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1.
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.  相似文献   

2.
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  相似文献   

3.
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.  相似文献   

4.
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  相似文献   

5.
正流注传播动力学特性随气压湿度的变化   总被引:1,自引:0,他引:1  
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。  相似文献   

6.
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  相似文献   

7.
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.  相似文献   

8.
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.  相似文献   

9.
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  相似文献   

10.
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.  相似文献   

11.
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  相似文献   

12.
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.  相似文献   

13.
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.  相似文献   

14.
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 Eg~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  相似文献   

15.
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  相似文献   

16.
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  相似文献   

17.
This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (⩽35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed  相似文献   

18.
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  相似文献   

19.
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  相似文献   

20.
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.  相似文献   

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