December 2002 PEi22 www.peimagazine.com

HVDC SystemsBo Normark, ABB Power Technologies

The Cross Sound Cable Interconnector uses HVDC Light technology to provide a vital submarine cable connectionbetween the transmission grids of New England and Long Island, New York.

istorically, the Long Island grid hasonly been connected to the US maingrid through New York City and

through a small interconnection in south-west Connecticut. But for at least 30years there have been plans to build asubmarine cable connection across theLong Island Sound to provide a directconnection with the New England trans-mission grid. Concerns about the poten-tial environmental impact of such aninterconnector, especially on the delicateaquatic ecosystem, have prevented previ-ous schemes from gaining the requiredstatutory approvals.

However, ABB together withTransÉnergie US has successfully devel-oped a $120 million project utilizing envi-ronmentally friendly High Voltage DirectCurrent (HVDC) Light technology to pro-vide a 330 MW, 40 km link of cable buriedbelow the ocean floor, with short under-ground sections connecting to converterstations on each shore.

Improved reliabilityThe Cross Sound Cable is a critical tool forthe Independent System Operators (ISO)in New York and New England since itwill enable them to improve the reliabilityof their power supply systems by increas-ing their capability to share generationplant capacity. It is estimated that it could

reduce the potential for blackouts inConnecticut by more than 50 per cent.

The Cross Sound Cable will provide eco-nomic benefits to the regions of Connect-icut, New York and New England by facil-itating electricity trading among powergenerators and customers and promotingmarket competition.

The need for access to more power isparticularly important to the Long Island

Power Authority (LIPA), which servesnearly 1.1 million electric customers andhas experienced major increases indemand in recent years. This is largely dueto a 20 per cent rate reduction in 1998 andis compounded by a real estate boom,which has sparked the development ofexpansive homes on the northeast andsoutheast forks of the island.

During the months of June, July andAugust 2002, LIPA delivered 6 609 112MWh of electricity, which is a ten per centincrease from last year’s record figures, andrepresents a 22 per cent increase in summerpower demand since LIPA took ownershipof the transmission system in 1998.

Merchant transmission linkThe builder, owner and operator of the

interconnector is Cross Sound CableCompany, owned by United CapitalInvestments, the unregulated subsidiary ofUnited Illuminating Company andTransÉnergie US, a subsidiary of the trans-mission division of Hydro-Quebec, whichconceived the project as a privately-ownedmerchant transmission link, rather than aconventional regulated transmission link.

So rather than being solicited or plannedby an ISO or other regulatory authority itis driven by market-based opportunities.The investors have assumed 100 per centof the financial risk of the project, forego-ing any guaranteed recovery of theirinvestment through mandatory charges toratepayers. Instead it will charge thosecustomers who use it.

An underwater cable routed across LongIsland Sound was the natural choice tolink the Long Island and New Englandpower grids since the path for a tradition-al land-based transmission line wouldadversely impact considerably more peo-ple and property along its path. HVDC,rather than conventional HVAC, is now

H

“Today, the combined transmission capacity of all the

sub-sea and underground HVDC cable links in the world is more than

8000 MW”

Cross sound goes underground

Figure 1. The Cross Sound Cable provides a high voltage subsea power interconnection linking the Long Islandand New England power grids. The 330 MW link uses HVDC Light technology

www.peimagazine.com 23PEi December 2002

well established as the optimum transmis-sion technology for this type of submarinecable link. Indeed, the world’s first com-mercial HVDC link – built by ABB nearly50 years ago – was via a submarine cableproviding a 20 MW interconnectionbetween the island of Gotland and themainland of Sweden. Today, the combinedtransmission capacity of all the sub-seaand underground HVDC cable links in theworld is more than 8000 MW.

HVDC LightABB’s new HVDC Light technology wasselected for the Cross Sound Cable. Whereasclassical HVDC is most cost effective in thehigh power range, above approximately 250MW, HVDC Light comes in unit sizes rang-ing from a few tens of MW up to 330 MW,and for DC voltages up to +/-150 kV.

HVDC Light consists of two elements:converter stations and a pair of advancedtechnology cables – the DC circuit is notconnected to ground so two conductorsare needed. The converter stations arevoltage source converters employing stateof the art turn-on/turn-off insulated gatebipolar transistor power semiconductors.The circuit is therefore quite different toclassical HVDC, which employs converterstations based on thyristor valves.

The HVDC Light cable is of extrudedconstruction and the selected materialsprovide high mechanical strength, highflexibility and low weight. They are pro-tected by solid insulation and contain noinsulating or cooling fluids, unlike someolder cable systems still in operation,which makes them environmentally friend-ly. The cables will have an installed life ofat least 40 years.

Unlike conventional HVDC, HVDC Lightdoes not rely on the AC network’s ability tomaintain a stable voltage and frequency.This means that less reinforcement is

required for the local grid as wellas providing extra flexibilityregarding the location of theconverters in the AC system.

The HVDC Light design isbased on a modular conceptwith a number of standardizedsizes. Most of the equipment isinstalled in enclosures at thefactory, which makes the fieldinstallation and commission-ing short and efficient (typical-ly three to four weeks com-pared with the three or fourmonths required for conven-tional HVDC).

HVDC Light stations arecompact and need little spaceand can easily blend into the local sur-roundings. The stations are designed to beunmanned and are virtually maintenancefree with operation either carried outremotely or even automated. No commu-nication links are required between theconverter stations. HVDC Light also

comes equipped with measurement andcontrol systems that enable power trading.

Prior to the Cross Sound Cable project,HVDC Light was already proven in projectssuch as Gotland (Sweden), Directlink (NewSouth Wales to Queensland, Australia) andEagle Pass (US to Mexico).

