IEEE Spectrum November, 2014 - 14

nOV 2014

nORTh aMERICan

720-km cables to the U.K., which will be
the world's longest subsea power cables.
ABB, of Zurich, which built Skagerrak 4's VSCs, says the tougher technology challenge was ensuring that the
VSCs function well alongside the older
HVDC lines. That's because current that
Skagerrak 4 carries south across the
strait must cycle back to Norway via
the Skagerrak 3 cable, which uses the
older, classic HVDC converters. This
will be the world's first paired operation of cables using VSCs and classic
HVDC converters.
Such a pairing gets interesting when
operators want to reverse the flow of
power-something that can happen up
to 1,000 times per year at Skagerrak as
winds and markets shift. VSCs normally
reverse power flow by reversing a line's
current, whereas classic HVDC converters must flip the line's voltage polarity.
So how to reverse power flow on both
lines? ABB's solution is a 5- to 10-second
process that uses coordinated actions
by both converter types and eight highspeed switches that reconfigure the wiring of the VSCs, flipping their polarity
so that the flow of power in Skagerrak 4
can change direction while its current
keeps flowing south.

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smootH opErAtors: Reactors on the Denmark
side of a new HVDC link even out the flow of power
stored in Norway. The country plans similar links to
the United Kingdom and Germany.

The process interrupts the circuit
by which current flows from one cable
to the other. But Lars-Erik Juhlin, an
HVDC expert at ABB, says there is no
meaningful loss or surge in power to
the AC grids.
The key, explains Juhlin, is the excellent electrical conductivity of seawater.
When the power-reversal scheme interrupts the circuit, the converters use subsea electrodes at either shore to feed the
return currents across the strait through
the water. Sending current through seawater can corrode subsea infrastructure such as natural gas pipelines, but
here, the dose makes the poison. "They
can accept even 2,000 amps for up to
2 hours. So for a short pulse, it's no problem," says Juhlin.
Statnett's follow-on interconnect ion projec ts cou ld move qu ick ly
because they will just be longer versions of Skagerrak 4. The first, a pair
of 500-kV VSC cables between Norway
and Germany, was nearing final regulatory approvals when IEEE Spectrum

aBB

weights Germany and the United
Kingdom. Construction on those
is expected to commence during
the coming year.
The existing Skagerrak interconnection, three HVDC cables
with a combined 1,000 megawat ts of capacit y, is a lready
showing the world just how well
wind and hydropower complement each other. According to the
Danish Energy Agency, such interconnectors are why Denmark can
accommodate the world's highest
levels of wind power, which met
41.2 percent of Danish demand in
the first half of this year. At times
wind power production even
exceeds the country's domestic
power demand.
"We store their surplus in the hydro
reservoirs and then feed it back on a
seasonal basis or a daily basis. This is a
very strong business case," says Håkon
Borgen , executive vice president at
Statnett, Norway's state grid operator.
Norwegian hydropower turbines throttle down as Norway consumes Danish
wind energy instead, leaving an equivalent amount of energy parked behind
dams. And when the weather shifts and
becalms the North Sea winds, the reservoirs and Skagerrak's cables feed that
stored energy back to Denmark.
Borgen says t he add it ion of t he
700‑MW Skagerrak 4 advances plans
to plug the U.K. and Germany into
Norway's batteries by pushing the
most flexible form of HVDC technology-voltage source converters (VSCs)-
to its highest voltage yet. VSCs' ability
to stabilize the voltage of the AC grids
on both sides of a cable makes the technology better suited than any existing
alternative for handling intermittent
f lows of renewable energy, he says.
Skagerrak 4's VSCs operate at 500 kilovolts each-30 percent higher than the
previous record holder. Borgen says
that voltage boost will be needed to
reduce losses on longer runs such as the

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Table of Contents for the Digital Edition of IEEE Spectrum November, 2014