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.     

Streamer propagation along insulating surfaces

Results concerning the propagation of streamers along insulating surfaces under uniform electric field are presented. The basic properties of streamers, namely the electric field required for a stable propagation and the propagation velocity, have been measured and compared with propagation in air alone as a reference. The results have shown that in the experimental arrangement used significant space charges due to streamer branching are absent, therefore the properties observed are considered as characteristic of a single streamer. Streamers propagate stably with an intrinsic propagation field and a characteristic velocity that depend on the nature of the insulating material. For electric fields higher than the minimum field required for a stable propagation, a streamer system propagates with a `surface' and an `air' component     

Simulation of streamer discharges as finitely conducting channels

This paper presents a two-dimensional simulation of positive streamers in air at atmospheric pressure in a quasi-uniform electric field. A streamer is assumed to consist of a hemispherical tip and a finitely conducting cylindrical channel with a constant axial potential gradient. The model predicts that E_g~450 kV/m and the radius is r~50 μm in air at standard atmospheric conditions. Moreover, the estimated number of positive ions in the streamer head of stably propagating streamers, agrees with estimations based on more advanced streamer models. The model is used to predict the behavior of streamers in electrical discharges in a semiuniform electric field and good agreement is found between experiment and theory. The computer simulation of such a simplistic model could be applied to predict the behavior of streamer discharges in complex electrode arrangements, including dielectric surfaces     

Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.     

天然酯绝缘油-纸界面的多分支流注放电仿真

油纸界面的局部放电是油浸式变压器中油纸绝缘失效的主要原因之一。本文在多分支流注放电数值模型基础上通过进一步考虑油纸界面的电荷输运特性,构建了油纸绝缘系统中的多分支流注放电模型,并利用有限元法分别对天然酯绝缘油/纸介电常数配比、针电极与纸板的间距影响下的沿面放电特性进行了研究。结果表明,针电极与纸板的间距及油/纸介电常数配比均会显著影响流注分支;针电极与纸板的间距越小,油纸界面处的流注分支间的抑制越明显,油纸界面的流注对z-轴流注的影响更显著;当绝缘油介电常数相比纸板大时,流注在油中的发展和分支更为显著,反之则流注更易沿纸板表面发展及在油纸界面聚集电荷。     

Electrostatic painting of insulating surfaces

The purpose of this work was to develop methods for painting insulating surfaces by the conventional electrostatic painting technique used to coat conducting samples. This technique is not applicable to insulators since the charges carried by the paint accumulate on insulating targets, thereby preventing a proper coating. A detailed investigation of the decay of charges deposited on insulators by a corona discharge simulating the charged paint has confirmed that a grounded counterelectrode (CE) on the back face of the sample speeds up the decay and that the better the contact, the faster the decay. Because a material CE with proper contacts is inconvenient for painting in production configuration, it was suggested that ionized air might behave as a satisfactory CE. Several ionizers have been tested, and good coatings have been obtained, provided that the flows of paint and ionized air are well separated. The possibility of self-creation of ionized air by the voltage of the early paint deposit has also been successfully tested     

Simulation of streamer discharges as finitely conducting channels[comments and reply]

The recent paper by Gao et al. [see ibid., vol.6, p35-42, 1999] is most interesting and develops a useful model. Three comments will be made: on the model itself and the `within-streamer' field used; on the interpretation; and on the streamer diameter     

Positive streamer propagation in large oil gaps: electricalproperties of streamers

This paper presents experimental data and calculations concerning the electrical properties of positive streamers in mineral oil at large gaps and HV. The experiments concern the measurement of charge, electric field, and the determination of the potential drop along streamers, either in the liquid alone, or for streamers guided within insulating tubes. Calculations of charge and field distribution around streamers are carried out by charge simulation. To do this, streamers are represented by objects with simple shapes (spheres or cylinders) equivalent to their macroscopic aspect. These models lead to a correct agreement with measured streamer charge and field on the plane electrode. Qualitative correlations are established between calculated field distributions and streamer behavior such as velocity, transitions between propagation modes. It is also concluded that the potential drop in streamers and branching both act as regulating mechanisms that help to keep the streamer tip field, and hence the velocity, constant over a wide voltage range     

