大气压下湿度对气隙流注放电及击穿的影响

空气中存在的水汽会使空气介电特性发生改变，严重影响输电线路外绝缘特性。为此通过理论及实验研究了湿度对流注放电的影响。理论方面通过Wieland近似法对湿空气的放电参数进行修正，对比分析了干洁空气与湿空气的放电参数，计算了不同湿度下针板气隙流注放电参数，提出了湿空气下流注起始、传播判据以及击穿电压计算方法。实验方面搭建流注放电室，进行棒板间隙流注放电实验，实验结果与理论计算结果相符。湿空气放电参数表明：低电场下(E/p<0.3V/(cm·Pa))湿度增大对电离系数影响不大，强电场情况下湿度对电离系数有着明显的促进作用；低电场时附着系数随湿度增大而增大，强电场时附着系数随湿度增大而减小；针板间隙流注计算结果表明湿度增大会使流注峰值电子密度、流注半径增大，流注传播速度加快；棒板击穿实验及击穿电压计算结果表明，当湿度较低时，湿度对击穿电压的影响不大，当相对湿度达到约94%时，湿度增大使得击穿电压明显提升。     

大气压下不同气体的流注放电特性

大气压下气体放电通常表现为丝状放电形式。流注或先导放电是其气体击穿的起始阶段,相对于低气压下暗放电或辉光放电具有更复杂的演化特性。为了研究不同气体的流注传播特性,采用流体模型对1 cm平板电极中大气压下氮气、氧气,以及氮气混合20%、1%和0.01%氧气的双向流注传播过程进行了仿真计算。光电离作为源项加在流体模型中,在数值仿真时采用解多组Helmholtz方程代替Zheleznyak积分计算。仿真结果表明:氧气含量较低时,流注会出现分叉现象;氧气中流注发展速度较快;氧气中正流注通道半径较大。     

纳秒脉冲电压下变压器油预击穿特性的仿真分析

以针—球电极间隙变压器油为研究对象,基于场致电离机理,建立用于表述液体电介质流注预放电过程中载流子的产生及输运的偏微分方程,结合电场泊松方程,以及热扩散方程,仿真研究纳秒脉冲电压下变压器油预击穿特性。得到了预击穿过程中电场强度随电压幅值、极性以及脉冲上升沿时间的变化规律。仿真结果表明:流注速度随电压幅值的增大而增大;负流注相比于正流注轴向传播距离小径向传播距离大;负流注起始放电电压高于正流注,且起始速度大于正流注;正脉冲上升沿时间越短所形成流注半径越大,上升沿时间大于50 ns的负脉冲条件下产生的流注易消散。本文的研究工作和取得的结论有助于加深对变压器油中放电起始、发展过程的认识以及对液体电介质放电机制的理解。     

低温次大气压不均匀电场空气流注放电特性研究EI北大核心CSCD

搭建空气放电实验平台,并构建仿真模型,通过实验和仿真对低温次大气压不均匀电场空气流注放电特性展开研究。结果表明:当约化场强E/N相同时,海拔高度与流注分叉数目、流注传播速度、电流峰值以及电流衰减时间均呈负相关,与流注半径呈正相关,但仿真得到的流注传播速度和电流与实际相比略有不同;预电离区域的存在导致针-板和针-针电极结构下的放电形态不同;随着方波电压频率的增大,二次放电通道对于一次放电通道的依赖性逐渐增大。随着方波电压占空比的增大,二次放电通道仅在一次放电通道尾部有所体现。     

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。     

直流电压下油纸绝缘沿面闪络特性及数值模拟

变压器油纸绝缘沿面放电起始与发展的物理过程较为复杂,放电机理尚未明确.本文通过对油间隙放电针-板电极模型和油纸绝缘沿面放电针-板电极模型进行试验测量和数值模拟,获取并分析油间隙放电和油纸绝缘沿面放电在不同纸板厚度、不同沿面距离下的放电特性和放电机理.结果表明:直流电压下,油纸绝缘沿面闪络电压低于油间隙击穿电压的原因在于绝缘纸板的存在不仅改变了电场的分布,使针尖处平行电场分量增大,流注放电的起始电压降低,还阻碍了空间电荷的扩散,加剧电场畸变程度,提升流注的发展速度.另外,由于绝缘纸板的存在,油纸绝缘沿面放电流注的起始过程也不同于油间隙放电流注的起始过程,存在着流注通道向绝缘纸板贴合的过程.增大纸板厚度可使针尖处平行电场分量增加,流注放电的起始电压降低,流注的发展速度变缓,沿面闪络电压降低.油纸绝缘沿面闪络电压随沿面距离的增加而增大,其呈现非线性变化是由于流注在向自持放电阶段发展的过程中,受外施电压的作用逐渐减弱,空间电荷畸变电场的作用占主导地位.     

