Power supply utilities in the countries in East and West Europe have operated with cross-border grid interconnections for more than 40 years, while the power connections between the two halves of Berlin and the surrounding area have until recently, remained severed. Bewag, the power-supply company supplying West Berlin, operated and managed an isolated power network. In 1994, Bewag's insular existence ended when it was connected to the regional interconnected grid. Part of this interconnector comprises a 106-mile (170 km), 380-kV transmission line from Helmstedt via Wolmirstedt substation near Magdeburg, to the Teufelsbruch substation in Spandau Forest, 0.7 miles (1.1 km) inside the city boundary. A 380-kV oil-filled cable system approximately 4.7 miles (7.6 km) long completes the inner-city interconnection to Reuter substation in Spandau. The 380-kV connection between Reuter and Mitte substations commissioned in 1978 enabled power to be supplied from the grid to Berlin's city center.
The Reuter-Mitte 380 -kV double circuit interconnector consists of a 1.6-mile (2.6 km) overhead transmission line and a 5 mile (8.1 km) underground cable. The 380-kV cable circuit comprises two parallel paper insulated, low-pressure, oil-filled cables with a cross-section of 1200 mm2. The cores of each circuit are installed in pipes and a direct water-cooling system gives each circuit a thermal rating of 1120 MVA.
Network Reinforcement Program In the early 1990s following the political changes, Bewag was made responsible for the power supply for Berlin and undertook load flow studies to determine the long-term reinforcement necessary to establish a reliable power system. The best strategy, based on technical and economic considerations, was to establish a 380-kV diagonal interconnection through the city that incorporated the existing 380-kV circuits. The first phase of the project involves installing a high-capacity 380 -kV cable connection between Mitte substation in the Tiergarten district and the Friedrichschain substation in Prenzlauer Berg district. This cable will link the power systems of the previously divided sectors of Berlin. In addition to restoring the firm capacity and reliability to the city's supply system, it will also ensure that the Bewag power plants can operate at optimum efficiency.
The second phase of the project includes a cableor, which is a gas-insulated line (GIL) connection between Friedrichshain substation and the proposed substation to be built in Marzahn on a site at the Berlin Institute Testing Station for High Performance Electric Systems (IPH). A 380-kV transmission line within the city boundary that required agreement from the district authorities and the public was also planned to form the interconnection between Marzahn and Neuenhagen (VEAG Energiewerke AG) substations to the east of Berlin. The completion of all phases of this 380-kV diagonal connection scheduled for the year 2000 will provide two high-capacity in-feeds to establish a reliable and economically efficient power system for Berlin. Figure 1 shows the existing power supply system for Berlin, the proposed 380-kV interconnector and the existing and proposed substations.
380-kV Cable Selection The strategic importance of this interconnector coupled with Bewag's limited knowledge and experience in the operation of 380-kV XLPE insulated cables resulted in the utility consulting cable manufacturers on the suitability of this cable design. Agreement was reached on a program of type testing the cable and accessories to assess the lifetime performance. The conditions that Bewag imposed for these tests were based on the CIGRE recommendations (September1992) for pre-qualification tests (CIGRE WG 21-03). In particular, it was necessary to demonstrate the operational suitability of the cable, straight joints and the SF6, indoor and outdoor sealing ends. This involved tests on their dielectric aging and thermo-mechanical properties under operating conditions. Table 1 compares the CIGRE and Bewag test parameters specified for the long-time tests on 380-kV XLPE cables. Practical experience was also required to ensure the cable was sufficiently robust to withstand the handling and various site conditions encountered during installation, i.e., open ditches, pulling into pipes and ducts. It was also important to establish space requirements for joint assembly and to confirm that the jointing could be done under building site conditions.
Pre-contract Cable Testing Program The tests were conducted at the Centro Elettrotechnico Sperimentale Italiano (CESI) in Milan. The center was chosen because it could undertake the technical work required at short notice. Six cable manufacturers took part in the first test series. Three manufacturers supplied cables with a laminated sheath and the other three supplied cables with an aluminum corrugated sheath. The specified test voltage of 400 kV was applied to the outdoor sealing ends of the six cables that were electrically connected in parallel. The three single-phase cables were coupled to form a single system in an SF6 sealing end that was fitted with disconnectors and earth connections. The installation was constructed to ensure that the test could continue irrespective of any defects occurring in one or more of the cable systems on test. The 400-kV test voltage applied for one year simulates the stress a cable installation operating at 230 kV is subjected to during some 50 years of operational service. Heating currents that were applied inductively via heating current converters generated the thermo-mechanical load.
