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 <strong>ABB</strong>, 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 the58<strong>Special</strong> <strong>Report</strong><strong>ABB</strong> <strong>Review</strong>
3Valve building in the Slupsk (Poland) converter stationof their lives and it is essential thattheir sense of direction remain unimpaired.Back in 1959 a study was carried outto determine how the Gotland cablehad affected the marine environment[2]. This was followed by exhaustivestudies on the Fenno-Skan link (Sweden–Finland)and the Baltic Cable(Sweden–Germany) [3]. The reportswere unanimous: marine life is affectedneither by the magnetic field nor byany chemical reactions. The facts speakfor themselves. Eels continue to findtheir way to the Baltic Sea, despitehaving to cross seven cables on theway [4, 5].eddy currents that flow in all powertransformers generate noise at a frequencyof 100 Hz. Converter stationsalso produce higher-frequency noisethat can be irritating to people livingnearby. It was clear that special soundproofingwould be necessary. Followingcalculations and measurement of thenoise level and noise propagation, thetransformers and reactors were enclosed.The filter capacitor cans are equippedwith a noise-reducing device.A magnetic field with minimal effecton the surroundingsWhenever electricity is transmittedthrough a conductor it generates amagnetic field around it. Since DC isused, the field is of the same type as theearth’s natural magnetic field. This iscompletely different to the AC fieldsnormally produced, for example,around overhead lines.Measurements have shown that the magneticfield around the cable at a distanceof six meters is equal in strength to theearth’s natural magnetic field, while at adistance of 60 meters its intensity dropsto just one tenth of that field.The magnetic field resulting from thecombination of 1 + 2 cables varies withthe depth and relative spacing of thecables. It is not practicable to lay thehigh-voltage cable at the same time asthe two return cables, which meansthat they cannot be laid right next toeach other. They also have to be separatedbecause of the heat they generate.In shallow water the HV cable islaid 5–10 meters from the returncables. The resulting magnetic fieldmeasured on the surface of the sea istypically 80% of that obtained around ahigh-voltage cable in a monopolarinstallation. The equivalent figure at100 meters depth with 20–40 metersseparation is typically 50%.Further away from the cables there isan even greater percentage reductionin the magnetic field, added to thefact that the absolute value of the magneticfield at this distance is insignificantcompared with the earth’s magneticfield. Thus, the use of returncables has no major effect on magneticfield strength. And, anyway, modernships no longer depend on magneticcompasses.But what are the possible effects onanimal life? Experience with previouscable links has shown that they donot affect fish or other marine organisms.Nor do they affect the vital homingability of eels and salmon. Thisis especially important since these fishmigrate regularly during the courseNo chlorine formationThe originally proposed monopolarsolution, which would have used electrodesto transmit the return currentundersea, has been replaced by analternative solution in which returncables form a closed circuit. Any concernsabout chlorine formation havetherefore been entirely eliminated,since no electrolysis can occur.34841 Conductor2 Conductor screen3 Insulation4 Insulation screen5 Metal sheathSwePol Link power cable, rated600 MW and 450 kV DC. Itsoverload capacity is 720 MW attemperatures below 20°C.1256796 Protection/bedding7 Reinforcement8 Armor9 Serving<strong>Special</strong> <strong>Report</strong><strong>ABB</strong> <strong>Review</strong>59