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.     

冰凌针-板间隙直流正极性下起始电晕特性的研究

覆冰是电力系统的严重自然灾害之一。覆冰绝缘子伞裙间隙未桥接时,冰凌尖端的放电是影响绝缘子闪络电压的主要因素之一。选择冰柱-冰板间隙模型,借助于紫外线成像仪等设备,在人工气候室内研究了冰凌-冰板模型的电晕特性,提出了用紫外线成像仪确定冰凌-冰板起晕电压的方法, 分析了影响冰凌起晕电压的因素。结果表明:冰凌-冰板间隙起晕电压与冰凌状态有关,湿冰的起晕电压较低,干冰的起晕电压较高;间隙较长,起晕电压较高;气压较低,起晕电压较低;冰凌起晕电压与覆冰水电导率没有明显关系。     

Control of corona characteristics with third electrode and biasvoltage

A novel corona control system consisting of a point-to-plate air gap with a cylindrical third electrode has been proposed. Negative and positive coronas from a point-to-plate air gap with a cylindrical third electrode set up close to the corona point of the needle have been investigated with emphasis on the role of the cylindrical third electrode, the bias voltage amplitude and polarity. It was found that the point-to-plate air gap with the cylindrical third electrode had a negative resistance characteristic on its I-V characteristic for negative polarity discharge, which is very different from the same electrode system without the third electrode. Discharge characteristics of the needle point, such as the corona onset voltage and the corona activity, could be controlled by adjusting the field intensity of the corona point by changing the corona point height from the third electrode and varying the bias voltage amplitude and polarity of the third electrode     

A Study of Early Discharges in Air Gaps

The corona pulses appearing in air gaps under high-voltage impulses are the bases for the complete discharge in these gaps. The various elements affecting the creation of these pulses and their consequent influence on flashover are investigated. The delay of corona pulses and the probability of their occurrence are related to those of breakdown. The results are used to form design grounds for the air insulation of high-voltage apparatus. The experimental work was performed on 50-cm rod plane air gaps at the High Voltage Laboratory of Cairo University.     

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     

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.     

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.     

冰棒—冰板间隙负极性电晕起始放电特性

国内外研究结果表明,绝缘子覆冰过程的一个显著现象是:在带电情况下,其表面冰凌并未完全桥接,而是在伞裙间形成一定的冰棱空气间隙。为研究这种空气间隙对放电发展的影响,利用冰棒-冰板空气间隙放电模型在人工气候室内进行了负极性直流电晕放电试验,并分析了不同间隙距离、不同大气压、不同覆冰类型(包括冰的表面状态以及覆冰水电导率)情况下的电晕起始放电特性。研究结果表明:随着空气间隙长度的减小和气压的降低,负极性电晕起始电压也明显下降;湿冰时的电晕起始电压高于干冰时的电晕起始电压。这些研究结果为进一步深入开展覆冰电极系统的电晕放电机理和覆冰绝缘子闪络研究打下了基础。     

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.     

Breakdown characteristics in air gaps with artificial floatingmetals under DC voltage

The DC breakdown characteristics of a sphere-to-sphere gap with conducting objects supported by an insulator rod in atmospheric air are investigated experimentally to provide fundamental parameters determining the breakdown voltage in the presence of floating objects around DC power apparatus by an air insulation. The shape of tested floating objects made of metal is plate, needle or sphere. Experimental results show that the main factors affecting the breakdown voltage are the shape and size of floating objects, the object location and the gap length. If the object is placed at the location giving the lowest breakdown voltage, any object lowers the breakdown voltage below that without the relevant objects     

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.     

Flow electrification phenomenon in forced air-cooled rotatingelectric machines

The factors affecting air-flow electrification under both HVDC and HVAC of variable frequency have been investigated experimentally on air flow between coaxial-cylinder and point-to-cylinder systems in a synthetic open cycle. Instantaneous measurements of the air flow electrification (AFE) `streaming current' and corona current `radial current' are presented as a function of mass flow rate, air temperature, pressure and relative humidity, electrode configurations, and voltage amplitude, polarity and frequency. The results reveal that both streaming and radial currents augment with voltage, relative humidity, mass flow rate, and temperature. On the other hand, both the streaming and the radial currents do not vary with pressure. Increasing the frequency of the applied HVAC significantly reduces the streaming current, while the radial current increases linearly. Similar trends of the results are observed for both electrode systems. For the point-to-cylinder system under negative HVDC, increasing the mass flow rate drastically reduces both corona inception and breakdown voltages, while positive HVDC gives only a slight effect. The present work confirms the significant role of the AFE on the field failures occurring inside forced air-cooled rotating electric machines. Therefore, such phenomena should be taken into consideration in the design of such machines     

