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The splitting and threshold currents of vacuum arc cathode spots in axial magnetic fields (AMFs) up to 220 mT are investigated experimentally. Measurements are made on electrodes of Cu and CuCr25 with electrode separations of 2 and 6 mm. The electrode diameter is 30 mm. All the experiments are conducted in a demountable vacuum chamber evacuated continuously by a turbo-molecular pump to a pressure of the order of 10−3 Pa. The spots are photographed using a high-speed digital camera with an exposure time of 2 μs. The AMF is applied by means of two permanent magnets placed symmetrically inside the chamber. The experimental results indicate that the splitting and threshold current decrease in the presence of AMF and tend to certain constant values when the field gets stronger. The splitting and threshold currents are considerably larger than Cu electrodes than those with CuCr25 electrodes. With increasing electrode separation, the splitting current decreases while the threshold current increases. Possible explanations are proposed to account for the experimental observations. [ABSTRACT FROM AUTHOR]

The dielectric recovery process is a crucial stage of current interruption in a vacuum circuit breaker (VCB). The postarc current is an important macroscopic electric characteristic of the dielectric recovery process, which has caused many concerns in the past few decades. In this paper, the postarc currents of VCBs with different types of contacts, such as butt contacts, axial magnetic field contacts, and transverse magnetic field contacts with different materials are measured with a high-resolution current-zero diagnostic system. Besides, the appearance of vacuum arcs is recorded using a high-speed camera in the experiments. The relationship between the arc memory effect and the postarc current is analyzed. What is more, the influence of the instability of the vacuum arc shape on the dispersion of postarc current is also studied. [ABSTRACT FROM AUTHOR]

A quench protection switch (QPS) is indispensable to protect the magnet coils from the damage of quench in a superconducting Tokamak. In this paper, a QPS based on artificial current zero is involved. The vacuum circuit breaker (VCB), which is driven by a high-speed electromagnetic repulsion mechanism, is used as the main circuit breaker (MCB). Two kinds of commercial vacuum interrupters (VIs), which have contacts generating axial magnetic field (AMF) and transverse magnetic field (TMF), respectively, are applied. The breaking current with an amplitude of 10~25 kA is generated by an LC oscillating circuit. The countercurrent with frequency in the range of 500–3000 Hz is provided by a commutation branch. The interruption performance of the two kinds of VIs under different breaking current and countercurrent frequencies is investigated. The experimental results indicate that the differences in interruption performance under the two types of contacts are increasing with larger frequencies of countercurrents. [ABSTRACT FROM AUTHOR]

The Robson drift phenomenon is a macroscopic reflection of the movement of a cathode spot (CS) in oblique magnetic field (OMF). Although it was discovered many years ago, its physical mechanism remains unclear. In this paper, the influence of OMF on the Robson drift of CS motion is simulated. A 2-D stepwise model of the movement of a single CS in OMF is established. It is assumed that the probability of generating a new spot in directions of Amperian force, anti-Amperian force, and directions perpendicular to Amperian force, is proportional to the magnetic pressure around the old spot. The drift angle and velocity of the CS in OMF are simulated. The simulation results show that the drift angle of the CS in OMF has liner relationship with the inclination angle of magnetic field, and it has no obvious relationship with the strength of the applied magnetic field and the amplitude of the arc current, which are in a good agreement with the experimental results. Besides, under the same angle of magnetic field, the velocity of spot motion increases linearly with the strength of OMF. Moreover, the simulation results show that the transverse magnetic field component has a much more predominant influence on the CS velocity than the axial magnetic field component, which are also in agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Fast interruption is vitally important in high-voltage direct current (HVDC) system to suppress the rapid increase of short current. In this paper, a multi-physics coupling simulation model of a high-speed electromagnetic repulsion mechanism (ERM) which is a part of HVDC breaker is proposed. The opening process with an electromagnetic buffer of ERM is investigated. The simulation results including forces, currents, displacements, and velocities are analyzed. Furthermore, a prototype of ERM is designed and developed. Results show that the efficacy of the proposed simulation methodologies is verified by the experimental results. The opening process of ERM could be divided into six stages. During an opening action, the impact of collision on the motion of ERM cannot be ignored. A good buffering effect can be achieved by reasonably setting the related parameters of the electromagnetic buffer. [ABSTRACT FROM AUTHOR]

