The effect of the electrode gap on breakdown in liquid dielectrics

It is a well known fact that the breakdown voltage of an insulating liquid increases nonlinearly with increasing electrode gap. Under nonuniform field dc conditions two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm or larger, one finds that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Recently, perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. In the case of the conventional hydrocarbon type oils, the positive streamer usually grows slightly faster than the negative one, but the streamer inception voltages of both are rather similar. In the case of perfluorinated polyethers the positive streamers move at least 10× faster than the negative ones, but they require much higher inception voltages than negative ones. At gaps ≳12 mm and the associated higher applied voltages this breakdown at negative polarity occurs at higher voltages than at the positive one. The negative ions of the perfluoro compounds are much more stable than those of conventional hydrocarbons. The discharge of the negatively charged perfluoro ions requires much higher fields than do those of conventional hydrocarbons. The implications of these observations are discussed in some detail     

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.     

Dielectric breakdown characteristics of cryogenic nitrogen gasabove liquid nitrogen

We measured the DC dielectric breakdown characteristics of cryogenic nitrogen gas above a liquid nitrogen surface for needle-to-plane and sphere-to-plane electrode configurations. Experimental results revealed that the DC breakdown voltage of cryogenic nitrogen gas increased, as the distance from the gap axis to the liquid nitrogen surface diminished, i.e. as the liquid nitrogen surface got closer to the gap. The breakdown voltage proved to be enhanced not only by the temperature drop of nitrogen gas due to the existence of liquid nitrogen, but also by an effect of vapor mist arising from vaporization of liquid nitrogen. For the quasi-uniform electrode configuration, the relation between breakdown voltage and the gas density times the gap spacing agreed well with the Paschen curve for nitrogen with both the temperature falling and the vapor-mist density considered. For the nonuniform needle electrode, positive breakdown voltage was higher than negative one; the polarity effect was interpreted in terms of the electric field relaxation at the tip of the positive needle resulting from partial discharges observed only for the positive needle     

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。     

Estimation of dc breakdown mechanisms in air gaps containingfloating metallic particles

The characteristics of breakdown voltages against the position of floating metallic particles in atmospheric air are investigated experimentally. The breakdown mechanism is estimated on the basis of electric field calculation and visual observation of discharges at different voltages and a set of flow charts for calculation of breakdown voltage is proposed. The studied gaps are needle-to-plane and sphere-to-sphere electrodes of 120 mm in length forming Poissonian and Laplacian fields respectively and the shape of tested floating objects is a needle or sphere. The theoretical breakdown voltages obtained by the flow charts agree reasonably with the experimental ones     

A combined engineering model for the analysis of switching surge flashover characteristics of long air gaps

The flashover characteristics of long air gaps submitted to switching surge voltages may be predetermined by use of a mathematical model of leader propagation. The present model is developed from each combination of previous engineering models, and is also applied to positive rod-to-plane geometries with gap lengths up to 20 m. However, four parameters to determine leader propagation with respect to our model are adjusted in the present calculation so as to fit experimental flashover voltages and calculated flashover voltages which depend on waveforms of the applied voltage and gap length. The calculations show that these parameters are only a function of gap length. Our combined engineering model using parameters that depend on gap length allows the flashover characteristics of long air gaps to be easily determined with a microcomputer.     

Influence of ground and corona currents on dielectric behavior of small air gaps

This paper investigates the influence of grounding on the field distribution and on the dielectric behavior of small rod-plate and rod-rod air gaps. This effect can be attributed to the grounding of one electrode. In the different arrangements with one electrode grounded or with the electrodes symmetrically charged the experimental results of the corona and breakdown are recorded and compared to the simulation results of the field distribution. It is resulted that the grounding influences significantly the electric field distribution, the dc corona and the dc breakdown. The field is less inhomogeneous in grounded rod - plate air gaps and in rod-rod air gaps with symmetrically charged electrodes, and consequently the values of the corona onset and the breakdown voltage are higher. In longer air gaps the corona current influences the field distribution and consequently the dc breakdown. The effect of grounding on the breakdown is weakened when the dc polarity of the voltage is negative, otherwise it is enhanced. In air gaps with lengths > 3 cm (negative dc polarity) or >8 cm (positive dc polarity) it is overlapped by the effect of the corona current. Relations between the field strength, the corona current, the corona onset and the breakdown voltage arise. The principle of action-reaction is valid.     

