
Benefits of a single US electrical grid with high-voltage direct current - mbrubeck
http://arstechnica.com/science/2016/01/making-a-single-us-electrical-system-boosts-renewables-lowers-costs/
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digerata
I was initially confused by the statement that direct current has lower
transmission losses over long distances.

My recollection was that a big argument for AC over DC going back to Edison v
Tesla was that DC required DC generation centers all over the place because of
transmission range:

"The primary drawback with the Edison direct current system was that it ran at
110 volts from generation to its final destination giving it a relatively
short useful transmission range: to keep the size of the expensive copper
conductors down generating plants had to be situated in the middle of
population centers and could only supply customers less than a mile from the
plant."

Source:
[https://en.wikipedia.org/wiki/War_of_Currents#Edison.27s_DC_...](https://en.wikipedia.org/wiki/War_of_Currents#Edison.27s_DC_company)

Yet, it appears this is no longer the case.
[https://en.wikipedia.org/wiki/High-
voltage_direct_current](https://en.wikipedia.org/wiki/High-
voltage_direct_current)

Why is that?

~~~
deelowe
Higher voltages and frequencies is how you get less loses as resistance is
coupled to current. The lower the current, the less the resistance affects
transmission. However you can only increase ac voltage and/or frequency so
much before you have another issue, the impedence of the air itself starts to
create loses Also, because AC doesn't fully penetrate the wire, you have to
run much larger wire sizes to achieve the same affect. At a certain point, you
simply can not push more power using AC without resulting to things like
superconductors.

DC OTOH, does not have the skin effect issue and so because more desirable in
certain cases. In fact, high voltage DC is how they electricity directly to LA
all the way from Oregon/Washington state:
[https://en.wikipedia.org/wiki/Pacific_DC_Intertie](https://en.wikipedia.org/wiki/Pacific_DC_Intertie)

~~~
amalag
I read further on wikipedia and it mentions that these lines can also be
underground. DC cables do not suffer from capacitance issues that AC cables
do: "Long underground DC cables have no such issue and can run for thousands
of miles."
[https://en.wikipedia.org/wiki/Electric_power_transmission#Un...](https://en.wikipedia.org/wiki/Electric_power_transmission#Underground_transmission)

~~~
deelowe
yeah. HVDC is pretty interesting. Lots of great applications.

------
cellularmitosis
Currently, the electricity you get at home has been stepped down in voltage
several times. The power station delivers it on the order of 100,000V, which
gets stepped down at a sub-station to something like 20,000V, and then stepped
down by a distribution transformer to 120/240V.

If the grid were DC, what would do all of that stepping down? Is he proposing
replacing all of those transformers with an army of industrial strength DC-DC
converters? Wouldn't that eat into the efficiency gains he is proposing (I
would also imagine that that gigantic active switching solutions are more
expensive and less reliable than gigantic transformers).

My first thought at a suggested alternative was "why not try increasing the
frequency of the distribution AC?". This redditor explains why that's not
ideal:
[https://www.reddit.com/r/askscience/comments/1u45rz/why_dont...](https://www.reddit.com/r/askscience/comments/1u45rz/why_dont_we_use_a_higher_frequency_in_the_ac/ceejje6)

~~~
jhallenworld
The current proposed UHV DC systems are not sophisticated switching
regulators. They still use 50/60 Hz AC transformers to change the voltage but
use electronic commutators to convert this to DC and back to AC (and is
reversible). The commutators actually use thyristors (they can only be turned
on, and depend on AC momentum to turn off). It means the receiving end of the
power must have large AC machines (power generators).

Take a look at:
[https://en.wikipedia.org/wiki/HVDC_converter](https://en.wikipedia.org/wiki/HVDC_converter)
It mentions IGBT based commutators which do not require AC machines, but which
are less efficient.

I think all the thoughts about the switching harmonics being horrendous are
correct.

------
jonrx
AFAIK Hydro-Quebec already does something like this.

[http://www.hydroquebec.com/learning/transport/grandes-
distan...](http://www.hydroquebec.com/learning/transport/grandes-
distances.html)

Granted, there's not much details :

« The technology used to transmit direct current is not the most common.
However, it can be advantageous for isolating alternating-current systems or
controlling the quantity of electricity transmitted. Hydro-Québec has a
direct-current line (which goes from the Baie-James region to Sandy Pond, near
Boston) as well as many direct-current interconnections with neighboring
systems. »

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Gravityloss
China has built a lot of ultra high voltage AC and high voltage DC
transmission lines, and is building more.

[http://dsius.com/cet/HVDCinChina_EPRI2013_HVDC.pdf](http://dsius.com/cet/HVDCinChina_EPRI2013_HVDC.pdf)

There's been some cartels in the power transmission scene.
[http://www.reuters.com/article/eu-transformercartel-
idUSL726...](http://www.reuters.com/article/eu-transformercartel-
idUSL726781420091007) It could be ripe for some "disruption".

------
neoCrimeLabs
Thomas Edison's PR powerhouse still trying to make direct current look good
and Nikola Tesla's alternating current look bad. When will the madness end?

Note: This is meant as humor. :-)

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it_luddite
Stopped reading at "while all the US grids operate on the same frequency,
those frequencies aren't always aligned—the peak in the AC of one grid may
line up with the trough in a neighboring grid". Practically every regional
grid is interconnected to it's neighbor. If they get this basic infrastructure
wrong, I'm unwilling to fact check the rest of their premise.

~~~
mbrubeck
The article doesn't claim that interconnection is impossible, just that it
requires extra AC->AC conversion hardware at each interface.

~~~
it_luddite
I didn't even read that far :0 The point of my comment was their lack of
understanding of how the existing transmission system is connected to their
neighboring regions. This is the base infrastructure of the electric energy
market (long term, day ahead, and spot markets). These interconnects between
regions (grids) requires that frequency is in synch and any deviation will
cause a disconnect.

~~~
mikeash
My understanding is that there are five separate grids which are not frequency
synchronized, as described here:

[https://en.wikipedia.org/wiki/Eastern_Interconnection](https://en.wikipedia.org/wiki/Eastern_Interconnection)

They are connected, but with DC or other techniques that don't require
matching phase.

Not so?

