A Brief Discussion on SF6 Load Switches and Gas-Fired Switchgear

Abstract: This article introduces the domestic and international product composition and development of SF6 load switches and gas-filled switchgear, as well as the necessary conditions and technical challenges for producing such products. Keywords: SF6 load switch, gas-filled switchgear, technical challenges. SF6 emerged in the early 20th century; it is a gas formed by the combustion of sulfur in fluorine gas. Under normal conditions, SF6 gas is a colorless, odorless, non-toxic, non-flammable, and chemically stable gaseous substance, with a density approximately five times that of air. Due to the negative charge of its molecules, it has a strong ability to adsorb free electrons and form negative ions, resulting in high dielectric strength and excellent insulation properties. Its low dissociation temperature and high dissociation energy also make it a gas with excellent arc-extinguishing properties. With its excellent insulation and arc-extinguishing properties, as well as its non-flammable nature, SF6 gas has gradually been adopted as a new type of insulating medium for electrical equipment. An SF6 load switch is a switchgear that integrates a load switch, disconnector, and grounding switch into a single unit, sealed within an SF6 gas-filled enclosure. Utilizing the excellent insulation and arc-extinguishing properties of SF6, it fulfills the three-position functions of closing, disconnecting, and grounding. It features simple structure, reliable interlocking, small size, low maintenance, and safe and easy installation, making it suitable for various applications. Because SF6 load switches, when used in conjunction with current-limiting fuses, provide better protection for transformers, they are widely used abroad. In industrialized countries (such as Europe and Japan), the ratio of circuit breakers to load switches is 1:5 to 6. In China, with the continuous development and upgrading of urban power grids, the application of load switchgear in 12kV distribution systems for ring network power supply and terminal power transmission is increasing. Currently, the majority of the domestic SF6 load switch market is still dominated by foreign companies (such as ABB and Schneider Electric), while dozens of domestic manufacturers have developed and produced various models of SF6 load switches (such as Wenzhou Kaiyuan, Ningbo Tianan, and Zhenjiang Longyuan). Domestic manufacturers, through digesting and absorbing rich design, manufacturing, and operational experience from abroad, have gained a more comprehensive and profound understanding of SF6 load switches, and the quantity and quality of domestically produced SF6 load switches have steadily improved. II. C-GIS Gas-Insulated Switchgear Cabinet-type SF6 gas-insulated switchgear (C-GIS gas-insulated switchgear) is a high-tech series of products that has emerged abroad in the last decade or so, gradually becoming an important type of metal-enclosed switchgear. China only began developing this technology in recent years. C-GIS gas-insulated switchgear is a medium-voltage complete set of equipment that encloses all high-voltage components, such as busbars, circuit breakers (or load switches), disconnectors, and instrument transformers, in a stainless steel shell filled with low-pressure SF6 gas. Its outstanding features include high safety and reliability, simple operation, maintenance-free operation, and convenient installation. Its greatest advantage is that it is unaffected by external environmental conditions such as condensation, dirt, small animals, and chemicals, making it suitable for harsh environments. Another important feature is that the use of high-performance SF6 insulation greatly reduces the external dimensions of the cabinet, facilitating miniaturization. Compared to air-insulated switchgear, SF6 gas-insulated switchgear has an installation area of ​​only 26% and a volume of only 27%. Furthermore, the use of high-performance oil-free switches significantly reduces maintenance and repair workload. It is suitable for both network nodes and user terminals, and meets the flexible use of compact switchgear in various secondary substations. It is particularly suitable for high-power applications such as airports, subways, and railways, as C-GIS adapts to the requirements of miniaturization and intelligence. Currently, several large research institutes and switchgear manufacturers in China have developed new cabinet-type SF6 gas-insulated switchgear through introduction, assimilation, and cooperation, such as Xi'an High Voltage Apparatus Research Institute, Xi'an Senyuan, Zhenjiang Longyuan, Tianshui Great Wall Switchgear Factory, Shanghai Tianling Switchgear Factory, and Guangdong Shunde Switchgear Factory. All products have passed type testing and are now in the stage of small-batch trial production, product improvement, and serialization. Due to the extremely high requirements for process equipment and the great difficulty in processing C-GIS gas-insulated switchgear, its promotion in China is not yet widespread. The extensive use of SF6 gas in switchgear has driven the rapid development of the switchgear manufacturing industry. However, SF6 gas decomposes during arc discharge to form sulfur-containing low-fluoride compounds. These decomposition products are both toxic and corrosive to many insulating and metallic materials. Furthermore, SF6 gas also has a certain impact on ambient temperature (greenhouse effect). In 1991, the former Ministry of Machinery and Electronics Industry issued the "Essential Conditions for SF6 High-Voltage Switchgear Manufacturers" to ensure the quality of SF6 switchgear production. This document outlines the essential conditions for manufacturers of SF6 high-voltage switchgear of 35kV and below. In brief: a) An assembly site with an air cleanliness level of 1 million or higher and a relative humidity below 75%, with good ventilation. b) An SF6 leak detection room, a parts cleaning room, and a drying device. c) A reliable SF6 charging, discharging, and recovery device. d) Hydrostatic testing and airtightness testing devices. e) Essential testing conditions and equipment: 1. Physical and chemical testing; 2. Insulation testing; 3. SF6 gas