ABB Review Special Report - ABB - ABB Group

1KarlshamnBornholmBaltic SeaSwePol Link between Swedenand Poland.RonnebyKarlskrona1 x 2100 mm 2HVDC2 x 630 mm 2ReturnUstkaDarlowoThe power cable (blue) and the return cables(red) run on the same route, spaced 5 to 10 mapart in shallow water and 20 to 40 m apart indeep water.Poland’s forthcoming admission tothe EU will reinforce measures to reduceits enviromental impact, therebypromoting the import of Swedishhydropower.2RinghalsMalmöThe 250-km SwePol Linkexchanges power between 400-kVAC grids in Sweden and Poland.HemsjöKarlshamnSlupskKrajnikOskarshamnDunowoGdanskThe converter stations are near Karlshamn insouthern Sweden and at Slupsk, 12 km fromthe Polish coast.Cable company is set upSwePol Link AB was formed in 1997 toinstall, own and operate the cable linkbetween Sweden and Poland. It is apower transmission company that willsell electricity transmission servicesacross the link.A Polish subsidiary was formed in1998 to handle the local business. Onthe Swedish side the link will be usedprimarily by state-owned Vattenfall,although other companies will be ableto sign transmission agreements withSwePol Link.The new link is approximately 250 kmlong. It runs from Stärnö, just outsideKarlshamn in Sweden, past the Danishisland of Bornholm, and returns to landat the seaside resort of Ustka on theBaltic coast of Poland 1 , 2 .The Swedish converter station was sitedat Stärnö because a 400-kV station andthe Swedish main grid are nearby. Thisavoided having to build new overheadlines that would have marred theSwedish countryside. The Polish converterstation 3 is connected to thePolish 400-kV grid at Slupsk, about12 km from the coast.The link involved around 2500 manyearsof work for ABB, primarily at itsplants in Ludvika and Karlskrona. Bothstations are unmanned, although on-callpersonnel will be able to provide coverageat short notice.DC circuitThe land-based power grid is, of course,an AC system. However, for long underwaterlinks DC is the only viable solutionon account of the high capacitanceof submarine cables.Most cable links are monopolar systems,in which the return current is carriedthrough the ground and the sea.Power is transmitted over a high-voltagecable. It is a common misconceptionthat sea-water carries the returncurrent because of its high conductivity.However, most of the current travelsat a considerable depth through theearth.In the case of the SwePol Link the returncurrent is carried by two insulatedcopper conductors rated at 20 kV. Theuse of these conductors eliminates theneed for environmentally controversialelectrodes.The high-voltage cable 4 is around140 mm in diameter, of which the centralconductor takes up 53 mm. Insteadof being solid, this consists of coppersegments to make it more flexible.The segments are shaped individually,then rolled as a unit to achieve an effectivecopper cross-section in excess of99%. The rest of the cable consistsof various layers of insulation, sealantand armor. The 250-km long cable ismade up of four sections which are laidindividually and joined by the layingbarge.Visible parts of the linkThe visible parts of the link are thetwo converter stations at Stärnö andSlupsk. Located just a small distancefrom the center of Karlshamn, theStärnö station is next to an oil-firedpower plant that completely dominatesthe landscape. By siting the tall valvebuilding in a former quarry some10 meters deep, the station’s impacton the skyline is reduced even more.The power cables run 2.3 km from thestation to the sea.At the Polish end, the valve building isa prominent, but by no means ugly,landmark in the flat agricultural countryside.The Slupsk station is just over20 meters high and is situated about12 km from the Polish coast.Both the high-voltage and return cablesrun underground almost all the waybetween the stations. On land thisrequired clearing a five-meter wideswathe through the landscape when thecables were being laid. This will soonbe hidden, partly thanks to forest replanting.At sea about 85% of the cablecould be laid in a trench about onemeter deep to avoid damage by trawlersand anchors.The converter stations cannot only beseen, but heard too. This is because the58Special ReportABB Review