A crucial step forward in the manufacture of extra high voltage XLPE cable systems has been taken in the installation of six XLPE cables rated at 525 kV in a hydropower station in China (Figures 1 and 2). Interconnection of underground substations promises to be an increasing market for high capacity XLPE cables.

The cables will connect the six Francis turbine – generator units of the 1350 MW Dachaoshan hydropower station on the Lancang River in Yunxian, Yunnan Province to the transmission network. The 525 kV cables, as well as two 220 kV cables running in parallel with the 525 kV cables will be supplied by Mannheim manufacturer ABB Energiekabel GmbH of Germany.

The approximately 2500 m long 525 kV cables will run through a 145 m deep vertical shaft to connect the Francis turbine-generator unit transformers with the SF6 gas insulated substations, 12 XLPE to SF6-GIS terminations being used in each case. The scheduled start up date for this gigantic project is October of 2003, although unit 1 will come on line at the end of 2001.

The hydropower plant

The Lancang River flows through the west of Yunnan Province and then runs southwards, eventually merging with the Mekong River on its way to the sea. The river is 1240 km long with a total water drop of 1780 m within the boundaries of Yunnan Province as shown on the location map, Figure 3.

Mean annual discharge is 1330 m3/s with design flood discharge of 18 200 m3/s. Energy potential is 25 500 MW and exploitable hydropower is reckoned to be 23 480 MW.

The Dachaoshan Dam site is 131 km upstream of the completed Manwan hydropower station and 600 km from Kumming City, the Capital of Yunnan Province. The entire discharge is used for electricity generation. Total installed capacity is 1350 MW and firm capacity is 363.1 MW. The mean annual energy output is 5931 GWh and will reach 7021 GWh after the completion of the upstream reservoir for the Xiaowan Hydropower Station.

The roller compacted concrete dam has a maximum height of 111 m above its foundations, with its crest level at elevation 906 m, a maximum crest length of 460 m including 254 m of dam section in the river bed. The vast reservoir holds a volume of 940 x 106 m3 of water, with a regulating storage capacity of 367 x 106 m3.

The underground powerhouse system shown in Figure 4 includes a main power house measuring 233.5 m x 62.33m x 26.4m, an auxiliary plant chamber, a generator transformer cavern, a bus bar gallery and other appurtenant chambers. The power tunnel system has its pressure power tunnels, draft tube tunnels, tail race surge chambers and tunnels configured as in Figure 5.

Maximum head on the six General Electric Canada Ltd 229.6 MW Francis turbine-generator units is 72.5 m. ABB Generation AB HPC640 digital governor systems are used for turbine control.

Classified under the Corporation Law of the Peoples Republic of China as an IPP, the Yunan Dachaoshan Hydropower Co. Ltd was established as a joint corporation of the State Development and Investment Corporation, Yunnan Hongta Industrial Co. Ltd, Yunnan Development and Investment Corporation and Yunnan Electric Power group Co. Ltd in a shareholding ratio of 5:3:1:1.

Substation connection

There are two 525 kV XLPE cable systems to carry the electrical output from the generator transformers to the 11/500 kV grid connection transformers and substation, each serving two turbine-generator units. These are three phase systems using a total of six cables, of 800 mm2 conductor cross section, with twelve specially designed, type tested and certified SF6-GIS terminations. Each of these systems is rated to carry the entire output of all four units in the event of an outage.

For the remaining two turbine generator units two 220 kV cable systems, which have just been ordered from ABB Energiekabel in Mannheim, are used to connect separate unit transformers to another substation. This is a highly innovative system giving great flexibility and redundancy with substantial operating margins.

Layout and cable routing

Of the six turbine-generators, units 1 and 2 and units 3 and 4 will be made as two paired connections by SF6 duct, through the breakers and main transformers, then through two 525 kV XLPE cables, then connected to the 525 kV GIS.

Units 5 and 6 will be made as two generator-transformer connections through two 245 kV XLPE cables, then connected to a 245 kV GIS. The six units and their auxiliary equipment are all installed in the underground powerhouse. From the main transformer hall to the 525 kV GIS building the cables will be laid through a 145 m high vertical shaft. Total route length is about 400m.

Cable design

XLPE cables of higher power capacity and transmission distance have already been used and are allowing the building up of a wealth of operation experience in such projects as the 420kV Copenhagen Metropolitan Link, TEPCO’s 500 kV central Tokyo underground transmission network, and the Berlin east-to-west underground cable link. This latter example has two three phase circuits of 400 kV cables rated to carry maximum loadings in excess of 1600 MVA apiece (see Modern Power Systems, March 1999 and Germany Supplement, September 2000). These examples are all for applications in densely populated urban electricity distribution systems, but the working voltage of 525 kV is a major step forward in underground XLPE cable technology for use in large power plant complexes, with particular economic advantages for the kind of application encountered in Dachaoshan where the maximum power should not exceed 500 MVA by much.

The extrusion of the dielectric is the key process in the manufacture of XLPE insulated cables. The quality of the insulation system including the semiconducting shields is largely dependent on the extrusion parameters and the die design together with the quality and processing of the raw materials for the continuous vulcanisation process.

Figure 6 illustrates the construction of the new cable, and the type testing process is shown in Figure 1. The copper conductor cross-section of 800 mm was determined for continuous load and is of standard round, stranded and compacted design.

The conductor is covered by the conductor screen, the insulation and the insulation screen. These three layers are extruded simultaneously by means of a triple extruder, thus achieving the smooth surface between insulation and conductor and insulation screens. The next layer consists of conductive tapes that are applied over the insulation screen.

The screen consists of copper wires and a counter helix of copper tape covered with swelling powder that forms a longitudinal water barrier when it comes into contact with water.

The cable features a radially water-tight laminated sheath as protection against external influences. The laminated sheath consists of a copolymer-coated aluminium tape fix-bonded to the outer polyethylene sheath. While the PE-sheath provides protection against mechanical damage, the aluminium tape prevents radial penetration of water or water vapour. A flame-retardant layer is extruded together with the PE sheath to protect the cables in case of fire.

The SF6 – GIS termination design is a further development of the concept so successfully tested in the one year 400 kV prequalification test for XLPE cable systems at CESI (Centro Elettrotecnico Sperimentale Italiano) conducted on behalf of BEWAG and featuring the stringent requirements laid down by CIGRÉ. The construction of the termination is is illustrated in the cutaway drawing in Figure 7.

Prequalification test

The following operational tests were carried out in December 2000.

Cable: 100 m XLPE cable, conductor cross-section 1×800 mm2

Accessories: GIS terminations, type EHSV 525 (with PD sensors)

Duration: 1 year, start 16.10.99, end 01.12.00

Voltage: AC voltage 500 kV (1.7 Vo)

Load cycles: > 180 cycles

Conductor temperature: 90°C (+5/-0)

Impulse voltages: Switching impulse test: +/- 1175 kV, 10 times Lightning impulse test: +/- 1550 kV, 10 times

Final examination of the cable system after completion of the tests above

Measurement of PD intensity at the GIS terminations.

Project schedule

Initial construction of the Dachaoshan hydropower Project, which represents an investment of RMBY 8.87 billion, commenced in August 1997. River closure was successfully achieved on the 10th of November 1997. The first generating unit is scheduled to be put into operation in 2001, three more units in 2002 and the last two units, with the 220 kV cables, in 2003. Cable manufacture and installation is reported to be on schedule to meet these dates.