Over the last decade, there has been an increasing trend in Europe to reduce the number of overhead extra-high-voltage (EHV) and high-voltage (HV) transmission lines and to replace them with underground cables. This also is the case in Denmark, where it was decided in the early 1990s that the power supply to the greater Copenhagen area needed restructuring. Modernization and improvement of the HV network in the power system operated by NESA, named the Metropolitan Power Project, will elim-inate almost all over-head lines from densely populated areas close to Copenhagen by the end of 2000. At the same time, reliability for consumers will improve as dependence on the central 132-kV Glentegard Substation is reduced.

NESA's HV System

The HV system in Zealand, Denmark is operated as a single power pool, which provides a basis for high supply reliability and low electricity prices. A central junction in this HV network is NESA's large Glentegard Substation, from which six 132-kV overhead lines link the metropolitan area with the rest of the network in Zealand. To remove these lines, it was necessary to construct two new 400-kV links, including new 400-kV substations in the northern and southern parts of greater Copenhagen.

In Europe, new cable technology combined with improved production processes and specifications of international long-term testing means that use of XLPE cables for EHV and HV applications is increasing. These considerations, together with the cable's proven reliability, were the factors that led to NESA's decision to install XLPE-insulated cables rather than fluid-filled cables, which require more maintenance.

More than 100 km (62.2 miles) of single-core 400-kV XLPE-insulated cables were needed to form the two independent power links. The southern route between the 400-kV station in Ishoj and the H.C. Oersted Power Plant was 22 km (13.7 miles), of which 18 km (11.2 miles) of cable was to be installed in NESA's supply area and 4 km (2.5 miles) in the area of Copenhagen Energy. Furthermore, the northern link in the NESA supply area consisted of12-km (7.5-mile) 400-kV XLPE cable section, which linked M plus lov to the Glent egard Substation.

Planning commenced in 1994 for the two 400-kV cable circuits, which were to be installed successively, making target commissioning dates of September 1997 for the South Link and December 1999 for the North Link.

Cable Contract

NESA decided to invite manufacturers to tender for the southern link cable (22 km or 13.6 miles), specifying a 400-kV XLPE-insulated cable circuit with a total transmission capacity of up to 1000 MVA. The tendered cable was a 1600-mm2 (3200-kcmil) Cu conductor with a lead or corrugated aluminium sheath. To achieve the required circuit rating, the cable system should be special bonded.

The contract assessment was based on a wide range of criteria, including references from similar cable projects, evaluation of technical knowledge, documentation of the quality system and satisfactory completion of type-test and long-term prequalification testing. Furthermore, NESA representatives inspected cable manufacturers, evaluating their production facilities and the way they handled materials-including the maintenance of XLPE compound purity and the correct placement of the conductor in the extrusion process.

The precontract test was conducted in Centro Elettrotechnico Sperimentale Italino in Milan, Italy. With load cycles, the test lasted one year, with an ac test voltage of 374-kV applied between the conductor and earth. During and after application of load cycles, impulse voltage tests were performed, and the cable from NKT passed the tests without breakdown. In addition, a type-test according to CIGRE Recommendation No. 151 was performed with extrapolated values to IECy840 to meet the tender specifications.

For routine tests, the CIGRE recommendations were followed but with a higher ac test voltage of 495-kV - 2.25 U0 for 1 hr, which results in a maximum field strength still below 27-kV/mm. The partial discharge test was performed according to the IEC 840, for example 1.5 U0 or 330-kV.

The contract for the southern link was awarded to NKT Cables of Denmark, which is one of the first cable manufacturers in Europe to carry out the prequalification test on the 400-kV XLPE cable. The cable is a further development of the 145-kV XLPE cable that NKT type-tested and delivered to a Danish utility in 1985 and 1986 and to a 245-kV XLPE cable export project in 1991. Apart from dimensions and current carrying capacity, the main differences in the design and manufacture of the 400-kV cable are the strict requirements regarding purity of the insulation material and smoothness of the semiconducting screens.

Cable Design and Production

Following acceptance, NESA closely monitored design and manufacturing. The 400-kV XLPE-insulated single-core cable comprises a longitudinal, watertight, copper conductor of 1600 mm (3200 kcmil). This is sufficient to limit the electrical stress far below critical limits, for example, a cable being hit by lightning when operating at normal system voltage. The cable core is then triple extruded on a Catenary Continuous Vulcanization (CCV) line with an insulation thickness of 29 mm (1.1 inches). The manufacturing is conducted in a controlled, clean environment to ensure that cable insulation has the highest purity and the most uniform dimensions.

To enable the use of prefabricated accessories in the installation it is important that the geometry of the cable is maintained within narrow limits. To guarantee this, X-ray equipment is installed to accurately measure the thickness of the layers in the cable core and verify that the cable construction satisfies the design specification. Following vulcanizing, the cable core is placed in a heated degassing chamber until the main part of the methane gas, which is created in the vulcanizing process, has been removed-a procedure that can take up to several weeks. Bedding tapes are then applied and the cable completed with a lead sheath and a polyethylene (PE) oversheath. To facilitate sheath integrity testing, an extruded semi-conducting layer isapplied on the PE oversheath.

Installing the Southern Cable Link

In Denmark, underground cables are normally direct buried at a depth that depends on the cable's voltage levels. For the 400-kV cable, NESA decided that a depth of 1.5 m (4.9 ft) was appropriate. At this depth, most of the existing lines and pipes for telephone, gas and water can pass over the cables. Furthermore, laying cable at this depth reduces the risk of third-party damages.

