Underground transmission lines for high power AC and DC transmission

The latest state of the art and ongoing research about high power gas-insulated underground transmission lines as well as the latest technical improvements are presented in this contribution. The basic technology is presented and the advances of the technology with respect to different installation options are discussed. A very high power transmission capability, a low capacitance, lower resistive losses compared to power cables and overhead lines, negligible magnetic fields, no ageing phenomena of the insulating system and a maintenance free design are the main benefits. In consequence, GIL technology for high power HVDC transmission is actually under development.