纳秒脉冲下变压器油两相流注放电仿真研究

为揭示液体电介质击穿过程中形成的气体放电通道对液体电介质放电过程的影响,以针—板电极间隙变压器油为研究对象,基于等离子体流体力学模型,引入了液体电介质放电过程中气相放电通道对电离机制及自由电荷迁移率的影响,建立了用于模拟脉冲电压下液体电介质放电过程的两相流体模型,仿真研究了纳秒脉冲下针板电极流注放电的起始与发展过程。仿真结果表明:采用Heaviside方程可以在模型的不同区域同时实现气相物理过程和液相物理过程的模拟与计算。气相物理过程的引入导致流注尾部电场显著降低,流注头部电场进一步增强,使流注通道的发展速度要高于传统液相模型,有助于加深对纳秒脉冲下液体电介质中预击穿流注的起始、发展过程的认识和理解。     

Study of streamer development in high-pressure electric discharge: Applications to excimer lasers

This work represents a study of the streamer formation in plasma for XeCl excimer laser at high pressure. It is based on a longitudinal mono-dimensional model of the cathodic zone. In this model, we show the possibility of the streamer development in the cathodic sheath and its propagation during the phase of plasma formation. The model gives the space and time evolution of the electron density and the discharge electric field in the presence of the streamer. The obtained results clearly indicate that, for conditions close to experiments for 50–100 ns laser pulse durations and electron power deposition in the MW/cm³ range in a 300 cm³ chamber, the streamer instability, related to the sheath evolution, patently appears. The drift velocity reaches a typical value of about 10⁸ cm/s.     

A new static calculation of the streamer region for long spark gaps

Different electrostatic approximations have been proposed to calculate the streamer region without going in deep details of the behavior of density of particles under the effect of high electric fields; this kind of approximations have been used in numerical calculations of long spark gaps and lightning attachment. The simplifications of the streamer region are achieved by considering it to be a geometrical region with a constant geometrical shape. Different geometrical shapes have been used, such as cones or several parallel filaments. Afterward, to simplify the procedures, the streamer region was approximated by two constants, one denoted K_Q, called the geometrical constant and in other cases K named as geometrical factor.However, when a voltage that varies with time is applied to an arrangement of electrodes (high voltage and grounded electrodes), the background electric field will change with time. Thus, if the background electric field is modified, the streamer zone could cover a larger or smaller area.With the aim of reducing the number of assumptions required in the calculation of long gap discharges, a new electrostatic model to calculate the streamer region is presented. This model considers a variable streamer zone that changes with the electric field variations. The three-dimensional region that fulfills the minimum electric field to sustain a streamer is identified for each time step, and the charge accumulated in that region is then calculated. The only parameter that is being used in the calculation is the minimum electric field necessary for the propagation of streamers.     

Mechanism for propagation of a positive leader

A model of leader breakdown in air is considered. The channel is formed due to heating of the streamer trace in the field of the streamer zone. A previous model of a streamer is generalized with allowance for recombination of charged particles. A mathematical model of heating of the streamer trace is developed. It is demonstrated that, at a given potential, the ignition of the channel is provided by streamers that possess a certain charge and the corresponding propagation velocity. This velocity determines the propagation velocity of a steady leader. The dependence of the leader velocity on the cloud potential is found. The results obtained are compared with the data from in-situ observations and laboratory studies.     

Local electron mean energy profile of positive primary streamer discharge with pin-plate electrodes in oxygen nitrogen mixtures

Local electron mean energy (LEME) has a direct effect on the rates of collisional ionization of molecules and atoms by electrons. Electron-impact ionization plays an important role and is the main process for the production of charged particles in a primary streamer discharge. Detailed research on the LEME profile in a primary streamer discharge is extremely important for a comprehensive understanding of the local physical mechanism of a streamer. In this study, the LEME profile of the primary streamer discharge in oxygen-nitrogen mixtures with a pin-plate gap of 0.5 cm under an impulse voltage is investigated using a fluid model. The fluid model includes the electron mean energy density equation, as well as continuity equations for electrons and ions and Poisson’s electric field equation. The study finds that, except in the initial stage of the primary streamer, the LEME in the primary streamer tip tends to increase as the oxygen-nitrogen mole ratio increases and the pressure decreases. When the primary streamer bridges the gap, the LEME in the primary streamer channel is smaller than the first ionization energies of oxygen and nitrogen. The LEME in the primary streamer channel then decreases as the oxygen-nitrogen mole ratio increases and the pressure increases. The LEME in the primary streamer tip is primarily dependent on the reduced electric field with mole ratios of oxygen-nitrogen given in the oxygen-nitrogen mixtures.     

