
Germany Takes the Lead in HVDC - EEGuy
http://spectrum.ieee.org/energy/renewables/germany-takes-the-lead-in-hvdc
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EEGuy
Until recent news such as this, I didn't know it was the "conceptually simple"
but practically difficult ability to switch (interrupt) a GW or so of DC
energy flow preventing the implementation of better than point-to-point HVDC
(i.e. multiple access point) power grids.

Evidently interruption's easier with AC because of the zero-crossings of
current every half-cycle.

Three alternate solutions to HVDC circuit interruption are described in this
article. Two are electronically integrated with the output circuitry of the
bidirectional AC-DC converters (bidirectional energy flow AC-DC converters
being an interesting, but off-topic subject in their own right).

I find it an ingenious use of an IGBT bridge surrounding a capacitor to throw
the cap's charge into reversed polarity so as to stop the current flow --
effectively opening the circuit. This tells me the cap would seem to be in
series with the DC side somehow. Were it in parallel, well, it wouldn't make
much sense to reverse the cap's polarity as you'd only get to do it once.

I also find it fascinating that the mathematics it takes to describe AC
circuits is far more complex than for DC -- one has to accept that there
genuinely exists a square root of negative one, "imaginary" though we may call
it.

As in programming, good nomenclature is often late to the table. I wouldn't
try (and I'm certainly not qualified) to "refactor" centuries, maybe millennia
[1] of mathematical terminology.

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[1] <https://en.wikipedia.org/wiki/Imaginary_number>

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kken
Very interesting article indeed!

>I also find it fascinating that the mathematics it takes to describe AC
circuits is far more complex than for DC

Why? The math is exactly the same as for DC once you substitute complex
numbers for current, voltage and impedances.

Things get more complicated in the time domain, which becomes relevant once
you want to look into problems like switching HVDC on and OFF. The
nonlinearities of the switching devices lead to differential equations that
can only be solved numerically.

~~~
EEGuy
Thank you kken for your clarifications.

I don't know the physics modelling switching devices or arcs, but I do recall
reading long ago that once an arc starts, its resistance drops dramatically,
continuing to drop as more current flows through it (within limits).

So an arc becomes a runaway situation if current flow isn't ballasted or
elsewhere interrupted. But if current flow is controlled, an arc has practical
application in fluorescent lighting and arc welding.

~~~
caf
A visual demonstration: <http://www.youtube.com/watch?v=9S64r-MjBN4>

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troymc
Germany also stores some energy by pumping water uphill for storage behind
dams, but there's only so much capacity (only so many dams). That will change
if other low-cost grid-level energy storage systems become available.

<https://en.wikipedia.org/wiki/Grid_energy_storage>

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igravious
Why mothball nuke plants when they are not in danger from tsunamis seeing as
how they are miles inland in Southern Germany? Was there a public outcry after
Fukushima? Seems like keeping the nuke plants running locally would obviate
the need for this high euro spend on HVDC as cool and all as the tech sounds.

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claudius
> Was there a public outcry after Fukushima?

Yes. Originally, the social-democrat/green government of 1998-2005 planned to
shut down all nuke plants until 2020 or so, but the conservative/liberal
democrats government that took over in 2009 stopped that, mostly against the
will of the public. After Fukushima, there was such a huge backslash that our
dear chancellor decided it might help her popularity more if she changed her
mind.

And while tsunamis are rather rare in Stuttgart, the upper rhine rift is an
active earthquake zone, and since we unfortunately are not allowed to just
sell the waste to Russia to dump in Siberia, we also have to dispose of it
somewhere in Germany. If you look at a population-density map, you will find
few places with few enough people to accept highly radioactive nuclear waste.

You are right, however, that it is economically somewhat senseless to shut
down the nuke plants basically shortly after the subsidies/initial costs paid
off, but, well.

~~~
eurleif
>You are right, however, that it is economically somewhat senseless to shut
down the nuke plants basically shortly after the subsidies/initial costs paid
off, but, well.

How is it more senseless than shutting them down any other time before the end
of their useful life? Sunk costs are sunk.

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InclinedPlane
You answer your own question. The amount of useful life they have left that
goes unused is an opportunity cost. The earlier they are shut-down the larger
that cost (which necessarily must be taken up in providing power through some
other means).

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junto
Germany has a problem. It has a desire to increase renewables (mostly from
wind), which is found in the north. The primary consumers of the energy
(manufacturing) are not in the north, in fact many are in the south. They need
to master the transmission of that energy from north to south.

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throwawayyyyyy
a couple of schematics of the various configurations of IGBT based converters
in the article would have helped a lot.

