Remote offshore power generation by offshore power plants and large scale offshore windpark installations are under discussion in some parts of the world. The generated power has to be transmitted to the onshore power grid and interconnected to the grid operator's facilities. This contribution introduces the latest technologies and applications for HVAC High Voltage Alternative Current and HVDC High Voltage Direct Current transmission technology and compares the different solutions from technical and economical aspects.
Environmental aspects like political requirements for reduced CO2/NoX reduction, reduced maintenance costs for gas turbines and reduced investment for spare units of conventional offshore located gas turbines and higher reliability become more and more important. These issues will transmission solutions to/from the main land more attractive for oil & gas field installations, where operation, accessibility and maintenance of local offshore generation becomes difficult and cost intensive. The use of XLPE (Cross Linked Poly Ethylene) type AC cables for sub sea installations reduces the dielectric charging currents of the cables and hence allows bridging larger transmission distances compared to the past solutions with older insulation materials with higher dielectric parameters. HVDC Classic transmission for bulk power (>2000 MW, +/- 500 kV DC) and HVDC using Voltage Sourced Converters up to 360 MW, +/- 150 kV DC power are the two proven technologies, where extremely long distances can be handled between the offshore and the onshore substations (Bergmann, 2001, and Schettler, 2001). HVDC transmission systems can be used for extremely large transmission distances (>600 km), theoretically without limitation of length.
Also other technologies such as AC transmission with low frequency (e. g. 16 2/3 Hz) and Cyclo-Converter technologies are under discussion to overcome the limitations of length for AC-Cable systems. Comparison will be made by introduction of the different schemes, physical limitations, space requirements, economical aspects and advantages/disadvantages.