00 ABB Review 4/2001 SwePol Link sets new environmental standard for HVDC transmission

Leif Söderberg, Bernt Abrahamsson

Six cable links – all of them HVDC (high-voltage direct current) – are currently in service between the power grids of continental Europe and the Nordic region, with another five planned. The latest to be brought on line is the SwePol Link, which connects the electricity networks of Poland and . It is unique in that, unlike previous installations that depend on electrode stations to transmit the return current under ground or under water, it uses 20-kV XLPE cable to carry this current. The high-voltage HVDC submarine cable used for the SwePol Link is designed for 600 MW at 450 kV.

he reason for all these links is the technically feasible to reverse the entire onds to prevent grid failure if the voltage T vital need to secure power system 600 MW power throughput of the in southern Sweden drops below 380 kV. reliability in each of the participating SwePol Link in just 1.3 seconds, although With all previous links of this kind, countries. They make it easier to this is not a feature that will be used in electrode stations off the coast transmit optimize power generation in an area in practice. Nevertheless, a typical emer- the return current under the sea, and this which different countries use different gency power measure could call for a has worked perfectly well (Table 1). The means of power generation and have ∆P ramp-up of 300 MW within a few sec- first such cable link was laid in 1954 different power demand profiles over a 24-hour period. Wet summers in the

Nordic region result in a considerable Table 1: Existing HVDC links across the power surplus, which can be sold to countries that rely on more expensive Name Capacity Year fossil fuel-fired power plants. Gotland 2 × 130 MW 1954, 1983, 1987 Conversely, any surplus power can be (converted to bipolar) sold back during periods of low load. Konti-Skan 250 + 300 MW 1965, 1988 Power system reliability in the region Skagerrak 2 × 250 + 440 MW 1977,1993 is increased by the addition of new Fenno-Skan 500 MW 1989 HVDC cable links. In the event of grid Baltic Cable 600 MW 1994 disruptions, the rapid power balancing 600 MW 1995 ability of these links can be used to SwePol Link 600 MW 2000 (first to use compensate for fluctuations in frequency return cables) and voltage. For example, it is

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1 SwePol Link between Sweden and Poland. The power cable (blue) and the return Karlshamn Ronneby Karlskrona cables (red) run on the same route, being spaced 5 to 10 meters apart in shallow water and 20 to 40 meters apart in deep water.

between Västervik, on the Swedish region to import Polish electricity 1 x 2100 mm2 HVDC mainland, and the Baltic island of generated from coal rather than start up Gotland. Since then, the power rating a condensing oil-fired power plant or a has been increased and the original gas turbine. Bornholm 2 x 630 mm2 mercury arc valves in the converter Polish imports of electricity via the Return stations have been replaced with link will in turn reduce environmental thyristor valves. The Gotland link has impact in that country. The predicted Baltic Sea shown excellent reliability and has annual net import of 1.7 TWh is expect- operated without interruption for eight ed to reduce emissions from Polish Ustka years since the switch to thyristor valves. power plants by 170,000 tonnes of sulfur Darlowo This link has also been converted from dioxide and 1.7 million tonnes of carbon monopolar to bipolar. dioxide, according to calculations by the In the case of the SwePol Link, return Swedish power company . between Sweden and Poland. It is a cables were chosen as an alternative to Polish exports of electricity over the power transmission company that will electrodes in order to pacify local link will also reduce environmental sell electricity transmission services resistance to the project, particularly impact in the area in the long term. across the link. around Karlshamn. The environmental The country’s forthcoming admission A Polish subsidiary was formed in issues that were raised during planning to the EU will reinforce this, since it will 1998 to handle the local business. On of this link may also apply to future have to introduce power plants that use the Swedish side the link will be used installations. modern flue-gas emission control primarily by state-owned Vattenfall, technology. although other companies will be able Lower emissions benefit the Poland is currently undertaking an to sign transmission agreements with environment extensive program of privatization. This SwePol Link. The power link between Sweden and requires that all power generation and The new link is approximately Poland is the latest example of the electricity distribution be privatized 250 km long. It runs from Stärnö, just growing economic cooperation between within three years. Following acquisition, outside Karlshamn in Sweden, past the the countries bordering the Baltic Sea. investors must guarantee an investment Danish island of Bornholm, and returns The cable, which was taken into program that involves upgrading to land at the seaside resort of Ustka on commercial service in June 2000, is a generating equipment to meet EU the Baltic coast of Poland 1 , 2 . step towards the large-scale power environmental standards. One example The Swedish converter station was distribution partnership that is known as of this is acquisition by Vattenfall of sited at Stärnö because a 400-kV station the Baltic Ring [1]. combined heat and power (CHP) plants and the Swedish main grid are nearby. The new link allows power in Warsaw, where the company has This avoided having to build new generation to be stabilized in both committed itself to invest at least overhead lines that would have marred countries, where the seasonal and daily USD 340 million in environmental the Swedish countryside. The Polish variations in demand can differ improvements over the next ten years. converter station 3 is connected to the considerably. The surplus that builds up Polish 400-kV grid at Slupsk, about in the Nordic region during wet years Cable company is set up 12 km from the coast. has already been mentioned. In a really SwePol Link AB was formed in 1997 to The total cost of the link is estimated cold year it makes financial sense for this install, own and operate the cable link at about US$ 250 million and has

