IEEE Power & Energy Magazine - March/April 2014 - 84

a 37.3-mi (60-km) experimental dc
line between Mellerud and Trollhättan in
Sweden. The power capability of the link
was 6.5 MW at 90 kV. Subsequently in
1954, the first commercial HVdc submarine Gotland link (20 MW, 100 kV) was
commissioned by ASEA (see Figure 2).
As HVdc mercury-arc valve technology
matured, two distinctively different valve
designs, Swedish and Russian, were in
use. The major difference was due to the
number of grading electrodes. The Russian valve used four grading electrodes
whereas the Swedish design used a large
number of grading electrodes, for example, 20 electrodes for a 125-kV valve.
The few grading electrode designs
offered a high current rating/anode but a
limited voltage withstand capability. The
Russian design employed a single-anode

bridges, and each valve in the bridge
was made up of three series-connected,
single-anode mercury-arc valves.
The Germans completed the ambitious
Elbe-Berlin HVdc project in April 1945
under the backdrop of World War II
but commissioning activities could not
be concluded. After the end of the war,
the entire system was disassembled and
moved to the USSR by Russians, preventing Germany from achieving the
feat of commissioning the first commercial HVdc project. The Elbe-Berlin
HVdc system was reinstalled in 1950 as
the Moscow-Kashira transmission system, which was used as a power transmission link and research facility.
Since then, several mercury-arcvalve-based HVdc systems were commissioned. In 1946, ASEA e ne rgi z e d

mercury-arc valve (see Figure 3) and,
for higher voltage, two such valves were
used in series. The Swedish design used
multi-anode mercury-arc valves with all
anodes mounted on the common cathode tank (see Figure 2). When voltage
higher than 125 kV was required, two
complete six-pulse bridges were placed
in series. The Swedish design used water cooling whereas the Russian design
used oil cooling for the cathode. In 1961,
English Electric company UK signed
an agreement with ASEA for the design
and manufacture of mercury-arc valves
and subsequently carried out refinements in the vacuum envelope. This resulted in the British development of the
Kingsnorth valve and later the Nelson
River I valve, which turned out to be the
most powerful and the last mercury-arc

table 1. HVdc transmission schemes (1954-1976).
HVdc System

Commissioned Rated Rated Rated SixAnodes/ Transmission
Valve
Power Voltage Current Pulse
Valve
Distance (km)
Type
(MW) (kV)
(A)
Bridge
Voltage
Line Cable Total
(kV)

Average
Utilization
Factor %
(1967-
1976)*

Gotland (Sweden)

July 1954

100

200

Merc

74.1

Cross Channel (United
Kingdom, France)

December
1961

160

±100

800

100

Merc

24.2

Volgograd-Donbass
(USSR)

October 1962- 720
May 1965

±400

900

100**

472

Merc

18.4

Benmore-Haywards
(New Zealand)

April 1965

600

±250

1,200

125

570

609

Merc

51.7

Konti-Skan (Denmark,
Sweden)

September
1965

250

±250

1,000

125

180

Merc

39.0

Sakuma (Japan)

October 1965

300

2 x 125 1,200

125

Merc

4.8

Sardinia (Italy)

June 1967

200

1,000

100

292

121

413

Merc

21.2

Vancouver, Pole 1
(Canada)

July 1968-
October 1969

312

±260

1,200

130

Merc

65.6

Pacific Intertie (United
States)

May 1970

1,440

±400

1,800

133

1,354 0

1,354 Merc

48.3

Nelson River Bipole 1
(Canada)

June 1972-
1976

1,620

±450

1,800

150

890

53.9

Thy

95.3

Merc

20.2

Eel River (Canada)

July 1972

320

2 x 80

2,000

Kingsnorth (United
Kingdom)

1975

640

±266

1,200

133

Cabora-Bassa
(Mozambique, South
Africa)

May 1975

960

±266

1,800

133

1,420 0

Merc

1,420 Thy

*G.S.H. Jarret and R.M. Middleton, "A ten year review of HVDC transmission systems 1967-1976," CIGRÉ Electra, no. 57, pp.
35-46, Mar. 1978.
**Two single-anode, mercury-arc valves in series.
Merc: mercury arc; Thy: thyristor
84

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