避雷针迎面先导发展物理过程仿真研究

开展避雷针迎面先导起始及发展过程的仿真研究对建立正确的雷电屏蔽分析模型具有重要意义。基于长间隙放电的物理机制,建立了包括正极性电晕起始与流注发展、先导起始、先导–流注体系发展等物理过程的迎面先导发展物理过程仿真模型,并使用实验室和自然雷电条件下的迎面先导发展过程观测结果对其进行了验证,最后采用该模型对避雷针迎面先导特性进行了分析讨论。结果表明:该模型的计算结果与实验室条件下和一次自然雷电条件下获得的正极性迎面先导发展过程观测结果相吻合;迎面先导起始时刻随着雷电流幅值和避雷针高度的增加而提前;避雷针迎面先导的发展过程主要受雷电流幅值、避雷针高度影响,其发展速度随着下行先导的趋近而逐渐增加;由实验室条件下的正极性棒–板间隙放电获得的先导起始特性直接用于自然雷电中正极性迎面先导起始的计算,以及在迎面先导发展过程的计算中假设迎面先导发展速度与下行先导发展速度成一固定比例是不合适的。     

3D imaging of lightning channel and leader progression velocity

Three‐dimensional imaging of lightning channels by means of UHF interferometers is performed. To realize this, a field campaign was conducted with two observation sites in Darwin, Australia. The distance between the two sites is about 14 km, and the triangulation scheme is applied to estimate locations of UHF radiation sources. Since the UHF radiation is more likely during the leader progression than in the return stroke phase, imaging of lightning channels is obtained mainly for downward leaders. Velocities of downward leaders are estimated and statistics are presented for first and subsequent strokes. The result is consistent with those of former investigations obtained by optical observations. The statistics for the velocity of leader propagation inside thunderclouds are presented herein. Using optical measurement, velocity estimation of leader progressions inside thunderclouds is impossible, and the presented results are concluded to be the first results of their kind. © 2001 Scripta Technica, Electr Eng Jpn, 137(3): 22–28, 2001     

Liquid microvortex generated around a laser focal point in anintense high-frequency electric field

A microvortex was discovered in liquid films when subjected to the combined action of a focused laser beam and a high-frequency electric field. The laser beam was perpendicular to both the surface of the liquid film and the direction of the electric field. The size of this opto-electrostatic microvortex (OEMV) in various liquids (water, ethanol, acetone, nitrobenzene) varied with the experimental conditions, but stayed in the 10-μm range. The velocity of the liquid flow in the vortex could be controlled by the intensity and frequency of the electric field (5.0×10⁴ to 0.8×10⁶ V/m, 2×10⁴ to 1×10⁶ Hz), by the laser power (0.01 to 2 W) and by the laser power density in the focal point. No microvortex was observed in benzene and chloroform. As an application of this phenomenon, single DNA molecules could be transported along the OEMV     

雷击风机叶片的跃变击距特性与定量表征

风机叶片对雷电的接闪是风机防雷研究中的关键内容,合理分析接闪时的击距特性意义重大。基于雷击过程的物理机制,将雷击发生时上、下行先导头部间的距离定义为跃变击距,并分析了多上行先导竞争对风机接闪和跃变击距的影响。基于先导起始和发展理论,根据下行先导的等效模型和稳定上行先导的判定方法建立了风机叶片接闪模型,利用该模型得到了随雷电流幅值变化的跃变击距公式,指出侧面距离等参数对跃变击距没有影响,实验和观测结果证明了模型的正确性。最后利用接闪模型提出了考虑稳定上行先导对周围电场畸变作用的对地击距公式,与其他学者计算的结果进行对比指出,现有对地击距公式或不适用于风机背景的防雷计算。     

Stratified gas-liquid two-phase electrohydrodynamics in horizontalpipe flow

Stratified gas-liquid two-phase electrohydrodynamics in pipe flow has been studied experimentally and numerically. Experimental studies were conducted using 1.27 and 1.9×10^-2 m-inner diameter horizontal tubes with air-water two-phase flow for the range of gas surface velocity of 10±²-10 m/s, liquid surface velocity of 10^-2-2×10^-1 m/s, and applied voltage of 0-20 kV. Experimental results are analysed by an area-averaged two-fluid one-dimensional model. The results show that the effect of the applied electric field is significantly influenced by the flow regime transition boundaries between stratified smooth-to-wavy and stratified wavy-to-intermittent flow. However, the time-averaged void fraction was not observed to be significantly influenced by applied electric fields     

