Distribution Of Cathode Spots Research Articles

Influence of the vapor shield potential control on the post-arc sheath development in vacuum interrupters

In tank-type multi-break vacuum circuit breakers (VCBs) and gas-insulated switchgear (GIS), the non-uniform distribution of vapor shield potential in the vacuum interrupters impacts the post-arc process significantly. To explore the effect of the vapor shield potential on the development of the post-arc sheath, a particle-in-cell model with shield potential control is proposed to obtain two-dimensional distributions of post-arc particles. Based on the synthetic test circuit and probe array, the test platform of the post-arc characteristic diagnostic is set up to verify the influence of vapor shield potential distribution on the post-arc sheath. The extinguishing process of the vacuum arc is also observed by the high-speed CMOS camera. Simulation and experimental results show that at shield potential of 0.5 transient recovery voltage, the post-arc sheath develops most rapidly and the distribution of cathode spots is the most uniform. These findings can provide valuable insights for the design of vacuum interrupters and the control of shield potential in tank-type GIS.

Research on the influence of grain size of CuCr contact on the characteristics of cathode spot and cathode surface erosion in vacuum arc

CuCr alloy is widely used in vacuum interrupters. The characteristics of cathode spot and erosion of cathode surface directly affects the lifetime and breaking capacity of vacuum interrupter. In order to control the cathode spot and crater, two types of contacts with different Cr phase size were used in this paper. The vacuum arc breaking experiments were conducted. The cathode spot images were recorded by ultra high speed camera and craters were measured by Scanning Electron Microscope and Profiler. It was found that the smaller the Cr phase size, the higher the expansion velocity of cathode spots, the larger the number of cathode spot, the smaller the cathode spot size and the lower the current carried by each cathode spot. The reduction of Cr phase size helps to improve the uniform distribution of cathode spots on the contact surface. Besides, the smaller Cr phase size leads to the decrease of radius and depth of crater. For CuCr contacts, thermal diffusion process was analyzed and there is a significant difference in the thermal diffusivity between Cu and Cr. The Cr phase uniformly distributed in Cu prevents the thermal diffusion process which reduces the size of crater and the erosion of cathode surface.

The motion and distribution characteristics of cathode spot and its influence on plasma parameters

Cathode spots and plasma are the key factors affecting the breaking capacity of vacuum switches. In order to analyze the motion and distribution characteristics of cathode spot and its influence on plasmas, the experiments of cathode spot observation and plasma diagnosis were conducted in this paper. The cathode spot images were captured by ultra high-speed camera. Based on the continuous spectrum theory, the electron density was measured by the colorimetric thermometry. According to the cathode spot images and method of cathode spot recognition, the parameters of cathode spots including number, size and current were obtained. The changes of these parameters with arcing time and current amplitude were analyzed. The dependence of the cathode spots average size on spot current and spot expansion velocity was discussed. The axial and radial distributions of electron density were obtained which is related to the cathode spot distribution on the cathode surface. The significant fluctuation in the radial direction for electron density was observed.

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.

Optical and mathematical method for studying electrode discharge spots with a liquid electrode

High-speed visualization of the discharge with a liquid cathode, color image processing were performed. The area of cathode spots concentration was identified. Statistical characteristics of the distribution of cathode spots were obtained in order to determine the range in which the intensity code of the green color channel changes, the polygon function of the empirical distribution of the intensity code for the green color channel. The graphical dependence on the frequency of the cathode spot indication hit into the specified interval of the color intensity code was created.

Simulation of vacuum arc cathode spot movement between transverse magnetic field contacts

Vacuum switches have been extensively used in the field of medium voltage. Carrying out relevant research on vacuum arc is particularly important for improving the performance and reliability of vacuum switches. The vacuum arc cathode spots provide metal vapor and electrons, which determine the breaking capacity of the vacuum switches to some extent. Therefore, based on the simulation model of the cathode spots between the transverse magnetic field (TMF) contacts, the motion phenomenon and distribution of the cathode spot were simulated and the characteristics of the initial diffusion stage of cathode spots between TMF contacts were studied. The simulation results showed that the probability of generating a cathode spot in the direction of retrograde motion increased with the increase in the TMF component and the direction of motion varied with the angle between the magnetic field and the contact. The diffusion process of cathode spots under wan-shaped contacts, cup-shaped contacts, and spiral-shaped contacts was simulated. The ring-shaped distribution of the cathode spots between cup-shaped contacts with a large current in the initial diffusion stage was formed by the self-generated magnetic field of cathode spots, while the axial magnetic field component and the clustered cathode spots destroyed the ring-shaped distribution. Taking the distribution of cathode spots and current from experiments as initial conditions, the simulation results of cathode spot distribution in the initial diffusion stage and the arc root radius were consistent with the experimental results.

