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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.
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.
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.
In hydropower stations, gas-insulated transmission lines can be used to transmit the generated current from the generators to the grid connection with minimal use of space, since they can be routed vertically or at any desired angle, without difficulty. Existing shafts and tunnels can also be used for the purpose, since GIL systems are not highly dangerous and do not give off hazardous radiation, and are therefore suitable, for example, for coal, oil, or gas-fired power stations.
Gas-insulated transmission lines (GIL) are ideally suited for providing a continuation for overhead lines underground. The insulation medium in gas form and the secure enclosure allow for auto-reclosure functionality with no external effects. There is, therefore, no need to separately monitor the GIL by means of distance protection systems. The high transmission performance of the gas-insulated line enables the phases to be connected 1:1 with the overhead lines. There is no need to double the lines. As a result, the space requirements at the transition point between the GIL system and the overhead lines are very low. The much lower electrical capacity compared to other underground transmission solutions also means that very long GIL sections can be combined with overhead lines with no need for reactive power compensation. This makes GIL an ideal means of providing a continuation for overhead lines underground.
Substations can be connected to the transmission network via gas-insulated transmission lines. Here, too, the high performance of GIL plays an important role. Because gas-insulated lines are used to transmit heavy currents, GIL systems can be used even for heavy-load connections with minimum use of space. Angle elements can easily handle the very tight changes of direction that are sometimes required in substations. This means that even complex routing can be achieved in a very narrow space. Flanged GIL elements are ideal for use in substations, since they can be assembled very quickly in this special environment.
Laying methods at a glance
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.
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