A. Processing of vacuum arcing images
When the experimental breaker broke a 5kA fault current, the high-speed camera captured the spatiotemporal evolution of the arcs at VCB1 (break 1) and VCB2 (break 2). The process of vacuum arcing can be divided into three stages: arc starting, arc burning, and arc extinguishing. Initially, the mechanism begins to act. Some spots of energy accumulation appear on the surface of the cathode contact, which is a typical phenomenon in the arc starting stage. With the development of zero zone arcing, a significant amount of plasma forms an arc channel between the contacts; this is the stable arc burning stage. When the gap between the contacts reaches a safe distance, the commutation circuit is activated, causing the main break current to decrease. When the arc energy decreases to a point where it is insufficient to maintain stable arc burning, the arcing process enters the arc extinguishing stage.
The evolution of the zero zone during vacuum arcing has a direct effect on the performance of vacuum breakers. On the micro level, the motion of vacuum arc plasma between contacts is influenced by the inter-electrode magnetic field. On the macro level, phenomena such as energy spots, arc column contraction and diffusion, and formation of metal droplets will appear during the arc burning process. To describe the spatial dynamic behavior of vacuum arc, we analyze the spatiotemporal evolution of arcing using both theory and experiment. A high-speed camera was used to capture the images of arc variation in the zero zone. The shooting covered the entire process of arcing, including arc starting, arc burning, and arc extinguishing. The images of vacuum arcing were processed using image processing techniques to reveal the spatiotemporal evolution of vacuum arcing and the distribution pattern of charged particles between the contacts. Edge detection techniques were used to detect and delineate the edges of charged particle nebulas in arc images, obtaining the arc area at each stage of the zero zone process and the charged particle zones formed by magnetic field action. The regions of different brightness levels in the arc images were color-coded through pseudo-color enhancement. Figure 3 depicts a comparison of original images and processed images.
B. Analysis of arc images captured under the condition of synchronous breaking
Vacuum plasma arc is mainly composed of metal vapor particles generated during the course of contact ablation. The plasma density distribution between the contacts and the energy dissipation velocity have a direct impact on the dielectric recovery capability and current breaking performance of the circuit breaker after the vacuum arc has been extinguished.
To investigate the effect of the gap difference between the two breaks of a DB DC VCB on the spatiotemporal evolution of the arc and the spatial distribution of plasma during the breaking process, we adjusted the gap difference by manipulating the action times of the two breaks using a control mechanism and captured the arc images during the zero zone process in each round of experimentation. During the first part of the experiment, the two breaks were controlled to act synchronously (hence no gap difference) during the breaking process, the arc evolution process in the zero zone process was recorded, and the arc images were processed. The images of arcs were subjected to a pseudo-color enhancement process in which regions with varying levels of brightness were color-coded. Different colors represent different particle concentration levels, hence different arc energy levels. High-brightness regions are the sites where arc energy accumulates, with the arc column possessing the highest plasma density and energy level. Under the action of the magnetic field, the plasma diffuses away from the contacts while dispersion and recombining continuously, causing the plasma density outside the arc column and the arc energy to drop. In pseudo-color-enhanced arc images, a red region indicates the highest plasma density level, while a blue region indicates the lowest plasma density level. From red to blue, plasma density level gradually declines. The arc images were created so that they could be subjected to the secondary treatment, which consisted of extracting regions of different colors based on the result of pseudo-color enhancement. Among them, the red region covering the arc column is the high-temperature zone, the orange and yellow regions are the medium-temperature zones, and the green and blue regions are the low-temperature zones. In the high-temperature zone of the arc column, both the arc energy level and the concentration level of charged particles are the highest. The medium and low-temperature zones are formed when the arc diffuses away from the contacts under the influence of a magnetic field, resulting in a gradual decrease in the concentration of charged particles. Some examples of processed images are shown in Figure 4.
To further investigate the distribution patterns of arc energy and plasma at the two breaks of the experimental breaker during the process of synchronous breaking, we extracted the high, medium, and low-temperature zones from the pseudo-color-enhanced images and performed edge detection in these regions. Based on the number of pixels in each region, the area of each region was calculated. Based on this, the plasma density distribution and energy dissipation pattern during the zero zone process of vacuum arcing were analyzed. The calculated areas of different temperature zones are shown in Figure 5.
