The 400-kV electrical interconnection between Spain and Morocco is the first link between Europe and Africa.

The short distance between Spain and Morocco and the feasibility of electrically interconnecting the two countries has interested system design engineers for the past two decades. The first studies for the interconnection of the European and Maghribian networks examined the technical and economic aspects, the power demands on the two systems and operational problems. The results of these preliminary studies confirmed the feasibility of the interconnection and the benefits to both countries' utilities, especially Red Electrica de Espana (REE), Madrid, Spain, which owns the national transmission system, and Office National de L'Electricite (ONE), Casablanca, Morocco, the country's national utility. The interconnection is currently operating with a power transfer capacity of 300 MW and will rise to 600 MW by the year 2000 after a decade of work on the project.

Project design work started in 1990 with the investigation of alternative schemes based on ac and dc technology to meet the interconnection transfer capacity requirement. REE and ONE's evaluation concluded that the 400-kV ac transmission scheme was the most economic, but the cables for this submarine crossing should be suitable for both ac and dc operation. The dual-purpose cable design offers the possibility of upgrading the interconnection to 2000 MW in the future via dc operation. This 400-kV circuit will be the first installation at this voltage in Morocco, where the existing transmission system comprises 2500 miles (4000 km) of 220-kV circuits. In Spain, similar transmission voltages are used, and the network is comprised of 8000 miles (13,000 km) operating at 400 kV and an additional 9600 miles (15,500 km) at 220 kV.

Three alternative corridors in the Strait of Gibraltar were identified, and a marine survey was performed on a 1.3 mile (2 km) wide corridor to investigate the bathymetry, geomorphology of the seabed, water salinity, temperatures and sea current profiles. The optimum routing and alignment of the power cables were determined by selecting the combination of conditions that would provide maximum reliability for the submarine cables and have minimal environmental impact. The route that best satisfies the conditions, Route B, has a length of 16 miles (26 km), a maximum seabed depth of 2000 ft (615 m) and is located between Tarifa, Spain, and Ferdiua, Morocco.

The ac/dc interconnect is designed to transmit 700 MW with a thermal overload capability of 900 MW for 20 minutes.

Submarine Power Cables The submarine power cables specified for the project were 1.24-sq inch (800-sq mm) copper-cored, self-contained, fluid-filled cables. REE and ONE awarded supply and install contracts to two manufacturers, Alcatel and Pirelli. The core in each cable has a 1-inch (24-mm) diameter central cylindrical oil duct. The inner and outer semi-conducting layers and paper insulation were applied in conditions to assure a very low relative humidity content. The cables are lead sheathed, reinforced with bronze tapes covered by a polythene anti-corrosion jacket and copper-wire armored. The cables have an overall diameter of 5.5 inches (139 mm) and weigh approximately 40 lb/ft (56 kg/m).

Alcatel and Pirelli each supplied two cable circuits and one factory-made flexible joint per circuit. Because of the different factory handling capacities, the positions of the four joints vary. The two Alcatel joints are positioned near the Moroccan coast at a depth of 1000 ft (300 m), and the two Pirelli joints are located in the middle of the Strait at around 2000 ft (600 m) deep. Prior to manufacture, a cable length and all accessories, including a flexible joint, were subject to a series of International Electrotechnical Commission and CIGR. standard mechanical and electrical type tests to verify compliance with the technical specification for ac and dc operation.

Oil Feeding System The oil feeding system for the power cables consists of a pumping station at each end of the circuit. These stations have the capacity to compensate for any oil volume change in the cables that may occur because of variations in temperature as a result of load changes and/or environmental conditions in the Strait of Gibraltar. Furthermore, the system has the design capacity to maintain the oil supply for a period of 21 days in the event of a cable being severed and exposed to seawater. During normal operation, the pressure control system maintains a positive oil pressure with facilities to automatically switch to flow control mode should the pressure fall below the predetermined (15-bar) value, compensating for transient conditions and preventing seawater penetration. The power to run the oil pumping stations is normally provided by the local network, but a diesel generator and dc power supply provide back up in emergency situations.

Submarine Optical Cables Two submarine optical fiber cables, whose design and construction satisfies the standards applicable to transoceanic connections, were also installed along the route. Each cable comprises eight single-mode fibers for high-capacity transmission in a loose structure reinforced with a steel strand, encased in a welded copper sheath with wire armor protection. These cables were bundled to two of the four power cables. Separate fiber-optic cables were used, as the technology to include the fiber optics within the cable armor was not fully developed. During deep-sea cable laying, the tension applied on the power cable is about 26 tons. In the longer Spanish land section, dielectric fiber-optic cables have been laid inside ducts, and therefore, a joint was made at the sea-land joint bay.

The telecommunication cables are used for remote control, protection system communication and voice communication between the two utilities.

Land Section Cable The cables for the two land sections of the interconnect have a copper core cross section of 2.5 sq inches (1600 sq mm) and are similar in construction to the submarine cables except that these cables are unarmored. The increase in cross-sectional area of these cables compensates for the thermal conditions that are more severe than the submarine section, thereby ensuring thatthe transmission capacity of the interconnection is maintained.

