THE NORNED HVDC CABLE LINK A POWER TRANSMISSION

THE NORNED HVDC CABLE LINKA POWER TRANSMISSION HIGHWAY BETWEENNORWAY AND THE NETHERLANDSJan-Erik Skog1Kees Koreman2 Bo Pääjärvi3Thomas Worzyk4Thomas Andersröd51jan.skog@statnett.no, Statnett, N-0302 Oslo Norway2k.koreman@tennet.org, TenneT, NL-6800 AS Arnhem, The Netherlands3bo.paajarvi@se.abb.com, ABB Power Technologies, SE-77180 Ludvika, Sweden4thomas.worzyk@se.abb.com, ABB Power Technologies, SE-37123 Karlskrona, Sweden5thomas.andersrod@nexans.com, Nexans, N-0509 Oslo, NorwayABSTRACTThe NorNed HVDC Cable Link is underconstruction. The transmission connects the hydropower based Norwegian grid with fossil fuelled acsystem in The Netherlands. The 700 MW, 580 kmlong link will be the longest cable transmission inthe world and will be in commercial operationtowards the end of 2007.1.Otherwise is injected into the DC cables. Themidpoint in Feda is isolated from earth with anarrester to protect the midpoint from over-voltages.To optimize the rating of the over-voltageprotection in the midpoints a RC damping circuit isinstalled in both midpoints.The configuration is shown in Figure 1.INTRODUCTION 450 kVDC-cableThis paper describes the novel technology used inboth converter stations and in the submarine cablesystem especially suitable for an extremely longcable HVDC connection.The flexibility and speed in controlling the leveland direction of the power flow gives a number ofbenefits like increased security of electricity supply,improved utilization of the power plants, reducedCO2 emissions and assistance in furtherdevelopment of renewable wind power.Further the functionality in power exchange and thetrading mechanism is outlined.2.TECHNOLOGY2.1Transmission configurationThe main circuit configuration consists of a 12pulse converter 450 kV with the mid pointearthed and two cables. The arrangement is a veryattractive solution for a single converter blockscheme for extremely long cable transmissions. Thetransmission voltage is effectively 900 kV givingfairly low cable current and low losses. The totallosses are 3.7 % at 600 MW load. The DC link isdesigned to operate continuously at 700 MW whenall converter cooling equipment is in operation.The converter midpoint earth in Eemshavenconstitutes the zero DC Voltage potential referencefor the DC side. This is accomplished by a midpointreactor which also blocks 6-pulse harmonic currentsEemshaven-450 kVFedaFigure 1. NorNed main circuit configuration2.2Converter stationsThe converter stations are located at the Fedasubstation in Norway and at the Eemshavensubstation in The Netherlands.In Feda the AC side equipment is connected to the300 kV AIS substation and placed outdoors. InEemshaven the AC side equipment is connected tothe 380 kV GIS station by cables and the AC filtersare located indoors. Further the DC side equipmentin Eemshaven is installed in a DC hall to avoidproblems with flashover due to salt contaminationfrom the sea.The converter transformers are of single phase,three winding type. Six double-valves in the valvehall are arranged to provide the - 450 kV twelvepulse converter. The twelve-pulse converter has avoltage rating of 900 kV while the DC voltage toground is 450 kV. Each single valve has 120thyristors including three redundant devices. Thesmoothing reactors are 700 mH, oil-insulated typewith the bushings protruding into the valve hall andDC hall(NL). As the DC cables are entering straight

into the converter stations there are no DC filtersrequired.The AC filters and shunt capacitor banks providestotally 485 Mvar of reactive power in Feda and 432Mvar in Eemshaven.The configurations are as follows:Feda Two 100 Mvar filter banks each consistingof one double-tuned 11th/13th filter branchof 48 Mvar and one HP24 branch of 52MvarOne HP3 filter bank of 95 MvarTwo shunt capacitor banks each of 95Mvar.Eemshaven Two 110 Mvar filter banks each consistingof one double-tuned 11th/13th filter branchof 50 Mvar and one HP24 branch of 60Mvar Two shunt capacitor banks each of 106Mvar.The cable starts at Feda in Norway and runsthrough a 1.4km long tunnel down to a jointingchamber. Here the cable is jointed to the submarinecable. From there the cable runs through a 150mlong micro tunnel down to the seabed approx. 45munder sea level. Further on the cable goes 156kmout the Fedafjord and is jointed to the ABB singlecore cable.The deep part MI type cable has been developedextensively over the last 15 years and it is certifiedfor transmission of 800 MW at 500 kV, accordingto Electra No 72. The mechanical test on this cablewas conducted for 500 m depth according to ElectraNo. 68.The design parameters of the MI cable are: Power to be transmitted The given voltage level The transient voltage level The maximum allowable electrical stressesin no-load and full-load conditions12342.35Cable system67General89The cable system consists of a mass impregnatedpaper insulated cable type. This cable technology ismore than 100 years old and it was employed in thefirst submarine HVDC cable in 1954. Althoughother insulation materials have been used time andagain the MI type of insulation is the one over 90 %of all HVDC submarine transmission projectsemploy.This cable system is split in two, a shallow partdelivered by ABB and a deep part delivered byNexans Norway AS. The system is built as abipolar system consisting of two HVDC cables of580km each. The maximum transmission capacityis 700 er(mm)Conductor, copperConductor screenInsulationInsulation screenFabrick tapeLead sheath, F3Polyethylene sheath, MDPEBedding tapeReinforcementBedding tapeArmour, galvanized steel wires,two layersOuter serving/PP yarn12Deep part systemThe deep part system consists of: 2x1.5km Tunnel Cable NODE-L 450kV1x760mm2. 2x156km Submarine Cable NOVA-L450kV 1x700mm2. 2xOil filled Sealing End EOPU 450kV 2xTransition Joint Submarine to Tunnelcable.Both the submarine and the Tunnel cable is a MItype of cable.Nominalthickness(mm)Nominal Cable weight: 35.2 kg/mFigure 2 Drawing of Submarine MI cableShallow part systemThe Shallow cable covers the largest part of the 580km transmission distance. Beside the electric data,the design of submarine HVDC cables must takeother parameters into consideration, such as: Water depth107

