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THE SKAGERRAK HVDC SCHEME

 

 

Between:      Denmark (Tjele) and Norway (Kristiansand).

 

Power Comp:             The Jutland-Funen Power Pool, Elsam, Fredericia and the Norwegian Power Grid Company, Statnett, Oslo.

 

Manufacturer:

              ABB (ASEA), Sweden, Pole 1 and Pole 2, 1976 - 1977

              ABB Power Systems, Sweden, Pole 3, 1993.

 

Commissioned:

              Monopole (Pole 1) in October 1976, Bipole (Pole 2) in September 1977.  In 1993 the third pole was commisioned as a monopole.  In December 1993 the current direction in Pole 2 was changed.

 

Main purpose:

              Peak power transmission to Denmark, with off peak return.  Energy supply to Norway in dry years.  Surplus energy supply to Denmark in years with ample precipitation.  Energy supply during emergency situations to support the AC systems in both countries i.e. in case of AC network disturbances or loss of generation etc.

 

Main data:

              Monopoles, 2 x 275 MW at 250 kV DC and 1100 A/pole.  Pole 1 and Pole 2 now work in parallel as two independent monopoles, and are connected in series with Pole 3.

 

              Monopole 500 MW at 350 kV DC and 1429 A/pole.  Overload capacity (temperature dependante) : Contineous current limit = 1570 A. 

             

              Maximum continous power transfer capability is 1050 MW. 

             

AC Networks:

              At all terminals two 3-phase converter transformers are used per pole and 12-pulse converter unit. 

              Tjele:

 

 

Pole

Vprim [kV]

Vsec [kV]

S [MVA]

Vreg [%]

1

150

109

154

+18.5 %, -6,5%

2

150

109

154

+18.5 %, -6,5%

3

400

143.9

256.4

+ 30 %, - 10%

             

              The six transformers are connected to the 150/400 kV AC system.  All poles have two transformers with YNy0- and YNd11-couplings. 

 

              Short circuit capacity:

             

              min. = 3000 MVA    Pole 1/Pole 2, 150 kV

 

              min. = 1700 MVA    Pole 3, 400 kV

              Extreme min. = 1000 MVA Pole 3, 400 kV

 

             

              Kristiansand:

             

Pole

Vprim [kV]

Vsec [kV]

S [MVA]

Vreg [%]

1

275

109

154

+ 24.5 %, - 2 %

2

275

109

154

+ 24.5 %, - 2 %

3

300

143.9

256.4

+ 30 %, - 10 %

             

              The six transformers are connected to the 300 kV AC system.  All poles have two transformers with YNy0- and YNd11-couplings. 

 

              Short circuit capacity: min. = 2500  MVA

 

HVDC system: 

              Route length = 240 km

 

Overhead lines:

              Denmark:

              Pole 1 and pole 2 have each a simplex ACSR conductor of  772 mm² (Martin).   Pole 3 has a duplex ACSR conductor of 2 x 772 mm² (Martin).

             

              Norway: 

              Pole 1 and pole 2 have each a simplex ACSR conductor of  772 mm² (Martin).   Pole 3 has a duplex ACSR conductor of 2 x 772 mm² (Martin).


 

              Length of overhead lines:  Denmark = 85 km

                                        Norway = 28 km.

 

              Line towers: They are self supporting lattice steel structures.

 

              Shieldings: The overhead lines are shielded in Denmark by a single steel wire, and in Norway by 2 steel wires (Portal towers).

 

              Insulator leakage:

             

              Denmark: min. 4.18 cm/kV ( 250 kV)

                        min. 3.93 cm/kV (350 kV)

 

              Norway:  min. 3.45 cm/kV (on 250 kV).

                        min. 3.75 cm/kV (on 350 kV)

 

Submarine cables: 

              Pole 1 and Pole 2

              There are one cable per pole, each with a single Cu-conductor of 800 mm². The cables have oilimpregnated paper insulation (solid type).

              Length = 127 km, weight = 48 kg/m in air.

              Armouring: a double contrahelically wrapped steel armouring of 5 and 7 mm respectively is used.

 

              Pole 3

              One cable with a single Cu-conductor of  1400 mm². The cable have oilimpregnated paper insulation (solid type).

              Length = 127 km, Weight = 59 kg/m in air.

              Armouring: a double contrahelically wrapped steel armouring of  7 and 6 mm respectively is used.

 

Land Cable:

              Pole 1 and Pole 2

                        One cable per pole with a single Cu-                           conductor of 1000 mm².

              Length = 3,5 km, weight = 35 kg/m in air.

              Armouring: Single wrapped steel armouring of 5.6 mm  is used.

 

              Pole 3

              The submarine cable is used.

              Length = 3,5 km

 

Electrodes:

              The electrode stations are located on the shore in Denmark and Norway.

              The distance from the converter stations is about 29-30 km.

 

              In Denmark the electrode is designed as a number of parallel connected graphite electrodes, arranged in concrete rings, each with d = 2.5 m and at a depth of 2 m. Coke backfill is used. R = 0.35 ohm.

 

              In Norway the electrode is designed in a similar way except that the graphite electrodes are placed in a wooden structure with coke backfill; R = 0.23 ohm.  The total number of electrodes on the norwegian side is 61.  Some of this are connected in series to give better current distribution in the cables to the electrode busbar.

 

              Electrode lines: In Denmark 3 x 454 mm² ACSR                   (Condor) conductors in parallel are used for                       the first 18 km, and supported by the                          overhead line towers. For the remaining 11                   km three parallel underground PEX-cables are                   used, each with a single Al-conductor of 500                       mm² shielded cable.

                        In case of a fault on one or two of the                        electrodelines, there are automatic runback                        to limit the current.

