Underground XLPE transmission cables, already working at 500 kV in Tokyo and over 400 kV in Copenhagen and Berlin, promise ever increasing realms of application. The contract for the second leg of BEWAG's 380 kV, 1100 MVA, cross-city diagonal in Berlin was awarded to ABB Energiekabel of Mannheim in January 1999.

As major cities around the world face up to the formidable complexities of trying to transmit ever increasing volumes of electricity through virtually impenetrable jungles of underground services networks, waterways, transport tunnels, deep structural foundations and unstable sub-strata, the urgent calls for dependable long term testing procedures for high voltage polymer insulated cables and accessories become ever more pressing. Overhead lines are now generally ruled out for environmental reasons.

Progress on type testing, prequalification testing, long term component testing and actual in-service experience is continuously reviewed within the study committees of CIGRE, and the subject was a major topic at the 37th annual session of the CIGRE Congress in Paris in September 1998.

Cables capable of carrying 1100 MVA and more at voltages of over 400 kV have already been installed over substantial distances and are working very successfully, albeit at great expense. The cables need large diameter air cooled tunnels, usually at considerable depths under the ground, to function successfully. What are the potential alternatives?

In spite of the present preoccupation with cross linked polyester insulated underground cable technology, there still seems to be increasing competition from SF6 insulated cable suppliers and, in the longer term, superconducting cables with enormous power capacities.

High temperature superconducting cables. This technology is at a very early stage of development and feasibility is far from certain. In the USA, Pirelli is constructing a 50 m long 115 kV, 400 MVA test cable which will carry some 2000 Arms, under an EPRI research programme. It is designed with a pipe type retrofit in the USA in mind.

The test assembly developed jointly by American Superconductor and Pirelli includes a joint, two terminations and a cryogenic refrigeration system to maintain the required temperature of 77 K. The results so far are described as “encouraging”.

  • Long, high capacity, SF6 cables. These are increasingly being used in major urban substations in London, Paris, some Japanese cities and many other urban conurbations around the world, but transmission distances tend to be very limited because of the high cost of the insulation system and the necessary maintenance. A good example is the 275 kV link between the Shin-Nagoya Power Station at Shinmeika to the Tokai substation line in Japan which will transport 2850 MW through the upper half of a 5.6 m diameter tunnel carrying LNG piping in the lower section. This system was developed under a CRIEPI programme which started in 1963.

    The developers point out that it takes only two circuits to carry this very large capacity – gas-insulated line (GIL) has four times the current carrying capacity of XLPE cables. The plan was to assemble some 1500 GIL units in a tunnel with 3.1 m headroom and very limited space for assembly in an area of seismic risk. The tunnel, which runs at a depth of 30 m under public roads, has four curves with a minimum radius of 150 m. The highly compact GIL has an outer diameter of only 460 mm.

    In France, Electricité de France is also putting much effort into the development of 420 kV GIL links for use on lines of greater than 1 km in length, but they also have development programmes with Alcatel and Pirelli on the optimization of 400 kV XLPE cables for bulk power transmission.

    500 kV XLPE in Japan

    After ten years of operation with underground XLPE insulated cables, and successful experience with 275 kV cables installed in great numbers over the last three years, Japanese utilities are now installing a 500 kV, 39.8 km long, double circuit XLPE cable to carry some 900 MW between Shinkeiyo and Toyusu for operation in the year 2000. These cables are mainly extruded by the “Mitsubishi – Dainichi vertical line for continuous vulcanisation” process.

    TEPCO is developing a 500 kV underground transmission system for metropolitan Tokyo with major underground substations. The compact XLPE lines will be expected to fit into the existing multi-utility tunnels intended to introduce the 275 kW cables.

    Second Berlin 400 kV link

    The second leg (Friedrichshain to Marzahn) of Berlin’s 400 kV, cross city transmission system diagonal, has now been ordered from ABB Energiekabel. Both links are designed to operate at 420 kV, but Bewag has now decided to run them at 380 kV, leaving plenty of latitude for short term overrating. The first leg (Mitte to Friedrichshain), which is now operating, was officially inaugurated on 7 December 1998.

    The new order comprises supply and installation of two 400 kV XLPE cable systems of around 5.4 km length and these two cable systems are scheduled to be completed in July 2000. Outstanding characteristics of the new project include:

  • Delivery lengths of more than 900 m with a cable drum flange of 4.5 m and a total transport weight of 30 t.

  • Cross-bonding of metallic screen including monitoring of sheath voltage limiters.

  • Partial discharge measurements of all accessories during high voltage tests after installation.

  • Optional continuous monitoring of partial discharge measurements in the accessories during operation.

