Iberdrola moves existing high-voltage and medium-voltage networks underground.
To satisfy increasing and future demands for energy in major cities — created by new industrial and residential development — often requires the reorganization of a city's existing electricity supply system. In Spain, there is an intense debate over the perceived adverse health effects of electromagnetic fields (EMF) and serious concerns regarding transmission towers and property devaluations.
To address these issues, Iberdrola, one of Spain's major utilities, embarked on a massive project to move the city of Madrid's existing high-voltage and medium-voltage overhead lines underground and to redesign the electricity transmission system and distribution network.
Iberdrola's large-scale project required the collaboration and support of many local administrations throughout all phases of the project. Replacing overhead lines with cable systems employing the latest technologies offered many benefits:
- Socially acceptable
- Negligible visual impact
- Narrow right-of-way and less land depreciation
- Negligible EMF
- No noise or corona
- Lower transmission losses
- Improved system reliability.
The Plan Madrid project included the construction of 16 compact gas-insulated substations; the installation of 180 km (112 miles) of 220-kV, 132-kV, 66-kV and 45-kV underground cable; and the dismantling of 125 km (78 miles) of overhead lines.
Scope of the Project
The routes for the underground cables were selected on a cost-effective basis to minimize the impact on the environment and social community. This required the consideration of many issues: land use, geotechnical and soil condition, traffic and parking, and health and safety, including EMF. The route selection process was undertaken in conjunction with public agencies and landowners.
As the underground cables had to be installed in an urban environment, the terrain features included different conditions, namely roads subject to heavy traffic flows, railways, motorway and river crossings, and areas of special interest.
Iberdrola decided to use cables with cross-linked polyethylene (XLPE) and hard-grade ethylene-propylene rubber (HEPR) insulation as their proven reliability is judged to be superior to other insulation materials. In agreement with the cable manufacturer, Iberdrola opted to specify cables with a simple traditional construction. The cable manufacturer was contracted to provide a comprehensive five-year warranty for the complete cable system.
For the project, SF6 and outdoor terminations were required. Those for outdoor installation were designed with composite insulation and not porcelain because of the safety risk associated with porcelain in case of terminal explosion. Lower weight and maintenance were additional reasons for the use of composite insulation. Accessories such as surge arresters and other substation equipment were procured by the utility and given to the contractor for installation. Cross-bonding and single-point earthing were adopted to minimize circulating currents in the metallic screens.
The World Health Organization has confirmed that, provided EMF exposure levels are below the limits recommended in the International Commission on Non-Ionizing Radiation Protection (1998) guidelines, electromagnetic fields do not produce any adverse effect on health. However, Iberdrola regards it as prudent to adopt the “as low as reasonably achievable” criteria.
The cables were installed in a trefoil configuration, as studies present this as the best arrangement to minimize the EMF when installed at a nominal depth of 1.5 m (5 ft). Optic fibers were also laid with the power cables at a different depth to aid maintenance; these fibers are used for substation and control center communications, with spare fibers for broadband communication.
Ducts were embedded in concrete to afford mechanical protection and provide the utility the opportunity to install the cables at times most favorable to the public. A duct spacer was installed to prevent duct movement and ensure adequate cover during the concrete-pouring process. Iberdrola designed a special duct spacer to ease and simplify the construction process. This spacer prevented duct bank flotation or movement during the pouring of the concrete, controlling the position of the ducts and ensuring sufficient concrete coverage of the ducts for the total length of the cable route.
In some locations, it was possible to install the cables in shared service galleries that offer economies in terms of easy installation, repair and maintenance, and superior protection from third-party damage. Other cable-excavation practices were adopted, such as the use of metal plate coverings, where the depth of lay was less than standard, and horizontal directional drilling under motorways, railways and the Manzanares River.
The existing cable joint pits were constructed in situ. But for the new cable network, new types of joint bays employing prefabricated components were designed and installed to ease the installation and substantially reduce the site construction time. These joint bays have sufficient height to allow comfortable jointing conditions without inconvenience to the public. The length of these joint bays is sufficient to accommodate two joints, enabling joint renewal or the completion of two new joints in the event of a joint failure, again without inconveniencing the public. The final stage of the development work was the tubular system used between the joint bay and the surface that simplifies the cable-laying process.
Collectively, the developments linked to duct laying and the prefabricated joint bays enable separation of the civil works and cable-laying processes, a change from traditional cable-laying practice. Collectively, these new processes allow greater flexibility in the construction schedule, minimize the impact on traffic and reduce public inconvenience. The joint bays are left empty so they can be regularly inspected. A rigid cable-clamping system is used in each joint bay, eliminating the need for thermal expansion loops.
The management of this major project was based on the creation of five teams, each having responsibility for key elements of the project:
Engineering — Responsible for the design, material processes and equipment specifications
Permitting — Responsible for negotiations with all the relevant bodies to secure all route permits; in Spain, it is mandatory to have state and local permits before starting construction work
Procurement — Responsible for all contracting strategies associated with the processes for service and materials purchasing
Management — Responsible for managing all construction works, materials and processes to ensure compliance with the project construction program and deadlines
Environmental, quality and safety control — Three security and health teams were responsible for the prevention, monitoring and mitigation of all safety issues that arose during the project.
Iberdrola adopted different working practices for construction works on high-voltage systems and medium-voltage networks. On medium-voltage networks, the internal resources of the Madrid region are used in a decentralized way, using the utility's global resources. For high-voltage systems that are more critical, a working team was established to act as a general contractor with complete responsibility for procuring the design, material and equipment and hiring contractors. This arrangement gives the project manager complete knowledge of each activity, enabling the allocation of resources to fulfill the master plan.
This project model used by Iberdrola is a proven solution for managing complex, large-scale projects involving multiple working teams, suppliers and contractors.
On completion of the Plan Madrid project, the city will have a flexible and secure transmission system and distribution network that has been redesigned and constructed to reduce the visual, physical and environmental impacts on the city.
High-Voltage Cable Features
Josu Orella Sáenz (firstname.lastname@example.org) received his industrial engineer degree in electricity from the ETSII Bilbao, and he is now studying for his Ph.D. degree in the field of underground power lines from the ETSII Bilbao. Currently, he is working for Iberdrola Engineering & Construction in the power line department. His particular research interests include power systems, high-voltage and electromagnetic fields, among others, which take place in the CIGRÉ Working Groups. In addition, he is a lecturer in the electrical engineering department at ETSII Bilbao.
Companies mentioned in this article:
Mikel Barriocanal Larrea (email@example.com) received his degree in mechanical engineering from the ETSII Bilbao, and since 2001, he has served as the director of Iberdrola Engineering & Construction in the transmission and distribution department. Previously, he was a project manager in Iberdrola Distribution for 11 years, so he has extensive experience in 132-kV, 220-kV and 400-kV transmission line and underground cable projects. During his career, he has been an active member on Working Groups for CIGRÉ, CENELEC and AENOR.
|Voltage||220 kV||132 kV|
|Conductor||2000 mm2 (3.10 in2 ) copper||1200 mm2 (1.86 in2) aluminium|
|Screen||206, 280, 315, 350 copper wires||102,172, 280, 350 copper wires|
|Rating||560 MVA||210 MVA|
World Health Organization www.who.int