Surface flashover of oil-immersed dielectric materials in uniform and non-uniform fields

The applied electrical fields required to initiate surface flashover of different types of dielectric material immersed in insulating oil have been investigated, by applying impulses of increasing peak voltage until surface flashover occurred. The behavior of the materials in repeatedly over-volted gaps was also analyzed in terms of breakdown mode (some bulk sample breakdown behaviour was witnessed in this regime), time to breakdown, and breakdown voltage. Cylindrical samples of polypropylene, low-density polyethylene, ultra-high molecular weight polyethylene, and Rexolite, were held between two electrodes immersed in insulating oil, and subjected to average applied electrical fields up to 870 kV/cm. Tests were performed in both uniform- and nonuniform- fields, and with different sample topologies. In applied field measurements, polypropylene required the highest levels of average applied field to initiate flashover in all electrode configurations tested, settling at ~600 kV/cm in uniform fields, and ~325 kV/cm in non-uniform fields. In over-volted point-plane gaps, ultra-high molecular weight polyethylene exhibited the longest pre-breakdown delay times. The results will provide comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage, pulsed-power machines.     

Flow visualization and image analysis of gas-phase AC corona discharge induced electrohydrodynamic liquid flow in a stratified fluid

A phenomenon of AC gas-phase needle-plate corona discharge induced electrohydrodynamic (EHD) liquid flow in a stratified fluid has been investigated experimentally. The flow visualization of flow pattern is carried out by a fluorescent dielectric liquid tracer, where the steady EHD liquid motion induced by the corona discharge is presented in visual impressions. The results show that the EHD liquid flow direction on an electrode axis is not from needle to plate electrodes axis when the needle position was above the liquid layer. For the case of the needle electrode tip immersed into the dielectric liquid, it is well known that the dielectric liquid flows in anticlockwise rotation by contraries. The flow velocity distribution is experimentally obtained by a particle image velocimetry with image processing, where the measurement system takes sequential digital images of flow field illuminated by a laser light sheet for computed tomography. The mechanistic model based on interfacial momentum transfer effects on liquid-phase fluid motion is discussed in detail.     

Fluid circulation in an enclosure generated by electrohydrodynamic conduction phenomenon

Fluid circulation/mixing inside an enclosure, nonmechanically, is of significant importance in various applications. This study investigates, theoretically, the circulation of an isothermal dielectric liquid inside an enclosure based on the electrohydrodynamic (EHD) conduction phenomenon. The theoretical model and the numerical solutions are presented in dimensionless forms to illustrate the effects of the controlling parameters on the induced flow. The EHD induced flow, based on the conduction phenomenon, is associated with the process of dissociation of the neutral electrolytic species and recombination of the generated ions. The charges generated by dissociation are redistributed in the region by the applied electric field resulting in heterocharge layers in the immediate vicinity of the electrodes. The Coulombic attraction between a given electrode and the charges within the corresponding heterocharge layer induces a fluid motion near that electrode. With a proper electrode design, a significant flow circulation/mixing can be achieved within an enclosure based on the conduction phenomenon.     

Effect of Discharge Electrode Parameters on the Flow Velocity Profile of the Wire?rod Type Electrohydrodynamic Gas Pump Exit

Experimental investigation has been conducted to study the effects of corona wire diameter, pipe length, and corona polarity on outlet flow velocity distribution profile of a wire-rod type electrohydrodynamic (EHD) gas pump. Upon applying negative or positive dc high voltage between a wire electrode (outer diameter (o.d.) 60 μm, 200 μm, or 300 μm) and a rod electrode (o.d. 3 mm) in atmospheric air, corona discharge occurs and EHD gas flow is generated in the direction from the wire electrode to the rod electrode through a cylindrical pipe (inner diameter (i.d.) 20 mm). For both polarities, the discharge current and average flow velocity increase monotonically on increasing the applied voltage before the onset of spark discharge. Using wire electrodes with a smaller diameter, stable corona discharge between corona onset and spark onset is generated in a wider voltage range, and the discharge current becomes larger, resulting in a higher flow velocity. The maximum average flow velocity of 2.0 m/s, corresponding to a flow rate of 38 l/min, was achieved with a wire of diameter 60 μm by applying a voltage of ?16 kV.     