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     

Unipolar charging of cylindrical insulating particles nearelectrode surfaces

Numerous papers have discussed the ionic charging of insulating spheres in uniform electric fields. However, in certain electrostatic technologies, such as separation and flocking, the particles are often cylindrical in shape, and they get charged on the surface of an electrode or in its proximity, so that existing formulas cannot be used. This paper addresses this problem from both a computational and an experimental point of view. The charge acquired by cylindrical particles of various dielectric constants was evaluated with an original computer program, based on the boundary-element method of field analysis. The computed results show that the position of the particle with respect to the electrodes changes the value of the saturation charge. The experimental setup simulated the charging conditions in a roll-type electrostatic separator. The unipolar space charge was generated by a needle-type electrode. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyethylene and polyvinyl chloride on a rotating roll electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward a particle self-discharge effect, at field intensities beyond a well-defined threshold. This kind of information may guide the design of the electrostatic technologies based on the corona charging of granular matter     

Decrease in ac partial discharges on insulating materials by use ofdye additives

The purpose of this study is to decrease PD at a triple junction by addition of a dye to an insulating material. PD is produced on a test specimen subjected to ac voltage under a sphere-plane electrode configuration. The specimen is composed of two layers: a PMMA film with additive, which is 100 μm thick; the lower layer a PMMA board 3 mm thick. The film with the additive was prepared by the solution-grown casting method. Five kinds of dye were used as the additive; p-aminoazobenzene (Ab), p-nitrobenzeneazoresorcinol (Nb), methyl yellow (My), methylene blue (Mb) and methyl red (Mr). In the case of the films with Ab and Mb at 0.05% wt, the magnitude and number of PD pulses were reduced to ~50% less than those in the case of the films without the additive. The decrease in PD was effective under an electric field strength slightly higher than the PD inception. In the case of the other dyes, decrease in PD was not observed. The surface voltage distribution due to the charge by PD depended on the kind of dye additive. The decrease in PD for the Ab and Mb films was explained from the dependence of the surface voltage distribution on the dye. It was concluded that ionization of the dye molecules in the film changes the distribution of surface voltage     

Sterilization of E. coli by underwater pulsed streamer discharges in a continuous flow system

Underwater pulsed streamer discharges using a wire‐to‐cylinder electrode contained in a 42‐mm‐diameter glass tube were investigated with respect to its spatial spread associated with voltage waveforms. A discharge reactor with a continuous flow system was employed to sterilize E. coli suspended in saline water (10 mS/m). The discharge electrode consisted of a 0.1‐mm‐diameter metal wire for the high voltage and a mesh cylinder for the ground electrode. Tens of disk‐shaped discharges emerged from the wire electrode and propagated toward the cylinder as long as the voltage lasted. The spatial expanse of the discharge was proportional to the cube of the pulse duration and to the amplitude of the voltage. The sterilization experiment showed that the survival ratio of bacteria over discharges was proportional to the average number of the exposures to the discharges while in the reactor. The minimum ratio of surviving bacteria was only 15% under 130 exposures, which corresponds to an energy expenditure of 35 J/ml. © 2008 Wiley Periodicals, Inc. Electr Eng Jpn, 164(1): 1–7, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20554     

Positive streamer propagation in large oil gaps: experimentalcharacterization of propagation modes

This paper presents an experimental study of positive streamer propagation in mineral oil, in large point-plane gaps under impulse voltage. A systematic investigation was done concerning the influence of gap distance (⩽35 cm), and voltage from streamer inception up to large overvoltages. The measurements presented concern breakdown voltage measurements, time to breakdown, visualization of streamers (streak and still photographs), electrical measurements (transient current, charge), light emission intensity. Streamers are characterized and then classified into different modes (2nd, 3rd and 4th modes) according to their propagation velocities up to more than 100 km/s. According to the applied voltage, transitions between modes are observed, and correlations are established between charge, shape, and streamer velocity. The physical mechanisms as well as practical consequences are then discussed     

