IEEE Power & Energy Magazine - March/April 2014 - 86

figure 5. A 100-kV, 1,000-A Siemens prototype thyristor valve (photo courtesy
of Siemens AG, Siemens Corporate Archives, Munich, Germany).

valve dc transmission installation (see
Figure 4). For the pioneering efforts of
Dr. Uno Lamm of ASEA in Sweden,
the Institution of Electrical Engineers
(IEE) in Great Britain honored him with
the title "father of HVdc power transmission" in 1965. In those days, his
HV mercury-arc valve was exclusively
being used in HVdc projects. The last
mercury-arc valve developed by ASEA
was for a current of 1,000 A/anode that
never went into production because of
the fast progress in the development of
thyristor valves.
Because of the high conversion costs
up to the mid 1960s, HVdc was favored
only in conditions where ac systems
encountered operational difficulties
like sea crossings. Volgograd-Donbass
86

ieee power & energy magazine

(USSR) was the first experimental and
commercial extra-high-voltage dc (EHVdc) (± 400 kV) overhead transmission system commissioned in stages
(initially at 100 kV, then at 200 kV,
and finally at ±400 kV) from 1962 to
1965; its utilization has been reported
to be low (see Table 1). The first longdistance EHVdc (± 400 kV) line in the
west (Pacific Intertie, United States)
was energized in 1970 after a gap of
16 years during which the mercury-arc
bridge capacity had increased 24 times.
This system was followed by other
long-distance dc links using mercuryarc valves.
Mercury-arc valves almost reached
the peak of their development by the
late 1960s when it was not possible to

further increase the blocking voltage of
a mercury-arc valve. The voltage and
power rating of dc systems were basically decided by two types of valve
design available: 1) a four-anode valve
with a six-pulse bridge rating of 133 kV,
1,200 A, 160 MW and 2) a six-anode
valve with a six-pulse bridge rating of
150 kV, 1,800 A, 270 MW (see Table 1).
The mercury-arc valve technology restricted the freedom of rating selections
for converter valves to take maximum
advantage of HVdc. Furthermore, the
arc-back phenomenon necessitated the
need for rapid switch-in and switch-out
of relatively small power blocks. The
major problems associated with mercury-arc valves that affected the performance of the converter station were
✔ arc-backs
✔ radio interference
✔ warm-up time
✔ a limitation in the rate of change
of load
✔ less flexibility in voltage rating
✔ the need for degassing facilities
✔ higher maintenance
✔ larger valve halls
✔ the need for bypass valves
✔ deterioration in service.
There were also environmental
issues associated with the operation
of mercury-arc valves. Each sealed
mercury-arc valve contained 2.64 qt
(2.5 l) of mercury. During operation
and maintenance of the valves, several pounds of mercury vapors were
released to the atmosphere each year;
hence careful monitoring around valve
halls was required to manage mercury
exposure. The mercury-arc-valve-based
dc transmission systems were limited
to the early 1970s (see Table 1). As of
today, mercury-arc valves in all HVdc
systems have been replaced by thyristor
valves, except for one link.

Thyristor Valve
Development
Selenium cells having rectification
properties were discovered in the
year 1883, but these became commercially available in the late 1930s only
after the introduction of copper-oxide
rectifiers. The drawbacks of metallic
march/april 2014

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