Flashover characteristics of vapor-mist dielectrics under lightning impulse and means to increase the flashover voltage

It has been reported that the flashover voltage of gaseous dielectrics may increase greatly when they are mixed with carbon-halide mist. However, the factors which bring about the increase of the flashover voltage still are not clarified fully. Based on the study on the basic flashover characteristics of vapor-mist dielectrics, the authors have suggested the scarcity of initial electrons as the predominant factor. This paper investigates further the effect of UV light irradiation, flashover characteristics under forced supply of initial electrons, and for lightning impulse superimposed on dc voltage. These results have verified that the scarcity of initial electrons is the decisive factor in increasing the lightning impulse flashover voltage of vapor-mist dielectrics. It is clarified also that the scarcity is attributed principally to: (1) the reduction of effective period (lifetime) of initial electrons, and (2) the suppression of photoelectric emission from electrodes.     

Electrophysicochemical characteristics of a waterpen point coronadischarge

Some interesting electrophysicochemical characteristics of waterpen point-to-plate discharges with DC and AC power have been investigated in a room atmosphere. The metal point electrode of a conventional point-to-plate air gap is severely bombarded by energetic ions from the discharge region near the point. Concurrently, the temperature of the point is greatly increased. There are different characteristics for the waterpen point discharge, because the surface temperature of the waterpen point stays low due to the evaporation of the moisture from the waterpen point. In this paper, the discharges and electrophysicochemical characteristics of a waterpen point and a Pt metal point have been studied to evaluate the differences. It was found that mists and sprays are dispersed from the waterpen point due to the bombardment of ions. Also, a positive corona is effective for mist dispersion, a negative corona disperses sprays, and AC corona disperses both mists and sprays. The AC discharge of the waterpen point generated more ozone of 56 ppm, while it was the negative DC discharge of the Pt point that produced more ozone of 30 ppm. The AC discharge of the waterpen point produced 0.11 ppm at 10 kV, and the Pt point produced 0.10 ppm at 15 kV. The DC discharge of the both points showed no NO formation     

Physical Aspects of Vapor-Mist Dielectrics

In earlier work it was shown that by acoustically cavitating a liquid dielectric, a dense mist (~2 x 105 droplet/cm3) of micron-size droplets could be formed which considerably increased the electrical strength of an insulating gas. The high electrical strength of these vapor-mist dielectrics results from a combination of the vapor from the droplets enhancing the gas strength and the droplets collecting electrons and ions. This latter effect helps prevent the formation of electron avalanches which precede breakdown. Within the vapor-mist dielectrics, complex mechanisms are involved, ranging from the enhanced vapor pressure at the droplet surface to surface tension effects and droplet charge which help prevent droplets from both freezing and evaporating. These aspects of vapor-mist dielectrics are examined and discussed in relation to the acoustic cavitation method used for generating the mist.     

Dielectric breakdown characteristics of cryogenic nitrogen gasabove liquid nitrogen

We measured the DC dielectric breakdown characteristics of cryogenic nitrogen gas above a liquid nitrogen surface for needle-to-plane and sphere-to-plane electrode configurations. Experimental results revealed that the DC breakdown voltage of cryogenic nitrogen gas increased, as the distance from the gap axis to the liquid nitrogen surface diminished, i.e. as the liquid nitrogen surface got closer to the gap. The breakdown voltage proved to be enhanced not only by the temperature drop of nitrogen gas due to the existence of liquid nitrogen, but also by an effect of vapor mist arising from vaporization of liquid nitrogen. For the quasi-uniform electrode configuration, the relation between breakdown voltage and the gas density times the gap spacing agreed well with the Paschen curve for nitrogen with both the temperature falling and the vapor-mist density considered. For the nonuniform needle electrode, positive breakdown voltage was higher than negative one; the polarity effect was interpreted in terms of the electric field relaxation at the tip of the positive needle resulting from partial discharges observed only for the positive needle     

直流负极性下雨滴对导线电晕特性的影响

高压直流输电线路电晕会带来严重的电磁环境问题.导线电晕已经成为影响特高压直流输电线路结构和建设费用的决定性因素.随中国电网的不断发展,特高压直流输电线路电晕问题开始受到人们的关注.在降雨过程中,导线表面会形成大量的雨滴,雨滴的存在严重影响导线的电晕特性.为了研究大气条件对直流输电线路电晕特性的影响,笔者利用电晕笼,研究...     

