The Mediterranean connection

The TEN energy programme aims to improve energy security, socio-economic development and price competition in the EU by supporting grid interconnection projects, and has identified the Mediterranean as a priority region for development. PEi looks at the Italy-Greece HVDC link, a key project in this area

Gaetano Pazienza,

Enel,

Venice, Italy;

Borje Jansson,

ABB Power Systems,

Ludvika, Sweden

In 2000, the Italy-Greece HVDC link between Galatina and Arachthos will be commissioned. The link, which uses an HVDC (high voltage direct current) undersea cable, is one of several similar links built or planned with support from the European Commission`s TEN programme.

The TEN programme was initiated by the European Commission as a means to establish and develop trans-European networks in three key areas: transportation, telecommunications and energy. This includes promoting interconnection and inter-operability between national networks as well as access to them. The TEN programme also stresses the importance of linking remote, isolated regions to the central regions of the European community.

The EU Commission supports a number of selected TEN Energy projects financially, and these projects were planned to have received a total of Euro112 million ($120 million) during the years 1995-1999. Some of these include electricity and gas links between EU countries and non-EU countries. This financial support is limited to a maximum of ten per cent of the total investment costs for each of the projects.

A common market

The European Commission`s directive on the liberalization of the electricity market entered into force in 1997 with a deadline of February 1999 for member states to begin opening their markets to competition. The directive is expected to increase the transfer of electric power across the national borders within the EU, and thus there is a need for improved or new transmission links.

In some areas, the lack of adequate transmission capacity is already a limiting factor in the trading of electric power .

The EU Commission`s TEN Energy programme is focused on high voltage lines, undersea cables and monitoring and protection systems for such links. It is supporting projects within the UCPTE (the western European power grid) and also between the UCPTE and CENTREL (the eastern European grid) and the Nordic network, Nordel. The programme has identified a number of `missing` electricity links between various regions of Europe. These are:

$#8226; The Baltic Countries with Nordel and with UCPTE/CENTREL

$#8226; The Balkan countries with UCPTE/ CENTREL

$#8226; The Eastern European countries with extended UCPTE

$#8226; Mediterranean countries with extended UCPTE

In the case of the latter category, Greece has borders with other Balkan countries in the north and with Turkey in the east. To the west of Greece, the southern tip of Italy is a close neighbour across the Adriatic Sea. Thus upgrading the existing links and adding new ones between the networks of Greece and its neighbours would mean a more efficient use of the networks and of the production capacity in this region.

Shared resources

A link between the Italian and Greek networks would greatly improve the capacity for electric power transfers in the central Mediterranean area. Thus, a link between the southern part of Italy and the western part of Greece became a priority project in the EU Commission`s TEN programme.

In June 1998, a consortium of Italian ABB group companies and ABB Power Systems in Sweden won the contract to design, build and commission an undersea HVDC link between the Italian and Greek power networks, and to supply the electrical equipment for the two converter stations. The 163 km undersea cable will be supplied by Pirelli. Enel and PPC are responsible for their respective civil constructions and also for the extensions to be made in their 400 kV AC systems.

The link will run between Galatina in the province of Puglia in southern Italy and Arachthos in the province of Epirus in western Greece, connecting the 400 kV AC networks owned by Enel in Italy and PPC in Greece. Commissioning of the link will take place at the end of 2000.

The power exchange via the Italy-Greece HVDC link will enable Italy and Greece to use their combined generating resources more efficiently. This is the main reason why this link has become a priority project and the first electric power transmission project implemented within the EU Commission`s TEN initiative.

The overall design and general specifications of the Italy-Greece link is typical for a modern undersea HVDC link. It will be similar in design to other recent links built by ABB. One example is the Baltic Cable, which has been in operation since 1994. This link is also an undersea cable link, connecting the networks in Sweden and Germany.

The Baltic Cable is part of a larger, future scheme, the Baltic Ring, which is a number of links connecting the networks around the Baltic Sea. Since the networks are often asynchronous and/or separated by the sea, HVDC is the ideal technology for these links.

The latest addition to the Baltic Ring is the SwePol link between southern Sweden and northern Poland. This link will be commissioned in late 1999, one year ahead of the Italy-Greece link. These two links are based on the same ABB technology, but the specific technical solutions differ depending on the network topologies and operator demands.

Station design

The main demands on the Italy-Greece link are a very high availability combined with a very low maintenance requirement. In terms of design, this translates into a proven, yet state-of-the-art design, with solutions and components based on the operating experiences from similar HVDC schemes, and a high degree of automatic operation.

In order to fulfil these demands, the design is based on ABB`s experiences from recent undersea cable projects in particular and also its range of HVDC projects in general. In recent years, ABB has designed and built eight undersea cable HVDC transmissions based on the same concept as that of the Italy-Greece interconnection.

