IEEE Power & Energy Magazine - September/October 2017 - 97

forming the cathode evaporated within
the glass envelope during operation but
quickly resettled back into a pool at the
bottom when finished. The shape of
the graphite-rod anode arms helped facilitate drainage back into the pool and
avoided a conductive path between the
cathode and anode (Figure 2).

transmission site using mercury-arc
valve was commissioned. At that point,
blocking voltage of the mercury-arc
valves could no long be increased.

the island of Gotland, Sweden, to the
mainland location of Västervik. The
connection was made by 90 km of undersea cable. The Gotland HVdc link
had a rating of 100 kV, 20 MW and
was the first commercial HVdc in the
world. This would be one of the many
firsts for the Gotland HVdc project.

Gotland HVdc
The Gotland HVdc link began operation in 1954, connecting Ynge on

Early Design Issues
Early mercury-arc valves were not without issues. Using mercury in fragile
glass envelopes was a constant risk in
the event that the glass envelope was
broken. Reports indicate that a number
of HVdc converter stations performed
extensive mercury remediations to
eliminate toxic buildups at the sites.
In addition, some mercury-arc valves
were prone to "arc back," according to
reports, where the valve conducted in
the reverse direction when the voltage
across it was negative. "Arc-backs can
be damaging or destructive to the valve,
as well as creating high short-circuit
currents in the external circuit, and are
more prevalent at higher voltages. One
example of the problems caused by
backfire occurred in 1960 subsequent to
the electrification of the Glasgow North
Suburban Railway where steam services had to be re-introduced after several
mishaps. For many years this effect limited the practical operating voltage of
mercury-arc valves to a few kilovolts,"
according to Wikipedia.
"The solution was found to be to
include grading electrodes between the
anode and control grid, connected to
an external resistor-capacitor divider
circuit. Lamm conducted pioneering work at ASEA in Sweden on this
problem throughout the 1930s and
1940s, leading to the first truly practical mercury-arc valve for HVdc transmission, which was put into service on
the 20 MW, 100 kV HVdc link from
mainland Sweden to the island of Gotland in 1954," according to an article in
Wikipedia. (See Figure 3.)
There would be ten more HVdc
projects constructed after the Gotland
HVdc link using mercury-arc valves.
Further enhancements were made until the early 1970s when the last HVdc
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2017