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Using magnetic fields to control the vacuum arc is a major way to improve the breaking capacity of vacuum interrupters. Besides the single transverse magnetic field (TMF) and single axial magnetic field (AMF), new type of TMF-AMF contact and TMF-TMF contact structures have been put forward to control the vacuum arc in recent years. In this paper, three different structural contacts are used in the experiments. The arc evolution was captured by an high-speed digital video camera, and at the same time, the arc current and arc voltage wave were recorded. The experiment was conducted under current ranging from 5 to 20 kA (root mean square) and 10 mm of gap distance. There are 12 current shots performed on one contact design. There are four current levels in the experiment which are 5, 10, 15, and 20 kA, and three tests are taken for each current level. The results showed that for TMF-AMF contacts, under low current (I = 5 or 10 kA), the arcs of the two kinds of contacts both kept diffused and rotating. Under high current (I = 20 kA), the arc of the TMF-AMF contact with six slots was constricted and rotated on the plate, and the melting was uniform and slight. As for the eight-slot contacts, the arc was constricted but did not rotate which resulted in significant melting in the local region. As for TMF-TMF contacts, the result is similar under low current, while the arc was constricted and rotated quickly on the inner plate under high current. The simulation results show that when the arc core is located on the outer plate; the total Lorentz force is near zero. When the arc core is located on the inner plate, the direction of total force is counterclockwise. [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]

In the interrupting process of a three-phase vacuum interrupter, there exists a transverse magnetic field (TMF) in the interelectrode region, which is produced by adjacent phases. The deflection of the vacuum arc caused by TMF will affect the interrupting process of vacuum circuit breakers. In this paper, the vacuum arc characteristics considering the action of TMF produced by adjacent phases is simulated based on a steady 2-D asymmetrical magnetohydrodynamic model. The simulation results show that the vacuum arc will swing around and is especially obvious at the smaller current moments near current-zero during one ac half-cycle, because of the changed direction of TMF produced by adjacent phases. This kind of swing phenomena can also be observed in the electrode erosion of unsuccessful interruption. At the moments near current-zero, axial magnetic field is relatively weaker, while TMF generated by adjacent phases is relatively stronger, therefore, the offset phenomenon of plasma parameters is more significant. Compared with smaller diameter electrode arc, larger diameter electrode has larger arc deflection distance, while its value of plasma parameters is much smaller. [ABSTRACT FROM AUTHOR]

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]

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]

The expansion speed of cathode spots (CSs) in the initial expansion stage of high-current triggered vacuum arc 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 \mus. The uniform constant axial magnetic field (AMF) was supplied by an external magnetic field coil. The combined influence of contact gap of 2–10 mm and AMF of 0–70 mT on CSs expansion speed in the initial expansion stage was mainly investigated. Investigation results indicated that the variation of contact gap could influence the CSs expansion speed by changing the plasma density inside the cathode arc root in two ways: loss of arc plasma and constriction of arc column. AMF could weaken these two effects caused by the variation of contact gap. In addition, experimental results indicated that there existed a critical current for a given AMF. If the arc current peak was less than the critical current, the given AMF could inhibit the arc column constriction effectively both at contact gap of 8 and 10 mm, which could maintain the dependence that CSs expansion speed increased with contact gap; otherwise, the constriction of arc column played a major role in the influence on CSs expansion speed. In this case, the CSs expansion speed was lower at contact gap of 10 mm than that at 8 mm. [ABSTRACT FROM AUTHOR]

The initial expansion process in a drawn vacuum arc and the influence of axial magnetic field (AMF) were investigated experimentally in a demountable vacuum chamber. Arc characteristics were investigated with the aid of a high-speed digital camera with an exposure time of 2 \mu\s . In a drawn vacuum arc, the arc sequence begins with the bridge column arc formed after the rupture of the molten metal bridge. This column evolves into the transition mode, which consisted of a central column with few or no cathode spots (CSs) outside the column, and then into the fully diffuse mode. Experimental results indicated that in transition mode arc, the expansion process could be characterized by the appearance of CSs outside the central column, and could be classified into two patterns, “slow” expansion and “quick” expansion according to the characteristics of the formation and motion of new CSs (conducting channels) outside the central column of arc. The influence of AMF and its distribution on the expansion process was also investigated. Investigation results indicated that AMF had two contrary effects, i.e., inhibiting effect and prompting effect, on the initial expansion stage of drawn vacuum arc. Furthermore, saddle-shaped AMF could encourage the arc transition into diffuse mode more effectively than bell-shaped AMF. [ABSTRACT FROM PUBLISHER]

Experiments on a drawn vacuum arc with forced current zero, which was formed by injection of a countercurrent with a frequency of 1 kHz, were conducted in a detachable vacuum chamber. The prospective peak main current was 3.5 kA. Arc characteristics were investigated with the aid of a high-speed digital camera with an exposure time of 2 \mu\s. In this paper, the influence of axial magnetic field (AMF) distribution and anode material on the evolution of vacuum arc in the initial expanding stage is mainly investigated. Three types of electrodes were used in the experiments. One of them was simple electrodes with butt contact plates, and the other two types were specially designed electrodes generating a conventional bell-shaped AMF profile and a saddle-shaped AMF distribution, respectively. OFHC copper and WCu10 were used as anodes for the electrodes with bell-shaped AMF. The appearance of vacuum arc in the forced current-zero stage is also investigated. [ABSTRACT FROM AUTHOR]

The anode melting in high-current vacuum arcs leads to the increase of metal vapor and liquid droplets around current zero and even the interruption failure. In our recent experiments of the cup-shaped axial magnetic field (AMF) contacts, an obvious clockwise swirl flow of the liquid copper was detected on the anode surface. However, if an additional external AMF which is reverse to the AMF generated by the contacts was introduced, the rotation direction of the liquid copper swirl flow on the anode surface could reverse to anticlockwise. These phenomena indicate that AMF has great influence on the performance of anode melting and its swirl flow. [ABSTRACT FROM AUTHOR]