IEEE Power & Energy Magazine - May/June 2019 - 117

in my view (continued from p. 120)
Fernando Valley. The operating staff
came from BPA (Celilo) and Los Angeles Water and Power (Sylmar).
All went smoothly until the morning of 9 February 1971, when a magnitude 6.5 earthquake destroyed about
half of the Sylmar converter station. In
roughly six months, half of the station
had been reassembled. In particular, the
converter valve mounting was changed.
Previously, the heavy valves sat atop
insulator stacks. In the restored building, the valves were suspended from
the valve hall ceiling. In the event of
another earthquake, they would swing
to and fro, rather than collapse in a pool
of mercury.
Subsequently, the converter assemblies would be increased in voltage and
power rating and changed to the new
solid-state thyristor technology, and
they would be able to accommodate
more voltage and a higher power rating. These assemblies would also be
suspended from the valve hall ceiling,
as all current-source valves have been
since then. From the original station rating of ±400 kV, 1,440 MW, it now reads
±560 kV, 3,800 MW (Celilo). One can
think of the converter station as comprising oversized plastic toy bricks, where
equipment and ratings can be modified
as needed. The Pacific Intertie illustrates how HVdc links, via a modular
design, can be modernized to accommodate new valve technologies and provide increased capacity.

Thyristor Converter
Valves Arrive
Let's backtrack slightly to the arrival of
the solid-state thyristor-based converters.
Mercury-arc valves were electrically efficient. However, modifying such valves
to handle increased voltage was difficult.
Additionally, the ongoing maintenance
had high accompanying costs, and the
environmental issues surrounding the
use of mercury needed to be addressed.
As solid-state thyristors appeared with
higher voltage ratings and the ability to
handle more current, they began to supplant the mercury-arc valves. In 1972,
may/june 2019

General Electric built the Eel River
320-MW back-to-back station linking
the United States to New Brunswick,
Canada. Two thyristors were parallel
connected to obtain the rated current,
and multiple pairs were series connected
to achieve the needed voltage rating.
In 1977, GE commissioned the Square Butte link [250 kV ± dc, 500 MW,
456 mi (730 km)] from North Dakota
to northern Minnesota. Two years later,
in 1979, ASEA commissioned the
CU link [400 kV ± dc, 1,000 MW,
427 mi (687 km)] from North Dakota
to east central Minnesota. Again, the
converters were thyristor based (and
air cooled).
Other applications are also worth
mentioning. One, in particular, is the
1965 Sakuma back-to-back link in Japan (125 kV, 300 MW), which interconnects a 50-Hz system with a 60-Hz one.
(This is not easily done with an ac-ac
system.) Other back-to-back systems
are found along the border of Texas and
the Rocky Mountains, which separate
the eastern from the western U.S. electrical grid systems. These regions now
all use thyristor-based converters.
In South America, the Brazil/Paraguay Itaipu system was commissioned
in stages from 1984 to 1987. Voltage
and power ratings increased to ±600 kV,
3,150 MW. The system carries power
for 488 mi (780 km). A parallel extraHV ac system was also installed. The
(electrically) small country of Paraguay operates at 50 Hz while the much
larger country of Brazil is 60 Hz. At
the dam, there are two 50-Hz generators
and 16 60-Hz generators. The HVdc
link enables the 50- and 60-Hz systems
to cooperate efficiently. The converter
valves were thyristor based as mercury-arc valves were supplanted by large
power semiconductors.

rectifier is at the James Bay hydroelectric complex in Canada. The primary
inverter station is at Sandy Pond outside of Boston. There is another converter station near Montréal, which can
be operated as needed. For example,
in January 1998, a massive ice storm
coated the Québec transmission network with 3-4 in of ice. By reversing
the normal flow of power on the HVdc
network, Sandy Pond was operated as a
rectifier, and the Montréal station operated as an inverter to begin the restoration process in Montréal.
The largest systems in operation or
under construction are now in China
and India. These include the massive
Three Gorges Dam in China, which
will deliver 22,500 MW over both ac
and HVdc systems. Other systems of
note in China include 600-kV and
800-kV HVdc systems with 11,000 kV,
10,000 MW under construction. (China is an active bidder on new projects.)
India has 800-kV links in operation,
including a three-terminal 6,000-MW
link. Various large HVdc links in the
United States are or have been under
consideration, including 500-kV networks
connecting huge windfarms in rural areas far from major population centers
(Wyoming) to large urban areas (Las
Vegas, Los Angeles). A link has also
been considered extending east from
the Oklahoma Panhandle toward large
Midwest networks.
The availability of thyristors with
high voltage and current ratings has
evolved over the last several decades to
the point where converter station losses
are in the range of 1% (per station) or
lower. For a long transmission line, the
line losses exceed the station losses.
There have long been reports of new
semiconductor materials (e.g., silicon
carbide), but they have not yet arrived.

Ever-Larger Systems

New Converter Technology

The first multiterminal system in North
America is the three-terminal system
linking Quebec, Canada, and New England in the United States. The system is
rated 2,250 MW, ±450 kV. The primary

Perhaps of greater interest is the evolution from Graetz bridge current-source
converters to VSCs that use gate turnoff power semiconductor technology.
One of the first VSC applications in
ieee power & energy magazine

117



IEEE Power & Energy Magazine - May/June 2019

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Contents
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