cable 400kv cleuson

7/29/2019 Cable 400kV Cleuson

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400 KV XLPE CABLES FOR THE EXTENSION OF CLEUSON-DIXENCE HYDRO POWER PLANTS

CABLES 400 kV EN XLPE POUR LEXTENSION DE LAMENAGEMENT CLEUSON-DIXENCE

T. Heizmann, C. Wyler, C. Biolley and A. MarroAlcatel Cable Suisse SACH-2016 Cortaillod, Switzerland

[email protected]

A. NicoletAmnagement Cleuson-Dixence, c.p. 439CH-1950 Sion, Switzerland

[email protected]

Rsum

La premire installation de cbles 400 kV en XLPEen Suisse a t mise en service en octobre 1998.Trois circuits cbls dune longueur denviron 400 m

relient les transformateurs situs dans la caverne delusine hydolectrique 1200 MVA de Bieudron une sous-station 400/220 kV sur lautre rive duRhne. Les cbles ont un conducteur en cuivre de800 mm

2et une gaine ondule en aluminium

extrud. Les extmits et les cbles sont remplis deSF6.

Abstract

The first commercial installation of 400 kV XLPEcables in Switzerland has been in industrial servicesince October 1998. Three cable circuits of lengths of

400 m link the transformers of the 1200 MVAunderground hydro power plant Bieudron with a400/220 kV substation located across the Rhoneriver. The cables have a conductor cross-section of800 mm

2copper and a metallic sheath made of

seamless extruded corrugated aluminium. The cablesas well as the terminations are filled with SF6 gas.

1. Introduction

The system of hydro-power plants of theAmnagement Cleuson-Dixence SA uses the

400 mio m3 of water stored in the Lac des Dix,which is located in Valais, Switzerland. The threeexisting power stations Chandoline, Fionnayand Nendaz use the head of 1745 m to producesome 780 MVA of electric power (see figure 1). Toenable a better response to the strongly fluctuatingdemand, the peak power production has beenincreased by the construction of the 1200 MVApower station Bieudron. This power stationcomprises the following components :

a new water intake bored in the GrandeDixence gravity dam

a headrace gallery of 15.5 km

a surge tank

a steel lined inclined shaft of 4.3 km

a power station with three hydroelectric groups(Pelton turbine 423 MW, generator 465 MVA).

The power station is installed underground in threecaverns with a total volume of 155'000 m

3(see also

[1] for more information on the installation).

With a head of 1883 m, a power of the Peltonturbines of 423 MW, and a power per pole of

35.7 MVA of the generators, the new hydro powerplant Bieudron sets three world records.

2. Electrical conception of power plant

Bieudron

Figure 2 shows a schematic of the electrical

installations. Each of the three groups has a 423 MWPelton turbine with 5 injectors running at 428 rpm.The generators have 7 pairs of poles and are cooledby purified water. The energy produced by thegenerators is lead to the transformers by means of asystem of coaxially shielded air insulated busbars at anominal voltage of 21 kV and a current of 15'000 A.The busbars have an aluminium cross-section of21'000 mm

2. Each group is provided with a three-

phase 21/410 kV transformer of 500 MVA which isalso installed in the cavern. There is no circuit breakerbetween the generators and the transformers. Threesystems of 400 kV cables link the transformers with a

400 kV gas-insulated substation (GIS), where also thecircuit breakers are installed. From there, the energyproduced is evacuated by a 400 kV overhead lineand, by means of a group of single-phased 400/220kV transformers of 600 MVA, by the 220 kV grid.

The basic characteristics of the 400 kV grid are givenin table 1.

Table 1 : Specifications of the 400 kV grid.

Max. operating voltage 435 kV

BIL 1550 kV

Short circuit current 55 kA / 3 s


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Figure 1 : System of hydro-power plants of Amnagement Cleuson-Dixence SA

3. Design of cables

After an extensive study of different types of cableswhich included oil-filled cable and polypropylene-laminated-paper insulation (PPLP), the customerpreferred XLPE to oil-filled insulation. This decisionwas based on the positive experience gained with400 kV cables with synthetic insulation. In addition,the maintenance is reduced and all risks of pollutionare avoided which is particularly important for acable that is crossing a river.

The 400 kV XLPE insulated cables (see table 2 forspecifications) have a copper conductor of 800mm

2, an extruded corrugated aluminium sheath,

which is protected with an anti-corrosion compoundand a jacket of flame-retardant polymer. To enable ameaningful oversheath voltage test, an additionalextruded semiconducting screen was applied on the

jacket. The cable is filled with SF6, which gives anadditional security by the gradual impregnation ofthe insulation by SF6, leading to an improveddielectric strength and aging behavior.

The cables were produced on a horizontal extrusionand vulcanization line (MDCV).

Table 2 : Specification of 400 kV cable.

