IEEE Power & Energy Magazine - March/April 2016 - 71

System performance since the Apollo refurbishment has
improved significantly. One of the main contributors to this
improvement was the elimination of highly sensitive controls
and protection that had previously safeguarded the highly
sensitive thyristors. The old system was designed to protect
a valve with "emergency firing," which involves inducing a
controlled commutation failure. With the large reduction in
the number of such events, the overall HVdc system operates much more smoothly. Overall, the improvement seems
disproportional to the fact that only one-half of the HVdc tie
at Apollo was refurbished. The long-standing oversensitive
control issues have probably also been resolved in Songo,
which may be an added benefit of the overall good performance to date of the new systems at Apollo.

In-Depth Description
of Some FACTS Upgrades
Hasle (Statnett, Norway)

figure 6. The thyristor valves at Hasle, Norway. (Image
used with permission from ABB.)

The work was performed while the other pole as well as
neighboring bridges were in service.
First the thyristor valves were assembled into the enclosures at the side while the containers were at ground potential. Then the valves were equipped with bushings and other
accessories.
The commissioning of the new converters was organized in such a manner that it was never necessary to
interface new and old control and protection systems in
any pole, although there were periods where old and new
equipment were operating in separate poles. Pole 2, the
first pole refurbished, was taken out of operation coincident with the decommissioning and installation of the
new equipment for the first ac filter. During this time, the
power had to be restricted to a level supportable by the
capacity of a single ac filter. After the first filter became
available, it was possible to operate any number of bridges
without restriction. The refurbishment of the second pole
(pole 1) was made on two bridges at a time with the others
in operation.
The refurbishment of pole 1 was successfully carried out.
By modifying the controls and the communication systems,
the power of an extra bridge (240 MW) was gained for a
period of some 60 days during a time that Eskom had severe
restrictions on its generating capacity. As soon as all the converters were available in Apollo, the system was returned to
normal bipolar configuration.
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ieee power & energy magazine

In 2008, recommissioning of the Hasle static var compensators (SVC) in Østfold County in southeastern Norway was
performed after a thorough rehabilitation and modernization
of the installation, which had been in operation in the country's 420-kV power transmission grid since 1981. The main
purpose of the SVC, rated 0-360 Mvar inductive at 420 kV,
is to control the grid voltage in electrical proximity to an oilfilled 420-kV submarine cable across the nearby Oslo Fjord.
The cable is part of a grid interconnecting the power transmission systems of Norway and neighboring Sweden. The
interconnection, having an overall transmission capacity of
approximately 2,000 MW in either direction, is a key facility
for power exchange between the two countries. The 420-kV
line from Hasle to Tveiten consists of an 11.7-km submarine
cable across the Oslo Fjord and 35.5 km of overhead line.
The reactive power generated in the cable and the connecting overhead line is 355 Mvar. The SVC increases the power
export capacity to Sweden, damps electromechanical power
oscillations over the interconnection, and protects the submarine cable from Hasle across the Oslo Fjord to Tveiten against
critical overvoltages in conjunction with switching operations or light loading of the cable. And vice versa, since the
cable is located in a mostly heavily loaded area, its capacitive
generation can be put to use by supporting the voltage during
high load.
The scope of the 2008 rehabilitation of the existing SVC
at Hasle involved replacement of the shunt reactors, thyristor valves, thyristor cooling water pump station, and control
and protection system. The new thyristor valve is shown in
Figure 6.

Insukamini (ZESA, Zimbabwe)
Since 1995, an SVC supplied by ABB and rated 100 Mvar
inductive to 200 Mvar capacitive at 330 kV has been operating in the Matimba-Insukamini 600-MW power interconnecting corridor between South Africa and Zimbabwe.
march/april 2016

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2016