This test program started in September 1993, but due to defects in the test cables it continued until April 1995, six months beyond the scheduled completion date. The failure of lapped joints to withstand the test voltage was the cause of the defects. The cable manufactured by ABB Kabel und Draht fulfilled all of the test requirements and pre-qualified them as a supplier for Bewag. The joint used by this manufacturer consisted of prefabricated and pre-tested slip-on fixtures. Figure 2 shows the cable testing facility at CESI.
The second series of tests commenced in August 1995. In contrast to the first tests in which only one cable manufacturer used a prefabricated and pre-tested composite joint, all six participating cable manufacturers adopted this type of joint. The test parameters remained the same apart from changes to the installation of the test cables. A cable tunnel and joint installed in a joint box were included in the cable circuits. This was a change introduced to simulate the cable tunnel conditions that had then been agreed for the circuit installed between Mitte and Friedrichshain substations. New measuring and sensor equipment for partial discharge measurement was also used for in-situ monitoring of accessories. The second test program concluded in March 1997 when the following five cable manufacturers were pre-qualified as suppliers of 380-kV XLPE cables and accessories to Bewag: Siemens AG, BICC, Felten and Guilleaume, Alcatel Kabelmetal Electro and KABELRHEYDT.
The positive outcome of the test series satisfied Bewag that XLPE insulated cables are a technically acceptable alternative to oil-filled cables and that the decision to use XLPE cable for the 380-kV connection between Mitte and Friedrichshain substations was correct.
Cable Tunnel Construction Originally, the conventional cut-and-cover method of cable installation was going to be used. In view, however, of the difficulty of route planning in a city center, a feasibility study confirmed that a cable tunnel could be economically constructed. The route and depth of the tunnel was determined on the basis of geological reports and test drills. The cable tunnel was constructed at a depth of 80-100 ft (25 to 30 m) beneath the surface, the external diameter of the tunnel is 12 ft (3.6 m). The vertical profile and cross-section of this cable tunnel are shown in Figure 3. The tunnel is approximately 3.9 miles (6.3 km) long and is divided into four sections by the construction of five shafts. Three tunneling machines worked simultaneously on the construction of the tunnel. The shafts provide tunnel access and ventilation. Figure 4 shows the tunneling machine at the base of a shaft prior to recovery. The use of a cable tunnel instead of the surface cable route reduced the overall length of the circuit route length by 0.70 miles (1.1 km).
Cable Installation from Mitte to Friedrichshain Contracts for the 380-kV XLPE cables were awarded to ABB Energiekabel and Siemens. The utility specified quality assurance checks during the manufacturing process. The key stages in the cable manufacturing and accessories were monitored by Bewag personnel. Two types of XLPE cable were used for these circuits: a laminated sheath cable (type: 2XS[FL]2Y) and an aluminum corrugated sheath cable (type: 2XKLD2Y). Figures 5 and 6 show a cross-section of the cables. Each has a cross-sectional area of 1600 mm2 (copper) with a nominal thermal rating of 1100 MVA, but forced air cooling is foreseen as a means of increasing the rating in the future.
The cables are attached to brackets at 24-ft (7.2 m) intervals in the tunnel and distance spacers are used between these brackets. Anti-fire protective paint was applied to all cables after installation. The installation of the cables started in September 1997 and after completion this summer, the cables will be subjected to a nine-week start-up test including a four-week period at operating current. Finally, partial discharge measurements will be taken on all parts of the installation at 400 kV using a resonance unit and PD measurement equipment. (These tests form a contract awarded to the Berlin Institut Pruffeld fur Elektrische Hochleistungstecknik [IPH - Berlin]). This 380-kV cable connection is due to go into operation this fall.
