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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.
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.
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.
|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|
|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|
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.
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.
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.
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
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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.
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
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.