Cable pathThe Cross Sound Cable is designed to pro-vide a 330 MW connection between NewEngland and Long Island, operating at +/-150 kV DC and 1175 A. The cable’s

southern terminal on Long Island is a newsubstation at the site of the decommis-sioned Shoreham nuclear power station inBrookhaven, New York, where the DC isconverted to 138 kV AC. A short underground land cable connects the station to the submarine cable.

The submarine cable heads north acrossLong Island Sound for 40 km, before com-ing to land on the eastern shore of NewHaven Inner Harbour. The cable route wascarefully chosen to minimize environmen-tal impacts. The northern terminal is anew converter station, which converts theDC to 345 kV AC.

Prior to burial, the two 125 mm diame-ter DC cables were bundled and laid onthe sandy seabed within a precisely definedcorridor across Long Island Sound. It wasdeployed by the specially designed sea-

HVDC Systems

“The submarine cable heads north across Long Island

Sound for 40 km, before coming to land on the eastern shore of New

Haven Inner Harbour”

Figure 2. The SmartJet hydraulic plough buried the cable 2 m belowthe sandy seabed. The cable laying process was completed in four days

Figure 3. The Shoreham converter station marks the cable’s southern terminal on Long Island

Table 1. HVDC cable transmissionoffers many technical advantages

compared with HVAC:

• For cable links longer than around 40 km,HVDC provides lower investment costs. ForCross Sound the saving gained frominstalling only two DC cables bundled in asingle trench instead of three AC cables,which have to be widely spaced in separatetrenches, more than compensates for thecost of the AC/DC converter stations.

• Long AC cables produce high amounts ofreactive power, requiring shunt reactors atboth ends. In extreme cases the reactive cur-rent may seriously reduce the active powertransmission capability.

• HVDC links can connect two asynchronouspower grids and are ideal for cases where it is notfeasible to establish a synchronous connection.

• In an AC system it is not possible to directlycontrol the power flow, while an HVDC systemallows rapid, direct control of both directionand quantity.

www.peimagazine.com 25PEi December 2002

spider vessel with a positioning systemcapable of maintaining its position to anaccuracy of less than 1 m. A hydraulic jetplough was then used to bury the cables toa depth of up to 2 m. This environmental-ly sensitive technique uses a controlled jetof pressurized water to create a trench,around 1 m wide. The trench is not exca-vated and back-filled as in land-based con-struction. Instead during the installationprocess the trench remains filled withloose, fluidized sediments into which thecables settle immediately.

Jet ploughing is widely acknowledged tobe a more controlled and less sediment-dis-turbing installation method than mechani-cal ploughing and dredging. It is alsoextremely fast and efficient, and the wholecable laying process was completed in justfour days. The seabed can self-restore to itsnatural contours very quickly.

Environmental considerationsRespect for the local environmental condi-tions were paramount throughout the

project, not least because an earlier projecthad been denied because it would havecrossed a large portion of cultivated shellfishbeds in New Haven Harbour. In response toconcerns the project was able to avoid allbut 230 m of the actively cultivated beds bylocating the cable within the federal naviga-tion channel (FNC) in the harbour, where no

shellfish cultivation takes place.For the 580 m section of the cable

between the FNC and landfall at NewHaven the cable installation used direc-tional drilling to create a tunnel beneaththe seabed so that the cable could behoused in a plastic conduit, and avoid disturbing the shoreline.

There will also be no adverse impact totemperatures in the seabed or surrounding

waters. The temperature increase at theseabed directly above the cable will be lessthan 0.1°C, with a related increase inwater temperature measured in millionthsof a degree. These increases are negligibleand well within annual temperature varia-tions in New Haven Harbour and LongIsland Sound.

The cable will not result in any majorchanges to magnetic fields affecting naviga-tion or animal life. This is because thetwin-cable design, with equal and oppositecurrent in each cable, largely cancel thecable’s magnetic field. The design will pro-duce only a slight magnetic field that willresult in a negligible magnetic compassdeviation of less than one degree in the nar-row area above the cable. Since it is a DCsystem the cable will not produce any ACelectromagnetic fields, which are frequently associated with health concerns.

InterconnectionThe Cross Sound Cable was completed inmid-2002 and commissioned in just fifteendays. Beyond this rapid completion, itscapability to transmit electricity over longdistances with efficient operation at a lowerinvestment level than conventional HVDCpower transmission systems, demonstratesthe applicability of HVDC Light technologyin larger MW applications.

Cross-Sound Cable’s completion was fol-lowed closely by another HVDC Lightinstallation called the Murraylink project inAustralia. Murraylink, the world’s longestunderground high voltage interconnection,can transfer 220 MW of power between thestates of Victoria and South Australia (seePEi May 2001). The interconnection wasdeveloped by Murraylink TransmissionCompany, also a subsidiary of TransÉnergie,was put in operation in October 2002 tohelp relieve South Australia’s anticipatedenergy supply shortages.

Both the Murraylink and Cross SoundHVDC Light systems are equipped withfast response control systems that enablepower trading over short dispatch peri-ods, making them especially suitable foroperation in deregulating and privatizedpower markets.

The Cross-Sound link is expected toimprove the reliability of power supply inthe Connecticut and New England powergrids, while providing urgently needed elec-tricity to Long Island. The HVDC Lightconnection is also designed to promote com-petition in the New York and New Englandelectricity markets by enabling electricity to be traded among power generators andcustomers in both regions.

HVDC Systems

“Both the Murraylink and Cross Sound HVDC Light

systems are equipped with fast response control systems that enable

power trading over short dispatch periods”

PEi

Figure 4. Directional drilling was used for the cable landfall at New Haven in order to create a 580 m tunnel underneath the shellfish beds. The cable is housed in a plastic conduit