The interconnection between Spain and Morocco, across the Straight of Gibralter, represents an outstanding example of cooperation between neighboring countries. The first electrical connection between the two countries — a 400-kV submarine cable link 28 km (17.5 miles) long laid at a depth of 615 m (2000 ft) — was commissioned in 1997. Commercial operation of the interconnector began in May 1998, supplying energy market agents on the basis of bilateral short-term energy contracts.

Since 1998, however, significant regulatory changes have occurred on both sides of the link. The European Union (EU) has deployed policies to strengthen cross-border transmission lines and to further develop the European electricity markets. In Spain, structural changes have been introduced and an electricity market has been in place for several years now. During the past three years, agents, operators and regulators have taken steps to launch the Iberian Electric Markets.

The Maghribian network, comprising Morocco, Algeria and Tunisia, is now being reinforced. This is part of an expansion plan conceived within a more ambitious Mediterranean Electricity Ring (MedRing) project. MedRing will interconnect the electrical networks of all Mediterranean countries: France, Spain, Morocco, Algeria, Turkey, Greece, Italy, Libya, Egypt and Jordan. With this new plan in place, the role of the submarine link between Spain and Morocco becomes an outstanding issue in terms of ensuring high-quality and secure grid connections.

The direction of power flow through the submarine cable has been substantially from Spain to Morocco since the line was commissioned (Table 1). However, the network stability and security contribution of the times of energy flow from Morocco to Spain has been of great value. These transfers have removed network constraints on the Spanish grid in the area of close proximity to the Mediterranean. The maximum transmission capacity through the interconnector is reduced from 700 MW to 400 MW, taking into consideration the neighboring network configuration and operational restrictions. This is known as the commercial rating of the link. This limit is reached several hours daily at times of system peak demand; hence, an increased rating would be valuable because there would be more opportunity for larger energy exchanges.

To increase the benefits available from this interconnection, the two system operators and owners, Red Eléctrica de España (REE; Spain) and Office National De L'Electricité, (ONE; Morocco), launched a project in 2001 to reinforce the existing link. This resulted in the design and installation of a second 400-kV ac submarine-cable link to operate in parallel with the existing circuit. This cable circuit provides the additional exchange capacity required and makes a significant contribution to system security and operational performance improvements. These two key objectives are vitally important when considered within the framework of the MedRing project, because the link between Spain and Morocco is the only Western transmission system interconnection between the northern and southern Mediterranean systems.

LAUNCHING THE PROJECT

REE and ONE jointly conducted research that defined and specified the reinforcement project. Their work revealed that the existing substations and transition stations located near the coasts had to be extended and a new submarine cable route on the rough sea bottom of the Straight of Gibraltar was needed. A new cable route corridor some 2 km (1.25 miles) wide westward of the existing circuit was studied and surveyed in 2002 (Fig. 2). These specialized survey vessels were equipped to identify the and seabed features with the aim of finding cable routes that would offer maximum stability, use sandy seabeds, optimize the significant number of cable crossings and minimize cable lengths.

The new cable routes proved to be longer than those used for the first cable link, but they satisfied most of the search objectives and solved the difficult task of the rocky Moroccan inshore approach. The total length of the new interconnector is 31 km (19.3 miles), including land sections at both ends, with 2 km (1.2 miles) in Spain and 0.25 km (0.16 miles) in Morocco. The maximum depth at which the submarine cable is laid is some 629 m (2064 ft), the sea depth in the middle of the straight.

With the experience gained from the first interconnector and the existing infrastructure available in each utility, the project was limited to the land and submarine cable sections, as the two utilities were able to manage the extension and adaptation of existing network facilities. An international invitation for tenders was launched in 2002 to attract the best and most experienced cable suppliers and contractors. Following assessment and evaluation of the bids received, a consortium comprising Prysmian Cables & Systems (Italy) and Nexans (France) were awarded the turnkey contract to design and install the second 400-kV ac circuit of the Spain-Morocco submarine electrical interconnection, with the value of this contract being 115 million euros (US$157 million).

A common-objective team, incorporating suppliers' and clients' project managers, formed in the execution phase during the second quarter of 2004, brought together the experience and enthusiasm to push forward this challenging project. The new circuit was commissioned in June 2006. After the new line successfully passed all contractual commissioning tests, the second interconnector was energized.

DESIGN AND CONSTRUCTION

Multinational projects require additional coordination because they are split by various disciplines that are being carried out in different countries by specialized engineering teams. A detailed project plan outlined at the beginning of this project served as a common guideline for all the engineers and specialists who contributed to the success of this project.