The Friedrichshain and Marzahn Connection The cable route between the 380 -kV substation at Friedrichshain and the proposed substation at Marzahn is approximately 3.23 miles (5.2 km). Following a European-wide invitation for tenders in 1997, a continuous cable tunnel was built similar to that constructed for the Mitte-Friedrichshain section. The civil work that is in progress began with the construction of the three additional new shafts, two of which will be built using the 'Caisson system.' This system involves the complete construction of the shaft above the surface. The shaft is then sunk into position underground. Figure 7 shows one of the 'Caisson system' shafts in this section under construction. The decisions on the type of cable transmission technology (oil-filled cable, XLPE-cable or GIL) and civil engineering methods for this section will follow a similar evaluation process as that used for the Mitte-Friedrichshain section where for comparable technical standards, tenders were accepted on a minimum-cost criteria. This new connection, which will complete the diagonal link-up, is scheduled to be commissioned in autumn 2000.
380 kV Line from Marzahn to Neuenhagen Substations The construction of the 380-kV overhead transmission line from Marzahn to Neuenhagen substation is 9.5 miles (12 km) long and is the final section of the west-to-east Berlin interconnection. The route for this circuit that is within the city boundary was only made possible after agreement on extensive compensatory measures. These measures required Bewag to dismantle four existing overhead transmission lines at 110 kV and 220 kV (route length 20 miles [33 km]) after the commission of the 380-kV line. The new circuit route gives due consideration to town planning and landscape preservation requirements, avoiding residential and recreational areas and bird flight paths. Although the line route passes over industrial land parallel to railway lines, the landscape preservation report specified the steps necessary to minimize damage to the environment, remedial measures and compensation. The foundations for the transmission line towers were installed using a vibration-free method and the space for construction site work was also reduced to a minimum. The line construction work was subject to environmental monitoring by a site manager from an independent landscape architectural company that was also responsible for compensatory matters.
The public and private law licensing procedures for the transmission line route took almost three years to complete (1993-1996). Bewag engaged in wide scale public consultation that assisted with the general acceptance of this overhead line project by the authorities, media and public. The work began in March 1997 and the 7.5-mile (12 km) 380-kV transmission line was completed in December 1997. Figures 8 and 9 illustrate the erection of the towers and conductor stringing on the 380-kV overhead transmission line.
Berlin's Energy Supply Structure for the 21st Century Bewag was assigned the responsibility to re-establish the energy supply network for Berlin because it previously operated an isolated network in the western half of the city. This capital city needed a secure power system to take advantage of the latest power delivery technology, while taking into account the huge investment costs and the energy costs for consumers. The increasing environmental concerns required alternatives to conventional construction methods and transmission technologies. Bewag adopted a professional approach to this project by combining its system operational experience with the expertise of cable manufacturers and civil engineering contractors. This has produced a cost-effective supply network that incorporates innovative alternatives that are compatible with the environment.
The 380-kV diagonal connection through Berlin's load centers comprising substations, cables and overhead lines including all civil engineering work, which is expected to cost US$226 million (DM400 million), will be completed by the year 2000. The existence of this double circuit system will enable power to be imported from the UCPTE grid, securing a reliable energy supply to the city and enabling the existing Bewag power plants to operate at optimum efficiency.
Claus Georg Henningsen is head of the Cable Technology Department at Berliner Kraft-und Licht (Bewag) AG. He studied electrical energy technology at the University of Lubeck and at the Technical University of Berlin where he received his Dipl.-Ing. degree. Claus started his professional career at Bewag in 1979 and was project leader for the 380 -kV connection of Bewag to the UCPTE grid.
Klaus Polster is head of the Cable and Transmission Line Planning Section at Berliner Kraft-und Licht (Bewag) AG. He studied electrical energy technology at Berlin University and joined Bewag in 1979. He was project leader for the 400-kV XLPE cable tests at CESI, Milan, and since 1996, he has been the project leader for this 400-kV diagonal connection.
Dietmar Obst is the HV cable projects engineer at Bewag and was involved in the 400 kV cable test program at CESI, Milan. Dietmar studied electrical energy technology at Berlin University starting his career with Bewag in 1991. He is responsible for all cable laying and testing within the Berlin tunnel projects. His expertise includes all practical knowledge cable selection, cable accessories and cable configurations for all types of installations.