Axial Magnetic Field Contacts Research Articles

Study on structural parameters of cup-type axial magnetic contacts on gap magnetic field and arc characteristics

The axial magnetic field (AMF) plays a crucial role in maintaining the diffusion of vacuum arcs within contact gaps, making it a key component in vacuum breaking systems. To enhance contact surface utilization, a novel vacuum cup-type AMF contact was designed, featuring a stronger axial magnetic field at the edge and weaker at the centre. Through Ansys Maxwell 3D simulations, the designed contact increased the effective utilization area by 18 %. Experimental analysis demonstrated that, compared to traditional contacts, the designed contact exhibited lower arc voltage and significant arc diffusion. Ablation of the designed contact was more concentrated at the edge, with scanning electron microscope (SEM) images revealing four microscopic morphologies: overlapping craters, bright metal droplets, various-sized flakes, and fine particles. Energy dispersive spectrum (EDS) analysis provided insights into the cathode surface's elemental composition and droplet growth. Additionally, the study explored the influence of different slotting parameters on vacuum arc characteristics, including contact plate configurations, cup bottom slotting, and cup wall tilt, focusing on the arc initiation stage, arc voltage rise, and arc shape transition.

Study on arc characteristics and influencing factors of cup-shaped axial magnetic field contact at different gap distances

The cup-shaped axial magnetic field (AMF) contact, integral to high-voltage vacuum interrupters, undergoes innovation by enhancing the slotting rotation angle of the cup wall. This adjustment significantly amplifies the gap magnetic field, promising breakthroughs in handling long-gap arcs within high-voltage vacuum interrupters. Comprehensive vacuum arc experiments were conducted on cup-shaped AMF contacts with varying slotting rotation angles at different gap distances. The research meticulously analysed the evolution of vacuum arc characteristics under diverse breaking currents. Scanning electron microscope (SEM) images of the cathode surface unveiled three distinctive microscopic morphologies: particle gullies, pore platforms, and particle platforms. Notably, the chromium content corresponding to these morphologies exhibited a progressive increase. Integrating the surface distribution of copper and chromium, the study proposed a plausible cathodic erosion mechanism. Extending the investigation, the research delved into the influence of the iron core, contact diameter, and slot number on vacuum arc characteristics, which expected to provide reference for the application of the cup-shaped AMF contact in high voltage vacuum interrupters.

Experimental investigation on the characteristic and voltage of low-current vacuum arc in vacuum OLTC

Due to vacuum interrupter's excellent interruption performance, an increasing number of on-load tap-changers (OLTCs) have chosen it as a load changeover switch in recent years. The stability of the arcing process is crucial for the interruption since the vacuum interrupter in OLTC must break current frequently with the requirement of long electrical life. In this paper, experiments on butt contacts and axial magnetic field (AMF) contacts applied in OLTC were carried out in a demountable vacuum chamber, and the arcing process was investigated by a high-speed camera. Investigations were conducted on the effects of opening speed, opening phase, current amplitude, and AMF on arc characteristics, e.g., arc expansion and arc voltage. The experiment findings demonstrated that expansion and instability of arc increased with higher opening speed, number of cathode spots moving to the inclined surface decreased with shorter arcing time, and that cathode spots' propensity to form group cathode spots increased with higher current. AMF lessened the impact of high opening speed and high current on the arcing process. The investigation carried out in this paper helps to better understand the characteristics of arc evolution and the influence of some key factors, which may serve as a guide for improving the design and vacuum arc control of vacuum OLTCs.

Experimental study and characteristic analysis of vacuum arc between cup-type axial magnetic contacts under different diameter-to-gap ratios

