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     

Temperature effect on the performance characteristics of beta raysopen air corona streamer counter

Open air three electrode (point-grid-plane) corona streamer counter with a preamplification gap has been utilized for detecting weakly ionizing beta rays. This is based on the fact that when the primary electrons produced by beta rays are multiplied in the external preamplification gap, they form a trigger electron swarm before entering the counter, produce the same effect as the direct incidence of strongly ionizing particle in the corona counter, and allow the counter to operate efficiently. However, operation of this counter in open air as one of its most attractive features requires a complete analysis of the counter response to the changes of ambient conditions. The effect of temperature on the beta particle corona counter performance is studied. A theoretical model is suggested to calculate the counter operating voltage as well as the beta particles triggered corona pulse characteristics at different temperatures (20°C-50°C). The temperature effects on the counter-energy response is explained. A compensation for the errors due to temperature effects is suggested. All the theoretical findings are confirmed by experiments     

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     

Effect of electrode ice layer on prebreakdown current in liquidnitrogen

The effect of ice on dc prebreakdown events was investigated using a needle-to-plane electrode system in liquid nitrogen at 77.3 K. It was found that H₂O ice has marked effects on prebreakdown current, corona onset voltage and breakdown voltage. The breakdown voltage at atmospheric pressure depends on microscopic cracks in H₂ O ice     

The growth of prebreakdown cavities in silicone fluids and thefrequency of the accompanying discharge pulses

Measurements have been made of prebreakdown cavities in silicone fluids, and of the current pulses that accompany cavity growth. These experiments were carried out in silicone fluids of 0.65, 10, 100 and 1000 cS viscosity. Cavity growth, driven by the electrostatic field, is limited at low viscosities by inertia, and at high viscosities by viscous drag. The electrostatic force on the cavity wall is related to the local field and to the space charge density in the liquid adjacent to the cavity. We are concerned with the relationship between the electrostatic force and the cavity growth, and with the discharges that accompany cavity growth. Discharges occur in well-defined pulse trains: the first pulse in a train generates the cavity, and subsequent pulses are due to discharges within the cavity. Knowing the scaling laws for cavity growth we can use the time between the first and second pulses to estimate the cavity size when the first cavity discharge occurs; this gives a cavity diameter of ~5 to 7 μm. The next pulse cannot occur until the charge from the previous discharge has dispersed. We find that the time between pulses Δt is strongly viscosity dependent; at high viscosities the average time between pulses at is proportional to fluid-viscosity, but in the low viscosity limit the dependence approaches η^1/3. To explain this viscosity dependence we consider three mechanisms: (1) a decrease in charge density due to increase in cavity size; (2) ion detrapping from the cavity wall and drift in the applied field; and (3) diffusion of an impurity species to the cavity surface, charge exchange to create a mobile ion, and its subsequent drift in the field. Our experimental results are consistent with the cavity expansion model, but there is evidence of diffusion effects in low viscosity liquids, and with ion-drift at high viscosities     

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     

Positive streamer discharges along liquid dielectric surfaces: effect of dielectric constant and surface properties

This paper presents investigations on positive streamer discharges propagating in air along the surface of two nonmixing dielectric liquids. By measuring the currents associated with the streamer discharge propagating along the surface of the two nonmixing liquids for varying ratio of their volume, we analyzed the effect of the dielectric constant (the capacitive effect) and the surface properties. Especially, we separated and quantified the magnitude of each contribution. Two different combinations were studied: silicone oil together with glycerol and transformer oil together with glycerol. For these material combinations, we found that the influence of the dielectric constant is about 3 times larger than the effect of the surface properties. Further, by using a simple model of the streamer discharge, we determined with a 3D field calculation program the capacitive behavior of the two nonmixing dielectric materials for varying ratio of their volume. We found that, in order to obtain the observed capacitive dependence, the streamer must propagate literally on the insulator surface.     

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     

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.     

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     

Calculations based on measurements of charge deposited by a streamer on a PTFE surface

Surface charge on an insulator's surface is studied using 3-dimensional finite element modelling. The calculations are based on measurements of surface charge deposited by a streamer on a PTFE surface. The surface electric field strengths associated with uniform surface charge density distributions over a 1 mm/sup 2/ region are calculated for a range of different measured charge densities. The effect of reducing the area of the surface charge whilst keeping the total amount of charge in the region constant is observed. The surface charge is then modelled with a Gaussian distribution of charge and compared to that of a uniform distribution. By assuming a maximum allowable surface electric field strength an estimation of actual charged path diameter is made.     

Formulation of the streamer breakdown criterion and its application to strongly electronegative gases and gas mixtures

Contents Following an examination of the definition of the effective coefficient of ionisation , a reformulation of the streamer breakdown criterion is undertaken. The application of this criterion to strongly electronegative gases and gas mixtures is considered, and a figure of merit is introduced to describe dielectric behaviour. By considering both ideal and practical electrode systems, this parameter is shown to be of fundamental importance with respect to the evaluation of the breakdown characteristics of such gases. Its value can be determined from a precision measurement of the linear portion of the Paschen curve.

