Gas-Insulated Transmission Lines
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Gas-Insulated Transmission Lines
Gas Insulated Lines (GIL) provide technical, environmental and operational features which make them a very good alternative wherever the transmission of extra high voltage (EHV) and extra high currents (EHC) is needed within restricted space, e.g. wherever overhead lines cannot be used.
GIL consist of two concentric aluminum tubes. The inner conductor is resting on cast resin insulators, which center it within the outer enclosure. This enclosure is formed by a sturdy aluminum tube, which provides a solid mechanical and electrotechnical containment for the system.
To meet up-to-date environmental and technical aspects GIL are filled with an insulating gas mixture of mainly nitrogen and a smaller percentage of SF6 (sulphur hexafluoride).
GIL can be used in a similar way as power cables, however extending the application range to higher voltages and higher transmission power. At the same time a better operational safety is reached.
GIL proved their reliability in more than 35 years of service experience.
The tubes are made of corrosion resistive aluminum alloy.
Due to their modular design the elements can be combined to deliberate length. Typical GIL installations have a lengths between 100 m and 60 km.
Due to their characteristics, GIL can navigate smooth curves down to 400 meter or sharp angular direction changes in any deliberate angle.
The system comprises mechanical expansion elements so that internal mechanical stress is kept at a minimum.
The tubes are welded together with a computerized orbital welding process. The seams are 100% ultrasonic tested to ensure that the system is absolutely gastight.
For typical applications up to 400 kV the insulation gas is composed of up to 80% nitrogen and only 20% SF6. The gas is not poisonous and can easily be detected.
Mostly GIL are used for transmission systems with 345 kV to 550 kV operation voltages and 2500 A to 5000 A operation current.
Due to their low electrical capacitance reactive compensation is not needed in general.
The enclosure is solidly grounded, thus it is safe to touch the GIL without risk and at the same time the electromagnetic field from the GIL to the environment is reduced to almost zero.
A particle trap is included in the tubes to make an increased lifetime of 50 years or even more possible.
|Nominal voltage (typical)||220 up to 500 kV|
|Maximum operation voltage||245 up to 550 kV|
|Nominal frequency||50 / 60 Hz|
|Nominal short-time current (rms., 1 - 3 s)||31.5 up to 63 kA|
|Nominal operation current (typical)||2000 up to 5000 A|
|Withstand voltage vs. earth:
AC withstand voltage 1 min
Nom. surge-impulse withstand voltage 1.2/50 µs wave
Nom. switching-impulse withstand voltage 250/2500 µs wave
|380 up to 620 kV
1050 up to 1675 kVpeak
850 up to 1175 kVpeak
|Housing and conductor material||Aluminium alloy|
80% N2 and 20% SF6
|Nominal pressure (absolute)||0.7 MPa|
|Environmental temperatures||-30°C up to +50°C|
|Weight per phase||approx. 50 up to 70 kg/m|
|Surge impedance||approx. 60 Ohm|
of conductor (typical)
|approx. 5300 mm²|
|Electromagnetic field||negligible µT|
|Gas tightness||Sealed for lifetime - no refeed|
The resistive losses of GIL are lower compared to cables and overhead lines. The dielectric losses of GIL are negligible. This reduces the operation costs and causes savings. Due to the large outer diameter, the heat dissipation is better than with cables and GIL normally do not require sophisticated cooling systems.
No electrical compensation
Due to its low capacitance, GIL do not require phase angle compensation even for a system length of 60 km or more.
Low electromagnetic fields
The conductor current induces in the enclosure a reverse current of the same size. Consequently the electromagnetic field outside the GIL is negligible. No special shielding is required even in areas which are critical with respect to EMC (e.g. airports or computer centres or in populated areas).
If an insulation failure would occur in a GIL, the fault arc would be safely enclosed within the outer housing. No influence would occur to the outside. The GIL are fire resistant and do not contribute to fire load. This means optimal protection of persons and environment.
