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     

The influence of surface charge on lightning impulse breakdown ofspacers in SF6

We have investigated the influence of surface charges on the discharge development across particle contaminated spacer surfaces under SF₆ for lightning impulse (LI) voltages (1.2/50 μs). Surface charges may be generated by dc, ac or lightning impulse stress. The discharge characteristic shows a strong reduction of the insulation strength if the applied voltage and the surface charge have opposite polarities. The investigations were performed with a needle protrusion attached to the bottom electrode to give severe field distortion. The bottom electrode was biased positively. The influence on the discharge process is observed by measuring the surface charge distribution and predischarge currents. The results reveal changes in streamer onset voltage, streamer to leader transition, and leader development     

Streamer initiation in mineral oil. Part II: influence of ametallic protrusion on a flat electrode

For pt. I see ibid., vol. 9, no. 1, p. 84-91 (2002). Presents a study of streamer inception in transformer oil, in a "semiuniform" geometry under impulse voltage. This geometry consists of parallel plane electrodes, with a thin triggering point of calibrated size emerging at the center of one plane. By reducing the length of the point, it is possible to move progressively from a point-plane geometry, up to a quasi-uniform geometry. Compared to a point-plane gap, the initiation of streamers is impeded due to the presence of a metallic plane behind the triggering point, that reduces the tip field. Field calculations allow to extrapolate the results obtained to the case of microscopic surface protrusions and particles. It is possible to predict the streamer initiation voltage, and hence the breakdown voltage under uniform field, from the shape of the protrusion and from streamer initiation fields measured in point-plane geometry     

正极性雷电冲击电压下流注起始特性研究

流注是长空气间隙放电的主要过程,其起始特性具有重要的理论价值与广泛的工程应用。以往对直流电压作用下流注起始特性的研究较多,对冲击电压特别是电压上升率较大的雷电冲击电压下流注起始特性研究很少。采用基于光电集成技术的高压侧电流测量系统与空间电场测量系统,对1 m棒–板间隙在正极性雷电冲击电压下流注起始特性进行研究,提出了新的流注起始观测手段,获得不同半径棒电极、不同电压上升率下的流注起始电压与场强,拟合得到考虑电压变化率的流注起始场强判据,并验证了所提出判据的广泛适用性。另外,还对正极性雷电冲击电压作用下流注起始时延的变化规律进行研究,定量测量了流注的起始时延,为研究流注起始时延的概率分布奠定基础。     

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     

Effects of additives on prebreakdown phenomena in n-hexane

Prebreakdown phenomena in n-hexane are observed in detail for positive and negative polarities by using simultaneously a high speed schlieren technique and an LED current measuring system, when an impulse voltage (1.1/225 μs) is applied to a point-to-plane electrode gap. Furthermore, the effects of several additives on the streamer propagation are investigated. Especially the effects of electron-trapping additives on negative streamer propagation and of low ionization potential additives on the positive streamer propagation, are examined, as is a correlation between the shape and the propagation velocity of the streamers     

Propagation characteristics of a positive surface streamer

The potential distribution of a developing positive surface streamer was measured by the Pockels effect. The transient potential distribution along a linear path was measured at minimum resolutions of 20 µm and 2 ns. Additionally, velocity of a streamer, average electric field along a streamer, and the developing length were measured while changing the insulator thickness from 0.8 to 5.0 mm. The potential along a positive surface streamer decreases linearly with the distance from the high‐potential electrode, and its gradient is about 500–650 V/mm, which is independent of the inception voltage and the insulator thickness. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Streamer propagation under impulse voltage in long point-plane oilgaps