The quench protection switch (QPS) is an indispensable component ensuring the safety of the magnet coils of a superconducting Tokamak when a quench happens. In this paper, the second commutation process of a QPS based on artificial current zero is investigated. In this process, the current, which has already transferred from the bypass switch to the main circuit breaker, is forced to commutate from the vacuum circuit breaker (VCB) to the discharge resistor by the countercurrent. An LC oscillating circuit is applied to generate oscillating current to simulate the direct current near its peak which is about 20 kA. The countercurrent with frequencies of 500 and 1000 Hz is provided by a commutation branch. The equivalence of the experimental interrupting process compared with the practical situation is analyzed. Two kinds of vacuum interrupters of the VCB are used in experiments. One kind is equipped with contacts generating transverse magnetic field, and axial magnetic field contacts are used in another one. The experimental results indicate that the initial process of the vacuum arc before injecting the countercurrent has crucial impacts on the interruption performance. The countercurrent with the frequency of 500 Hz is easier to interrupt the current than the case of 1000 Hz. [ABSTRACT FROM AUTHOR]

The residual axial magnetic field (AMF) at current zero, which is caused by large \mathrmdi/\mathrmdt during the forced current-zero stage of DC interruption based on the technique of artificial current zero, has a significant influence on the interruption performance of the vacuum circuit breaker. In this paper, a model of AMF contacts system is developed by finite-element-analysis software Ansys. The eddy current induced in the contacts system is also analyzed by an equivalent circuit. The influence of the frequency of countercurrent and the characteristics of saturable reactor, which results in the change of \mathrmdi/\mathrmdt before current zero, are investigated. The results demonstrate that the frequency of countercurrent has a significant influence on the residual AMF at current zero. The residual AMF decays exponentially after current zero with a time constant independent of the countercurrent frequency. The residual AMF can be reduced by using a saturable reactor. The effect of a saturable reactor depends mainly on the duration of the low \mathrmdi/\mathrmdt stage before current zero. [ABSTRACT FROM AUTHOR]

The motion of cathode spots plays a crucial important role in removing oxide layer on a metal surface by vacuum arc. In this paper, the characteristics of the motion of a single cathode spot on metal surface with oxide layer are investigated experimentally. Experiments are conducted in a detachable vacuum chamber. A hollow copper anode with a hole of 10~\text mm \times 10 mm is used in order to observe the 2-D motion of the cathode spot. The motion of the cathode spot during the descaling process is photographed by a high-speed digital camera with an exposure time of 2~\mu \texts . Experimental results indicate that there are slow motion and fast motion of the cathode spot during descaling process, and the slow motion is the basic characteristic in most of the descaling time. The probability distribution of the cathode spot’s displacement and resident time, the average velocity, and the movement parameter S^2/t are analyzed quantitatively. The results indicate that with the increase of gap distance or the decrease of oxide layer thickness, the cathode spot becomes more active. [ABSTRACT FROM PUBLISHER]

In this paper, the influence of an external transverse magnetic field, in parallel with the cathode surface, on the ignition probabilities of a cathode spot (CS) in the Amperian and retrograde directions is developed assuming that the ignition probability of a new CS is proportional to the magnetic pressure around the old one. Furthermore, both stepwise and statistical models are established to simulate the random and retrograde motions of a single CS of vacuum arc, assuming that the 2-D motion of the CS in the $xy$ plane is equivalent to the synthesis of two independent 1-D movements along the $x$ - and $y$ -axis, respectively. With both models, the random and retrograde motions of a single Cu CS are simulated step by step and statistically, respectively. It was found that stepwise simulation results agree well with the statistical results, and they are also consistent with the relevant experimental results. The difference in retrograde motion velocity between the present stepwise model and that assuming that a new CS can be ignited in any direction around the old one is discussed. [ABSTRACT FROM PUBLISHER]

