Progress of long-distance DC electrical power transmission

Progress of long-distance DC electrical power transmission systems is reviewed. Pros and cons are analyzed and compared regarding different technologies. Major electrical power transmission systems may consist of overhead line (OHL), extruded cable, mass-impregnated (MI) cable, oil-filled (OF) cable, gas insulated line (GIL), superconducting cable and superconducting transmission line systems. These systems make it possible today to transmit electrical power over long distance with the transmission capacity up to the order of 12 gigawatts (GW). High voltage direct current (HVDC) technology is favored in a long-distance power transmission because it reduces electrical losses and costs (after the transmission distance is longer than a break-even distance) in comparison with high voltage alternating current (HVAC) technology. The benefits are able to be increased further with the suitable increase of rating voltages, such as, with the further use of ultra-high voltage direct current (UHVDC) technology. Converter stations are needed in order to control and manage the transmission power at the start, the end, and even in the middle points in a DC power transmission system. Two types of converter stations are reviewed and discussed in this paper, i.e. voltage source converters (VSCs) stations on the basis of insulated-gate bipolar transistors (IGBTs), and HVDC classic stations with the use of converter transformers. The technologies addressed above and the progress of the technologies are studied and summarized.