IEEE Power & Energy Magazine - March/April 2014 - 90

required, where each thyristor had
to be gated, cooled, and protected.
There were problems associated
with the firing of series-connected thyristors and voltage sharing
during steady-state and transient
conditions. Losses associated with
the thyristor valve were 50-100%
higher than those of mercury-arc
valves. To reduce the valve losses
and cost, it was essential to reduce
the number of thyristors in a valve.
The advances in the thyristor
manufacturing process improved
thyristor characteristics and allowed rapid growth in device rating in the 1960s. By the late 1960s,
thyristors with a blocking capability of 1.6 kV and a current rating of
900 A, equivalent to the current of
a three-anode mercury-arc valve,
were developed. It made solidstate technology an economical
proposition for HVdc applications
up to a voltage of 50 kV per valve,

90

ieee power & energy magazine

figure 7. One prototype thyristor valve for a
GEC 220-kV, 360-MW, three-phase bridge converter (from Proc. American Power Conference,
1967, vol. 29, p. 995).

above which thyristors were still
inferior to mercury-arc valves because of higher losses.
Different manufacturers all
over the world started developing HV thyristor valves, and
there were more than three large
prototype valves on test in different countries. In 1963, an HVdc
transmission (HVDcT) working
group composed of AEG, BBc,
and Siemens was formed to look
at the possibility of the series
connection of smaller mercuryarc valves, which concluded that
future HVdc converters should
be based on thyristor technology.
The series connection of smaller
mercury-arc valves was also being
explored in the mid-1960s in Great
Britain; there were serious drawbacks due to the multiplication of
voltage drops and the additional
requirement of grading circuits.
consequently, two members of

march/april 2014



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2014

IEEE Power & Energy Magazine - March/April 2014 - Cover1
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IEEE Power & Energy Magazine - March/April 2014 - Cover3
IEEE Power & Energy Magazine - March/April 2014 - Cover4
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