SF6 appeared in the early 20th century as a gas formed by the combustion of sulfur in fluorine gas. SF6 gas is colorless under normal conditions. Odorless. non-toxic. A non combustible, 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 electrical strength and excellent insulation performance. It has a low dissociation temperature and high dissociation energy, and is also a gas with excellent arc extinguishing performance. SF6 gas is gradually being used as a new type of insulation medium for electrical equipment due to its excellent insulation and arc extinguishing properties, as well as its non combustible characteristics.
SF6 load switch
SF6 load switch is a type of load switch. Isolating switch. The grounding switch is integrated and sealed in a box filled with SF6 gas, utilizing the excellent insulation of SF6. Arc extinguishing performance, meeting the requirements for closing. Isolation. Switchgear with three station grounding function. Has a simple structure. Interlocking is reliable. Less volume calibration and maintenance. Advantages such as safety and easy installation, suitable for different application scenarios.
Due to the better protection of transformers by the combination of SF6 load switches and current limiting fuses, they have been widely used abroad. In industrialized countries such as Europe and Japan, the ratio of circuit breakers to load switches is 1:5-6. In China, with the continuous development of urban power grid construction and renovation, the application of load switching equipment in the ring network power supply and terminal power transmission of 12kV distribution systems is also increasing. At present, the majority of the domestic SF6 load switch market is still dominated by foreign companies (such as ABB, Schneider, etc.), and dozens of domestic manufacturers have already developed it. Produce various types of SF6 load switches (such as Wenzhou Kaiyuan, Ningbo Tian'an, Zhenjiang Longyuan, etc.). Domestic manufacturers digest through digestion. Absorbing rich designs from abroad. manufacture. Through operational experience, I 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 increased.
C-GIS Inflatable Cabinet
Cabinet style SF6 gas insulated switchgear (referred to as C-GIS inflation cabinet) is a high-tech series of products that have been newly developed abroad in the past decade, gradually becoming an important variety of metal enclosed switchgear. Our country has only started to take off in recent years. The C-GIS inflatable cabinet integrates all high-voltage components such as busbars. Circuit breaker (or load switch) Isolating switch. Medium voltage equipment such as transformers enclosed in stainless steel casings filled with lower pressure SF6 gas. Its prominent feature is safety. High reliability, easy operation, and maintenance free. Easy installation, etc. Its biggest feature is that it is not affected by external environmental conditions, such as condensation. Dirty. Small animals and chemicals can be used in harsh environments. Another important feature is that the use of high-performance SF6 insulation greatly reduces the external dimensions of the cabinet, which is conducive to the development towards miniaturization. Compared with air insulation, the installation area of SF6 gas filling cabinet is 26% and the volume is 27%. At the same time, due to the use of high-performance oil-free switches in the gas filling cabinet, the workload of maintenance and repair is greatly reduced. It is suitable for the requirements of network nodes or user terminals, and also meets the flexible use needs of compact switchgear in various secondary substations. Especially suitable for regular subway trains. C-GIS has adapted to miniaturization in applications with high electricity requirements, such as railways. The requirement for intelligence. Currently, there are several large research institutes in China. The switch manufacturing factory is introduced. Digestion. We have collaborated to develop a new type of cabinet style SF6 gas insulated switchgear, such as the Xigao Institute. Xi'an Senyuan. Zhenjiang Longyuan. Tianshui Changcheng Switchgear Factory. Shanghai Tianling Switch Factory. Guangdong Shunde Switch Factory, etc. All products have passed the type test and have now entered small-scale trial production and product improvement. Series stage. Due to the extremely high requirements for process equipment in C-GIS inflatable cabinets. The processing difficulty is very high, and currently the promotion in China is not very extensive.
The extensive use of SF6 gas in switchgear has driven the rapid development of the switch manufacturing industry. However, SF6 gas decomposes into low fluoride sulfur compounds during arc discharge, which are both toxic and corrosive to many insulation and metal materials. In addition, SF6 gas also has a certain impact on environmental temperature (greenhouse effect). In 1991, the former Ministry of Mechanical and Electrical Engineering issued a notice entitled "Essential Conditions for SF6 High Voltage Switchgear Production Plants" to ensure the quality of SF6 switchgear production. Relevant requirements are proposed for the necessary conditions of SF6 high-voltage switch production plants with 35kV and below. Simply put: a. It should be installed in a site with an air cleanliness level of over 1 million and a relative humidity below 75%, and have good ventilation conditions.
b. There is a SF6 leak detection chamber. Parts cleaning room. Drying device. C. There is a reliable SF6 filling, discharging, and recovery device.
d. Water pressure test and air tightness test device.
e. Essential testing conditions and equipment: 1. Physical and chemical testing; 2. Insulation testing; 3. SF6 gas detection;
f. Establish sound quality management. Process management system.
