Sulfur hexafluoride enclosed combined electrical apparatus, internationally known as "Gas Insulated Switchgear", abbreviated as GIS. It integrates all primary equipment in a substation, except for transformers, into a single unit through optimized design. These devices mainly include: circuit breakers, disconnectors, busbars, voltage transformers (PT), current transformers (CT), and inlet and outlet bushings. All live parts of the GIS equipment are enclosed by a metal shell, which is made of aluminum alloy, stainless steel, and non-magnetic cast steel. The shell is grounded with copper busbars, and the interior is filled with sulfur hexafluoride gas under a certain pressure.

  Next, we will introduce each component and its function separately. Circuit breaker. The circuit breaker assembly consists of a three-phase common box circuit breaker and an operating mechanism. Each phase arc extinguishing chamber is enclosed by an independent insulation cylinder. The arc extinguishing chamber is of single-pressure type, using the principle of axial synchronous bidirectional arc blowing, with a simple structure and strong breaking capacity.

  Disconnector and grounding switch. The grounding switch can be equipped with manual, electric, or electric spring mechanisms. Manual and electric mechanisms are mainly used for maintenance grounding switches; electric spring mechanisms are used for grounding switches that have the capability to open and close electromagnetic induction currents, electrostatic induction currents, and need to close short-circuit currents. At the same time, the grounding switch can be used as a primary connection terminal, therefore, under the condition of not needing to release gas, it is used to check the changes of current transformers and measure resistance, etc.

  Lightning arrester. The lightning arrester is of zinc oxide type enclosed structure, using sulfur hexafluoride insulation, with vertical or horizontal interfaces, mainly composed of the tank body, basin insulator mounting base, and core, etc. The core is made of zinc oxide resistance sheets as the main component, which has good volt-ampere characteristics and large current capacity.

  Gas compartment. Each compartment of the GIS is divided into several independent sulfur hexafluoride gas chambers by non-ventilated basin insulators (gas compartment insulators), i.e., gas compartment units. Each independent gas chamber is electrically connected to each other, but is isolated from each other in the gas path.

  Compared with traditional electrical equipment, GIS has the following characteristics:

  1. Miniaturization. Due to the use of sulfur hexafluoride gas with excellent insulation performance as the insulation and arc extinguishing medium, the volume of the substation can be greatly reduced, achieving miniaturization. The three-phase common box compact GIS has a minimum compartment width of 0.8m, and the standard compartment occupies only 2.9m2. Generally, the footprint of 220kV GIS equipment is 37% of conventional equipment; the footprint of 110kV GIS equipment is about 46% of conventional equipment.

  2. Reliable and safe. Since all live parts are sealed in inert sulfur hexafluoride gas, not in contact with the outside, and not affected by the external environment, the reliability is greatly improved. In addition, since all components are combined into a single unit, it has excellent seismic resistance. At the same time, because the live parts are sealed in a grounded metal shell, there is no risk of electric shock. Sulfur hexafluoride gas is a non-combustible gas, so there is no fire hazard. Also, because the live parts are enclosed by a metal shell, electromagnetic and electrostatic shielding is achieved, with low noise and strong resistance to radio interference.

  3. Strong adaptability to the environment. GIT is suitable for harsh environmental conditions, such as severe pollution, hail, snowstorms, dew, high altitude, and earthquake-prone areas.

  4. Easy installation and maintenance. Due to the miniaturization of GIT, the installation period is short, and it can be assembled and tested as a whole unit in the factory, then transported to the site in the form of units or compartments, thus shortening the on-site installation period and improving reliability. At the same time, due to its reasonable structural layout and advanced arc extinguishing system, the product's service life is greatly extended, so the maintenance cycle is long, the maintenance workload is small, and due to miniaturization, it is close to the ground, making daily maintenance convenient.

  V. The Future of Sulfur Hexafluoride and the Exploration of New Insulating Gases

  Although sulfur hexafluoride has played an important role in the power industry due to its superior performance. But everything has two sides, while we take advantage of its strengths, we should also pay full attention to its shortcomings. The biggest hazard of sulfur hexafluoride gas is the greenhouse effect. Although sulfur hexafluoride gas does not destroy the ozone layer, it has a particularly large impact on global warming. With the increase in the use and emission of sulfur hexafluoride gas, the concentration of sulfur hexafluoride gas in the atmosphere is also increasing year by year, and its concentration varies with location and season. Therefore, reducing the emission of sulfur hexafluoride gas to improve our living environment has become a huge issue.

  Research on alternatives to sulfur hexafluoride gas, from the 1970s to the 1980s, the EPRI (Electric Power Research Institute) in the United States actively carried out work in this area, although during this period there were many proposals for expert insulation, but ultimately the view that there is a gas superior to sulfur hexafluoride in terms of insulation characteristics and arc extinguishing performance was negated. In terms of insulation characteristics, there are several gases with higher discharge voltage than sulfur hexafluoride gas at the same pressure, but from the comprehensive consideration of liquefaction temperature, toxicity, and stability, there is no gas superior to sulfur hexafluoride. Considering the new characteristic of global environmental impact, the search for new gases to replace sulfur hexafluoride has recently started again. Considering the destruction of the ozone layer, a new control condition of not containing Cl or Br elements has been added to the research, and the exploration is ongoing. So far, it seems that no gas has been found that can replace sulfur hexafluoride. In the relevant literature, various possible combinations of elements have been processed one by one, and the results show that the only gases that can be used without causing adverse environmental impacts are air and nitrogen.

  The mixed gases that are now widely studied are almost all combinations of sulfur hexafluoride and nitrogen, although they can reduce the use, storage, potential emission, and leakage of sulfur hexafluoride, there are also shortcomings. First, the electrical strength of mixed gases is lower than that of pure sulfur hexafluoride gas, to increase the breakdown voltage, the gas pressure must be increased, thereby increasing the size of the equipment; second, liquefaction recovery is difficult. Since gases like nitrogen are not easy to liquefy, it is extremely difficult to improve recovery efficiency by liquefying them. Based on the above two points, the result is an increase in the emission rate of greenhouse gases.

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