High-Voltage DC Breakthrough Could Boost Renewable Energy

http://news.nationalgeographic.com/news/energy/2012/12/121206-high-voltage-dc-breakthrough/

by amalag • 14 years ago

57 35 14 years ago

news.nationalgeographic.com

dkhenry• 14 years ago

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• 14 years ago

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• 14 years ago

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.

kens• 14 years ago

A friend's father worked on a massive DC power line project in Manitoba in the 1960s [1], so HVDC technology has been around for a long time. The article discusses a potential incremental improvement that might make HVDC useful in more circumstances, but the article seems to be making HVDC sound like a big new breakthrough.

[1] http://en.wikipedia.org/wiki/Nelson_River_Bipole

gus_massa• 14 years ago

DC is better for very long distances and underwater connections: http://en.wikipedia.org/wiki/High-voltage_direct_current

EEGuy• 14 years ago

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).

stcredzero4• 14 years ago

The government should be building out infrastructure like this.

sageikosa3• 14 years ago

Or maybe someone who could profit from it.

alexkus• 14 years ago

Gigawatt-hour storage does exist but it is limited in capacity.

Dinorwig can store ~11GWh and can provide that at 1.8GW (going from 0W to 1.8GW in under 2 minutes) and is ~75% efficient as a store, but it's only got a 6 hour capacity due to the limited size of the upper reservoir.

Something similar built at Itaipu or Three Gorges scale could provide some more serious capacity.

marshray3• 14 years ago

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.

eigenvector• 14 years ago

A switch-mode DC/DC converter obtains the same result as an AC transformer with efficiency approaching that of a well-designed AC transformer. It is a solved problem.

marshray3• 14 years ago

The century-old methods are still competitive at this high a voltage and power.

http://en.wikipedia.org/wiki/High-voltage_direct_current#Dis...

HVDC is less reliable and has lower availability than alternating current (AC) systems, mainly due to the extra conversion equipment. Single-pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault-tolerant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98%.[19]

The required converter stations are expensive and have limited overload capacity. At smaller transmission distances, the losses in the converter stations may be bigger than in an AC transmission line. The cost of the converters may not be offset by reductions in line construction cost and lower line loss.

breckinloggins• 14 years ago

I'm wondering if it would be cost-feasible (eventually) to incorporate pure sine wave inverters at the "last mile point" and do AC to the premises.

Does anyone know?

ChuckMcM2• 14 years ago

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.

marshray3• 14 years ago

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.

forgottenpaswrd• 14 years ago

"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."

This is a long ago solved problem for AC and DC:

http://en.wikipedia.org/wiki/Sulfur_hexafluoride (Look at the "Applications")

It it more expensive but totally worth it for high voltage lines.

lutorm• 14 years ago

Yes, you have to run both at high voltage, but I thought the efficiency difference was because DC has no inductive losses.

CapitalistCartr3• 14 years ago

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.

stcredzero4• 14 years ago

Just do it!

http://en.wikipedia.org/wiki/Nickel%E2%80%93iron_battery

Wingman4l7• 14 years ago

Interesting -- I'd never heard of this battery tech -- but I thought most personal off-grid battery banks were lead acid. Google "lead acid vs nickel iron" and you'll get some discussions on solar power / off-grid forums. I skimmed these, and it seems like any advantage nickel iron has doesn't justify their greater cost.

grecy2• 14 years ago

Tons of people up here have elaborate solar setups, usually 24V or 48V DC. Then they have an expensive 4kW inverter to 110V AC, and the vast majority of their appliances just convert back to 12V DC.

A big plan for my off-grid setup is straight DC.

meaty• 14 years ago

A big chunk of your energy will disappear in heat via resistance unless you run silver wire everywhere or keep the runs short. DC cabling has some weird voltage divider and load characteristics as well causing drop outs. This is particularly visible with high current, low voltage loads.

48v dc is pretty dangerous as well.

gvb• 14 years ago

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...

CapitalistCartr3• 14 years ago

"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.

amalag• 14 years ago

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.

specialist• 14 years ago

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?

High-Voltage DC Breakthrough Could Boost Renewable Energy

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