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     

Laser-induced breakdown characteristics of nonuniform dc coronadischarge fields

Laser-induced breakdown of nonuniform field gaps in atmospheric air was caused by an XeCl excimer laser. This laser beam was focused on the center of a rod-to-plane, needle-to-plane or hemisphere-to-plane gap with dc voltage. The laser energy dependency of breakdown voltage was investigated for positive and negative applied voltages and gap lengths of 15 to 25 mm. It was found that the positive breakdown voltage was higher than the negative one in the hemisphere-to-plane gaps or the rod-to-plane gaps with low voltages, where there is little or no space charge. However, the effect of polarity is reversed in the rod-to-plane gaps with large spacings and the needle-to-plane gaps. On the basis of the dependence of the discharge mode on this polarity effect, it can be seen that it was caused by the difference in development of positive and negative streamers in the corona discharge     

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     

Nanosecond, electrical triggering of water switches

Spark gaps, which use water as a switching medium, allow fast closing and high repetition rate operation for high power and high voltage switching. They are usually operated in the pulse-charging mode and have a large jitter. To reduce the jitter, a trigger electrode with positive polarity is used to initiate breakdown between a negative high voltage electrode and a ground electrode. The trigger electrode consists of a tungsten wire with a diameter of 50 μm, enclosed in glass tubing. At the triple point where the three dielectrics (glass, water and metal) meet, the electric field is greatly enhanced. From the triple point, a trigger voltage of +16 kV at a pulse duration of 100 ns produces two streamer branches traveling towards the high voltage and ground electrodes. These two streamer branches bridge the main gap and serve as preionized channels for the initiation of the breakdown between the main electrodes. Arc transition develops along the two channels and completes the switching. It allows for the gap to be fired at various voltages independent of the gap?s self-breakdown voltage. The delay between application of the trigger pulse and the electrical breakdown is on the order of hundred nanoseconds. The jitter in breakdown is on the order of ten nanoseconds. It is less by more than an order of magnitude than that obtained with pulse charging.     

Impulse insulation characteristics of a composite insulation system having a wedge gap in SF6 gas

Impulse insulation characteristics were investigated in a composite insulation system having a wedge gap in SF₆ gas. The partial discharge inception voltages of wedge gaps with various types of film were measured and compared with the calculated breakdown voltages estimated from Paschen's curve of SF₆ gas. Also discussed is how the charge accumulated on the film surface due to a partial discharge had an effect on the creepage breakdown voltage. Partial discharge inception voltages in wedge gaps were higher with higher SF₆ gas pressures and with lower film permittivities. Creepage breakdown voltages depended little on gas pressures or on creepage distances. The dependency of breakdown voltages on gas pressures and the effect of polarity on the breakdown voltage differed with the types of film. This may be partly because the charge on the film due to partial discharge had an effect on the discharge propagation, and that charging of the film differed with the types of film.     

棒-板长间隙正极性流注生长概率模型及应用

为研究长空间隙放电分散性和放电路径随机性,建立了结合传统流注放电理论和分形生长理论的正极性流注生长概率模型。首先基于经典的流注起始判据,计算了棒-板间隙流注起始电压;选择空间电位和电子崩形成时间作为流注生长控制变量,结合泊松方程、电荷连续性方程和欧姆定律描述流注通道内部的电荷转移,仿真流注的持续发展过程;当流注到达板电极后,选择放电树枝中电位梯度最大的通道作为最后的主放电通道。分析表明,该模型可描述流注区空间电荷随时间的积累过程,棒-板间隙击穿时间的分散性以及放电路径的随机性;由其计算得的棒-板间隙主放电通道分形维数与试验结果相吻合,同时基于该模型对棒-导线-板间隙放电选择概率分布的计算结果也与试验所得规律一致。     

Dielectric Strength of Compressed Insulating Gases and their Mixtures

A method is developed for computing the discharge threshold voltages in SF6, N2 as well as their mixtures above atmopheric pressure. In electronegative gases, and for both uniform and nonuniform field gaps of positive polarity, there is a critical pressure above which field emission from the cathode surface has a significant effect. The method of calculation is based on the condition necessary for the onset streamer to propagate. The effect of increased pressure over the critical value is that spark breakdown takes place within the discharge gap unpreceded by any corona. The computed values are in good agreement with those measured experimentally at pressures below and above the critical value for both uniform and nonuniform field gaps. The present calculations confirm those predicted experimentally, namely, there is no significant difference in the behavior of pure SF6 and mixtures up to 50 percent of N2. The fact that mixtures can show a higher breakdown voltage at the same total pressure than pure SF6 is also confirmed.     

Corona Inception and Breakdown in Nonuniform Field with Insulating Support in Air

Corona inception and breakdown was investigated in air for point-to-plane gaps with cylindrical spacers under 60-Hz ac voltages. The effect of a series gap, between the high-voltage point electrode and the spacer, on corona inception and breakdown voltages was also studied. For epoxy spacers the corona inception voltage decreased but the breakdown voltage increased, compared to an equivalent point-to-plane air gap without a spacer. The corona inception voltage characteristic was found to exhibit a maximum at a certain value of the series air gap. The results are interpreted in terms of recently reported observations of spacer surface charging in air and in other insulating gases and insulating barriers.     

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     

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。     

Propagation of positive and negative streamers in oil with andwithout pressboard interfaces

Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at ~2 to 4 mm/μs at voltages to 2× the minimum breakdown voltage. Above this voltage, a fast event (>100 mm/μs) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm/μs range. The negative streamers have ~2× the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown     

Comparisons of direct-current breakdown characteristics in dry air and SF6 gaps using a parallel-plane arrangement with variable height protrusion

Breakdown voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to nonuniform gap in dry air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge crossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the dc voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset. The experimental observations have been explained qualitatively on the basis of a streamer model and precise electric field calculations of gap.     

