IEEE Power & Energy Magazine - March/April 2016 - 69

just 15 days. Similar control and protection upgrades can be
done with a minimal loss of power transfer capability.
In 2014, a control system upgrade was delivered for
the Skagerrak 3 HVdc link. The delivery also included a
new bipole control enabling Skagerrak 3 and Skagerrak 4
to operate as a true bipolar connection, for example, with
earth current minimization. Skagerrak 4 is a new VSC and
Skagerrak 3 is a line commutated converter (LCC), delivered
in 1993. It is the first time that a VSC system and an LCC
system have been combined to run as a bipole.

Apollo HVdc Station in the Republic
of South Africa
figure 1. The Fenno-Skan 2 HVdc station in Finnböle,
Sweden. The HVdc station on the Swedish side is located
at Finnböle, 70 km from the Fenno-Skan 1 station, located
at Dannebo. The separation of the stations was necessary
due to constraints in the ac grid. (Image used with permission from ABB.)

Norway

Kristiansand

Denmark

Tjele

figure 2. The Skagerrak HVdc link.

çam

biq

Songo

South
Africa

Apollo

Apollo is the inverter end of the Cahora Bassa HVdc scheme
shown in Figure 3. It is situated in the Republic of South
Africa, while the rectifier is at the Songo Station, 1,414 km
to the north, in Mozambique. The present rating of the HVdc
link is 1,920 MW at ±533 kV.
The link provides power transmission capacity from a
hydropower plant on the Zambezi River in northern Mozambique and was put into service in three stages, from 1977
to 1979. The HVdc system were built by the ZAMCO consortium [AEG-Telefunken, Brown Boveri Company (BBC),
and Siemens]. The power link is owned by the power utilities
Hidroeléctrica de Cahora Bassa in Mozambique and Eskom
in South Africa. The system comprises two parallel monopolar lines across a 1,400-km-long route, 900 km of which
is in Mozambican territory.
In 2008, the Apollo converter station of the Cahora Bassa
HVdc link was upgraded with new outdoor air-insulated,
water-cooled valves (shown in Figure 4) replacing the old oilcooled valves; new control and protection systems including
measuring equipment; and new ac filters. The upgrade will
boost transmission capacity from 1,920 MW to 2,500 MW
and also prepare the installation for a future upgrade to
3,960 MW. The power increase possibilities would utilize
the full capacity of the transmission line but would of course
require similar upgrades of the terminal in the other end of
the transmission.
Due to the structure of the Apollo station with eight seriesconnected six-pulse bridges, it was possible to perform the
installation and commissioning in two six-pulse groups at
a time to minimize the disturbance of the power transfer.
The system performance since refurbishment has markedly
improved, with fewer forced outages and higher availability,
as a result of the new valves and the new control equipment.

figure 3. The Apollo-Songo HVdc link.

Thyristor Valves

a control system upgrade of Skagerrak 3 to enable these two
installations to operate together as a bipole connection.
In 2007, a successful upgrade of the Skagerrak 1 and 2
HVdc transmission link was installed, demonstrating how
control and protection upgrades can be completed during a
very short outage of the power transfer; in this case, it was

The first-generation thyristors in the original design were
critically rated because, at the time, the thyristor valve design
had to match the top power and current ratings of the mercury arc technology at its zenith. Various extreme control and
protection measures were therefore applied. For example,
two parallel thyristors, each rated 900 A, were required to
reach the 1,800 A rating; the limited voltage capability of

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