Siemens is developing a gas-insulated transmission line designed to meet future challenges in the transmission system with its ability to carry large amounts of power at high DC voltages. Berlin Technical University and Dresden University of Applied Sciences, among others, are working under the lead management of Siemens on developing the technology (known as the DC CTL - Compact Transmission Line for Direct-Current High Voltage) for laying underground. It is designed to transmit up to 5 GW of power per system at up to 5000 A. The German Federal Ministry for Economic Affairs and Energy (BMWi) is providing €3.78 million in funding for the development project.
The DC CTL will be based on the technology of the existing gas-insulated transmission line, which consists of two concentric aluminum tubes. A mixture of gases is used as the insulating medium. Until now, the GIL has only been available for AC. The challenge for the DC CTL is posed by the special design of the components, taking into account their specific properties at high direct voltages. Because of its significantly higher current carrying capacity the DC variant of the GIL would not only be able to transmit the required amounts of electrical power more efficiently compared with cable in the future grid upgrade, it would also make power transmission routes more environmentally compatible as well as cheaper.
The specific need being met by DC transmission lines is the necessary expansion of the grid if 80 percent of demand for electrical energy in Germany is to covered by renewable energy sources by 2050. The power generated by wind turbines in the north of the country and off the German coast will need to be transported as efficiently as possible to the load centres in the south. DC transmission is the method of choice because of its low electrical losses compared with AC. It is believed that developing the grid using high-voltage direct-current (HVDC) transmission technology with overhead power lines and gas-insulated DC transmission lines buried underground over certain sections could be implemented using considerably less resources than three-phase technology.
Integration of renewable energy sources in existing power transmission and distribution systems poses a major challenge for Germany's energy transition. The continuation of overhead power routes in sections underground in the form of a GIL represents a key piece of the route planning mosaic, since possible corridors for new overhead lines are restricted due to existing buildings. Gas-insulated DC lines can handle the power from an overhead line with the same number of conductors. This means that the substations and transmission corridors can be designed to take up less space, which makes them more cost-efficient.
Use of the DC-GIL will also allow the advantages of gas-insulated transmission technology to be exploited for the new HVDC multi-terminal systems and networks, making the vision of a transmission grid with superimposed DC a reality. Siemens believes that HVDC systems in conjunction with GIL DC lines routed in sections underground are perfectly suited as a key technology for this.
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