Differences Between Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS)

Modern substations primarily utilize two types of switchgear: Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS). AIS relies on air as the insulating medium, while GIS uses SF₆ gas in a sealed environment. Each type has unique advantages suited to different applications. This article highlights the key differences between AIS and GIS substations across five critical aspects.

1. Construction

The primary distinction between AIS and GIS lies in their construction and insulation medium.

• AIS Construction: AIS employs air for insulation, meaning that its electrical components are exposed to the surrounding environment. It consists of circuit breakers, disconnectors, transformers, and other equipment mounted on steel or concrete structures.
• GIS Construction: In contrast, GIS uses SF₆ gas within a fully enclosed metal structure. Its components—such as busbars, circuit breakers, and disconnectors—are housed in sealed, gas-insulated compartments, providing excellent dielectric strength.

Since GIS is completely enclosed, it offers better protection against environmental factors and electromagnetic interference. In contrast, AIS is more exposed, making it vulnerable to external conditions and requiring more frequent maintenance.

2. Space Requirements & Suitable Environments

Space Efficiency

• GIS is significantly more compact than AIS, requiring much less space for installation. This makes it ideal for urban areas, underground substations, industrial facilities, tunnels, and offshore platforms where space is limited.
• AIS requires a larger footprint due to the need for greater air insulation distances. It is better suited for rural and suburban areas, where space is less of a constraint.

Environmental Adaptability

• GIS is well-suited for harsh environments such as coastal regions, deserts, and areas with high humidity due to its sealed design that prevents contamination.
• AIS performs best in moderate climates, as air insulation is more susceptible to humidity, pollution, and extreme temperatures. In highly polluted or humid environments, AIS insulation can degrade, increasing the risk of flashovers and requiring additional maintenance.

3. Safety & Reliability

Both AIS and GIS are designed with safety and reliability in mind, but they differ in how they manage risks.

• GIS Safety: The sealed design of GIS significantly reduces arc flash risks, ensuring better personnel protection and reducing direct exposure to high-voltage components. The use of SF₆ insulation enhances reliability by lowering the risk of dielectric failures.
• AIS Safety: Since AIS is exposed to environmental factors, it is more susceptible to lightning, pollution, and moisture, which increases maintenance requirements and operational risks.

While both switchgear types are built for reliability, GIS generally requires less maintenance and experiences fewer unexpected outages due to its enclosed nature.

4. Scalability & Expansion

Scalability is an important factor when planning future substation expansions or modifications.

• AIS Scalability: AIS substations are relatively easy to expand because they are built in an open-air layout. Adding extra bays, transformers, or switchgear can be done with minimal disruption to existing operations, making AIS a cost-effective choice for utilities planning future expansions.
• GIS Scalability: GIS expansion is more complex due to its compact, modular design. Adding new gas-insulated modules requires specialized handling and can be expensive. However, leading manufacturers like CHINT are developing modular GIS solutions that improve scalability through plug-and-play designs and digital monitoring.

5. Environmental Impact

Environmental considerations are crucial in modern substation design.

• GIS & SF₆ Impact: GIS relies on SF₆ gas, a potent greenhouse gas with a high global warming potential (GWP). Although GIS is designed to prevent leaks, emissions can still occur due to equipment aging or mishandling, contributing to climate change. As a result, manufacturers are researching SF₆-free alternatives using dry air and other eco-friendly gases.
• AIS & Environmental Considerations: AIS has a lower direct environmental impact since it does not use SF₆ gas. However, it requires more land, potentially leading to deforestation or habitat disruption for large installations.

In response to environmental concerns, many power companies and regulators are encouraging SF₆-free technologies, pushing manufacturers to develop greener GIS solutions.

Conclusion

Both AIS and GIS have distinct advantages depending on space constraints, environmental conditions, safety requirements, and future scalability needs. GIS is preferred for compact, high-reliability applications, while AIS is a cost-effective choice where space is not a constraint and environmental exposure is manageable.

As the industry moves toward sustainable solutions, advancements in SF₆-free switchgear technologies will further shape the future of substation design.

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