SF6 in Switchgear: SF6 Load Break Switch and SF6 Gas Insulated Switchgear

SF6 gas, also known as sulfur hexafluoride or sulphur hexafluoride, emerged in the early 20th century. This colorless, odorless, non-toxic, and non-flammable gas boasts exceptional chemical stability and a density five times that of air. Thanks to its molecules’ knack for attracting free electrons and forming negative ions, SF6 gas boasts impressive electrical strength and acts as an exceptional insulator. Furthermore, it breaks down at a low temperature but requires a lot of energy to do so, making it a superb arc quencher. Given these outstanding insulating and arc-extinguishing abilities, coupled with its non-combustible nature, SF6 gas has steadily become the go-to insulating medium for electrical equipment.

SF6 Gas used in Load Break Switch

The SF6 load break switch is an all-in-one switching device, combining load switching, isolation, and grounding functions within a sealed enclosure filled with SF6 gas. Leveraging SF6’s superior insulation and arc-quenching capabilities, it smoothly handles closing, isolating, and grounding in a three-position design. These switches boast a simple build, reliable interlocking, compact size, minimal upkeep, and straightforward, safe installation, making them a great fit for various applications.

Because SF6 load break switches work so well with current-limiting fuses to protect transformers, they’re a common sight in developed nations like Europe and Japan, where the ratio of circuit breakers to load break switches is around 1 to 5 or 6. In China, as city power grids expand and upgrade, these load break switches are increasingly used in ring networks and for end-of-line power delivery in 12kV distribution systems. While the Chinese market is largely dominated by international load break switch manufacturers like ABB and Schneider, many domestic manufacturers are now developing and producing their own diverse range of SF6 load break switches. By learning from extensive international experience in design, manufacturing, and operation, Chinese companies have deepened their understanding, leading to a steady rise in both the number and quality of domestically produced SF6 load break switches.

SF6 Gas used in Gas Insulated Switchgear

Cabinet-type SF6 gas-insulated switchgear (SF6 GIS) is a relatively recent development, hitting the scene in the last decade and quickly becoming a key player in metal-enclosed switchgear.

Think of MV GIS switchgear as a complete medium-voltage setup where vital high-voltage components – like busbars, circuit breakers (or load break switches), disconnectors, and voltage current transformers – are tucked neatly inside a stainless steel shell filled with low-pressure SF6 gas. This design brings a lot to the table: enhanced safety, top-notch reliability, simple operation, zero maintenance, and hassle-free installation. Better yet, it shrugs off tough environmental conditions like condensation, dirt, critters, and chemical exposure, making it ideal for harsh settings. A major plus is how SF6’s excellent insulation allows for significantly smaller cabinets, pushing the trend towards miniaturization. Compared to traditional air-insulated setups, GIS takes up only about a quarter of the space and volume. Plus, pairing it with high-performance, oil-free switches drastically cuts down on maintenance and overhauls. Whether it’s for network hubs or end-user connections, GIS fits the bill and aligns with both domestic and international trends in secondary SF6 load break switches and GIS development, addressing the necessary production requirements and technical challenges. SF6 gas-insulated switchgear perfectly meets the demand in substations for flexible, compact switchgear and is a particularly good fit for high-power applications like airports, subways, and railways, readily embracing the move towards smaller and smarter equipment.

Challenges in SF6 Switchgear Manufacturing

The widespread adoption of SF6 gas in switchgear has really fueled the growth of the switchgear manufacturing. However, there’s a catch: when an arc occurs in SF6, it breaks down into sulfur fluorides, which are not only toxic but can also corrode various insulating and metallic materials. Plus, SF6 contributes to the greenhouse effect, so it’s not entirely environmentally benign. Because of these factors, manufacturers of SF6 switchgear have to meet certain stringent production standards. In a nutshell, high-voltage switchgear manufacturers producing SF6 load switches and gas-insulated switchgear face key technical hurdles, primarily:

Leakage has always been a major concern for both manufacturers and end-users. The acceptable annual leakage rate for SF6 load switches and gas-insulated cabinets is typically less than 1%. Given that these switches are expected to last around 30 years, the sealing needs to be robust, considering the compression, lifespan, aging, and overall reliability of the sealing components.

For gas sealing, SF6 load switches commonly use O-rings for both static and dynamic seals, a method seen in ABB’s SFG and Schneider’s SM6 switches. Some opt for a combination of stainless steel welding and O-rings, like in VEI’s SF6 switches. GIS enclosures employ various sealing techniques, including welding, O-rings, and metal bellows. When welding is used, ensuring a hermetic seal is crucial, often requiring specialized equipment like argon arc or laser welders to guarantee weld integrity.

Beyond this, manufacturers must consider how environmental factors, such as extreme high or low temperatures, might affect the sealing performance.

The moisture content in SF6 gas is a critical quality indicator for SF6 switches, directly impacting their insulation capabilities. For new SF6 gas, the initial moisture level must be checked to ensure it’s within the specified limits. During the vacuuming and filling stages of SF6 switchgear production, high-purity nitrogen or SF6 gas is used to purge the piping, controlling moisture levels. Standardized production processes for SF6 switchgear are essential.

The quality of individual parts is paramount to the overall product quality. SF6 load switches and gas-insulated cabinets are designed to be maintenance-free or require very little upkeep. If any component within the sealed enclosure fails, the entire switchgear unit could become unusable. Ensuring part quality involves strict control over materials, manufacturing processes, and inspection methods, verifying that mechanical strength, electrical performance, and other parameters meet product specifications. Incoming parts undergo rigorous checks, and the entire assembly process demands tight process control and thorough inspections at each stage to guarantee correct and consistent assembly. Notably, the lubrication of moving parts and contact surfaces within the sealed enclosure must remain effective for the entire lifespan of the product. Contact pressure must be carefully controlled, and the resistance of the main current path should remain within technical specifications throughout the switch’s service life.

With the rapid growth of the economy and increasing urban electricity demands, the need for reliable power supply is ever greater. High-performance ring network distribution systems, multi-circuit distribution, and switchgear have become indispensable in secondary distribution networks. Compact and expandable SF6 switchgear, in particular, has become the dominant technology in this area. With the increasing application of digital, integrated, and mechatronic technologies, the development of SF6 switchgear is set to accelerate further.