GIL technology has proven its reliability since over 35 years in service without any failure up to now.
The insulation system of GIL shows no ageing phenomena, neither electrical nor thermal. There is practically no maintenance needed on the GIL for up to 50 years or more.
Operation like an overhead line
The GIL are the ideal addition to overhead lines. The high transmission capability of GIL allows to continue overheadlines underground with one GIL tube per phase, which minimizes space consumption. The GIL allow auto-reclosure switching cycles, consequently no major changes in the operation- and protection schemes of the grid are necessary.
Maintenance free design
Thanks to the technological clear-cut, logical design and the use of high quality materials a practically maintenance free product is achieved which requires external inspection only. The GIL can stay in operation during regular inspection activities.
GIL can be used in all common laying arrangements, which makes them suitable for numerous applications. Be it feeder connections in power plants, transmission systems in tunnels beneath cities or directly buried for overland transmission systems, where overhead lines cannot be used for specific reasons.
GIL can be used horizontally, vertically or in any inclined arrangement.
During service, the fully encapsulated design ensures optimal protection of the GIL against environmental influences.
Since 1975, the long-proven Siemens GIL technology has provided solutions in up to now 37 projects with an overall tube length of about 91,000 meters to customers all over the world. Please find below selected references about Siemens' activities in GIL projects worldwide from 1975 until today. For more detailed information please get in contact with our competence center (contact address on the right-hand side).
Whether above ground, tunnel or directly buried installation - we have the adequate solution for all kind of challenges.
Customer: Stadtwerke München
Tube length: 2610 m (single-phase)
Requirements: Creating space for the relocation of the brewery from downtown, replacing a 400-meter segment of a 380-kV overhead line, where the Paulaner Brewery constructed new modern facilities, tight schedule of only 11 months, two curves over a length of 400 meters necessary because of construction requirements
Solution: Tunnel-based gas-insulated transmission line (GIL) for 380 kV, two 3-phase tubular conductor systems carrying max. 2,300 MVA each, elasticity of the GIL enabled adaptation to space curves along the line
Customer benefit: Excellent EMC, no fire risk, maintenance-free, high transmission capacity of GIL systems reduces significantly spare requirements compared to conventional cable solutions – narrow line width.
The tunnel is designed to enable later expansion of the transmission capacity to a total of 9,200 MVA with 4 GIL systems
Customer: Yalong River Hydropower Development Company
Location: Liangshan Yi district of the Sichuan province in China
Tube length: 3375 m (single-phase)
Requirements: To connect the world’s tallest hydropower dam to the State Grid of China through 230m deep vertical shafts
Solution: 3 three-pole state-of-the art welded GIL systems, transmission capacity 2,200 MVA each
Customer benefit: A most reliable connection of the cavern power plant to the overhead lines above with the GIL ready to be energized on schedule despite challenging conditions (installation interrupted by massive landslides). Project targets could be achieved even under extreme and rapidly changing site conditions.
Customer: China Three Gorges Project Corporation
Tube length: 12,500 m (single-phase)
Requirements: To connect the world’s longest and highest-capacity connection to date using gas-insulated transmission lines (GIL), to transport the electricity from the turbines in the power plant cavern below up about 460 meters to the outgoing overhead lines, challenging conditions on site
Solution: Seven three-pole GIL systems, directed vertically upwards in four shafts, each transmitting 3900 MVA, 550 kV operating voltage
Customer benefit: State-of-the-art welded GIL installation as optimum solution for high-power transmission, sending first time hydro-generated electricity to a distant consumer location, fewer systems required thanks to GIL’s large transmission capacity, reduced space requirements compared to HV cables, increased earthquake resistance. In the event of a malfunction, GIL do not represent a fire load and thus pose no hazard to operating personnel and other equipment.
