IEEE Power & Energy Magazine - May/June 2019 - 120

in my view

Willis Long

evolution of HVdc links
mercury-arc valves, thyristors, and more

PARTICIPATING IN THE ACTIVIties of the I EEE Power & Energy
Society and CIGRE is of great value,
particularly for the opportunities they
provide for meeting and collaborating with engineers and scientists with
similar interests. These benefits cannot
be overemphasized, as I know from experience during my long career that encompassed high-voltage (HV) dc work
in academia and industry. I strongly
urge new and experienced engineers to
get involved in committees and working groups.
From my perspective, new HVdc applications have continued to evolve with
improvements in equipment, and the
outlook for the development of new technologies in the U.S. market is promising.
This column traces developments from
the earliest mercury-arc valve currentsource converter applications to today,
when new turn-off power semiconductor
technology has ushered in voltage-source
converters (VSCs) with voltage ratings of
more than 500 kV.
In the late 1880s and early 1890s,
there was a "battle of the currents." On
one side was Thomas Edison, who preferred dc for the distribution of electric
energy. On the other side was Nicola
Tesla (and George Westinghouse), who
understood the advantages of ac. The
transmission of large blocks of electric energy by dc was uncommon for a
half century. The definitive technical
breakthrough occurred in Sweden in
1954. In 1970, the first U.S. HVdc link
was commissioned.
Digital Object Identifier 10.1109/MPE.2019.2896692
Date of publication: 17 April 2019

120

ieee power & energy magazine

Gotland Island 1954
The first "modern" HVdc link went
into service in 1954. It connected the
island of Gotland to the Swedish
mainland (96 km, i.e., 60 mi) and accommodated 20-MW transmission at
100 kV via a single conductor cable.
(The return f low was through water and land.) The ac-dc and dc-ac
converter equipment comprised two
six-pulse (Graetz bridge) valve groups,
each rated 50 kV, 200 A, and 10 MW.
The groups were arranged in series
on the dc side. Each valve had two
anodes in parallel.
A few words on the term valve are
in order. To enable the conversion of
ac to dc and dc to ac, an HV switching device was required. In the early
days of HVdc transmission, the converter valve was a mercury-arc device.
A pool of mercury in the bottom of the
valve served as the cathode. During the
positive portion of the ac waveform,
the valve would conduct if a trigger
pulse (gate pulse) was provided. The
valve would cease conducting (block)
if a current zero occurred after the ac
voltage reversed its polarity. (It is far
easier to envision this if the switching
element is a thyristor and not a mercury-arc valve!)
Swedish scientist Uno Lamm is
credited with designing the first successful mercury-arc valve, which led
to the development and application of
the valve for the Gotland Island link.
The normal means of operation was
for the positive converter voltage to
be on the mainland (the rectifier or
ac-to-dc converter) and for the inverter (dc-to-ac converter) to be on Got-

land Island. Reverse operation was
possible but seldom used.

Pacific Intertie
HVdc System
In the early 1960s, a small group of
engineers/planners at the Bonneville
Power Administration (BPA) in the
northwest region of the United States
began examining ways to send excess
hydroelectric power from the Columbia River dams south to the Los
Angeles basin. The expectation was
that, in the summer, there would be
a need for air conditioning and other
load and that the hydroelectric energy from the Pacific Northwest would
be less expensive than the oil-based
generation then used in Southern
California. The exchange would then
reverse in the winter. The engineers
found the idea of HVdc appealing because the distance was roughly 850 mi
(1,370 km), and there was no need to
drop part of the energy midway.
BPA, as a government entity, took
the lead, and their technical staff convinced the utilities in the Los Angeles
basin to join them. The link, Pacific
Intertie, was commissioned in the fall
of 1969, with equipment provided by
ASEA (now ABB) of Sweden and General Electric of the United States. This
equipment included modern mercuryarc valves, the first to be used in the United States. The ratings were ±400 kV (line
to ground), 1,800 A, and 1,440 MW.
The operating substations were at Celilo, Oregon, near The Dalles Dam,
and at Sylmar, California, in the San
(continued on p. 117)
may/june 2019

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