Spain's Madrid Barajas Airport is being redeveloped with the construction of two new runways, terminal and satellite areas. Overhead lines in close proximity to airports are regulated by law, because the towers and conductors present a hazard to aircraft takeoff and landing routes and can cause radio electric interference with automatic navigation systems (LLZ, GP and ILS).

The transmission system operated by the Spanish utility Red Eléctrica de España (REE) included a double-circuit, 400-kV overhead transmission line located within the area to be developed. This circuit plays an important role in the transmission system, because it closes the 400-kV ring around the Madrid metropolitan area; at the same time, it links generation areas in northwest Spain to high-demand areas located in the Mediterranean area east of Madrid. To eliminate future interference with airport operations, a 12.2-km (7.6-mile) section of the 400-kV transmission line had to be placed underground.

Alternative solutions were considered, and although an overhead line would have been the most economical, it was unfeasible to find a suitable route close to Madrid because the land is scheduled for development and acquiring the right-of way would have been very difficult and time-consuming.

Underground cable and gas-insulated line (GIL) schemes were considered for the underground section, which required ratings of 1390 MVA/1720 MVA (summer/winter, respectively) to maintain the load-transfer capacity of this section of the bulk transmission system. The GIL scheme was some 40% to 50% more expensive than cable and involved more logistic problems, such as less flexibility, tube manufacture in specific lengths, as well as problems associated with gas handling (SF6 and N2). Hence, the pre-investment technical and economic studies concluded that a cross-linked polyethylene (XLPE)-insulated cable system was the most competitive solution for this project. The underground section has a route length of 12,700 m (41,890 ft), and REE decided to install two single-core cable circuits, one cable per phase, inside a prefabricated concrete tunnel. To extract the heat generated by cable losses from the high-power rating of these circuits, the tunnel is equipped with a forced cooling system.

Sizing the Solution

The technical solution that met the project's electrical and thermal requirements consisted of a 2-m by 2.25-m (6.5-ft by 7.4-ft) tunnel buried at an average depth of 2 m below ground level. The photos below show the civil works for the tunnel and the external jacketing of the tunnel. Two cable circuits (one cable per phase) were supplied by two manufacturers, ABB (Mannheim, Germany) and Pirelli (Gron, France). The specification was for single-core cables to be installed in a vertical configuration, one circuit on each side of the tunnel with a separation of 0.5 m (1.6 ft) between the single-core cables. Apart from differences in the cable screen, the performance of the two cables in terms of rating and losses are identical. REE decided to assign a single-circuit contract to each manufacturer to reduce the technical risk on a project using such large conductors for the first time. Also, REE was able to benefit from the increased competitiveness in the cable market, securing favorable delivery times with the cables and accessories ready for installation within six months.

The criteria used for the cable-system design was based on the equalization of the current ratings of the overhead line and cable sections, as well as on the optimization of the circuit electrical losses. The 400-kV XLPE-insulated cables have 2500-mm2 (3.88-inches2) copper conductors and aluminum screens: 529 mm2 (0.82 inches2) on Pirelli's cables and 250-mm2 (0.39-inches2) copper wires with an aluminum-foil screen on ABB's cables.

Cable-sheath bonding consists of a combination of 15 cross-bonding sections 810 m long and two single-point bonded lengths of 300 m (984 ft) at one end and 400 m (1312 ft) at the other end (Table 1). This configuration was specified to ensure the cable manufacturer of 90 drums of identical length. But, these cable ends have absorbed eventual route changes that were adopted during civil-construction works.

The high circuit ratings required by REE exceeded the design ratings of the large cross-section cables. Hence, a forced cooling system had to be installed to ensure thermal stability. The cooling system installed includes five fan stations, distributed temperature sensing (DTS) to measure circuit and tunnel temperatures, and a real-time thermal rating (RTTR) system that commands and integrates all subsystems.

Cable Design

The main characteristics of the two cables (Pirelli and ABB) are shown in Table 2. Maximum circuit losses are indicated in Table 3. (The values are for the two circuits in operation at maximum ratings.)

The technical solution, choice of cables and design of the prefabricated cable tunnel were selected to satisfy the following basic project data.

