HVDC Italy–Corsica–Sardinia

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.[2]

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.

The scheme consists of three overhead line sections: one on the Italian mainland with a length of 22 kilometres (14 mi), one on Corsica with a length of 156 kilometres (97 mi) and one on Sardinia with a length of 86 kilometres (53 mi). The overhead line sections use twin conductors (each rated at 1000 A dc) connected in parallel. Each aluminium conductor has a cross section of 628 mm² in Sardinia and Italy, and 755 mm² on Corsica.[1]

In addition to this, there are two submarine cable sections: 105 kilometres (65 mi) between Italy and Corsica and 16 kilometres (9.9 mi) between Corsica and Sardinia. The conductor cross-sections are 1080 mm² on land and 420 mm² for the submarine lengths.

The scheme was designed to export power from coal-fired power stations on Sardinia to the Italian mainland, and thus was only required to operate in one direction. Although the converter stations are inherently capable of operating in either direction of power transmission, advantage was taken of the unidirectional power-flow requirement to economise on the design of the sea electrodes. The sea return current enters the sea at the Sardinian end (anode) and leaves the sea at the mainland end (cathode). The mainland electrode consists of bare copper (a section of insulated copper cable with the insulation stripped off), which would corrode rapidly if used as an anode but suffers no degradation when used as a cathode. However, the anode electrode on Sardinia required a more sophisticated design using platinum-coated titanium pipe, subdivided into 30 sub-electrodes, in order to prevent corrosion.[3]