Production of oil/water type uniformly sized droplets using aconvergent AC electric field

The authors proposed a new method to produce uniformly sized insulating liquid droplets (such as kerosene or plastic monomer) in immiscible liquid media (distilled water) by means of an applied convergent electric field generated using AC or pulsed voltage. The disintegration mechanism of the liquid column was observed precisely using video images and still photographs. Kerosene droplets with an essentially uniform diameter ranging from 100 to 250 μm were produced synchronously with the applied AC frequency using a nozzle diameter of 100 μm. When the flow rate of the co-flowing liquid surrounding the oil-phase liquid jet was increased, the synchronous frequency became higher and the size of the resulting droplets was decreased due to the elongation of the liquid jet. The disintegration mechanism is most likely the forced oscillation of the liquid jet stimulated by each cycle change of the applied voltage. The droplet size can be widely controlled by varying the AC frequency, nozzle diameter, liquid flow rate and velocity ratio between the oil-phase and co-flowing water     

热老化对纤维素绝缘纸的空间电荷及陷阱特性的影响

为理解变压器用纤维素绝缘纸在其热老化过程中的空间电荷特性演变规律及其影响机制,采用电声脉冲法研究不同老化程度绝缘纸在不同电压等级(30、15和5kV/mm)下的加压和去压后的电荷密度分布规律,分析了绝缘纸热老化程度与其空间电荷特性演变规律间的联系。结果表明:纤维素绝缘纸体内电荷注入量及注入速率与外施电场等级成正比关系。热老化能够降低纤维素绝缘纸空间电荷注入的起始阈值,并减缓绝缘纸体内电荷注入和迁移的速率,从而削弱了体内电荷的复合现象。随着热老化程度的增加,纤维素绝缘纸体内陷阱的分布密度和陷阱能级的深度都将增大,致使材料体内残留电荷不仅总量逐渐增大,而且衰减速率逐渐降低。研究成果为理解热老化对纤维素绝缘纸空间电荷积聚和消散的影响机制提供了理论支撑。     

钛酸钡对聚乙烯试样中空间电荷的影响

在高场强下用电声脉冲法(PEA)测量经BaTiO3表面处理后的聚乙烯试样中空间电荷的分布，比较纯聚乙烯试样与经BaTiO3表面处理后的聚乙烯试样中的空间电荷分布，试验结果表明，用BaTiO3表面处理的聚乙烯可以有效的抑制电荷的注入和空间电荷的形成。     

等离子体射流快速改性促进表面电荷衰减

随着高压直流输电迅猛发展,绝缘材料在直流电压下表面电荷积聚现象严重威胁直流输电系统的安全可靠运行。为加快绝缘材料表面电荷的消散,采用大气压等离子体射流,以TEOS为前驱物,在环氧树脂表面沉积SiO_x薄膜。对改性前后材料表面化学组成、表面电导率、表面电荷特性、表面陷阱分布以及耐压特性进行多参数测量,研究等离子体射流改性前后环氧树脂表面特性。实验结果表明:等离子体处理在环氧树脂表面引入大量以Si-O-Si及Si-OH基团为主的无机基团,且表面电导率提高2个数量级。随着改性时间的延长,表面电荷的初始积聚量减少,消散速度加快,陷阱能级深度变浅;沿面闪络电压呈现先增后降的趋势,在改性180s时闪络电压提高到最高值9.0k V。研究结果表明:通过大气压等离子体射流在聚合物表面沉积薄膜能够提高环氧树脂绝缘性能,为其工程应用提供了有效的改性方法。     

Surface charging and flashover on insulators in vacuum

Investigations have been carried out on right cylindrical Teflon spacers in vacuum under dc stress to study the effect of cathode geometry on surface charging and flashover. Three cathode support arrangements were used. Type 1 involved a recess in the cathode into which the spacer was located, type 2 was a simple flat plate cathode, while type 3 had a raised insert on the cathode surface over which the spacer was located. In all cases the surface charge density and distribution is primarily dependent upon the magnitude of the electric stress. Using the type 1 arrangement inception for charge deposition was always ~10 kV even when the spacer had been subjected to repeated flashovers. At applied voltages higher than this the density of the charge, which was fairly uniformly distributed around the surface, was more or less proportional to the applied voltage. For the type 2 and type 3 arrangements, a distinct, uniformly distributed negative charge of ⩽20 μC/m² was always detected at low values of applied stress and at a fairly well-defined transition voltage this gave way to a distribution which was substantially uniform in the case of type 2 but quite filamentary in type 3, and both involved the deposition of positive charge     

