Gas Insulated Lines (GIL) DC

Direct Current Gas Insulated Lines (GIL), known as DC CTL – Compact Transmission Line for HVDC, is a newly under development transmission technology. The challenges for the DC CTL is posed by the special design of the components, considering their specific properties at high direct voltages. In DC CTL similarly to AC GIL, the inner conductor is located in a pipe of approximate dimeter of 50 cm. It is kept central using disc or support epoxy resin insulators. The pipe is filled in with insulating gas.

Technology Types

The technology type is defined by:

  • The gas or gas mixture as insulating medium
  • 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)

• Elongation compensators

• Angle components

Advantages & field of application

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

Electromagnetic fields The construction of AC GIL results in a shielding of electric fields and in 15 to 20 times smaller magnetic fields in the vicinity of the installation than with conventional power transmission systems.

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.

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

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

Technology Readiness Level

For EHV applications:

2020: TRL 4-5 *(N2/SF6 mixtures)

2025: TRL 6

2030: TRL 8

* based on ~30 IEEE publications in this domain, mostly coming from out of Europe and published after 2016

Research & Development

Current fields of research: Finding a substitute to SF6 and adapting basic design accordingly; the design of the epoxy resin insulators and their long term dielectric performance; particle-free assembly or particle treatment; measurement of electrical performance (partial discharges); speeding up construction and assembly time on-site.

Best practice performance

Currently, DC GIL have not yet been realised in extra high voltage network. DC GIL with following technical specifications are under development and are expected to be available in the future:

Maximum capacity: 5 GW

Current rating: 5 kA

Voltage rating: 525 kV

Best practice application


Starting 2014 and still ongoing

In collaboration with three German universities, Siemens is developing and testing the first DC GIL in Germany designed to transfer 5 GW. The project is also funded by the federal German government.

The DC CTL is based on the technology of the existing gas-insulated transmission line (GIL), which consists of two concentric aluminium tubes. A mixture of gases is used as the insulating medium. The challenge for the DC CTL is posed by the special design of the components taking into account their specific properties at high direct voltages.

Enabling compact DC transmission of electricity. HVDC systems in conjunction with DC GIL will the transmission grid with superimposed DC a reality. It will also allow space saving design of substations and transmission corridors.


[1] Siemens. Siemens develops gas-insulated transmission line for direct current. [Link]

[2] Modern Power Systems. German industry developing DC GIL. [Link]