Since vacuum circuit breakers gradually advanced to higher voltage levels, axial magnetic field (AMF) contacts have drawn a great deal of attention due to their excellent breaking ability. The cup-type AMF contact is a common kind of AMF contact, which has much potential in contact design and application of high voltage grade systems due to the advantages of strong structural strength, uniform magnetic field distribution, lower resistance, etc. This study analyzes the arcing characteristics of a cup-type AMF contact with a large slotted rotation angle at various diameter-to-gap ratios (DGRs). The arcing process is divided into five stages as follows: initial diffusion, contracting, fully constricted, re-diffusion, and extinguished. Arc self-rotation and anode separation phenomena in the re-diffusion stage appear when the DGR is 58/24. The reasons for these occurrences were discussed and explained with regard to the magnetic field vector's spatial distribution. The duration of each stage and the current threshold of a fully constricted arc will both differ with the change of the DGR. The structural parameters of the fully constricted arc were computed through the method of imaging the luminous intensity distribution after the arc was fully constricted. The source of the change in the arc voltage can be seen in the variation of arc structural parameters, which also reflect anode activity intensity to a certain extent. The transient magnetic field simulation method was used to explain why the arc under the same instantaneous current shows variable morphology at the extinguished stage and contracting stage in one arcing process. The research results presented in the article can be used as a reference for developing high-voltage cup-type AMF contacts.

Design and Study of the Key Characteristics of a New DC Vacuum Interrupter With a High Peak and Low Residual Magnetic Field

With the rapid development of multiterminal high-voltage direct current (HVdc) transmission systems, the research and development of high-speed and stable high-voltage dc circuit breakers have received attention. Among them, mechanical dc circuit breakers based on manual zero-crossing dc breaking technology have been applied in practical projects, owing to their small size, low on-state loss, and low price. However, this method adds a high-frequency current, which deteriorates the magnetic field between the poles of the interrupter when the arc is broken. Therefore, the development of HVdc circuit breakers that maintain high peak magnetic fields and low residual magnetic fields at high frequencies has become the key to the development of multiterminal HVdc transmission. To solve this problem, this article proposes a new interrupter for a vacuum dc circuit breaker based on artificial zero-crossing technology and studies its key characteristics, such as temperature rise, electromagnetic field, and arc column plasma distribution characteristics. The results reveal that the new interrupter proposed in this article has a low-temperature rise, uniformly distributed electric field, high peak value and low remanence of the longitudinal magnetic field, and uniformly distributed plasma in the arc column, and exhibits arc diffusion. Its ability to cope with high-frequency current is significantly higher than that of the traditional cup-shaped axial magnetic field (AMF) contact system interrupter, which is conducive to the high-speed and stable breaking of the interrupter.

Experimental investigation of vacuum arc characteristics subject to axial magnetic field contacts with different ratios of diameter to gap distance

In this study, to investigate vacuum arc characteristics under different diameter-to-gap distance ratios (D/g), experiments are conducted on axial magnetic field (AMF) contacts with varying D/g. The peak current value ranges from 15 to 35 kA. The experimental results show that the ability of AMF contacts to control the arc decreases with D/g. Furthermore, the 2/3 coil contact can control the arc under long gap distances, whereas the cup-shaped contact cannot effectively control the arc under long gap distances. The arc energy and noisy period duration decrease with increasing D/g. The arc voltage of the 2/3 coil contact at the peak moment is always the lowest compared to the cup-shaped contact and the 1/2 coil contact, and the arc voltage of the 2/3 coil contact at the peak moment has an approximate power function relation with D/g. The simulation results show that the AMF distribution changes from saddle shape to bell shape with decreasing D/g. The 2/3 coil contact is more suitable for high-voltage vacuum interrupters (VIs), while the cup-shaped contact is more suitable for medium-voltage VIs.

Vacuum arc characteristics of an AMF–TMF composite contact with fixed AMF contacts under different opening velocities

The objective of this paper is to determine the vacuum arc characteristics of a novel axial magnetic field (AMF)–transverse magnetic field (TMF) composite contact under different opening velocities. The AMF–TMF composite contact consists of a pair of outer fixed AMF contacts and a pair of inner movable TMF contacts. Drawn arc experiments were carried out in an L–C discharging circuit with this composite contact system to observe the differences in the arc development process under different opening velocities. The opening velocity of the inner movable contact was set to 1.3, 1.8, and 2.3 m/s. The arc current was up to 20 kA rms. In the experiments, the arc behavior was recorded using a high-speed camera. The arc current of each contact and the arc voltage were measured. Then the arc characteristics, including the arc current transfer characteristics, diffused arc duration, and mental droplet distribution before zero current, were quantitatively analyzed. Furthermore, magnetic field calculation was carried out to explain why the arc characteristics are different under different opening velocities. Experimental results show that with an increase in the opening velocity from 1.3 to 2.3 m/s, the diffused arc duration before zero current increases from 1.05 to 1.75 ms when the cathode is on the fixed side and increases from 1.30 to 1.90 ms when the anode is on the fixed side. In addition, the quantity of mental droplets between contacts before zero current is less with high opening velocity. It indicates stronger short-current breaking capacity under higher opening velocity. From the simulation results, it can be seen that during the arcing period, more current transfers to the fixed AMF contact under higher opening velocity and generates a stronger AMF. It could explain the earlier diffusion of the vacuum arc column under higher opening velocity.

