Thermal processes in a streamer discharge in water

The propagation features of a streamer discharge in water have been investigated. Based on the experimental data obtained in the study of water discharges in a nonuniform electric field, due propagation of streamers is explained as the evaporation of water at the tip of the streamer and around it. The energy balance in the process of the streamer propagation is calculated for a sub-microsecond discharge in distilled water. It is shown that the energy released in the pre-breakdown process is sufficient to evaporate the liquid in the streamer channels. Similar velocity of the streamer propagation in both tap and distilled water substantiates negligible effect of ionic current density onto the streamer propagation process. These estimations, based on experiment, have relevance to the discussion of the nature of the dielectric breakdown of water     

Positive streamer propagation in large oil gaps: electricalproperties of streamers

This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range     

正流注传播动力学特性随气压湿度的变化

为了解流注传播电场测量中大气参数的影响,利用“三电极系统”研究了不同气压湿度条件下正流注的传播电场和传播速度。利用两个光电倍增管检测流注在平行极板间的传播,获得了流注传播电场的概率分布及平均传播速度,得到了无外加脉冲电压时流注传播的“稳态电场”与相对空气密度、湿度间的经验公式,并和其它研究者提出的公式进行了比较。研究表明:流注传播的“起始电场”和“稳态电场”随着外加脉冲电压的减小而增大,随着气压和湿度降低而减小;电场强度相同时,流注平均传播速度随着气压和湿度的增大而减小,其数量级为105m/s。     

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.     

High field conduction and prebreakdown phenomena in dielectric liquids

The present paper is mainly devoted to phenomena occurring in point-plane electrode geometry, where breakdown is the result of the initiation and propagation of prebreakdown phenomena called "streamers". In this configuration, an investigative study of the streamer initiation processes, requiring very high electric field strengths (/spl sim/ MV/cm), and of propagation (requiring low electric field, /spl sim/ kV/cm) can easily be carried out for negative streamer development as well as for the positive case. From analysis of experimental results in pure liquids the physical processes connected with streamer initiation and propagation, particularly the electronic ones, are presented and discussed. Estimations of the main parameters of slower subsonic streamers and of the faster filamentary ones (such as field strength at the streamer tip, field inside the channel, charge density, etc.) have been obtained from qualitative considerations and compared to experimental data.     

Streamer propagation along insulating surfaces

Results concerning the propagation of streamers along insulating surfaces under uniform electric field are presented. The basic properties of streamers, namely the electric field required for a stable propagation and the propagation velocity, have been measured and compared with propagation in air alone as a reference. The results have shown that in the experimental arrangement used significant space charges due to streamer branching are absent, therefore the properties observed are considered as characteristic of a single streamer. Streamers propagate stably with an intrinsic propagation field and a characteristic velocity that depend on the nature of the insulating material. For electric fields higher than the minimum field required for a stable propagation, a streamer system propagates with a `surface' and an `air' component     

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.     

Properties of streamers in transformer oil

The electrical characteristics of creeping discharges and single creeping streamers in transformer oil first are compared with those of streamers developing in the liquid bulk. The distribution of electric potential along the channel of a single negative creeping streamer is determined using a capacitive probe technique. Then the distribution of the space charge associated with each streamer channel is discussed and the electric field around the channels is estimated. A strong correlation between the mean potential gradient and the capacitance of the streamer channels is found. The different results and considerations tend to support the hypothesis of the same basic physical mechanism for both creeping discharges and streamers developing in the bulk     

Simulation of streamer discharges as finitely conducting channels

This paper presents a two-dimensional simulation of positive streamers in air at atmospheric pressure in a quasi-uniform electric field. A streamer is assumed to consist of a hemispherical tip and a finitely conducting cylindrical channel with a constant axial potential gradient. The model predicts that E_g~450 kV/m and the radius is r~50 μm in air at standard atmospheric conditions. Moreover, the estimated number of positive ions in the streamer head of stably propagating streamers, agrees with estimations based on more advanced streamer models. The model is used to predict the behavior of streamers in electrical discharges in a semiuniform electric field and good agreement is found between experiment and theory. The computer simulation of such a simplistic model could be applied to predict the behavior of streamer discharges in complex electrode arrangements, including dielectric surfaces     