AC particle-triggered corona discharge in low pressure SF/sub 6/ gas

This paper deals with AC particle-triggered corona discharge as a follow-up to our previous research with DC voltage to clarify the particle-triggered corona discharge process in SF₆ gas. Corona current pulses, charges associated with a corona current pulse, and corona light pulses were observed with an aluminum ellipsoidal particle suspended in a parallel plane electrode system under a SF₆ gas pressure range of 30 kPalesPles50 kPa by changing the particle position. Corona mode, phase (Phi)-charge (q) characteristics as well as corona discharge processes were discussed and the following results were obtained. AC corona mode depended on the instantaneous applied voltage, voltage gradient as well as the particle position and then, the Phi-q characteristics were also affected by those parameters. Charges flow into the floating particle due to coronas on the both tips of particle and excite field fluctuations around the opposite side of particle in addition to the applied AC field. The field fluctuation in SF₆ gas by the corona charges was about 4% of the applied field and much lower than that in air gap which was about 70%. That is, the corona development was suppressed effectively by high electron affinity of SF₆ gas even in the case of floating particle. The less effective interference between coronas on the both side of particle in SF₆ gas results in an obscure local minimum in the breakdown voltage characteristics as the particle is in the vicinity of electrode as contrasted with a drastic fall in the breakdown voltage by the particle in air     

Influence of humidity on current waveform and light emission of alow-frequency discharge controlled by a dielectric barrier

The aim of this work is to determine the effect of humidity on a silent discharge in air. A point to plane configuration has been used. Current and emission spectra of the discharge have been recorded in dry and humid air for short and long inter-electrode gaps and with the plane electrode coated by a silicone rubber plate alone or under a polypropylene film. Results show the typical corona discharge regimes. For a long gap, humidity decreases the number of breakdown streamer pulses. For a short gap, the behavior observed depends on the surface conduction of the dielectric in contact with the discharge. These results show that the presence of humidity in the discharge cell not only modifies the gas but also the surface states of the dielectrics coating the electrodes. The surface charge decay appears as a major parameter, since it is related to the local field value which results from the applied field and the reverse field induced by charge accumulation during previous discharges     

基于支持向量机的棒-板空气间隙击穿电压预测方法及其应用

空气间隙的击穿电压与放电起始前的电场分布特征存在多维非线性关系。为了实现空气间隙的击穿电压预测,以电场特征集作为输入,以间隙在加载电压下是否击穿作为输出,采用支持向量分类机建立击穿电压预测模型。针对极不均匀电场空气间隙的击穿特性受电晕影响的问题,提出两种修正方法:通过增加受电晕影响的训练样本数据,提高预测模型的泛化性能;或基于"电晕云"的思想进行二次电场计算及特征量提取,对预测模型的输入特征进行修正。采用修正后的模型对极不均匀电场下棒-板间隙的工频击穿电压及棒-板长空气间隙的操作冲击放电电压进行预测,预测值与试验值吻合良好。该方法有利于减少试验次数,降低试验成本。     

交直流复合电压下棒-板电极起晕电压实验分析

为了获得复合电压作用下电极的起晕特性,在室内搭建了复合电压下无锈棒-板电极起晕特性实验平台,采用串联的复合加压方式,即一个电极上施加直流电压,另一个电极上施加交流电压,以紫外光子计数的突变作为电晕起始判据,获得了直流和交流复合电压下的电极起晕特性。同时对比了不同直流电压极性、电极加压方式和不同复合电压升压方式下电极起晕电压的特点,探究复合电压下棒电极半径和棒板间隙对电极起晕电压的影响。实验结果表明:在相同的大气条件及电极布置下,由于空间电荷影响,棒电极正直流、板电极交流加压方式下的起晕电压比棒电极交流、板电极正直流加压方式下的起晕电压高;在棒-板气隙间距0~5cm范围内,定直变交复合升压方式下,交直流分量有效值叠加后的起晕电压随着直流分量的增加而升高,定交变直方式下,该值随着交流分量的增加而下降,且交直流分量间互为线性关系。本文的研究成果可以为进一步研究复合电压作用下电极的电晕机理及起晕电压的准确预测奠定坚实基础。     

基于紫外成像技术的极不均匀电场电晕放电

为监测电气设备外绝缘缺陷、灵敏观察和准确定位电晕放电,以紫外光谱特征量为量化电晕放电强度的表征参数,以相同工频电压等级下的放电水平和电晕脉冲电流峰值作为量化比较参数,在基本相同的大气条件下,利用紫外成像技术试验研究了极不均匀电场工频电压下电晕放电。结果表明,电晕放电强度量化表征参数与量化比较参数之间的对应关系很好;表征参数可反映电晕放电的几个阶段;结合紫外成像反映的放电形态,可分辨电晕放电过程的各阶段,且紫外光谱特征量在电晕放电过程各阶段的分布具有一定的统计规律。最后提出选取流注放电阶段的紫外光谱特征量交集作为空气间隙击穿的注意范围判据。     

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.     

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。     

Role of air gaps on AC withstand voltage of an ice-covered insulator string

The influence of the number and position of air gaps on the 50% AC withstand voltage, V₅₀, of a short string of 5 IEEE standard insulator units covered with ice was experimentally investigated and numerically simulated. The results show that the V₅₀ is affected by the number and position of the air gaps. The results also show that the presence of a partial arc along the air gap close to the high voltage (HV) electrode leads to a redistribution of the voltage along the ice-covered insulator. Based on the obtained results, suggestions for improving the performance of line insulators under icing conditions were proposed