Dynamic characteristics of cathode spots during expansion process in a vacuum arc

Cathode spot is the source of plasma and metal vapor which affects the breaking capacity of vacuum circuit breakers. In this paper, the cup-shaped axial magnetic field contacts were used and the images of cathode spots were obtained by an ultra high-speed camera in the experiment. The dynamic characteristics of cathode spots during expansion process in vacuum arc were investigated. Under the conditions of different current amplitudes, different distributions of cathode spots were observed. During the expansion process of cathode spot, there are group spots and individual spots. The group spot only occurs in the cold electrode. The electrons emission characteristics of cathode spot as a function of temperature and electric field was calculated. The formation of group spot is related to the current and electron emission of cathode spot. The cathode spots were identified by Matlab. It was found that the variation of number of cathode spots is consistent with the current. However, at different stages, the reasons for the increase in the number of cathode spots are different, such as the increase in the area of cathode spots in the initial arcing stage and the splitting of cathode spot. It was observed that the cathode spots are not completely evenly distributed on the cathode surface. The farther away from the center of the contact, the longer the distance between the cathode spots. It is mainly effected by the transverse magnetic field (TMF) which is generated by the arc column. The closer to the contact edge, the higher the TMF at the cathode spot and the higher the expansion velocity of cathode spot.

A Relationship Between Vacuum Arc Characteristics and Short-Circuit Current Interrupting Capability at Minimum Arcing Times

The study is to explore a relationship between vacuum arc characteristics and short-circuit current interrupting capability of a fast vacuum circuit breaker (FVCB) at a minimum arcing time. Both a short-circuit current interruption test and an arc mode transition observation test were carried out to explore the relationship. Firstly, an FVCB with ceramic enclosure vacuum interrupter (VI), in which a pair of axial magnetic field (AMF) contacts were arranged, was operated with three different opening speeds to interrupt short-circuit currents. A Weil synthetic circuit provided short-circuit currents of 20, 30, and 40 kA, and a transient recovery voltage (TRV) of 92 kV/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$120~\mu \text{s}$ </tex-math></inline-formula> for the short-circuit current interruption capability experiment. Then, the arc mode observation experiment was carried out under the same conditions as the short-circuit current interruption capability experiment, except for two differences, that is, a glass enclosure VI without a shield was used for observing the arc with a high-speed camera, and the other point was that only the short-circuit current was applied and no TRV was applied. Experimental results showed that the increase in the opening speed resulted in a decrease in the minimum arcing time, the critical arc extinguishing gap, and the critical arc diffusion time, but led to an increase in the maximum arc column diameter. In contrast, the increase in the short-circuit current would increase the minimum arcing time, the critical arc extinguishing gap, the critical arc diffusion time, and the maximum arc diameter. The diffusion of the arc column is essential but not sufficient for the successful interruption. The contact gap plays a significant role. On the one hand, a larger contact gap promotes arc diffuse. The arc diffusion can decrease the contact surface temperature and the local particle density at current zero. On the other hand, a larger gap distance leads to a smaller electric field intensity between fixed and moveable contacts, which reduces the breakdown probability during the postarc dielectric recovery. The critical arc energy density for a successful interruption of the FVCB in short arcing times was obtained to range from 0.54 to 1.01 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times \,\,10^{8}$ </tex-math></inline-formula> J/m3.