In this paper, the relationship between the external transverse magnetic field (TMF) and the ignition probability of new cathode spot in different directions is developed assuming that the ignition probability of a new cathode spot is proportional to the magnetic pressure around the old one. Furthermore, a method is established to simulate the 2-D motion of a cathode spot step-by-step. With this approach, stepwise simulations of the 2-D retrograde motion of a single Cu cathode spot are carried out in a large range of flux density of TMF ( Bem ). Simulation results indicate that, if the self-generated magnetic field Bsm exceeds the applied magnetic field Bem , i.e., B\textrm {em}\le B{sm} , there is a linear relationship between the velocity of the retrograde motion and Bem , in certain range of Bem , after which the velocity of the retrograde motion increases slowly and nonlinearly with further increase of B\textrm {em} . This phenomenon agrees well with relevant experiment results. In the linear stage, the dependence of velocity constant on the spatial step and the temporal step of the motion of cathode spot is investigated as well. Moreover, when Bem>Bsm , the velocity of cathode spot decreases with the increase of B\textrm {em} , even the macroscopic motion is still in the retrograde direction. This effect might also contribute to the reversal of cathode spot motion from the retrograde direction to the Amperian direction at strong TMF besides the predominant effect of the deflection of arc column toward the Amperian direction. [ABSTRACT FROM PUBLISHER]

Cathode spots (CSs) dynamics in the initial expansion stage of high-current triggered vacuum arc and the influence of axial magnetic field (AMF) was investigated experimentally. Experiments were conducted with butt contacts in a demountable vacuum chamber. Images of CSs were photographed with a high-speed digital camera with exposure time of 2 \(\mu \) s. The uniform and constant AMF in the intercontacts region was supplied by external magnetic field coil. In high-current triggered vacuum arc, CSs expanded across the cathode surface in basically a ring-shaped pattern. Experimental results showed that the expansion ring consisted of group CSs (GCSs) and individual CSs at the beginning of the expansion process. The GCS moved outward faster than individual CS. Research results indicated that an AMF could reduce the tendency of formation of GCS. The effect of local plasma and global plasma, which could be significantly influenced by AMF, on the motion pattern and distribution of CSs in the initial expansion stage was investigated. [ABSTRACT FROM PUBLISHER]

In the Special Issue, we are very pleased to present the expanded versions of selected papers from the 27th International Symposium on Discharges and Electrical Insulation in Vacuum (ISDEIV). It was held at the Xi’an Jiaotong-Liverpool International Conference, Suzhou, China, from September 18–23, 2016. [ABSTRACT FROM PUBLISHER]

This work investigates the dynamic characteristics of vacuum arc in the initial expansion process of a direct current (dc) circuit breaker. Based on the commercial cup-type axial magnetic field (AMF) contact, a 3-D transient magnetohydrodynamic (MHD) model is established. In the model, transient equations are used to study the transient characteristics of the vacuum arc. The effect of magnetic field hysteresis is also considered in the method. The dynamic mesh technique is adopted to simulate the electrode separation, which can make the simulation results more accurate. The simulation results show that, during the initial expansion process, with the increase in the current, the arc gradually expands in the electrode gap. The ion pressure, ion number density, axial current density, and energy flux density to the anode gradually decrease. Ion rotation can be observed under the influence of the magnetic field, and the rotation speed will increase with the increase in the current. Two different arc ignition positions are also compared. The result shows that igniting the arc at the contact center is more helpful to the success of the interruption of the dc circuit breaker. The appearance of the arc predicted by the model is consistent with the experimental results. [ABSTRACT FROM AUTHOR]

In real power system, when the three-phase short-circuit fault occurs, the vacuum arc in one phase will be influenced by the transverse magnetic field generated by neighbor phases and bus bars. This kind of effect is the main cause of the unstable arc and deflected erosion of contact plates, which leads to the failure of vacuum circuit breakers interruption. The objective of this paper is to get more insight into the influence of external transverse magnetic field (ETMF) on vacuum arc’s behavior. The experiments were conducted in a demountable vacuum chamber with pressure about 10^-4 Pa. The cup-type axial magnetic field contacts were used, whose material was pure copper and the diameter is 35 mm. The uniform ETMF in the arc region was generated by two parallel bulk permanent magnets. The experiments were conducted under different ETMFs (0, 15, and 25 mT) and current levels (1-, 2.5-, and 4-kA rms) with different gap distances (6, 8.5, and 11 mm). The videos of arc column were recorded by a high-speed charge-coupled device camera. Under the action of ETMF, the vacuum arc got deflected. Due to the retrograde motion of cathode spots and Ampere force acting on arc column, the deflection behaviors during three typical periods (that is, initial, peak value, and close-to-current-zero period) were different. Moreover, the deflection level of vacuum arc at current peak value moment was greatly impacted by ETMF and gap distance. Larger the ETMF and gap distance were, higher the deflection level at current peak value moment was. The simulation results of arc deflection based on magnetic-hydrodynamic model were in agreement with experimental results in trends. [ABSTRACT FROM AUTHOR]