Burial depthThermal resistivity of the sea bottomAnnual variation of the sea bottomtemperature and sea waterData related to the sea bottom conditions can onlybe achieved by a comprehensive route survey.Based on these data, and operational and testrequirements, the dimensions of the constructivelayers of the cable can be designed. As conditionschange along the route, the optimum cable designwould also be different for different route sectors.However, it is prudent to keep the number ofvariations small in a given project. For the NorNedsubmarine cable, three different designs from twosuppliers were chosen.The shallow part is covered by two different ABBcable designs:a) the single core HVDC cable. A pair of this cablewill run through approx 154 km of North Sea in upto 70 m of water. The cable is very similar to theDeep part cable depicted in Figure 2.b) the double-core FMI ( ”Flat Massimpregnated”) cable shown in Figure 3. This cablecounter-helical steel wire armouring protects theflat cable. It offers an extremely low outsidemagnetic field as the conductors with counteracting currents are only 100 mm apart. Also, bothcores can be laid in a single operation. Due to thesmall distance between the heat generatingconductors, the ohmic losses in the FMI cable arereduced by using a slightly larger cross section (790mm2) compared to the single-core cables (700mm2).The FMI cable will run along approx. 270 km ofwater.Although the tensional forces during cable layingare very moderate in the shallow waters, both ABBsubmarine cable types are provided with a strongdouble-helical steel wire armouring. The wirearmouring is a part of a cable protection systemwhich, together with burying of the cable into thesea bottom, protects the cable from external impactsfrom e.g. anchors and fishing gear. Statistics showthat the overwhelming majority of submarine cablefaults are caused by external impact. A convincingprotection system is important to keep availabilityhigh.A 28 km portion of the FMI cable is equipped witha fibre-optical temperature sensor embedded intothe armouring. See Figure 3. It enables a distributedtemperature measurement along the cable routeclosest to the Dutch coast. This route portion isparticularly interesting for temperature monitoringas the sea bottom morphology is prone toconsiderable changes. Temperature measurementscan potentially also reveal changes in the cableenvironment.Land cablesBeside the submarine cable the shallow part alsocomprises a pair of 790 mm2 single-core landcables. As the requirements for tensional strengthand impact protection are considerably lower thanfor the submarine cables the land cables have onlyone layer of steel wire armouring. The length of theland cables is approx. 1500 m. They are laid in anopen trench between the FMI landfall and theconverter station in Eemshaven.Figure 3. Submarine FMI cabledesign is a novelty. It comprises two independentcable cores each representing a complete electricsystem. The FMI cable comprises two independentcable cores each rated 450 kV dc put side by sideinto a common steel wire armouring. Each cablecore has its own insulation, metallic sheath andplastic sheath. An extruded plastic profile betweenthe cores supports the cable cores. A common3.AC SYSTEM CHARACTERISTICS3.1NorwayThe Norwegian AC system is characterised by itsextreme amount of hydropower. The highest

density of plants is found in the west. The systemtherefore is characterised by strong transmissionlines west – east to the most densely populatedareas around Oslofjord.When considering linking of the Dutch and theNorwegian systems it is important to note that thecoupling in fact is to the Nordic system (Nordel),ref. Figure 4.Figure 5 Actual production since 1975This great difference as compared to the continentalpower generation system is one of the main driversmotivating NorNed.3.2The NetherlandsThe converter station will be connected to theDutch AC-network in Eemshaven. This substationis an indoor GIS substation based on a breakerand-a-half scheme.Two generators are connected to the 380 kVsubstation in Eemshaven in addition to the HVDCconverter. Next to the two transformers, 220kV/380 kV with a rated power of 750 MVA each,connect the 380 kV substation to the adjacent 220kV substation.A single line diagram is given in Figure 6.EC-7EC-6HVDCFigure 4 Scandinavian gridIt is easy to see that the Norwegian grid is notparticularly strong in the south. This is a challengein the import situation and is now carefully beingstudied as part of the detailed engineering ofNorNed.An already planned upgrade of the grid betweenEvje and Holen north of Kristiansand will when inplace, improve the situation drastically.TR-1Meeden-1Meeden-2TR-2Figure 6 Schematic diagram substation EemshavenThe Norwegian AC system is also characterised bythe large variance in power production caused bythe greatly varying precipitation, ref. Figure 5.A four circuit 380 kV line, two of which arecurrently operated at 220 kV, connects thesubstation with the main 380 kV network in TheNetherlands. All circuits have a rated capacity of2500 MVA (4000 A). The circuits terminate insubstation Meeden where a coupling with theUCTE network exists.TWh/yearThe connection of the NorNed converter station tothe 380 kV network can be considered a strongconnection. The impedance of the system is lowleading to a high short circuit level of 18.5 GVA.Actual productionDemand forecast10% wet year inflow14012010090% dry year inflow80Total demandAverage hydropower production6040200Year1975 1980 1985 1990 1995 2000 2005 2010 2015 2020