**[b]1. Problem Statement[/b]** In power supply systems, voltage abnormalities occur due to various reasons such as power loss. To maintain voltage and power factor and reduce power loss, reactive power compensation is often used to maintain the normal operating voltage of the power grid and ensure power quality. In 12kV-40.5kV systems, the most practical and economical method for reactive power compensation is to install cosine capacitor banks. In power systems, oil-minimum circuit breakers were initially used to control cosine capacitor banks; however, they cannot fully meet the requirements for the frequent switching of capacitor banks. In recent years, vacuum circuit breakers have been widely used in power systems due to their long service life, frequent interruption capability, oil-free operation, and low maintenance. Therefore, power systems also hope to replace oil-minimum circuit breakers with vacuum circuit breakers for switching capacitor banks. As is well known, vacuum switches generate various complex operational overvoltages during capacitor bank switching. If the performance of the vacuum circuit breaker is not perfect, it is prone to reignition, generating high overvoltages that seriously damage power equipment, thus limiting the application of vacuum circuit breakers in this area. In recent years, with vacuum switches gaining a dominant market share in the medium-voltage field, this demand has become more prominent and urgent. According to data from the Shaoxing test station, in the 10kV field, the reignition rate of domestically produced vacuum switches switching capacitor banks has remained at around 1% in recent years. This data falls short of user expectations and also lags behind the test statistics of similar foreign products. Therefore, how to improve the capacitor bank switching capability of vacuum circuit breakers is a problem that vacuum switchgear manufacturers must face. The vacuum interrupter, as the "heart" of the vacuum circuit breaker, plays a crucial role in controlling the vacuum arc to complete the switching action between conductors and insulators, and is also vital for the successful switching of capacitor banks. Therefore, how to improve the capacitor bank switching capability of vacuum switches is also a problem that interrupter manufacturers need to seriously study. 2. Characteristics of Reignition Phenomenon When Vacuum Switches Switch Capacitor Banks What exactly causes reignition when vacuum switches switch capacitor banks? This requires analysis of the characteristics of the reignition phenomenon that occurs when vacuum switches switch capacitor banks. We found that the reignition phenomenon occurring when a vacuum switch closes a capacitor bank exhibits the following characteristics: 2.1 The occurrence of reignition is highly random and unpredictable; Multiple observations of reignition show that the timing of reignition, the number of interruptions, the magnitude of the interrupting current, and the phase in which reignition occurs are all completely unpredictable. It is impossible to predict at which interruption, in which phase, or after the arc has extinguished. The highly random and unpredictable nature of reignition, possessing only a certain probability, increases the difficulty of studying the reignition phenomenon when a vacuum switch closes a capacitor bank. 2.2 Re-ignition often occurs between 40 and 100 ms after current interruption. When the vacuum switch is switching capacitor banks, the voltage waveform shows a significant difference in timing between the voltage re-breakdown phenomenon and the re-ignition phenomenon during typical short-circuit current interruption. Re-ignition during short-circuit current interruption generally occurs a few milliseconds after arc extinguishing, at most no more than one cycle. However, re-ignition during capacitor bank switching often occurs between 40 and 100 ms after arc extinguishing, sometimes even longer. Therefore, it can be concluded that the re-ignition mechanism of the vacuum switch when switching capacitor banks differs from that during short-circuit current interruption. 3. Analysis of the Causes from Traditional Aspects Affecting Vacuum Interruptor Performance Since the reignition phenomenon occurring when vacuum switches are switching capacitor banks exhibits the above characteristics, we attempt to analyze the causes from several traditional aspects that may affect the performance of vacuum interruptors: 3.1 Vacuum Degree As is well known, vacuum interruptors use vacuum as the insulating medium, giving them advantages such as strong arc-extinguishing ability, fast dielectric strength recovery speed, and high insulation strength. Therefore, maintaining a necessary vacuum degree within the vacuum interruptor is crucial. When the vacuum degree within the vacuum interruptor is too low, the arc may fail to extinguish or reignite during the switching process. However, reignition caused by low vacuum should be continuous and approximately repeatable. Reignition occurring when vacuum switches are switching capacitor banks is discontinuous and accidental. Moreover, even when the vacuum degree of the vacuum interruptor used for switching capacitors is measured before the test, or even when a specially manufactured and selected vacuum interruptor with a high vacuum degree is used, reignition still occurs. After the test, vacuum degree testing of the interruptors that exhibited reignition was found to be good. Therefore, the view that low vacuum causes switch reignition is clearly untenable. 