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Die Formulierung des Streamer-Durchschlagskriteriums und dessen Anwendung auf stark elektronegative Gase und Gasmischungen

Übersicht Nach einer Diskussion der Definition des effektiven Ionisationskoeffizienten wird eine Neufassung des Streamer-Durchschlagskriteriums aufgestellt. Die Anwendung dieses Kriteriums auf stark elektronegative Gase und Gasmischungen wird besprochen, und ein das dielektrische Verhalten beschreibender Qualitätsparameter wird eingeführt. Es wird gezeigt, daß dieser Parameter für die Berechnung von Durchschlagskennlinien solcher Gase von entscheidender Wichtigkeit ist; dabei betrachtet man sowohl idealisierte als auch praktische Elektrodensysteme. Den Wert des Parameters erhält man durch Präzisionsmessung des linearen Teils der Paschenkurve.

     

轴流转桨式水轮机轮缘间隙空蚀的试验研究

本文对轴流转桨式水轮机轮缘间隙流动进行试验研究,咩细研究了不同工况下轴流转桨式水轮机轮缘间隙流动的空蚀形态。受桨叶角度和开度的影响,轮缘间隙流动有五种的空蚀形态,这几种空蚀形态随开度的变化,可能有一种或两种同时出现。其中侵蚀危害性最严重有Ⅰ、Ⅱ和Ⅴ三种空蚀形态。在负的桨叶角度,协联工况的左边运行时,在叶片的背面轮缘处产生第Ⅰ中泄漏涡带,同时在叶片进口正面头部轮缘处产生泄漏流动和二次流动漩涡相互作用所引起的小面积的强烈空蚀空泡为第Ⅱ种,它是叶片头部缺角的主要原因;在正的大桨叶角度、大开度情况下,间隙泄漏流动主要发生在叶片的背面轮缘处尾部第Ⅴ种空蚀形态,泄漏涡带与主流相互作用形成一个长长的涡带扫射下部转轮室,是叶片背面尾部和转轮室空蚀破坏的主要原因。     

扫描探针显微镜针尖电容的测量与应用

本文基于扫描探针显微镜（SPM）建立了微区电容的低频测量系统,最小测量电容为30aF,工作频率为20～100kHz。试验测量了SPM导电针尖与金属样品之间的电容Ctip与它们之间的距离z关系曲线,并同时通过光学系统测量了针尖的关于频率的一次受力信号Fw与z的关系曲线。推导了两种测量方法所获曲线的内在关系,并获得了针尖与样品的表面势之差。并且,利用此系统测量了Al0.3Ga0.7N/GaN薄膜的微区C-V曲线,获得和宏观C-V曲线趋势一致的结果。     

加湿对PEMFC性能的影响

研究了在重力作用下,反应气体的湿度对质子交换膜燃料电池(PEMFC)性能的影响.通过控制加湿温度,控制反应气体的相对湿度;通过改变阴极和阳极的相对位置,来改变PEMFC内部的水管理.阳极不加湿(阴极加湿)时,PEMFC的性能最差;阴极不加湿(阳极加湿)时,PEMFC的性能最好.阳极在上时,重力对阴极排水有积极的作用;阴极在上时,重力阻碍阴极排水.     

蓄电池生产工艺对充电接受性能的影响

起动用铅酸蓄电池的充电接受性能是其电气性能的一个重要指标。其含义是表示已放电的蓄电池在定电压下易于充电的特性。随着汽车工业技术水平的不断提高,蓄电池的这一性能指标显得愈来愈重要。如果电池放电后不能在一定时间内充电以恢复其荷电性能,那么不仅影响到其下一次的放电性能,也影响到电池的使用寿命,特别是在低温条件下更是如此。我们在80年代后期从国外引进的蓄电池技术,目的是要生产符合日本ＪＩＳ标准和德国ＤＩＮ标准的铅酸蓄电池。由于采用同一铅膏配方和生产工艺制造符合两种标准的电池,因此根据既要符合日本ＪＩＳ标准的充电…     

铈对铅锑合金电化学性能的影响

采用循环伏安法、腐蚀失重法、交流阻抗法研究了铈对铅锑合金板栅电化学性能的影响。结果表明 ,铈的添加有助于改善铅锑合金腐蚀层结构 ,对合金的耐腐蚀、电化学稳定性产生好的影响。     

铈对铅锑合金板栅性能的影响

采用循环伏安法、腐蚀失重法、交流阻抗法研究了铈对铅锑合金板栅电化学性能的影响。结果表明,铈的添加有助于改善铅合金腐蚀层结构,对合金的耐腐蚀、电化学稳定性产生好的影响。     

The effect of porosity on the electric strength of heat-resistantconcrete

In many applications where both high temperature and high electrical stress exist, little is known about the interaction of the electrical and thermal properties of the materials involved. This paper is a study of this interaction for a material which we call a concrete, and is a fine alumina powder mixed with a cement, i.e. calcium aluminate (CaOAl₂O₃) slurry, which cures in the same manner as a concrete. The porosity of the product depends upon the relative volume of the hydrated cement to the volume of the spaces between the alumina particles, and the shapes of the pores may be changed by choosing the mix of various particle sizes of alumina particles. In this study, specimens of this material were cast between uniform field electrodes in one case, and between nonuniform field electrodes in the other, cured and dried to remove residual moisture, and tested for electric strength. It is well known that the electric strength of porous materials is low and nearly the same as the electric strength of the gap between the same electrodes in air in the absence of the solid porous material. This paper shows that for this material and only at porosities <20%, as measured by a mercury porosimeter, does the electric strength become larger than that of air and become a significant function of the porosity and of the distribution of pore volume among pores of differing equivalent pore diameter. The uniformity of the field had no noticeable effect