A model for streamer propagation

A model is presented to describe the propagation of positive corona streamers in the low field region of a non-uniform field gap in atmospheric air. It has been assumed that the growth is a property solely of the streamer tip, uninfluenced by the channel conductivity. Calculations from the model indicate that the criterion for propagation of a streamer in zero external field is that the number of ions in the tip be 10⁸ and the radius about 3×10^?3 cm. It is proposed that the streamer ceases to propagate as a result of the loss of energy of the tip due to the formation of ion pairs in the channel. The results of previous experimental observations of streamers are compared with calculations derived from the model, and a prediction from the model of the lifetime of streamers after voltage removal is discussed.     

正极性纳秒脉冲电压下变压器油中流注放电仿真研究

建立了二维轴对称流体模型, 仿真研究了正极性纳秒脉冲电压下变压器油中针-板电极流注放电的起始与发展过程, 得到了不同的外施电压幅值、脉冲上升沿时间与电极间隙距离下油中流注放电的形貌、 电场强度与空间电荷密度分布等. 仿真结果表明: 空间电荷加强了流注头部前方电场, 使流注通道更易于向前推进, 形成"电离波"; 随着外施电压幅值升高, 流注发展的平均速度显著变大; 较陡的脉冲上升沿形成的放电半径较大, 对应的最大电场强度值变小; 随着电极间隙距离的增加, 流注发展平均速度变快. 仿真显示纳秒脉冲下放电中油温无明显升高, 表明此类放电过程没有明显的油气化现象. 我们认为, 场致电离是油中带电粒子产生的主导机制; 空间电荷效应增强流注前方电场使得电离进一步发展, 最终导致击穿. 本研究有助于加深对变压器油中放电起始、发展直至击穿过程的认识以及对液体电介质中电离机制的理解. 关键词： 变压器油 流体模型 流注放电 空间电荷效应     

Spatially hybrid computations for streamer discharges : II. Fully 3D simulations

We recently have presented first physical predictions of a spatially hybrid model that follows the evolution of a negative streamer discharge in full three spatial dimensions; our spatially hybrid model couples a particle model in the high field region ahead of the streamer with a fluid model in the streamer interior where electron densities are high and fields are low. Therefore the model is computationally efficient, while it also follows the dynamics of single electrons including their possible run-away. Here we describe the technical details of our computations, and present the next step in a systematic development of the simulation code. First, new sets of transport coefficients and reaction rates are obtained from particle swarm simulations in air, nitrogen, oxygen and argon. These coefficients are implemented in an extended fluid model to make the fluid approximation as consistent as possible with the particle model, and to avoid discontinuities at the interface between fluid and particle regions. Then two splitting methods are introduced and compared for the location and motion of the fluid-particle-interface in three spatial dimensions. Finally, we present first results of the 3D spatially hybrid model for a negative streamer in air. Future applications of the hybrid model lie in effects of electron density fluctuations on inception, propagation and branching of streamers, and in accurate calculations of electron energies at and of electron run-away from the streamer head. The last is relevant for hard radiation from streamer-leader systems and possibly for Terrestrial Gamma-Ray Flashes.     

Mechanism for the streamer propagation toward the anode and cathode due to background electron multiplication

A simple mechanism for the propagation of an ionization wave in a dense gas due to the multiplication of background electrons in a nonuniform electric field is proposed. The mechanism does not depend on the sign of the field projection onto the streamer propagation direction. The streamer propagation is caused by the enhancement of the electric field at the streamer head. It is shown that, in a prebreakdown field, the intense multiplication of electrons takes place in both electropositive and electronegative gases. The prebreakdown multiplication can provide a fairly high density of background electrons; this allows one to treat the background as a continuous medium when considering streamer propagation as a multiplication wave. The initial ionization is enabled by the natural background of ionizing radiation and cosmic rays. An analytical expression for the velocity of the ionization front is obtained based on a simple equation for the multiplication of background electrons. This expression is in good agreement with numerical simulations performed within both a simple model of background electron multiplication and a more comprehensive drift-diffusion model. In particular, the drift-diffusion model predicts the propagation of the ionization front from a small-radius anode to the cathode due to the multiplication of background electrons. The velocity of the ionization wave front is calculated as a function of the electric field at the streamer head for helium, xenon, nitrogen, and sulfur hexafluoride. It is shown that some features of streamer propagation (e.g., its jerky motion) can be related to the recently found nonmonotonic dependence of ionization frequency on the electric field.     

