Xiaochuan Song

Investigation on DC interruption based on artificial current zero of vacuum switch

Direct current interruption utilizing vacuum switch based on artificial current zero is investigated. A high-speed vacuum switch, which is driven by electromagnetic repulsion force, is adopted as the main switch. The injection of high-frequency countercurrent is controlled by a triggered vacuum switch. The main experimental current is supplied by a LC circuit. The overvoltage generated during interruption is suppressed by metal oxide arrester. The interruption process is also simulated via a circuit model, with which the two important current commutation processes are analyzed, i.e., the current commutation from main switch to countercurrent branch, and the current commutation from countercurrent branch to arrester branch. Simulation results are compared with measured waveforms of voltage and current.

Experimental Investigation on the Initial Expansion Process in a Drawn Vacuum Arc and the Influence of Axial Magnetic Field

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 μs . 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.

The Influence of Axial Magnetic Field Distribution on High-Current Vacuum Arc

The influence of the distribution of axial magnetic fields (AMFs) was investigated experimentally with three pairs of specially designed testing electrodes generating a conventional bell-shaped AMF profile and different saddle-shaped AMF distributions. The characteristics of high-current vacuum arcs (HCVAs) up to 20 kA (rms), e.g., the shape of the arc column and the distribution of cathode spots, were investigated with the aid of a high-speed digital camera with an exposure time of 2 mus. Experimental results further manifested the distinct influence of AMF distribution on the characteristics of HCVAs. Saddle-shaped AMFs could resist the constriction of HCVAs more efficiently than traditional bell-shaped AMFs. Furthermore, cathode spots in saddle-shaped AMFs distributed more uniformly than that in bell-shaped AMFs. It was found that AMFs in the central region of saddle-shaped AMFs should not be too weak. Otherwise, cathode spots would be out of control in the central region as observed in experiments. The ldquooptimalrdquo minimal AMF in the central region was estimated to be about 3-4 mT/kA.

The Influence of Axial-Magnetic-Field Distribution on the Initial Expansion Process in Triggered Vacuum Arc

In this paper, the initial expansion process after trigger of vacuum arc was investigated experimentally with two typical axial-magnetic-field (AMF) distributions, i.e., bell-shaped AMF generated by traditional cup-shaped AMF electrodes and saddle-shaped AMF generated by specially designed coil-type AMF electrodes. The motion of cathode spots (CSs) in the initial expansion process was investigated by high-speed digital camera with exposure time of 2 mus. Experimental results indicated that CSs expanded faster under saddle-shaped AMF than under bell-shaped AMF at 5 (rms), 10, 15, and 20 kA. Furthermore, the motion of CSs slowed down between 15 and 20 kA in the case of bell-shaped AMF, whereas CSs tended to quicken up with the increase of arc current under saddle-shaped AMF. Based on the images of CSs, it was proposed that, besides direct influence of AMF on the retrograde motion of CSs, the concentration of CSs at relatively high current and strong central AMF could also inhibit the outward motion in the initial expansion process.

Experimental Investigation on the Characteristics of Drawn Vacuum Arc in Initial Expanding Stage and in Forced Current-Zero Stage

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 detach able 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 μs. 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.

Investigations on the Motion of High-Current Vacuum-Arc Cathode Spots Under a Magnetic Field

Much research has been done on the motion of low-current vacuum-arc cathode spots (CSs). The relationship between the motion velocity of CSs and the magnetic field, particularly the relationship between the direction of CS motion and the ratio of Bz/Bt, has been revealed. However, little investigation on the motion of CSs in high-current vacuum arcs has been conducted before. In this paper, the motion of CSs of high-current vacuum arcs under different axial magnetic fields was experimentally investigated. Simultaneously, calculation of the motion of CSs based on the empirical formula has been done, and comparison with experiments and analysis were given. Experiments were conducted in a detachable vacuum chamber with an arc current of 10 kA (root mean square). Images of CSs were photographed with a high-speed digital camera with an exposure time of 2 μs. With computer-aided numerical processing, the spatial distribution of the CSs was obtained, based on which the magnitude and direction of the CS motion velocity were calculated, and the CS motion trajectories around the current peak were plotted. Results indicated that the properties of the CS motion in high-current vacuum arcs are similar to those in low-current vacuum arcs.

Investigation on the Inclination of Cathode Plasma Jets in High-Current Vacuum Arcs in Magnetic Field

The theoretical and experimental studies of cathode plasma jets in vacuum arc and the effect of magnetic field on them have been under way for several years. In this paper, different axial magnetic field electrodes were tested in a vacuum chamber, obvious individual cathode plasma jets were observed, and the cathode-jet inclination was also detected. Based on the numerical calculation of magnetic field in interelectrode region and the measurement of cathode-jet inclination in experimental results, it is proven that the inclination angle of composite magnetic field (the combination of the axial, the azimuthal, and the radial components) is consistent with the inclination angle of cathode plasma jets, which indicates that the cathode plasma jets in the arc column flow along the magnetic-field direction when the high-current vacuum arc is diffused or even constricted slightly. Meanwhile, it was also found that plasma jets far from the cathode surface mixed with each other and became ambiguous when the electrode gap increases to a certain distance.

Experimental investigation on the motion of cathode spots in removing oxide film on metal surface by vacuum arc

The motion of vacuum arc cathode spots has a very important influence on the efficiency of removing the oxide film on the metal surface. In this paper, the characteristics of cathode spot motion are investigated experimentally. Experiments were conducted in a detachable vacuum chamber with ac (50?Hz) arc current of 1?kA (rms). A stainless steel plate covered by an oxide layer was used as the cathode. The motion of cathode spots during the descaling process was photographed by a high-speed digital camera with an exposure time of 2??s. Experimental results indicate that the motion of cathode spots is influenced by the interaction among individual cathode jets and the position of the anode as well as the surface condition. The waveform of arc voltage is also influenced by the motion of cathode spots.

Self-Magnetic Field Calculation in Modeling the Current-Carrying Electrode System With Plasma Jet and Cathode Spot Motion in a Vacuum Arc

The method of magnetic field calculation and its influence on simulating the motion of cathode spots (CSs) in a vacuum arc were investigated in this paper. A method based on coordinate transformation and magnetic field superposition principle was proposed. In this method, the influence of current passing through an electrode rod and a contact plate was taken into account. Comparison was made between the proposed method and two simplified methods which were on the basis of the Biot-Savart law. Moreover, simulation results of CS motion with magnetic field calculated by different methods were shown and compared. Investigation results indicated that current passing through the contact plate and electrode rod could significantly influence the magnetic field and thus influence the simulation results of CS motion. Furthermore, it was shown that the proposed magnetic field calculation method had higher calculation accuracy with acceptable consumption of calculation time.

The Combined Influence of Contact Gap and Axial Magnetic Field on the Expansion Speed of Cathode Spots in High-Current Triggered Vacuum Arc

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 μs. 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.