Abstract: This paper briefly describes the development of vacuum circuit breakers and compares and analyzes them with SF6 circuit breakers. It also discusses the development path of vacuum circuit breakers towards 750 kV ultra-high voltage levels. Keywords: Vacuum circuit breaker; SF6 circuit breaker; 750 kV ultra-high voltage; photoelectric control mode Currently, China is constructing its first 750 kV ultra-high voltage line in the Northwest, laying the foundation for future construction of higher voltage-level ultra-high voltage lines. The equipment will primarily utilize equipment that China is capable of designing and manufacturing independently. Currently, the global trend in circuit breaker development is to vigorously develop high-voltage and ultra-high-voltage vacuum circuit breakers, gradually phasing out SF6 circuit breakers. In China, a high-level 126 kV vacuum circuit breaker will be successfully trial-produced by the end of this year, and research and development of 750 kV ultra-high-voltage vacuum circuit breakers are actively underway. 1 Overview of Vacuum Circuit Breaker Development 1.1 Historical Review The use of high-voltage circuit breakers to protect power systems has a long history. From the initial oil circuit breakers to compressed air circuit breakers; in the early 1960s, SF6 circuit breakers and vacuum circuit breakers entered the power system simultaneously, gradually phasing out oil circuit breakers and compressed air circuit breakers, and dominating the entire high-voltage power system. Looking back at history, research on using vacuum as the arc-extinguishing and insulating medium of circuit breakers predates that on SF6 gas. Research on SF6 gas began in the 1940s [1]. Due to the unique thermochemical properties and negative charge of SF6 gas, its insulation and arc-extinguishing performance are also particularly good, and it is widely used in high-voltage switchgear as an insulating and arc-extinguishing medium. In 1959, the first practical SF6 circuit breaker was launched. Currently, it has become the main type of high-voltage circuit breaker and occupies almost the entire share in the high-voltage and ultra-high-voltage fields. Since the beginning of the 18th century, some people have begun to imagine using some characteristics of vacuum to interrupt current [2-4]. In 1893, Rittenhause of the United States designed the first simple vacuum interrupter and published it as a patent, which attracted the attention of professors and experts. In 1920, the Swedish company Birka manufactured the first vacuum circuit breaker. Although its breaking capacity was extremely small and had no practical value, it attracted considerable interest. Around 1923, Sorenson and Mandenhall began research on breaking current in a vacuum at the California Institute of Technology in the United States, and successfully broke a 926 A power frequency AC current at 41 kV. They published their research results in 1926. Subsequently, some electrical companies in the United States and Germany devoted themselves to the research of vacuum circuit breakers, but because the scientific and technological requirements for vacuum circuit breakers were not urgent at the time, the research results were not very significant. In the early 1950s, research on vacuum circuit breakers made rapid progress. In 1961, General Electric (GE) of the United States produced a vacuum circuit breaker with a rated voltage of 15 kV and a breaking capacity of 12.5 kA. In 1966, it further successfully trial-produced vacuum circuit breakers with ratings of 15 kA, 25 kA, and 31.5 kA, thus officially introducing vacuum circuit breakers into the power system. To date, the rated current of vacuum circuit breakers has reached 6300 A, and the rated voltage of single-break vacuum interrupters has reached 123, 126 kV, and 145 kV respectively, with a maximum breaking current capacity of 63 kA. 1.2 A Brief Introduction to the Development of Vacuum Circuit Breakers in my country my country began research on vacuum arc theory and the development of vacuum circuit breakers around 1958. In 1965, Xi'an Jiaotong University, in collaboration with the former Xi'an High Voltage Switchgear Rectifier Factory, successfully developed the first vacuum interrupter. Shortly after, Xi'an Jiaotong University independently developed a 10 kV three-phase vacuum circuit breaker capable of interrupting the capacitive current of a 1500 A power capacitor bank. In 1967, the Xi'an High Voltage Electrical Apparatus Research Institute successfully developed a 10 kV, 2000 A single-phase fast vacuum circuit breaker. Driven by these achievements, factories such as Factory 4401, Factory 777, Factory 779, and Factory 771, formerly under the Ministry of Electronics Industry, also began developing vacuum interrupters, sparking a surge