Research on the Postarc Sheath Growth Process Considering the Plasma Motion and Distribution

In this article, the sheath growth of the vacuum circuit breaker and its initial conditions of the calculation are introduced. The images of cathode spots distribution and waveforms of postarc current under different frequencies are obtained by medium- to high-frequency current breaking experiments. The quantitative relationship of $D$ and $di/dt$ with the variation of current frequency $f$ and current amplitude $I_{m}$ is obtained by fitting the data. The reasons for the change of postarc current parameters are reasonably explained by the analysis of equivalent circuit and plasma movement process. Combined with the continuous transition model (CTM) and its initial calculation conditions, the influence of the cathode spots distribution diameter $D$ and $di/dt$ after the current zero crossing on the sheath growth is analyzed. An initial postarc plasma density calculation model which is suitable for each current frequency is proposed which makes the calculation of the sheath growth more accurate, especially when applied to medium- to high-frequency current conditions.

Experiment research of post-arc current and cathode spots distribution in medium-high frequency vacuum arc

In this paper, the breaking process of the vacuum circuit breakers is briefly described, and the main factors that influence the post-arc process are analyzed. Based on the detachable vacuum interrupter, the experimental platform of AC breaking is built, and the experiments of the post-arc current measurement and cathode spot observation are completed. The voltage and current during the period of current zero are measured, and the images of cathode spots during the arcing process are obtained. The experimental results show that at the same frequency, the post-arc current Ipac and the post-arc charge Qpac increase with the increase in didtcz(didt at current zero). Under the same conditions of didtcz, Ipac and Qpac decrease with the increase in frequency, but both are unchanged with frequency when the frequency is high enough. With the increase in current frequency, the distribution diameter of cathode spots decreases under the condition of the same didtcz value. According to the characteristics of cathode spot distribution, the metal vapor density under different conditions is calculated. Comparing the calculation results of metal vapor with the experimental results of post-arc current, the concept of metal vapor ionization is put forward. An ionization process of metal vapor after the current zero crossing occurs. The ionization of metal vapor will cause an increase in the number of electrons after the current zero crossing, which affects Ipac, Qpac, and sheath growth.

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

AbstractWith the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de‐mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high‐speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi‐cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.

Investigating cathode spot behavior in argon alternating current tungsten inert gas welding of aluminum through experimental observation

The behavior of cathode spots in stationary argon alternating current tungsten inert gas welding of aluminum was clarified in this study. The temporal variation in cathode spot distribution during electrode positive (EP) polarity was, for the first time, revealed quantitatively based on high-speed video camera observation with a frame rate of 500 000 fps. In addition, the weld pool surface temperature at the start and end of EP polarity was also measured using the two-wavelength temperature measurement method to clarify the relationship with the cathode spot behavior. As a result, the general tendency of the life cycle of the cathode spot was clarified. The first cathode spot appeared and divided outside the weld pool. Following this, they moved rapidly toward the weld pool center within 100 µs. After that, the cathode spot group became an uneven and ring-shaped distribution, which expanded outward with time with a velocity of approximately 1 m s−1. The cathode spots have a random movement with the average velocities of each cathode spot on the liquid and solid surface being about 142 ± 59 m s−1 and 87 ± 44 m s−1, respectively. The ratio of the number of cathode spots located in the weld pool to the total number of cathode spots decreased from 100% to 37.4% with elapsed time, except right at the beginning. Furthermore, the temperature of the center surface of the weld pool was 1400 K, and this gradually increased to a slightly higher temperature than the melting point of aluminum around the weld pool boundary at the start of EP polarity. The maximum temperature at the center is considered to be linked with absence of a cathode spot. The temperature at the center decreased by 150 K at the end of EP polarity due to the temporal expansion in the ring-shaped distribution of the cathode spot group leading to a decrease in heat input around the center.

Arc Appearance and Cathode Spot Distribution in a Long-Gap High-Current Vacuum Arc Controlled by an External Axial Magnetic Field

An experimental study of a high-current vacuum arc generated between two static CuCr25 contacts spaced 20 or 30 mm apart was conducted to characterize the arc appearance and the cathode spot (CS) distribution. The arc was ignited from the lateral surface of the cathode and controlled using an external axial magnetic field. Under the investigated experimental conditions, three distinct arc modes have been observed: multiple arc, diffuse, and diffuse columnar modes. The latter appeared at low $B_{\mathrm {AMF}}/I_{\mathrm {arc}}$ values, typically below 4 mT/kA. The temporal evolutions of the spot spatial distribution and the associated distribution of the current density were analyzed by processing high-speed video images of the cathode. Various types of distribution depending mainly on current were identified. At low currents (up to 13 kA), the CS distribution covered only a fraction of the cathode surface. At intermediate currents (in 17.2–31.1-kA range), CSs were present on the whole circumference of the cathode and the CS distribution included a closed region without spot. The latter was progressively filled by CSs, yet it totally disappeared before the end of arcing only when its location was off-centered with respect to the cathode axis. At high currents (up to 36.3 kA), the whole cathode surface was occupied by CSs at current peak. The azimuthally averaged radial distribution of the current density was found to be relatively uniform in the regions occupied by CSs. An average spot current of 36.5 ± 2.5 A has been estimated.

