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THE SQUARE BUTTE HVDC SCHEME

 

 

Between :                   Center, North Dakota and Arrowhead (Duluth), Minnesota.

 

 

Power Co :                 Minnesota Power & Light Co. (MP & L) Duluth - Minnesota.

                                   Minnesota Power Cooperative (MPC) Grand Forks - North Dakota.

 

 

Manufacturer :          G.E. , USA.

 

 

Commissioned :         4th of May 1977.

 

 

Main Purpose :          Base load, bulk power transmission from Center to Arrowhead ; reversal transmission is possible. Reinforcement of the AC system in North Dakota.

 

 

Main Data :

                                   Bipole, 500 MW at ± 250 kV DC and 1000 A/pole

 

                                   Overload capacity :

 

                                                           1100 A continuously

                                                           1250 A for 100 sec.

                                                           1400 A for  10 sec.

                                                           1800 A for   1 sec.

 

 

AC Networks :

                                   At both terminals, single phase converter transformer units are used.

 

                                                           + 15

                                   Center : 52 MVA, 235 kV      %/110 kV

                                                           -  5

                                   Connection to the 230 kV AC system

                                   Short circuit : min. 3000 MVA.

 

                                                           + 10

                                   Arrowhead : 48 MVA, 234      %/102 kV

                                                           - 10

                                   Connection to the 230 kV AC system

                                   Short circuit : min. 2500 MVA.

 

 

 

HVDC System :         Overhead lines : 749 km

 

                                   Line towers : Self supporting lattice aluminium structure ; guyed towers are used in bogs.

 

                                   Conductors : 1 x 1439 mm2 ACRS/pole.

                                   Shieldings : 1 x   70 mm2 steel wire.

                                   Insulator leakage : 2.72 cm/kV (of 250kV)

 

 

Electrodes :                At Center designed as circular trench, containing electrodes surrounded by coke ;

                                   resistance = 0.03 ohm.

 

                                   Electrode lines :

                                        2 x 380 mm2 Al, 15 kV underground cable

                                        length = 10 km.

                                        R = 0.3 ohm.

 

                                   At Arrowhead designed by using 800 prepackaged vertical anodes.

                                        R = 0.3 ohm.

 

                                   Electrode lines :

                                   1 x 483 mm2 ACSR,

                                   length = 40 km

                                   R = 2.5 ohm

                                   On-line metallic return switching is possible.

 

 

DC Filters :                are double tuned to the 12th and 24th harmonic.

 

 

HVDC Valves :         the thyristor valves are housed indoor, and they are designed as "Quadruple" valves. Three quadruple valves form a 12-pulse converter unit.

 

                                   One valve has 12 modules, each with 12 thyristors in series connection, giving the total of 144 thyristors in series connection/valve arm and none in parallel. This makes 576 thyristors in one quadruple valve or 1728 thyristors in one 12-pulse converter unit for 250 kV DC and 1000 A.

 

                                   The valve structure is air insulated and air cooled.

 

                                   The smoothing reactor is placed on the high voltage side and is designed for 250 kV DC, 1000 A and 0.06 Henry.

 

 

 

A.C. Filters :

                                   Harmonic filters are provided equal designed at each terminal.

 

                                   Center = Arrowhead : Two 11.+13. harm. filters, each 39 MVAr with :

                                  

 

Q

Harm.

MVAr

C = µF

L = mH

12.5

11

23.5

1.16

50.0

12.5

13

15.5

0.77

53.5

 

 

                                   Two HP filters, each 30 MVAr with :

                                   C = 1.5 µF ; L = 9 mH ; R = 166 ohm

 

 

 

References :              Electra N° 63 (1979)

 

                                   "Harmonic filter and reactive compensation for HVDC"

                                   A general survey by SC14-WG03.

 

 

 

System correspondent :

 

                                   Mr Lynn Crane

                                   Minnesota Power

                                   30 West Superior Street

                                   DULUTH

                                   Minnesota 55802

                                   USA

 

                                   Telex :

                                   Telephone : + 1 218 722 2641

                                   Telefax :