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.     

Creepage discharge characteristics over solid-liquid interfaceswith grounded side electrode

The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of ~20% from the streamer tip. The streamer appeared to progress with a constant mean velocity     

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.     

Streamer propagation under impulse voltage in long point-plane oilgaps

A 67 mm point-plane gap immersed in transformer oil, at atmospheric pressure, stressed with a 1/180 μs impulse, has been studied by recording gap current and light emission. Time-resolved images were obtained by use of an image converter camera. Minimum breakdown voltages were twice as high for negative as for positive points. All streamers at these and higher voltages were supersonic, with negative streamer velocities ⩽50 km/s. The speed of the positive streamer gradually increased to 19 km/s with increasing voltage, where it saturated. At high stresses the propagating positive streamer has been observed to switch to a faster mode which propagates with speeds in the 65 to 200 km/s range. Positive streamers were bush shaped to 2× the minimum breakdown voltage, the branching diminishing with increased voltage. Negative streamers were coarsely bush shaped or usually tree shaped. For both polarities, light emission consisted of a background light, with superposed pulses corresponding to 15 ns wide current pulses of up to 10 A. The pulses are caused by very bright, brief re-illuminations of single streamer channels. The tips of positive streamers are considerably more luminous than the channels, indicating that electron multiplication may take place at the tips. A model of the streamers as being plasma filled channels may explain the re-illuminations     

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     

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.     

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 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     

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。     

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。     

The effect of tip curvature on the prebreakdown streamer structurein cyclohexane

In this paper the effect of tip curvature on the generation and propagation of prebreakdown density change streamer in cyclohexane was investigated. The streamer inception voltage increased with tip curvature, which was higher for a positive point than a negative one. The initial structure of the streamer was a single filament for both polarities in cyclohexane. For a negative point, the streamer structure changed from sphere to hemisphere, pagoda and bush-like form with increasing tip curvature and applied voltage. A slim channel connecting the streamer with needle tip was clearly observed for tip radii >5 μm, while it was not for those <3 μm. For positive streamers, on the other hand, it changed from slow bush-like form to fast filamentary with increasing tip curvature and applied voltage. Both magnitude and propagating speed of the streamer at its inception voltage increased with a tip curvature. After the growth stopped, they broke up into small bubbles and contracted with damped oscillation. From the spatial distribution of the field strength between the point and plane calculated by the finite element method, it was found that there exists a specific distance from the tip where the field strength is equal under any tip radii used in this study for both polarities. This distance and field strength may considered to be the minimum length and field for an electron avalanche in liquid     

Effects of chain length on prebreakdown phenomena in straight-chain saturated hydrocarbon liquids

Prebreakdown light emission, currents, and streamers in straight-chain saturated hydrocarbon liquids, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane, are investigated under nonuniform field conditions. The light inception voltage and the breakdown voltage increase with the chain length and the liquid's density. For a negative point, the breakdown voltage is much higher than the light inception voltage in all liquids but the difference decreases with the chain length. This voltage difference is not observed for a positive point except in n-pentane and n-hexane. The difference between the negative and positive breakdown voltages decreases with the chain length. Regarding the polarity effect, the negative breakdown voltage is much higher than the positive one in n-pentane, but there is no difference in n-decane. A negative density change streamer has a bushlike structure and propagates in stepped manner at an average speed between 70 and 160 m/s, while for a positive one, the structure is treelike having an average speed of a few kilometers per second. Only in n-pentane and in n-hexane, a slow positive bushlike streamer is observed whose average velocity is about 200 m/s, slightly faster than the negative bushlike one. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 8–16, 1998     

Streamer propagation in mineral oil in semi-uniform geometry

Presents a study of streamer propagation in transformer oil, with point-plane and semi-uniform geometry. The latter is made of parallel plane electrodes, with a thin triggering point of calibrated size. By reducing the length of the point, it is possible to move progressively from a point-plane geometry to a quasi uniform geometry. The propagation of streamers is impeded by the presence of a metallic plane behind the triggering point, that reduces the field on propagating streamers. The effect varies widely according to the streamer type considered. The propagation of negative and fast positive streamers is nearly quenched, whereas slower filamentary positive streamers (usually responsible for breakdown in oil) are less affected. This shows that many results obtained in point-plane geometry can not be simply extrapolated to the more realistic case of uniform field     

Optical and electrical characterization of creeping discharges over solid/liquid interfaces under lightning impulse voltage

This paper deals with the experimental characterization of discharges propagating over solid insulators immersed in mineral oil, under impulse voltages using a point-plane electrode arrangement. It's shown that the nature and the thickness of the solid insulator significantly influence the characteristics of creepage discharge and especially the final length L/sub f/ and the density of branches /spl rho/. For a given thickness, L/sub f/ increases quasi-linearly with the voltage; it decreases when the thickness increases. So /spl rho/ increases with the dielectric constant of the solid insulator; it also increases when the thickness is reduced. This indicates the important role of the capacitive effect in the propagation mechanism. The main discharge is followed by a secondary one of opposite sign to that of the main discharge.     

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。     

Surface profile effect on streamer propagation and breakdown in air

In a uniform field arrangement, under direct voltage, positive streamer propagation and breakdown are investigated along cylindrical insulators with different profiles, inserted perpendicularly between two parallel plane electrodes. The basic properties of streamer propagation and breakdown, namely the electric field required for a stable propagation together with the associated velocity and the breakdown field together with time to breakdown, are measured as influenced by the pulse voltage amplitude used for the streamer initiation and by the insulator profile. It is shown that a strong relation between streamer propagation and breakdown exists, because the insulator profile exerts a similar influence on the breakdown and propagation fields. The effect of a shed on an insulating surface, forming an `obstruction' to streamer progress, is to increase the stability for propagation and breakdown fields, and to reduce the propagation velocity at all applied fields compared with those in the case of a smooth insulator. Along the surface of a smooth insulator, a streamer system propagates with a `surface' and an `air' component; however, a shed on an insulating surface modifies this system, resulting in only one component reaching the cathode     

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.     

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     

Streamer initiation in mineral oil. Part II: influence of ametallic protrusion on a flat electrode

For pt. I see ibid., vol. 9, no. 1, p. 84-91 (2002). Presents a study of streamer inception in transformer oil, in a "semiuniform" geometry under impulse voltage. This geometry consists of parallel plane electrodes, with a thin triggering point of calibrated size emerging at the center of one plane. By reducing the length of the point, it is possible to move progressively from a point-plane geometry, up to a quasi-uniform geometry. Compared to a point-plane gap, the initiation of streamers is impeded due to the presence of a metallic plane behind the triggering point, that reduces the tip field. Field calculations allow to extrapolate the results obtained to the case of microscopic surface protrusions and particles. It is possible to predict the streamer initiation voltage, and hence the breakdown voltage under uniform field, from the shape of the protrusion and from streamer initiation fields measured in point-plane geometry     

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.