in the trial production of vacuum circuit breakers by numerous domestic switchgear factories, including Beijing Switchgear Factory and Suzhou Switchgear Factory. Currently, my country possesses a number of specialized factories producing vacuum interrupters and vacuum circuit breakers, as well as research institutes and universities dedicated to studying vacuum arc theory and improving the performance of vacuum interrupters. These efforts play a positive role in promoting the diversification and performance enhancement of my country's vacuum circuit breakers. Currently, China can produce vacuum circuit breakers with rated voltages of 12–40.5 kV, rated currents up to 6300 A, and breaking currents of 63 kA. The product quality and parameters of China's vacuum circuit breakers are basically the same as those of advanced foreign products, with almost no gap. However, further improvements are needed in reliability and appearance quality. The annual output of high-voltage vacuum interrupters in China is increasing year by year with the vigorous development of the power industry, as shown in Figure 1. As shown in Figure 1, by the end of 2003, China's annual output of vacuum interrupters had reached more than 600,000 units, accounting for about half of the world's total output, and had begun to be introduced into the international market. Figure 1: Annual output of high-voltage vacuum interrupters in China. 2. Comparative Analysis of Vacuum Circuit Breakers and SF6 Circuit Breakers Vacuum circuit breakers and SF6 circuit breakers are two major types of power switches that began to develop in the 1960s, making significant contributions to the development of the power industry. This section focuses on evaluating the future development trends of these two types of circuit breakers from the perspectives of human survival and environmental protection. 2.1 Vacuum Circuit Breaker Vacuum circuit breakers rely on vacuum interrupters to interrupt current in a vacuum. Undoubtedly, strong X-rays will be generated during the interruption of current. Whether these strong X-rays are harmful to the human body has been debated many times. At the height of the debate, it almost caused the production of vacuum circuit breakers to stop. In order to eliminate this impact, in 1983, Toshiba Electric Corporation of Japan, on behalf of the Japanese government, reprinted a legally binding solemn statement in the journal Electrical Review based on international law [5]. In accordance with the ANS1 C3-85 standard, under the supervision of a notary unit designated by the United Nations, they carefully conducted actual tests on X-rays and concluded that they were not harmful to the human body. The test results are shown in Table 1. Table 1 X-ray test results In 2002, SIEMENS AG of Germany measured the X-ray emission dose rate of an 84 kV vacuum interrupter under normal working conditions according to the international standard spark scintillation counter at a distance of 700 mm from the X-ray focus. The result is shown in Figure 2 [6]. The conclusion was that there was no harm to human health. Figure 2 Measurement of X-ray emission dose rate 2.2 SF6 circuit breaker Global warming can lead to abnormal weather events, such as heat waves, droughts and the spread of diseases. Protecting the Earth and the ecological environment has become a great historical mission for all countries in the world today [7]. Greenhouse gases, represented by CO2, make it easy for short-wavelength sunlight to pass through, but they also easily absorb longer wavelengths of light (such as infrared rays) from the Earth's surface. A large amount of greenhouse gases are released into the air and form a gas layer in the Earth's atmosphere. The infrared rays absorbed by these gases cannot pass through the gas layer to radiate outward, thus causing a significant rise in atmospheric temperature and warming. This phenomenon is called the greenhouse effect. In the 20th century, the global average temperature rose by about 5-9°C [8]. The main reason for the rise in global temperature is the greenhouse gases CO2, CH4, NO2, HFCS, PECS and SF6 emitted by humans. Although SF6 does not destroy the ozone layer, it has a particularly large impact on global warming. The simplest quantitative indicator of the greenhouse effect of gases is the global warming potential (GWP). Global warming is determined by the infrared absorption spectrum of gases. The concentration in the atmosphere depends on the lifespan of the gas under investigation and the number of years of warming. The characteristics of several major gases are shown in Table 2 [9]. Table 2 Characteristics of various gases As can be seen from Table 2, the GWP of SF6 gas is much larger than that of other gases. However, its impact on global warming is related to the product of its