Production of chemically active species in the air by a singlepositive streamer in a nonuniform field

We report the results of a two-dimensional (2-D) simulation of a cylindrically symmetric positive streamer in atmospheric pressure air between a positively stressed point and a grounded plate. A hydrodynamic diffusion drift model was used. It is shown that a streamer propagates with the constant velocity and constant field at the tip (≃140 kV/cm) and constant electron density in the head (≃10¹⁴ cm^-3). Special attention has been paid to the generation of excited molecules, radicals, and atoms [chemically active species (CAS)] by the streamer, 2-D distributions of CAS are obtained. The total number of CAS linearly increases with time (except excited oxygen molecules) in accordance with the constant velocity of streamer propagation. The mean energy cost of each active specie is 35 eV     

光电离速率影响大气压空气正流注分支的机理研究

大气压空气中的流注放电有广泛的理论和应用研究价值,包括雷电机理、输变电系统空气绝缘理论以及材料表面改性等.流注是一个快速发展的强电离区域,在传播过程中存在着一种重要的特点—分支现象.光电离为正流注发展提供必要的自由电子,且实验结果表明分支特征与流注头部的光电离速率密切相关.本文基于新的流注分支判据,采用了粒子网格单元与蒙特卡罗碰撞相结合(PIC-MCC)的三维放电模型(Pamdi3D)进行数值仿真验证.为了研究光电离速率对正流注分支的影响,仿真了毫米尺度间隙针-板电极正流注发展,系统研究了不同光电离参数的影响.当减小氮气-氧气比例、光子吸收截面或光电离效率系数后,流注均更早地出现分支现象.这些计算结果表明大气压空气中流注头部光电离速率的降低将导致其发生分支的概率更高.     

Streamer propagation mechanism on the basis of plasma oscillations

A mechanism explaining streamer propagation to the anode and to the cathode without photoionization of the gas is proposed. It is shown that the velocity of the anode streamer is determined by the electron mobility in the field of the end of the streamer. The accelerated electrons appearing at the anode end of the streamer excite longitudinal plasma oscillations in the streamer, whereupon streamer propagation to the cathode is explained. An explanation is given of the phenomenon of streamer self-propagation after the disappearance of the external field because of the stored energy of the plasma oscillations. The conditions for the transition of an avalanche into a streamer is defined as the condition for the appearance of accelerated electrons at the anode end of the avalanche.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 34–39, July, 1977.     

Untersuchungen über die Entwicklung des Kanaldurchschlags in Gasen

The temporal development of the current of a discharge leading to breakdown was investigated under static and homogeneous field conditions with wide band oscilloscopic techniques. The discharges were started by 10³ to 10⁵ electrons released within some 10^?9 sec along traces of single α-particles parallel to the electric field. Measurements have shown that streamer mechanism occurs in gases as CH₄ and CO₂ at static breakdown, if a sufficient density of space charge is produced by a great number of overlapping avalanches. The gas amplification of thesingle avalanche only has a value of about 10⁵. Also in O₂ and dry air at highpd-values breakdown develops with streamer mechanism. — In electronegative gases as freon 12 and O₂ a prolongation of the time necessary for streamer formation is caused by the reduction of the effective total space charge by the negative ions. The results of a detailed study of the time necessary for streamer development and the time constant of the increase of the current leading to breakdown confirm the model of streamer mechanism.     

Streamer radius model and its assessment using two-dimensionalmodels

The variable radius method is proposed to approximate the radius of the ionization channel in the one and a half-dimensional (1.5-D) fluid models for studying streamer development, in which the unreasonable constant radius in the traditional 1.5-D fluid models is corrected. The streamer development and propagation between the 1.5-D fluid model with the variable radius method and the two-dimensional (2-D) fluid model using the same initial and external conditions are compared. The radius in each stage of streamer development from the 1.5-D fluid model with the variable radius method shows agreement to a certain degree with that of the 2-D fluid model. The purpose of this paper is not to negate the role of the 2-D fluid models, but to explore the potential of the 1.5-D fluid models and make them more useful and accurate as well as to understand the evolution of streamer radius. The streamer development from the 1.5-D fluid model with the variable radius method not only maintains simplicity of the 1.5-D fluid models, but also presents agreement with the 2-D fluid models for streamers     

Steady-state leader breakdown. Nitrogen atmosphere

Physical arguments about the possible mechanism of formation of a leader channel are presented. A mathematical model describing steady-state leader breakdown is constructed. An algorithm for determining the propagation velocity, dimensions, and electric field in the streamer zone is developed. A numerical simulation of the channel formation stage in the plasma of a streamer zone a nitrogen atmosphere is performed. The dependence of the leader velocity on the potential is obtained. The tentative model proposed here can be used to describe the leader breakdown of a long gap at high positive potentials. Zh. Tekh. Fiz. 68, 37–44 (June 1998)     

Streamer Rest Mass and its Limit Velocity

Based on the nonlinear plasma-waveguide model of the electrical breakdown of gases, experiments on neutron generation in electric discharge processes are considered. An analysis of these processes makes it possible to determine the lower limit of the streamer mass as 10^?23 g and the existence of the upper limit of the streamer propagation at the level of 10⁹ cm/s.     

