
High-Voltage DC Breakthrough Could Boost Renewable Energy - amalag
http://news.nationalgeographic.com/news/energy/2012/12/121206-high-voltage-dc-breakthrough/
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marshray
Ohms law: V = IR

Power: P = VI

Power loss due to resistive heating in the wires: P = I^2 * R

Note that current (I) quickly dominates the loss calculation because it is
squared. So for transmission lines, efficiency is obtained by keeping the
current as low as possible. Therefore, to transfer the same power you need to
make the voltage as high as possible.

The I^2*R heating loss applies equally to DC and AC transmission lines. So the
DC lines still need to be high voltage for efficiency as well.

By using AC we can 'transform' high-voltage-low-current to low-voltage-high-
current power very easily. It's literally just two coils of wire around a
chunk of iron. Step-down AC transformers are used both at your neighborhood
substation and on the pole outside your house.

But the article didn't say how they planned to transform the high-voltage-low-
current DC into anything usable.

~~~
ChuckMcM
Historically the limit on voltage has been the dielectric constant for air.
Once you exceed it, the air ionizes, and you get a short. Visible as
'lightning' going through the air from source to its ground counterpart.
Transmission line effects limit the ability to send AC power long distances as
the every time you switch the potential you generate magnetic fields and some
of the energy is radiated off into the space around the wire.

AC has been more desirable to date because of a relatively easy infrastructure
for converting it, and for sharing it on the same line. Transforming DC into
AC is easily done by driving a generator set but for most things you'd want
the genset to convert to 'standard' three phase AC and then re-use existing
city wide infrastructure.

What bothered me about the article was that nowhere does it explain how you
can make a DC breaker that can actually disconnect at those voltages. Since
the dielectric constant is a function of distance, when the breaker first
opens there is an arc because, the current really really wants to keep
flowing. And the dielectric constant of ionized air is quite small. "Regular"
breakers have a fan that kicks in once they get to a minimum distance apart to
"blow out" the arc (it pushes it out to extend it and thus 'break' it, see the
video here <https://www.youtube.com/watch?v=hIkNY5xjy5k> for how this works)

Anyway, would love to hear actually what it was they invented.

~~~
marshray
As ususal, looks like the best description made it into Wikipedia
<http://en.wikipedia.org/wiki/High-voltage_direct_current>

_The ABB breaker contains four switching elements, two mechanical (one high-
speed and one low-speed) and two semiconductor (one high-voltage and one low-
voltage). Normally, power flows through the low-speed mechanical switch, the
high-speed mechanical switch, and the low-voltage semiconductor switch. The
last two switches are paralleled by the high-voltage semiconductor switch.

Initially, all switches are closed (on). Because the high-voltage
semiconductor switch has much greater resistance than the mechanical switch
plus the low-voltage semiconductor switch, current flow through it is low. To
disconnect, first the low-voltage semiconductor switch opens. This diverts the
current through the high-voltage semiconductor switch. Because of its
relatively high resistance, it begins heating very rapidly. Then the high-
speed mechanical switch is opened. Unlike the low-voltage semiconductor
switch, which is only capable of standing off the voltage drop of the closed
high-voltage semiconductor switch, this is capable of standing off the full
voltage. Because no current is flowing through this switch when it opens, it
is not damaged by arcing. Then, the high-voltage semiconductor switch is
opened. This actually cuts the power. However, it only cuts power to a very
low level; it is not quite 100% off. A final low-speed mechanical switch
disconnects the residual current._

~~~
ChuckMcM
Oh, I like that. A pretty clever design.

------
dkhenry
If I am not mistaken we already use HVDC on high power runs due to its much
lower loss ? How does this beaker change that ?