GWP and the concentration of the gas in the atmosphere. Therefore, the impact of SF6 gas is much smaller than that of CO2. However, a larger GWP value means a greater potential impact on warming. If the emission of SF6 gas continues to increase in the future, its influence may become quite significant. In 1995, the world's SF6 production was about 8,000 to 9,000 tons, and it is estimated that it has now exceeded 200,000 tons (including storage) [7]. Among them, the annual consumption of SF6 in the power industry is about 7,000 tons, mainly used in circuit breakers and other power transmission and distribution equipment. In theory, SF6 gas can be recycled and reused, and it is absolutely forbidden to leak into the atmosphere. However, there are large leaks in actual operation. The annual leakage of SF6 gas in the United States is equivalent to about 8 tons of CO2. Ten years ago, the concentration of SF6 gas in the atmosphere was almost imperceptible, but now the content is about 32 ppt [8]. A large part of these leaks are attributed to the power industry. With the increase in the use and emission of SF6 gas, the concentration of SF6 gas in the atmosphere is also increasing year by year. Its concentration varies with location and season. The industrialized Northern Hemisphere is about 0.4 ppt higher than the Southern Hemisphere, close to 4 ppt [10]. However, in recent years, the concentration of SF6 gas in the atmosphere in the Northern Hemisphere has shown a linear upward trend. At present, the situation of SF6 gas leakage in the operation of SF6 switchgear in my country is as follows [11]: (1) According to the statistics of literature [12], from 1989 to 1997, among the leakage faults of 220 kV and above SF6 circuit breakers and GIS, there were 26 leaks in the equipment body, of which 3 were imported equipment. (2) According to the statistics in reference [13], the leakage problem of domestic SF6 circuit breakers is very prominent. In 1993 alone, 11 out of 11 SF6 circuit breakers used by Beijing Power Supply Bureau had leakage in 11 phases, and a total of 18 gas replenishments were made throughout the year. (3) According to the statistics in reference [14], there were 16 leakage incidents in SF6 circuit breakers in 1994. This problem exists in both domestic and imported equipment. (4) According to the statistics in reference [15], there were 22 failures of domestic 500 kV SF6 circuit breakers between 1988 and 1995. The main problem was the sealing quality, which accounted for almost half of the failures. (5) According to reference [16], the International Conference on Large Electric Systems (CIGRE) 23-03 Special Working Group investigated 29 GIS units from 7 manufacturers of 11 Brazilian users. The results showed that in some substations, the annual SF6 leakage rate of the equipment exceeded 3%, and in some cases it was as high as 10%. Most 100-200 kV GIS systems in Brazil have SF6 leakage rates very close to the permissible limit of 1%. Between 1991 and 1993 alone, GIS installations in Brazil resulted in the release of at least 5,623 kg of SF6 gas into the atmosphere due to leakage problems. Typical calculations regarding the concentration of SF6 gas in the atmosphere and its compaction effect have been reported. These reports also present the trend of atmospheric temperature increases [10], as shown in Figure 3. Based on this, assuming an annual SF6 emission of 6,800 t after 2000 (curves 1 and 2 in Figure 3), or 10,000 t after 1990 (curves 3 and 4 in Figure 3), and assuming an infinite lifetime for SF6 gas in the air, the predicted SF6 concentration in the air by 2010 would be 8 ppt (or 10 ppt if annual emissions were 10,000 t after 1990). As can be seen, under the most stringent conditions (curve 4), atmospheric temperature will rise by 0.0043 °C by 2010 and by 0.02 °C by 2100. Similarly, the greenhouse effect caused by CO2 gas also depends on future emissions. It is estimated that by 2010, the atmospheric temperature rise caused by CO2 will be 0.8 °C, and by 2100, the atmospheric temperature rise will be 2–5 °C. Figure 3: Greenhouse effect prediction of SF6 gas. Note: Curves 1 and 3 are predictions of the greenhouse effect when the lifetime of SF6 gas is 200 years; curves 2 and 4 are predictions of the greenhouse effect when the lifetime of SF6 gas is infinite. Therefore, from the perspective of global environmental protection, CIGRE proposed a draft decision at the 1997 Kyoto Conference in Japan, namely the Kyoto Protocol to the United Nations Framework Convention on Climate Change (hereinafter referred to as the "Kyoto Protocol"). Annex A of the Protocol lists six greenhouse gases: carbon dioxide (CO2), methane (CH4), nitrous oxide (NO2), hydrofluorocarbons (HFCS), perfluorocarbons (PECS), and sulfur hexafluoride (SF6), and lists the sectors and