Emulsification and Demulsification Processes in Liquid–Liquid System by Electrostatic Atomization Technique

A new electrical emulsification and demulsification apparatus using an electrostatic atomization technique was developed, and the investigation of both processes in a liquid–liquid system was conducted. First, fine water droplets were generated in silicone oil using a nozzle electrode with ac high voltage, and then, a stable water-in-oil (W/O) emulsion was produced without surfactants. The mode of electrostatic atomization and the trajectory of the fine droplets' flow were observed. The diameter of fine droplets and their distributions were measured as a function of the applied voltage and its frequency. Experiments indicated that electrohydrodynamic (EHD) induced liquid flow with water droplets played an important role in the formation of the emulsion. Next, the demulsification of W/O emulsion was carried out by two injections of oppositely charged water droplets, using the method of electrostatic atomization. The test liquid, which was prepared by the electrostatic atomization technique, was maintained in a state of emulsion for a longer time if no electric field was applied. When the positive- and negative-charged droplets were injected into the test liquid, the emulsion, with a dull color, gradually became transparent as time elapsed. The aggregated droplets fell down due to gravity and the EHD flow effect, resulting in the separation of water and oil. It was found that two injections of oppositely charged water droplets were effective for resolving the emulsion.      

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。     

Properties of EHD Pump with Combination of Rod-to-Rod and Meshy Parallel Plates Electrode Assemblies

A high pressure required to drive the liquid can be generated by the compact electrohydrodynamic (EHD) pumps with multiple pairs of the rod-to-rod and meshy parallel plates electrode assemblies arranged in series. In this work, the performance of EHD pumps with electrodes up to nine pairs has been examined as a function of the applied voltage using a semi-insulating liquid. The pressure generated by energized electrodes gives rise to the vigorous liquid flow and the liquid ejects from the pump outlet. The flow direction always is from the cathode toward the anode. The pressure increases with nearly the square of the applied voltage and in proportion to the number of electrode pairs. The maximum pressure head achieved with a series of nine electrode pairs is 25.3 kPa at 18 kV with maximum power consumption of 7.2 W. The generated pressure is maintained stably at constant level of the corresponding value. EHD pumping is attributed to conduction pumping based on an unbalance of heterocharges with single ionic polarity in the vicinity of the anode and cathode.     

Impulse creeping discharge characteristics in insulating oils with EHD function reinforced by added HFC gas components

The basic properties of electrohydrodynamic (EHD) pumping and dielectric strength were examined for two insulating oils, rapeseed ester oil and silicone oil, which were denatured by a bonding of hydrofluorocarbon (HFC)134a gas components decomposed using a barrier discharge reactor. In this work, the mechanism of a pure conduction pumping contributed to the evaluation of the electrical pumping in sample oils. Dielectric strength of oils was evaluated from the impulse creeping discharge characteristics and the 60 Hz breakdown voltage test. Experimental results revealed that the creation of radicals such as C-F and C-H in denatured oils led to a slight deterioration of the insulation ability compared to standard oils, but the pumping ability was reinforced significantly. Such additional EHD function may be utilized as a new cooling method in oil-filled power apparatuses in view of the dielectric strength.     

Effect of Extension Length in a Surface Barrier Discharge on the Driving Force of Electrohydrodynamic Gas Flow

A surface barrier discharge generated between an exposed and a buried electrode induces a unidirectional gas flow, known as an electrohydrodynamic (EHD) gas flow. The discharge behavior and flow characteristics have been investigated. When the exposed electrode is an anode, several streamers propagate above the buried electrode, whereas when the exposed electrode is a cathode, weak light emission is uniformly observed above the buried electrode. The maximum discharge length increases as both the length of the buried electrode and the amplitude of the applied voltage increase. Independent of the buried electrode length, the velocity of the EHD flow increases monotonically as the discharge extends farther, and is uniquely determined by the maximum discharge length.     

Streamer initiation in mineral oil. Part I: electrode surfaceeffect under impulse voltage

Presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 μm points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S^-0.17 (S: electrode surface area, cm²). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage     

Theoretical/numerical study of electrohydrodynamic pumping through conduction phenomenon

In an isothermal liquid, only the Coulomb force which is the force acting on the free charges, can contribute to the net electrohydrodynamic (EHD) motion. In the absence of a direct charge injection or induction, the charges can be generated through the dissociation process of the fluid. The generated charges by dissociation are redistributed by the applied electric field, resulting in the heterocharge layers around the electrodes. The pumping of an isothermal liquid without ion injection is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes. This type of pumping is referred to as the conduction pumping. This paper investigates the pressure head generated by the conduction pumping mechanism theoretically through the numerical solutions. For this purpose, a theoretical model for the static case (i.e., without a fluid motion) is established and a numerical code using finite volume method is developed. Electric potential, electric field, charge density, and electric body force distributions for the selected electrode configuration are presented. The generated pressure as a function of the applied voltage is also presented. The numerical results confirm the EHD conduction pumping concept theoretically.     