64 ABB Review 4/2001 Crossing the seas with HVDC

In any long AC power cable link, the reactive power bipolar configuration. It is actually two monopolar flow due to the high cable capacitance will limit the systems combined, one at positive and one at negative maximum possible transmission distance. As a result, polarity with respect to ground. Each monopolar side over a 40-km or so stretch of AC submarine cable, the can operate on its own with ground return; however, charging current supplied from shore fully loads the if the current at the two poles is equal, each pole’s cable and leaves no room for transmitting real power. ground current is cancelled to zero. In such cases, With DC there is no such limitation, which is why, for the ground path is used for short-term emergency long cable links, HVDC is the only viable technical operation when one pole is out of service. alternative. Another good reason for using DC cable is In a monopolar metallic return system, return that it is much cheaper than AC cable. current flows through a conductor in the form of a In an HVDC system, electric power is taken from medium-voltage cable, thus avoiding potential one point in a three-phase AC network, converted to problems associated with ground return current. DC in a converter station, transmitted to the receiving point by submarine cable and then converted back to AC in another converter station and injected into the receiving AC HVDC sea cable network. HVDC power transmission cable AC system AC system schemes can be variously configured. The basic HVDC cable transmission Ground return scheme is a monopolar installation that Sea electrode anode Cathode uses the earth and sea to return the Simplified single-line diagram of a typical modern current. The sea return reduces the cost monopolar HVDC cable link with ground return of the interconnection since only one cable is necessary between the two converter + HVDC stations. Losses are also kept to a minimum as the return path has a huge cross section, which makes the resistance negligible. The only losses are due to the 50% transmission power AC systemin emergency AC system voltage drops at the anode and the cathode. The electrodes have to be located well away from the converter - HVDC stations and the main HVDC cable to avoid corrosion of pipelines or other Simplified single-line diagram of a bipolar HVDC cable transmission link metallic structures in the vicinity as well as direct current pick-up in transformer

neutrals. The good conductivity of the HVDC earth and seawater makes it easy to AC system AC system design the electrodes, and it can be said that field experience with monopolar transmissions has been excellent. <5 kV DC A further development of the Simplified single-line diagram of a typical modern monopolar transmission scheme is the monopolar HVDC cable link with metallic return cable

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2 The 250 kilometers long SwePol Link exchanges electrical power between the 400-kV AC grids in Sweden and Poland. The converter stations are located near Karlshamn in southern Sweden and at Slupsk, 12 kilometers from the Polish coast.

DC circuit water carries the return current because

Ringhals The land-based power grid is, of course, of its high conductivity. However, this is an AC system. However, for long Oskarshamn not the case; most of the current travels Hemsjö underwater links DC is the only viable at a considerable depth through the earth. Karlshamn solution on account of the high capaci- In the case of the SwePol Link the tance of submarine cables. The SwePol return current is carried by two insulated Malmö Link basically builds on ABB’s proven copper conductors rated at 20 kV. The Slupsk Gdansk HVDC rectifier technology, which was use of these conductors eliminates the first used in 1954. In the meantime, need for environmentally controversial Dunowo Krajnik performance has been further improved, electrodes. losses have been reduced and control Another way to reduce any negative methods have been continuously side effects of electrodes is to use bipolar involved around 2500 man-years of work upgraded. technology, which becomes financially for ABB, primarily at its plants in Ludvika Most cable links are monopolar systems, viable when the capacity exceeds 1000 and Karlskrona. Both stations will be in which the return current is carried MW. In this case the current carried by unmanned, although on-call personnel through the ground and the sea. Power the electrodes and earth is due entirely will be able to provide coverage at short is transmitted over a high-voltage cable. to the small imbalance that can never be notice. It is a common misconception that sea- entirely eliminated by the converters.