Streamer discharges in liquids and their applications

Industrial applications using pulsed power technologies have been developed in many fields. One of them is the treatment of exhaust gas by using uniformly produced streamer discharges. Streamer discharges in liquid also generate extremely high electric fields at the tip of streamers, as well as high energy electrons, ozone, other chemically active species, ultraviolet rays and shock waves. All of these may be utilized to sterilize microorganisms and to decompose molecules and materials. Large-volume streamer discharges in water have been produced in order to realize industrial applications. A wire to plane electrode configuration has been used. The discharge in water shows similar optical radiation to that in an atmospheric gas as provided by evidence from still photographs. The influence of polarity, conductivity, electrode geometry and hydrostatic pressure on the streamers in liquid, the mechanism of streamer discharges and possible industrial applications of streamers in liquids, are discussed in the present work     

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.     

Streamer propagation along profiled insulator surfaces

Experimental results on the propagation of positive streamers along contoured, axially symmetric, polytetrafluoroethylene (PTFE) insulation surfaces are presented. A plane parallel electrode configuration provides a substantially uniform electric field for streamer propagation and a point electrode at the ground plane initiates the avalanche process. Basic streamer properties of velocity and propagation probability with field strength are measured and compared with the corresponding characteristics of air and cylindrical insulators. Several insulator profiles are investigated and the occurrence of multiple streamer paths is demonstrated, which are generally distinguishable as surface and air components with different propagation velocities. Comparative data of the breakdown fields for the various insulators is included     

Determination of the streamers characteristics propagating in liquids using the electrical network computation

This work is devoted to the modeling of branching streamers propagating in transformer oil using an equivalent electrical network and the electrical network computation. The proposed model enables one to determine the different characteristics of the streamer (i.e., the associated current and the electrical charge, the power and the energy injected in the liquid, the local electric field at the streamer head, the streamer shape and its velocity, the mobility of the charge carriers within the streamer channels, the local viscosity and temperature). It's shown through the simulated values of the mobility of charge carriers, the local viscosity and temperature that both electronic and gaseous mechanisms are implicated in the streamer development. The gaseous nature of streamers and the role of the local electric field are evident. The influence of the conductivity and additives as well as the electrode gap on the propagation velocity of positive streamers is analyzed.     

Positive Streamer Propagation and Breakdown in Air: the Influence of Humidity

The influence of humidity on streamer propagation at conditions from the threshold for propagation to those for streamer-induced breakdown was investigated in a uniform field in air at atmospheric pressure. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap, with an auxiliary needle in the earthed anode. Positive streamers were initiated by applying at the needle electrode a pulse voltage which varied in amplitude. These propagated towards the upper plane electrode which was stressed by a negative dc voltage. Under natural atmospheric conditions, propagation and breakdown probability curves were obtained for several values of absolute humidity in the range between 5 and 22 g/m³. Thus, distributions of the electric field required for streamer propagation and breakdown were obtained and the associated velocity of propagation and time to breakdown were measured. Besides humidity, the amplitude of the voltage used for streamer initiation and the ambient electric field were considered as influencing parameters on streamer properties. Empirical equations are presented expressing the effects of the above parameters on the intrinsic streamer properties. A comparison with previous work in the literature is made and this leads to the conclusion that the influence of humidity on streamer propagation and breakdown can be placed in a sounder quantitative basis.     

Measurement of streamer charge distribution on an insulator in SF6 Gas

The charge density produced by streamers on an insulator surface in SF₆ has been investigated by using a probe method with a high-speed temporal resolution. Concentric circular probes, which also act as a plane electrode, are used in this probe method. Probe signals are observed oscilloscopically and converted into the charge densities through a numerical calculation. This method reveals the charge distribution before a disturbance caused by the “back discharge.” The charge density thus obtained ranges from several nC/cm² up to about 60 nC/cm². The density depends on the pressure, voltage height and the position of the streamer. The electric field on the insulator is analyzed numerically taking into account the surface charge. The internal electric field of the streamer is found to be 40 ～ 50 kV/cm · atm when the streamer ceases its propagation. However, it partly exceeds the critical one (89 kV/cm · atm) during the propagation.     