Incipient electric field determination for bush and streamer stage in dielectric liquid under energy balance condition

This paper introduces a new approach for streamer advance mechanism in dielectric liquid. The existing of bush-like streamer shape early and then a tree structure shape after that has been given an over view and definition by devising a breakdown index for dielectric liquid which reach a value of 25. The deviation of streamer velocity from low values of tens of meter per second, until several kilometer per second from bush-like shape, until complete breakdown has been discussed too. These different stages have been studied on an energy balance concepts. In this paper using energy balance analysis, different critical applied fields have been obtained. These values reach 2.18 MV/cm for one branch channel in bush-like streamer shape and 21.5 MV/cm, for tree streamer. After that, the initial streamer velocities concerning these stages have been introduced. From these analysis the dissociation of dielectric liquids starts when the streamer velocity reach the sound speed in air, 331 m/s. In addition, the dissociation field depends mainly on the physical values of the dielectric medium, such as density, and permittivity have been introduced. In this paper the dissociation starts at an electric field value of 21.5 MV/cm for nearly all dielectric liquids, This result is equal to tree streamer inception value, which can be considered as a new introduced finding. A new energy equation relating injected energy electric field, velocity and new deduced breakdown index in dielectric liquid has been devised. The streamer may stop or continue its advance until complete breakdown. According to many published data for streamer, there is no clear explanation for streamer stopping and continuing it advance. In this paper, the streamer must advance ahead of the bush zone in the gap toward the opposite electrode when the prospective electric field at 66% of the gap achieves a breakdown index of 25. This result can be considered as a new criterion for streamer growth until crossing the gap. These new equations and findings have been applied to several experimental works and achieve good results.     

针-板空气间隙流注放电起始过程的三维PIC/MCC仿真研究

流注放电作为自然界中闪电传播的预电离机制、高压输变线路间长空间间隙放电的重要初始阶段,在工业领域存在诸多潜在应用,近年来引起人们越来越多的关注.流注放电具有典型的多尺度、非线性的放电特征,实验观测中多呈现出分叉等不规则结构.为了研究其微观结构特性和行为特征,本文采用三维粒子仿真模型(PIC/MCC),着重研究了流注从针型正电极的起始和发展过程.模型采用了可变自适应网格、可变粒子权重以及并行计算等技术,有效地降低了三维粒子仿真的计算时间.通过调节针型电极上的施加电压幅值、改变气体组分及调整电极形状尺寸等,研究了放电参数变化对流注放电的分叉结构、半径等行为的影响.模拟结果表明:随着电压的升高,流注的半径及分叉数目增加;对比不同气体组分(纯氧以及不同比例氮氧混合气体),发现其对流注的分叉数目影响较为显著;针型电极结构直接影响了流注的起始时间和形貌.     

An integral approach to considering the evolution of a microwave streamer

We present a model of a microwave streamer based on analytical relations that allows its evolution to be described both during its elongation along the external electric field and after its stop. The equations for the electric field amplitude at the streamer center, the equations that describe the dynamics of the longitudinal and transverse sizes, and the equations of plasma chemical kinetics are self-consistently solved in terms of this model. Comparison with numerical 2D simulations of the electrostatic stage in air is made. We derive simple analytical relations that allow the power released in a plasma channel to be estimated on the fly. Almost the entire energy contribution is shown to be made after the termination of the streamer elongation.     

Step propagation of a streamer in an electronegative gas

It is shown on the basis of numerical simulation and an experimental investigation that a streamer can propagate in a step manner in an electronegative gas. The experiments and most calculations were performed for air under close to normal conditions. The step motion is associated with the appearance of a secondary ionization wave near the electrode and propagation of this wave along the channel of the streamer; this wave maintains the channel in a conducting state and allows for the propagation of the streamer in a nonuniform external field over distances which are inaccessible under ordinary conditions of a streamer discharge. Simulation in heated air, oxygen, and SF₆ demonstrated that the phenomenon studied is common for various gases and that the special features of its manifestation remain in a wide range of decay rates of the streamer channel.