The HVDC Italy–Corsica–Sardinia (also called SACOI; Sardinia–Corsica–Italy) is an HVDC interconnection used for the exchange of electric energy between the Italian mainland, Corsica and Sardinia. It is unusual, having more than two converter stations as part of a single HVDC system, and (as of 2012) is one of only two multi-terminal HVDC systems in operation in the world (the other multi-terminal scheme being the Quebec – New England Transmission system linking northeastern United States with Quebec in Canada). The scheme is a monopole using a mixture of overhead line and submarine cable for the 200 kV high voltage conductor, and sea return for the neutral current. The overhead lines and submarine cables are duplicated, with both circuits being installed on the same towers. The scheme was constructed in three phases. SACOI 1 - Mercury arc system (1968) Converter stations When originally completed in 1968 by English Electric, the scheme comprised two converter stations, at San Dalmazio in Tuscany on the Italian mainland, which was situated close to a production area and offered a connection to two 220 kV lines, and Codrongianos on Sardinia. Each converter station was rated at 200 kV, 200 MW and consisted of two 6-pulse converter bridges in series. Normal operation was with both bridges in service but the scheme could be operated at 50% of rated voltage and power by bypassing one converter group at each end, to allow maintenance to be carried out. Each 6-pulse converter bridge consisted of 6 main mercury-arc valves plus a 7th for high-speed bypass operations. Each valve was rated at 1000 A dc and had four anode columns in parallel, with air cooling used for both the anodes and cathodes of the valve. The original scheme was commissioned during 1967 and put into commercial operation in January 1968. The mercury arc valves were as well in San Dalmazio as in Codrongianos situated in a hall with a length of 60 metres, a width of 20 metres and a height of 15 metres. The valves are still in these halls, however the mercury was removed, after the decommissioning of the mercury arc system in 1992. Transmission lines and cables The scheme consists of three overhead line sections: one on the Italian mainland with a length of , one on Corsica with a length of and one on Sardinia with a length of . The overhead line sections use twin conductors (each