POWER June 2022 - 16

TRANSMISSION & DISTRIBUTION
The Vital Link: How HVDC Is
Modernizing the Grid
Significant advances in high-voltage direct current (HVDC) transmission are in step
with rapid changes to energy systems worldwide.
Sonal Patel
S
hortly after POWER magazine began
publication in 1882, the competitive
technology battle between
Thomas Edison and Nikola Tesla-the
legendary " War of the Currents " -began
to brew. Edison had developed direct
current (DC), which essentially runs
continually in a single direction like a battery
or a fuel cell, allowing early generating
stations to deliver electricity over
copper wires. Though revolutionary, the
method was so inefficient, power plants
remained tethered within a mile of the
load they served. And though DC was
widely adopted, it was not easily converted
to higher or lower voltages.
Tesla posited alternating current (AC)
was a better solution. Reversing direction
a certain number of times per
second (60 in the U.S.), AC could be
converted to different voltages relatively
easily using a transformer, enabling the
transmission of power over long distances
using smaller wires at a higher voltage.
AC for the most part proved to be
the more cost-effective and flexible highvoltage
transmission solution, and it was
quickly adopted as a hallmark of power
networks everywhere.
Why HVDC?
Most high-voltage direct current
(HVDC) lines are point-to-point lines
that typically carry power, unidirectionally
and in a controlled manner, from
a large power plant or agglomeration
of power plants to a single converter
station in a distant alternating current
(AC) grid. An HVDC link typically comprises
its generation sources (except
in intertie applications), AC transmission
lines (to provide a dedicated collector
system), an AC to DC converter
station, HVDC transmission lines, a
DC to AC converter station, AC transmission
and distribution lines, and, finally,
its end-users.
" An AC cable and a DC cable have
a pretty large electrical capacitance, "
or the ratio of the amount of electric
charge stored on a conductor to a difference
in electric potential, Roger
Rosenqvist, vice president of business
development for Hitachi Energy's
HVDC division in North America, explained
to POWER in May. " And so, if
you connect an AC cable to the grid,
before you even start to push any
power through that cable, you will see
a pretty high current in the cable because
it's constantly charged and recharged
with the power frequency. At
a certain critical length, you have essentially
used up the whole capacity
rating of the cable just for the charging
current, [leaving] no room to transmit
real power in the system, " he said.
The War of the Currents was not de1.
In 1954, ASEA, the predecessor of ABB,
used this classic technology utilizing mercury
arc valves to construct the world's first commercial
high-voltage direct currect (HVDC)
link between Västervik, on the east coast of
Sweden, and Ygne, on the island of Gotland in
the Baltic Sea. The original Gotland link could
transfer 20 MW over a 98-kilometer-long submarine
cable with a voltage of 100 kV. Courtesy:
Hitachi Energy
16
finitively won, however. Attempts to
transform DC voltage to higher or lower
levels continued. In 1954, after some
marked demonstration triumphs, Swedish
engineering firm ASEA opened the
world's first high-voltage direct current
(HVDC) link between mainland Sweden
and the island of Gotland (Figure 1),
based on a 1929-patented low-pressure
mercury-arc converter designed by Dr.
Uno Lamm (who is largely considered
the " Father of HVDC " ).
Advancements came fluidly thereafter.
In the 1960s, General Electric introduced
www.powermag.com
" When you want to increase the
capacity rating on an electrical circuit,
you typically do that by going to
higher voltage, but when you go to
a higher voltage on an AC cable, you
increase the charging current. So, a
high-capacity AC circuit needs to go
to a high voltage like 345 or 500 kV,
but a 500-kV cable has a shorter practical
distance than a 345-kV cable or a
230-kV cable. " And while AC circuits
can transmit power
at
higher
distances
at lower voltages, that is compounded
with the associated reduced
capacity in the cable, which often requires
multiple cables. " So, there is
a maximum length at which you can
transmit electric power in the submarine
or underground cable using AC
technology, " he said.
For DC, " you charge the electrical
capacitance of the cable when
you energize it, but then the voltage
stays constant, so there is no routine
charging and recharging of the
cables, " Rosenqvist said. " Once a DC
cable has been energized, you have
the capacity of the electrical conductor
in the cable available for transmitting
real power, and there is really no
practical length limitation on how far
you can go with a DC cable. " DC cables
can enable transmissions of up
to 2,000 MW at very high voltages in
a single circuit, " which is not possible
using AC technology, " Rosenqvist
said.
silicon-controlled rectifiers (now known
as thyristors), a more efficient solid-state
semiconductor converter
technology.
While thyristor valves have ultimately
replaced many of the HVDC lines with
mercury valves (which are known as s
line-commutated converters), thyristor
valves still require an external AC circuit
to turn them off or on.
In the 1990s, ABB, a company that
evolved from ASEA, pioneered a voltage
source converter (VSC) under its HVDC
Light brand, and subsequently installed
the world's first VSC link in Gotland.
VSCs, which are more compact than
POWER | June 2022
http://www.powermag.com

POWER June 2022

Table of Contents for the Digital Edition of POWER June 2022

POWER June 2022 - Intro
POWER June 2022 - Cover1
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