Experimental investigation on the evolution of vacuum arc in the quench protection switch based on forced current zero

In this paper, the evolution of vacuum arc under different conditions in the quench protection switch (QPS) based on forced current zero was investigated experimentally and analyzed quantitatively. Experiments were conducted with cup-shaped axial magnetic field (AMF) contacts in a demountable vacuum chamber. Images of the arc column were photographed through the observation window with a high-speed digital camera with exposure time of 2 μs. Arc appearance was analyzed quantitatively through digital image processing. Quantified arc appearance and arc voltage characteristics indicated that the high-current vacuum arc evolution in the QPS could be divided into four stages: arc formation stage, arc column merging stage, diffusing stage, and fast extinguishing stage. The influence of AMF on the evolution of high-current vacuum arc in the diffusing stage was also studied. Experiment results indicated that the vacuum arc at relatively low current had a simpler evolution process and the arc column merging had less correlation with voltage stability. It was found that the vacuum arc with a long gap distance developed faster and the time required to enter the stable burning state was shorter. In addition, the vacuum arc with long arcing time tended to constrict again, which is not beneficial to current interruption.

3D transient MHD simulation of DC breaking vacuum arc based on artificial current zero

In the vacuum DC circuit breaker based on the artificial current zero, the change of arc luminosity delays the change of current value significantly in the fast current-zero stage. This phenomenon has impacts on the breaking capacity of the DC vacuum circuit breaker. Therefore, it is necessary to study the dynamic characteristics of the arc plasma under the above condition. In this work, a 3D transient magneto-hydro-dynamic model based on the commercial cup-type axial magnetic field (AMF) contact is established to study this phenomenon. The simulation results show that the fast current-zero stage is too short for the plasma to diffuse. The changes of the physical characteristics of the arc plasma lag the change of current value. Moreover, the magnetic field hysteresis caused by the eddy current on the electrode occurs in this stage, resulting in a continuous stronger AMF. It makes the distributions of the plasma uniform, while the diffusion of the arc plasma reduced at low currents, which is detrimental to the post-arc dielectric recovery stage. In addition, in the fast current-zero stage, the AMF near the contact slot is smaller than in other areas. This indicates that the slotting is effective in suppressing eddy currents and avoiding the negative effect of excessive AMF on plasma diffusion at this stage. The simulation results are consistent with the experimental results.

3-D Dynamic Simulation of the Initial Expansion Process of Vacuum Arc Plasma in DC Interruption

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.

Experimental investigation on the velocity of cathode spots in a vacuum arc with high di/dt

With the development of DC transmission and aerospace technology, the demands on the breaking capacity of vacuum circuit breakers (VCBs) under high condition are getting higher and higher. The breaking capacity of VCBs is determined by cathode spots, which provide electrons and metal vapor to maintain the arc. In this paper, the experiments of cathode spot observation were carried out based on the detachable vacuum interrupter and the cup-shaped axial magnetic field contacts. The images of cathode spot distribution were obtained by an ultra high-speed camera. In the initial expansion stage of arcing, it was discovered that the distribution of cathode spots is ring-shaped and symmetrical. The initial expansion velocity of cathode spot ring is approximately proportional to the value of and is independent of frequency. During the whole arcing process, the expansion velocity of cathode spot ring rises first and then drops with arcing time. The effects of the magnetic field and metal vapor density on the expansion process of cathode spot were analyzed. Based on the experimental conditions and results, the values of magnetic field and metal vapor density were calculated which can reasonably explain the variation of expansion velocity of cathode spot.

Capacitive making of double‐break vacuum circuit breaker: Transverse magnetic field contacts are superior to axial magnetic field contacts

Previous studies mainly focused on the behaviours of single-break vacuum circuit breakers (VCBs) in switching capacitor banks. The prestrike characteristics of double-break VCBs subjected to axial magnetic fields (AMFs) and transverse magnetic fields (TMFs) in capacitive making operations, to find a proper structure of double-break VCBs for capacitive switching are experimentally investigated here. Special attention is given to the prestrike gap, inrush current interruption, prestrike voltage, prestrike electric field strength and erosion on contact surfaces of double-break VCBs. The results show that the 50% prestrike gap d50 and the scatter in prestrike gaps are significantly reduced by nearly 50% by the TMF double-break VCB compared with the AMF double-break VCB. The 50% prestrike arcing time ta50 of the TMF double-break VCB can be reduced by 37.0% compared with the AMF double-break VCB. The voltage distribution ratio between two vacuum interrupters (VIs) is more balanced in the TMF double-break VCB (6:4) than in the AMF double-break VCB (7:3) when prestrike occurs. The TMF double-break VCB can reduce the erosion caused by inrush currents on contact surfaces compared with the AMF double-break VCB. In switching capacitor banks, the TMF double-break VCB is superior to the AMF double-break VCB.

