A phase-related investigation of AC corona in air

A computer-based partial discharge detector has been used to measure AC corona pulses in air in a point-cup electrode system. The pulse height distribution was measured as a function of phase angle and pulse height for a range of voltages. The onset voltages for both positive and negative half-cycles are the same, and significantly lower than for the DC case, due to space charge effects. In all other respects the patterns for the two half-cycles are different: pulses only occur at the onset voltage (rising) for the positive half-cycle, but continuously for the negative half-cycle, except for the higher applied voltages which exhibit a glow discharge transition near the (negative) peak. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given, together with calculations showing that space charge from one cycle can linger in the gap between half-cycles.     

特高压双回直流架空线路电晕起始电压的计算分析

为研究特高压双回直流架空线路的电晕起始特性,本文考虑线路排列方式和绞线的实际结构,结合气体放电理论建立了特高压双回直流线路的起晕电压计算模型。根据正负极起晕的不同机理,在利用优化模拟电荷法对绞线表面空间电场求解的基础上,计算了各极子导线的起晕电压。分析并讨论了导线布置、温度与气压及绞线参数等因素对起晕电压的影响。计算结果表明：-+/-+排列方式时线路起晕电压值较高。另外,导线半径的增大、气压的升高、分裂数的增加都能显著提高起晕电压。导线对地高度、极间距、上下层导线高度及分裂间距也都会不同程度影响起晕电压。绞线绞入率逼近于0时,绞线起晕电压接近光滑导线的起晕电压。     

Positive corona in air using a point/cup electrode system

The authors have previously published some measurements of negative corona currents in air using a spherically-symmetric point/cup electrode system, and also the related simulations. In this paper the corresponding current/voltage measurements and simulations for positive-point corona, again using the highly symmetric electrode system, are described. Although the corona onset voltages are again predicted with reasonable accuracy for different point radii and air pressures, the currents are very much lower, presumably due to the effect of positive space charge accumulating close to the point electrode (since the simulations assume the absence of space charges). In agreement with this assumption, the streamer pulses are only seen at or just above the onset voltage. Only avalanches occur from about 1 kV above the onset voltage.     

Charging of Polymeric Surfaces by Positive Impulse Corona

This paper focuses on analysis of charging of polymeric surfaces by means of impulse corona discharges in air. Internal (space charge densities and electric fields) as well as external (circuit current) characteristics of corona in a point-plane electrode configuration are investigated by means of computer simulations. Two types of onset positive corona modes, namely positive glow corona and burst pulse corona are identified. The developed and verified computer model is further used to study corona charging of a 2 mm thick polymeric material sample. Both the mechanism of charge deposition and distribution of deposited charges on the surface are dependent on the mode of the corona discharge used. In the case of glow corona, charge generation is limited to the anode region and the generated charges move towards the sample surface under applied electric field. Thereafter the deposited charge cloud expands radially along a portion of the surface with fairly constant concentration. In the case of burst pulse corona, series of positive charge clouds start from the anode and move towards the sample surface in a wave-like manner. Each burst contributes to the deposited charge, which spreads over the surface less extensively than that observed during glow corona charging.     

±800 kV管母线的电晕起始特性研究

目前我国向家坝-上海±800 kV特高压直流输电工程正在紧张的设计和建设中,作者拟通过试验和仿真计算对±800kV直流工程用管母线的直流起晕特性进行系统研究。首先在北京、西宁和拉萨3地进行了多种直径模拟管母线的可见电晕对比试验,然后利用模拟电荷法对不同直径管母线表面电场分布进行仿真计算,并结合气体放电理论得到管母线负极性电晕的起晕电压。将试验数据和仿真结果进行对比,两者较为吻合。且由试验和仿真数据均可看出,母线起晕电压随着对地高度和母线直径呈线性变化关系;仿真计算还表明,随着对地高度和母线直径的进一步增加,母线起晕电压呈现出饱和的趋势。文中还针对海拔对管母线起晕电压的影响进行了讨论,给出了工程适用的海拔校正方法。     

邻近直流导线时交流电晕电流脉冲特性试验

本文搭建交直流并行线路电晕放电试验平台,研究邻近直流电压时交流单点电晕放电下的电晕电流脉冲特性。并对交流电晕电流脉冲的分布模式、重复频率以及波形等特征参数进行统计分析。试验结果表明,邻近的正极性直流电压会促进交流负半周电晕放电而抑制交流正半周电晕放电,邻近的负极性直流电压作用正好相反。直流电压对交流负半周电晕电流脉冲波形参数无明显影响,而正半周电晕电流脉冲的幅值会随负极性直流电压的增加有所升高。同时发现,随着交流电压幅值的升高,促使其正半周发生电晕放电所需要的负极性直流电压幅值也相应地升高,并基于电晕放电的发展过程中电离强度的计算提出种子电子的缺失是造成该现象的原因。最后,在电晕放电离子云模型基础上,提出脉冲序列发展的过程中电离区域电场强度恒定不变的假设,从而对邻近直流电压影响下的交流电晕电流脉冲波形参数特性给出了合理的解释。     