~~~
surbas
According to the article HVDC is only used for point to point electrical runs,
because managing grids of it is very hard. What the breaker does is allow for
easier managing of the grids by allowing parts under going surge, like from a
lighting strike, to be segmented off.

~~~
crusso
Exactly. The modern AC power grid is very resilient due in part to the multi-
interconnect nature which allows for parts of the grid to go down without
taking down the whole network.

On an AC grid, there are breakers that automatically disconnect and often
reconnect.

<http://en.wikipedia.org/wiki/Recloser>

I guess that this HDVC hybrid breaker accomplishes more or less the same
thing.

------
EEGuy
DC doesn't have phase and frequency issues AC does. Presumably a shared DC
grid would be constant-voltage. What surprises naive me is that gigawatt
inverters seem to be a done deal. Once we have gigawatt-hour ("utility-")
scale _storage_ , then we can have a fully renewables-based future (again,
naive me thinks).

~~~
stcredzero
The government should be building out infrastructure like this.

~~~
sageikosa
Or maybe someone who could profit from it.

~~~
yarrel
Rent seeking doesn't lead to innovation.

~~~
sageikosa
Hence why the government shouldn't be involved. I live in a state
(Pennsylvania) with energy choice. The one with the most "efficient" system
gets the lowest rates, which allows me to preserve my own little slice of the
"social wealth". If I so choose, I can go for an eco-friendly solution as
well, if that sort of thing made me feel good.

~~~
stcredzero
By this logic, the interstates, the railroads, and the internet should not
exist.

~~~
CapitalistCartr
That makes no sense. First, he's advocating private ownership of electrical
infrastructure, not the elimination of it. Built by private interests doesn't
equate to non-existence.

Second, advocating private ownership of electrical infrastructure doesn't
equate advocating private creation and ownership of all infrastructure.

I rather like a good straw man; this one isn't.

~~~
stcredzero
_> Built by private interests doesn't equate to non-existence._

Sometimes it does.

------
CapitalistCartr
I would love to have my house run on DC. Almost everything I own either
switches to DC internally, or doesn't care. If it were practical to run the
grid on DC, or we all go to solar for residential and light commercial, maybe
I'll get my wish.

~~~
gvb
The stuff in your house that "switches to DC internally" isn't switching to
100,000 to 800,000 volts[1], which is what the HVDC is that the article is
talking about. Running DC in your house isn't going to eliminate any voltage
conversions that are already going on, they will just be DC-DC converters with
48vDC (or whatever) input rather that 120vAC input.

Most of the stuff in your house isn't even running on the same voltages
internally... most of the electronics nowadays use many voltage "rails":

* 17-19v laptop chargers (the voltage has to be higher than the battery's voltage in order to charge the battery)

* 12v for older hard drives

* 5v for not quite so older hard drives and older logic

* 3.3v for newer logic and newer hard drives

* A veritable plethora of sub-3.3v rails for CPU chips and other high tech chips.

* 120vDC for "universal" motors unless you replace all your appliance motors with 48vDC (or whatever) motors

In addition, the current requirements of running at a non-lethal DC voltage
inside your house would require much larger wire gauges than your current
wiring in order to keep the resistive losses reasonable[2]. The rewiring costs
would be staggering.

[1] <http://en.wikipedia.org/wiki/High-voltage_direct_current>

[2] Charts for 12v wiring, 48vDC would be 4x better, but still not good -
[http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvi...](http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorView?langId=-1&storeId=11151&catalogId=10001&page=Marine-
Wire)

~~~
CapitalistCartr
"The stuff in your house that "switches to DC internally" isn't switching to
100,000 to 800,000 volts[1], which is what the HVDC is that the article is
talking about. Running DC in your house isn't going to eliminate any voltage
conversions that are already going on, they will just be DC-DC converters with
48vDC (or whatever) input rather that 120vAC input."

Being an electrician, I'm aware of that. The reason I don't have a DC house is
because the grid couldn't transmit DC long distance, back when the War of the
Currents was fought. Now, maybe I'll get my D/C house soon.

------
specialist
When I was a kid, Scientific American had a cover featuring a high voltage
transistor, which was supposed to revolutionize power transmission.

My very weak understanding is that energy transmission is directed by a
delicate balancing of loads. Which is inefficient. Where using a solid state
switching solution would be more efficient.

Does this sound familiar to anyone?

------
amalag
I didn't follow why the article stated that AC is not ready for the grid of
the future. This DC grid seems to be an incremental improvement, 10%? But IMO
not a huge breakthrough.

