Gas-insulated transmission lines


Maximum power, minimum impact

Gas-insulated transmission lines (GIL) are the safe and flexible alternative to overhead lines and take up much less space while providing the same power transmission. Since they hardly impact on the landscape and their minimal electromagnetic radiation means they can also be used close to, or even within buildings, GIL can be considered for a wide range of applications. They are suitable for providing a continuation for overhead wires underground, connecting power stations to the power network, or as a space-saving way to connect major industrial plants to the public grid.

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Powerful alternatives

Proven technology for maximum performance

The energy system is in transition. Whereas previously, electricity was generated where it was needed, now, large-scale generation using renewable energy sources means that electricity often has to be transported long distances, depending on requirements. Transportation must involve the minimum possible losses, and requires major technical input. Since building more new overhead lines demands a lot of space and is often negatively viewed by the public, power supply companies are looking for alternatives. 

Structure of a gas-insulated transmission line
GIL structure

Benefits of gas-insulated transmission lines

Gas-insulated transmission lines (GIL) have proven their worth for a number of decades now, since they offer outstanding safety in operation and even in the event of failure, in addition to their very good transmission performance. Any impact on individuals or neighboring resources can be largely ruled out. That means GIL systems can also be used in existing tunnels or in, or close to, buildings. 


Gas-insulated transmission lines consist of two concentric aluminum tubes. The inner lead rests on cast-resin insulators, which center it within the outer sheath. This casing is formed from a stable aluminum tube, which ensures a solid mechanical and electrotechnical encapsulation for the system. To satisfy the latest environmental and technical aspects, GIL systems are filled with an insulating gas mixture consisting mainly of nitrogen and a smaller proportion of SF6 (sulfur hexafluoride).

The tubes are made of a corrosion-resistant aluminum alloy, and are provided with an additional coating if they are laid directly in the ground. Their modular design enables them to be combined to any length. Typical GIL systems have a modular length of up to 1.2 km. These sections can be multiplied to suit requirements, which can make very long lines possible with no impact on system availability. The properties of GIL systems mean they can describe smooth curves with a radius of up to 400 meters, or even changes of direction involving any angle, no matter how sharp. The system includes mechanical expansion elements that will safely cushion mechanical loads.

GIL is a reliable solution: the gas insulation creates a physical similarity to an overhead line, which means these two types of system can be combined very well from an operational perspective. There is no need for costly devices at the interfaces between the GIL system and the overhead lines. Cost-efficiency is boosted by the system’s lengthy service life (60 years). Electrical losses are kept comparatively low thanks to the large cable cross section of the GIL system.

  • Low losses
    Resistance losses with GIL are lower than with cables or overhead lines. The dielectric losses with GIL are negligible, which helps reduce the operating costs. Heat emissions are better than with cables, thanks to the larger external diameter. With GIL, there is normally no need for highly refined cooling systems.
  • No electrical compensation
    The low capacity of GIL systems means that they only need phase angle compensation devices once the system length exceeds about 70 km.
  • Low electromagnetic fields
    The phase current induces an almost identical opposite current in the sheathing. This means the magnetic field outside the GIL is negligible. Even in EMC-sensitive areas (such as close to residential areas or hospitals), there is generally no need for special shielding.
  • Greater safety
    Even in the event of an internal insulation failure, the internal arc would be safely enclosed within the outer sheath. There are no external impacts on humans or other parts of the system. GIL systems are non-flammable and do not contribute to the fire load. The result is the best possible protection for both people and the environment. 
  • High reliability
    GIL technology has proven its reliability in more than 40 years of operation, with no failures to date.
  • No aging
    The GIL insulation system is not prone to either electrical or thermal aging. GIL systems are almost maintenance-free.
  • Used in the same way as overhead lines
    GIL systems are the ideal complement to overhead lines. The high transmission capacity offered by GIL makes it possible to provide a continuation for overhead lines underground, with one GIL tube per phase, which minimizes the amount of land the system will take up. GILs can tolerate brief interruptions, which means energy utility companies are not required to make any major changes to their operational or protection strategies. 
  • Maintenance-free design
    The physical properties and the use of high-quality materials make this an almost maintenance-free product. Routine maintenance is limited to an external inspection, and the line can remain in operation for the duration.
  • µÊ½


