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.     

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.     

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。     

正极性雷电冲击电压下天然酯绝缘油油纸沿面流注动态变化规律研究

油纸绝缘复合电介质沿面放电是电力设备内绝缘的研究基础,其在雷电冲击电压下的绝缘特性是变压器绝缘设计的重要参数之一。为了获得油纸沿面流注传播与消散过程中电学、空间电荷分布演化规律及其关联关系,以交界面平行于施加电场方向的油纸系统为研究对象,通过构建适用于绝缘油油纸沿面流注动态变化特性的试验观测系统,可同步获得正极性雷电冲击电压下流注传播和消散过程中的电压、放电电流和放电通道流注阴影图像。利用该平台还测量了油纸沿面正极性雷电冲击击穿电压。试验结果表明,在正极性雷电冲击电压下天然酯绝缘油油纸绝缘相对介电常数差异并不会促进油纸沿面流注的传播过程,而粘度对于油纸沿面流注侧向分支影响显著。粘度越低,空间电荷在迁移过程中所受到的阻力越小,流注头部空间电荷在受到表面电荷的斥力后越容易往油中扩散,空间电荷在绝缘油中所形成的空间电场使得油纸沿面流注的主分支能够在绝缘油中传播,增加了油纸沿面流注传播距离,从而使低粘度天然酯绝缘油油纸沿面正极性雷电冲击击穿电压略高于其纯油击穿电压。     

金属颗粒对绝缘油流注发展特性的影响研究

变压器在制造、运行和维修等过程中,其内部绝缘油难免会引入金属颗粒,为探究金属颗粒对油纸绝缘性能的影响,文中搭建并调试了油纸绝缘雷电冲击放电试验平台等,开展了金属颗粒对绝缘油流注发展特性影响试验研究,研究了不同浓度金属颗粒对绝缘油流注起始电压与发展过程中形态的影响,并通过COMSOL仿真平台进行验证。结果表明,金属颗粒会促进油中流注的起始与发展过程,流注停止长度和发展速度与颗粒浓度呈正相关,而绝缘油流注起始电压与击穿电压与颗粒浓度呈负相关,金属颗粒的引入会使得流注形态更加发散。分析认为油中金属颗粒与流注的相互作用是降低流注起始电压,加速流注发展,进而降低绝缘油击穿电压的主要原因。     

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     

考虑载流子密度扰动的植物绝缘油中初始流注放电仿真

在现有泊松方程及电流连续性方程的放电模型基础上,通过引入载流子密度波动源建立具有多分支流注形貌的放电改进模型;通过COMSOL软件仿真分析了雷电冲击电压为120kV、160kV和200kV时的放电结果。结果表明:改进模型的流注分支形貌比原模型更接近试验结果;最大场强主要分布在流注头部,且均超过2×108 V/m,其中200kV时由二次流注引起的场强和电荷变化最为明显。此外,3种电压下的z轴流注发展长度和速度还表明流注分支间电荷的相互排斥对流注有抑制作用。而对比电压上升沿时间分别为5ns、10ns、50ns和120ns时的流注形貌还发现:电压上升沿时间越短,流注半径越大,分支越多。该文的研究成果有助于分析放电过程中载流子密度波动对流注分支的影响机理。     

Creepage discharge characteristics over solid-liquid interfaceswith grounded side electrode

The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of ~20% from the streamer tip. The streamer appeared to progress with a constant mean velocity     

Deterioration characteristics of insulating spacer model in repeating impulse voltage

To discuss the UHV GIS test voltage, the deterioration characteristics of the GIS insulating spacer model were studied by repeating lightning or switching impulse voltage. The deterioration factor indicating the decrease of lightning or switching impulse breakdown voltage applied up to 1000 times was found to be 35.7/45.5. The relationship between the partial discharge inception voltage and the breakdown voltage after 1000 impulse applications was also studied. It was found that the partial discharge inception voltage increases despite a decrease in breakdown voltage due to the impulse voltage. In addition, to elucidate the deterioration phenomenon, a study was made of the breakdown characteristics of models with various boundary conditions between the electrode and epoxy resin, as well as the relationship between the boundary conditions. Microdischarge was also studied. The results of the investigation suggest that the deterioration by repeating voltage is caused by microdischarge due to electric field intensification on microprotrusions of the electrode surface.     

Effect of lightning impulse prestress on AC partial discharge characteristics in cast resin transformer

It is crucial for cast resin transformer to detect void and delamination existing in the cast resin because these defects reduce insulation performance of cast resin equipment. Furthermore, defects are likely to be mixed in cast resin because of several surface boundaries between resin and conductor. It is considered that detecting partial discharge (PD) is effective to diagnose equipment of power equipment. However, it is reported that withstand lightning impulse voltage test may give influence on AC partial discharge test, especially partial discharge inception voltage. This paper deals with accumulated charge in a void under AC voltage to investigate the effect of the impulse voltage prestress on subsequent AC PD characteristics in cast resin transformer. AC PD characteristics were compared before and after impulse application. In addition, recovery of PD characteristics was measured to investigate charge decay. As a result, the mechanism of prestress effect is revealed and surface charge density is estimated quantitatively in cast resin transformer.     