高压静电液体雾化技术

高压静电雾化具有雾滴粒径细小、粒径尺度单一、空间弥散程度广等优点,广泛应用于农牧林业病虫害防治、工业喷涂、燃烧、脱硫除尘及材料薄膜制备等领域。为有效提高燃油燃烧效率、烟气脱硫效率及药剂灭菌效率等,从破碎动力学、不稳定理论及雾化模式等出发对高压静电雾化理论进行了详细的阐述,测试了平口雾化喷嘴在针-环状组合电极下的喷雾特性。获得了雾化角、射程、沉积量分布及雾滴荷质比、索太尔(SMD)平均直径和PIV雾化流场图象等信息,并进行了分析。实验结果表明:高压静电减小了液体的表面张力和粘滞阻力,使液体容易破碎成更为细小的液滴,使雾滴尺寸分布更均匀。雾滴荷电后,带电雾滴在高压静电场的作用下容易发生二次雾化,进一步减小雾滴粒径;同时带电雾滴在电荷之间斥力作用下,弥散程度加大,且能在目标物感应出与本身电荷极性相反的电荷,从而在极化力、引力等作用下更容易被目标物所捕获。     

DC conduction associated with electric field-induced motion in mineral oils

The contributions of the electrohydrodynamic (EHD) motion to the dc electric conduction in insulating oils are presented. The effects of solid/liquid interfaces on the EHD motion have been studied with a mineral oil-polypropylene mixed dielectric and a mineral oil single dielectric. The time dependence of current, schlieren images of EHD motion induced in the oil layer and response signals of density change in the vicinity of the electrode/oil interfaces after application of step voltage are observed. The results are as follows: (1) the EHD motion occurs at the positive electrode/oil interfaces when a voltage above critical voltage is applied; (2) a time delay exists between the application of step voltage and the onset of EHD motion. The time is inversely proportional to the applied voltage; (3) the EHD motion increases current and produces a peak phenomena in the current waveform; and (4) the time difference between peak current time and the delay time is inversely proportional to applied voltage.     

Measurement protocol for planar lipid bilayer viscoelastic properties

This paper describes how to estimate planar lipid bilayer's elasticity module E and surface tension σ by means of measuring its breakdown voltage and using Dimitrov's viscoelastic model of electric field-induced breakdown of lipid bilayers. Planar lipid bilayers (BLMs) were made of two components: 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC) and 1-palmitoyl 2-oleoyl phosphatidylserine (POPS) in five different compositions. Folding method for forming planar lipid bilayers in the salt solution of 100 mM KCl was used. Breakdown voltages U_br and membrane life times t_br were measured by means of applying linear rising voltage signals of seven different slopes. Specific capacitances c_BLM of bilayers were measured with charge pulse method. Then Dimitrov's viscoelastic model was fitted to measured data allowing for estimation of elasticity module and surface tension of the lipid bilayer. Our results show that onecomponent bilayers composed from POPS were more stable and thus having higher breakdown voltages and elasticity moduli then bilayers composed of POPC. Surface tension values were similar regardless of the membrane composition. Values of the elasticity (E) and surface tension (σ) are comparable to those published in the literature. We conclude that the protocol used, though time consuming, is an alternative to other methods used for determination of bilayer's mechanical properties.     

Charge acquired by a spherical ball bouncing on an electrode: comparison between theory and experiment

A conducting ball immersed in a poorly conducting liquid between two horizontal plates acquires a charge when subjected to a dc voltage. When the applied voltage is high enough for the electric force to overcome gravity, the ball rises, moves up through the liquid and falls down as its charge leaks away. The threshold of voltage, as well as the time of flight between contacts, depends on the charge acquired by the ball during the contact with the electrode. Experiments have been done using liquids of different conductivities and it was shown that the conductivity does not affect the lift-off voltage. When the ball gets close to the electrode a discharge occurs and a current pulse is registered by the external circuit. The charge carried by the pulse is an order of magnitude smaller than the charge transferred to the ball from the electrode. We have made a detailed study, with different balls and liquids, of the charge transferred to the external circuit. The dependence on the different parameters of the ratio charge transferred to the circuit-charge acquired by the ball is explained using the coefficients of capacity of the electrostatic system.     