球-球电极气液两相体直流击穿现象的研究

研究了空气和雾组成的气液混合两相体在球-球电极中的直流击穿电压。实验结果表明:"!电极上的水滴会使击穿电压降低,在准均匀和非均匀电场中,降低的比例差别不大;随着喷雾量的增加,降低的比例增加;"#空气中的雾会使击穿电压提高,在准均匀电场中(间隙较小时),提高的比例较大,而且随着雾量的加大,提高的比例增加;在非均匀电场中(间隙较大),提高不明显。     

CO2和细水雾对地下变电站变压器火灾抑制作用研究

为了研究CO2和细水雾对地下变电站变压器油火灾爆炸的抑制作用及其化学动力学机理,利用实验和Chemkin软件数值模拟研究了不同体积分数的细水雾和CO2对化学当量比条件下20% H2-CH4-air混合物爆炸的影响。实验和模拟结果表明：细水雾对H2-CH4-air混合物的火焰结构和爆炸压力的抑制作用随着细水雾体积分数的增加而愈发明显;细水雾蒸发产生的水蒸气分子和CO2在反应体系中主要起稳定的第三体作用,从而减缓了爆炸链式反应的发展。另外,CO2是CH4氧化反应的主要产物,加入CO2会削弱甲烷氧化的基元反应;反应体系中加入细水雾会通过降低基元反应温度敏感性来抑制反应体系升温,并且加入CO2会促进消耗·H自由基的基元反应发生。这也是CO2和细水雾共同作用抑制效果优于细水雾单独作用的原因。以上研究结果可为地下变电站变压器油火灾爆炸防治技术的发展提供理论依据。     

雾霾对冲击放电路径影响特性的分析

由于经济规模的迅速扩大和城市化进程的加快,雾霾现象日趋严重,这些雾霾多发地区是我国输电走廊或用电高负荷密度地区。为研究雾霾对放电发展的影响,利用水雾、粉煤灰等模拟雾霾天气,采用雷电冲击电压进行了雾霾对击穿电压和放电路径影响的实验研究。研究结果表明:当雾霾的粒径范围<0.01mm时,雾霾间隙的击穿电压始终较空气间隙的高,放电路径选择雾霾的概率<50%;当粒径范围为0.01～0.1mm时,正电压下选择概率<50%,雾霾间隙的击穿电压较空气间隙的高,而负电压下选择概率>50%,雾霾间隙的击穿电压较空气间隙的低。雾霾中放电发展是电场的畸变和雾霾颗粒与粒子间作用联合影响的结果。     

磷酸铁锂电池模组过充热失控特性及细水雾灭火效果

磷酸铁锂电池模组的热失控及灭火是大规模应用急需解决的问题。文中分别以单个磷酸铁锂电池模组和簇级磷酸铁锂电池模组为试验对象,在恒流过充方式下研究单个磷酸铁锂电池模组与簇级磷酸铁锂电池模组的热失控特性,并使用细水雾作为灭火剂,研究细水雾对磷酸铁锂电池模组的灭火效果。试验结果显示:簇级磷酸铁锂电池模组燃烧后温度急剧上升,18 s内温升速率达42.74℃/s,温度峰值近1000℃,明显高于单个模组的峰值温度(600℃);细水雾持续喷洒100 s后,2组试验模组温度迅速降低,明火完全扑灭无复燃,灭火效果极佳。试验结果可为磷酸铁锂储能电站的安全和消防提供有效的理论与试验支撑。     

运动水滴在球──球直流电场中的放电研究

研究了运动水滴在球－球直流电场中的放电现象,对于含有水滴的电场分布进行了计算,水滴的引入使得水滴外表面的场强跃变为无水时场强的３倍左右,从而水滴在电场中会在较低的电压下诱发电晕,比较了正负极性电压下起晕现象。为水处理技术中的混合两相体放电问题提供了依据。     

运动水滴在球一球直流电场中的放电研究

研究了运动水滴在球－球直流电场中的放电现象，对于含有水滴电场分布进行了计算，水滴的引入使得水滴外表面的场强跃变为无水时场强的３倍左右，从而水滴在电场中会在较低的电压下诱发电晕，比较了正负极性电压下起晕现象。为水处理技术中的混合两相体放电问题提供了依据。     

