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The Last DC Power Grid Shut Down in NYC
cell-block-9 writes "Today the last section of the old Edison DC power grid will be shut down in Manhattan. 'The last snip of Con Ed's direct current system will take place at 10 East 40th Street, near the Mid-Manhattan Library. That building, like the thousands of other direct current users that have been transitioned over the last several years, now has a converter installed on the premises that can take alternating electricity from the Con Ed power grid and adapt it on premises.' I guess Tesla finally won the argument."
http://en.wikipedia.org/wiki/War_of_Currents
For short distances and for use within IC it's quite useful. The conversion from AC to DC at lower voltages for use within computers produces quite a bit of heat (hence the fan in your PSU, yes I realize that even DC from a higher voltage to DC at a lower voltage produces quite a bit of heat) and so you find that some data centers are moving to converting from AC to DC outside of the cases and transporting DC directly to the servers.
/. about this a while back and perhaps somebody who'd like to be modded up a bit can post the link.
There was an article on
internet like monkeys'
When I lived in Cambridge, I sometimes visited friends in Boston who had 600VDC elevators using power from the city.
Later elevators still used 600VDC but used a dynamotor; that whine you used to hear when you pressed an elevator button elsewhere was the dynamotor starting, to convert to 600VDC from the 120VAC line current. Eventually, elevator manufacturers stopped using it, but when you hear that whine in a medium-old elevator, you know what is is.
AC's advantage of high voltage transmission doesn't apply to subways as 1200V seems to be the limit for third rail. 2400VDC was tried in 1915 on the Michigan Railways (an electric interurban in central Michigan) with abysmal results - the voltage was changed to 1200V within a year of the initial installation.
As related here high-voltage DC transmission is more efficient than high voltage AC transmission for a number of reasons, and it has other benefits as well in allowing potentially unsynchronized AC systems to transfer power. The main problem is efficient voltage conversion, which requires more infrastructure than an AC system with equivalent power transfer capability.
Less is more.
DC is actually used extensively in modern power grids, the main advantage being that there is no need to synchronize the phase from different generating stations or subgrids. For example, the Pacific Intertie transmits three gigawatts of direct current between Los Angeles and eastern Washington state. (Power is sent from LA to Washington in the winter, covering the demand of electric heating in the pacific northwest; and from Washington to LA in the summer to power our air conditioners.)
for those who don't get it http://www.roadsideamerica.com/pet/topsy.html
Do you have transmission lines that are three blocks in diameter? Then it's more efficient to convert at the source. What? You don't? Then I guess converting at the point of use is more efficient. Transporting the 5V and 12V levels that most consumer electronics use internally would be insane over more than a few feet because of voltage drop.
See Electric power transmission: History for more information.
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It keeps being repeated, even in this article which says "it can be transmitted long distances far more economically than direct current", that AC is more efficient. This is not really true. The advantage (and pretty much the only advantage) that AC has over DC is that it is relatively simple to change voltages.
Over the short-haul, this is good since losses are primarily resistive and losses are related to the amount of current flowing in the conductors. Power in my neighborhood is delivered at 12,000V and down-converted to 120/240 by transformers located every few houses. Delivering power at 120V would require 100 times the current and massively larger conductors. Once it gets to my house, with the exception of some motors and some lights, everything from TV to stereo to computer ends up having to take that power and reconvert it to DC.
But AC has far higher losses through capacitance and inductance which become severe over long distances. This is why some current and other planned long-haul transmission routes use DC. A good example of this is the 800-kilovolt DC line that connects into the Sylmar Terminal Station near Los Angeles.
Apparently, the use of Extra High Voltage DC is being proposed for a number of new long-haul transmission systems and it is the high losses incurred by AC over long distances that is driving the use of DC.
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"You are not remembered for doing what is expected of you." - Atul Chitnis
It would be more efficient to transmit DC, if we are talking about the same voltages. AC is impeded by inductance as well as resistance, so in addition to the inefficiencies of converting, you also are better off transmitting DC if it is the same voltage.
The trick is, transmitting at higher voltages is more efficient than transmitting the same power at lower voltage. This is because to send the same power at low voltage, you must send more current, and more current means more energy wasted as heat from the resistance of the line. So voltages from the generator are stepped way up before being transmitted.
And the reason AC won out is that it is much, much cheaper and easier to step up AC voltage (you just need a transformer, which is nothing but a couple coils of wire around an iron core) than to step up DC voltages (which requires a boost converter, which at its heart is a giant transistor [big enough to survive the voltages and currents of a power plant in this case] and a huge inductor [big fat coil of wire] along with timing and firing circuits to control the action of the transistor).