A study of thermal bubble behavior in the simulated electrodesystem of HT superconducting coils

Bubble behavior is studied theoretically and experimentally with an electrode system which consists of concentric coil layer-to-cylindrical electrodes with insulation barrier and spacers immersed in liquid nitrogen for the simulation of the insulation environment in high temperature superconducting coils at the quenching state. The results show that bubble behavior is affected severely by 60 Hz electric field and pressure; it is categorized into two types according to the width l_c of the cooling channel between coil layer and cylindrical electrode. When l_c is larger than the diameter of bubble released from the coil electrode, bubbles rise by buoyancy at a low applied voltage, but they are trapped in grooves between coil-turns at higher applied voltages. Trapped bubbles move along the groove if there is no obstruction, but otherwise move out of the groove. When l_c is smaller than the bubble diameter, the bubbles are trapped in the groove due to surface tension, and now along the groove even at no applied voltage. At higher applied voltages, the bubbles are trapped tightly and an electrohydrodynamic (EHD) instability is excited on the bubble surface if the spacer exists. Bubbles stream from the groove due to the effects of EHD instability and buoyancy     

GIS中不均匀直流电场下球状自由导电微粒运动分析

针对气体绝缘系统(GIS)中自由导电微粒无害化的问题,研究导电微粒在GIS中不均匀电场作用下的运动规律,建立球状自由导电微粒在楔形不均匀电极系统中的受力模型,采用Runge-Kutta方法对微粒运动方程进行求解,并对球状自由导电微粒在不均匀直流电场中的运动轨迹进行仿真,同时考虑电压波动和电极表面粗糙等随机因素对微粒运动轨迹的影响。另外,研究了自由导电微粒的运动轨迹与施加电压、微粒初始位置、微粒和电极材料的性质及电极表面状况的关系。研究结果表明:在一定的电场条件下,微粒运动会处于一种谐振状态,而施加电压的幅值、波形以及电极表面的反射系数对微粒运动轨迹有显著影响     

高介碳酸钡陶瓷沿面闪络伪火花开关的研究

为了满足脉冲功率技术和脉冲电流试验技术对放电开关的要求,笔者设计了BaTiO3高介电常数的电介质沿面闪络伪火花开关的触发装置。通过对表面放电触发器的实验发现:在伪火花开关的工作气压范围内,高介沿面闪络触发器显示出很强的电荷发射能力和快速的电荷注入能力,在气压为7 Pa时,触发器能够在30 ns内释放1.5μC,触发电流的上升陡度为1.2×1011A/s,发射的电子数达到9.4×1012。对自放电电压为28 kV的伪火花开关,可靠工作的最低电压可降低到130 V,放电时延为35～100 ns,最小抖动为6～25 ns。     

管-管和管-板电极介质阻挡放电特性研究

电极结构对介质阻挡放电(DBD)的放电特性有重要影响,研究和比较不同电极结构DBD的放电特性,对优化DBD反应器结构和提高放电效率具有重要意义。笔者实验研究和比较了大气压空气中管-管电极和管-板电极DBD的放电特性,比较了它们电压电流波形图、李萨育图形以及发光图像的区别,研究了不同电压幅值下放电参量的变化,并从放电机理上对实验结果给出合理解释。结果表明:管-板电极DBD的电气特性和发光特性与管-板电极DBD有明显的区别,相对于管-管电极DBD,管-板电极DBD的放电更稳定,放电细丝分布更均匀;随着外加电压幅值的增加,两种电极结构DBD的放电持续时间、电流幅值、放电功率和传输电荷量都增加,在相同外加电压幅值下,管-板电极DBD的各参量均大于管-管电极DBD。     

Influence of co-field motion on the conduction current in oil undernonuniform dc fields