Location: Frankfurt airport area
Tube length: 5400 m (single-phase)
Requirements: Replacement of a 420-kV-OHL by an underground transmission solution, create space for the new airport runway at Frankfurt airport, ensure minimal influence to the environment and public by minimizing EMI, and trench width, ensure maximum operation safety
Solution: Gas-Insulated Transmission Line (GIL) for 420kV, directly buried, two systems with max. 3100 A transmission current
Customer benefit: Underground transmission on high power level on very small trace, low losses, installation without additional tunnel. Environmentally friendly integration into the landscape.
Location: Geneva expo area
Tube length: 2560 m (single-phase)
Requirements: Replacement of an existing OHL by an underground solution. Create space for new buildings and an expo-hall. Ensure minimal influence to the public. Ensure maximum operation safety. Ensure minimum EMI, (<1mT government requirement)
Solution: Gas-Insulated Transmission Line (GIL) for 300kV, tunnel installation, two systems with 2000 A transmission current capacity.
Customer benefit: Utmost reliability, very small space consumption, integration into OHL grid without separate protection mechanism. Unparalleled low EMF.
Customer: Austria Hydro Power
Location: Kaprun mountain area
Tube length: 465 m (single-phase)
Requirements: Connection of a 400-kV-OHL to an underground GIS with an underground solution. Small tunnel-width required due to expensive tunneling work.Very high safety requirements with regard to fire protection, as the tunnel is used also as ventilation shaft. Maximum reliability needed for the connection. Maximum operation safety and minimum EMI demanded as tunnel serves as emergency access way for personnel.
Solution: Gas-Insulated Transmission Line (GIL) for 400kV, tunnel installation, one system with 1000 A transmission current
Customer benefit: Underground transmission on highest safety level. Small tunnel size resulting in low tunneling costs
Customer: Schluchseewerk AG
Location: Wehr mountain area
Tube length: 4000 m (single-phase)
Requirements: Connection of a 400-kV-OHL to an underground transformer using an underground transmission solution. Overcome an altitude difference of 160 meter. Very high safety requirements with regard to fire protection. Maximum reliability needed for the connection.
Solution: Gas-Insulated Transmission Line (GIL) for 400kV, tunnel installation, two systems with 2500 A transmission current
Customer benefit: Underground transmission on highest safety level. Small trace, low losses and high reliability.
Customer: Electricity Generating Authority of Thailand
Location: Bangkok switchyard
Tube length: 3510 m (single-phase)
Requirements: Connection of an OHL with a Gas insulated switchgear under space restrictions. Reduce protection requirements for grid operation. Ensure maximum operation safety
Solution: Gas-Insulated Transmission Line (GIL) for 550kV, two systems with 4000 A transmission current capacity
Customer benefit: Utmost reliability, low losses, very small space consumption, integration into OHL grid without separate protection means.
Customer: National Grid Transco
Country: Great Britain
Location: Elstree, London, switchyard
Tube length: 750 m (single-phase)
Requirements: Ensure high power connection under restricted space conditions. Cope with several laying forms (partially underground and partially above surface). Ensure minimal protection efforts for gird operation. Ensure maximum operation safety
Solution: Gas-Insulated Transmission-Line (GIL) for 420kV. Two systems with 4000 A transmission current capacity.
Customer benefit: Utmost reliability, very small space consumption, integration into OHL grid without separate protection mechanism, low losses
Customer: Electricity Power Corporation
Location: Cairo, power plant
Tube length: 1850 m (single-phase)
Requirements: Ensure sufficient electrical clearance between transmission line and traffic lane. Cope with restricted space conditions. Ensure simple integration into power grid. Ensure maximum reliability. Withstand environmental requirements (55°C)
Solution: Gas-Insulated Transmission-Line (GIL) for 245kV. Two systems with 4000 A transmission current capacity
Customer benefit: Utmost reliability, very small space consumption, integration into OHL grid without separate protection mechanism.
Tel.: +49 (9131) 7-44510