The vertical axis in the figure represents the number of pixels calculated after image processing. The arc area is represented by the number of pixels, and the calculation results can be used for qualitative analysis of the sizes of different temperature zones.
The processed arc images show: At 0.36ms after the contacts separate, energy-accumulating cathode spots appear on the contact surface. At this time, the energy level is low at these spots. As the contacts moves further apart, a large number of metal vapor particles are produced as a result of the field emission effect, causing the spatial plasma and arc energy to gradually increase. At 0.48ms, an arc channel is formed, as indicated by the high-temperature zone on the arc image.
When the gap between the contacts increases to the safe width, the commutation circuit applies a reverse current to create an artificial zero-crossing point. At this point, there is no energy input to the break, and the arc moves rapidly along the arc path until it extinguishes under the influence of the magnetic field. During the course of arc channel disappearance, rapid recombination of metal vapor particles and formation of a sheath layer take place, until the dielectric between the contacts is completely restored. A comparison of the arc images' temperature zones reveals: In the scenario of synchronous breaking, the arcs generated at the two breaks have different shapes due to the influence of the magnetic field. However, the calculated area values of the column region (red region) indicate that the two arcs are basically consistent in terms of arc energy and plasma density, and their diameters are close to one-half of the contact area. In addition, under the influence of the magnetic field, the vacuum arcs expand toward the outer sides of the contacts, and the distribution pattern and variation trend of plasma at each arcing stage are consistent between the two breaks. During the stage of arc extinguishing, the two arcs break at different locations, which has to do with the shape of the arc column and magnetic field distribution. The plasma density is lower in regions far from the arc column, and the particle density and area of the residual particle zone are comparable for the two breaks.
Analysis of the arc images captured during the breaking period of 0 to 0.36 ms reveals that immediately after the contacts separate, there is no obvious bright region; at 0.36 ms, a bright region appears, indicating that energy accumulates at the cathode spots. Then, multiple arc channels are formed at the time of arc starting. As a result of energy interaction between the arc channels, the arc channels eventually merge into a single arc channel, or arc column; as arc burning time passes, the diameter of the arc column increases. The arc column zone has the highest energy level, and the energy level declines along the radial direction of the contacts.
When the arc is located on one side of the contacts under the control of a magnetic field, the energy level on the side of the arc column is significantly higher than that on the side without an arc column. Additionally, metal droplets form on the contact surface on the side of the arc column. An observation of the arc energy distribution in the entire arcing process reveals that under the action of a transverse magnetic field, the location of the arc generation point is random, but its motion trajectory is influenced by the magnetic field. As the arc moves along the contact surface, there is a correlation between the plasma density distribution and the arc motion trajectory. During the arc-burning process, the area of the medium-temperature zone remains relatively stable, whereas the areas of the high- and low-temperature zones continue to expand at different speeds.
The images captured in the period of 2.96-3.04ms show: When the commutation circuit is activated, the arc energy begins to decrease; When the energy between the contacts is insufficient to maintain stable arc burning, the arc breaks from the center of the arc column. Due to the presence of an electric field on the contact surface, the energy level in the contact gap's center is lower than that on the contact surface. Within a very short period after the extinguishing of arc, there are still a large quantity of residual particles in the contact gap, and the residual particles have a certain amount of energy. This period is the post-arcing dielectric recovery phase, which involves the diffusion and disappearance of residual particles.
A comparison of the sizes and emergence times of the two breaks' various temperature zones reveals: At 0.36ms, a low-temperature zone appears; At 0.42ms, the plasma density increases, and a medium-temperature zone appears; At 0.48ms, a high-temperature zone appears; When there is no gap difference between the two breaks, the arcs at the two breaks have different shapes due to slight differences in the roughness of the contact surface, cathode spot occurrence locations, and magnetic field, but the sizes of the temperature zones are approximately the same for both breaks. After image processing, the obtained area values can be used to determine the growth rate and development trend of arc plasma density, as well as the sizes of particle concentration distribution regions and the plasma diffusion range. It is believed that the higher the plasma density, the greater the arc energy level in the region, and that there is a direct correlation between arc energy level and plasma quantity.
