Gas Insulated Lines (GIL) AC

In Gas Insulated Lines (GIL), the inner conductor is located in a pipe the approximate diameter of which is 50cm. It is kept central using disc or support epoxy resin insulators. The pipe is filled in with insulating gas. Compared to OHL and underground cables, the electric and magnetic field is very low. Nowadays, GIL are mainly used in short lengths within substations, in densely populated areas or to connect industrial / power plants to the transmission network transferring current.

Technology Types

The technology type is defined by:

  • The gas mixture used in a GIL:

    • 100 % SF6 insulated – majority of projects commissioned between 1970 and 2000
    • Gas mixture: N2 and SF6 mixture – majority of projects commissioned after 2000
    • Alternative (synthetic) gases – currently under development
  • Disc insulator design

  • Assembly for the 15-20 m GIL pipe sections:

    • welding
    • flanged connection
  • Application type:

    • Air insulated substations
    • Tunnels
    • Direct burying in earth

Components & enablers

  • Outer sheath (usually aluminium tube of around 50-60 cm diameter)
  • Inner aluminium conductor
  • Epoxy-resin insulators
  • Insulation gas (5-7 bar)
  • Additional outer coating for direct burial
  • Elongation compensators
  • Angle components

Advantages & field of application

GIL are an alternative to OHL or underground cables if high capacity in narrow and complex routing is required.

Reactive compensation

Due to capacitance comparable with overhead lines, GIL systems typically do not require compensation.

Electromagnetic fields

The construction of GIL results in a complete shielding of electric fields and many times smaller magnetic fields comparing to cables and over-head lines.

High safety

GIL are fire resistant and do not contain flammable material, nor do they emit noxious fumes under fire conditions. In short circuits inside the GIL system, toxic products may occur, but the arc does not leave the GIL compartments.


The gas used as insulating media does not age. In typical application, the doubling of lifetime comparing to conventional VPE cables can be expected.


Realisation of 90 degrees, vertical, curved sections with narrow spacing of phases is possible.

Technology Readiness Level

For EHV application:

2020: TRL 7 - GIL (SF6 free)

2025: TRL 8 - GIL (SF6 free)

2030: TRL 9 - GIL (SF6 free)

*GIL by pure SF6 or SF6 mixture is a mature technology. TRL is 9.

Research & Development

Current fields of research: finding substitute to SF6 and adapt design accordingly, speeding up construction and assembly time on-site.

Best practice performance

Maximum capacity: 2850 MVA (at 500 kV)

Maximum current rating: up to 4,500A

Typical voltages: 245 kV-500 kV

Longest distance: 3.3 km (realised, 275 kV)

Energy losses: 0.0015% km at 500 kV and 260 MW

Standard definition: IEEE Std C37.122.4-2016 - IEEE Guide for Application and User Guide for Gas-Insulated Transmission Lines, Rated 72.5 kV and Above

Expected lifetime: > 60 years

Best practice application

South East of England


National Grid applied; a SF~6~-free 420 kV GIL. The 230 meter long, gas-insulated circuits connects the substation to overhead line in the ElecLink project, via a 1 GW HVDC cable.



Shinmeika-Tokai line, Japan


Connection of a power plant to the grid to deliver energy into the Nagoya high industrial region.

Two three-phase 275 kV welded GIL, designed to transport 1x2,850 MVA. It is one of the longest GIL in the world, with a length of 3.3 km each. The GIL is installed in a tunnel where space for the assembly work is limited and atmosphere is dusty. Gas: 100 % SF~6~.

High voltage power transmission was made feasible given the geographic constraints.

Frankfurt, Germany


The underground connection of GIS substation close to Frankfurt Airport was needed. The aim of the project was to demonstrate the alternative to cable and reduce the trench size.

Two three-phase 380 kV welded GIL systems were laid directly in the ground with a length of 900 meters each. Gas mixture consists of 80% N2 20% SF6. Both conductor tube and enclosing tube are made of aluminium.

No need for reactive power compensation.


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[2] N. Takinami, S. Kobayashi and A. Miyazaki. Application of the world's longest gas insulated transmission line in Japan. [Link]

[3] Siemens. Siemens is presenting the mobile factory - a mobile tube-laying platform for gas-insulated transmission lines (GIL). [Link]

[4] Siemens. Experience with 2nd Generation Gas-Insulated Transmission Lines GIL. [Link]

[5] GE. NationalGrid, SF6-free 420 kV Gas-Insulated Line. [Link]

[6] 3M. 3M™ Novec™ Dielectric Fluids SF6 Alternatives for Power Utilities. [Link]

[7] ee publishers. Gas-insulated transmission lines: the next generation of power transmission? [Link]

[8] T&D World. Transforming the Transmission Industry. [Link]

[9] Koch, H. Gas insulated transmission lines (GIL). [Link]

[10] Cigre. Application of Long high capacity gas-insulated lines in structures. [Link]

[11] Cigre. Factors for Investment Decision GIL vs. Cables for AC Transmission. [Link]

[12] Siemens. Go-ahead for power pipeline: Siemens and RWE to install extra-high voltage line at Frankfurt Airport as a buried twin gas-insulated line. [Link]