Gas-insulated transmission lines

Maximum power, minimum impact

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 lines 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 over long distances, depending on requirements. Transportation must involve the least 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 shifting their focus towards alternative power transmission solutions. 

Structure of a gas-insulated transmission line
GIL structure

Benefits of gas-insulated transmission lines

Gas-insulated transmission lines (GIL) have already proven their technical reliability for a number of decades, 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 conductor 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 kilometer. 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 (40 years and above). Electrical losses are kept comparatively low due to the large conductor 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 to 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 capacitance of GIL systems means that they only need phase angle compensation devices once the system length exceeds about 70 kilometers.
  • Low electromagnetic fields
    The phase current induces an almost identical reverse current in the enclosure. 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 housing. 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 space consumption. GILs allow automatic reclosure, hence do not require major changes of operation and protection schemes of the grid. 
  • 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 during inspection activities.
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Nominal voltage (typical ) 220-500 kV
Max. operating voltage 245-550 kV
Nominal frequency 50/60 Hz
Nominal short-circuit current (rms, 1-3 s) 31.5-63 kA
Nominal operating current (typical) 2,000-5,000 A
Withstand voltage vs. earth:
AC-rated power frequency withstand voltage 1 min 380-620 V
Norm. surge impulse withstand voltage 1.2/50 µs wave 1,050-1,675 kV peak
Norm. switch impulse withstand voltage 250/2,500 µs wave 850-1,175 kV peak
Enclosure and conductor material Aluminum alloy
Insulating gas Gas mixture, 80% N₂ and 20% SF₆
Nominal 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
Surge impedance Approx. 60 Ω
Electrical cross section of conductor (typical) Approx. 5,300 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 installed 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 tubes and connections safely against corrosion.

Siemens is the only provider to offer a fully automated orbital welding process that produces completely gastight welded seams. In addition, every seam is individually tested using an ultrasonic process. The welded lines are characterized by a strong 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 meters, 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 high-power transmission capacity, low space requirements and minimal electromagnetic impact associated with gas-insulated transmission lines result in various application fields for GIL.

Laying methods at a glance

Download table as: PDF Excel
Power generation
Power transmission
Above ground
Direct burial
Tunnel installation
Vertical installation
Gas-insulated transmission lines at a glance
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Power Transmission Service

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

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GIL success stories

Gas-insulated transmission lines (GIL) are an ideal alternative to overhead lines and high-voltage 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

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 supports you in planning and implementing your projects with a complete portfolio and decades of experience in the areas of product design, engineering, and project management.

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