3.2 Structural Design of the Arc-Extinguishing Chamber If the structural design of the arc-extinguishing chamber is not reasonable enough, resulting in an uneven electric field distribution or insufficient arc control, the arc-extinguishing performance of the chamber will be poor. This will cause the arc to fail to extinguish or withstand the transient recovery voltage during the breaking process, leading to breaking failure. However, this type of breaking failure mainly manifests in the breaking test of the ultimate short-circuit current, where the short-circuit current generally reaches tens of kiloamperes. The characteristic of breaking failure is that the arc is extinguished only briefly and then quickly reignites, accompanied by a huge release of energy, which can even burn out the switch or arc-extinguishing chamber. Vacuum switches or vacuum interrupters used for capacitor switching generally undergo type testing, including short-circuit current breaking, proving their rated short-circuit current breaking capacity. The breaking current when switching capacitor banks typically does not exceed a few hundred amperes, and vacuum interrupters have no difficulty breaking this level of current. Therefore, the structural design of the interrupter is not directly causally related to the reignition phenomenon that occurs when the switch switches are switching capacitor banks. 3.3 Contact Material: Under otherwise constant conditions, the contact material of a vacuum interrupter has a significant impact on its breakdown voltage. Early interrupters used contact materials such as copper-bismuth-cerium and copper-bismuth-aluminum. These materials, containing low-melting-point metals (bismuth), have relatively low breakdown voltages and are prone to voltage breakdown. However, since the use of copper-chromium materials with superior overall performance, the withstand voltage of the products has improved, allowing the interrupter to break larger currents with a smaller opening distance. Our current circuit breaker interrupters use copper-chromium contact materials, ensuring good withstand voltage performance. Furthermore, breakdowns caused by the contact material will manifest within milliseconds after the arc is extinguished, which differs from the reignition characteristics when the switch is closing the capacitor bank. Therefore, the performance or selection of the contact material is not the fundamental reason for the high reignition rate of domestically produced arc-extinguishing chambers. 4. Analyzing the Causes of Reignition from the Process of Switching Capacitor Banks Let's analyze the causes of reignition from the process of switching capacitor banks: Before switching, the capacitor bank is charged and has a certain voltage. After the switch receives the tripping command, it opens, the arc is extinguished, and the external circuit is an AC voltage with constantly changing polarity over time. Therefore, after the switch opens, it will be subjected to a reverse voltage, requiring the switch contact to withstand up to twice the normal circuit voltage, which is more severe than a typical interruption process. Moreover, the current when closing the capacitor bank is generally small, not exceeding several hundred amperes, and the current interruption process itself is not difficult. Therefore, reignition when closing the capacitor bank is mainly due to voltage breakdown. Voltage breakdown mainly occurs in two ways: field emission and particle breakdown. We know that the breakdown time caused by field emission in a vacuum is very short, generally not exceeding a few milliseconds. This is inconsistent with the phenomenon observed when the capacitor bank is switched, which takes tens of milliseconds to break down. Therefore, the possibility of reignition caused by field emission can be ruled out. So what about the possibility of reignition due to particle breakdown? We know that during the manufacturing process of a vacuum interrupter, the vacuum level inside the chamber must be maintained, and the chamber must be kept clean. However, it's unavoidable that various particles will always be present inside. Small particles will always adhere to the electrode surfaces, and parts will always have some burrs. During assembly, oil, sweat, and cotton fibers will inevitably be introduced. These particles, under the influence of the electric field, will acquire a charge and may be released during the switching process, generating movement and possessing a certain kinetic energy. If the electric field is strong enough, and the particle diameter and mass are suitable, they will have significant kinetic energy by the time they pass through the gap to reach the other electrode. Upon collision with the other electrode, the kinetic energy is converted into heat, causing the particles to evaporate into vapor and diffuse, rapidly increasing the local particle density. These particles then collide with electrons emitted by field