Cable Installation Submarine Cable. The submarine cables were manufactured at the Pirelli factory in Arco Felice, near Naples, Italy, and at the Alcatel factory near Calais, France, before being transferred to large rotating storage plants close to the dockside landing piers adjacent to each plant. The cables were loaded for transportation to the construction site and for installation using a linear pulling machine and rollers to the power cable laying vessel operated by Alcatel. This dynamically positioned (DP) vessel is capable of carrying 7,000 tonnes on its cable-rotating platform. The pulling capacity during cable laying, provided by the cable capstans, is 50 tonnes.

To commence cable laying, the vessel was positioned approximately 2500 ft (800 m) from the landing beach, paying out the cable that was supported at regular intervals by floats. On the Tarifa landing, which has a sandy beach, the floating cable string was kept in position by rubber boats to maintain the cable as straight as possible between the vessel and shore. Cast iron shells were used to protect the cable on Tarifa's sandy beach. On the rocky Moroccan landing, the cables were pulled into four pre-installed steel pipes, each approximately 330 ft (100 m) long. On completion of the cable landing operation, the laying vessel commenced its track along the planned route, laying the submarine cable on the sea bottom. During laying, the vessel position and the cable tension, which is a function of water depth, were constantly controlled. A remote operated vehicle (ROV), supported by the DP vessel, continuously followed the touch down point on the cable catenary on the sea bottom. The ROV transmitted video image s of the cable status on the seabed to the deck of the laying vessel. This constant real-time monitoring allowed minor deviations from the route to avoid the cable resting on top of isolated obstacles or long cable 'free spans' due to very localized changes in the seabed features.

On completion of the submarine cable laying operation, a complete as-laid survey was conducted using the ROV to determine the number and length of 'free spans.' All free spans exceeding the values specified by REE and ONE were rectified by dumping small rocks beneath the cable using a DP vessel equipped with a fall pipe system capable of operation in water depths of more than 2000 ft (615 m). Handling the fall pipe in strong currents acting in opposite directions proved to be particularly challenging.

Different forms of protection for the submarine cables near the shore on both sides of the Strait were used. On the Spanish side, the cable burial method used Alcatel's CAPJET machines. These machines use pressurized water injected into the soil by nozzles distributed on two vertical 'stingers' inserted into the seabed, one on each side of the cable. As the machine progresses along the route, the cable is dropped into the excavated trench. The cable was buried at a depth increasing from 3 ft (1 m) at a distance of 260 ft (80 m) off shore to 10 ft (3 m) at the on-shore-jointing bay. The rocky and uneven sea bottom on the Moroccan side resulted in the need to protect the submarine cable for the final 5200 ft (1600 m) of the route using steel pipes and concrete mattresses, which extended to a water depth of 100 ft (30 m).

To accomplish this cable laying operation across the busy shipping lanes in the Strait of Gibraltar, three vessels were used at the same time: the cable laying vessel, the support vessel with ROV and a supply vessel in charge of the surveillance especially at the transit section. All activities were coordinated with the marine traffic control center located in Tarifa.

Land Cable. The submarine cables are directly connected to the land cables via transition joints, constituting a single electrical and fluid feeding system. Figure 5 shows the submarine/land cable transition joints in the jointing bay on the Spanish side. The land sections, 1.3 miles (2.1 km) in Spain and 0.16 miles (0.25 km) inMorocco, are terminated with outdoor sealing ends incorporating porcelain insulators of a design that will permit dc operation of the interconnection in the future.

Supporting Transmission System In addition to the submarine and underground cable sections and terminal stations at each end of the interconnect, the following transmission system work was also required:

n 400-kV overhead lines in Spain and Morocco. n Extension of the 400-kV substation at Pinar del Rey, Spain, to accommodate the switchgear for the Tarifa overhead line, and two 150 MVAR reactor banks. n Construction of a new 400-kV substation at Mellousa, Morocco, adjacent to an existing 220-kV substation. This includes switchgear to control the Fardioua overhead line, two 125 MVAR reactor banks and two 400/220-kV, 375 MVA transformers.

Power Transfer Contracts The power transfer contract between REE and ONE, which became effective in May 1998, is for the supply of 95 MW from the Spanish to the Moroccan grid. In late 1998, ONE signed a contract with ENDESA, headquartered in Madrid, for the supply of an additional 200 MW to the Moroccan network via the submarine link. While operating as an ac link, additional benefits of the interconnect include frequency control and the capability to respond to power deficiencies in the event of a generator or transmission line failure. To date, the short-term peak load transfer has reached 750 MW.

Technical and Economic Benefits The Spain-Morocco electrical interconnection is an important project that demonstrates that advancing technology enables countries and continents to benefit from power system interconnection.

The project advances the development of the electrical infrastructure in Morocco and significantly improves the stability of voltage and frequency in the region. Both countries are now able to benefit from the improved technical and economic exploitation of the existing power generation and transmission systems in this joint venture that establishes an energy trading facility between Europe and Africa.

Ramon Granadino graduated with an industrial engineering degree in 1990 at the Polytechnic University of Madrid. In 1993 he was awarded the MSECE by the University of Massachusetts at Amherst, U.S. He joined REE in 1994 as a project manager involved with the development of the 220-kV and 400-kV Spanish transmission system. Granadino acted as assistant project manager for the electrical interconnect project between Spain and Morocco.

Haddou Amerdoul graduated in electrical engineering in 1979 from the Ecole Mohammedia Des Ingenieurs of Rabat. He joined ONE in 1979 to work on the planning and maintenance of the power transmission network. In 1995 Amerdoul became the manager of the realization department and is responsible for the development on ONE's 60 kV, 225 kV and 400 kV networks.