Capacitive making of vacuum circuit breakers: Double-break vacuum circuit breaker is superior to single-break vacuum circuit breaker

Previous studies mainly focused on the prestrike phenomena in single-break vacuum circuit breakers (VCBs) under capacitive switching. However, experimental investigations on prestrike characteristics under capacitive making operations of double-break VCBs were not addressed in literatures, which were important for the strategies of capacitive controlled switching. The objective of this paper is to experimentally investigate the prestrike characteristics of single-break and double-break VCBs in switching capacitor banks, to determine the most suitable type of VCBs in capacitive switching. The double-break VCB consists of two commercial 12 kV vacuum interrupters (VIs) with cup-type axial magnetic field (AMF) contacts, which is the same VI in the single-break VCB. The results show that the 50% prestrike gap d50 of each VI in the double-break VCB, which was calculated based on the complementary Weibull distribution, was significantly reduced by 10.5% compared with the single-break VCB. Due to the voltage-sharing effect in the double-break VCB, scatters in prestrike gaps of the double-break VCB were smaller than the single-break VCB. With the application of the double-break VCB, both the inrush current interruption and the prestrike arcing time can be reduced. The double-break VCB can reduce the erosion caused by the inrush current of the low-voltage side VI. In capacitive switching, the double-break VCB is superior to the single-break VCB, especially in capacitive controlled switching.

Numerical Simulation of Multispecies Vacuum Arc Subjected to Actual Spatial Magnetic Fields

A three-dimensional (3-D) model for multispecies vacuum arc considering ionization and recombination process has been built in this article. With the model, the arc behaviors and species characteristics subjected to actual spatial magnetic fields of cup-shaped axial magnetic field (AMF) contact and transverse magnetic field-axial magnetic field (TMF-AMF) contact are investigated and compared. Furthermore, the validity of the model is verified by comparing with the experiments. Simulation results show that contact structures affect the species distributions of the arc by changing magnetic field and current density distributions. More triple charged ions are generated through ionization under higher electron temperature so that the triple charged ions gather where current constricts. The heat flux density is large in the current contraction region, where the contribution from triple charged ions is large as well. Spatial magnetic field distribution affects the Lorentz force acting on ion components. The force distribution is relatively uniform and the species are subject to inward force on the arc edge under cup-shaped AMF contact. For TMF-AMF contact, the double charged ions are subject to the force pointing to the TMF contact arms, whereas the triple charged ions are subject to the force pointing to the arc center. In addition, the consideration of arc species can improve the accuracy of the simulation results of the heat flux distribution, leading to a good agreement of the erosion condition between experiments and simulation.

Research on the current-zero period of vacuum arc interruption and equivalent model of post-arc gap

In this paper, the current breaking experiments of vacuum interrupter are conducted and the waveforms of voltage and current are obtained. The results show that there is current instability phenomenon when the value of is low and the value of instability-initiation current decreases as the value of increases. The number of voltage spikes will also decrease and irregular oscillation will weaken. According to the experimental results and theoretical analysis, the ‘memory characteristic’ of plasma is proposed and a new mathematical model of post-arc initial plasma density is estabilished which will provides a new method for the calculation of the sheath growth. The equivalent circuit model of post-arc gap is established to analyze the transient recovery voltage. The equivalent parameters in the circuit are obtained by the calculation of sheath growth and plasma density and verified by comparison with measurements. Through the comparison of experimental results between the axial magnetic field (AMF) contact and the butt contact, the effects of AMF on post-arc plasma density and sheath growth are analyzed. The results show that AMF can make the post-arc plasma density higher and the sheath growth slower.