testing; f) Sound quality management and process management systems. [b]A high-voltage switchgear manufacturer needs to produce SF6 load switches and gas-insulated switchgear, and mainly needs to solve the following technical problems:[/b] 1. Sealing Issues: Leakage rate has always been a focus of attention for manufacturers and users. The annual leakage rate of SF6 load switches and gas-insulated switchgear is below 1%/year. The service life of the switches is generally around 30 years. Sealing must take into account factors such as the compression, lifespan, aging, and reliability of the sealing elements. When the SF6 gas pressure inside the enclosure drops to atmospheric pressure, it seems that the internal and external pressures are equal, and there is no gas leakage. However, from the perspective of permeation, the SF6 gas concentration inside the enclosure is obviously higher than outside, and SF6 gas still seeps out. Conversely, the air concentration outside the enclosure is higher than inside, and air will enter the enclosure through gaps. Over time, although the gas pressure inside the enclosure is still atmospheric pressure, the gas composition changes. The percentage of SF6 gas decreases. When the SF6 proportion drops to 40%, it may lead to accidents due to a decline in insulation performance. Therefore, sealing issues must be taken seriously. SF6 load switches generally use sealing rings to achieve dynamic and static gas sealing (such as ABB's SFG and Schneider's SM6 switches); a very few use a combination of stainless steel welding and sealing rings (VEI's SF6 switches); C-GIS gas-filled switchgear uses sealing methods such as welded seals, sealing ring seals, and metal bellows seals. When using welded seals, ensuring the airtightness of the weld is crucial. General welding equipment and techniques are often insufficient; at least argon arc welding or laser welding equipment is required (several domestic companies have imported welding robotic production equipment from abroad) to guarantee weld quality. There are two common methods for SF6 gas leak detection: qualitative leak detection, i.e., finding the leak point; and quantitative leak detection, i.e., measuring the gas concentration in the sealed volume after 24 hours to estimate the annual leakage rate. Additionally, the impact of environmental factors on sealing performance must be considered, such as whether the sealing performance meets requirements under high or low temperature conditions. 2. Gas moisture content: The moisture content of SF6 gas is one of the main quality indicators for SF6 switches, and its level directly affects the insulation level of the switch. When the moisture content in SF6 gas is below the saturation value at its operating temperature, the moisture exists in a gaseous state within the SF6 gas. Tests have shown that the insulation performance of SF6 switches is essentially unaffected in this state. However, if the moisture content exceeds the saturation value, some water vapor will condense on the surface of the solid insulation material inside the switch, thereby reducing its surface flashover voltage and affecting the switch's arc-extinguishing performance. Therefore, DL/T596-1996 "Preventive Testing Procedures for Power Equipment" stipulates that the moisture content of SF6 switches after new commissioning and major overhauls should not exceed 150 ppm, and during operation, it should not exceed 300 ppm. For new SF6 gas, the moisture content of the gas itself must first be tested to ensure it does not exceed the specified value (generally 15 ppm). An appropriate amount of dried adsorbent should be placed in the sealed enclosure. The adsorbent is mainly used to adsorb moisture to ensure that the moisture content inside the sealed enclosure does not exceed the standard requirements. All components (including the sealing enclosure, transmission components, contact blades, etc.) and standard parts within the sealed enclosure must be baked strictly according to their respective baking temperatures and times. The moisture content of the components must be strictly controlled, and the sealing time must be guaranteed. During evacuation and gas filling of SF6 switchgear, the filling pipelines are cleaned with high-purity nitrogen or sulfur hexafluoride gas to control the moisture content. A standardized production process for SF6 switchgear must be established. 3. Component Quality Issues: The quality of components directly affects the quality of the product. SF6 load switches and gas-filled switchgear are maintenance-free or low-maintenance switchgear. If any component within the sealed enclosure has quality problems, it could very likely lead to the scrapping of the entire switchgear. Ensuring component quality requires strict control over the materials, processing technology, and inspection methods of the components, ensuring that their mechanical strength, electrical performance, and other parameters meet product requirements. More stringent inspections must be conducted on the components to be installed, and the entire assembly process must be subject to stricter process control and in-process inspection to ensure correctness and consistency in assembly. In particular, the lubrication of the transmission links and contact surfaces within the sealed enclosure should be permanently effective throughout the entire product lifecycle. The contact pressure of the contacts should be strictly controlled, and the resistance value of the main circuit of the switch should meet the product's technical requirements throughout its lifecycle. [b]Conclusion[/b] With the rapid development of the social economy and the continuous growth of urban electricity load, the requirements for power supply reliability are becoming increasingly stringent. High-performance ring network distribution, multi-circuit distribution, and switching stations have become indispensable and important equipment in secondary power distribution systems. In particular, compact and scalable SF6 switchgear has become the dominant equipment in secondary power distribution systems. With the widespread application of digital technology, integration technology, and mechatronics technology, SF6 switchgear will experience even greater development.