Phase relationship of PD pulses in dielectric liquids under acconditions

Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of ⩾15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether     

不同海拔下电晕笼分裂导线起晕电压的计算分析

为了研究海拔高度对电晕笼分裂导线起始电晕电压特性的影响,建立电晕笼钢芯铝绞线起始电晕电压的计算模型,并开展相应试验研究。采用模拟电荷法计算钢芯铝绞线的空间电场强度。依据极不均匀电场下自持放电判据,建立不同海拔高度电晕笼分裂导线电晕起始电压的计算模型。在超/特高压人工环境气候试验室内,以500 m海拔高度为间隔,系统开展19~4-000-m海拔高度范围内六分裂导线起始电晕电压的试验研究。试验获得超高压电晕笼不同海拔高度下6-LGJ—400/50、6-LGJ—500/45分裂导线的起晕电压。计算获得不同海拔高度、分裂间距、导线分裂数及绞线表面粗糙系数下的导线起晕电压曲线族,以及不同绞线半径及最外层铝绞线股数的表面粗糙系数计算结果。分析结果表明:计算模型能够较好地计算电晕笼内绞线的起晕电压;在350~500-mm分裂间距范围内,分裂导线起晕电压随着分裂间距的增大而降低,随着导线分裂数的增加而升高;绞线表面粗糙系数与绞线最外层铝线半径与绞线半径之比相关。     

Corona power loss determination on multi-phase power transmission lines

This paper presents a method to calculate the corona power loss and the ground-level electric field values for three- and six-phase power transmission lines. The charge simulation technique is applied in which the surface charge on the stressed conductors as well as the space charge around each conductor are represented by two discrete sets of unknown infinite line charges. Satisfying properly chosen boundary conditions at the transmission-line conductor surfaces results in a system of linear algebraic equations whose solution evaluates the unknown line charges. The emission of ions from a conductor surface is assumed to take place when the magnitude of the charge simulating its surface charge exceeds an onset value based on a pre-defined corona onset field value for both positive and negative half cycles. The simulation space line charges are displaced back and forth to the conductor by the action of the electric field whose magnitude and direction depend on the simulation charges in the conductor and in the surrounding space. Recombination process takes place whenever charges of opposite polarities meet together. The calculated corona power loss for three-phase power transmission lines agreed satisfactorily with those reported earlier, and for the first time, the six-phase corona power loss will be presented.     

空气湿度对导线电晕起始电压的影响

我国特高压工程中,输电线路要经过山地、湖泊等高湿度地区,空气湿度对架空输电线路电晕放电有较大影响。为研究湿度对导线电晕放电的影响,利用同轴线-筒电极研究了不同大气湿度对交流以及直流正负极性电晕起始电压的影响规律。试验结果表明:湿度对直流正极性电晕起始电压影响较小,而对直流负极性以及交流电压电晕起始电压影响较大:在相对湿度达50%～60%时,直流负极性与交流起晕电压最高。分析认为,空气湿度对空间电荷分布的影响以及湿度对导线表面状态的影响是导致起晕电压变化的主要因素。     

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     

超/特高压交流输电线路电晕损失的数值仿真研究

电晕决定输电线路的电磁环境特性。采用模拟电荷法计算交流输电线路的电晕损失,交流导线用多根线电荷表示,导线表面场强超过起晕场强时令一定量电荷由导线表面发射到空间中。将交流周期分为若干时段,在每一时刻都考虑了导线表面电荷发射、空间电荷运动、空间电荷复合等效应,重复计算若干周期直至离子流场稳定。在已有方法的基础上改进了起晕条件和电荷发射的计算方法,考虑了导线表面电场不均匀性对电晕放电的影响,从而可以对多相多分裂导线离子流场进行仿真计算,进而计算得到线路电晕损失。对三相8分裂特高压交流线路电晕损失计算结果与试验结果有较好的一致性。     

Space charge behavior in SF6 gas and sequentialgeneration of PD pulses

We observed time-resolved partial discharge (PD) characteristics under ac and dc conditions in order to demonstrate the effects of space charge behavior and the corona stabilization effect in SF₆ gas. From the experimental results, we found that the charge magnitude of the first PD in the positive half cycle of applied ac voltage depended only on the instantaneous voltage. Under dc conditions we found that the magnitude of the PD charge related closely to the time interval between PD pulses. Space charge behavior and the PD generation mechanism are interpreted diagrammatically, and we describe how positive ions reduce the magnitude of PD charge and activate the corona stabilization effect and that the negative ions shorten the time interval between PD pulses     

