GIS combined appliance
GIS combined appliance, full name Gas Insulated Switchgear, is a kind of high-voltage switchgear that organically combines most of the primary equipment in a substation except the main transformer (such as circuit breakers, disconnectors, earthing switches, instrument transformers, surge arresters, busbars, etc.), and encloses them all in an earthed metal housing filled with sulfur hexafluoride (SF₆) gas at a certain pressure as the insulating and arc-extinguishing medium.
Core structure and working principle
GIS equipment uses modular design to combine electrical components with different functions into several "bays". Its core structure and working principle are as follows:
Core components: It integrates circuit breakers (responsible for breaking and closing currents), disconnectors and earthing switches (responsible for safe isolation and earthing), current/voltage transformers (responsible for measurement and protection), surge arresters (responsible for overvoltage protection), and busbars (responsible for power collection and distribution).
Insulating and arc-extinguishing medium: It is filled with excellent insulating and arc-extinguishing gas (usually SF₆ gas). SF₆ gas not only has excellent insulation performance, greatly reducing equipment size, but also has strong arc-extinguishing capability, making the circuit breaker structure more compact.
Gas compartment division: The interior of GIS is divided into several independent gas compartments. For example, the circuit breaker compartment usually has a higher pressure, while busbars, voltage transformers, surge arresters, etc., each have their own independent compartments. Each compartment is equipped with a density controller to monitor gas pressure in real time, ensuring insulation performance.
Three-phase common enclosure and separate enclosure structures: Depending on the voltage level and design requirements, GIS can be divided into three-phase common enclosure type (mostly used for busbars or medium/low voltage levels, saving space and reducing eddy current losses) and separate enclosure type (mostly used for high voltage levels, where insulation issues are easier to handle).
Main features and advantages
Compared with traditional open-type substation equipment (AIS), GIS has significant advantages:
Extreme compactness, saving land: Thanks to the excellent insulation performance of SF₆ gas, the footprint of GIS is only 15% to 50% of that of conventional open-type equipment, making it particularly suitable for city centers, underground power stations, and areas with limited land.
High reliability and safety: All live parts are sealed inside an earthed metal housing, unaffected by the external environment (such as pollution, ice and snow, salt spray, high altitude, etc.). At the same time, the metal housing effectively prevents electric shock hazards to personnel, and SF₆ gas is non-flammable, eliminating fire risks.
Anti-interference and environmental protection: The fully enclosed metal structure effectively shields electromagnetic and electrostatic interference, resulting in low noise and extremely low radio interference levels, making it very suitable for installation in urban areas and near residential areas.
Low maintenance workload: Internal components are not eroded by the external environment, and the maintenance cycle is extremely long (the maintenance interval for major components can usually reach 10 to 20 years), making routine maintenance very convenient.
Limitations and operational precautions
Although GIS has superior performance, its fully enclosed structure also brings some inherent challenges:
Difficult fault location and maintenance: Due to the highly sealed equipment, once an internal flashover or insulation fault occurs, locating and repairing the fault point is very complicated, and the average outage time for repair after an accident is usually longer than that of conventional equipment.
Extremely high requirements for manufacturing processes: The breakdown characteristics of SF₆ gas are extremely sensitive to electric field uniformity. If there are conductive impurities, metal particles, or burrs on the conductor surface inside, it is very easy to cause partial discharge or even insulation breakdown. Therefore, strict cleanliness control must be maintained during manufacturing, transportation, and on-site installation.
Gas and moisture management: During operation, the density and moisture content of SF₆ gas must be strictly monitored to prevent insulation strength degradation due to gas leakage or external moisture ingress. In addition, decomposition products generated under the action of arcs are corrosive, and internal adsorbents need to be regularly treated or replaced.
Higher cost: The initial cost of GIS is higher than that of conventional open-type equipment, and it requires more SF₆ gas. It also demands higher professional skills and special tools (such as SF₆ gas recovery devices) from maintenance personnel.