The maximum lengths of cable that could be installed were limited by the drum size. In this case, drums with a record diameter of 4.95 m (16.2 ft), the largest drum that can pass under most road bridges in the densely populated Copenhagen area, were constructed. This imposes maximum lengths of about 880 m (2890 ft) for each cable. To handle supply lengths of this magnitude, a special installation vehicle was bought and modified to fit project requirements. This truck could carry more than 45 tons (40.8 t) and was flexible enough to drive on even the smallest local roads.

The 400-kV cables were laid in flat formation in an open trench that was 1.1 m (3.6 ft) wide. At the bottom of the trench, 20 cm (7.9 in) of concrete was placed. The concrete layer enables other services to pass under the cable system and provides a solid base for the equipment, which is necessary for pulling the cables. Rollers and mechanized rollers were placed and secured to the concrete bedding, and a cable winch was used to pull the cable into position. The winch continually measured and controlled the pulling force, ensuring that the cable would not be damaged.

The first layer of backfill on top of the cable was 0.3 m (1 ft) of special cement (weak mix), followed by normal soil and surface reinstatement. This procedure was used in all of the trenches except in places where the use of an open trench was impossible, for example, when crossing roads and railways. In these instances, either pipe-jacking or directional drilling was used.

Only one major problem occurred while using pipe jacking. One of three cable lengths got stuck in a 700-m (2300-ft) length of PE pipe used when crossing an area with shallow water. Inspection revealed some minor deformation of the pipe, but it did not prevent the cable from being installed without damage.

Directional drilling, which is less expensive and faster than pipe-jacking, was used at more than 10 crossings in the southern link, with the lengths varying from 50 m to 500 m (165 ft to 1650 ft) depending on soil type. The longest and most challenging drilling was in the 500-m (1650-ft) section to cross a moor in an environmentally protected area. By drilling from both sides of the moor and connecting the two 250-m (825-ft) tubes in the middle, the crossing was completed in less than one month.

Establishment of Jointing Facilities

One of the most critical points of a 400-kV XLPE cable installation is that jointing must be performed in an environment free from dust and with controlled temperature and humidity. So, jointing was undertaken in special containers that were placed on a 2.5-m by 15-m (8.2-ft by 49-ft) concrete foundation that was positioned before the cable was installed. The foundation consisted of five specially designed concrete elements that were placed on sand bedding and then waterproofed. After the cable was laid and placed inside the foundation, three purpose-designed 6-m (20-ft) containers from NKT Cables were placed on top of the foundation, providing a jointing area in which they could be installed in the final position. At the beginning of the installation, it took five weeks to complete three single-core joints, but with the jointing area, that time was greatly reduced to three weeks by the final stage.

Accessories

The insulated cable joints were made with prefabricated and pretested components. This prefabricated joint with sheath sectionalizing comprised an epoxy sleeve and two stress cones, which, together with the cable insulation, formed the main insulation system. The epoxy sleeve embeds an electrode of aluminium alloy, as the coefficient of thermal expansion equals that of the epoxy material. A spring-loading system maintains the pressure required to ensure the electrical integrity needed in the XLPE/stress cone and stress cone/epoxy interfaces. Sheath sectionalizing is effected by means of an insulating flange with a coaxial bonding lead to enable sheath cross-bonding or sheath earthing as required. A glass fiber housing filled with compound protects the joint, and metallic sleeves encasing the epoxy sleeve and a spring-loading system secure water tightness. Both sides are overlapped and are plumbed to the cable's lead sheath.

The terminations were of the outdoor and GIS type, filled with degasified synthetic oil kept under pressure and a stress cone of silicone rubber to control the electric field. A thermic weld that is radiographically inspected by X-ray joins the top connector of the termination and the cable conductor.

Project Construction Timetable

Site work on the southern link started in April 1996 by laying in the first 820-m (2700-ft) cable section. Before the end of 1996, 19 of the 26 three single-core cable sections were installed. After winter break, work on the remaining seven sections began in April 1997. Therefore, the installation, including all joints, was completed ahead of schedule in August 1997.

The two-week site testing of this installation at system voltage (400 kV) started in September. Following the precommissioning testing, the southern route of this cable system was put into commercial operation. This installation, comprising 66 km (41 miles) of 400-kV XLPE single-core cable, 72 prefabricated joints, nine GIS and three outdoor terminations, was, at the time of commissioning, the first in the world in which an XLPE cable system including cross-bonding joints was used at this voltage level.

The Northern Link

In May 1997, with the southern link installation still in progress, NESA invited tenders for the northern link. Manufacturers whose cables had satisfied precontract tests for the southern link were invited.

Again, NKT Cables offered the best commercial and technical solution. Additionally, NESA had the benefit of this manufacturers' references from their first contract. This meant that the constructive dialogue established between the project managers from NESA and the manufacturer could be continued. At present, NKT Cables has manufactured and laid 36 km (22 miles) of 400-kV XLPE single-core cable for the northern link without any technical problems.

Prospects

During the past six years, Denmark has replaced many HV transmission lines with underground cables. NESA's Metropolitan Power Project, which traverses rural and urban areas, penetrating into Copenhagen's inner city, is a further example of this. This project is a culmination of a long-term joint development, demonstrating the environmental benefits that can be achieved by placing transmission systems underground in densely populated areas. By installing this EHV/XLPE-insulated cable system, NESA has further enhanced the technical and economic benefits of this rapidly developing XLPE cable-insulation technology.