Which gas is used in GIS?

The primary gas used in GIS (Gas-Insulated Switchgear) is Sulfur Hexafluoride (SF₆).

Here's a breakdown of why SF₆ is used and other relevant gases:

Sulfur Hexafluoride (SF₆):

Why it's dominant: SF₆ has exceptional dielectric strength (about 3 times higher than air at the same pressure), making it incredibly effective at insulating live high-voltage components within the compact GIS enclosure. It also has excellent arc-quenching properties, crucial for safely interrupting high currents during switching operations or faults.

Advantages: Enables much more compact switchgear designs compared to air-insulated equipment (AIS), ideal for space-constrained locations (cities, indoor substations, offshore platforms). It's non-flammable, non-toxic, chemically inert under normal operating conditions, and provides stable performance over a wide temperature range.

Major Drawback: SF₆ is an extremely potent greenhouse gas (GHG). It has a Global Warming Potential (GWP) 23,500 times greater than CO₂ over a 100-year period, and it has a very long atmospheric lifetime (up to 3,200 years). Leaks from GIS contribute significantly to its presence in the atmosphere.

SF₆ Gas Mixtures:

To reduce the environmental impact while maintaining performance, mixtures of SF₆ with other gases (like Nitrogen - N₂, or Carbon Trifluoride - CF₄) are sometimes used, especially in lower voltage applications or specific components.

These mixtures dilute the SF₆ content, lowering the overall GWP of the insulating gas blend, but often require slightly higher operating pressures to achieve the same dielectric strength as pure SF₆.

Alternative "SF₆-Free" Gases:

Due to the severe environmental concerns surrounding SF₆, significant research and development has focused on finding viable alternatives with much lower GWPs. These are increasingly being deployed:

g³ (green gas for grid): A mixture based on 3M Novec™ 4710 fluid (fluoronitrile) and CO₂. It has a GWP of less than 1 and performs similarly to SF₆ in HV applications. Major manufacturers offer GIS filled with g³.

AirPlus™: ABB's alternative, primarily a mixture based on C₅-fluoroketone and buffers like air or CO₂/N₂/O₂. Also has a very low GWP (<1).

Novec™ 5110 Insulating Gas (3M): A fluoronitrile-based gas used in mixtures, similar to g³.

Pure Nitrogen (N₂) or Dry Air: Sometimes used in lower voltage GIS or specific compartments where the dielectric strength requirements are less stringent. They are environmentally benign but require larger equipment sizes or higher pressures compared to SF₆ or its alternatives for equivalent HV ratings.

In summary:

SF₆ remains the overwhelmingly dominant insulating gas in existing and many new GIS installations due to its unmatched technical performance.

Environmental regulations (F-Gas regulations in EU, etc.) are driving a rapid shift towards SF₆-free alternatives like g³ (Novec™ 4710 + CO₂) and AirPlus™ (C₅ FK + air/CO₂) for new installations, especially in regions with strict GHG controls.

SF₆ mixtures are a transitional option to reduce GWP impact.

Nitrogen or Air are used where performance requirements allow, primarily at lower voltages.

So, while the answer is overwhelmingly Sulfur Hexafluoride (SF₆), the landscape is actively evolving towards environmentally friendlier alternatives.