Nominal system voltage: 400 kV

Maximum system voltage: 420 kV

Minimum system voltage: 380 kV

Winter rating: 2 × 1720 MVA

Summer rating: 2 × 1390 MVA

Short-circuit current: 50 kA, 0.5 sec

Installation type: Tunnel

Impulse levels

  • Switching: 1050 kV
  • Lightning: 1425 kV

Air temperatures

Exterior maximum: Summer 42°C (108°F); Winter 25°C (77°F)

Exterior minimum: Winter -10°C (14°F)

Exterior average maximum: Summer 35°C (95°F); Winter 22°C (71.6°F)

Exterior average minimum: Winter -3.5°C (25.7°F)

Tunnel maximum: 50°C (122°F)

Maximum air velocity in the tunnel: 5 m/sec (16.4 ft/sec).

Installation of Cable in the Tunnel

The tunnel was installed on a route where water was present for some 30% to 40% of the total route length, and the 6500 prefabricated concrete sections comprising the tunnel weighed 75,000 tons. Due to the high mechanical stresses created by the variable load conditions, a flexible cable-laying system (snaking system) was chosen to minimize the longitudinal and radial stresses on the accessories.

The cables that weigh 40 kg/m (27 lb/ft) are fixed on metallic supports fixed to the tunnel at 6-m (19.6-ft) intervals, and the cable rests on saddles and brackets. Three-phase rigid aluminum spacers are positioned intermediately between the supports to ensure cable separation and minimize the maximum electrodynamic forces generated in the event of short-circuit forces. The sag applied to the cables after installation is equal to 0.25 m (0.82 ft).

Spanish contractors were awarded all the project installation contracts. SAGLAS constructed the cable tunnel, and the main project installation contractor COBRA was supported by Spanish contractors ELECNOR, INABENSA and SAMPOL. These Spanish contractors did the majority of the cable installation under the supervision of REE and ABB-Pirelli, but the two cable manufacturers completed the cable joints and terminations. In total, the tunnel cable installation (shown in photos on page 64) took more than 7000 hours to complete.

The Tunnel's Forced Cooling System

The design of the tunnel's forced cooling system was based on the following boundary conditions:

  • Maximum air speed: 5 m/sec
  • Winter air intake temp: 22°C
  • Summer air intake temp: 35°C
  • Maximum tunnel air temp: 50°C

Circuit losses and tunnel air temperatures with a margin of safety were determined based on steady-state loading conditions — 1390 MVA (summer) and 1720 MVA (winter) — the latter being the worst-case scenario.

The factor used to design the tunnel environment was the temperature of the 400-kV cable joints that effectively determined the maximum distance between fans and air outlets and the maximum power required at each fan station. With airflow of 5 m/sec and two circuits operating at maximum load, the tunnel length between fan stations was set at 2480 m (8100 ft). Each of the five fan stations is equipped with three fans (two on-load and one spare) each rated at 38.3 kW to inject fresh air into the tunnel. At the midpoint between the fan stations — the hottest position in the tunnel — air outlets are installed.

Tunnel temperatures, measured at the top of the tunnel and on the hottest cable circuit, are continuously monitored by a distributed temperature-sensing system using two fiber-optic cables attached to the serving of the power cables. A third fiber-optic cable positioned at the top of the tunnel measures the air temperature. The operation of the fans, with inverters to regulate fan speed, is controlled by an automatic RTTR system.

Real-Time Thermal Rating System

The function of the RTTR is to integrate the DTS and fan stations automatically controlling the tunnel cooling system. The RTTR interface together with the DTS regulation of the fan speeds controls and maintains tunnel and cable temperatures within design limits. The system is redundant and has a conventional temperature gage system to back up the DTS in the event of malfunction.

The operation of the fan stations is normally controlled by the RTTR system, but a duplicate system using conventional temperature gages is available for use should exceptional conditions arise. Circuit parameters, such as load, temperatures and surrounding environment conditions, are continuously monitored by the RTTR system that calculates in real time the circuit maximum load (steady-state and short-term overload values) that could be applied to the circuits under safe conditions.

