Investigation of cathode spot characteristics in vacuum under transverse magnetic field (TMF) contacts

With the development of DC transmission and aerospace technology, the demands on the breaking capacity of vacuum circuit breakers under high di/dt condition are getting higher and higher. The breaking capacity of vacuum circuit breakers 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 (di/dt)1/2 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.

Section: Study On the Initial Expansion Velocity Of Cathode Spot Ringmentioning

confidence: 78%

Section: Introductionmentioning

confidence: 99%

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

Liu¹,

Yuan²,

Chen³

et al. 2022

“…At the same time, since the extinction of cathode spots will significantly affect the plasma distribution, their dynamic behavior is suggested to include cathode spot motion, splitting, and extinguishing phenomena. In order to obtain more realistic simulation results of the extinction process, the actual magnetic field configuration [30][31][32] is also considered in this work. In practice, an axial magnetic field (AMF) can keep the high-current vacuum arc in a diffuse mode, which is beneficial to arc interruption [33,34].…”

Section: Introductionmentioning

confidence: 99%

Three-dimensional hybrid simulation of vacuum arc extinction considering dynamic behavior of cathode spots

Li¹,

Cao²,

Cao³

et al. 2022

Vacuum arc extinction directly determines the plasma distribution at the current zero point, which affects the post-arc insulation recovery. In that regard, studying the extinction process to improve the post-arc insulation recovery ability is of great significance. In the extinction process, as the current decreases, cathode spots are extinguished and their distribution exhibits apparent discreteness and asymmetry, while the plasma density decays drastically. On the one hand, the discreteness of cathode spots makes the simulation setup regarding the whole cathode surface as a continuous emission source no longer applicable. On the other hand, the drastic decay of plasma density requires a simulation tool that can balance the computational efficiency and accuracy of the extinction process. To deal with these problems, the dynamic behavior of cathode spots is considered in this paper, and a three-dimensional hybrid plasma model is established. In order to simulate a more realistic situation, two applied axial magnetic field configurations for practical applications ─ bell-shaped and saddle-shaped configurations ─ are introduced. Both configurations are used to compare the position distribution and extinguishing sequence of cathode spots, as well as the evolution of plasma. More importantly, the method applied in this study can be employed to evaluate the effect of different magnetic field conditions on vacuum interruption, which would enable one to simulate the extinction process in a realistic manner.

“…The cathode spots move predominantly in the anti-Amperian, or retrograde direction under the action of transverse magnetic field (TMF) [7][8][9]. Based on the TMF contacts, the ring-shaped and star-shaped cathode spot distribution were discovered [10]. Besides TMF, axial magnetic field (AMF) is also important for the distribution of cathode spots.…”

Section: Introductionmentioning

confidence: 99%

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

Liu¹,

Yuan²,

Chen³

et al. 2022

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 obversed 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 which is generated by the arc column. The closer to the contact edge, the higher the transverse magnetic field at the cathode spot and the higher the expansion velocity of cathode spot.