emission, ionizing them and ultimately leading to discharge breakdown of the gap. The presence of mobile particles is not directly related to the vacuum level of the tube, the tube structure, or the performance of the contact materials. Their fall is accidental, and the discharge time is brief. Therefore, it can be determined that the reignition phenomenon of the vacuum switch during capacitor bank switching is caused by the breakdown resulting from the release of particles in the arc-extinguishing chamber due to mechanical vibration during the opening operation. The probability of reignition is only related to the amount of particles present in the tube, and the possibility of reignition during capacitor bank switching increases significantly with the increase of system voltage. The performance of the arc-extinguishing chamber in switching capacitor banks is not directly related to its short-circuit breaking performance. Its performance is closely related to the design parameters of the arc-extinguishing chamber, the surface finish of the parts, the cleanliness of the arc-extinguishing chamber, and its manufacturing process. 5 Explanation of various phenomena in the process of switching capacitor banks by vacuum switch using particle breakdown theory According to particle breakdown theory, many phenomena that occur in the process of switching capacitor banks by vacuum switch can be well explained, and the factors that affect the test results can also be analyzed: (1) According to a large amount of experimental statistical data, the probability of reignition of vacuum interrupters produced by different manufacturers is different, and sometimes there is even a large gap. This is because the reignition rate of vacuum switch is related to the manufacturer or process of vacuum interrupter. Different manufacturers have certain differences in their production process, and the process control in the production process is also different. Therefore, the number of particles brought into the interrupter is also significantly different, which will inevitably have a significant impact on the statistical results of the test. (2) In the experiment, such a phenomenon was also found. When the same batch of arc-extinguishing chambers were assembled on the circuit breaker for capacitor cutting test, one of them repeatedly experienced reignition and breakdown. Even after replacing the arc-extinguishing chamber with one that had passed the test on another circuit breaker, it still did not work. Finally, it was found that the conductive rod of the arc-extinguishing chamber was not very straight during assembly. After adjusting the assembly to make the conductive rod straight, the test was finally passed smoothly. Therefore, the assembly quality of the circuit breaker has a great influence on the reignition rate. When the conductive rod is not assembled correctly, the switch will inevitably cause a concentrated release of energy during the closing process, causing the contacts to bounce violently. At this time, the particles originally attached to the arc-extinguishing chamber will become mobile, which will easily cause reignition during the opening process. Moreover, when the conductive rod is not assembled correctly, the distance between the contacts and the shielding cylinder will inevitably change, which may cause distortion of the local high voltage electric field, thus leading to the breakdown of the arc-extinguishing chamber. (3) In numerous tests, it was also found that the opening and closing speed of the circuit breaker affects the test results. Because if the opening speed is too high, it will produce a large opening bounce, and if the closing speed is too high, it will produce a large closing bounce. These operations will cause the particles in the arc-extinguishing chamber to change from adhesive to mobile, which will lead to re-breakdown during the test. (4) In the tests, it was found that the probability of re-breakdown decreases with the increase of the number of switching. In the first few switching, the probability of re-breakdown is relatively high, and the probability of re-breakdown decreases significantly with the increase of the number of switching. This is because the mobile impurity particles inside the pipe are burned off, and the number of particles gradually decreases. (5) To investigate the causes of reignition and even explosions of capacitor banks during capacitor bank switching by vacuum circuit breakers, the Guangdong Provincial Electric Power Research Institute used five batches of ordinary or high-cleanliness vacuum interrupters as samples. These were installed sequentially on the same set of vacuum circuit breakers to test the switching of the same set of capacitor banks. The mechanical characteristic parameters of the vacuum circuit breaker were kept consistent after each interrupter replacement. The results showed that the internal cleanliness of the vacuum interrupter is a crucial factor affecting the re-breakdown rate of capacitor banks during capacitor bank switching. The results are shown in Table 1. (The vacuum interrupter used in the test was BD401.) Based on the technical evidence provided by the test verification, the Guangdong power operation department replaced all the original ordinary vacuum interrupters with high-cleanliness vacuum interrupters to prevent capacitor bank explosions and similar accidents. After the replacement, the system operated for many years without any accidents. This also provides strong practical support for the particulate breakdown theory. 