Effect of Barrier Materials on Discharge Properties in Air at Low Pressure

Dielectric barrier discharge(DBD) is widely investigated in order to obtain uniform low-temperature plasma.Many studies have proved that some barrier materials,especially electrets,can improve the uniformity of discharge.It is regarded as an available way to get atmospheric pressure glow discharge(APGD).In this paper,discharge forms with 4 different barrier materials(alumina,quartz,PTFE and PET) are investigated,and the transition of discharge form depending on the air pressure are recorded to estimate the influence of barrier materials on discharge.By using electrets as barrier materials,homogeneous discharges can be obtained in a more wide pressure range.Under the same experimental conditions,discharges with electrets are more uniform or have larger uniform areas due to the storage and desorption of charges on the surface of electrets.The electrons deposited in the surface layer can be released on next half cycle when the polarity of the applied voltage changes,and provide a number of seed electrons,which makes the discharge more homogeneous.The capacitance and the permittivity of barrier materials have no effect on the discharge form directly.     

Dielectric Liquid-in-Liquid Dispersion by Applying Pulsed Voltage

Liquid-in-liquid dispersion, such as organic liquid in water or water in organic liquid, has been performed using dc or ac voltage applied between nozzle and ground electrode. In the present study, pulsed high voltage was applied to produce droplets with controlled diameter in wide range. The high voltage pulse source was capacitor discharge type with 20 - 50 Hz and ranged from 0 to several kV. Water glass was atomized in alcohol solution into diameters ranging from several mum to sub-mm, depending on applied voltage. The atomized water glass droplets were solidified by removing water molecules from the water glass. Synchronized droplet formation with pulse frequency was possible by controlling pulse voltage, width and frequency, which produced uniform sized droplets successively. When the pulse voltage was raised, the droplet formation mode changed from the synchronized formation to dispersion mode through transient mode. In the dispersion mode, droplets of several mum diameter having high uniformity were produced. Utilization of high voltage and high-speed pulse to liquid-liquid dispersion could make it possible to atomize in a conductive liquid without electrolysis.     

锂离子电池碳负极材料的研究(1)--镀膜对碳材料电化学性能的改善

对锂离子电池碳负极材料表面进行镀膜处理,并对处理前后的电极进行电化学容量测试和扫描电镜形貌分析.结果表明,镀膜处理后的电极的充放电容量提高了10%～15%,而充放电效率和电压平台基本没有改变.结合电镜结果对这一现象进行了分析和讨论.     

The effect of the electrode gap on breakdown in liquid dielectrics

It is a well known fact that the breakdown voltage of an insulating liquid increases nonlinearly with increasing electrode gap. Under nonuniform field dc conditions two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm or larger, one finds that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Recently, perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. In the case of the conventional hydrocarbon type oils, the positive streamer usually grows slightly faster than the negative one, but the streamer inception voltages of both are rather similar. In the case of perfluorinated polyethers the positive streamers move at least 10× faster than the negative ones, but they require much higher inception voltages than negative ones. At gaps ≳12 mm and the associated higher applied voltages this breakdown at negative polarity occurs at higher voltages than at the positive one. The negative ions of the perfluoro compounds are much more stable than those of conventional hydrocarbons. The discharge of the negatively charged perfluoro ions requires much higher fields than do those of conventional hydrocarbons. The implications of these observations are discussed in some detail     

Surface discharges on polymeric insulator shed surfaces

This paper describes results of an experimental investigation into surface discharges occurring on the surface of polymeric insulators. Discharges on partially hydrophobic surfaces were found to occur across well defined dry bands, while discharges on hydrophobic surfaces appeared as small points of light occurring between discrete water drops. These discharges were observed to produce a localized loss of hydrophobicity. It is shown that these discharges are stable atmospheric-pressure glow discharges. The discharges are characterized by a high cathode voltage fall, dependent on the electrical conductivity of the water drop, and a voltage gradient in the positive column, dependent on the discharge current. A technique is presented for measuring the rate at which surface hydrophobicity of polymeric materials is lost due to the action of this type of discharge. It was found that the loss of hydrophobicity occurred predominantly in the region near the cathode     

