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-------------------------------- J IC A B L E '9 9A1.SDesign of new 150 kV XLPE'cable system for the Belgian electrical networkC O UN ES ON P ., B R IJ S B ., T rac te bel, B rux elle s, B elgiu mA RG A U T P ., M A R T IN B ., S A GE M S A , M ontereau, F rance

ResumeLe secteur electrique beige a annonce en novembre1992 une nouvelle politique d'implantation desreseaux Haute Tension impliquant un recours plusfrequent a des liaisons souterraines de 30 a 220 kV.De nouveaux systemes de cables ont etedeveloppes pour proposer des solutions pluseconorniques et plus fiables. A titre d'exemple, cerapport detaille les composants des liaisons 170 kVet les essais realises pour leur validation.

1-lntroductionGiven the increasing resistance of both thepopulation and the administration responsible fortown and country planning to the idea of newoverhead line projects, the Belgian electrical sectorannounced in November 92 a new policy for theimplementation of HV networks.Among its etements, this policy provides that the totallength of overhead lines be frozen at its 1992 level,in the 30 to 220 kV range.This decision had lead to more frequent use ofunderground cables at these voltage levels.As an example, taking into account the. need of atransmlssion power rating increasing for these newunderground links, a new 170 kV cable system with atransmission capacity of 24p to 290 MVA (continuousrating) was considered. Besides, as the costs of anunderground link are significantly higher than anequivalent overhead line, it is of the greatestimportance to improve the reliability of theunderground systems as much as possible and alsoto take advantage of the present state-of-art for thehighest voltage levels to reduce, by this way, thecosts of the HV underground links.In order to achieve these purposes, differentdevelopments were conceived and tested. The mostsignificant ones are described here after.

AbstractThe Belgian electrical sector announced in november1992 a new policy for the implementation of HighVoltage networks, leading to a more frequent use. ofunderground cables from 30 to 220 kV levels. Newcables systems have been developed to offer 1110reeconomical and reliable solutions for undergroundlinks. As an example the paper details the 170 I

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Radial and longitudinal water tightnessInsulation: cross-linked polyethylene

< !I Over sheath = HOPE (e ~ 4.5 mm)(!) After a long duration prequalification test, othertests according to IEC 840(!) Routine testing: the test voltage shall be 3.5 Uoprovided that the electrical stress shall not

exceed 25 kVlmm during the test.2-1-2- Cable dimensioning2-1-2-1-lnsulationThe insulation wall thickness dimensioning is madetaking into account:a) the maximum AC stress in service operatingstress on the conductorb) the external stress allowed by accessories(operating stress over insulation)c) the acceptable AC breakdown risk level inserviced) the maximum lightning impulse stress (on theconductor)e) the AC stress for the routine test170 kV cables are extruded in a vertical linecommonly used to manufacture 245, 420 and 525 kVcables, of the same design and using the samecompounds : one can thus expect the sameperformances, with regard to the acceptableelectrical stresses. Cable systems including cableand accessories have been long term tested withstresses of 34 kV/mm on the conductor and14 kV/mm over insulation.These results have enabled to validate the possibilityof operation under high service stresses as specifiedby the French standard C 33-253 for cables up to290/500 (525) kV.In this way, for the 170 kV level, electrical stresses of10 kV/mm on the conductor and 4.5 kV/mm overinsulation allow an important reduction of theinsulation wall thickness while keeping a good safetymargin, as proved by the highly satisfactory servicerecords obtained on hundreds of 130/225 (245) kVcables designed with the same stresses. Highervalues of the operating stresses are even notexcluded in the future if the stresses considered forthe 400 kV - 500 kV level would be used. Thissecond step requires nevertheless new developmentof accessories and for this reason was notconsidered at (his stage.

Lightning impulse stressIn order to take into account the lightning impulse atincreased temperature ( 95C), a moderate value ofmaximum electric strength equal to 85 kV/mm wasselected, based on many short term test carried outon XLPE 245, 420 and 525 kV cables.

Routine testingSince the electrical stress at 3.5 Uo correspondsapproximately to 25 kV/mm, if an external operatingstress of 4,5 kV/mm is adopted, the routine testduration provided by the IEC 840 standard Gould berespected.2-1-2-2- Conductor sizingTo provide the transmission capacity specified, twoconductors have been considered :1200 rnrn" copper2000 rnrn" aluminumThese two conductors offer approximately the sametransmission capacity.Due to the lower weight of the cable and its lowercost, the 2000 rnm- aluminum conductor wasselected.2-1-2-3- Optical fibre integrationOptical fibre, placed in a stainless steel tube areinserted in a lapping of semi-conducting. fillershelically laid on the cable core. Besides, layers ofswelling tapes are wounded under and above thefillers providing for the longitudinal water-tightnessbarrier of the cable.2-1-2-4- Radial water-barrier and metallic screenTo prevent the ingress of moisture and to provide theshort-circuit current carrying capacity, a 3 mm thicklead sheath is extruded over the cable core and theswelling tapes. layer. Finally, an HDPE over sheath isextruded over the lead sheath.