电晕笼交流单根导线电晕损失的计算分析

电晕损失是导线电晕特性研究的重要内容之一。为了建立电晕笼内导线电晕损失的计算模型,将模拟电荷法应用于电晕笼内导线电晕损失计算。采用线电荷模拟交流导线,对正方型截面笼壁分别作镜像处理。当导线模拟电荷量超过起晕电荷量时,分别计算导线上每个模拟电荷点向空间发射的电荷量。考虑空间电荷的影响,计算电晕笼空间的合成电场。仿真模拟电荷发射、迁移、复合过程,计算电晕笼中空间电荷运动,计算迁移过程中电荷运动产生的能量损失。进行电晕笼单根光滑导线、单根钢芯铝绞线LGJ—300/40、单根钢芯铝绞线LGJ—400/35电晕损失测量试验。仿真结果与试验结果对比,结果基本一致。因此采用模拟电荷法能够较好地建立电晕笼内单根导线电晕损失的计算模型。     

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.     

特高压直流输电线下的直流离子流电场

针对特高压直流输电导线发生电晕的情况,采取测量试验来探究直流导线产生电晕以后导线下方的离子流对地面电场的影响。通过试验,得出直流导线产生电晕以后导线下方的合成场强、标称场强、空间电荷场强分别与导线电位成线性关系,正、负极导线放电电晕情况有差异以及标称场强在合成场强中的比重随高度变化而改变等。     

直流电晕电流时域脉冲特性的统计分析

为了研究直流正负电晕电流时域脉冲的特性,基于实验室开发的高频电晕电流高压端测量系统,开展了不同条件下直流正负电晕的线板放电实验。基于大量实验数据,统计、对比了正、负极性电晕脉冲的幅值、上升时间、半波时间和重复频率等特征参数随导线电压及导线半径变化的规律。实验结果表明：导线电压对单个正、负脉冲波形的影响较小,主要影响脉冲的重复频率;负脉冲的上升时间平均值为20~30 ns,半波时间的平均值为90~110 ns;正脉冲的上升时间平均值为40~50 ns,半波时间的平均值为140~160 ns;随着导线电压的升高,正、负脉冲重复频率均迅速增大,但相同电压下,正脉冲的重复频率显著小于负脉冲的重复频率,负脉冲重复频率约为正脉冲的15~25倍。     

The effect of space charge on the initial growth of electrical trees in polyethylene

This paper describes the influence of space charge on the growth of an electrical tree which is an important factor of the dielectric breakdown in polyethylene-insulated power cables. Polyethylene and ethylene vinyl-acetate copolymer with a semi-conducting needle were used as the treeing specimens. The statistical analysis showed that the growth rate and the unbalance of positive and negative partial discharges increased as the vinyl-acetate content increased, and that the growth per cycle decreased with the increase in frequency. A model of the tree growth accompanied by the space charge behavior was proposed considering the above experimental results. The tree growth is considered to be caused by the dielectric breakdown due to the field enhancement at the tree tip during the positive half cycle of the applied voltage, the field enhancement being due to residual negative space charge injected during the previous negative half cycle. The negative space charge in polyethylene would be mainly supplied through the partial discharge inside the tree.     

多针电极结构双极电晕放电伏安特性

为提高电晕放电的能量密度和放电稳定性,提出了针阵列电极结构的双极电晕放电方式并研究了多针电极结构双极电晕放电的伏安特性。实验得出放电电流I随针尖半径a和电极间距d的增大而减小,随相邻针尖间距s的增大而增大,但当s≥40 mm时,相邻针尖的相互作用已很小,I几乎不变;d对火花击穿电压的影响较大,a对其的影响较小。由于电极结构的对称性,高压电极的极性对放电无明显的影响,正负电晕放电的伏安曲线和火花击穿电压均较为接近。将多针电极双极电晕放电电流I等效成电极间距为d/2的多针对板正、负电晕放电电流I1和I2相加,分析了I>I1+I2的原因,并推知其电离区内电子密度也有相应规律。     

运动水滴在球──球直流电场中的放电研究

研究了运动水滴在球－球直流电场中的放电现象,对于含有水滴的电场分布进行了计算,水滴的引入使得水滴外表面的场强跃变为无水时场强的３倍左右,从而水滴在电场中会在较低的电压下诱发电晕,比较了正负极性电压下起晕现象。为水处理技术中的混合两相体放电问题提供了依据。