Electromagnetic Fields

Studies have confirmed that the electromagnetic fields (EMF) generated by the cable circuits reach the specified limit of 100 µT within 2.5 m (8.2 ft) of the tunnel center. Hence, at ground level immediately above the cable circuits, the magnetic field strength is always less than 100 µT. In practice, with the selected cable arrangement, the strength of the magnetic field 1 m (3.1 ft) above ground level on the center line of the tunnel is less than 20 µT.

Project Planning and Construction

Following completion of the project studies designed to identify the best technical and economic solution, REE completed the engineering specifications, and a project timetable was established (Table 4).

In addition to the 400-kV XLPE-insulated cable contracts awarded to ABB and Pirelli, the project included the installation of a further 76-km (47-mile) 12-/20-kV XLPE-insulated cable. The 12-/20-kV cable ring is installed in the tunnel on the upper tray interconnecting seven auxiliary MV/LV substations that supply the ancillary systems, such as tunnel lighting system, drains and pumps, fire detection, DTS- and RTTR-monitoring systems. The five intermediate MV/LV substations are equipped with 250-kVA dry transformers and substations at either end of the tunnel with 160-kVA dry transformers.

The photo on page 64 shows one of the transition towers that was erected at each end of the cable tunnel, and the photo above shows the dismantling of the existing 400-kV transmission line.

Commissioning Tests

HV tests and partial discharge measurements were performed on all cable circuits and accessories, according to the IEC 60067 specification. An ac voltage of 260 kV was applied for one hour to each cable phase using three variable-frequency mobile resonant generators. During the test, 18 accessories (two terminations and 16 joints) were simultaneously connected by means of partial discharge base units via mobile telephone modems with a control room located at REE headquarters close to the site.

The total estimated expenditure on the Barajas Airport 400-kV XLPE-insulated underground cable project is 80 million euros. This REE project was well planned. Apart from a 10-day delay, the result of a minor joint defect identified during partial-discharge testing, circuit energization was achieved in February 2004 on target. Completion of this large high-profile project marks the end of a great team effort between REE and all the contractors. Despite the many site difficulties, commissioning on schedule demonstrates the effectiveness of astute project-management techniques.

Ramón Granadino graduated in industrial engineering in 1990 at the Polytechnic University of Madrid and received the MSECE in 1993 from the University of Massachusetts at Amherst. He has been with Red Eléctrica de España since 1994 and has managed projects for the development of the 220-kV and 400-kV Spanish transmission system. rgranadino@ree.es

Table 1. Cross-Bonding Scheme
Induced Voltages in Cable Sheaths Sheath-Ground (V) Sheath-Sheath (V)
Normal service 263 ÷ 317 605
Three-phase short circuit 5.296 ÷ 6.387 12,286
One-phase short-circuit cross bonding 1.198 ÷ 7.092
One-phase short-circuit single-point bonding 4.425 ÷ 7.524
Table 2. Main Characteristics of Pirelli and ABB Cables
Overall Diameter Pirelli Cable mm (inches) ABB Cable mm (inches)
Milliken conductor (six segments) 65.0 (2.56) 65.0 (2.56)
Extruded semi-conductive screen 71.6 (2.82) 70 (2.76)
XLPE insulation 122 (4.80) 125.9 (4.96)
Extruded semi-conductive screen 126 (4.96)
Water swelling tape 128.3 (5.05) 129.7 (5.11)
Aluminum screen/copper screen laminated AL sheath 130.9 (5.15) 138.8 (5.46)
PE jacket 142.5 (5.61) 148.0 (5.83)
Table 3. Maximum Circuit Losses
Cable Component Losses (W/m)
Winter: 1720 MVA Summer: 1390 MVA
Conductor 384.3 247.4
Insulation 15.6 15.6
Screen 48.7 31.7
Total 448.6 294.7
Overall average 371.7
Table 4. Project Timetable
Project Activities Timetable
Contracts Awarded:
XLPE cable systems September 2001 - January 2002
Civil works and installation February 2002 - June 2002
Civil works August 2002 - July 2003
Cable laying and accessories March 2003 - December 2003
Commissioning and overhead line removal December 2003 - February 2004