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     

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     

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.     

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。     

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.     

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.     

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 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     

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.     

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     

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     

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.     

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     

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     

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 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     

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     

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.     

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     

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。