石灰石-石膏湿法脱硫系统水耗计算及节水分析

石灰石-石膏湿法系统的运行是电厂耗水量增大的主要原因。利用物料平衡方法,计算了某600MW机组脱硫系统的耗水总量,以及对脱硫系统的废水排放、石膏携带水、烟气携带液滴、吸收塔内水分蒸发等水耗情况进行了统计。根据计算结果,指出吸收塔内水分蒸发是造成脱硫系统高水耗的主要原因,并提出了减少烟气量、降低锅炉排烟温度、合理调整脱硫用水水源等措施,实现脱硫系统的节水目的。     

纳秒脉冲表面滑闪放电的电气和光学特性

为了研究纳秒脉冲表面滑闪放电特性,本文采用一种新型三电极结构的激励器,通过纳秒脉冲叠加负直流的混合激励模式产生表面滑闪放电。实验研究了电压脉冲分量、电压直流分量及两者的差值对纳秒脉冲表面滑闪放电特性的影响。实验结果表明,当脉冲电压幅值固定时,直流电压幅值的改变对脉冲侧电流的影响较小,但对直流源侧电流却影响显著,直流源侧电流随直流电压幅值的增加而增加,发生表面滑闪放电后峰值和速度均增加。直流电压幅值越大,直流源侧电流出现时刻越早。当直流电压幅值固定时,脉冲侧电流和直流源侧电流均随着脉冲电压幅值的增加而增加。实验中存在一个电压阈值(脉冲分量和直流分量电压差值)使纳秒脉冲表面滑闪放电发生,该阈值为22k V。此时发生表面滑闪放电,瞬时功率峰值、单脉冲能量峰值和稳态能量均迅速增加。脉冲直流电压差值相同时,脉冲分量主导脉冲侧电流的大小,直流分量主导直流源侧电流的大小,脉冲分量所占比例的大小对功率和能量损耗的影响较大。此外,利用数码相机拍摄放电图像研究了纳秒脉冲表面滑闪放电的光学特性,放电图像表明,在电极间施加合理的脉冲电压和负直流电压均可产生表面滑闪放电,实现等离子体的拉伸效果,在阻挡介质表面获得大面积的等离子体。     

空气中电晕放电与绝缘液体中流注放电的测量和分析

Insulation is one of the most important parts in a high voltage equipment.There are gaseous,liquid and solid insulations which are commonly used.In a high voltage transformer for example the insulating materials are all used.During operation of a high voltage equipment high electric stress may occur.Under extreme condition failure of the insulation may take place.Excessive electric field in air may cause corona discharges while in liquid insulation discharges may take place in the form of streamer.This paper reports experimental results on the corona and streamer discharges in air and silicone oil.The discharges were artificially generated around a needle tip in a needle-plane electrode system with gap length of 4 mm under sinusoidal and triangular voltages.The needle was made of steel with tip radius of 3 μm and curvature angle of 30°.The needle was made by Ogura Jewelry.The discharge pulses were measured using personal-computer based partial discharge(PD)measurement system with sensitivity of better than 0.5 pC.The system is able to measure discharge in time sequential.Phase-resolved analysis of the discharges was done to interpret the physical processes behind the discharges.The experimental results showed that corona discharges took place at negative half cycles.The discharges were concentrated around 270° of phase angle of applied voltage.The discharge magnitude and discharge number of corona clearly dependent on the instantaneous of applied voltage.These were strongly supported by the application of triangular voltage.Streamer discharges occurred at both positive and negative half cycles.The discharges pulses concentrated around the peak of applied voltage at phase angle of 90° and 270°.Experimental results under sinusoidal and triangular voltages revealed that streamer discharge magnitude as well as probability of occurrence was strongly dependent on the instantaneous applied voltage.     

非均匀场中气固两相体雷电冲击U50的实验研究

通过对几种典型固体颗粒的气固两相体放电实验研究,发现其击穿电压具有极性效应,负极性条件下体积比对雷电冲击U50影响显著,使雷电冲击U50降幅达到30%左右,并且小颗粒的击穿电压比大颗粒的击穿电压低;正极性条件下击穿电压随体积比变化呈小波动变化特性,体积比及颗粒大小对击穿电压影响不显著。另外,与空气放电一致的是,相同体积比下正极性雷电冲击击穿电压低于负极性。