Impulse breakdown of insulators and air gaps of similar electrodeconfigurations: effects of temperature

Comparative measurements are reported on the breakdown, under lightning and switching impulses, of a concentric electrode arrangement in which the inter-electrode space is spanned either by an insulator surface or by air. Temperature in the system has been varied in the range 10m, where 0.81.0 for negative polarity at the highly stressed electrode. Variations are the same, in the presence or absence of the insulator surface, but absolute positive flashover voltages are lower across the surface, than in air, while negative flashover voltages are higher. Times to breakdown decrease with increasing temperature     

特快速瞬态过电压和雷电冲击作用下特高压GIS绝缘特性

随着电压等级的提高,气体绝缘开关设备(GIS)的额定雷电冲击(LI)耐受电压和特快速瞬态过电压(VF-TO)水平的差异越来越小,由VFTO引起的绝缘事故也日益严重。为研究特高压GIS在VFTO下的绝缘耐受水平,设计了VFTO模拟产生装置,研究了不同气压下SF6棒-板和球-板间隙在VFTO和雷电冲击下的绝缘特性。实验结果表明:稍不均匀电场间隙在VFTO下的击穿电压高于雷电波;对于高气压下棒-板间隙,负极性VFTO下的击穿电压和伏-秒特性曲线位于雷电波之下;雷电冲击下棒-板间隙极性效应在一定的气压下会出现反转现象。分析表明:在不同冲击波前和振荡波尾的VFTO和雷电波作用下SF6气体间隙击穿特性的差异与放电过程中空间电荷行为的差异有关。     

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.     

SF_6混合气体负离子稳定性对冲击击穿机理的影响

在利用辅助电极的负直流电晕放电向枫不均匀场间隙注入负离子电荷条件下,测试了SF_6及其混合气体的正雷电冲击击穿电压。依据实验结果,分析了SF_6混合气体负离子的稳定性对冲击击穿机理的影响。     

低气压棒-板间隙操作冲击放电特性及电压校正

为了研究高海拔地区空气间隙的操作冲击特性,在大型人工气候室内对0.25～2m的棒-板间隙的操作冲击50%放电电压与气压p、间隙距离d的关系进行了系统的实验研究。结果表明:操作冲击50%放电电压U50随着气压p的下降呈幂指数关系下降,间隙长度不同,气压影响指数n不同;通过分析得到了不同间隙长度下的U50气压校正公式;U50与不同间隙距离d之间的关系与电压极性有关:正极性时U50与间隙距离d的线性关系较好,负极性时有明显的饱和性,且U50的气压影响特征指数n与电压极性有关。试验结果显示操作冲击50%放电击穿时间与电压极性和间隙距离有关。     

Flashover phenomena in positive rod-to-plane air gaps under impulse and DC voltages

Discharge phenomena in positive rod-to-plane air gaps under lightning impulse and dc voltages were investigated. Under impulse voltages, the 50 percent flashover voltages agree with the 50 percent corona inception voltages only in a gap shorter than a certain value d_c which increases with the rod diameter φ. The 50 percent flashover voltage at d_c rises at a rate of about 14 kV/cm with d_c in the range of d_c < 2.8 cm (φ ? 1.5 cm), and then it rises at a rate of about 5 kV/cm. Experimental results show that the flashover process in the former region (termed the Gp process) is different from that in the latter region (the Lp process). Under dc voltages, the critical gap length, at which the first corona may lead to flashover without any succeeding streamer corona, corresponds to d_c under impulse voltage. In a gap shorter than d_c flashover occurs through the Gp process (φ ? 1 cm) or Lp process (φ ≥ 2 cm), but in gap longer than d_c flashover may occur through another process which cannot be seen under impulse voltages. In this paper, those flashover characteristics and processes are described on the basis of experimental results and photographic observations with an image converter camera.     

负极性冲击电压下油–屏障绝缘的放电行为

考虑到运行过程中油浸式电力设备会承受双指数型与双极性振荡型冲击电压的作用,该文旨在通过研究两类冲击电压下油–屏障绝缘的放电行为,为油–屏障绝缘结构的优化与油纸放电机理的探究提供参考.利用搭建的油纸绝缘光电联合诊断平台,该文基于阴影成像法拍摄了负极性双指数冲击电压与双极性振荡冲击电压下油–屏障绝缘中流注的典型发展过程,并...     

Electrical breakdown characteristics of nitrogen and air at cryogenic temperature in a quasi‐uniform electric field

The breakdown voltages of the longer‐gap configurations in gaseous nitrogen and air that are necessary in designing superconducting electrical power apparatuses are measured at temperatures of 293 and 93 K. The quasi‐uniform electric field made by a sphere‐to‐sphere electrode with a diameter of 150 mm and a gap length of about 10 to 100 mm is used in the measurement of the breakdown voltages. When 50‐Hz ac and dc voltages are applied to the sphere‐to‐sphere gap, the breakdown voltages in nitrogen and air obey Paschen's law even at cryogenic temperatures (93 K). When a 1.4/50‐μs lightning impulse voltage is applied to the gap, the 50% breakdown voltage of nitrogen also obeys Paschen's law under UV irradiation of the cathode electrode. However, the breakdown voltage in air at 93 K is higher in the case of lightning impulse voltage applications, and the delay from impulse voltage application to breakdown occurrence is apparently longer at 93 K than at 293 K. © 2000 Scripta Technica, Electr Eng Jpn, 132(4): 28–33, 2000