hv xlperformance cable technology

Deregulation of the electricity supply markets and growing environmental awareness are creating

exciting new markets for power transmission solutions based on extruded cable

technology. At the same time, improvements on all

fronts are extending the use of XLPE (cross-linked

polyethylene) insulated cable systems up to

500 kV. Today’s cable system applications are

often competitive with overhead lines, while

new manufacturing methods are enabling

submarine cables with integrated optical

fibers and flexible joints to be supplied in

longer lengths than ever before. Further devel-

opment of extruded insulation systems is also

contributing to the success of ABB’s innovative

HVDC LightTM concept.

47Special ReportABB Review

High-voltage cable systems rated 220 kV and above have become

part of the very backbone of modern-day power transmission infrastructure.This importance carries with it, howev-er, a special responsibility on the partof the suppliers to ensure that thesystems exhibit the highest reliabilityand, because of the high electricalstresses at such voltage levels, that thecables and accessories are properlycoordinated.

Deregulation – changing the rulesIn today’s deregulated electricity mar-kets, the rules that used to govern gen-eration, transmission and distributionhave changed for both the power utili-ties and the suppliers. Suddenly, it isthe customer who is in the spotlight.

Accordingly, the market has to listenmore to public opinion, and there is a strong possibility that this will in-clude a call for a less ‘visible’ T&Dinfrastructure.

All the actors in this new market haveto reduce their costs and at the sametime guarantee high reliability for thetransmission and distribution systems. A likely scenario is that new cable in-terconnections will be built and opera-tional margins will be utilized morefully in order to get maximum technicaland economic benefit from the electri-cal network.

Extruded cable systems have a majorpart to play in this new, competitiveenvironment, especially when it comes

to replacing overhead lines with under-ground cables. XLPE cable systemscosts have decreased during the lastdecade and are likely to fall even fur-ther. At the same time, XLPE cable per-formance has increased enormously.The new message is therefore thatXLPE cable systems are able to competewith overhead lines, technically, envi-ronmentally and commercially. This isparticularly true in the voltage range of12–170 kV .

Extruded insulation – performance and improvementsThe well-established trend toward asmaller insulation thickness will contin-ue, resulting in a leaner cable withmany advantages, among them longerdispatch lengths, fewer joints, easier

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Björn Dellby, Gösta Bergman, Anders Ericsson, Johan Karlstrand

XLPERFORMANCEHigh-voltage

cable technology

48 Special ReportABB Review

installation, and reduced thermal con-traction/expansion of the insulating ma-terial. Experience accumulated duringEHV XLPE cable system development,improvements made in materials andprocesses, and the excellent servicerecord of XLPE, have reduced the thick-ness of cable insulation to 12–15 mmfor 132-kV cable systems. This placesthe XLPE cable systems versus over-head line transmission scenario in anew light, where the cable solutionoften can be an attractive alternative.

Underground cables versus overhead linesThere are, of course, many operational,security, environmental, reliability and

economic parameters that distinguishXLPE cable systems from overheadlines [1]. For modern XLPE cable sys-tems, the reduced cost ratio and envi-ronmental and reliability benefits arethe most obvious and important con-siderations. Due to their larger cross-sectional areas, cables usually exhibitfewer losses per MVA than comparableoverhead lines. A summary of the ben-efits of XLPE cable systems is given inthe table on page 52.

The ratings of the overhead lines aresometimes dictated by high winterloads, which include a lot of electricalheating equipment. During hot summerdays the overhead line carries some

50% less electricity than in winter, mak-ing them less attractive if load profileshave to be smoothed out in the future.In areas where there are many air-con-ditioning units, for example, the bene-fits of XLPE underground cables makethem a genuine alternative .

Underground transmission lines alsohave a better overload capacity forperiods of time shorter than 90 minutesdue to the high thermal mass of thesurrounding soil.

Qualification of 400–500 kV cablesystemsThe IEC emphasizes the importance ofreliability and coordination of the cablesand accessories by recommending thatthe performance of the total system,consisting of cable, joints and termina-tions, be demonstrated. The compre-hensive test program, including a ‘pre-qualification’ test, is described in detailin IEC 62067.

ABB qualified as a supplier of cablesystems for the 400-kV voltage level in1995.

Quality, materials and manufacturingOnly certified suppliers are contractedto deliver essential materials. All ABBmanufacturing sites for HV cables andaccessories are ISO 9001 and 14001 cer-tified. The XLPE cable core is producedon a dry curing manufacturing line. Thecable insulation system, including theconducting layers, is extruded in a sin-gle process using a triplex extrusioncross head located, together with thethree extruders for the insulating andconducting materials, in a clean-room .

Cable designshows a 400-kV XLPE cable. The

cable’s copper conductor, which has across-sectional area of 2500 mm2, isdivided into five segments to reduceskin effect losses. ABB uses segmented(Milliken) conductors made of strandedwires for cross-sections greater than1000 mm2. For cross-sections smallerthan 1000 mm2, the conductors arehighly compacted to obtain a rounder,smoother surface.