Effect of vacuum arc cathode spot distribution on breaking capacity of the arc-extinguishing chamber

A DC circuit breaker performs a key function in breaking an intermediate-frequency (IF) current since breaking a pure IF current is equivalent to breaking a very small DC with a reverse IF current. In this study, it is found that cathode spots show a ring-shaped distribution at 2000 Hz. An arc with an uneven distribution of cathode spots has been simulated. The simulation results show that the distribution of cathode spots significantly affect the microparameter distribution of arc plasma. The current distribution on the anode side differs from that on the cathode side under the total radial electric field. Specifically, the anode current distribution is both uneven and concentrated. The applied axial magnetic field, which cannot reduce the concentrated anode current distribution effectively, might increase the concentration of the anode current. Finally, the uneven distribution of cathode spots reduces the breaking capacity of the arc-extinguishing chamber.

Evolution of the Cathode Spot Distribution in an Axial Magnetic Field Controlled Vacuum Arc at Long Contact Gap

The distribution of cathode spots in a CuCr25 vacuum arc controlled by an axial magnetic field and ignited on the lateral surface of the cathode is investigated for long gap distances, from the processing of high-speed video images. The processing method includes also estimating the current carried by a single spot and reconstructing the distribution of the current density at the cathode. Various distributions depending partly on the arc current are described.

Cathode spot motion in a vacuum arc with a long roof-shaped cathode under magnetic field

Vacuum arc cathode spot motion was investigated on a “roof-shaped” aluminum cathode under an external magnetic field. The cathode included a flat roof-top and four sloped sides, inclined by an angle α. A high speed camera was used to observe cathode spot motion. The spot velocity on the slopes and the distribution of cathode spots on the roof and slopes was determined. It was obtained that under a magnetic field, the spot motion on the roof was slow (<1 m/s) and mainly random, while on the slopes fast retrograde motion was observed. This velocity increased linearly with the magnetic field and decreased slightly with α.

Effect of Fe on vacuum breakdown properties of CuW alloys

CuW alloys with different Fe additions were prepared by the infiltration method. The microstructures of the CuW alloys prepared were characterized by a transmission electron microscope (TEM). The effect of Fe content on the surface erosion morphology was observed by a scanning electron microscope (SEM). The distribution of cathode spots was analyzed by a digital high-speed video camera. The results show that Fe addition can improve the relative density and thermal conductivity of CuW alloy with less than 0.6 wt.% Fe. However, excessive Fe addition can deteriorate the relative density and thermal conductivity. TEM results show that Fe addition can promote the formation of CuW solution. The area and depth of the erosion pits on the surface of CuW alloy depend on the breakdown position. Fe addition can strengthen the interface bond of Cu and W, improve the arc stability and the division of the cathode spots and, thus, avoid the concentration erosion.

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.

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.

Distribution of Cathode Spots in Vacuum Arc Under Nonuniform Axial Magnetic Fields

Recent results on the distribution of vacuum arc cathode spots (CSs) in nonuniform axial magnetic field (AMF) are presented. Based on previous studies, we deem that two contrary influences of AMF, inward effect and outward effect, are attributed to CSs distribution. With this notion, we have analyzed the controlling effectiveness of nonuniform AMF on CSs distribution. Experiments were conducted in a detachable vacuum chamber with iron-style AMF electrodes. Images of vacuum arc column and the distribution of CSs were photographed with a high-speed charge coupled device (CCD) camera. Experimental results agreed well with the theoretical analysis.

Numerical simulation of vacuum arc under different axial magnetic fields

Based on the improved two-dimensional axisymmetric magnetohydrodynamic model, the vacuum arc characteristics under different distributed axial magnetic fields (AMFs) were analysed in this paper. First, a uniform cathode spot distribution was assumed, and the influences of the azimuthal self-magnetic field and different radially distributed AMFs on the vacuum arc characteristics were analysed. Then, in order to simulate the real situation, according to the experimental results of other researchers, a non-uniform distribution of cathode spots was considered in the simulation. The simulation results showed that the effect of the current density of the cathode side on the vacuum arc characteristics (such as distribution of axial current density at the anode side and heat flux density to the anode) was significant.

Cathode spots on the surface of nanostructured W alloys

The tracks of cathode spots produced after arcing on the surfaces of nanostructured W alloys were investigated using a scanning electron microscope (SEM). For nanostructured W–2%ThO 2 cathodes, the distribution of cathode spots was dispersed and their geometrical properties were completely controlled by the shape and size of ThO 2. For nanostructured W–20%Cu cathodes, the spots were also separated on Cu-rich phase. The results showed that cathode spots on the surface of nanostructured W alloys were quite different from those on the surface of commercial W alloys with coarse grain size, and the microstructure of the cathode materials had significant influence on the cathode spots propagation. Also, space-charge regions induced by internal electric field at the interface which responded to the effect of microstructure on cathode spots were suggested.