交流电压下变压器油中流注发展时空演化的光-电信号特征研究

流注放电多具有电离程度高、发展速度快及树状分叉等典型特征,是介质击穿前发生的各类放电起始、发展现象的统称。研究变压器油中流注现象对于加深液体介质放电机理的认识以及提升油纸绝缘性能具有重要意义。本文通过搭建流注光-电特性检测平台,获取了交流电压下针板电极结构中流注放电的光-电脉冲电流序列和光学图像。结果表明,负流注主要包括单个脉冲、幅值递增序列、幅值递减序列及幅值先增后减序列四种典型序列模式。负流注通道形状为球状,但较难向前发展,导致击穿电压较高;正流注电流脉冲序列为正脉冲簇形式,通道形状为细丝状且发展过程中伴随着随机分叉现象。结合流注发展电流序列和空间发展图像,对正、负流注起始与时空演化机理进行阐述,根据电容模型和流注通道压力平衡方程计算了正、负流注发展时的半径、速度特征参量,得到负流注通道半径约为80?m,正流注直径为60～80?m,平均速度约为3km/s,计算结果与实验现象相符。     

厘米级短间隙不均匀场正流注数值仿真分析

为了克服二维不均匀场流注仿真求解中数值求解复杂及难收敛的问题,笔者提出了一个准一维流注发展等效模型,实现了不均匀场中流注发展仿真。基于该流注发展等效模型,对典型的厘米级棒—板间隙中正流注发展微观物理过程进行研究,获得了空间电场、电势、电荷密度分布及流注发展速度等重要微观物理量的变化特性。同时讨论了等效模型中流注通道外形对流注放电物理特性的影响,确定了合理选择流注通道外形的原则。     

双极性振荡冲击电压下变压器油放电特性

当外部过电压侵入油浸式变压器绕组内部时,其匝间绝缘会承受双极性振荡冲击电压的威胁。在此通过双极性振荡冲击电压下变压器油放电特性的研究,为绕组纵绝缘的合理设计及液体放电机理的探究提供参考。首先通过可调电路元件,严格控制外施电压波形参数,实验获得了稍不均匀场与极不均匀场中变压器油在单一波形变量下的冲击放电特性,统计得到了放电电压概率分布与击穿点分布特征。然后利用分幅相机拍摄了双极性振荡电压下油中流注形貌,阐述了流注极性转换等主要发展特征。最后实验结果表明:无论电场分布是否均匀,振荡系数的增加、振荡频率的降低或波前时间的缩短均会导致变压器油50%击穿电压下降,且当振荡系数高于0.55时存在低于标准雷电耐受水平的可能;当振荡频率较低时,击穿多发生在第1或第2个正极性波峰内,而随着频率的升高,击穿点逐渐向后序波峰偏移,击穿于负极性波峰内的概率显著增加。结合光电倍增管记录的流注发光信号,从流注发展角度分析了变压器油击穿电压与击穿点变化的原因。     

短空气间隙放电流注分叉特性实验研究

短间隙流注放电通道在发展过程中产生的流注分叉现象能使放电所输出的能量和产生的活性物质分布更加均匀。为此,研究了短空气放电间隙长度和电极形状对流注分叉的影响。使用输出电压峰值为52 kV,电压上升时间约35 ns的脉冲电源分别对间隙长度≤5 cm的针–板、针–棒间隙进行放电,实验测试了不同间隙长度下流注的分叉特性。实验结果表明:间隙长度大于某一值(针–板间隙为3.5 cm,针–棒间隙为3 cm)时,观察到的流注分叉数目和分叉的发光强度随着间隙长度的增大而减少;而当间隙长度小于该值时,所拍摄的放电照片中流注通道很宽而且明亮,且很少有分叉。另外,相对于针–板间隙而言,在针–棒间隙中分叉的流注更容易向棒电极两端运动,产生重连接现象的概率更高。     

Thermal processes in a streamer discharge in water

The propagation features of a streamer discharge in water have been investigated. Based on the experimental data obtained in the study of water discharges in a nonuniform electric field, due propagation of streamers is explained as the evaporation of water at the tip of the streamer and around it. The energy balance in the process of the streamer propagation is calculated for a sub-microsecond discharge in distilled water. It is shown that the energy released in the pre-breakdown process is sufficient to evaporate the liquid in the streamer channels. Similar velocity of the streamer propagation in both tap and distilled water substantiates negligible effect of ionic current density onto the streamer propagation process. These estimations, based on experiment, have relevance to the discussion of the nature of the dielectric breakdown of water     

