Study on transition process of vacuum arc under transverse magnetic field

The transverse magnetic field (TMF) drives the vacuum arc to move along the surface of the contacts to prevent the local overheating and melting of the contact surfaces. The arcing process has great influence on the breaking capacity of short‐circuit current. In this paper, the arcing process between three types of TMF contacts was investigated. The transition process of an arc from the ignition stage to the diffusion stage was discussed. The transition moment, transition gap distance, and transition current were obtained. It was found that the axial magnetic field component of TMF contacts affected the arc transition process.     

Study on arc mode and shape transformation in the arc motion stage between transverse magnetic field contacts

Transverse magnetic field (TMF) contacts drive the vacuum arc moving along the contact surface by producing a magnetic field perpendicular to the arc current flow, which makes the arc energy distribute uniformly and prevents the contact from serious ablation due to local overheating. The characteristics of arc motion between the TMF contacts directly determine the surface temperature distribution and contact erosion. Also, it is closely related to the breaking ability of the contacts. In this paper, the arc motion between spiral‐type contacts was recorded by a high‐speed camera. By analysing the arc voltage and arc images, the characteristics of arc shape transformation in the motion stage under different currents were studied. At the same time, the influence of arc duration and arc characteristics before the motion stage on the arc motion is discussed. It is found that the arc behaviour before the motion stage did influence the shape transformation and range of arc rotation. After entering the motion stage, there was always a rapid transformation process of the arc shape and the arc would return to where it first became constricted after a short movement. After a continuous movement, it might rotate in a small region where the arc stagnated before the motion stage. In addition, the arc behaviour was also related to its duration. When the opening time was varied from 2 to 5 ms, the arc velocity decreased. Meanwhile, it was more likely that multiple transformations of the arc aggregation degree occurred.     

Investigation on arc characteristics in cup-shaped contacts with different structure for transverse magnetic field vacuum circuit breaker

The cup-shaped transverse magnetic field (TMF) contacts contain radial components and tangential components in the TMF generated when the current is interrupted. The tangential force generated by the radial magnetic field component drives the vacuum arc to rotate, and the tangential magnetic field component generates a radial force that causes the vacuum arc to move radially outward. In this paper, in order to study the influence of the arc force direction on the arc characteristics, the influence of the contact structure parameters such as the inclination of the inner wall on the arc force direction is simulated, and the breaking tests of different levels of current are carried out on the contact with different structure parameters. It is found that the direction of the Lorentz force has a significant effect on the breaking characteristics of the current, and the tangential and radial force components have varying degrees of influence on the motion characteristics of the arc during the start process and the metal droplet splashing.     

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

With the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de-mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high-speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi-cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.     

纵磁结构真空灭弧室电磁场及电弧运动特性

为了了解纵向磁场下的电弧运动过程,建立了真空灭弧室的3维仿真模型。采用PIC模拟方法对12 kV灭弧室内的电磁场、电弧运动特性进行计算仿真。通过改变触头间距、屏蔽罩尺寸和触头开槽宽度,研究了灭弧室内的电场、磁场分布;对不同触头间距下随时间变化的电弧运动过程和触头表面的电弧分布情况进行了模拟计算。计算结果表明:在真空灭弧室中适当设置屏蔽罩,可有效改善灭弧室内的电场分布;触头铜基上的开槽宽度对磁场会产生影响,宽度越大,磁场强度越大。     

Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

Physical and theoretical aspects of a new vacuum arc controltechnology-self arc diffusion by electrode: SADE

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment     

纵磁作用下真空电弧单阴极斑点等离子体射流三维混合模拟

真空电弧的特性直接受到从阴极斑点喷射出的等离子体射流的影响,对等离子体射流进行数值仿真有助于我们深入了解真空电弧的内部物理机制.然而,磁流体动力学和粒子云网格仿真方法受限于计算精度和计算效率的原因,无法有效地应用于真空电弧等离子体射流仿真模拟.本文开发了一套三维等离子体混合模拟算法,并在此基础上建立了真空电弧单阴极斑点射流仿真模型,模型中将离子作宏粒子考虑,而电子作无质量流体处理,仿真计算了自生电磁场与外施纵向磁场作用下等离子体的分布运动状态.仿真结果表明,单个阴极斑点情况下真空等离子体射流在离开阴极斑点后扩散至极板间,其整体几何形状为圆锥形,离子密度从阴极到阳极快速下降.外施纵向磁场会压缩等离子体,使得等离子体射流径向的扩散减少并且轴线上的离子密度升高.随着外施纵向磁场的增大,其对等离子体射流的压缩效应增强,表现为等离子体射流的扩散角度逐渐减小.此外,外施纵向磁场对等离子体射流的影响也受到电弧电流大小的影响,压缩效应随电弧电流的增加而逐渐减弱.     