corresponding industries related to these greenhouse gases. The energy and manufacturing industries are specifically mentioned as being involved in the production and consumption of SF6. The Kyoto Protocol stipulates that 2008–2012 is the first period of emission limits and reduction commitments. Annex B lists the emission limits or reduction commitments of the relevant parties (based on 1990 as the base year, and 1995 as the base year for the latter three gases, including SF6). Currently, SF6 gas is mainly used in high-voltage power equipment as an insulation and arc-quenching medium. The restrictions on SF6 gas use stipulated in the Kyoto Protocol have drawn the attention of relevant departments in China, including environmental protection and power sectors. It is hoped that technical personnel working with high-voltage switches can propose specific suggestions for limiting and reducing SF6 gas emissions. 3. The Path to Developing Vacuum Circuit Breakers to 750 kV Ultra-High Voltage Levels Most electrical experts believe that high-voltage and ultra-high-voltage circuit breakers with single-break breaking capacity are the best performing circuit breakers. Currently, among ultra-high-voltage circuit breakers, only SF6 circuit breakers can achieve a single-break structure. However, due to the severe greenhouse effect of SF6 gas, it will be gradually phased out and eventually disappear from the market. Currently, countries around the world are moving towards using single-break vacuum interrupters connected in series to form ultra-high voltage level vacuum circuit breakers. Furthermore, 123 kV, 126 kV, and 145 kV single-break vacuum interrupters are already available internationally. Analysis shows that using 126 kV single-break vacuum interrupters connected in series to form a 750 kV ultra-high voltage vacuum circuit breaker has become a feasible development direction. Currently, my country has initially successfully developed a 126 kV single-break vacuum interrupter, which is expected to be put into production in the second half of this year. Xi'an Jiaotong University is researching and designing a 750 kV ultra-high voltage vacuum circuit breaker composed of six single-break 126 kV vacuum interrupters connected in series. They have equipped each 126 kV vacuum interrupter with a permanent magnet operating mechanism to achieve synchronous operation of the six vacuum interrupters. This type of ultra-high voltage vacuum circuit breaker with a permanent magnet operating mechanism and using intelligent photoelectric control mode electronic circuitry to achieve automatic remote control function is the first attempt of its kind in China. It is hoped that it will be supported and selected by relevant departments for use in my country's first 750 kV line currently under construction. 3.1 Design Scheme of 750 kV Ultra-High Voltage Vacuum Circuit Breaker The 750 kV ultra-high voltage vacuum circuit breaker composed of multi-break vacuum interrupters connected in series can be summarized into two structures: porcelain column type and box type. The type of porcelain column vacuum circuit breaker currently under development in my country is shown in Figure 4. Each pole consists of six 126 kV vacuum interrupters connected in series. A control box is installed between each vacuum interrupter, and a voltage equalizing capacitor is arranged in each vacuum interrupter. The control box contains a permanent magnet operating mechanism and capacitors, as shown in Figure 5. The bottom of the six series-connected porcelain columns is supported by a steel base for the entire circuit breaker. Several insulated cables are used for tensioning to ensure the reliable and stable operation of the vacuum circuit breaker. The ultra-high voltage power supply is input through terminal A and output through terminal B. Figure 4: Layout diagram of a porcelain column type 750 kV ultra-high voltage vacuum circuit breaker. Figure 5: Layout diagram of vacuum interrupters and control box. The opening and closing operations of the vacuum circuit breaker are completed by the vacuum interrupters, the overtravel spring combination unit, and the control box located in the middle. The control box consists of intelligent components such as photoelectric and electronic controllers. When the photoelectric and electronic controllers receive external signals, the capacitor discharges to the permanent magnet operating mechanism coil, pushing the pull rod to drive the overtravel spring combination unit to move rapidly, causing the moving contact of the vacuum interrupter to close or open, thereby realizing the opening or closing of the vacuum circuit breaker. 