Download table as: PDF Excel
Rated voltage (typical ) 220-500 kV
Max. operating voltage 245-550 kV
Rated frequency 50/60 Hz
Rated short-circuit current (rms, 1-3 s) 31.5-63 kA
Rated operating current (typical ) 2000-5000 A
Dielectric strength to ground:
AC rated power-frequency withstand voltage 1 min 380-620 V
Rated wave impulse withstand voltage 1.2/50 µs wave 1050-1675 kV peak
Rated switch impulse withstand voltage 250/2500 µs wave 850-1175 kV peak
Casing and lead material Aluminum alloy
Insulating gas Gas mixture, 80% N₂ and 20% SF₆
Rated pressure (absolute) 0.7 MPa
Ambient temperatures -30°C to +50°C
Weight per phase approx. 50-70 kg/m
Capacitance 55 nF/km
Inductance 0.205 mH/km
Wave resistance approx. 60 Ω
Electrical cross-section of wire (typical) approx. 5300 mm²
Electromagnetic field negligible µT
Gas tightness Sealed for life, no refilling required

All requirements covered

Depending on demand, gas-insulated transmission lines can be produced with a range of different connection types. For short distances involving many changes of direction, the components are normally flanged together, for example for use in transformer substations or when connecting to longer sections. If a GIL is laid directly in the ground, this will use a welded, flangeless system with an extra coating to protect the GIL pipes and connections safely against corrosion.

Siemens is the only provider to offer an automated orbital welding process that produces completely gas-tight welded seams. In addition, every seam is individually tested using an ultrasound process. The lines welded in this way stand out on account of their particular mechanical stability and are also suitable for routing with a bending radius of up to 400 meters. The short installation times that this connection technology involves also ensure that systems for large-scale projects can be fitted at a reasonable cost. 

Versatility in application and laying methods

Due to their unique properties GIL systems have become well established in all parts of the world, to solve high-power transmission tasks in complex routings. GIL installations have been realized in every conceivable layout, with shafts mastering straight vertical distances of more than 200 m, overcoming steeply inclined slopes, passing around buildings both above and below ground, and smoothly following serpentine routings with no need for angle units.

Transfer of three-phase and DC electricity

Gas-insulated transmission lines have proven their worth in the transmission of three-phase power for a number of decades now. Siemens installed a GIL in a tunnel in the Wehr pumped-storage power station in the Black Forest as long ago as 1975. An inspection after 30 years showed that all components were still in top condition, even after such a lengthy period. 

It would also make sense for this long life cycle, high transmission power and the system-based operating benefits offered by GIL to be available for high-voltage direct-current transmission. Siemens is currently researching the necessary solution in collaboration with several universities. 

Fields of application for gas-insulated transmission lines

The extensive transfer capacities, low space requirements and minimal electromagnetic radiation associated with gas-insulated transmission lines mean they can be used in many different ways.

Laying methods at a glance

Download table as: PDF Excel
Power generation
Power transmission
Above ground x x x
Direct burial x
Tunnel installation x x
Vertical installation x

Downloads, support and services

Additional information

This is where you can find our brochures, contact details for individual consulting interviews and further information.

Power Transmission Service

Siemens support the development of the power transmission and distribution grid with its strong team of experts and a number of services

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Power Transmission Service

Siemens support the development of the power transmission and distribution grid with its strong team of experts and a number of services

Learn more


GIL success stories

Gas-insulated transmission lines (GIL) are an ideal alternative to overhead lines and traditional underground cables. Siemens offers a complete portfolio for efficient, future-proof routing of GIL systems, and has a long history of successfully implemented projects.


Connecting the new Paulaner brewery to the power network

One of the most powerful GIL systems in China

High voltage at Frankfurt Airport

Innovation corner

In development: innovative technologies

Gas-insulated transmission lines have proven their worth for a number of decades now. Even so, Siemens engineers and university scientists are engaged in research to improve the technology, to make it even more efficient, reasonably priced and environmentally friendly.

Environmentally friendly solution

Well connected with GIL

Gas-insulated transmission lines (GIL) are the right choice in cases where overhead lines or underground cables are not sufficient to meet your requirements. Siemens helps you plan and implement your projects with a complete portfolio and decades of experience in the areas of product design, engineering and project management.

Find out more about the benefits of GIL
Download White Paper