3 Valve building in the Slupsk (Poland) converter station

66 ABB Review 4/2001 5 Only the cooling assemblies of the fully enclosed and soundproofed transformers are visible in the Slupsk station.

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4 SwePol Link power cable. The of various layers of insulation, sealant Both the high-voltage and return HVDC submarine cable is rated 600 MW and armor. The 250-km long cable is cables run underground almost all the and 450 kV DC. Its overload capacity is made up of four sections which are laid way between the stations. On land this 720 MW at temperatures below 20°C. individually and joined by the laying required clearing a five-meter wide swathe 1 Conductor barge. through the landscape when the cables 2 Conductor screen were being laid. This will soon be hidden, 3 Insulation Visible parts of the link partly thanks to forest replanting. At sea 4 Insulation screen The visible parts of the link are the two about 85% of the cable could be laid in a 5 Metal sheath converter stations at Stärnö and Slupsk. trench about one meter deep to avoid 6 Protection/bedding Located just a small distance from the damage by trawlers and anchors. 7 Reinforcement center of Karlshamn, the Stärnö station is The converter stations cannot only 8 Armor next to an oil-fired power plant that be seen, but heard too. This is because 9 Serving completely dominates the landscape. By the eddy currents that flow in all power siting the tall valve building in a former transformers generate noise at a fre- quarry some 10 meters deep, the quency of 100 Hz. Converter stations The high-voltage cable 4 is around station’s impact on the skyline is reduced also produce higher frequency noise 140 mm in diameter, of which the central even more. The power cables run 2.3 km that can be irritating to people living conductor takes up 53 mm. Instead from the station to the sea. nearby. It was clear that special sound- of being solid, this consists of copper At the Polish end, the valve building proofing would be necessary. Following segments to make it more flexible. is a prominent, but by no means ugly, calculations and measurement of the The segments are shaped individually, landmark in the flat agricultural noise level and noise propagation, the then rolled as a unit to achieve an countryside. The Slupsk station is just transformers and reactors were enclosed. effective copper cross-section in excess over 20 meters high and is situated about The filter capacitor cans are equipped of 99%. The rest of the cable consists 12 km from the Polish coast. with a noise-reducing device5 .

ABB Review 4/2001 67 A magnetic field with minimal cables. It is not practicable to lay the use of return cables has no major effect effect on the surroundings high-voltage cable at the same time as on magnetic field strength. And, anyway, Whenever electricity is transmitted the two return cables, which means that modern ships no longer depend on through a conductor it generates a they cannot be laid right next to each magnetic compasses. magnetic field around it. Since DC is other. They also have to be separated But what are the possible effects on used, the field is of the same type as the because of the heat they generate. In animal life? Experience with previous earth’s natural magnetic field. This is shallow water the HV cable is laid cable links has shown that they do not completely different to the AC fields 5–10 meters from the return cables. The affect fish or other marine organisms. normally produced, for example, around resulting magnetic field measured on the Nor do they affect the vital homing overhead lines. surface of the sea is typically 80% of that ability of eels and salmon. This is Measurements have shown that the obtained around a high-voltage cable in especially important since these fish magnetic field around the cable at a a monopolar installation. The equivalent migrate regularly during the course of distance of six meters is equal in strength figure at 100 meters depth with their lives and it is essential that their to the earth’s natural magnetic field, 20–40 meters separation is typically 50%. sense of direction remain unimpaired. while at a distance of 60 meters its Further away from the cables there is Back in 1959 a study was carried out intensity drops to just one tenth of that an even greater percentage reduction in to determine how the Gotland cable had field. the magnetic field, added to the fact that affected the marine environment [2]. This The magnetic field resulting from the the absolute value of the magnetic field was followed by exhaustive studies on combination of 1 + 2 cables varies with at this distance is insignificant compared the Fenno-Skan link (Sweden–Finland) the depth and relative spacing of the with the earth’s magnetic field. Thus, the and the Baltic Cable (Sweden–)