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。     

非均匀场中气固两相体雷电冲击U50的实验研究

通过对几种典型固体颗粒的气固两相体放电实验研究,发现其击穿电压具有极性效应,负极性条件下体积比对雷电冲击U50影响显著,使雷电冲击U50降幅达到30%左右,并且小颗粒的击穿电压比大颗粒的击穿电压低;正极性条件下击穿电压随体积比变化呈小波动变化特性,体积比及颗粒大小对击穿电压影响不显著。另外,与空气放电一致的是,相同体积比下正极性雷电冲击击穿电压低于负极性。     

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.     

Effect of the small arc current generated by a poor contact on the breakdown voltage in SF6 gas

One of the failures related to gas-insulated switchgear is caused by the poor contact between conductors. This causes breakdown between the contact and the wall of chamber, initiated by the small discharge due to the poor contact. In this paper, the modeled experimental apparatus for the contact portion of the gas-insulated switchgear assumed a rod-to-plane gap with small arc current. The influence of the arc current on the breakdown into the chamber was investigated experimentally, and the breakdown mechanism between the contacts and the wall was studied. It was clarified that the breakdown voltage between the contact and the wall decreased by increasing the current, and the small arc current in the contact portion played an important role in the breakdown between the contact and the wall. The reduction ratio of the breakdown voltage increased with increasing gap length of the small gap. The breakdown voltage was different depending on the duration of the small arc current.     

静态模拟气固两相体起晕和击穿的实验研究

研究了非均匀电场中的静态模拟气固两相体放电的击穿、起晕特性及固相物介电常数、占空比、放电间隙、电压极性和种类对击穿电压与起晕电压的影响。结果表明:实验范围内,正极性大占空比时,随放电间隙的增加,两相体的起晕电压逐渐低于空气的起晕电压。正极性和交流电压下,两相体的击穿电压比空气高。     

The Sparking Characteristics of Needle-To-Plane Coronas

The sparking voltage (Vs) and maximum presparking corona current (Imax) of needle-to-plane coronas have been measured as independent functions of polarity, tip radius (r), and needle-to-plane spacing (S). For a negative needle, Vs and Imax increase with S but are independent of r. For positive polarity, Vs and Imax increase with both S and r. Thus to increase the corona current while avoiding a spark, one should increase the spacing and voltage. At large spacings, Imax is appreciably greater for negative polarity than for positive. Analysis of the data reveals that for negative polarity the maximum presparking current may be written in the approximate form Imax?Vs(Vs-VO)S-2 where VO is the corona threshold voltage. This is an extrapolation of a relationship previously obtained for Trichel pulse corona. The analysis shows that the transition from negative corona to sparking occurs when the linear space charge density in the gap reaches a critical value Qs/S ?10-10 coul/mm, where Qs is the total space charge in the gap just before sparking occurs. It is shown that this condition amounts to spatial overlap of the multiple ion clouds in the gap, thereby creating a continuous ionized channel between the electrodes.     

风沙条件下的平板模型直流沿面放电特性

The influence of sand dust on discharge of external insulation has caused widespread concern.At present,the research results show wind-sand electricity has a remarkable effect on the discharge characteristics of insulator and has little influence on the discharge characteristics of air gap.The flashover of insulator strings occurs along the insulator surface and air gaps,and the sand dust deposited on the insulator surface may affect the flashover characteristics of insulator strings.This paper studies the flashover characteristics of flat plate model under DC voltage in wind-sand condition.The experimental results show that under positive polarity voltage,the flashover voltage of the flat plate model has a maximum value,while under negative polarity voltage,the flashover voltage of the flat plate model has a minimum value with a certain degree of sand dust deposition.The wind or sand in sand-dust weather has an important effect on the flashover characteristics of the flat plate model.In certain variation range of electric charge,electric charge of sand dust has little effect on the flashover voltage of flat plate model under DC voltage.The deposition of sand has significant influence on the flashover process of flat plate model,which is related to the deposition density and moisture content of sand particle.     

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.     

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.     

Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschen's law at cryogenic temperature for 50 Hz, AC, DC and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in nonuniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.     

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...     

Breakdown Process in Rod-to-Plane Gaps with DC Voltages

The breakdown process in rod-to-plane gaps, of which gap spacings were up to 80 cm under dc voltage, was investigated with multiple techniques using an image converter camera, five photomultipliers, and two still cameras, as well as with the measurements of voltage and current. The combination of a photomultiplier with an image converter camera allowed us to view for the first time the leader development with dc voltage. It is shown that breakdown proceeds, after well-known corona discharges, through streamers bridging the gap, leader development, and their bridging the gap to induce the intense return ionizing waves, to lead the formation of a spark channel. The charge density was estimated to be 2.7-24.5 pC/cm in corona discharges for the applied voltage of 170-410 kV and to be on the order of 10-8 C/ cm in the leader head.     

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