The Italy-Greece HVDC interconnection was commissioned in July this year. Not only does the link connect Greece to other EU countries for the first time, it also features the deepest submarine cable used anywhere in the world.

In July 2002, Enelpower and Enel-Terna of Italy, together with Greece’s Public Power Corporation (PPC), inaugurated the 500 MW high voltage direct current (HVDC) link between Greece and Italy. The direct link is the first of its kind linking Greece with Western Europe, and was developed as part of the European Commission’s Trans-European Networks (TEN) programme (see PEi May 1999, Vol 7 Issue 4).

The TEN initiative aims to develop cross-border infrastructure systems to support the single European market. The project was financed in part by European Commission grants and European Investment Bank loans.


Figure 1. The Italy-Greece HVDC link was inaugurated in July 2002
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In addition to connecting Greece with the EU, the link has also realised other key objectives: providing Albania and Turkey with access to EU electricity markets; reinforcing the ‘Mediterranean Ring’; and fostering east-west energy exchanges. It will also allow Italy and Greece to share thermal and hydro generating resources and increase the reliability and flexibility of their electricity systems.

The project brought some unique challenges to the developers: it required 163 km of submarine cable, much of which lies at a depth of 1000 m, a world record depth for a power link; in addition, the terrestrial section in Italy connecting Galatina with the coast is one of the longest land links of high-tension oil-filled cable ever installed.

System description

The Italy-Greece HVDC link consists of a mono-polar link with sea return, with a rated voltage of 400 kV, a rated current of 1250 A and a rated power of 500 MW, which can flow in both directions between Italy and Greece. The 204 km link consists of a 43.5 km, 400 kVdc underground cable between Galatina substation in the centre of Italy’s Salento region and Otranto on the coast, and a 163 km submarine cable which crosses the Straits of Otranto, passing to the north of Corfu island.

The energy cable comes ashore on the steep underwater slopes at Aetos in Greece near the Albanian border. From there, the connection continues via 110 km of overhead DC lines to Arachthos, where the conversion station is located. The link also includes two conversion systems – from 400 kV AC, 50 Hz to 400 kV DC – located in the Galatina and Arachthos stations which include power transformers, smoothing reactor, and filters, and two submarine electrodes and related ground connections at the Italian and Greek coasts.


Figure 2. The schematic bathymetric profile
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The link was realized by Enel-Terna and PPC with Enelpower acting as main contractor using the engineering support of CESI. It was designed and manufactured by ABB and Pirelli.

Deep waters

The submarine portion is subdivided in three sections: shallow water 28 km long, deep water 71 km long, shallow water again for the remaining 61 km up to the second land-sea joint on the Greek coast at Aetos, facing Corfu island.

The cable conductor is made up of a central copper rod surrounded by four shaped copper segments to give nominal cross sectional area of 1250 mm2. For the insulation, special high density paper tapes were used and impregnated with viscous compound. The protective sheath is made of polyethylene and has been applied immediately on the lead sheath to act as anticorrosion protection. Polyester tapes and a reinforcement made of galvanized steel tapes (two layers) are applied over it. The armour consists of two layers, counter-helical applied, of galvanized high strength steel flat wires.

The length of the submarine link required that it be installed in two laying campaigns using the Pirelli cable ship ‘Giulio Verne’, equipped with a 7000 t-capacity turntable. The capacity of the turntable allowed the entire connection to be made using only one intermediate field joint 60 km from the Greek coast.

The cable was embedded 0.6-1 m into the seabed down to a water depth of 150 m. This will protect the cable against external damage by fishing equipment, for example. Below a depth of 150 m, the cable was allowed to settle on the seabed.


Figure 3. The project presented developers with several challenges
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Different land cables have been used in Italy and Greece. An oil filled cable was used in Italy, with a nominal cross section of 1200 mm2. For the insulation, low viscosity insulating oil was used for impregnation. Lead alloy sheath was chosen. For the reinforcement and external protective sheath, bronze tapes and medium density polyethylene were adopted.

On the Greek side a mass impregnated cable was used, with characteristics similar to the submarine cable, but with a cross section increased to 2000 mm2 to protect it from extreme thermal conditions. The submarine armour has been substituted by a protective polyethylene jacket.