The results of a calculation based on the processed images indicate that when there is no gap difference between the two breaks, the area of the medium temperature zone is small, while the area of the low-temperature zone is the largest. Given the positive correlation between arc energy, brightness, and particle concentration level, it is possible to calculate that, as time passes, the gap widens, the area of the high-temperature zone increases slowly, the area of the medium-temperature zone remains essentially unchanged, and the area of the low-temperature zone increases significantly. It can be concluded that during the stable arc burning stage, the arc moves along the arc path under the action of the transverse magnetic field, which has a positive effect on the dissipation of arc energy. The area of the arc column zone increases slowly, but the widening of the gap, the elongation of the arc, and the shrinking of the arc column are conducive to the interruption of the arc. As a result, the velocity of plasma diffusion from the high-density area to the low-density area increases (as indicated by the increase in the area of the low-temperature zone), which increases the dissipation and recombination of particles.
C. Analysis of arc images captured under the condition of asynchronous breaking
When there is a gap difference between the two breaks of a DB DC VCB, the beginning and development of the arc will be affected. The arc images were captured and processed when the gap difference between the two breaks of the DC VCB was 0.4mm (when the commutation circuit is activated, the gap width of break 1 is 6mm, and the gap width of break 2 is 5.6mm). The area curves of high, medium, and low-temperature zones of the two breaks plotted based on the calculated area figures are shown in Figure 6.
The calculated areas of distinct temperature zones on the processed arc images indicate: At 0.3ms, cathode spots appear on the surfaces of breaks 1 and 2 to form low temperature zones. At 0.36ms, a medium temperature area is formed at break 1, while medium and high-temperature zones appear simultaneously at break 2, which can be explained by the following causal chain: The gap width of break 2 is smaller than that of break 1 → The travel distances of charged particles are shortened → The field emission effect on the cathode surface is stronger → The arc energy is more concentrated → Medium and high-temperature zones are formed earlier. Comparing the area curves of different temperature zones reveals that the areas of the high, medium, and low-temperature zones differ between the two breaks. Specifically, the area of the medium-temperature zone of break 2 is close to that of break 1; the area of the low-temperature zone of break 2 is smaller than that of break 1. The area of the high-temperature zone of break 2 becomes increasingly larger than that of break 1 with the elapse of time. The gap of break 2 is smaller than that of break 1. However, its high-temperature zone is larger than that of break 1, indicating that the arc energy of break 2 is greater than that of break 1, that the high plasma density zone (arc column) of break 2 is larger, and that the arc energy of break 2 is more concentrated. The area of the low-temperature zone of break 2 is smaller than that of break 1, indicating that the arc energy dissipation velocity of break 2 is less than that of break 1.
When the gap difference between the two breaks of the DC VCB is 0.8mm (When the commutation circuit is active, the gap width of break 1 is 6mm, and the gap width of break 2 is 5.2mm), arc images are captured and processed. The arc evolution is depicted in Figure 7.
The results obtained from image processing and calculation indicate that at break 1, a low-temperature zone appears at 0.24ms, while medium and high-temperature zones appear at 0.3ms. At break 2, whose action lags behind break 1, low, medium, and high-temperature zones appear simultaneously at 0.42ms, indicating that arcing occurs immediately after the separation of contacts under these working conditions, and that the arc energy is very high. The experimental results indicate that when the contacts of break 1 have moved a certain distance apart and the contacts of break 2 remain static, the formation of high, medium, and low-temperature zones and arc channels is significantly accelerated compared to synchronous breaking scenarios. After the contacts of break 2 move apart, an arc channel is formed between the contacts rapidly, which elongates the overall arc burning time and aggravates the contact ablation. Comparing the area curves of various temperature zones reveals that the high-temperature zone of break 2 is significantly larger than that of break 1, while the low-temperature zone is significantly smaller. The size of the medium temperature zone is close to that of break 1. The areas of the high and low-temperature zones of break 1 are close to one another, and both are larger than the medium-temperature zone. The high-temperature zone of break 2 is the largest, while the medium-temperature zone is the smallest.
To further study and compare the distribution patterns of arc energy and plasma at the two breaks of the experimental breaker during the asynchronous breaking process, we compared the areas of the low, medium, and low-temperature zones of the normal break and smaller-gap break under various gap difference conditions. The comparison results are shown in Figures 8 and 9.
