ABB Review Special Report - ABB - ABB Group

SwePol Linksets new environmentalstandard for HVDC transmissionLeif Söderberg, Bernt AbrahamssonSix cable links – all of them HVDC (high-voltage direct current) – arecurrently in service between the power grids of continental Europeand the Nordic region, with another five planned. The latest to bebrought on line is the SwePol Link, which connects the electricitynetworks of Poland and Sweden. It is unique in that, unlike previousinstallations that depend on electrode stations to transmit the returncurrent under ground or under water, it usescables to carry this current.The reason for all these links is thevital need to secure power systemreliability in each of the participatingcountries. They make it easier to optimizepower generation in an area inwhich different countries use differentmeans of power generation and havedifferent power demand profiles overa 24-hour period. Wet summers in theNordic region result in a considerablepower surplus, which can be sold tocountries that rely on more expensivefossil fuel-fired power plants. Conversely,any surplus power can be soldback during periods of low load.Power system reliability in the region isincreased by the addition of new HVDCcable links. In the event of grid disruptions,the rapid power balancing abilityof these links can be used to compensatefor fluctuations in frequency andvoltage. For example, it is technicallyfeasible to reverse the entire 600 MWpower throughput of the SwePol Linkin just 1.3 seconds, although this is nota feature that will be used in practice.Nevertheless, a typical emergency powermeasure could call for a DP ramp-up of300 MW within a few seconds to preventgrid failure if the voltage in southernSweden drops below 380 kV.All previous links of this kind use electrodestations off the coast to transmitthe return current under the sea, andthis has worked perfectly well. The firstsuch cable link was laid in 1954 betweenVästervik, on the Swedish mainland,and the Baltic island of Gotland.Since then, the power rating has beenincreased and the original mercury arcvalves in the converter stations havebeen replaced with thyristor valves.In the case of the SwePol Link, returncables were chosen as an alternativeto electrodes in order to pacify localresistance to the project, particularlyaround Karlshamn. The environmentalissues that were raised during planningof this link may also apply to futureinstallations.Lower emissions benefit theenvironmentThe power link between Sweden andPoland is the latest example of thegrowing economic cooperation betweenthe countries bordering the Baltic Sea.The cable, which was taken into commercialservice in June 2000, is a steptoward the large-scale power distributionpartnership that is known as theBaltic Ring [1].The new link allows power generationto be stabilized in both countries, wherethe seasonal and daily variations in demandcan differ considerably. The surplusthat builds up in the Nordic regionduring wet years has already been mentioned.In a really cold year it makesfinancial sense for this region to importPolish electricity generated from coalrather than start up a condensing oilfiredpower plant or a gas turbine.Polish imports of electricity via the linkwill in turn reduce environmental impactin that country. The predicted annualnet import of 1.7 TWh is expectedto reduce emissions from Polish powerplants by 170,000 tonnes of sulfur dioxideand 1.7 million tonnes of carbondioxide, according to calculations by theSwedish power company Vattenfall.Special ReportABB Review57