和相对空气密度以及湿度之间的经验公式。建立流体模型,对流注传播的动力学特性进行了仿真计算,得到的流注稳态传播电场及平均传播速度和实验测量得到的结果吻合得很好。结果表明：流注的传播场强随湿度、气压的增大而增大;在相同的电场强度下,流注平均传播速度随湿度、气压的增大而减小。利用仿真计算模型计算得到流注传播过程中的各个参数,分析了流注传播动力学特性随气压湿度变化的机制。     

Studying branching of a cathode-directed streamer in air by means of 3D modeling

A cathode-directed streamer in air is modeled using the hypothesis that branching is initiated by large electron avalanches that develop in a strong electric field in the front of streamer head. A series of streamer discharge simulations are carried out using a three-dimensional numerical model. The possibility of streamer branching is shown as a result of the interaction with two electron avalanches that arise in front of its head. They are directed to the sides from streamer propagation direction. Such a mechanism of branching is brought about by the fact, that at the moment of contact with the streamer, the avalanches have already undergone an avalanche–streamer transition or are close to it. The equality of the number of electrons in these avalanches at the moment of contact is not important. If this equality is violated, branching is asymmetric. If one of the avalanches interacting with the streamer is far from the avalanche–streamer transition, it does not trigger perfect branching, but it may produce an underdeveloped branch that does not evolve further. The streamer trajectory then deviates toward a larger avalanche, interacting with it.     

正极性雷电冲击电压下天然酯绝缘油油纸沿面流注动态变化规律研究

油纸绝缘复合电介质沿面放电是电力设备内绝缘的研究基础,其在雷电冲击电压下的绝缘特性是变压器绝缘设计的重要参数之一。为了获得油纸沿面流注传播与消散过程中电学、空间电荷分布演化规律及其关联关系,以交界面平行于施加电场方向的油纸系统为研究对象,通过构建适用于绝缘油油纸沿面流注动态变化特性的试验观测系统,可同步获得正极性雷电冲击电压下流注传播和消散过程中的电压、放电电流和放电通道流注阴影图像。利用该平台还测量了油纸沿面正极性雷电冲击击穿电压。试验结果表明,在正极性雷电冲击电压下天然酯绝缘油油纸绝缘相对介电常数差异并不会促进油纸沿面流注的传播过程,而粘度对于油纸沿面流注侧向分支影响显著。粘度越低,空间电荷在迁移过程中所受到的阻力越小,流注头部空间电荷在受到表面电荷的斥力后越容易往油中扩散,空间电荷在绝缘油中所形成的空间电场使得油纸沿面流注的主分支能够在绝缘油中传播,增加了油纸沿面流注传播距离,从而使低粘度天然酯绝缘油油纸沿面正极性雷电冲击击穿电压略高于其纯油击穿电压。