The design of the two converter stations is kept as simple as possible with regard to the specifications. As far as the local circumstances permit, the stations are designed to be as similar as possible. The philosophy is to use simple, straightforward solutions which are both proven and typical of a transmission of this type and size.

The AC switchyards will be almost identical in both of the stations, but due to differences in ground area allocation and local requirements, the DC switchyards will be different. The DC yard in Galatina will be quite small, with only a few pieces of equipment. In Arachthos, however, filtering requirements necessitate a larger DC yard.

The Italy-Greece link will have a power rating of 500 MW in either direction, with reference to the inverter station in each case. The operating voltage will be 400 kV DC, and the AC sides of the converter stations are connected to the 400 kV AC networks of Enel in Italy and PPC in Greece, respectively.

The link will be monopolar with a 163 km undersea HVDC cable at the bottom of the Adriatic Sea, combined with a return path via sea. The DC line between the undersea cable and the converter station at Galatina will be a 44 km underground HVDC cable. On the Greek side, the DC line between the Arachthos station and the undersea cable will be a 110 km overhead HVDC line.

Converters

Both converter stations consist of converter transformers, a valve hall with thyristor valves, control system and an AC and a DC yard.

The thyristor valves will be of the well-proven ABB designed with three branches with a quadruple valve in each branch, each one fed from a single phase, three-winding transformer. The same valve design will be used in both converter stations.

Each quadruple valve consists of four identical single valves, connected in series. The quadruple valves will be suspended from the ceiling of the valve hall, with their high voltage connections at the top. Suspending the valves from the ceiling is a standard solution used by ABB, a design which gives the valves an excellent structural integrity both statically and dynamically. This design fulfils the moderate dynamic seismic withstand capability required at the Arachthos station. For the Galatina station, there is no such seismic requirement, but using the same valve design is an advantage from a maintenance point of view.

The transformers are housed immediately adjacent to the valve hall with their valve side bushing extending through the valve hall walls into the valve hall. This arrangement protects the DC side components from the outdoor environment and reduces the radio interference.

The AC switchyards will be very similar in the two stations. The DC yards will differ in their designs.

On the AC side, both stations have similar demands of reactive power generation and harmonic filtering. Thus, the AC yards will be almost identical. Both the maximum limits of switching overvoltages and the requirements on the harmonic suppression are satisfied with a design with two filter banks. The filters are tuned to the 12th and 24th harmonics, combined with a high pass characteristic. Each filter bank will supply 90 Mvar of reactive power.

At Galatina, the converters feed directly into the HVDC underground cable, eliminating the need for DC filtering. At Arachthos, however, the converters are connected to a 105 km HVDC overhead line, which makes DC filtering necessary in order to suppress the harmonics. The filter is one branch of the triple-tuned type.

The control system uses a fully redundant design, which complies to the very high demands on availability and reliability of the plant specification. The control system, which combines the functions for control, supervision and protection of the link, is based on ABB`s Mach family of control equipment. This includes both the hardware and the software needed.

The entire control system, including the protection, is physically redundant and divided into two identical parts. One part is the operating part, while the other one is in a hot stand-by mode.

The normal operating mode for the link is constant power control. In addition, other control functions which are typical for modern HVDC transmissions are available, including constant current control, frequency control, decreased DC voltage and automatic filter switching.

The link will be designed for totally automatic operation from either one of the two converter stations, Galatina or Arachthos. Both of the stations are also prepared for remote operation from the dispatch centres of Enel or PPC.

Availability and reliability

The main design objective is to achieve a high availability and reliability combined with a low maintenance requirement. The down time due to planned maintenance, and the duration of the maintenance, have to be kept to a minimum. This requires careful planning for preventive maintenance and proper training of the service personnel. Also, the low maintenance requirement is made possible due to the modern design and proven technology.

If a problem leads to a forced outage, the events causing the outage are recorded by a monitoring system. Coupled to a documentation system which presents the chain of events in a clear and easily understandable way, the cause of the problem can be tracked efficiently. Thus the unit causing the problem can quickly be found and replaced or repaired.

In order to reach the specified demands on availability and reliability, the design uses a high degree of redundancy. Almost all of the critical auxiliary systems are duplicated in order to reach the necessary level of redundancy.

As the first project built with support from the EU Commission`s TEN programme, the Galatina-Arachthos link will benefit both the Italian and Greek electricity markets. It will make it possible for the two operators, Enel and PPC, to share production resources with benefit to both parties. The operator`s strict demands of very high availability and low maintenance needs are met by using a proven, yet state-of-the-art HVDC technology.

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Figure 1. The Mediterranean is a priority area for development

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Figure 2. The Italy-Greece HVDC link between Galatina and Arachthos

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Figure 3. Single line diagram

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