Gas-insulated transmission line (GIL) technology is in use since more than 40 years. Most applications are related to very high voltages of 420 kV, 550 kV and 800 kV with high current ratings of 4000 A, 5000 A and up to 8000 A in special cases. This makes GIL a powerful electric energy transmission technology. Starting in the 1970s with the first technical generation of GIL with pure SF6 as an insulat- ing gas and AC voltages, the second generation of GIL technology followed in 2001 with N2/SF6 gas mixtures, automated orbital arc welding and using of elastic bending to follow bends down to 400 m radius for better laying options. The typical fields of applications are inside substations, hydro and thermal power stations or as part of overhead lines when they need to be underground.
Today the third generation of AC GIL is entering the market with new developments in welding technology and automated assembly onsite. The welding technology is based on the friction stir welding (FSW) process, which offers high robustness against ambient conditions and by factor 3-4 reduced times for connecting two aluminum pipes. The second innovation is related to the mobile factory which provides an automated laying process onsite including the pipe handling, weld connection and laying into the trench or tunnel. These developments are now in the final stage.
The AC GILs are seen as an alternative to the high voltage cable e.g. because of high transmission capacity, low space requirement, flexible routing, or negligible low electro-magnetic field, or in cases when overhead lines are not possible e.g. because of restrictions coming from public opposition, restricted space in densely populated areas, or by environmental protected areas.
The DC technology for gas-insulated transmission lines can be used as an alternative for DC cable technology in cases of limited access to the laying area because of the weight of cable transportation units or in steep areas for underground laying. DC gas-insulated lines can transmit high power ratings of up to 5 GW at ±500 kV rated voltage. The use of HVDC technology for long distance power transmission is worldwide increasing. Typically, the end of the high power line will be in a densely populated area with limitations in space, where the DC gas-insulated substation (DC GIS) can be used.
The DC gas-insulated substation is a new technology to use the compactness for application in HVDC switchyard for earthing, disconnecting, voltage and current measurements, surge arresters and connection modules to overhead lines or cables. Circuit breaker function is not required. The DC GIS will allow new technical designs at much smaller spaces. Compared to the air-insulated switchyard of HVDC converter stations, the gas-insulated compact substation will save (80 ... 90) % of the space.