For several years, the Berlin utility Berliner Kraft und Licht (Bewag) AG has been working on a 400 kV transmission link diagonal to ensure the security of power supply for Berlin. The section between the Mitte and the Friedrichshain substations is now under construction. A tunnel of 6.3 km in length, approximately 25 m below the surface was drilled into which two 400 kV XLPE cable systems will be installed. ABB Energiekabel GmbH, Mannheim, was awarded with supply and installation of one of these cable systems.

In contrast to former low-pressure oil-filled cable 400 kV connections, the new cables are insulated with cross-linked polyethylene (XLPE).

The 400 kV XLPE cable design is a well-established construction with a laminated sheath that has proven its quality and reliability for more than 20 years in the range of 110 kV, as well as for numerous 220 kV, 275 kV and 400 kV cable systems. The conductor cross-sectional area of 1600 mm2 was configured for 1100 MVA power transmission capacity in a cable laid in the tunnel cooled by air circulation. A new generation of joints will be used which are designed as prefabricated and pretestable units to control the extra high stresses and to allow a faster assembly. The key characteristic of the joint is the epoxy-resin insulator, terminated at both ends with field control cones made of silicone rubber.

Due to the lack of specific experience on 400 kV XLPE cable systems, especially on 400 kV joints for this type of cable, Bewag decided to launch a prequalification test according to the Cigre recommendations.

This test was conducted on behalf of the Bewag utility at the Centro Elettrotecnico Sperimentale Italiano (CESI) in Milan. The test was carried out on a complete cable system including SF6 and outdoor sealing ends and one joint. ABB Energiekabel was the only contender out of six participants that passed this one year test successfully, being the only company to be successfully prequalified in 1995 at the time of the order negotiations for this project. The joint concept was a newly developed type which was an essential part of the one year 400 kV prequalification test for XLPE cable systems at CESI conducted on behalf of Bewag.

The cable systems installed were tested with an operating voltage at a value of 1.7 times the nominal operating voltage of 230 kV and load cycles up to 92¬įC for a total of 8760 hours including eight switching impulse tests of 850 kV at the maximum admissible conductor temperature. The test was concluded by a lightning impulse test with a peak value of 1175 kV followed by a final switching impulse test of 950 kV.

Cable laying

After extensive preparation works, such as the installation of lights and cable supports in the tunnel, the first cable was laid by ABB Energiekabel on October 22, 1997. The laying was performed in a 35 m deep shaft immediately adjacent to the Mitte substation. The single cable length laid was 750 m. The cable, manufactured by ABB Energiekabel, has an overall diameter of 135 mm and weighs 26 kg/m.

The cable was pulled from a drum trailer and guided down the vertical section inside the shaft by a plastic pipe.

The drum trailer is equipped with a braking device. In addition, four specially designed braking elements were installed into the plastic pipe in order to prevent the cables from slipping through.

In the transition area at the bottom of shaft to the horizontal tunnel the cables are led in an arched steel pipe. Within the tunnel, the cables are installed on a patented cable supporting system consisting of cable saddles with supporting length of 600 mm for the cables.

The saddles are fixed to a wall bracket installed at the tunnel wall to the cast-in C-profiles every 7.2 m. A steel tape binding with rubber insert installed at every saddle and a short-circuit spacer of stainless steel in the middle of each span are installed in order to ensure short-circuit proof fixation.

The basic advantage of the saddle system is that the cable can be pulled directly into its final position by means of installation rolls which are plugged directly on the cable saddles. In order to balance length expansions in the cable which can be caused by temperature changes during load cycles, the cables are pressed to form sags of around one cable diameter.

The cable laying started with the three phases of System Length 2 supplied by ABB Energiekabel. After laying the first three lengths, a sag of 100 mm occurred allowing the cables to be pressed into their final position manually. Starting from the centre of the system length the sags were formed and, finally, the spacers and steel tape bindings were installed to the saddles.

It can be concluded that the cable installation system that was chosen as well as the cable guidance and the braking device in the vertical laying zone definitely fulfilled all expectations. The parties concerned expect to complete the cable systems in good time by 1998 in spite of a very tight time schedule.

New joints

As opposed to the site-extruded joints or the wrapped joints usually employed in the 220 kV range, the new technology introduced by ABB Energiekabel features a joint with prefabricated and pretested components and short installation times for the 220 to 400 kV range. The new joint is maintenance-free since it does not contain any gaseous or liquid components.

The joint consists of an insulating body of epoxy resin with integrated electrode. The ‘high voltage sealing’ of the cable insulation and the epoxy resin body is achieved by prestressed cones of silicone rubber with integrated stress control. The prestressing is accomplished by the use of spring packets that guarantee equal pressure at all boundaries for all operational conditions of the cable system.

The standardized solid joint was tested in adaptation to IEC 840, with an impulse voltage of 1425 kV and an AC voltage of 2 U0 (460 kV). The impulse voltage testing of the sheath insulation of the cross-bonding sectionalizing joint was performed according to the Cigre recommendations in Electra 128 with 63 kV to earth and 125 kV between the two joint sections.

In three-phase cable systems, conductor currents at power frequency induce voltages in the cable screens. The voltages depend on the geometrical positions of the three phases and the magnitude of the conductor currents. If the cable screens are solidly bonded at both ends, induced currents will flow in the cable screens causing additional heat losses in the screens.

Regular transposition and cross bonding of cable screens may eliminate or substantially reduce the screen circulating currents and thereby also reduce the heat losses generated in the screens affecting the current carrying capacity of the cable circuit.

Growing experience

Germany’s first 110 kV XLPE cable system was installed by ABB Energiekabel in 1973. Further milestones followed, like supply and installation of the first XLPE high voltage cables with conductor cross-sectional areas of 1600 mm2 and 2000 mm2.

Germany’s first commercial order for the supply and installation of a 420 kV XLPE cable link went to ABB in 1993. In 1995, ABB Energiekabel signed a contract with a Russian PUB for 56 km of 220 kV XLPE cable with a cross-section of 1000 mm2 and received a further order for a 3.5 km 220 kV cable from Nordostschweizerische Kraftwerke (NOK) in Switzerland.

After the successful commissioning of the first commercial 400 kV XLPE cable circuit in Europe at Neckarwerke Elektrizitätsversorgungs-Aktiengesellschaft, Esslingen in 1996, the Berlin installation emphasises the viability of XLPE underground cable technology.

Table 1. Technical data and test requirements for the 400 kV XLPE cable and accessories qualification test by CESI