Study on Arc Evolution Behaviour of Breaking Short Circuit Current for a Fast Vacuum Circuit Breaker

Abstract A fast vacuum circuit breaker (FVCB), driven by an electromagnetic repulsion mechanism, has an extremely short opening time, and in coordination with the controlled switching technology contributes to breaking a short circuit current with the minimum arcing time. The objective of the paper is to determine the arc evolution behaviour of fast vacuum circuit breakers with a minimum arcing time. A 40.5 kV vacuum interrupter (VI) was used, which was equipped with a pair of CuCr50 (Cr 50 wt%) axial magnetic field (AMF) contacts. The diameters of the contact were Φ50mm, Φ58 mm and Φ65 mm respectively. The arc evolution behaviour in the drawn arc experiment was photographed by a high-speed camera through a glass envelope of the VI. The experiment results showed that an initial intense arc tended to evolve into a diffuse arc when the arcing time is larger than a critical arc diffusion time under a certain opening velocity. The critical arc diffusion time for the vacuum arc is defined as arc evolving from the instant of contact separation to an appearance of separated cathode spots. For a Φ50 mm cup-type AMF contact, the critical arc diffusion time decreases from 0.7 ms to 0.46 ms, with the increase of the opening velocity from 2.52 m/s to 4.65 m/s. What’s more, for a Φ58 mm cup-type AMF contact, the critical arc diffusion time decreases from 1.0 ms to 0.58 ms with the increase of the opening velocity from 2.52 m/s to 4.65 m/s. For a Φ65 mm cup-type AMF contact, the critical arc diffusion time decreases from 1.30 ms to 0.64 ms with the increase of the opening velocity from 2.52 m/s to 4.65 m/s. The critical arc diffusion time increases with the increase of the diameter of the contact under the same opening velocity.

Development and Current–Voltage Characteristic of Arc on AMF Contacts at Different Opening Speeds

This article presents the results of the investigation of high-current vacuum arcs (HCVAs) on axial magnetic field (AMF) contacts at an average opening speed of 2 m/s and the comparison of the obtained results with similar results received earlier at a lower average opening speed of 1 m/s. We compared the voltage oscillograms and the current-voltage characteristics (CVCs), as well as the arc development process of the discharges at different opening speeds. The discharge was ignited by opening the contacts in a demountable vacuum chamber and was fed with the half-sine current pulse of 10 ms in duration. For analyzing the arc development processes, high-speed video recording of the arc column and the cathode surface was conducted throughout the entire current pulse. We discovered that the process looks similar for both the investigated contact speeds: a discharge is initiated at one or more points and covers the entire contact surface in a few milliseconds. Only quantitative differences are observed due to a higher arc voltage and, accordingly, energy input at a higher speed of contact movement. The CVC of the developed arc in both cases turned out to be linear in a wide range of currents. The analysis of the CVC showed that, in the developed arc, the ratio of the resistance of the column to its length does not depend on the length of the gap and the opening speed of the contacts. Thus, we can assume that the plasma parameters in the column change little with an increase in the current and in the opening speed of contacts. This assumption is supported by the results of a discussion of the literature on the CVC of HCVAs.

Optimizing the Opening Velocity for a Vacuum Circuit Breaker With Cup-Type Axial Magnetic Field Contacts

This article extends the quantitative method of optimizing the opening velocities for a vacuum circuit breaker (VCB) with cup-type axial magnetic field (AMF) contacts. The material of the AMF contact is Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Cr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> . We define v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> as the initial opening velocity over half the full contact gap and v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> as the average velocity over the full contact gap of 20 mm. We optimize v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> using the transition of anode-discharging modes, of which we combine the footpoint mode and anode spot mode into a large-gap high-current (LGHC) mode. The experiment shows that v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> should be higher than a threshold velocity v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> , which corresponds to a peak critical contact gap d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> for the transitions of an intense arc mode into a diffuse arc mode, in high-current interruption. The higher v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> is than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1_th</sub> , the faster the intense arc mode evolves into the diffuse arc mode. This helps successfully interrupt the short-circuit current in a short arcing time. Another threshold velocity, v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> , corresponds to the maximum critical contact d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">II_th</sub> for the formation of an LGHC mode in the second arc current half-wave. A high v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> design needs v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> to be lower than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> . The lower v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is than v <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2_th</sub> , the better the VCB avoids the LGHC mode in high-current interruption. This helps interruption over a long arcing time. We propose an intelligent opening and breaking operation for VCBs with cup-type AMF contacts, whereby the VCB opens with different opening velocities according to the short-circuit current level.