Type XAluWET-Tsc (Nof)

Cross section 800 mm2

Nominal voltage Un / Uo 400 / 230 kV

Max. operating voltage 435 kV

Insulation XLPE

Insulation thickness 32 mm

Field strength at 400 kV 12.3 kV/mm

Sheath seamless extruded andcorrugated aluminium

Outer sheath flame retardant polymer

Diameter 142 mm

Weight 21.6 kg/m

Minimum bending radius 3000 mm

Min. radius during pulling 5000 mm

GIS G T

400 kV

High Voltage

cables

400 m

XAluWET

1x 800 mm Cu

400/230 kV 3-phases transformer

410/21 kV, 500 MVAGenerator

465 MVA

Pelton turbine

423 MW

Coaxial aluminium busbar

21 kV / 15'000 A

Figure 2 : Schematic of the electrical installations of the hydro-power plant Bieudron


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4. General description of cable circuits

Nine cable lengths of approximately 400 m link the21/410 kV transformer in the cavern with a gasinsulated switchgear on the other side of the Rhoneriver. Along most of the route the cables are laid intrefoil formation. As there is no separate duct for thecables, they are fixed on the wall or on the floor of

access galleries by means of supports (see figure 3for a typical laying arrangement). The route ischaracterized by about 10 changes of direction,many of them with radii of only 3 m, which is theminimum allowed for this type of cable. The crossingof the river is done by a metallic bridge with a lengthof approx. 100 m. Close to both ends of the cablesspare loops are provided, enabling the mounting ofa new termination or a joint in the case of a failure(see figure 5). The cables are provided on bothsides with SF6-type terminations, i.e. both of theintroductions into the GIS and the transformers isdone by 90 angles of gas-insulated busbar of

lengths of about 6 - 10 meters. The terminations aswell as the cable itself are filled with SF6 gas, thepressure of which is monitored on both sides andthe corresponding alarms are transmitted todispatching.

Figure 3 : Fixation of cable circuits in vault ofaccess gallery.

5. Mechanical aspects

The cables are laid in trefoil formation and are fixedon supports by means of aluminium clamps every 6meters. To cope with the short-circuit forces thecables are provided with an intermediate clampbetween two supports. To ensure a controlleddilatation and to avoid unacceptable stresses on thesupports, each cable was pre-formed with a sag of10 cm, which will increase to about 16 cm by the

heating caused by the nominal current.The three cable circuits are separated one againstthe other with plates consisting of a sandwich of

18 mm of a fire resistant material (F30 : 30 min at1000 C) between two plates of 5 mm thickness ofaluminium. Additional mechanical protections wereprovided in locations with a risk of damage, e.g. bycirculating trucks (see figure 6).

6. Terminations

The cable circuits were provided on both sides withSF6-type terminations (see figure 4) according to IEC859 with silicone stress cones filled with SF6. Thealuminium sheath of the cables is directly weldedunder Argon to the entrance tube of the terminations.

Figure 4 : SF6-type terminations at the entrance ofthe GIS.

7. Ampacity / thermal design

The current carrying capacity has been calculatedaccording to IEC 287. As it can be seen from table 3,the conductor temperature at the nominal current ismoderate, thus reducing transmission losses. Inreality the cables are operated at even lower

temperatures taking into account the cyclic loading foronly a few hours per day.

Table 3 : Ampacity of the cable circuits at 20 Cambient temperature.

Nominal current In 782 A

Cond. temp. at In 49 C

Total losses at In 70.8 kW/km

Max. adm. current Im 1207 A

Cond. temp. at Im 90 C

Total losses at Im 182 kW/km


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8. Admissible short-circuit currents

Table 4 shows the admissible short-circuit currentsin the conductor and the sheath (initial / finaltemperature : 90 / 250 C).

Table 4 : Admissible short circuit currents.

Conductor 67 kA / 3 sSheath 64 kA / 3 s

Due to the high aluminium cross-section of 1114mm

2the requested short-circuit current of 55 kA /

3 s can also be carried by the sheath withoutadditional parallel conductors.

Figure 5 : Spare loop and mounting of the cables tothe transformer.

9. Sheath bonding / EMC

The cable feeders are operated with single pointsheath bonding on the side of the GIS. The opensheath end close to the transformer is protected withsurge arresters with a rated voltage of 6 kV. Duringthe first tests, air discharges on both ends of thecable circuits were observed when disconnectors inthe GIS were activated. To cope with this problem,

high-voltage, high-frequency capacitors weremounted in parallel to the surge arresters on the sideof the transformers.

Similar capacitors were also mounted on the otherside in parallel to the ground connections of thesheath (lengths of about 1.5 m because of currenttransformer used for protection), as their impedance

at high frequencies was not low enough to avoiddischarges.

Figure 6 : Mechanical protection of cables inarrangement as shown in figure 3.

10. Tests

The routine tests in the factory comprised of an ACwithstand test at 460 kV for 60 min and a partialdischarge test at 345 kV. For commissioning of thethree cable circuits an on-site AC withstand test of460 kV / 15 min was performed. The capacitive powerof 2.3 MVA necessary to energize one cable wasprovided by a series resonance test set at a frequencyof 62 Hz.

References

[1] A. Nicolet, Amnagement Cleuson-Dixence,Equipements lectriques, IAS (Ingnieur

Architecte Suisse), no. 3, pp. 30-40, February1999.

cable 400kv cleuson

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