HVDC is much more efficient than AC transmission in long-distance applications, which is even more important for
projects now in development. HVDC systems feature lower
line resistance, which leads to significantly reduced line losses
in long-distance applications. This also means HVDC designs
can make full use of their conductors’ carrying capacity, so they
can move more power with similarly sized conductors. While
AC transmission requires three conductors, HVDC plans only
need two, which means narrower right-of-way requirements
for transmission towers and other equipment.
“Generally, a DC line of the same transmission capacity is
cheaper to build,” said Roger Rosenqvist, business development
vice president in ABB’s HVDC and HV cable division, based in
Cary, N.C. “And, the value you put on the energy that’s basically
burned off to the environment also plays into the picture.”
This added efficiency, along with reduced right-of-way
needs, is driving a surge in HVDC development in China,
Europe and the United States. China, especially, is turning
to the technology to carry enormous amounts of electricity
from resource-rich rural areas to its dense population centers.
The country has recently completed a 1,600-mile, ultra-high-
voltage DC line with a capacity of 1, 100 MW at 1, 100 kilovolts
(kV)—current U.S. AC transmission systems top out at 765 kV.
A large network of similar 800– 1,100-kV lines is in the planning and construction stages. In the wake of Japan’s Fukushima
nuclear disaster, Germany also is developing an HVDC network
to use wind-power sources to replace the nuclear generation
it shut down.
Largely due to the fact that interstate transmission lines
require approval from each state they cross, the United States
has been somewhat slower in such efforts. Planning and permitting long-distance transmission across multiple states is
difficult and expensive. However, several U.S. projects that
promise to enable bulk transmission of wind-generated electricity from the West and Midwest to the East, and hydropower
from Canada to the Northeast, are slowly moving forward.
The key technology for HVDC systems, and the point where
the greatest advances have come, is at the converter station.
It is at this facility that AC power from the supply point is
converted to DC for transmission. Just as semiconductors
have shrunk to enable moon-mission-level data crunching
on a device as small as a smartphone, converter-station space
requirements have become significantly less, even as capabilities have climbed.
47 ELECTRICALCONTRACTOR | JUN. 16 | WWW.ECMAG.COM
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