正极性雷电冲击电压下空间电荷电场测量研究

很多绝缘问题本质上都与电场相关,电场测量是高电压领域的重要研究手段。空间电荷导致电场畸变是流注理论的基础,因此定量测量空间电荷的泊松电场对于研究空气间隙的放电机制有重要意义。利用光电集成电场传感器测量了1m棒–板间隙未发生放电条件下各点的轴向电场,并与数值仿真结果进行对比,验证了电场传感器进行定量测量的可靠性;对3种电极条件下1m棒–板间隙空间电荷区域外部轴线上各点的电场时域波形进行测量,将空间电荷形成的泊松电场与外加电压形成的拉普拉斯电场分开,定量测量了不同电压幅值下、不同空间位置的泊松电场幅值与变化规律。研究结果为建立并验证空间电荷模型奠定了基础。     

基于Kerr电光效应的冲击电压下液体电介质空间电荷高速CCD测量

空间电荷的存在直接影响液体电介质的绝缘击穿性能,为了更好地研究空间电荷的动态变化过程,建立了基于Kerr电光效应的冲击电压下液体电介质空间电荷的高速电荷耦合器件(CCD)测量系统。运用高精度光电传感器的测量方法,得到了室温下液体碳酸丙烯酯的Kerr常数。进而通过高速CCD的动态光电测量,并结合图像处理技术,求取了冲击电压下平行板铜电极间电场和空间电荷的动态分布曲线。实验研究表明:在平行板铜电极间,当液体介质中电场达到一定程度时会出现空间电荷的注入与输运,使得极间电场呈现出中间高两边低的分布情况。另外还分析了空间电荷的分布和输运对液体绝缘特性的影响,该研究为分析冲击电压作用下液体介质中空间电荷的产生机制及空间电荷对流注放电的影响奠定了基础。     

Electrode conditioning characteristics in vacuum under impulse voltage application in nonuniform electric field

In this paper, we quantitatively investigated the impulse conditioning mechanism under nonuniform electric field electrodes in a vacuum. A negative standard lightning impulse voltage was applied between rod and plane electrodes whose materials were Cu-Cr, stainless steel and Cu and the gap lengths were d=5 and 10 mm, respectively. Experimental results revealed the transition of the breakdown (BD) sport region on the rod electrode and the corresponding BD field strength in the conditioning process. As a result, we found that the BD spot region started at the tip of the rod electrode and moved to the wider region of the rod electrode with lower electric field as the shots of the voltage application increased before the saturation of the BD voltage. Finally, by analyzing the results with an electric field strength, we propose that "the conditioning degree" along the electrode surface distributed directly proportional to the electric field distribution under a nonuniform electric field in a vacuum, irrespective of the electrode materials     

面电荷模型求解变电所边界外工频电场强度

为预测计算变电所边界外空间工频电场强度,采用面电荷法推导了电场强度计算模型,理论和实测值比较的结果表明,面电荷模型准确性和可靠性较高,完全能反映变电所外空间工频电场强度的分布。     

Numerical design of HV insulators based on charge accumulation invacuum

A MCS (Monte Carlo simulation) method of charge accumulation based on the SEEA (secondary emission electron avalanche) mechanism is applied to the design of effective corrugation on an insulator surface used in vacuum. The electric field distribution on the cathode in the vicinity of a triple junction is analyzed taking the SEEA charge accumulation into account. Considerable relaxation of field strength takes places due to negative charge accumulation with appropriate corrugation. The flashover voltage obtained experimentally increases as the field strength decreases. The results of the MCS agree well with experiment     

基于模拟电荷法的绝缘子电场非接触式测量法

根据模拟电荷法的基本原理和绝缘子片的实际形状,采用环线形离散模拟电荷等效分布在绝缘子表面的束缚电荷,使模拟电荷产生的空间场的边界条件满足绝缘子的束缚电荷产生的空间场.测量绝缘子串附近一定距离范围内的几点(对于110 kV,可只测量3点)的电场强度,根据测量点的实际电场强度和采用模拟电荷法建立的等效模型,结合一定的搜索方法,可计算出绝缘子串的实际分布电压值.将绝缘子串的实际电压分布值与国家标准给定的电压分布值相比较,可判断出有无劣质绝缘子片及其位置,从而完成绝缘子的预防性检测,避免电力事故发生.与目前国内的电场测量法相比,这种方法采用程序计算来代替部分测量工作,可以带电检测,测量点少,操作方便,实现了一定距离的非接触式测量,避免测量装置直接与高压绝缘子接触,降低了操作的危险性.     