Comparing the appearance times of different temperature zones at breaks 1 and 2 reveals that as the gap between the two breaks increases, low, medium, and high-temperature zones appear earlier and earlier, which can be explained as follows: In the scenario of synchronous breaking, in which the contacts of the two breaks operate simultaneously, the energy of the entire breaker is distributed evenly between the two breaks (i.e., both breaks have the same contact surface energy). Under these conditions, the arcing process between the two breaks is consistent. In the scenario of asynchronous breaking, break 1 acts normally (at t1), break 2 acts with a delay (at t2). During the interval between t1 and t2, the energy of the entire circuit breaker is concentrated at break 1, thereby accelerating the arc generation process and causing cathode spots to appear earlier. By the time the contacts of the smaller-gap break (break 2) separate, the dielectric property of the medium between the contacts of the normal break (break 1) has been degraded, so break 2 carries the majority of the system's energy. Consequently, arc channels are formed rapidly between the contacts of break 2, causing arcing to occur earlier and arc energy to be more concentrated than in the synchronous breaking scenario. This working condition is not conducive to the dissipation of arc energy as well as the dispersion and particle recombination of the plasma, which will further hinder the post-arcing dynamic dielectric recovery.
A comparison of the areas of break 1's various temperature zones reveals that under the working condition of zero gap difference, the area of the low temperature zone is the largest and the area of the medium temperature zone is the smallest. As the gap difference increases from 0 to 0.8mm, the areas of the high and medium temperature zones of break 1 increase gradually and slowly, while the area of the low-temperature zone decreases significantly. The following conclusions can be drawn from the results of the above experiments: In the scenario of synchronous breaking, the plasma diffuses rapidly under the influence of a magnetic field, and the rate of arc energy dissipation is the highest. In the scenario of asynchronous breaking, as the gap width of the smaller-gap break decreases, the area of the arc column zone increases, while the area of the low-temperature zone decreases. It indicates that a change in working conditions alters the distribution patterns of arc energy and plasma, and that the increase in plasma generation rate and decrease in plasma dissipation and recombination velocities slows the arc energy dissipation velocity.
Comparing the areas of the temperature zones of break 2 reveals that as the gap width decreases, the area of the high-temperature zone increases, the area of the low-temperature zone decreases, and the area of the medium-temperature zone changes only slightly. In the scenario of asynchronous breaking, as the gap width increases, the arc energy and plasma become more concentrated, the magnetic field's ability to regulate the arc decreases, and the arc diffusion velocity decreases, which is not conducive to the breaking of the arc between the contacts and the post-arcing dielectric recovery.
An examination of the analysis results of Figures 8 and 9 reveals that the arc energy levels at the two breaks of the dual-break DC VCB affect each other. When the gap width of one break is smaller than that of the other break, the distribution patterns of arc energy and plasma at the smaller gap break will change, and the other break will be affected by these changes. As a result, the contact ablation becomes more severe, leading to a greater concentration of arcs at the two breaks, slower plasma recombination and diffusion, and slower arc energy dissipation. It can be concluded that asynchronous breaking reduces the ability of DB DC VCBs to break arcs. A comparison of the emergence times of the two breaks' low, medium, and high-temperature zones reveals that as the gap difference between the two breaks increases, the low, medium, and high-temperature zones will appear earlier, and the arc diffusion time and plasma density will increase at the two breaks. The increase of gap difference also causes changes in all stages of arcing: In the initial stage of arcing, the unevenness of energy distribution between the two breaks worsens, the arc column zones at the smaller-gap break become more concentrated, the arc diameter increases, the high-density plasma zone expands significantly, and the diffusion of plasma towards the edges of the contact gap slows; In the stable arc burning stage, the unevenness of energy distribution between the two breaks aggravates, the arc energy between the contacts of the small gap break increases, the contact surface ablation intensifies, more metal vapor particles are generated, spots appear on the anode surface, and metal droplets are formed on the contact surface In the post-arcing stage, the densities of residual particles at the two breaks increase to differing degrees (the area of the high-density residual particle zones of the smaller-gap break is significantly larger than that of the normal break), and the post-arcing dielectric recovery capability weakens to differing degrees for both breaks. For the DB DC VCB as a whole, the gap difference between the two breaks will affect its performance in the following aspects: The gap difference leads to an increase in arc diameter, weakens the ability of magnetic field to regulate the arc, intensifies contact ablation, which is not conducive to arc extinguishing and fault current breaking; After the arc is extinguished, the initial concentration of residual metal vapor particles increases, the velocity of energy dissipation decreases, and the dielectric recovery ability is weakened, which reduces the service life and current breaking performance of the entire breaker. The greater the gap difference, the more severe the degradation of the DB DC VCB's current breaking ability.