3.2 Advantages and Disadvantages of Porcelain-Pillar Type Ultra-High Voltage Vacuum Circuit Breakers The advantages of porcelain-pillar type circuit breakers are low manufacturing cost, simple structure, easy installation, debugging and maintenance, and easy replacement of parts in case of accidental damage. The disadvantages are that they are easily damaged during earthquakes, and equipment such as current transformers, surge arresters, and disconnecting switches need to be purchased separately and require additional space for installation. 4 Intelligentization of Ultra-High Voltage Vacuum Circuit Breakers A simulation virtual test was conducted on the specially designed permanent magnet operating mechanism. The results show that its force output characteristics and speed characteristics are suitable for 750 kV ultra-high voltage vacuum circuit breakers. The maximum stroke of the permanent magnet operating mechanism can reach 84 mm, the load attraction force at the closing position is approximately 10,000 N per pole, and the average opening speed can reach 3.5 m/s. Each permanent magnet operating mechanism is equipped with eight parallel capacitors, with a pre-charge voltage of 100 V and a total capacitance of 0.8 F. When six 126 kV vacuum interrupters are combined to form a 750 kV ultra-high voltage vacuum circuit breaker, synchronous operation of the permanent magnet operating mechanisms of the six vacuum interrupters can be achieved. The dispersion of their action times generally does not exceed 100 μs. Therefore, the designed permanent magnet operating mechanism can meet the requirements of a 750 kV ultra-high voltage vacuum circuit breaker. Furthermore, the automatic control and remote control testing device is equipped with photoelectric automatic controllers that receive optical signals and photoelectric wave energy. These controllers are located in the control box and are used to control the pre-charged capacitor bank to discharge to the closing or opening coil of the permanent magnet operating mechanism, thereby closing or opening the vacuum interrupter contacts. This automatic controller can also operate the vacuum circuit breaker by receiving signals via a dedicated telephone line or wireless telephone. The photoelectric automatic controller is equipped with reliable electromagnetic shielding facilities to resist external interference. Figure 6 shows the framework structure diagram of the photoelectric control mode of the 750 kV ultra-high voltage vacuum circuit breaker (patent pending). It mainly consists of vacuum interrupters, overtravel spring assemblies, permanent magnet operating mechanisms, energy storage capacitors, electronic optical control systems, fiber optic cables, and signal receivers. Each phase of the 750 kV ultra-high voltage vacuum circuit breaker can be independently interrupted before the current crosses zero, and its short-circuit current breaking capacity is estimated to be increased by 15% to 20%. [align=center]1—Permanent magnet operating mechanism; 2—Energy storage capacitor; 3—Fiber optic cable; 4—Overtravel spring assemblies; 5—Electronic optical control systems; 6—Signal receiver Figure 6 Frame structure diagram of the photoelectric control mode of the 750 kV vacuum circuit breaker[/align] Its operation process is as follows: When the signal receiver receives a power station operation signal (such as a fault signal received by the vacuum circuit breaker during normal operation), it immediately transmits the signal to the electronic optical control system via fiber optic cable 3 for identification and issues a command to discharge the energy storage capacitor bank to the coil of the permanent magnet operating mechanism, causing it to operate. Thus, the six vacuum interrupters connected in series simultaneously open (or close). 5. Conclusion The use of series-connected vacuum interrupters to form ultra-high voltage vacuum circuit breakers is a current development trend. It is understood that research on using series-connected single-break vacuum interrupters to form ultra-high voltage vacuum circuit breakers is also underway abroad. It is hoped that relevant departments will provide strong support for the development of porcelain column-type ultra-high voltage vacuum circuit breakers in China. Regarding intelligentization, with the rapid development of current electronic technology, the adoption of automated photoelectric control technology and synchronous phase selection closing schemes is feasible. 6 References 〔1〕 Wang Jimei, Qian Yugui. On the successful development of 126 kV outdoor high-voltage vacuum circuit breakers in China and suggestions for further solving the problem of vacuum interruption chambers at high voltage levels. High Voltage Apparatus, 2003(1):3. 〔2〕 Wang Jimei. Vacuum Switch Theory and Its Application [M]. Xi'an: Xi'an Jiaotong University Press, 1986. 〔3〕 Wang Jimei, Wu Weizhong, Wei Yijun, et al. 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