Studies on electrostatic surface discharges on corona-chargedpolymer surfaces

Electrostatic surface discharge phenomena on corona-charged thin polymer films such as FEP or PTFE Teflon, with surface potentials from 1 to 5 kV in both polarities, were studied. A grounded sphere electrode, 5 mm in diameter, approached the charged surface by a computer-controlled step motor until the surface discharge occurred. Discharge-current waveforms, with a very short risetime (a few nanoseconds), were observed to be related to the length (5-30 cm) or type (single wire or plain mesh) of the earth line. The discharge gap length between the film surface and the grounded electrode was also calculated. The results were strongly dependent on the polarity of the surface charge and were widely divergent, indicating the complicated surface discharge mechanism     

Electric field and energy computation on wet insulating surfaces

The paper presents the results of electric field and energy computation on insulating surfaces covered by water in the form ranging from small discrete droplets to large wet patches. The goal of this study was to understand the differences in the flash-over performance of outdoor insulators, in particular nonceramic insulators, under contaminated conditions as a function of surface wettability. The computational algorithm is based on the charge simulation method. A decrease in the maximum electric field at the water droplet tip is obtained as the droplet size increases, however, the electrostatic potential energy increases with the size of the water droplet, which might increase the surface discharge intensity. The role of the coalescence of small water droplets into larger droplets and filaments on the electric field and energy has been investigated. The location of such larger water bodies with respect to the electrodes has been examined. These computations shed some light not only on the role of hydrophobic surfaces but also on the location of hydrophobic surfaces along the insulator on the flashover performance of outdoor insulators. It has been shown that these computations provide quantitative data that compliments information obtained from simpler techniques such as visual observation and surface resistance measurements, for the purpose of assessing the performance of insulators in service     

局部放电对粗放的绝缘材料生产和应用方式的挑战

综述了近10年来发电机主绝缘、变频电机匝间绝缘、变压器主绝缘、交联聚乙烯电缆绝缘的老化机理研究进展,分析了局部放电的发生及其对绝缘破坏的主要原因,认为随着电力电气设备向着大容量、高电压发展,局部放电导致的绝缘破坏已对我国粗放的绝缘材料生产和应用方式形成挑战。指出今后我国绝缘材料的生产和应用应从提高原材料的纯度、更新工艺和装备、实行严格的质量管理、创造洁净的生产环境等方面入手,才能把绝缘中的气隙、杂质等绝缘薄弱点控制在较低水平,才能延长绝缘寿命和提高绝缘可靠性。     

A study of partial discharges from water droplets on a siliconerubber insulating surface

This paper presents measurements of partial discharge (PD) from water droplets on a silicone rubber insulating surface in an ac field. Tests are done by placing droplets on the surface using a micro-pipette, and by condensing water in a controlled humidity chamber. The field enhancement factor due to droplets placed on the insulation surface is experimentally and numerically determined. Values in the range of 3.8 to 6.3 are obtained depending on the size and number of droplets. The time resolved analysis of PD from water condensation indicates a threshold field, between 3.0 and 3.5 kV/cm, above which the PD activity increases with time, and brings the insulation to a surface breakdown. The corresponding field enhancement factor obtained for the condensation experiments is between 5.8 and 6.8. The results therefore suggest that the PD activity on the hydrophobic surface can be used effectively to learn about the insulation surface conditions     

Positive discharge development in insulating oil: opticalobservation and simulation

Results of optical investigations of positive discharge channel development in transformer oil under divergent field configuration and impulse voltages to 650 kV are presented. The propagation of two different discharge structures (bush-like, tree-like) and the transition between these structures were observed. The conditions of their development and boundary of the bush-to-tree transition were determined. Anomalous behavior of the breakdown time dependence on applied voltage, consisting of a sharp drop in time at a critical voltage, was found. The dielectric breakdown model modified by introducing the varying channel conduction is presented. In the model discharge channel propagation is described by stochastic development of current and field instabilities. The model describes adequately the main spatial and temporal features of the various discharge structures observed in the experiments. The current and field characteristics of the simulated discharge channels which belong to different structures are compared. The bush-to-tree transition and the anomalous dependence of the breakdown time on voltage were explained by current instability in bush-structure channels