6 Measures and countermeasures to reduce the re-ignition rate Since we have basically determined the mechanism of re-ignition and analyzed the factors affecting it, we can take targeted measures to improve the reliability of the vacuum switch when switching capacitor banks and reduce the probability of re-ignition: (1) Improve and control the production process of the arc-extinguishing chamber, and reduce the number of particles in the tube from the aspects of component manufacturing and production process. 1) In the processing of metal parts, try to avoid and remove burrs from the parts; improve the surface quality of the parts and ensure the surface smoothness of the parts. 2) Before the assembly of the whole tube, insist on effective ultrasonic cleaning of the components, which can achieve obvious results; continuously improve the cleaning process so that the particles in the arc-extinguishing chamber are removed as cleanly as possible through cleaning. 3) In the production process, maintain good vacuum hygiene and work habits, and effectively control the air humidity and the number of suspended particles in the air in the operating room. 4) Organize production scientifically, so that the components or contacts of the arc-extinguishing chamber are processed and stored for as little time as possible, and are assembled into the furnace in time to reduce the probability of oxidation and contamination of the components. 5) Appropriately increasing the voltage of the vacuum interrupter used for switching capacitor banks and performing power frequency voltage aging and lightning impulse withstand voltage aging can reduce the breakdown weaknesses in the interrupter, improve its voltage withstand capability, and increase the reliability when switching capacitor banks. 6) Performing low current aging on the interrupter can use the high temperature of the arc to remove a thin layer of material on the electrode surface, burn off the burrs on the electrode surface, and remove the gas, oxides and impurities on the electrode surface at the same time, which plays a role in cleaning the electrode surface and improves the electrical performance of the interrupter to a certain extent. Therefore, the interrupter should be subjected to appropriate current aging before leaving the factory. 7) Performing parallel capacitor aging on the interrupter can quickly and significantly improve the withstand voltage capability of the product. (2) Improve the design quality and assembly quality of the circuit breaker and control its mechanical motion characteristic parameters within a reasonable range. 1) The design of the circuit breaker should be reasonable, ensuring that the moving conductive rod of the interrupter is installed vertically and is easy to adjust. 2) The assembly quality of the circuit breaker should be reliably measured and well controlled. The closing output power and opening output power of the operating mechanism should be appropriate, and the opening and closing speeds should be adjusted within a reasonable range to minimize the opening bounce and closing bounce. 3) Sometimes, even though the circuit breaker has been finalized, the output power of the operating mechanism is fixed, and other assembly conditions are good, there may still be occasional large opening bounce or closing bounce that cannot be adjusted down. This may be related to the oil pressure or viscosity of the buffer system, or it may be related to the loss of control over the overall form and position tolerance of the circuit breaker frame. Therefore, the quality control of the parts or accessories of the circuit breaker is very important. All parameters of the circuit breaker must be adjusted to the ideal state before it can be put into use or tested. (3) After the vacuum switch is assembled, a certain number of no-load operations can be performed to stabilize the mechanical parameters of the switch; and appropriate voltage and current aging of the switch as a whole can reduce the new burrs generated in the arc-extinguishing chamber and reduce the re-ignition rate of the vacuum circuit breaker switching capacitor bank. References: 1. Principles and Applications of High Voltage Circuit Breakers, Xu Guozheng, Zhang Jierong, Qian Jiali, Huang Yulong, et al., Beijing: Tsinghua University Press, 2000. 2. Current Status and Countermeasures of Vacuum Circuit Breaker Capacitor Bank Switching Performance, Li Dian, Jin Bairong, Hong Jinqi, Qiu Yong, *High Voltage Electrical Appliances*, 2003, No. 5, pp. 44-46. 3. Experimental Verification of Vacuum Circuit Breaker Capacitor Bank Switching, Chen Jinqing, Li Duanjiao, *Guangdong Electric Power*, 2002, Vol. 15, No. 4. 4. Study on Re-breakdown Phenomenon in Vacuum Switching Capacitive Load Opening and Closing Process, Jiang Wenquan, Master's Thesis, Xi'an Jiaotong University, Y146293. 5. Research on New Voltage Aging Technology of Vacuum Interruptor, Yang Lanjun, et al., *High Voltage Engineering*, 1999, No. 1, pp. 88-90.