Gas-Insulated Transformers: A Safe Solution for Urban Installations
A gas-insulated transformer (GIT) swaps insulating oil for SF₆ gas. That single substitution is why these units show up in places where oil-filled equipment is effectively banned: basements, underground caverns, building-integrated substations, rail depots, and data center campuses where a fire event is not an acceptable risk.
The fire argument is straightforward. SF₆ does not burn, and an internal arc produces far less pressure rise than an oil fault, so the tank is not a fuel-loaded explosion source. What matters more for urban projects is what disappears around the transformer: no oil containment pit, no fire suppression skid, no separation distances between banks. Toshiba's field data puts the transformer room height reduction at roughly 2 to 2.5 meters once the conservator is gone, and the removal of auxiliary fire systems lowers both design and installation cost. In a city where floor area is the expensive part, those savings are the business case.
Service experience is not thin. The first GIT was delivered in 1967, and several thousand units are in operation worldwide, with ratings up to 400 MVA and 345 kV. IEC 60076-15:2015 is the governing standard for gas-filled power transformers; its second edition added explicit clauses on safety and environmental requirements, including leak containment through joint design, gasket selection, and corrosion prevention.
One thing to write into the specification rather than assume: SF₆ is a potent greenhouse gas. The operational safety gain is real, but the environmental burden moves from spill containment to gas monitoring and end-of-life recovery. Name the recovery procedure and the monitoring scope in the project documents.
How the numbers compare
The table below summarizes published comparisons between oil-immersed and gas-insulated designs.
| Item | Oil-immersed (OIT) | Gas-insulated (GIT) |
|---|---|---|
| Insulating / cooling medium | Insulating oil | SF₆ gas |
| Conservator | Required | Not required |
| Fire suppression system | Required | Not required |
| Oil containment system | Required | Not required |
| On-load tap changer | Arc switching in oil | Vacuum interrupter |
| Foundation cost | Higher | Lower |
Source: Toshiba GIT technical data.
Installation efficiency moves in the same direction. Mitsubishi Electric reports that GIT installation time is about 25% shorter than OIT and transport volume is about 25% smaller; the on-load tap changer's electrical life extends from 200,000 hours to 500,000 hours.
The most concrete field data comes from a 275 kV substation project in Japan. Against an oil-immersed scheme, the GIT design cut installation area by more than 40%, reduced overall weight by about 20%, and lowered losses by about 25%. Compared with a conventional air-insulated substation, the footprint shrank by more than 80% and total project cost fell by roughly 45%.
Where GITs make sense, and where they don't
The fit depends on what the site costs you. In a metro traction substation where the fire separation distance to the adjacent platform cannot be met with an oil-immersed bank, the GIT is often the only design that clears code without rebuilding the station. The same logic applies to a basement plant room in a commercial tower, or a data hall where a suppression system failure is not a scenario the operator will accept. Confined space, high fault-current exposure, expensive floor area - the pattern keeps pointing to gas.
The counter-case is just as clear. If the site is a fenced outdoor yard with room to spare, an oil-immersed unit is cheaper to buy, cheaper to maintain, and does not carry the SF₆ recovery obligation at end of life. For distribution-level ratings in that setting, the economics rarely close, and forcing a GIT into the specification adds cost without buying anything the site actually needed.
For procurement, four items drive the specification: rated power and voltage class, gas monitoring and leak-detection scope, end-of-life SF₆ recovery provisions, and the delivery timeline against the civil works schedule. Get those onto one page before requesting quotations, and the comparison against oil-immersed and dry-type options becomes a straight numbers exercise rather than a technology debate.
TIG - Trust in Grid builds transformers and supplies core materials. Oil-immersed and dry-type power and distribution transformers, plus grain-oriented electrical steel, amorphous alloy cores, and magnetic materials with grade selection and supply chain support. If you are evaluating transformer specifications, core grade, loss targets, or supply options for a transformer project, send us your specs: efficiency target, transformer type, volume, and timeline. You will get a straight answer.
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References
[1] IEEE Transformers Committee, "Gas Insulated Power Transformers," Tank Rupture Working Group Presentation, 1995.
[2] BSI, "BS IEC 60076-15:2015 Power transformers – Gas-filled power transformers," 2015.
[3] Toshiba, "Gas Insulated Transformer (GIT) Brochure," Technical Data.
[4] Mitsubishi Electric, "Gas Insulated Transformer (GIT) Technical Data."
[5] H. Okubo et al., "Realization of the Compact Outdoor Substation by Advanced Gas-insulated Transformers," IEEJ Transactions on Power and Energy, vol. 138, no. 5, pp. 399–411, 2018.