Electrostatic Spraying of Liquid Insulators

A method for generating drops of liquid insulators using electrostatic spraying is reported. This method differs from normal electrostatic spraying in the method of charging the liquid. A sharp needle placed in the liquid near the end of a capillary is raised to a high potential, thus injecting charge into the liquid. As the voltage is raised, a jet is formed electrostatically with the size of the jet decreasing with increasing voltage. A theoretical prediction of the jet radius as a function of current is obtained by finding the amount of electrical energy which goes into mechanical energy. Experimental results of the spraying of silicone oils are given and compared with the theory.     

Partial discharges in a cavity at variable applied frequency part 2: measurements and modeling

In this paper partial discharges (PD) in a disc-shaped cavity are measured at variable frequency (0.01 ? 100 Hz) of the applied voltage. The measured PD phase and magnitude distributions, as well as the number of PDs per voltage cycle, changed with the varying frequency. A charge consistent model is presented and used to dynamically simulate the sequence of PDs in the cavity. The simulation results show that the properties of the cavity surface, mainly the surface conductivity and the surface emission of electrons, change with the varying applied frequency. This is interpreted as an effect of the difference in time between consecutive PDs at different applied frequencies. This is the second of two papers addressing the frequency dependence of PD in a cavity. The first paper described how the PD frequency dependence changes with the applied voltage amplitude, the cavity size and the cavity location.     

Modeling of electrostatic-based pesticide spray systems

A microscopic model for the electrostatic spraying system is presented to investigate the effect of the voltage applied to the induction nozzle on the droplet's charge, mobility, and charge-to-mass ratio. The variation of these parameters along the jet was also included. The model also lays particular emphasis on the effect of the applied voltage on the spray current and the charge density at the nozzle. A macroscopic model for the electrostatic spraying system is also presented. The objective of the model was to study the spatial distribution of the droplet charge density, transit time, and trajectory in the region between the nozzle and the target in terms of the flow velocity of the spray and the space-charge-produced electric field. On the macroscopic scale, both the droplet charge density and the spray current increase with the voltage applied to the charging electrode. With the decrease of the spray flow velocity, the space-charge-produced electric field becomes dominant and tends to: contract the droplet trajectories toward the axis of the spray system, and hence enhance the droplet deposition efficiency; and decrease the charge density at the target with the possibility of minimizing back-ionization. On the microscopic scale, induction charging eliminates the ion current from the current to the target and the associated back-ionization. The calculated droplet charge and charge-to-mass ratio which increase with the voltage applied to the charging electrode, agreed with the values reported previously     

Real-time observation of surface charging on a cylindricalinsulator in vacuum

By using an electrostatic probe located close to the triple junction on the cathode surface, we have conducted real-time observations of the change in electric field due to surface charging of an insulator in vacuum. A cylindrical sample made of polymethylmethacrylate (PMMA) or Al₂O₃ has been exposed to HVDC. An axisymmetric simulation based on the secondary electron emission electron avalanche (SEEA) theory has been performed. The measured results agreed with the simulation concerning the polarity of the accumulated charge as well as the field strength. The simulation also predicts the inception of charging at a voltage well below the measured flashover voltage. These results clearly support the SEEA theory as a macroscopic mechanism of surface charging of an insulator in vacuum     

螺旋针-环结构等离子体射流放电过程分析

为降低气体的击穿和维持电压,设计一种螺旋针-环电极结构的等离子体射流装置,研究不同电压下的放电电压、电流波形。研究表明,氦气等离子体射流放电可以分为电晕放电、介质阻挡放电和射流放电三个阶段,并且可以通过放电电压、电流波形的特征进行区分。分析可知,射流管管径较大时,采用螺旋针状内电极结构可以减小电极间的平均气体间隙距离,从而降低气体的击穿和维持电压,使放电更加容易进行;电压反向过程中残留电荷使得合成电场得到加强,气体将"提前"发生放电;由于电极结构的不对称,气体在正半周期更容易发生放电,放电产生的电流脉冲数目更多,电流值更大;随着外加电压增大至14k V,放电最终过渡到丝状放电状态。