The land connection is composed of a high voltage cable, two medium voltage return cables for the marine electrodes, a triple conduct for the telecommunication cables and a pilot cable for the check and surveillance of the line. All components are installed in the same trench. In some sections along the land cable route a new installation technique, called Mechanized Laying, was used. The basic concept is to dig the trench and to lay the cables in one operation

Electrodes and converter stations

The Italy-Greece mono-polar link re-closes the connection with the sea acting as the return cable. This practice, common in submarine HVDC connections, takes advantage of the high conductivity of the sea water. For the current return through the sea two metallic electrodes have been designed and manufactured. The anode is located in Greece off the Corfu strait, the cathode is located in Italy, near Otranto Cape.

Besides AC yard breakers and disconnectors, current and voltage transformers and HV bus-bars, the DC converter station is characterized by standard design AC filters, DC filters (at Arachthos only), converter transformers (three single-phase units rated at 200 MVA), and air-core smoothing reactors. The converter system includes a 12-pulse thyristor bridge, with three water cooled sections with four valves each. The converter system also includes arresters, valve hall grounding switches, DC busbar system inside the valve hall for the pole and electrode interconnections and gas-insulated wall bushings.

The system is fully microprocessor based and adopts a high performance distributed architecture. The system configuration is symmetric for both the stations. It allows the link operation in one of the two admissible control modes: at constant power regulation, in normal operating conditions, or in frequency control, under abnormal network conditions.

The normal operation is in ‘Automatic Link Mode’ (ALM), where the two terminals are automatically coordinated by telecommunication and manoeuvred through synthetic commands ordered locally or remotely at the master station. Without telecommunication the ALM allows a reduced set of operational manoeuvres.

In ‘Automatic Terminal Mode’ (ATM) each terminal is operated on its own, always through synthetic commands, ordered after a phone coordination by both the operators at the Galatina and Arachthos converter stations. This operating mode is used in case of long telecommunications failures.

The ‘Manual Mode’ (MAN) is only used for maintenance, commissioning and testing. The monitoring and diagnostic system includes specialized features like event reporting, transient recording and harmonics analysis.

Besides the nominal performances the system has a minimum operation limit (50 MW) and can be operated at reduced voltage – at 320 kV DC with a reduced current of 1000 A, the reduced power is 320 MW. At the rated power of 500 MW and with an ambient temperature of 40°C the guaranteed conversion losses are about 7 MW while the DC line losses are about 14 MW.

According to the above defined performances, the link operation is allowed with at least one 400 kV AC line in service with a minimum short circuit capacity of 3600 MVA in Italy and 2800 MVA in Greece. The system performance is automatically reduced in case of one electrode line out of service (the maximum current is reduced to 900 A), thyristor over-temperature (the transmitted power is progressively reduced with steps of five per cent), ambient over-temperature or spare cooling system unavailability (the current or power order is immediately frozen).

Design challenges

At the planning stage of the system, it appeared that the Italy-Greece HVDC interconnection would have presented one major difficulty, the installation across the Otranto channel with a water depth of 1000 m, never previously reached for a power cable. The transition joint between the oil filled land cable and the mass impregnated submarine cable and the huge porcelain insulators, suitable for a very high salinity level, were also challenging accessories to be developed.

An important part of the preliminary feasibility study was a detailed marine investigation to collect data relevant to characteristics and profile of the seabed and to define the possible risks due to fishing activities and anchoring. On the basis of the acquired data the best route was selected and the protection for the cable was defined. The marine survey was performed in 1991. A complete sea-trial, utilizing 3.5 km of cable, was performed in order to check the full suitability of the equipment and procedures to meet all the installation and protection conditions foreseen.

The test was performed in 1995 and included the following main phases: embedding machine test in shallow water (about 30 m); embedding machine test down to 150 m water depth; test of cable laying, including repair joint, at 1000 m water depth; recovering of the cable and transfer to Arco Felice factory; electrical test at -600 kV DC for 15 min.

Reference

A. Giorgi, R. Rendina, Enel-Terna, Italy; G. Georgantzis, PPC, Greece; C. Marchiori, G. Pazienza, Enelpower, Italy; S. Corsi, C. Pincella, M. Pozzi, CESI, Italy; K. G. Danielsson, H. Jonasson, ABB Utilities, Sweden; A. Orini, R. Grampa, Pirelli, Italy. ‘The Italy-Greece HVDC Link’, Cigré Session 2002