68 ABB Review 4/2001 [3]. The reports were unanimous: marine titanium mesh and copper cables for the it mostly occurs in molecular form. In life is affected neither by the magnetic cathode. The following competing time it breaks down into its component field nor by any chemical reactions. The reactions take place at the anode: parts. facts speak for themselves. Eels continue There was a suspicion that the ⇒ + - to find their way to the Baltic Sea, 2 H2O 4H + O2 (g) + 4e chlorine gas and hypochlorous acid that despite having to cross seven cables on are formed would react with biological - ⇒ - the way [4, 5]. 2 Cl Cl2 (g) + 2e material in the vicinity of the electrodes, resulting in the formation of compounds No chlorine formation (g) in the above formulae indicates that such as polychlorinated hydrocarbons, The originally proposed monopolar these elements are in gaseous form. which include PCBs. Studies on the solution, which would have used The amount of chlorine gas generated Baltic Cable have ruled out this concern electrodes to transmit the return current depends on the temperature, the [3]. No accumulation of organic chlorine undersea, has been replaced by an chloride content of the seawater and the was observed in the surrounding alternative solution in which return reaction energies. It reacts almost biomass. cables form a closed circuit. Any exclusively with water as follows: To put things in the right perspective concerns about chlorine formation have it is worth comparing the described ⇒ - + therefore been entirely eliminated, since Cl2 (g) + H2O HClO + Cl + H process with the common chlorination of no electrolysis can occur. drinking water, in which the hypochlorite The electrodes that would have been At low pH the hypochlorous acid that is concentration is at least 100 times higher used have an anode made of fine formed could be ionized, but in seawater than the value measured at the anode.

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Benefits versus cost Table 2: Identified metal objects on Swedish coast that could Using the described return cables does, have been affected by the SwePol Link of course, have some advantages, among Object length Distance from electrode Example them the reduced magnetic field strength along the cable route and the fact that More than 25 m Less than 5 km Cable support they cause neither chlorine formation More than 200 m 5 – 10 km Sewage pipe Ø 1.2 m nor corrosion of underground metal More than 1000 m 10 – 20 km 10-kV cable objects. And they also allowed a solution District heating that addressed the environmental Copper shield concerns of various groups of society. In around building any final count, however, these benefits More than 5000 m 20 – 50 km Protective shield (Cu) have to be measured against the extra cost. In the case of the SwePol Link, for example, they added about 5% to the cost of the project. No corrosion SwePol Link a list was therefore made of The return cables used for the SwePol all the metal objects that could be at risk Link eliminate the risk of corrosion, and (Table 2 ). this would seem to be the only tangible Objects that are at risk of corrosion advantage they offer. due to leakage currents require some Authors DC cable links that use electrodes, form of active protection, such as Leif Söderberg do lead to leakage currents in the earth. sacrificial or cathodic protection. SwedPower AB The return current passing through the The return current can also find a SE-162 16 Stockholm Sweden earth takes the shortest path. On its route through other power distribution [email protected] route between the electrodes some of systems that have multiple earth points Fax: +46 (0) 8 739 62 31 the current may pass through long metal close to the electrode. This gives rise to Bernt Abrahamsson objects, such as railway tracks, gas pipes a DC component in the AC grid, which ABB Power Systems AB and cable shielding. Electrolytic reactions can lead to undesirable DC magnetiz- SE-771 80 Ludvika Sweden could occur between this metal and its ation of transformers. The problem can [email protected] surroundings, possibly leading to generally be solved by modifying the Fax: +46 (0) 240 807 63 corrosion. During the planning of the grounding of the AC system.

References [1] The making of the Baltic Ring. ABB Review 2/2001, 44–48. [2] W. Deines: The influence of electric currents on marine fauna. Cigré study committee no 10, 1959. [3] Anders Liljestrand: Kontrollprogram bottenfauna, bottenflora (Inspection program: bottom flora and fauna). Baltic Cable. Marin Miljöanalys AB, 1999. [4] Håkan Westerberg: Likströmskablar, ålar och biologiska kompasser (DC cables, eels and biological compasses). Fiskeriverkets Kustlaboratorium, 1999. [5] E. Andrulewicz: Field and laboratory work on the impact of the power transmission line between Poland and Sweden (SwePol link) on the marine environment and the exploitation of living resources of the sea. Sea Fisheries Institute Report, Gdynia, Feb 2001.

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