 

              In Norway the first 20 km of the electrode lines are likewise supported by the line towers whereas the remaining 10 km are designed as a separate overhead line on wooden poles. The conductors used are 2 x 344 mm² Al in parallel on the first 20 km, and 2 x 344 mm² Al in parallel on the separate overhead line.

 

DC filters:

              Denmark:

              On the Danish side there is a filter between the neutral bus and ground, made up by four 1 F capasitors.  There is also an active filter.  The active part feeds into the DC-line through the passive part.  The principles of the active filter is that harmonics in the DC-lines of pole 1, 2 and 3 are measured and added together.  A controller reproduces an equal current in counterphase.  This signal is then amplified in a high power amplifier and fed into the neutral bus end of the passive DC-filter by a transformer, thereby removing the disturbances.


 

 

Units

Pole 3

Pole 1 and 2

C1

F

0.923

1.25

L1

mH

15.8

11.25

C2

F

1.349

5.0

L2

mH

28.0

37.0

R1

ohm

 

150

C3

F

 

1.9

 

              Norway:

 

              The DC-filters of conventional design is                       used.

    

 

Units

Pole 3

Pole 1 and 2

C1

F

0.923

1.25

L1

mH

37.0

11.25

C2

F

1.897

5.0

L2

mH

28.0

 

R1

ohm

1400

 

 

 

HVDC valves: 

              In all three poles the thyristor valves are housed indoor, and they are designed as "Quadruple" valves. Three quadruple valves form a 12-pulse converter unit.

 

              Pole 1 and Pole 2

              Each quadruple valve has 4 valve sections in series connection. One valve section has 24 modules, each with 6 series connected thyristors, giving the total of 144 thyristors in series connection per valve arm. This makes 576 thyristors for one quadruple valve or 1728 thyristors in one 12-pulse converter unit for 250 kV DC and 1000 A. Parallel connection of thyristors is not used.

 

              The valve structure is air insulated and air cooled.

 

              Pole 3

              Each quadruple valve has 4 valve sections in series connection. One valve section has 10 modules, each with 6 series connected thyristors, giving the total of 60 thyristors in series connection per valve arm.

             
This makes 240 thyristors for one quadruple valve or 720 thyristors in one 12-pulse converter unit for 350 kV DC and 1429 A. Parallel connection of thyristors is not used.

 

              The valve structure is air insulated and water cooled.

 

Valve cooling:

             

              Pole 1 and pole 2:

              The valves are cooled by filtered air.  The heated air is transferred to a water/glycol secondary circuit through heatexchangers.  The secondary circuit is cooled in outdoor heatexchangers.

 

              Pole 3:

              The valves are cooled by deionized water circulated in a closed loop system.  The heat is transferred to a secondary circuit which is cooled in outdoor heatexchangers.

 

Smoothing reactors:

 

              Pole 1 and pole 2:

The smoothing reactors (equal in both terminals)  are placed on the neutral side and is designed for 0.5 H at 1000 A.

 

              Pole 3:

              The reactors are aircoils and placed on the high voltage side.  They are designed for 225 mH at 1260 A.


             

AC Filters:

              Harmonic filters are provided at each terminal:

 

              Denmark: Two 11. + 13. harm. filters each

                        20 MVAr with:

       

Harm. order = n

Q = MVAr

C1=[F]

C2=[F]

L1=[mH]

L2=[mH]

R1=[]

R2=[]

11

20

2.47

 

34

 

0,71

 

13

20

2.47

 

24.3

 

0.48

 

 

              Furthermore one shunt capacitor bank = 80 MVAr (changed from HP in Dec. 1980); C = 9.95 uF phase.

              Pole 3: 4 x HP 11/24 harmonic filters.

 

Harm.

order = n

Q = MVAr

C1=[F]

C2=[F]

L1=[mH]

L2=[mH]

R1=[]

R2=[]

12/24

65

1.66

2.013

32.26

17.80

648

383

 

 

              Norway: Two 11. + 13. haromonic filters each with 83 MVAr, and two HP filters, each

              90 MVAr.

 

Harm.

order=n

Q=MVAr

C1=[F]

C2=[F]

L1=[mH]

L2=[mH]

R1=[]

R2=[]

11

83

1.468

 

57.04

 

9856

 

13

83

 

1.468

 

40.84

 

8340

24

90

3.183

 

5.526

 

104

 


 

             

References:

 

            Cigré 1978, papers 14.04. The Skagerrak HVDC Transmission scheme.

            "System design features and service experiment".  O. Hauge, S. Vikanes, E. Andersen and G. Styrbro

            Electra no. 63 (1979).

            "Harmonic filter and reactive compensation for HVDC".A general survey by SC14-WG03.

            Cigré 1993 Int. Colloquium New Zealand

              "New Features of the Skagerrak 3 HVDC Intertie", B. Jansson, D. Norman, P.L. Thomsen, A. Strandem.

           Cigré 1994. "Installation of Skagerrak 3 - a 350 kV HVDC Link between Norway and Denmark".  A. Strandem, J. E. Larsen    

 

 

 

 

System correspondents:

        

Mr B. Sehman

ELSAM - Tjele

DK - 8822 Örum Sdrl.

DENMARK

 

Telex:   66207 ELSAMT DK

Telephone:    + 45 6 652222

 

Mr. O.T Eide

Statnett SF

Technical Operations departement

Regional Operations Southern Norway

P.O. Box 70 Smestad

0309 Oslo

NORWAY

 

Telephone:    + 47 22 52 75 00

Fax      + 47 22 52 75 01

 

 

 

Mr B. Tonstad

Statnett SF

Kristiansand Converter Station

4652  Øvrebø

NORWAY

 

Telephone:    47 380 21 201

Fax:     47 380 23 162