Search

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]

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 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]

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]

In this paper, the influence of an axial magnetic field (AMF) on the expansion speed of cathode spots (CSs) in the initial expansion stage of high-current triggered vacuum arc is investigated experimentally in a demountable vacuum chamber. Butt contacts with contact plate material of oxygen-free high-thermal-conductivity Cu and CuCr25 were used in experiments. Images of CSs are photographed with a high-speed digital camera with an exposure time of 2 \mus. The uniform constant AMF in the intercontact region is supplied by external magnetic field coil. Experimental results indicate that, in an arc stabilized by AMF, CS expansion speed also increases with the increase of di/dt, which is the same as that in arc burning freely. In addition, quasi-quantitative analysis of the experimental results show that the bending and lengthening of CS motion trajectory in AMF are not sufficient to explain the decrease of CS expansion speed with AMF. Another reason might be related to the inhibitory effect of AMF on the loss of arc plasma, which could also contribute to the decrease of CS expansion speed. [ABSTRACT FROM AUTHOR]

This paper analyzes how removing oxide layer on metal surface by vacuum arc could be a green substitute for the traditional acid-washing and mechanical cleaning technique widely used in the metallurgical industry, which is harmful to the environment. In this paper, the effects of vacuum arc on removing oxide layer on the surface of carbon-steel tubes, which were used as the cathode, is investigated experimentally. External coils are used to generate transverse magnetic field (TMF) driving cathode spots to rotate on tube surface. Experiments are conducted in a vacuum chamber with Ar as the protection gas under different TMF flux densities. Arc images are captured by a high-speed digital camera with an exposure time of 2 \mus. The influence of transverse magnetic flux density on the characteristics of arc and descaling effect is mainly investigated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Removing an oxide layer on a metal surface by vacuum arc has been preliminarily used in the metallurgical industry as an environment-friendly substitutor for the traditional acid-washing technique. In this paper, the influence of protection gas pressure on descaling process is investigated experimentally. Arc images taken by a high-speed digital camera indicated that the descaling rate was closely related to the motion pattern of cathode spots in different gas pressures. The decrease of descaling rate with the increase of gas pressure could be partly attributed to the formation of cathode-spot group and its less mobility at high gas pressure. The influence of gas pressure on the power density of cathode spot was also analyzed. It seemed that higher gas pressure would lead to the decrease of the ratio of power input into the cathode spot to the total arc power, which could also contribute to the decrease of the descaling rate. [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]

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 \mu\s. 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. [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]

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]