Observation of multiple stroke and multipoint discharges by means of UHF interference

Results of interferometric observations of cloud‐to‐ground flashes in Darwin (Australia) have been analyzed to investigate some features of the lightning phenomenon. Our study focused on comparison between multipoint and multiple negative cloud‐to‐ground flashes. The speed of leaders was estimated with submillisecond resolution for both multipoint and multiple strokes. Leaders preceding the first stroke in multiple‐stroke flashes progress in a stepped fashion, and their behavior exhibits the same features as leaders in multipoint flashes. The estimated average speed of the leaders is on the order of 10⁵ m/s, and the mean step length is found to be 96 m with a mean pause time of 73 μs. The running time of the leader found from results of these observations has a mean value of 89 μs. The duration of UHF radiation bursts generated by leaders propagating along previously formed channels ranges from 0.2 to 55.3 ms with a mean value of 7.5 ms. By contrast, the duration of UHF radiation in leaders preceding multipoint strokes ranges from 21.1 to 90.6 ms with a mean value of 47.5 ms. The time intervals between strokes that strike at the same point as the previous stroke range from 22.5 to 330.6 ms with a mean value of 75.1 ms, while intervals between strokes in multipoint flashes range from 55.6 to 633.7 ms with a mean value of 149.1 ms. All leaders preceding multipoint strokes start in the same region inside the cloud. © 2001 Scripta Technica, Electr Eng Jpn, 134(4): 62–69, 2001     

Simulation of energized Franklin rods for lightning protection

In this paper, the onset criterion of the upward streamers from an energized Franklin rod is formulated as a function of the geometry of the rod and the height and current of the downward leader. The electric field in the vicinity of the lightning rod is calculated using the charge simulation technique. The dependency of the radius of protection on the amplitude of the pulse voltage applied to the Franklin rod, the downward leader current, and the tip radius and height of the rod is investigated     

Nonequilibrium electron distributions and high-field electrontransport in an AlxGa1-xAs-based p-i-nnanostructure semiconductor-a picosecond Raman probe

Electron transport in an Al_xGa_1-xAs (x=0.3) based p-i-n nanostructure semiconductor under the application of an electric field has been studied at T=80 K by picosecond transient Raman spectroscopy. Single-particle excitations associated with spin-density fluctuations were used to directly measure electron distribution functions and drift velocities under various electric field intensities. Extremely nonequilibrium electron distributions were observed. Specifically, for an injected carrier density of n≅1×10¹⁸ cm^-3, a drift velocity V_d as high as 2.5×10⁷ cm/s was measured for an electric field intensity E=18 kV/cm. These experimental results are in good agreement with Ensemble Monte Carlo calculations     

Diagnostic dielectric spectroscopy methods applied to water-treedcable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10^-5 to 10⁵ Hz. Insertion loss measurements were also carried out in the frequency range of 3×10 ⁵ to 3×10⁹ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables     

基于长间隙放电研究雷电屏蔽问题的进展

长间隙放电是研究地面物体雷电屏蔽问题的最有效手段之一。首先介绍了国内外在雷电击距、直击雷防护措施的屏蔽性能试验和雷电迎面先导过程研究3个方面所取得的进展,并结合最新开展的长达6 m间隙尺度的放电试验观测,对现阶段上述3个方面研究所存在的问题进行了分析。认为基于雷电击距建立的电气几何模型(electric geometry model,EGM)仅适用于小尺度目标物的雷电屏蔽性能分析,现有的雷电屏蔽模拟试验方法仅能近似模拟不存在雷电迎面先导时的雷击过程,无法完全证明以提前流注发射模型装置为代表的非传统防雷装置具有比传统措施更优越的屏蔽性能。大尺度目标物的雷电屏蔽问题应聚焦于雷电迎面先导过程的研究,建立并完善雷电迎面先导过程的模拟试验方法和物理仿真模型。     

