ENTSO-EGas-Insulated Lines (GIL)
Overview
Gas-insulated lines (GIL) describe technology for the transmission of electrical energy using conductors located in pipes filled with compressed gas as an insulating medium. The inner conductor is made of aluminium and is kept at the centre of the pipe using disc or support epoxy resin insulators. The pipes are made of aluminium or aluminium alloys to avoid magnetisation losses. Pipe diameters vary depending on the voltage level: 15 cm to 50 cm for 110 kV and 400 kV, respectively. GIL are commonly applied in tunnels or in open air as an extension of gas-insulated substations (GIS) bays. There is also a project with GIL buried directly in soil [1]. GIL technologies can be differentiated based on:
- Voltage level and voltage type (Alternating Current (AC) or Direct Current (DC)).
- Number of conductors in a pipe (1 for higher voltages, 3 for 110 kV).
- Applied insulating gas (SF6, SF6 mixed with N2, new SF6-free gases as C4FN or compressed air).
- Joints of the pipes and inner conductors (long (up to 500 m) welded sections or screwed sections (up to 20 m)).
- The mechanical support of the inner conductor.
Benefits
The benefits of GIL are listed below:
- GIL do not emit an electric field or noise. Compared to overhead lines (OHL) and cables, the magnetic field is much lower.
- The diameter of the inner conductor enables the conduction of very high currents (above 3000 A) with relatively low electrical losses. For example, two cables are required for the same amount of current.
- The pipe thickness is approximately 1 cm, which contains potential short circuit arcs within the pipe. As a result, maintenance work is less restricted compared to cables.
- GIL allow for the realisation of very long, steep or vertical connections, as well as connections with sharp angles (e.g. 90°).
- A properly designed GIL can have a lifetime exceeding 50 years, as gas insulation does not degrade over time.
Current Enablers
GIL enablers include:
The decisive aspects of GIL application are planned right-of-way routing, available space, installation time and cost. The cost of GIL is significantly higher than OHL and somewhat lower than the two-cable option. However, the assembly time for GIL is much longer compared to other technologies.
The AC GIL has electrical capacitance comparable to that of cables. As a result, additional reactive power compensation may be required for longer sections.
R&D Needs
To further enhance the GIL technology, suggested R&D activities are listed below:
- The introduction of new fluorine-free insulation gases requires redesigning the existing AC and DC GIL solutions. The shape and material of the supporting insulators, as well as pipe diameter, are key areas of R&D. In addition, the partial discharges in fluor-free installations are not yet fully understood.
- Assembly and installation time for GIL must be reduced to more closely match that of cables.
- A sustainability evaluation comparing GIL to OHL cables does not exist.
The technology is in line with milestone “SF6-free solutions operating in high voltage and extra high voltage grids” under Mission 1 of the ENTSO-E RDI Roadmap 2024-2034.
TSO Applications
Examples
| Location: London, UK [2] | Year: 2022 |
|---|---|
| Description: Bengeworth Road substation will be supported with a 420 kV SF6-free gas-insulated lines. This is a part of the London Power Tunnel project 2, a 32.5-km-long tunnel in south London connecting crucial substations. | |
| Design: The SF6-free gas mixture for the insulation of lines consists of fluoronitriles (C4-FN), carbon dioxide (CO2) and oxygen (O2). | |
| Results: Throughout the life cycle of operation, the gas-insulated lines, along with the switchgear in the substation, will save around 150,000 tons of CO2. | |
| Location: Southeast of England, UK [3] | Year: 2017 |
|---|---|
| Description: National Grid applied an SF6-free 420 kV GIL. The 230-m-long, gas-insulated circuits connect the substation to the OHL in the ElecLink project via a 1 GW High Voltage Direct Current (HVDC) cable. | |
| Design: The g3 gas mixture is blended using 4710 Insulating Gas with a balanced percentage of carbon dioxide to optimise performance while maintaining high dielectric performance. | |
| Results: The first non-SF6 insulated line was energised and will save 11,676 tons of CO2e emissions during the operation period along an operationally critical part of the UK’s transmission. | |
| Location: Frankfurt, Germany [4] | Year: 2011 |
|---|---|
| Description: An underground connection for the GIS substation near Frankfurt Airport was required. The aim of the project was to demonstrate an alternative to cable and reduce the trench size. | |
| Design: Two three-phase 380 kV welded GIL systems were laid directly in the ground, with a length of 900 m each. Gas mixture is 80% N2/20% SF6. Both the conductor tube and the enclosing tube are made of aluminium. | |
| Results: Repair procedures have been successfully tested. Both systems have been in continuous operation for many years. Maintenance works focus on the cathode corrosion protection system. GIL itself is maintenance-free. | |
Technology Readiness Level The TRL has been assigned to reflect the European state of the art for TSOs, following the guidelines available here.
- For applications above 245 kV:
- TRL 9 for AC GIL using SF6 or SF6 mixture.
- TRL 7 for AC GIL using new insulating gases.
- TRL 7 for DC GIL using SF6 or SF6 mixture.
- TRL 4 for DC GIL using new insulating gases.
References and further reading
S. Poehler and P. Rudenko, “Directly buried gas-insulated transmission lines (GIL)”, PES T&D 2012, Orlando, FL, USA, 2012, pp. 1–5, doi: 10.1109/TDC.2012.6281707.
National Grid, “SF6-free 420 kV gas-insulated line”
Lixon, “Hitachi Energy’s EconiQ powers the London Tunnel Project with Linxon’s Bengeworth Road Substation contract” Jan. 8, 2024.
Clarion Energy Content Directors, “Siemens to lay 1km gas insulated, 380 kV overhead line at Frankfurt Airport”, Factor This Power Engineering, June 15, 2009.