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01986 2000199819961994199219901988

CR

400 kV

130 kV

Comparison of cost ratio (CR) for XLPE cable systems and overhead transmission lines

1

Air/soil temperature

Am

pac

ity

Soil

OHTL

XLPE cables

Rating of overhead line (OHTL) versus underground XLPE cable. The dashed lineindicates that higher powers could be transmitted if the daily load cycle profile istaken into account.

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The metallic screen consists of copperwires on a bedding of crepe paper to re-duce the mechanical and thermal impacttransferred to the insulation. The numberof wires and the total cross-section de-pend on the short-circuit requirements ofthe network. Longitudinal water tightnessis achieved by filling the gaps betweenthe screen wires with swelling powder.

External protection against mechanicalimpact and corrosion is provided by atough, extruded, laminated sheath madefrom HDPE (high-density polyethylene).A bonded metal foil on the inside of thesheath stops water from diffusing intothe cable.

The resulting lean, low-weight cable hasseveral advantages: a greater length ofcable can be wound onto any givendrum; high eddy-current losses in thecable sheath are avoided; the current-carrying capacity is optimized.

Possible oversheath options are:An extruded conductive layer forouter sheath measurementsAn extruded flame-retardant layer forextra safety in hazardous environ-ments

Another option the cable design offers isspace-resolved temperature monitoringwith optical fibers. The fibers are con-tained in a stainless-steel tube, approxi-mately the same size as a screen wire,which is integrated in the cable screen.Monitoring the temperature in this wayenables the cable load to be optimized.

Cable accessoriesIn the early 1990s ABB developed pre-fabricated joints for HV and EHV cableswhich are totally dry, ie with neithergaseous nor liquid materials, and main-tenance-free. The main electrical partscan therefore be pre-tested in the facto-ry, speeding up on-site installation andreducing the attendant risks. The jointshave integrated sheath insulation inorder to comply with the CIGRE recom-mendation contained in Electra 128,which requires them to withstand im-pulse voltages of 125 kV between thetwo joint sections and 63 kV to earth.This permits cross-bonding of the cable

screen, which reduces the inducedscreen currents and losses in the AC ca-ble system. The complete cable systemwith joint, outdoor terminations and GISterminations, fulfills the requirements ofIEC 62067 in every respect.

Testing of 220–500 kV cable systemsIn the case of medium-voltage cables itis usual to think in terms of compo-nents. Even if these come from differentsuppliers, they can be joined together

Vertical extrusion of XLPE cable insulation3

400-kV XLPE cable. The copper conductor is divided into five segments to reduceskin effect losses.

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and the system as a whole will stillwork. This is why limits are given forthe electrical stresses in the constructionrequirements in IEC 60502.

HV and EHV cables and accessories, onthe other hand, are designed as systems.No construction requirements exist forcables for these voltage levels, just thetest requirements in IEC 60840 and IEC 62067.

400-kV XLPE cable projectsIn 1996 ABB received an order from thepublic utility Bewag (now VattenfallEurope) to supply and install a 400-kVXLPE cable system in a 6.3-km longunderground tunnel in the center ofBerlin. The ventilated tunnel is situated25 to 35 meters below ground and has adiameter of 3 meters . The cable sys-tem, with a 1600 mm2 segmented copperconductor, has a transmission capacity of1100 MVA and forms part of a diagonaltransmission link between the transmis-sion grids west and east of the capital.

5

The cable is installed with the threephases arranged vertically, one abovethe other, on specially designed cablesaddle supports7.2 metersapart, with ashort circuit-proof spacer in the middle of each span.The cable routewas dividedinto nine sec-tions, eachapproximately 730 meters long. GISterminations were installed at the twosubstations and the new ABB joint wasused to interconnect the cable lengths.The laid cable consists of three maincross-bonded sections, with three mi-nor sections within each main section.The cable circuit went into service inDecember 1998.

The Bewag utility subsequently award-ed a second 400-kV XLPE cable con-

tract to ABB, this time for a 5.4-kmlong system, again in an undergroundtunnel. This cable circuit completes the

diagonal link be-tween the transmis-sion grids west andeast of Berlin, andwas handed over to the customer inJuly 2000.

Further 345–400 kVcable projectsawarded to ABB

include orders for 200-km XLPE cables,accessories and installation. Commis-sioning of these projects is scheduled to take place during 2003–2004.

New submarine cable projectsIn 1998 ABB was awarded the ChannelIslands Electricity Grid Project, whichreinforces the power supply fromFrance to Jersey and, for the first time,connects Guernsey to the Europeanmainland grid. The submarine part

400-kV cable system in a 6.3 km underground tunnel running through Berlin’s city center5

Greater lengths of low-weight XLPE cable canbe wound onto anygiven drum, while higheddy-current losses inthe sheath are avoided.


of this project was completed in July2000.

The main components delivered for theproject were:

Submarine cables between Franceand Jersey and between Jersey andGuernsey (approx 70 km)Underground cables on Jersey andGuernseyGIS substationsNew transformers and reactors

The two submarine cables are of thesame basic design, ie three-core, sepa-rate lead-sheathed, and with triple-extruded XLPE insulation. Each has afiber optic cable with 24 fibers integrat-ed in it for system communication andinter-tripping. The cables have doublewire armor (ie, an inner layer of tensilearmor and an outer, so-called rockarmor) to protect them from damagethat could be caused by tidal currentsand fishing.