Image quality analysis for dual energy subtraction imaging with afemtosecond laser-based hard X-ray source

We present a study of image quality for dual energy subtraction imaging using an iodinated contrast agent and a femtosecond laser-based hard X-ray source. The INRS CPA laser (400 fs pulse focused on solid targets in a 3 μm spot at 4 × 10¹⁸ W cm^-2) was used to create a bright hard X-ray source (conversion efficiency of 10^-5 in the characteristic K_∝ line emission, 12 μm X-ray source diameter). A model of image quality has been developed and been benchmarked with specific experiments using specially made angiography phantoms     

正极性上行先导特性的模拟试验方法

开展负极性雷电地闪过程中地面物体正极性上行先导特性的模拟试验研究是完善雷电屏蔽分析模型的基础,选择正确的模拟试验方法十分必要。分析了采用棒–棒、棒–板及板–棒等典型放电间隙结构的模拟试验方法的电场特征,研究了各种方法获得的正极性先导特性,并通过与自然雷电下的电场特征及上行先导特性进行对比,评价了各模拟试验方法的等价性。结果表明:采用棒–棒间隙和棒–板间隙无法模拟自然雷电下地面物体附近的电场空间分布特性;采用板–棒间隙可以模拟自然雷电下地面物体附近的电场空间分布特性,但传统的冲击电压发生器无法产生与自然雷电下行先导趋近地面过程中电场时变规律相同的冲击电压。因此,提出了研发一种基于电力电子技术、能够模拟自然雷电下电场时变规律的高压任意波形发生器,并结合板–棒间隙结构,形成正确有效的正极性上行先导特性模拟试验方法。     

高压输电线路雷电绕击分析方法及模拟试验研究*

大量线路雷电跳闸故障统计资料显示,雷电绕击是引起电压等级为500 kV及以上输电线路雷击跳闸的主要原因。综述了几种有代表性的输电线路雷电绕击分析方法,并介绍了在南方电网昆明特高压基地开展的雷电绕击模拟试验研究。试验模拟了下行雷电先导接近线路时输电线路上行先导起始和发展的过程。试验结果表明,导线、地线会产生上行先导放电,且地线上行先导放电起始易于导线,导线、地线上行先导发展速度约为1.2～2.4 cm/?s。该结果可为雷电绕击分析提供了试验基础和物理参数。     

Spectral analysis of the light emitted by streamers in hydrocarbonliquids

This paper presents the results of a spectroscopic study from 200 to 850 nm of the light emitted by streamers initiated in cyclohexane and n-pentane under step voltage in point-plane geometry. Experimental spectra of the light emitted by bush-like and filamentary streamers are composed of the H_α-Balmer line, the C₂ Swan band system and a background continuum. The diagnostic method we used for evaluating rotational and vibrational temperatures of excited C₂ in streamers was first tested on high-pressure corona discharges in nitrogen. For streamers in cyclohexane and n-pentane, it was impossible to determine the rotational temperature of C₂ and consequently the effective temperature of molecules in the streamer. Moreover, we found that vibrational populations of excited C₂ do not follow Boltzmann statistics. This indicates that excitation processes are due to chemical reactions. Electron densities deduced of the Stark broadening of H_α are in the range 4×10 ¹⁶ to 7×10¹⁶ cm^-3 for filamentary streamers and 2 to 6×10¹⁷ cm^-3 during the breakdown phase. For slow bush-like streamers, the electron density is not measurable     

Molecular dielectrophoresis of biopolymers

Dielectrophoresis (DEP) has found wide applications in manipulations of micrometer-sized particles such as biological cells. However, it has been considered less effective for nanometer-sized particles such as biological macromolecules where DEP effects may be overwhelmed by Brownian motion. Here, the authors use the fluid integrated circuit (FIC) concept to fabricate micrometer-sized electrodes, with which a very high-intensity, steady-state, high-frequency field, over 1×10⁶ V/m at 1 kHz to 10 MHz, is created. The observation of molecular DEP using avidin (68 kD) and other biopolymers is made, and it is found that DEP occurs at the field strength of 0.4-1.0×10⁶ V/m, which is substantially lower than DEP theory predicts. As applications of molecular DEP, a novel molecular patterning method and “dielectrophoretic chromatography” are proposed. In the latter, biopolymer solution is moved through a high-intensity field region created in a FIC, and the polymer is analyzed and/or separated by the difference in the DEP attraction     