Fig 1 - Cable construction

2-1-3- Cable manufacturingAs 245, 420 and 525 kV XLPE cables, 170 kV cableis manufactured on a vertical line installed in a shaftof 100 m depth.

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Insulation is achieved following a Completely DryCuring and Cooling process (C.D.C.C.) with commonextrusion of the three layers in a-triple head.After insulation , the cable is treated in an oven toeliminate some by-products; after this treatment, thewater content of XLPE is < 100 ppm.2-2- AccessoriesPrefabricated accessories for supertension extrudedcables have beendeveloped more than 10years agoand described in many occasions. Referring to thework prepared by the CIGRE WG 21-06, theproposed accessories for the 170 kV level are of thefollowing types :a) terminations:stress cone and composite or porcelaininsulator for the outdoor termination;as variant, completely synthetic terminationfor the outdoor termination;stress cone and insulator or directlyimmersed for the Metal Enclosed GIStermination.b) joints:one piece premoulded joint.2-2-1- Outdoor terminationa) With insulatorOutdoor terminations include a factory premouldedsilicon stress cone, either oil or SF6 gas as insulatingfluid and are housed in a composite insulator asshown in fig 2 with a rigid core of conical shape ontowhich elastomeric sheds have been extruded. Suchkinds of insulators Werechosen for their safety, easeto handle and shock resistance during transportationand installation as also for their excellent behaviourin polluted areas. The standard connecting box,placed under the termination, allows the linking ofoptical fibres inserted in the 150 kV cable to theSUbstationbuilding through a common optical fibrecable.b) Without insulator (synthetic termination)Synthetic terminations have been considered andhave been adopted for 170 kV provisional linksoperatedsince November 1997for evaluation of theirbehaviour. This type of termination will probably-b-e-the solution in a near future.2-2-2- Metal Enclosed GIS terminationsUsing the same elastomeric factory premouldedstress cone, two designs are available.with insulator to comply with IEC 859 . The SF6gas inside the insulator can be provided by theGIS by means of a passage way through theinsulator or can be segregated.without insulator, as for example described andspecified in French standard C 33-062 or tocomply with any special customer requirement.

Fig 2 - 170 kV Outdoor Terminationwith composite insulator2-2-3- JointsUsing the same premoulded components as the "onepiece" premoulded joints in use in several 245 kVgrids since 1989 with good outcome of experience,joints can include a shield-break to allow cross-bonding connections. In addition, the joint also_enables the splicing. of the optical fibres inserted inthe two cables sections to joint with the req-l.liredshield-break insulation level.

3- Tests on cables and accessories3-1- Test according to IEC 840 with customer'sspecial requirements.Main following tests were performed:

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3-1-a) Routine testsPartial discharge testVoltage testElectrical test on non metallic sheath

Requirements ResultsPartial discharge Sensitivity s 1 pC Sensitivity 1 pCtest Magnitude s 2 pC Magnitude 1,2 pCunder 2Uo at 174 kV(174 kV)Voltage test 30 minutes at 30 minutes at

3.5 Uo (304.5 kV) 304.5 kVACno failureElectrical test on 1 minute at 25 kV 1 minute at 25 kVnon metallic DC DCsheath cable immersed in cable immersedwater in water

no failureTable 1

3-1-b) Special testsAll special tests included in lEG 840 werecarried out with success.

3-1-c) Water penetration testWater penetration test was made on the cableequiped with optical fibres, after 3 bendingcycles.

Requirements ResultsBending test 3 bending cycles 3 bendingDiameter of the test cyclescylinder not greater Diameter of

than 25 (0 + d) + 5 % the cylinder(3801 mm) 3600 mmWater 24 hours room 24 hours roompenetration test temperature + temperature10 heat cycles 10 heat cyclesno water may emerge no waterfrom the ends emerged

(at 1.5 m)Table 2

3-1- d) Type testsTests on complete cable system : heatingcycles voltage test according to lEG 840

Requirements ResultsPartial Sensitivity s 1 pC Sensitivity 0.9 pCdischarge test Magnitude S2 pC Magnitude 1.5 pCunder 2 Uo under 174 kV(174 kV)Bending tests 3 bending cycles 3 bending. cyclesDiameter of the test Diameter of thecylinder not greater cylinder 3600 mm

than25 (D + d) + 5 %(3801 mm)Partial Sensitivity s 1 pC Sensitivity 0.9 pCdischarge test Magnitude S2 pC Magnitude. 1.5 pCafter bending under 2 Uo under 174 kV( 174 kV)Partial Sensitivity s 1 pC Sensitivity 0.9 pCdischarge test Magnitude s 2 pC Magnitude 1.5 pCafter joint under 2 Uo under 174kVinstallation (174 kV)Partial Sensitivity S 1 pC Sensitivity 0.9 pCdischarge. test Magnitude s 2 pC Magnitude 1.1 pCon complete under 2 Uo under 174 kVset up before (174 kV)heating cyclesvoltaoe testHeating 2 Uo during 480 h 174 kV during 542 hcycles voltage 20 heat cycles. for 22 heat cyclestest 95C S6.s 100C 9SC