Characteristics of the effect of an impulse supply voltage on the structure and characteristics of a surface discharge

Results of an experimental investigation of surface barrier discharge in a three-electrode system are presented. An analysis is done of the flow intensity of charged species extracted from the plasma layer of the surface discharge by the field of high dc potential of the third electrode. A high unipolar periodic impulse voltage of 15-kHz frequency, impulse duration from 0.5 to 45 ms, and 150-ns impulse-front duration is used to form the surface discharge. A It is revealed that the voltage-impulse duration has a significant effect on the intensity of charged species flow (extracted current), the character of the influence being dependent on the voltage-impulse polarity. The influence of voltage-impulse duration on the surface-discharge current is revealed as well. Pictures of the structure of surface discharge at the rise and the fall of the voltage impulse made by means of a high-speed camera are presented. The difference of the discharge structure for different polarities of voltage impulse is revealed. It is suggested that the surface-discharge characteristics are dependent on a combination of a number of factors, such as the parameters of the impulse voltage, volume charge in the gas gap, and the charge on the barrier surface, as well as the material and surface structure of the barrier.     

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.     

大气中短空气间隙流注放电过程数值仿真

介绍了气体放电过程的流体数值模型,并将有限元(finite element,FE)和通量校正传输(flux-corrected-transport, FCT)法结合求解该模型。针对求解时间过长的问题,引入网格自适应剖分法和并行计算方法,提高了计算精度和效率。采用所提出方法仿真了棒板间隙流注放电过程,与实验结果吻合较好。基于仿真方法研究了电压大小、电极形状对放电的影响。基于气体放电现象的物理本质,从微观上研究了气体放电过程,得到了流注过程中空间电荷、电场分布等难以直接测量的微观参量。     

±500 kV直流输电线路雷电屏蔽模拟试验研究

模拟试验是研究超/特高压输电线路绕击特性的一个重要手段,针对直流线路标称电场对线路雷电绕击特性的影响问题开展模拟试验研究,首先从模拟试验比例尺的确定、地闪先导过程近区电场的模拟和模型线路标称电场模拟三方面论证了试验的物理等价性;在计及直流电压的条件下,通过放电试验得出了±500kV输电线路G4–40型杆塔绕击空间分布;试验中观测到主放电击中导线(或避雷线)的同时,避雷线(或导线)上残存的迎面流注。依据试验现象,定性分析了标称电场对绕击特性的影响机理;对试验结果定量分析表明,计及直流电压后G4–40型杆塔正极绕击率为不考虑直流电压时的1.79倍。     

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     

电晕离子流对先导放电路径的影响

为探明当空气中含有电晕产生的离子时(离子背景),对雷电先导的发展、击穿过程和路径的选择有何影响,分别采用两种不同的实验方法研究了正、负离子流对先导放电进展路径影响的特点及规律,两种方法得出的一致结果表明,有离子背景的雷电先导放电路径具有选择负离子背景而回避正离子背景的特点和规律并运用电子崩发展和电场作用两方面原理,分析了离子背景的影响机制。这对于特高压输电线路与设备的防雷及其绝缘设计的进一步研究具有重要参考意义。     

Dynamic observation of needle-plane surface-discharge using theelectro-optical Pockels effect

Dynamic observation of surface charge distribution is the main advantage of the electro-optical Pockels effect technique over the dust figure technique, the photographic Lichtenberg figure technique, and the static potential probe scanning method. This technique is demonstrated here to observe the surface charge distribution deposited by partial discharge during application of one period of an 8 kV sinusoidal voltage to a needle-dielectric plane electrode system with zero gap spacing. Polarity effect and backdischarge are prominent in ac surface discharges. The observed polarity effect in the charge pattern shows that if the dielectric surface is initially free of surface charge, positive `streamer' channels burst out intermittently and radially from the needle tip resulting in a spoke-like surface charge distribution, while negative streamers expand almost uniformly in all radial directions resulting in a nearly circular surface charge distribution. This behavior results in quasi-permanent positive surface charges. Residual negative surface charge from prior discharges has a considerable influence on the trajectory of subsequent positive streamer discharges, i.e. causing deviation of positive streamer channels from the radial direction; whereas residual positive surface charge has little influence on the radial development of surface charge from subsequent negative discharges. This measurement technique has a potential for widespread application in investigating the dynamics of surface charging phenomena     

Influence of insulating barrier on the creepage discharge intransformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point     

基于长间隙放电研究雷电屏蔽问题的进展

长间隙放电是研究地面物体雷电屏蔽问题的最有效手段之一。首先介绍了国内外在雷电击距、直击雷防护措施的屏蔽性能试验和雷电迎面先导过程研究3个方面所取得的进展,并结合最新开展的长达6 m间隙尺度的放电试验观测,对现阶段上述3个方面研究所存在的问题进行了分析。认为基于雷电击距建立的电气几何模型(electric geometry model,EGM)仅适用于小尺度目标物的雷电屏蔽性能分析,现有的雷电屏蔽模拟试验方法仅能近似模拟不存在雷电迎面先导时的雷击过程,无法完全证明以提前流注发射模型装置为代表的非传统防雷装置具有比传统措施更优越的屏蔽性能。大尺度目标物的雷电屏蔽问题应聚焦于雷电迎面先导过程的研究,建立并完善雷电迎面先导过程的模拟试验方法和物理仿真模型。     

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。