两次仅有连续电流的负极性人工引发雷电特征分析

对2006年8月28日在广州从化人工引发的两次雷电(F184626和F185503)的特征进行了分析。这两次触发闪电都是经典负极性慢型放电过程,整个放电过程仅有连续电流没有回击,但连续电流过程中包含有多个M分量。两次过程触发时地面电场分别为-6.1 kV/m和-8.2 kV/m,触发高度分别约336 m和244 m。资料分析表明,两次人工引发雷电形成稳定上行正先导之前,出现多次明显的双极性脉冲,脉冲间隔平均值分别为18.12 ms和16.64 ms;在上行正先导产生初始阶段出现明显的阶梯特性,梯级先导脉冲的平均间隔分别为20 ms和24 ms,总持续时间都约为250 ms,脉冲的宽度约1 ms。8月28日的两次触发闪电与以往北方正极性的慢型放电过程相比有连续电流持续时间较长等特征。     

InGaN-based blue laser diodes

The continuous-wave (CW) operation of InGaN multiquantum-well (MQW) structure laser diodes (LDs) was demonstrated at room temperature (RT) with a lifetime of 100 h. The threshold current and the voltage of the LDs were 50 mA and 5 V, respectively. The threshold current density was 8.8 kA/cm². The carrier lifetime and the threshold carrier density were estimated to be 3.5 ns and 1.8×10²⁰/cm³, respectively. The Stokes shift of the energy difference between the absorption and the emission energy of the InGaN MQW LD's were 140 meV. Both spontaneous and stimulated emission of the LD's originated from this deep localized energy state which is equivalent to a quantum dot-like state. From the measurements of gain spectra and an external differential quantum efficiency dependence on the cavity length, the differential gain coefficient, the transparent carrier density, threshold gain and internal loss were estimated to be 5.8×10^-17 cm², 9.3×10 ¹⁹ cm^-3, 5200 cm^-1, and 43 cm^-1 respectively     

A Modified Version of the Rolling Sphere Method

The calculation of the striking distance can estimate the probability of lightning strike on a structure and thereby evaluate the effectiveness of a lightning protection system (LPS). The dimensioning and the positioning of air-termination on structures is often performed with the Rolling Sphere Method (RSM). RSM originated from the electric power transmission industry and is based on the well-known Electrogeometric Model (EGM). The EGM relates striking distance to the prospective peak stroke current. To apply this technique, an imaginary sphere is rolled over the structure. All surface contact points are deemed to require protection, whilst the unaffected volumes are deemed to be protected. The main drawback of this method is that it disregards the upward leaders? development and assumes the same probability for attachment to the ground, to a structure, and to a LPS. The proposed model is based on physical phenomena leading to the formation and the development of positive upward leader in the field produced by the negative downward leader charge distribution and by some other competing upward leaders. Its purpose is to develop a 3-D numerical model in order to improve the interception efficiency of the Lightning Protection System.     

Effects of liquid conductivity upon gaseous discharge of droplets

The effect that liquid conductivity has upon gaseous breakdown and conduction between a droplet and a sharp grounded metal point was investigated as a function of the droplet charge level and point-to-droplet gap. A uniform stream of equally spaced 1210 μm droplets was studied in passing the point at 2.05 m/s at a rate of 465/s. The negative droplet charge was set at 24, 40, 55, and 60% of the Rayleigh hydrodynamic instability limit (i.e. 3×10^-10 C) for liquid conductivity values in the 10^-4-10¹ S/m range characterizing electrostatic crop sprays. No significant conductivity effect was found for charges up to the 55% level; the most conductive liquid exhibited a significantly (α<0.10) higher discharge current at the 60% charge level. For close gaps, droplets departed the grounded point region with a reversed charge, indicating they were overneutralized by a positive-ion flux from the grounded point