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4- Developments concerning the cable system4-1- Temperature measurement sensors.To get a better knowledge of the actual thermalstresses during the operation of high voltageunderground cables, it was decided to use opticalfibres for temperature measurement along the entirelength of scheduled high voltage lines.These optical fibres (both monomode andmultimode) are inserted in one of the three cables ofeach circuit.As breaks in the fibre could occur during laying, andin order to check the appropriate design of the cable,a special type test program including bending testsfollowing IEC 840 and additional pulling and bendingtests representative of on site handling wasperformed on a full 500 m cable length (see detailshere under).For each cable length. equipped with optical fibres,the optical characteristics of the fibres shall bemeasured in the factory and on site before and afterlaying operations :It attenuation by refiectometer at 1310 nm and850 nm (for e = 20C) for each multimode fibreIt attenuation by refiectometer at 1310 nm and1550 nm (for e = 20C) for each monomode fibre.Maximum acceptable values of attenuation shall be:

Type of 0ptical fibreMultimode 50/125 urn Monomode 9/125 _jJm850 nm 1310 nm 1310 nm 1550 nmIn thefactory 0.4 0.25nd 3.0 1.0beforelayingAfter 3.5 1.15 0.45 0.30layingTable 5

m

R=-2,1m1m

R=-2,1m

4-1-1- Tests on optical fibres temperaturemeasurement system4-1-1-1- Tests during manufacturing processAttenuation measurements have been made atdifferent steps of the manufacturing process: beforeand after laying of the stainless steel tubes ( housingoptical fibres ) on the cable core , after lead sheathextrusion , and after jacketing . After the completemanufacturing process , the values comply with thecustomer requirement given in Table 5.

4-1-1-2- Tests during laying processSpecial laying tests have been performed both instraight line route and bending route on the samecable length .Attenuation measurements have been made beforeand after laying of the cable

a) straight line laying:520 m of cable have been pulled withsynchronized power drive rollers installed at100 m intervals.b) bending line laying:Four curves with a bending radius of 2085mm (17 to 18 times the outside diameter ofthe cable) were prepared along the cable routeaccording to the sketch given in Fig 3.

Table 6 gives compared results of the measurementsmade after laying in both configurations. All valuescomply with customer requirements. and proved thatno mechanical stress was induced on optical fibersin the different life stages of the cable construction asalso during the laying operations.

Fig 3 - 8ending route for laying tests

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AttenuationTube O.F A (dB/km) Requirements(nm) Straight Bending ~route route

1 1310 0,372 0,372 01 1 1550 0,248 0,206 -0,042

Monomode 2 1310 0,386 0,386 02 1550 0,270 0,218 -0,052 MonomodeOF1 850 2,428 2,432 +0,004

2 1 1310 0,512 0,522 +0,010 1310 nmMultimode 2 850 2,304 2,304 0 < 0,45 dB/km

2 1310 0,496 0,500 +0,0042,462 1550 nm1 850 2,480 -0,018 < 0,30 dB/km

3 1 1310 0,490 0,528 +0,038Multimode 2 850 2,338 2,252 -0,114

2 1310 0,462 0,522 +0,0601 1310 0,362 0,356 -0,006

4 1 1550 0,158 0,200 +0,042Monomode 2 1310 0,398 0,394 -0,0042 1550 0,220 0,204 -0,016 MultimodeOF1 850 2,462 2,558 +0,096

5 1 1310 0,498 0,520 +0,022 850nmMultimode 2 850 2,282 2,352 +0,070 < 3,50 dB/km

2 1310 0,490 0,492 +0,002 1310 nm1 850 2,400 2,486 +0,086 < 1,15 dB/km6 1 1310 0,496 0,518 +0,022

Multimode 2 850 2,268 2,324 +0,0562 1310 0,490 0,506 +0,016

Table 6

4.2- Cross-bondingTo improve the transmission capacity of thesupertension cable systems, a new cross-bondingtechnique has been adopted, and is described inFig. 4.

One or two ground cables (depending. on the lengthof the circuit) are laid in parallel with the screens ateach joint bay between each ternary section and tothe ground at both ends of the circuit. Surgearresters are connected in star formation and thecommon point of the arresters is not connected tothe ground cable nor to any local earth. They arehoused in an external box and are dimensioned towithstand the DC sheath test without triggering.5- Conclusion

New components have been developed to offermore economical and more reliable undergroundlinks to meet the need of the Belgian electricalsector regarding XLPE cables systems in voltageranges up to 220 kV.As an example, 170 kV components have beensuccessfully qualified according to a testingprocedure prepared on the basis of lEG 840 and itsfuture amendments and including severe additionalrequirements concerning partial discharges leveland temperature measurement system.

EARTHING CABLESHV POWER CABLES

Fig 4 - Cross Bonding System