The effect of enforced co-field motion on conduction current in transformer oil was studied using a point-to-plane electrode geometry. Tests were carried out on degassed oil and gas-saturated oil for negative and positive point polarities using both a solid and a mesh-plane electrode, and gaps ranging from 200 to 900 μm. For stationary oil the presence of oxygen decreased the conduction current for both polarities of the point electrode. For degassed and O₂ saturated oil the axial flow produced a significant reduction in the conduction current; the faster the flow the larger the reduction in current. With oxygen in solution, current fluctuations were much larger than for degassed oil. An unexpected result was the consistent finding that for gas-saturated oil an increase in pressure produced an increase in the conduction current, but only for a solid plane electrode. There is an excellent correlation between the effect of flow on conduction current and on the breakdown voltage     

纳秒脉冲表面放电等离子体影响因素的实验研究

在大气环境条件下,以环氧为介质阻挡材料,基于单极性ns脉冲电源进行了表面介质阻挡放电实验,研究了电压幅值、电极宽度、电极间距和重复频率对放电等离子体的影响。结果表明ns脉冲表面介质阻挡放电是丝状放电,放电发生在电压脉冲的上升沿阶段;放电电流主要包括两部分脉冲,与放电丝分布的均匀性有着一定的内在关系,外加电压对放电的均匀性以及产生等离子体的长度起作用;电极宽度和间距对放电电流和产生等离子体的发光强度影响不大,电极宽度和间距越小,放电丝分布越均匀,电极宽度存在一个最优值,使得激励器的放电稳定且产生等离子体相对均匀;脉冲重复频率仅对等离子体强度起作用,对放电特性的影响较复杂,不同电极参数下这些影响与放电丝的分布状态有关。     

变压器油基础油烃组成与其脉冲击穿电压和电阻率的关联性研究

针对脉冲功率装置的Marx脉冲发生器绝缘用油的特点,开展了变压器油基础组成与其电阻率和脉冲击穿电压大小的关联性研究。结果表明:在直流电场下,温度与电阻率的大小成反比关系,变压器油的芳烃含量是影响其电阻率的关键因素。在正负性脉冲电场下,不同芳烃含量的变压器油表现出极大的差异性,在高压负脉冲电场下,变压器油的芳烃含量是影响其脉冲击穿电压的主要因素,芳烃含量的高低与负脉冲击穿电压的大小成反比关系。     

An EHD Gas Pump Utilizing a Wet Porous Point Electrode

A point-to-mesh type electrohydrodynamic (EHD) gas pump, utilizing a wet porous point electrode, has been proposed, and the effect of the wet porous point electrode on the gas flow velocity and flow generation yield of the EHD gas pump was investigated. The wet porous point is used as a corona discharge electrode, and the mesh is used as an ion collecting electrode. It was observed that the EHD gas pump with the wet porous point electrode can generate a higher gas flow velocity and flow generation yield than the same design EHD gas pump with a non-porous metal point electrode. As a result with the wet porous electrode, a peak gas flow velocity of V_W=2.54, 2.74, and 1.93 m/s has been obtained for positive, negative, and ac corona discharges. This is 1.95, 1.31, and 1.30 times higher than the velocity V_W =1.30, 2.09, and 1.49 m/s measured with the same design EHD gas pump and a non-porous) metal point electrode. At 0.1 W of input corona power for dc and ac powers, the flow generation yields of Y=13.88, 12.80, and 11.70 m/s/W were obtained with the wet porous point electrode for the positive, negative, and ac corona discharges, which are 1.23, 1.24, and 1.15 times higher as compared with those of the metal point electrode with Y=11.32, 10.29, and 10.20 m/s/W, respectively. These enhancements may be due to the elevated input corona powers and the water particles generated from the wet porous point electrode.     

大气压氦气多脉冲辉光放电径向光学演化过程

介质阻挡放电(dielectric barrier discharge,DBD)的径向演化及其影响因素对研究低温等离子体的机制具有重要的参考价值。采用铟锡氧化物(indium tin oxide,ITO)透明电极,进行了大气压氦气介质阻挡三脉冲辉光放电试验,利用增强型电子耦合器件(intensified charge-coupled device,ICCD)拍摄放电的侧面短时曝光照片来诊断放电模式,拍摄放电的底面短时曝光照片并绘制其发光灰度值在电极表面的三维分布图来研究放电的径向演化过程。结果表明:放电首先在圆形电极的边缘区域发生,逐渐向电极中心发展,最后又转向电极边缘并逐渐减弱;随着电流脉冲次序的增加,放电在电极中心发展越来越充分;电极结构的限流作用同外施电压和介质表面电荷一样,对径向位置上不同时刻的放电演化过程起着重要作用。