    To exclude the occurrence of identical system failures in the 400 kV XLPE cable systems applied for the first time in urban power supply, the cables are to be supplied and installed by two independent cable manufactures. In the first leg, ABB Energiekabel and Siemens were the cable suppliers, while for the second leg ABB Energiekabel and Alcatel are the cable suppliers. ABB Energiekabel GmbH, Mannheim and Alcatel Kabel AG & Co, Hannover, formed a consortium with ABB in charge technically due to the experience with the former Bewag project and Alcatel in charge of commercial order handling. Each of them is still responsible for their own design of cables and accessories.

    The Berlin project has benefitted from previous experience with a major 420 kV underground XLPE cable installation feeding metropolitan Copenhagen in Denmark, supplied by NKT Cables, Brøndby, Denmark. A new extension of this system, to the north of Copenhagen, has recently been ordered.

    In the Berlin case the conductor cross-sectional area is 1600 mm2 configured for 1100 MVA power transmission capacity in a cable laid in the tunnel cooled by forced air circulation. Ventilators in the end shafts blow ambient air into the tunnel which leaves via the central shaft. During undisturbed normal service both cable systems each transmit half of the load (2 x 550 MVA). Should there be a fault in one system, the other takes over the total load (1 x 1100 MVA). In a further extension stage each of the systems will transmit 800 MVA (2 x 800 MVA) so that, should a fault occur, one system would take up the load of 1 x 1600 MVA. Temporary operation with 2 x 1100 MVA without redundancy is also possible.

    In substations at each end of the cables, the 400 kV XLPE cables are connected to SF6 insulated switchgear by sealing ends in accordance with IEC 859. One end (Termination A) has capacitive stress control using a taped stress cone with conductive inserts, while the other end (Termination B) uses a prefabricated slip-on stress cone made of silicone rubber (SIR) with integrated deflector.

    Termination A is built on the composite-joint principle with the body of the joint made of epoxy resin (EP) and the two lateral stress cones of silicone rubber. The pressure exerted on the interfaces between EP-SIR and XLPE-SIR is produced by metallic spring packages, whose special characteristics ensure equal pressure distribution and high electrical strength of the interfaces within the whole temperature range.

    Termination B is a one-piece slip-on joint of silicon rubber which is prefabricated as a complete body including all stress controlling devices. The advantage of this principle is that all the electrically relevant components of the joint are exactly positioned and inseparably connected with each other under defined conditions at the manufacturer’s.

    Type testing

    First test series. The cables and the requirements of the first prequalification tests at the CESI test laboratory in Milan were reported in Modern Power Systems, March 1998. The result of this long duration test on synthetically insulated 400 kV cables and their accessories showed that the initially applied taped joint technology did not fulfill the test conditions. Only one manufacturer with a prefabricated pretestable composite joint satisfied all requirements of the long-duration test over 8700 hours. Faults on cables did not occur during the test.

    Second test series. In parallel to the selection of a straight tunnel for Mitte to Friedrichshain, Bewag decided to carry out a second prequalification test on 400 kV XLPE cable. In addition to the first test, each participant had to install his cable in a tunnel duct and a joint in a joint chamber.

    The conditions for participation in the second test series were the application of prefabricated prestestable solid joints. All the accessories (SF6 sealing end, outdoor sealing end and two joints) were equipped with sensors for partial discharge measurements.

    In August 1995, the second test series started. In November 1996, the tests were successfully concluded with the prequalification of five participants. Only one manufacturer had to break off the test early after 5600 hours. All components, especially the prefabricated solid joints, had proved their performance and operational applicability.

    Progress with XLPE

    Since 1952, Bewag had to cover the power demand by its own power stations using an adapted network infrastructure. So, an extensive 110 kV transmission and distribution network has evolved within the city. In 1978, the first double-system 400 kV link was erected using 400 kV low-pressure oil-filled cables with 1200 mm2 cross-section copper conductor. The route length of this line was 10.6 km.

    Direct water cooling of the single-core cables permitted a guaranteed load of 1100 MW per system. The field experience gained with the oil-filled cable system over the last 20 years is reported to have been excellent. So far no faults have occurred on cables or accessories.

    Germany’s first 110 kV XLPE cable system was installed by ABB 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.

    Today, many more cable makers are gearing up, type testing, qualification testing and long term endurance testing. Pirelli are active in many countries, particularly in Italy and the USA in conjunction with EPRI programmes. BICC Supertension and Subsea Cables at Erith in the UK is greatly advancing the technology. LG Cable & Machinery is preparing to meet a major market in Korea for 400 kV cable products that they have already developed, and there are others already active in the field including the large Japanese cable suppliers working in conjunction with the utility groups.

    Diagonal route


    Table 1 Long distance XLPE transmission cable lines under construction in Japan
    Table 2 Transmission load cases for Berlin diagonal link
    Table 3
    Long distance XLPE transmission cable lines supplied in Europe