A 67 mm point-plane gap immersed in transformer oil, at atmospheric pressure, stressed with a 1/180 μs impulse, has been studied by recording gap current and light emission. Time-resolved images were obtained by use of an image converter camera. Minimum breakdown voltages were twice as high for negative as for positive points. All streamers at these and higher voltages were supersonic, with negative streamer velocities ⩽50 km/s. The speed of the positive streamer gradually increased to 19 km/s with increasing voltage, where it saturated. At high stresses the propagating positive streamer has been observed to switch to a faster mode which propagates with speeds in the 65 to 200 km/s range. Positive streamers were bush shaped to 2× the minimum breakdown voltage, the branching diminishing with increased voltage. Negative streamers were coarsely bush shaped or usually tree shaped. For both polarities, light emission consisted of a background light, with superposed pulses corresponding to 15 ns wide current pulses of up to 10 A. The pulses are caused by very bright, brief re-illuminations of single streamer channels. The tips of positive streamers are considerably more luminous than the channels, indicating that electron multiplication may take place at the tips. A model of the streamers as being plasma filled channels may explain the re-illuminations     

Computer analysis and observation of streamer growth at adielectric interface

A simulation study of streamer growth within a dielectric liquid at a solid interface was undertaken. Using the finite element method with computer graphics, the shape of the growing streamer was determined and plotted. In addition, the changing voltage and electric field values within the dielectric were calculated, and simulated images of these parameters produced. The behavior of the streamer is based on the formation of a localized gas phase discharge and the growth of the resulting plasma within the liquid. The model determines an inception site and predicts a growth path that depends on the electric field immediately surrounding the streamer tip. Two liquids, n-hexane and water, both with an interface, were investigated using the simulation model. The effect of the choice of the liquid and the solid dielectric, in addition to the geometry, was explored. For mixed solid and liquid insulation, the results show that streamer growth depends on the permittivity mismatch between the two materials. Experimental findings are presented that show streamer growth for different interfacial conditions. A good correlation is observed between the theoretical and experimental work     

Effects of chain length on prebreakdown phenomena in straight-chain saturated hydrocarbon liquids

Prebreakdown light emission, currents, and streamers in straight-chain saturated hydrocarbon liquids, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane, are investigated under nonuniform field conditions. The light inception voltage and the breakdown voltage increase with the chain length and the liquid's density. For a negative point, the breakdown voltage is much higher than the light inception voltage in all liquids but the difference decreases with the chain length. This voltage difference is not observed for a positive point except in n-pentane and n-hexane. The difference between the negative and positive breakdown voltages decreases with the chain length. Regarding the polarity effect, the negative breakdown voltage is much higher than the positive one in n-pentane, but there is no difference in n-decane. A negative density change streamer has a bushlike structure and propagates in stepped manner at an average speed between 70 and 160 m/s, while for a positive one, the structure is treelike having an average speed of a few kilometers per second. Only in n-pentane and in n-hexane, a slow positive bushlike streamer is observed whose average velocity is about 200 m/s, slightly faster than the negative bushlike one. © 1998 Scripta Technica. Electr Eng Jpn, 122(4): 8–16, 1998     

Study of the Development of Positive Streamers Along an Ice Surface

This paper presents the results of fundamental investigations on the inception and propagation of corona discharge on an ice surface stressed with a standard lightning impulse voltage. High-speed photography and photomultiplier techniques were used to observe and record the propagation of the streamers. The effects of several experimental parameters such as freezing water conductivity and HV rod electrode radius on the streamer inception parameters were investigated. Moreover, time to first streamer, inception voltage and corresponding field, as well as streamer propagation velocity and charge deposited by a streamer on ice surface were measured. The results are discussed and emphases are laid on the main factors influencing the development of positive streamers on ice surface.     

Initiation from a point anode in a dielectric liquid

Many previous studies of electrical breakdowns in dielectric liquids in point-plane geometry have examined the relationships among the breakdown structure (or speed), the electrode geometry (point radius, gap length), and/or voltage. This paper explores the hypothesis that, for streamers initiating from a point anode, the critical volume model used for similar geometry in gaseous dielectrics is useful in liquids. The assumption of the critical volume is shown to be consistent with experimental data. Specifically, a critical volume of 0.4-1.0 mum ³ is consistent with the location of streamer initiation, with the independence of the initiation voltage for the 2nd anode mode from the tip radius for sharp tips, and the measured free paths of electrons in cyclohexane for the energies of interest     

