Development of electrical power supplies began more than one hundred years ago. At the beginning, there were only small DC networks within narrow local boundaries, which were able to cover the direct needs of industrial plants by means of hydro energy. With an increasing demand on energy and the construction of large generation units, typically built at remote locations from the load centre, the technology changed from DC to AC. Power to be transmitted, voltage levels and transmission distances increased.
DC transmission and FACTS (Flexible AC Transmission Systems) has developed to a viable technique with high power ratings since the 60s. From the first small DC and AC "mini networks", there are now systems transmitting 3 - 4 GW over large distances with only one bipolar DC transmission: 1.000 - 2.000 km or more are feasible with overhead lines. With submarine cables, transmission levels of up to 600 – 800 MW over distances of nearly 300 km have already been attained, and cable transmission lengths of up to 1.300 km are in the planning stage.
As a multiterminal system, HVDC can also be connected at several points with the surrounding three-phase network. FACTS are applicable in parallel connection or in series or in a combination of both. The rating of shunt connected FACTS controllers is up to 800 Mvar, series FACTS devices are implemented on
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