Adaptation of the Control Equipment to Permit 3-Terminal Operation of the HVDC Link between Sardinia, Corsica and Mainland Italy

The Sardinia-Corsica-Italian mainland HVdc link (called SA. COl.) has been in commercial service with ENEL since 1967. At present, it is the only link between Sardinia and mainland Italy. After more than 20 years service as a conventional 2-terminal HVdc link, the Sardinia-Mainland Italy HVdc link (200MW, 200kV) is now operating as a three-terminal system. The original control equipment has been modified to allow the introduction of a tapping station on the line for 3-terminal operation. The original wayleave agreement between the French and Italian authorities, Electricite de France (EDF) and ENEL respectively, provided for the possibility of a tapping station in Corsica at such time as the technology might permit, and the need for this has been emphasised by the recent growth of demand in Corsica. Recently EDF initiated the construction of a parallel tapping station in Corsica, which has been supplied independently using thyristor valves in contrast to the original mercury arc. Its rating is 25% of that of the original (main) stations. The Corsica station controls its own current, whether acting as a rectifier or an inverter. The ordered current is adjusted to meet the constant power or frequency control requirements of Corsica. This minimises the need for interstation communication. Reversing high-speed isolators allow the Corsica station to operate either as rectifier or inverter, regardless of the polarity imposed by the main stations.