An initial detailed engineering phase allowed the confirmation of the pre-engineering performed by each utility and allowed the start of manufacturing activities that have a relatively long lead time for this type of project. Extra-high-voltage and fiber-optic cable (submarine and land) installation procedures, control and protection systems — including a temperature-monitoring system with distributed temperature sensing and oil-feeding systems — were designed and cross-checked before manufacturing actually began. Special efforts were made to maximize the interoperability of both links using modern electronic control systems, as they share one spare power cable. Changes to the original control system were necessary to achieve this objective. The interconnector is designed to transmit 700 MW with a thermal overload capability to allow a 900-MW load for 20 minutes. The general features of this double-circuit link are listed in Table 2.

Inshore and offshore operations are of great sensitivity in this type of project, as they involve specialized cable-laying vessels, equipment and personnel for a relatively short period of time. Two specialized power cable-laying vessels, Skagerrak (Nexans) and Giulio Verne (Prysmian), were used to lay the submarine cables in two separate laying campaigns in November 2005 and February 2006, respectively. As winter is not the best period of the year for offshore operations in the Straight of Gibraltar, the two campaigns had to cope with high winds and strong currents, increasing the challenge of these outstanding tasks.

The required support for the marine activities, landing operations and cable protection and burial were secured with the presence of very experienced marine contractors such as Impresub D&MC (Italy), Asso Divers (Greece) and Mika (Norway). Figure 1 shows the Skagerrak cable-laying vessel and the onshore cable installation at Tarifa (Spain), and Fig. 3 shows the cable-laying vessel Giulio Verne used to install the first cable. The seabed cable protection is undertaken by jetting (Fig. 4) and the use of cast-iron shells (Fig. 5).

Onshore, the extension of the transition terminal stations at Tarifa (Spain) and Fardioua (Morocco) and the land sections of the cable link had to be completed before the submarine cables were installed, because the oil-feeding-system continuity was required on a 24-hour basis. The completion of this task was ensured by the contributions of contractors Nexans, Elecnor (Spain), Cegelec (Morocco) and Arco Beton (Morocco). The overall coordination of skilled workers within the two locations in the south of Spain and north of Morocco, geographically close but at the same time distant from each other, was one of the keys to the success of this project.

ENVIRONMENT AND CORPORATE RESPONSIBILITY

Life-cycle assessment of new electrical infrastructures and social acceptance of project stakeholders are increasingly important topics for utilities worldwide. The Straight of Gibraltar is a site of great environmental value, representing the link of two continents, two cultures and one project. It is now one of the world's major bird-watching locations, because it serves as the main migration corridor between Europe and Africa. EU directives such as “Sites of Community Interest within Natura 2000 Network” protect some special areas of the straight. Within their respective countries, the two utilities have undertaken a large number of actions to promote community participation in the preparation, discussion and final approval of environmental impact assessment reports.

Ramón Granadino earned his degree in industrial engineering in 1990 at the Polytechnic University of Madrid, Spain, and his M.Sc.ECE degree in 1993 from the University of Massachusetts at Amherst (U.S.). He has worked at Red Eléctrica de España since 1994, managing projects for the development of the 220- and 400-kV Spanish networks. rgranadino@ree.es

Fatima Mansouri graduated in electricity and electrotechnical engineering at the National Higher School for Electricity and Mechanics (Casablanca, Morocco). Working with Office National De L'Electricité since 1992, she has been involved in managing projects for the development of the 60-, 225- and 400-kV Moroccan transmission networks. mansouri@one.org.ma

Table 1. Energy exchanges through the interconnection.
Period 2000-2003 2004 2005
Spain to Morocco (MWh) 6,639,295 1,567,641 898,367
Morocco to Spain (MWh) 59,153 21,190 110,405
Table 2. General features of the 400-kV ac interconnector.
Rated voltage (ac/dc) 400/450 kV
Maximum transmission capacity (ac) 2 × 700 MW
Maximum overload capacity 2 × 900 MW (for 20 minutes)
Maximum sea depth 620 m (2016 ft)
Number of submarine power cables 7 (6 +1 spare)
Number of submarine fiber-optic cables 4
Submarine route length 26 km (16 miles)
29 km (18 miles)
Land section length
Spanish side
Moroccan side
2.1 km (1.3 miles)
0.25 km (0.16 miles)