SPAIN'S ELECTRICITY REGULATION — THE 54/1997 ACT AND THE ROYAL DECREE 1955/2000 — establishes the premise on which its extra-high-voltage (EHV) transmission network must be developed and operated to guarantee the reliability and quality of electricity supplies at minimum cost while taking environmental protection into consideration. As the transmission system operator (TSO) and EHV grid manager, Red Eléctrica de España (REE) is responsible for submitting technical proposals to the government that take into consideration not only the needs of traditional utilities and independent power producers (IPPs), but also the needs of organizations and regional administrations.

Transmission network planning is becoming a real challenge as REE works to comply with regulations. There is a high degree of uncertainty in the location of new power plants, as generation planning is only indicative and not a real commitment for utilities and IPPs. In general, new generation plants have construction and commissioning dates far shorter than the time required to construct an HV line, even if the time involved with the social rejection and environmental issues associated with new HV transmission corridors were ignored. Additionally, load growth is not homogeneous, and in some substations load increases of 20% per annum have been recorded. In this business environment, traditional forecasting methods are not normally accurate enough to provide good estimations of what future values will be for system variables, mainly because of the lack of detailed data for available model methodologies.

Faced with these challenges, REE's plans for the mid-term development of the country's EHV transmission system now include a considerable number of 400-kV and 220-kV projects.


In order to plan for the future, exhaustive data-collection campaigns are required to obtain good quality data — including load behavior, distribution network expansion plans and generator characteristics — to aid a utility in developing accurate dynamic models. For example, Spain's peak demand in 2004 was 43,378 MW, but requests in the pipeline for new generation include some 30,000 MW of combined-cycle power plant capacity and new renewable generation — mainly wind farms — totaling 50,000 MW. On the demand side, an increase of approximately 15,500-MW singular loads is forecast (such as for high-speed trains, 1000 MW), with supplies to large industrial consumers and new urban development all expected to request access to the main transmission system. With this multitude of uncertainties, what would be the most probable scenario in Spain for the next 10 years?

To cope with this situation, REE has adopted a flexible approach for transmission network expansion planning. The approach is based on a multi-scenario analysis where the most influencing variables are taken into account — expected wind dispatch, hydraulic conditions, fuel prices, international interchanges and expected demand — together with a distribution of probability for each variable. The results produce several plans where priorities are determined based on the admissible risk assessment, taking into consideration both physical layout restrictions and environmental aspects.

REE is not only tackling this challenge with a new approach to transmission expansion planning, but also with an improvement in the construction times for EHV transmission lines and substations by using reliable standard designs with a larger number of factory-tested components, reduced maintenance needs and faster commissioning. REE's new 400-kV modular substations at Palos and Fuendetodos are equipped with unified cabinets for protection equipment and standard transformers. Several units of special polytransformers (400 kV/220 kV/132 kV/110 kV) have been successfully installed. Similarly, 400-kV and 220-kV double-circuit transmission lines are an outstanding achievement in terms of standardization that now determines the designs used to satisfy environmental and construction restrictions.


The planned major transmission system projects in Spain are as follows:

  • The ASGA project (Asturias-Galicia) is the first part of the planned northern corridor that will provide a link between the western generation facilities and the demand located in the eastern part of Spain. The EHV line from Soto to Penagos is the last section of this link. It is expected that wind generation and conventional thermal power plants (combined cycle and other technologies) will benefit from this project that will also provide the region of Asturias a link that has been in the development plan for a long period. Finally, the Vitoria-Muruarte-Castejon 400-kV transmission line will join the northern corridor to the EHV system. Figure 1 shows the existing 400-kV transmission line between Galapagar — San Sebastián de los Reyes supported by tubular poles, and Fig. 2 shows the transition point between the overhead line and underground sections on the San Sebastián de los Reyes — Loeches Morata 400-kV circuit

  • The SUMA project (Madrid-Valladolid) will reinforce the main transmission corridor in Spain. This corridor exports the surplus generation located in the northwest of Spain to the heavily loaded city area of Madrid. At the same time, this reinforcement will provide the electrical supply to the high-speed train route from Madrid to Valladolid.

  • The second international interconnection from Spain to Morocco, a submarine cable interconnector across the Straits of Gibraltar, will duplicate the capacity supplied by the existing circuit. France and Portugal exchange energy capacity with Spain, and the transfer capacity will be increased via upgrading of the existing circuits and the construction of new HV lines, such as with the projects to construct a new eastern interconnection with France and the Tajo, Miño and Duero River corridor with Portugal. The Spanish terminal substation at Tarifa (Cadiz) for the first Spain-Morocco submarine cable interconnector is shown in Fig. 3.

  • Among the high-priority projects is the duplication and reinforcement of the Cataluña-Levante interconnection required due to the anticipated concentration of new generating power plants in Aragon and Cataluña. These projects are the uprating of Vandellós-La Plana 400 kV and Nueva Teruel-Morella-La Plana 400 kV. Supplementary connections are being studied as longer-term projects.

  • The 220-kV network in the eastern part of Spain (Levante) requires interconnection to the 400-kV transmission system, a project to be commissioned shortly. At the same time, the construction of interconnections between the center and east of Spain will offer the wind-farm generation at La Mancha as well as conventional generation power plants direct access to the energy market. These long-term 400-kV proposals include a 250-km (156-mile) transmission line from Romica-La Paloma-Puertollano that would also help to substantially improve the system under emergency conditions.

  • Southern-area 400-kV projects, such as the Arcos-Lucena and Litorial-Caparacena-Guadame corridors, will cater to the energy needs of the high-speed trains and give HV grid access to wind farms and combined-cycle projects. Simultaneously, the EHV transmission system load flow pattern will reduce the strong north to center dependency.

  • The principal urban areas such as Madrid and Barcelona will be reinforced, mainly with additional 220-kV substations. In the Madrid area, a project to underground a section of the existing 400-kV line via XLPE cables in a gallery has been recently commissioned to allow for further runway development at Barajas Airport in Madrid (Fig. 4).


Together with these planned projects, extensive plans for line uprating, reactive compensation at distribution networks, new generation facilities and demand substations will require REE and distribution utilities to develop a transmission system in harmony that encompasses all the different interests. This will ensure that Spain has universal electricity supply as required and enforced by its Electricity Act. If the electricity system is considered one of the most complex human realizations in history, the Spanish transmission system is set to be an excellent example.

Fernando Soto heads the Network Planning Department for Red Eléctrica de España, the transmission system operator and extra-high-voltage grid manager in Madrid, Spain.

Baldomero García heads the Installations and Systems Department for Red Eléctrica de España, the transmission system operator and extra-high-voltage grid manager in Madrid, Spain.