Streamer current in a three-electrode system

A study has been conducted on positive streamer discharges in air at atmospheric conditions for a three-electrode system, The electrode system consisted of two parallel planes (one grounded and one supplied with a negative dc voltage) and a small, insulated needle, sticking out from the center of the grounded plane. A triggering positive square impulse voltage of 5 μs duration was applied to the insulated needle and the currents associated with the streamer discharge were measured simultaneously on all three electrodes. During the streamer propagation, the current measured at the needle was the conduction current while the other two were the displacement (or capacitive) currents generated by the movement of charge in the electrode gap. The objective of this study is to identify the three currents and to investigate if simple representations of the streamer can reproduce the displacement currents measured at the plane electrodes. Two models for the streamer were applied: (1) a charged sphere moving in the background field and (2) a channel with a constant voltage gradient extending in the gap. In both models it was assumed that the streamer propagated with a constant velocity, which was estimated from the measurements. The motion of the streamer was simulated by a series of electrostatic calculations, using a field calculation program. Comparison of the measurements with the simulations indicates that the charge of the streamer is confined to a spherical region (i.e. streamer head) and it is increasing continuously during its advancement in the electrode gap. A discussion on advantages and disadvantages with the two investigated models (sphere vs. channel with potential gradient) is conducted, and a possible hybrid model is suggested. In the proposed model, features from both considered streamer representations are included     

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.     

Breakdown conditioning characteristics of long gap electrodes in a vacuum

An improvement in dielectric strength is required in vacuum circuit breakers (VCBs) intended for use in higher voltage systems. In order to develop higher voltage VCBs, it is important to improve the dielectric strength in a vacuum based on consideration of the vacuum breakdown mechanism. Particularly for gaps longer than 10 mm, little is known about the breakdown mechanisms and their quantitative analyses in a vacuum. This paper discusses the breakdown conditioning characteristics of long gap electrodes, under a non-uniform electric field in a vacuum. We treat gap lengths of up to 50 mm in this paper. The conditioning characteristics are investigated under impulse voltage applications. A negative standard lightning impulse voltage was applied to rod-plane electrodes made of Cu-Cr and SUS304 for different tip radii and gap distances until the conditioning effect was completed. We observed illumination spots on electrodes at each breakdown during the conditioning process and calculated the corresponding breakdown field strengths. Experimental results revealed that the tendency of breakdowns associated with long gaps is different from that for the short gaps. As a result, we clarified that the breakdown field strengths are nearly constant at 110-120 kV/mm at the distances longer than 10 mm, and the breakdown field strength is at its maximum when the gap distance is about 5 mm.     

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.     

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     

Inception voltage of positive streamer and its length on PMMA inair

A positive surface discharge on a solid insulator in air consists of many branched streamers. The length of the streamers is controlled by the applied voltage at the instant when the streamers occur. The relationship between the inception voltage and the streamer length depends on the thickness of the solid insulator and the number of streamer branches. At a low voltage, the length of a streamer decreases with increasing the thickness of the insulator, but at a high voltage, it increases with thickness. The length of a streamer with a large number of branches is shorter than that with a few branches. This phenomenon can be explained as follows: the electrons generated in each branch flow into a positive high-potential electrode through the stem of the streamer, and the potential drop in the stem is enhanced due to this electron flow     

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     

聚乙烯中水树枝-电树枝的相互转化

通过合适的手段模拟水树枝存在情况下材料中的电场强度分布,对理解水树枝向电树枝的转化和评估水树枝的危害程度有重要的实用价值。在总结水树枝-电树枝相互转化已有研究成果的基础上,用有限元方法模拟了不同半径水树枝存在情况下材料中的电场分布。模拟结果表明:水树枝存在的情况下,材料中的最大电场强度出现在水树枝尖端;在外施电压一定的情况下,材料中的最大电场强度随水树枝半径的增大先减小后增加;较短的水树枝弱化了材料中的局部电场强度。