Anode material seriously influences the characteristics of vacuum arc and further affects the performance of medium-voltage vacuum switches when the interruption current is high. There are many materials used for electrode manufacture, and different materials are selected for different switches. For a pure metal, its performance usually cannot satisfy the actual requirement. To improve switch’s performance, an alloy is usually used as an electrode material. In this paper, thermal processes of six kinds of metal anodes (including pure metal and alloy anodes) are simulated and researched. The physical parameters of the pure metals all come from experiment results directly or are fitted by the experimental data. The physical parameters of the CuCr alloys are derived from Cu and Cr parameters. Two kinds of temperature calculation methods are used, which are called melting and solidification model and equivalent specific heat method, respectively. Simulation results show that W and Mo anodes have the higher temperature than Cu, Cr, CuCr25, and CuCr50 anodes. A pure Cr anode has the largest melting width and highest saturated vapor pressure and evaporation energy. A Cu anode has the biggest melting depth. A W anode has the smallest melting width and depth. Axial temperature gradient is related to the thermal conductivity, and the Cr anode has the largest axial temperature gradient. The thermal characteristics of CuCr25 and CuCr50 anodes are located between the pure Cu and Cr anodes. There are two melting points appearing in the results of CuCr alloys, and between the two melting points, the alloy anodes are in solid–liquid mixture state. [ABSTRACT FROM PUBLISHER]

Based on a steady-state 3-D magnetohydrodynamic (MHD) model, the high-current vacuum arc (HCVA) under combined effect of actual magnetic field (MF) and external transverse MF (ETMF) is simulated. The actual MF is generated by cup-type axial magnetic field contact system commonly used in commercial vacuum circuit breakers. The ETMF may cause the deflection of arc column, which is the main reason of the contact deflected erosion. According to some experimental results, the electron temperature in HCVA is assumed to be uniform and equal to 3 eV. Therefore, the MHD model is simplified by neglecting the electron energy equation to improve the simulation efficiency. With the three conservation equations (mass, momentum, and energy) of ion flow coupling solved, the spatial distributions of some flow parameters can be obtained. The influence of all three components of the MF is inserted by solving the magnetic transport equations sequentially. Proper boundary conditions are set on the cathode and anode side, which separated the cathode spots mixing region and anode sheath region from computation domain, respectively. Under the influence of the ETMF, the deflection of the plasma flow can be predicted, which may be helpful to understand the mechanism of the contact deflected erosion. [ABSTRACT FROM PUBLISHER]

In this paper, high-current vacuum arc (VA) breaking experiments in vacuum interrupters were conducted using two kinds of high-capacity (larger diameter) axial magnetic field (AMF) electrodes with contact gap distance changing from 10 to 40 mm. Triggering arc mode and fixed gap distance were adopted. Experimental results showed that during the initial diffusion stage, the arc voltage of high-current VA got a peak value. The change in high-current VA appearance would also affect the curve of arc voltage. With the increase of contact gap distance, the voltage of high-current VA increased and the VA near the anode appeared contracted, and its shape was changed from cylinder to cone. In the shorter gap distance, for cup electrodes, the plasma jets mix with each other and the arc column appears like a bright cone, whereas for coil electrodes, the plasma jets are clear and the column appears like a cylinder, and the transfer of arc appearance affects arc voltage distribution. Moreover, arc twist and rotation often occur under longer gap and higher current, which is due to the larger radial magnetic field component generated by AMF coils, and arc voltage appeared high-frequency and high-amplitude noise, and ultimately resulted in the distortion of the arc current in the current experimental conditions (lower charging voltage of capacitor). [ABSTRACT FROM PUBLISHER]

Based on a 3-D steady magnetohydrodynamic model, the high current vacuum arc subjected to a realistic spatial magnetic field generated by the cup-type contacts was simulated. The influence of the realistic spatial magnetic field profile on plasma's rotation behavior was analyzed. The distribution of the azimuthal velocity on anode side under realistic spatial magnetic field profile was quite different from that under uniform axial magnetic field. [ABSTRACT FROM PUBLISHER]