Variation of the Dynamics of Positive Streamer with Pressure and Humidity in Air

The dynamics of positive streamer in air under different pressure and humidity conditions was studied. The widely used three-electrode system was used to obtain the streamer propagation probability and average propagation velocity as a function of background field. A simulation was performed based on the 1.5D fluid model. Simulated intrinsic stability field for streamer propagation and corresponding average propagation velocity were in agreement with those deriving from experimental results. The intrinsic stability fields obtained with the empirical equations proposed by the author and other investigators were compared. It was analyzed that the effect of the amplitude and duration of voltage pulse applied on the needle. The statistical distribution of propagation field was explained by the dispersion of discharge time delay. The decrease of intrinsic stability field with the reduction of pressure and/or humidity was due to the increase of the conductivity of the streamer channel as a result of the weakening of attachment and/or electron-ion recombination effect.     

Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.     

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.     

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。     

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。     

空气中电晕放电与绝缘液体中流注放电的测量和分析

Insulation is one of the most important parts in a high voltage equipment.There are gaseous,liquid and solid insulations which are commonly used.In a high voltage transformer for example the insulating materials are all used.During operation of a high voltage equipment high electric stress may occur.Under extreme condition failure of the insulation may take place.Excessive electric field in air may cause corona discharges while in liquid insulation discharges may take place in the form of streamer.This paper reports experimental results on the corona and streamer discharges in air and silicone oil.The discharges were artificially generated around a needle tip in a needle-plane electrode system with gap length of 4 mm under sinusoidal and triangular voltages.The needle was made of steel with tip radius of 3 μm and curvature angle of 30°.The needle was made by Ogura Jewelry.The discharge pulses were measured using personal-computer based partial discharge(PD)measurement system with sensitivity of better than 0.5 pC.The system is able to measure discharge in time sequential.Phase-resolved analysis of the discharges was done to interpret the physical processes behind the discharges.The experimental results showed that corona discharges took place at negative half cycles.The discharges were concentrated around 270° of phase angle of applied voltage.The discharge magnitude and discharge number of corona clearly dependent on the instantaneous of applied voltage.These were strongly supported by the application of triangular voltage.Streamer discharges occurred at both positive and negative half cycles.The discharges pulses concentrated around the peak of applied voltage at phase angle of 90° and 270°.Experimental results under sinusoidal and triangular voltages revealed that streamer discharge magnitude as well as probability of occurrence was strongly dependent on the instantaneous applied voltage.     

气体成分对水中脉冲放电产生过氧化氢的影响

通过化学方法检测了水中脉冲放电产生的H2O2,通过拍摄照片研究流注长度的变化。实验发现,在分别通入氮气和氧气的情况下,随着气体流量和放电时间的增加,H2O2的浓度逐渐增加;在电极间距为5cm,所加电压为35kV时,通氮气比通氧气生成的H2O2多,通氮气时流注长度较通氧气时要长。在电极间距为3cm,所加电压为21kV时,通氧气比通氮气生成的H2O2多,而流注长度差异不大。     

PD characteristics as a stochastic process and its integralequation under sinusoidal voltage

This paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal waveform voltage conditions based on a simple PD model (an equivalent circuit model). The stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of PD delay time after the discharge inception voltage is built up across a discharge gap. The distribution of delay time is assumed to obey an exponential distribution and the dependence of the delay time on the overvoltage is assumed to be expressed by an exponential function of the overvoltage. The integral equation has been solved numerically under the low voltage condition in which only one PD can occur in a half AC voltage cycle. It is found that the proposed integral equation can provide the basic characteristics such as PD pulse repetition rate vs. applied voltage phase angle     

Computer analysis and observation of streamer growth at adielectric interface

A simulation study of streamer growth within a dielectric liquid at a solid interface was undertaken. Using the finite element method with computer graphics, the shape of the growing streamer was determined and plotted. In addition, the changing voltage and electric field values within the dielectric were calculated, and simulated images of these parameters produced. The behavior of the streamer is based on the formation of a localized gas phase discharge and the growth of the resulting plasma within the liquid. The model determines an inception site and predicts a growth path that depends on the electric field immediately surrounding the streamer tip. Two liquids, n-hexane and water, both with an interface, were investigated using the simulation model. The effect of the choice of the liquid and the solid dielectric, in addition to the geometry, was explored. For mixed solid and liquid insulation, the results show that streamer growth depends on the permittivity mismatch between the two materials. Experimental findings are presented that show streamer growth for different interfacial conditions. A good correlation is observed between the theoretical and experimental work