Voltage of the vacuum arc with a ring anode in an axial magneticfield

The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40° in the zero magnetic field case and approaches a value of about 20° with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found     

Separation of spiral contacts and the motion of vacuum arcs at highAC currents

A framing camera is used to photograph the vacuum arc between separating spiral-petal vacuum interrupter contacts. The rupture of the molten bridge between the contacts first leads to a high-pressure, transient arc column. This arc motion can become constricted for several milliseconds before it goes diffuse as the current decreases to zero. The current through the spiral contacts produces a magnetic field perpendicular to the arc column, which forces the arc to move outward and run along the periphery of the petals. Several vacuum arc modes occur during the half-cycle of high current arcing. Movies, gap-current curves, and arc voltage traces are used to study the development of the arc motion and how it is affected by the contact structure. This information is used to generate arc appearance diagrams in which the arc form and motion are correlated to instantaneous values of current and gap for a wide range of peak currents. Appearance diagrams are shown for two ranges of opening delay from current onset     

Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing charge-coupled device camera of 1 μs exposure time. The experiments were performed with cup-shaped contacts, with sinusoidal currents of amplitudes between 20 and 100 kA, and a sine halfwave duration of 10-12 ms. The arcs were drawn by contact separation and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. The anode and cathode arc roots behave reproducibility and arc scaleable within the range of currents investigated. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points are in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For currents of 20-100 kA, mean current densities of 81-121 and 41-60 kA/cm ² were found in anode and cathode arc roots, respectively. Estimations of their temperature and vapor densities were performed. For the investigated current range T_A≈3300-3600 K, n_A ≈1.6*10¹⁹-2.2*10¹⁹cm^-3 and T _C≈3200-3400 K, n_C≈0.8*10¹⁹-1.2*10 ¹⁹ cm^-3 were found for anode and cathode, respectively     

Characteristics of a vacuum switching contact based on bipolaraxial magnetic field

Axial magnetic field (AMF) contacts can increase the interruption capability of vacuum interrupters. Depending on the design, the principles of the local axial field arrangement are different. For unipolar arrangements the direction of the axial magnetic field is the same within the whole contact area. For bipolar arrangements, the polarity of the field changes once. In this paper investigations have been carried out to characterize a bipolar AMF contact system and to test its interruption performance. The influence of the bipolar AMF on the arc development and the thermal stress is described by high speed camera and contact surface pictures. In addition, three-dimensional AMF simulations have been performed by means of a finite-element program to estimate the influence of slots within the contact plates on the AMF performance. The high interruption capability of the bipolar AMF contact system has been verified in different test laboratories up to 12 kV/80 kA_RMS (symmetrical) and 36 kV/40 kA_RMS (including 40% DC) by three and single phase tests. The investigations are completed by measuring the post-arc current and the shield-potential during recovery period, both describing the switching behavior of the contact system     

The behavior of vacuum arcs between spiral contacts with small gaps

The application of small gaps in high-current vacuum interrupters highlights the interdependence of the contact design, the contact gap, and the arc behavior. In this investigation, a framing camera was used to record the appearance and motion of drawn vacuum arcs between spiral-petal contacts with final gaps of 2 to 3 mm. After the rupture of the molten metal bridge, a high-pressure arc column formed and expanded across the width of the spiral arm. With a single arc column for the duration of the half-cycle, an intense anode spot formed if the peak current exceeded ~15 kA. Compared to results previously obtained at larger gaps, the arc motion was greatly reduced, and severe contact damage was observed at lower currents     

Experimental observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps     

Angular distribution of ion current emerging from an aperture anodein a vacuum arc

The ion current distribution emerging from a vacuum arc between a Cu cathode and a conical ring anode was measured by a set of five probes. It was found that: (1) the total ion current emerging through the anode was 8.5% of the arc current; (2) the measured ion distribution without a magnetic field was a slightly flattened cosinusoidal function; (3) with an axial magnetic field, the ion current distribution became peaked along the z axis; (4) the total ion current extracted through the anode aperture slightly increased with the magnetic field; and (5) an anode with a larger aperture exhibited less magnetic collimation     

Motion of high-current vacuum arcs on spiral-type contacts

Motion of vacuum arcs on spiral-type contacts is not only controlled by self-induced magnetic fields, but also by heating phenomena. An expression is derived which permits the calculation of the speed of the arc from a computation of the time needed to heat the surface up to boiling temperature. The heat flux density of the constricted arc at the anode is required as input for the calculation. Good coincidence is achieved with experimental data. The speed of the arc varies from 5 to 400 m/s, depending on experimental conditions     

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.      

Axial magnetic field contacts with nonuniform distributed axial magnetic fields

It is well known that axial magnetic fields (AMFs) can keep vacuum arc in diffuse mode at high current. According to our recent research and other published papers, it has been found that vacuum arc can be maintained in high-current diffuse mode at much higher current if nonuniform AMF is applied, that the axial magnetic field is higher at contact periphery than at center. The influence of spatial distribution of AMF on vacuum arc is mainly studied in this paper. Furthermore, two types of AMF contacts with new structures to generate nonuniform AMF are presented.     

The influence of contact surface microstructure on vacuum arcstability and voltage

The authors present the results of DC vacuum arc lifetime and voltage measurements for contacts with different surface microstructures. This was realized by treating the contact surface with emery paper of varying roughness. The contact surface microstructure was found to have a large effect on the DC arc lifetime (several tens of times difference at the most) and arc voltage (30% difference at the most). The mechanism of the surface microstructure effect on DC arc stability was analyzed and is explained preliminarily. The rougher the contact surface is, the longer the arc lifetime and the more stable. The arc itself has a tendency to condition the surface in a way that is unfavorable for a sustained arc lifetime. The crater size for a rough surface is smaller than for a smooth one. The rougher the contact is, the lower the arc voltage (both DC and HF components). The arc erosion has a tendency to increase the arc voltage. It is probable that type I cathode spots (fast motion, small crater size) correspond not only to the surface contamination, but also to the surface roughness