High-Voltage DC Breakthrough Could Boost Renewable Energy
news.nationalgeographic.com
news.nationalgeographic.com
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.
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.
Does anyone know?
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.
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.
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.
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.
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.
Sometimes it does.
Many railroads (in the US of A) may have been state chartered and sanctioned, but generally were built and operated by profit-seeking enterprises (such as the Pennsylvania Railroad), negotiating land-sale or lease for right-of-way. The most notable exception is the Transcontinental Railroad, which took an act of Congress to get it started, and another one to end its construction phase. The government didn't build the railroads, they just regulated many of them to death. The (US Federal) government only runs Amtrak, whose primary purpose is intercity passenger, and that's only seen a surge in recent times because air-travel has gotten a bit more onerous.
The early internet protocols may have started as a DARPA project, but the Internet you connect to isn't a single thing, it is a private provider with private packet passing agreements with other private providers. The name resolution system is probably the biggest most visible remnant of government "creation", and one that arguably shouldn't be left with it.
By definition, that means the service will be more expensive than necessary.
What's wrong with the government charging a fee that is just enough to break even year after year?
b) the cost of something (in a competitive market) is determined by supply and demand, not by the amount of profit a supplier makes (or doesn't make).
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.
A big plan for my off-grid setup is straight DC.
48v dc is pretty dangerous as well.
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...
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.
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?