正极性雷电冲击电压下流注起始特性研究

流注是长空气间隙放电的主要过程,其起始特性具有重要的理论价值与广泛的工程应用。以往对直流电压作用下流注起始特性的研究较多,对冲击电压特别是电压上升率较大的雷电冲击电压下流注起始特性研究很少。采用基于光电集成技术的高压侧电流测量系统与空间电场测量系统,对1 m棒–板间隙在正极性雷电冲击电压下流注起始特性进行研究,提出了新的流注起始观测手段,获得不同半径棒电极、不同电压上升率下的流注起始电压与场强,拟合得到考虑电压变化率的流注起始场强判据,并验证了所提出判据的广泛适用性。另外,还对正极性雷电冲击电压作用下流注起始时延的变化规律进行研究,定量测量了流注的起始时延,为研究流注起始时延的概率分布奠定基础。     

Inception voltage of positive streamer and its length on PMMA inair

A positive surface discharge on a solid insulator in air consists of many branched streamers. The length of the streamers is controlled by the applied voltage at the instant when the streamers occur. The relationship between the inception voltage and the streamer length depends on the thickness of the solid insulator and the number of streamer branches. At a low voltage, the length of a streamer decreases with increasing the thickness of the insulator, but at a high voltage, it increases with thickness. The length of a streamer with a large number of branches is shorter than that with a few branches. This phenomenon can be explained as follows: the electrons generated in each branch flow into a positive high-potential electrode through the stem of the streamer, and the potential drop in the stem is enhanced due to this electron flow     

Influence of molecular structure on propagation of positive streamer discharge in dielectric liquids

In this paper, streamer discharges in dielectric liquids which have various molecular structures, such as a double bond or a triple bond in the molecular formula, are observed in detail for positive point polarity under the application of an impulse voltage to the point electrode. As test liquids, four straight‐chain hydrocarbon liquids—n‐hexane, n‐heptane, n‐octane, and n‐decane—are selected and in addition the influence of a double or triple bond included in their molecular formula on the propagation of positive streamers is investigated in detail by simultaneously using a high‐speed schlieren method and an LED current measurement system. Some differences in the mean propagation velocity and the propagation shape of positive streamers are evident. Their shape is mainly filamentary in test liquids including a double or triple bond. Also, positive streamer propagation is promoted in these liquids. It is thought that this is due to the effect of π bonds which have smaller bond energies than σ bonds. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 149(1): 15–21, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10366     

A new model for the primary process of electrical breakdown inliquids

A new model for the generation of electrical streamers in insulating liquids is proposed, It is based on the mechanical stress generated by the electric field and its influence on the cohesive properties of the liquid. In fields of 10⁸ to 10⁹ V m^-1 the stress is sufficient to enhance significantly the thermal generation of sub-microscopic rupture surfaces (holes) in the liquid which has solid-like properties in the short time of streamer development. Using the well-known Griffith concept of mechanically-generated crack propagation in solids, it is then argued that, when the population of sub-microscopic holes becomes sufficiently large, the same stress encourages macro-crack development which has all the hallmarks of streamer growth, In this model electrical discharges do not have a traditional primary role although they will have an important secondary role once macro-cracks have developed     

Effects of additives on prebreakdown phenomena in n-hexane

Prebreakdown phenomena in n-hexane are observed in detail for positive and negative polarities by using simultaneously a high speed schlieren technique and an LED current measuring system, when an impulse voltage (1.1/225 μs) is applied to a point-to-plane electrode gap. Furthermore, the effects of several additives on the streamer propagation are investigated. Especially the effects of electron-trapping additives on negative streamer propagation and of low ionization potential additives on the positive streamer propagation, are examined, as is a correlation between the shape and the propagation velocity of the streamers