What is GIS?
Its full name is Gas Insulated Switchgear. Gas Insulated here refers to sulfur hexafluoride (SF6). Switchgear is the abbreviation of a combination of circuit breakers, disconnectors, grounding switches, transformers, lightning arresters, busbars, connectors and outgoing terminals. These devices or components are all enclosed in a metal grounded casing, and filled with SF6 insulating gas at a certain pressure, so it is also called SF6 fully enclosed combination. So, GIS is to combine the high-voltage components in the electrical system according to the connection method of the electrical wiring, and install them together in a metal shell of SF6 with excellent insulation and arc extinguishing ability to form a closed high-voltage switchgear.
Characteristics of SF6
SF6 is an artificially synthesized inert gas born in France. Pure SF6 gas is colorless,
tasteless, odorless, non-flammable, and has stable chemical properties at room temperature. It is an inert gas. The gas density is 5.1 times that of air.
SF6 gas has a dielectric strength equivalent to that of transformer oil at a pressure of 0.29 MPa, and its arc extinguishing ability is 100 times that of air. It is currently the most widely used electrical insulating gas.
Comparison between AIS and GIS
Floor space
GIS, a completely sealed metal package with a compact structure, occupies only 10% of the floor space of AIS for 220kV GIS and only 5% of the floor space of AIS for 500kV GIS. This is why GIS is the first choice in places where land demand is high, land availability is very lo,w or land prices are very high. Of course, it does not mean that land is the only criterion for choosing GIS. There are many other factors, and this is just one of them.
Altitude
Relationship between altitude and dielectric strength:
As altitude increases, atmospheric pressure drops significantly, resulting in a decrease in air density. The air becomes “thin”. The dielectric strength of air (its ability to resist electrical breakdown and prevent arcing) depends directly on its density. Air with lower density (high altitude) has larger molecular spacing and longer molecular free paths. This makes it easier for electrons to accelerate in the electric field and gain enough energy to collide and ionize other molecules, making it easier to cause electrical breakdown (arc generation). Therefore, at the same voltage, the insulation performance of air at high altitudes is significantly lower than that at low altitudes or sea level.
When the switchgear is installed at a higher altitude, the maximum voltage it can withstand without arcing is reduced, so the maximum voltage at which it can operate safely is also reduced. This is why, for conventional air-insulated switchgear, if it needs to be installed above 1000 meters, equipment with higher dielectric strength is required. GIS is protected by SF6 gas, so its installation will not be affected by altitude.
Safety
In terms of safety, all high-voltage components are sealed in a fully enclosed metal casing that is grounded. The casing forms a natural electromagnetic shield (Faraday cage effect), and even if the casing is directly touched when the equipment is energized, electric shock will not occur, achieving intrinsic safety protection. AIS adopts an open structure, and the high-voltage conductor is directly exposed to the air. When operating or approaching accidentally, personnel must strictly maintain a safe distance (usually more than several meters). There is an inherent risk of accidentally touching energized equipment, and physical isolation barriers and operating specifications must be relied on for protection.
But this does not mean that AIS is less safe or not safe at all. In fact, 70% of substations worldwide are composed of AIS technology. Regular cleaning of insulators, installation of bird protection devices, and de-icing equipment can also maintain safe operation.
Substation expansion
When designing a substation, engineers usually reserve land area to meet the needs of future load growth. In air-insulated substations (AIS), this expansion is usually relatively straightforward and flexible. Since AIS equipment (such as circuit breakers, disconnectors, transformers, busbars, etc.) is open-type installation, they mainly rely on air insulation and physical spacing between each other. Adding new bays or extending busbars usually only requires installing new standardized equipment units in the reserved space and ensuring sufficient electrical distance. There is less dependence on the specific model or manufacturer of the newly added equipment. Equipment of different brands or models (as long as they meet the parameter requirements) can usually be easily integrated into existing stations or run in parallel.
However, for gas-insulated substations (GIS), expansion is much more complicated and highly dependent on specific manufacturers. This is because GIS highly integrates high-voltage components and seals them in a modular metal housing filled with insulating gas (such as SF6). Expansion usually means that new, fully matched gas-sealed spacer modules must be added. These modules need to be strictly compatible with existing equipment in terms of size, interface (mechanical and electrical), gas system, internal connectors (such as plug-in contacts), housing design, and monitoring and control systems. This compatibility requirement makes it almost necessary to use specific model modules of the same manufacturer, or even the same series and design era as the original equipment for expansion to ensure the success of airtightness, insulation reliability, mechanical matching and system integration. Therefore, the expansion planning of GIS is highly dependent on the initial equipment selection and the long-term support of the manufacturer.
Environmental protection
Cost
The procurement cost of AIS equipment is about 30%-50% of that of GIS, which constitutes a core price advantage. The land cost pressure is prominent: sufficient electrical safety distance needs to be reserved for exposed conductors, resulting in a large area. In urban areas or high-value plots, the land acquisition cost may exceed the equipment investment. The following table is a comparison of the initial investment, operation and maintenance costs, and the entire life cycle costs of AIS and GIS.
| Dimension | AIS | GIS |
| Initial investment | Low equipment cost (about 30%-50% of GIS) High land cost | High equipment cost (including sealed gas chamber/SF₆ gas) Land saving 50-70% |
| Operation and maintenance costs | Frequent cleaning of insulators anti-fouling flashover coating/de-icing equipment investment | Basically, maintenance-free SF₆ gas monitoring is the main cost |
| Full life cycle | Low technical threshold, simple but continuous maintenance | High initial investment, operation and maintenance costs can be reduced by 40%+ within 20 years |
Gas-Insulated Switchgear (GIS) provides numerous advantages in terms of space savings, safety, and reliability, making it a key technology for modern power systems. If you want to explore the working principle, main components, key advantages, and real-world applications of GIS, check out our next article: Gas-Insulated Substations (GIS): Compact and Reliable Power Solutions for Modern Grids