The cables have a diameter of ap-proximately 250 mm and weigh about 85 kg/m in air.

Both cables were delivered by the facto-ry in their full lengths .

Because of the risks posed by fishingactivities, the cables between Jersey andGuernsey and the fiber optic cables be-tween Jersey and France were jetted in-to the seabed for extra protection.

Another submarine cable project is theMa Wan and Kap Shui Mun Cable,which crosses a channel in Hong Kong.Due to the heavy traffic in this channel,it was decided to forgo conventionalinstallation of the 132-kV and 11-kVsystems, which would probably havedisturbed shipping even if modern tech-niques were used. The problem wassolved by drilling under the seabed andinstalling ducts through which thecables were pulled. This has the extraadvantage of allowing upgrades to becarried out in the future.

Separate control systems were installedto monitor operation of the cable link,which was completed in 2003.

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A new submarine cable project hasrecently been awarded to ABB byAramco. The cable, which is 53 kmlong, is rated 110 kV with 3 × 500 mm2

copper conductors. It will be commis-sioned in 2004.

HVDC LightTM

HVDC LightTM, which was launched in1997, is another ABB innovation in theT&D field that incorporates advancedHV cable technology. High VoltageDirect Current (HVDC) cables are em-ployed for bulk power transportationover long distances, mainly underwater.

Traditional cable technology is basedon paper insulation systems impregnat-ed with highly viscous oil. While thesecables have many technical advantages,the manufacturing process is slow andthe end-product is mechanically sensi-tive. The industry had therefore beenlooking for a long time for an extrudedHVDC cable of the kind used in ACsystems.

With HVDC Light [2], ABB has intro-duced to the market an extruded cablesystem, together with new transistor-based converters, that makes HVDC

Storage of submarine cable prior to laying6


transmission competitive even at lowpower ratings. The first commercialsystem, a link rated at 50 MW, wasinstalled on the Swedish Island ofGotland, where it transmits power froma wind power plant to the town ofVisby [3].

Other major projects that have beencompleted are:

The Directlink, rated 180 MW at 80 kV, which transfers power be-tween the states of New South Walesand Queensland in Australia.The Murraylink, rated 200 MW at 150 kV, built to transfer power be-tween Victoria and South Australia [4].The Cross Sound Cable, rated 330 MW at 150 kV, which transferspower between New England andLong Island.

The latest HVDC Light project, to supplypower to an offshore platform in theNorth Sea (Troll A), is due to be com-missioned in 2004. (See article startingon page 53.)

Applications for HVDC Light include:Feeding of isolated loads (eg, off-shore platforms)Asynchronous AC grid connection

Transmission of power from smallgeneration units (eg, wind powerplants)DC grids with multiple connectionpointsNetwork reliability enhancementthrough voltage stability and blackstarts

Tomorrow’s electrical infrastructure – here nowExtruded cable systems are available astotal solutions, with a ‘cradle to grave’supplier commitment. Such systems areturnkey offerings in the commercial aswell as the technical sense. They maystart with the permit application, contin-ue with the removal of the overheadlines and the supply and installation ofthe cable system, and end with theenvironmentally friendly disposal of theold equipment.

Complete cable system applications canalso be seen as intelligent combinationsof monitoring equipment, converters,load-sharing devices, series and/orshunt compensation devices. Financing,too, can be arranged; here, leasing anda new type of availability guaranteecould resolve several commercial uncer-tainties.

Together, these ‘thumbnail’ sketches of the future add up to a new cus-tomer-value-based market. Extrudedinsulated cable system applications aredestined to play a key role in thisevolving market by meeting not onlythe transmission and distribution net-work requirements of today but alsothose of tomorrow.

Environment

No visual impact

Low/no electromagnetic fields

High level of personnel safety,

low risk of flashover in air

Good working conditions

Grid security

Not affected by wind, snow, ice,fog, etc

Nothing can be stolen

Economy

Low maintenance

Minimum investment for lake/river crossings

Land use minimized

Value of land/buildingsunaffected

Operation

High availability, few faults

Usually low losses/MVA

High short-time overloadcapacity

References

[1] D. Karlsson: Comparison of 130 kV XLPE cable systems and OH lines – loading capability, reliability and planning criteria. CIGRÉ 2002.

[2] K. Eriksson: HVDC Light DC transmission based on voltage sourced converters. ABB Review 1/98, 4–9.

[3] M Byggeth, et al: The development of an extruded HVDC cable system and its first application in the Gotland HVDC Light project. JICABLE 1999.

[4] T. Worzyk, et al: The Murraylink Project – the first commercial 150 kV extruded HVDC cable system, JICABLE 2003.

Dr. Björn Dellby

Gösta Bergman

Dr. Anders Ericsson

Johan P. Karlstrand

ABB Power TechnologiesHigh Voltage Cables

SE-371 23 KarlskronaSweden

[email protected]

Benefits of underground transmission lines

hv xlperformance cable technology

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