Boost converters are expensive, but over a long enough run of transmission line the advantages of DC over AC do make up the difference (as I recall, the break-even point is about a 400 mile long line). So you do find some long distance transmission lines that are DC. I know there is one out here in Sylmar, California that runs up to Washington state somewhere.
Automated teller machine machine?
Personal identification number number?
Direct current current?
See wiki .
I had but a simple dream, to destroy all humans.
http://en.wikipedia.org/wiki/General_Electric
GE's divisions include GE Commercial Finance, GE Industrial, GE Infrastructure (including GE-Aviation and Smiths Aerospace), GE Consumer Finance, GE Healthcare, and NBC Universal, an entertainment company.
In Chicago it's ComEd, for Commonwealth Edison. In New York, it's Con Ed for Consolidated Edison. I think Massachusetts used to have a ComEd, though not sure if that was the same company as in Chicago, it stood for Commonwealth Edison (but these days it's called NSTAR in Mass).
There are some advantages (source: http://en.wikipedia.org/wiki/HVDC#Advantages_of_HVDC_over_AC_transmission):
- Undersea cables, where high capacitance causes additional AC losses. (e.g. 250 km Baltic Cable between Sweden and Germany[9]).
- Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas.
- Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
- Allowing power transmission between unsynchronised AC distribution systems.
- Reducing the profile of wiring and pylons for a given power transmission capacity.
- Connecting remote generating plant to the distribution grid, for example Nelson River Bipole.
- Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current.
- Reducing line cost since HVDC transmission requires fewer conductors (i.e. 2 conductors; one is positive another is negative)
Shoot, it's used in the US and UK (in the "Chunnel").Here's a list of notable places that use it:
http://en.wikipedia.org/wiki/List_of_HVDC_projects
DC doesn't require high voltages, a DC line at a given voltage is slightly more efficiant than an AC line of the same voltage.
DC has two main problems
1: it is a pain to voltage convert. Voltage conversion is pretty vital to our modern use of electricity, you don't want 11KV in your home but you don't want to be transmitting 240/415 three phase or worse 120/240 split phase any significant distance. You also want much lower voltages for loads of equipment.
For equipment power supplies it isn't so bad, they generally don't have particularlly high efficiancies anyway, they tend to run at fairly low power and they tend to be in a nice indoor environment but building a DC equivilent of a pole pig with similar efficiancy and reliability would get pretty expensive.
2: DC is a pain to switch, switches and breakers would have to be either much bigger or much more complex for a given DC voltage than for the same AC voltage (the zero crossings of AC tend to break arcs).
note: i'm known as plugwash most places but i screwd up registering that here somehow in the past and now can't register
This story explains what the original FA obscures; that some old buildings had elevators and pumps designed to run on DC. Sue me if the link doesn't work. http://query.nytimes.com/gst/fullpage.html?res=940CE7DF173DF93BA25750C0A9679C8B63&sec=&spon=&pagewanted=print
BSA: "Would you like a free Software Audit"? me: "No, thanks. My software is all Free".
Uhm, no, everything is *not* technically DC. Yes, anything that uses electronic controls or is electronics will have either an internal transformer/rectifier or an external wall-wart, but lots of things use AC directly. Motors in power tools, dishwashers, refrigerators, vacuum cleaners, and most power electric appliances (not to mention modern elevators) are AC motors. Incandescent *and* flourescent lighting is a direct AC user (but LED's use DC, of course). Fans (not the PC kind), blowers, electric lawnmowers, even electric heat, all use AC directly.
Yes, DC wins when using cables with high capacitance and also between power grids of different frequency. About 5 mins walk from my house is the anchor point for the UK France cross-channel HVDC link. The converter station is at Sellinge, about 10 miles away, and can ship up to 2GW in either direction. However, due to higher capacity on the French side (they have more nuclear), I understand the UK are net consumers of energy.
Although the UK and France both operate at a nominal 50Hz, it is normal for actual grid frequency to vary slightly throughout the day. The way this was explained to me was fairly intuitive: when you load the grid, you load the generators and in turn they load the turbines, which slow down ever so slightly. Because turbines have very large inertia, their response time to step loads is rather long. In order to pump power into the grid, a generator has to have a phase lead to overcome its self inductance. A shift in the phase means a shift in the power being pumped.
France is 1 hour ahead of the UK and have different norms regarding hours of work, cooking etc, all of which mean they have a different load profile. Combining the UK and French grids would only be possible if the link were much greater than 2GW thus able to cope with the difference in power swing. The link was never intended to serve that purpose. Perhaps, like Concord[e] and the Chunnel, it was more a punctuation of the ongoing Entente Cordiale than a technical necessity.