D. Analysis of arc images captured in the process of failed current breaking
When the gap difference between the two breaks of the experimental DC VCB is 1mm (When the commutation circuit is activated, the gap width of break 1 is 6mm, and the gap width of break 2 is 5 mm), arc images are captured and processed. The area curves of the high, medium, and low-temperature zones of the two breaks plotted based on the calculated area figures are shown in Figure 10.
The following can be observed from Figure 10: After the commutation circuit has been activated, the current passing through the two breaks reaches zero, thereby stopping the energy input into the arc. As a result, the areas of the high and low-temperature zones of breaks 1 and 2 decrease, while the area of the medium temperature zone remains nearly unchanged. After the arc extinguishes, a substantial amount of plasma remains between the contacts, while the residual energy and metal vapor slowly dissipate. At this time, the TRV is borne by the two breaks. As the dielectric recovery ability of break 2 is weak due to the slow plasma dissipation velocity, a breakdown weak point is formed, resulting in re-ignition of the arc at break 2 when break 2 cannot withstand the portion of TRV applied to it. Due to the current continuity of the two breaks, arc re-ignition also occurs at break 1. After the arc is re-ignited, arc channels are once again formed at the two breaks, resulting in rapid plasma generation and re-expansion of the high and low-temperature zones. Subsequently, the main break and the commutation capacitor start the charging-discharge process, causing the current passing through the break to cross zero again. At this point, the energy input into the arc ceases, and the TRV continues to be supported by the breaks. As a result of the previous arc re-ignition increasing the amount of residual plasma between the contacts, the dielectric recovery ability weakens, and arc re-ignition occurs again under the influence of the TRV. After repeated charging-discharging processes of the main break and the communication capacitor, as well as the arc extinguishing and re-igniting processes at the breaks, it is evident that the areas of the high and low temperature zones at the two breaks increase gradually, indicating that more plasma is generated and left between the contacts, the contact ablation becomes more severe, and the dissipation of a large amount of metal vapor particles is slow. Consequently, the dielectric recovery capability of the circuit breaker rapidly degrades, ultimately leading to breaking failure.
As shown in Figure 11, the arc images of break 2 captured after zero-crossing of current under synchronous and asynchronous breaking conditions are processed to obtain the distribution pattern of residual particles.
In the figure, the vertical axis indicates the color scale value (ranging from 0 to 255) obtained through pseudo-color enhancement, which reflects the plasma density level. The greater the value, the denser the plasma. Figure 11 reveals the following details: After the current reaches zero, there is still a substantial amount of residual plasma between the contacts, but the areas of all temperature zones decrease significantly compared to the arc burning period. When the gap difference is equal to zero, the area of the high-temperature zone on the cathode surface decreases significantly, metal droplets slide down the contact surface, and anode spots are obvious on the anode surface. When the gap difference is 0.4mm or 0.8mm, there is a certain amount of residual energy and a high-temperature plasma region with a certain level of density, the quantity and volume of metal droplets increase, and the area of the anode spots on the anode surface increases significantly (in the case of a 0.8mm gap difference, multiple anode spots aggregate into a single spot); When the gap difference is 1 mm, it is evident that after the arc is extinguished, the arc is broken and there are no complete arc channels between the contacts. At this time, the residual energy on the cathode and anode surface, as well as the area and volume of high-temperature plasma, increase significantly, and there is still a very high level of residual energy and a high-temperature plasma zone, forming a breakdown weak point, so there is a high probability of arc re-ignition under the action of TRV. Comparing the area of the high-temperature zone in the center of the contact reveals that, after the current crosses zero, the high plasma density region at the center of each contact of the smaller-gap break also increases gradually with the increase of gap difference. Therefore, it can be concluded that the velocity of arc energy dissipation and the plasma recombination velocity decrease with the decrease of the gap width.