Among the many choices that must be made during project planning and development for high-voltage substations, one might look fairly straightforward on first glance: the choice of switchgear.

Air-insulated switchgear (AIS) has been used in the U.S. for more than a century. It is proven technology that maintenance personnel are familiar with, and the cost of the equipment is significantly lower than the alternative, gas-insulated switchgear (GIS).

But when one steps back and takes a more holistic view, there are a number of factors that might counterbalance the GIS sticker shock and highlight a more involved decision to make.

What is GIS, and what are some of its advantages?

Gas-insulated switchgear encapsulates electrical equipment in an insulating gas, typically sulfur hexafluoride (SF6). SF6 is a synthetic chemical that is nonreactive, noncorrosive and nontoxic, offering several significant advantages over ambient air for the insulation of electrical equipment.

The gas has several properties that make it well-suited for insulating electrical equipment. It is very effective at arc quenching and has higher dielectric strength and better thermal conductivity than air. These properties combine to allow for greatly reduced electrical clearances compared to equivalent AIS equipment.

GIS equipment requires a significantly smaller footprint than an equivalent AIS installation. This can be advantageous in locations where real estate is scarce or exceptionally costly.

Reliability is another big difference. GIS equipment has a mean time between failure (MTBF) several times higher than AIS equipment. That translates into lower costs for failure and repairs over the lifetime of the installation, not to mention the potential economic losses from power interruptions that are prevented. These reduced life cycle costs help to offset the larger installation cost of GIS equipment.

GIS equipment can be safer to operate. Interlocks prevent incorrect switching operations and, with all equipment being fully encapsulated, operators are fully protected in the unlikely event of a flashover or other failure. It also can offer physical security benefits because it can be installed inside a building. This removes any line-of-sight exposure and provides six walls of protection.

When might GIS be preferable to AIS?

The compact nature of GIS is beneficial in several scenarios. In dense urban areas or other situations where real estate comes at a premium, a smaller footprint can reduce the cost of land acquisition, grading and permitting. It also could enable additional site alternatives to be considered, which could reduce transmission line costs if a smaller site is better situated.

The acreage necessary for an AIS substation, particularly at extra high voltages (greater than 230-kV), can be substantial. Those projects especially might realize significant savings from the smaller GIS footprint.

For brownfield projects, GIS offers more flexibility in design layout than AIS equipment. That means it can accommodate unconventional layouts to accept transmission lines, and it can be arranged around legacy AIS equipment to maintain service at an existing substation during upgrades.

Since GIS can be installed indoors or outdoors, it is possible to put the substation entirely inside a building. Besides offering security protection helpful to meet current and future industry requirements, this also might be more aesthetically pleasing and thereby facilitate siting, permitting and public acceptance.

Although GIS technology has been around for decades, U.S. utilities and developers have tended not to use it often, except perhaps when site geography demands the smaller footprint. It’s easy to look at the upfront cost of the equipment and decide to stick with established, proven AIS technology that operations and maintenance personnel are more familiar with. Installation can be challenging, and GIS does require more of a learning curve to operate and maintain, but the flexibility, reliability and long-term savings can make comparisons a closer call.

Each substation project will have unique considerations, and there is no single correct blanket choice. But for those who are less familiar with GIS or fear the initial price tag, it might be worth another look to analyze all benefits and other variables over the asset’s life span that could tip the balance.


Needing to increase capacity at a vital substation on a tight brownfield site, a Northeastern utility chose to upgrade to GIS. Learn how our engineer-procure-construct (EPC) approach helped deliver results.

Read the Case Study

Nicholas Matone, PE, is an electrical engineer in the substation department at Burns & McDonnell. His utility industry experience involves substation conceptual engineering, detailed substation physical design, grounding and lightning protection studies, construction details, standards development, gas-insulated switchgear, and major equipment specification and purchasing. He also serves on the IEEE PES Gas Insulated Switchgear Committee.