High voltage switch manufacturers need to produce SF6 load switches and inflatable cabinet switchgear, mainly to solve the following technical problems:
1. Sealing issue:
Leakage rate has always been a focus of attention for manufacturers and users. The annual leakage rate of SF6 load switches and gas filling cabinets is below 1%. The service life of switches is generally around 30 years. Sealing should take into account the compression capacity of the sealing components. life. ageing. Factors related to reliability. When the SF6 gas pressure inside the box 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 infiltration, the concentration of SF6 gas inside the box is obviously higher than outside the cabinet, and SF6 gas still leaks out. Conversely, the concentration of air outside the box is higher than inside the box, and air will enter the box through gaps. Over time, although the gas pressure inside the box remains at atmospheric pressure, the gas composition changes. The percentage of SF6 gas decreases, and when the SF6 ratio drops to 40%, accidents may occur due to a decrease in insulation performance. Therefore, sealing issues must be taken seriously. SF6 load switches commonly use sealing rings to achieve dynamic and static gas sealing (such as ABB's SFG. Schneider's SM6 switch); There are also a few that use a combination of stainless steel welding and sealing rings for sealing (VEI's SF6 switch); The sealing method of C-GIS inflatable cabinet includes welding sealing. Sealing rings and metal bellows seals, etc. Welding sealing is adopted to ensure the airtightness of the weld seam. Generally, welding equipment and welding technology cannot meet the requirements. At least argon arc welding or laser welding equipment should be used for welding (currently, several domestic companies have introduced welding robot production equipment from abroad) to ensure welding quality.
There are two commonly used SF6 gas leak detection methods: one is qualitative leak detection, which involves identifying the leak point; The second is quantitative leak detection, which measures the gas concentration inside the sealed volume after 24 hours of sealing to calculate the annual leakage rate.
In addition, the impact of environmental factors on sealing performance should also be considered, such as whether the sealing performance meets the requirements under high or low temperature conditions.
2. Gas moisture content issue:
The moisture content of SF6 gas is one of the main detection indicators for SF6 switch quality, and its size directly affects the insulation level of the switch. When the moisture content in SF6 gas is lower than the saturation value at that temperature, the moisture exists in the SF6 gas in a gaseous state. Tests have shown that the insulation performance of SF6 switches is basically not affected at this time; If the moisture content of the gas exceeds the saturation value, some water vapor will condense on the surface of the solid insulation inside the switch, thereby reducing its surface flashover voltage and affecting the arc extinguishing performance of the switch. Therefore, DL/T596-1996 "Preventive Testing Regulations for Power Equipment" stipulates that the moisture content of SF6 switches after new operation and major repairs shall not exceed 150ppm, and during operation, it shall not exceed 300ppm.
For new SF6 gas, the first step is to check the moisture content of the gas itself, which should not exceed the specified value (generally a moisture content of 15ppm). Put an appropriate amount of dried adsorbent into the sealed box, which is mainly used to adsorb water to ensure that the moisture content inside the sealed box does not exceed the standard requirements. All components inside the sealed box (including the sealed box body, transmission parts, contact blades, etc.) Standard parts must be baked strictly according to the baking temperature of each component. Baking time is used to strictly control the moisture content of the components themselves and ensure the packaging time of the components. SF6 switchgear vacuum pumping. When inflating, clean the inflation pipeline with high-purity nitrogen or sulfur hexafluoride gas, control the moisture content in the pipeline, and develop a standardized SF6 switchgear production process.
3. Quality issues with components: The quality of components directly affects the quality of the product. SF6 load switches and gas filling cabinets are maintenance free or low maintenance switchgear. Once there are quality problems with the components inside the sealed box, it is highly likely that the entire switchgear will be scrapped. To ensure the quality of components, it is necessary to strictly control the materials of the components. Processing technology and inspection methods, their mechanical strength. The electrical performance and other parameters meet the requirements of the product. Conduct stricter inspections on the required components to be installed, and provide stricter process control and inspection control for the entire assembly process to ensure the correctness and consistency of the assembly. Especially the lubrication of the transmission links inside the sealed box and the surface of the contacts should be permanently effective throughout the entire product life cycle. The contact pressure of the contacts should be strictly controlled, and the resistance value of the switch main circuit should meet the technical requirements of the product during the life cycle. With the rapid development of social economy and the continuous increase of urban electricity load, the demand for power supply reliability is becoming higher and higher, and high-performance ring network distribution is needed. Multi circuit power distribution. Switching stations have become one of the irreplaceable important equipment in secondary distribution systems, especially compact ones. Scalable SF6 switchgear has become the dominant equipment in secondary distribution systems. With the advancement of digital technology. The widespread application of integrated technology and mechatronics integration technology will lead to greater development of SF6 switchgear.
