As we know, Thomas Edison was a major proponent for using direct current and he argued it was the only choice for household electrical power. As we also know George Westinghouse (working from the ideas of Nikola Tesla), argued for alternating current.
The big advantage of AC over DC is that AC “travels” better. AC can be stepped up to higher voltages sent along power lines and then stepped down. This allows for the use of thinner power lines and allows AC to travel much farther (further?) than DC (no, not the city). I think direct current can only travel a few miles before it loses sufficeient power to be usable.
So, how did Edison plan on generating and “delivering” direct current for homes? It seems he would have had to build a power station for each city and town. How extensive were Edison’s direct current electric companies before it was realized that AC was the way to go?
There were a lot of rather interesting systems developed in the early days of electricity. You can set up motor-generator sets which will effectively create DC “transfomers”, but obviously these are a lot more expensive and complex than AC transformers. There was even one distribution setup where a bunch of motor/generator sets were run in series in one really big loop. The advantage of this of course was that there was only one wire going around the loop. The disadvantage was also pretty obvious. If anything in the loop breaks the entire system goes down.
At really high voltages, DC has smaller losses due to capacitance and corona discharge, which can offset the expense of more complex transformers being required at either end (not to mention that at some point you have to convert the power to AC to make it play nice with the rest of the power system). The cost of the transformers doesn’t make it practical for neighborhood type power distribution, but if you need to get a whole lot of power from over here to someplace way over there, DC can be used, and in fact is used in several places around the world.
An example of a non-trivial DC power line is the Pacific NW-SW Intertie system. The original line “only” goes from the Columbia river in N. Oregon to near L.A. 845 miles as the juice flies.
There are some advantages now that we have the technology to do it at 1.5 million volts.
You aren’t using current through the resistance of the line to continually charge and recharge the line capacity. No radiation loss. And that’s just off the top of my head.
However, I’m not sure the advantages are enough to make it worth while. The Pacific Intertie DC line is now something like 30 or 40 years old and it’s still all by itself as a long distance DC transmission line in the US.
The line runs down the eastern side of the Sierra Nevada range about 15 miles west of here. It crosses a campground on the Owens River up north of here and I think I’ve mentioned that when I am under it I never erver point my finger at it when telling people what it is. Why take even a super remote chance on raising the voltage gradient in your vicinity?
I dug out one of my old EE textbooks which has a couple of pages about DC power transmission. The book was printed in 1982 so some of it is a bit out of date, but here are a few excerpts you might find interesting:
From “Elements of Power System Analysis” fourth edition, by William D. Stevenson, Jr. (1982)
The advantages of DC (aside from cost) according to that book are:
Voltage regulation is less of a problem since at zero frequency inductance is no longer a factior, whereas it is the chief contributer to voltage drop in an ac line
The transmission line can be operated monopolar in an emergency if one of the lines gets grounded
(no the book does not say Hi Opal)
DC works better underground over long distances, like in the English Channel
The distances between conductors is smaller, which in addition to the cost savings for towers also means a smaller right of way of land is needed
Since AC has to be insulated for the highest peak to peak voltage, the insulation required for DC is much less for the same average voltage
The book also mentions that ac circuit breakers can extinguish the arc that develops between the conctacts because there is a zero crossing twice on each cycle. No dc equivalent of this exists (or didn’t when the book was written).
Interesting. I knew about the Russian line but the North Dakota one is news.
Another advantage is that less conductor is needed. For example a 1 million volt DC line carrying 1000 amp. is a 1000 megawatt line and consists of two wires. The same capacity 3-phase AC line at 550 kV would have 1050 amp but would use 3 lines of the same size as the DC line.
All else being equal, it is inherently more efficient to transfer power via DC (vs. AC) when you only consider the conductors. There are many reasons for this, as other posters have stated. I believe the most significant factor is that there are no reactive losses at DC.
Here’s another reason:
Let’s say your power source is operating at 60 Hz and a foreign customer located 1000 miles away wants to buy your power at 50 Hz. To convert the 60 Hz to 50 Hz, you first have to convert the 60 Hz to DC, and then convert the DC to 50 Hz. For reasons stated above, it would be more efficient to convert the 60 Hz to DC at the power source, transfer the power over long distance at DC, and then covert the DC to 50 Hz at the customer’s location.
And it’s not only an issue of frequency, but also phasing. All the AC on particular grid must be in phase, or bad things happen when generators are connected. A DC intertie allows two non-synchronized grids to share power.
As an aside, the 430 mile DC line from central North Dakota to the Twin Cities was the subject of huge controversy with rural Minnesotans sparking a rebellion in which, at its peak in 1978, nearly 50% of the state’s highway patrolmen were tasked with protecting the line. The original course of the line appeared to have been decided upon by simply drawing a line without regard to inconvenience to landowners and farmers. Supposedly, in the original plan, a structure was to be placed in someone’s front yard. After energization of the line, ‘bolt weevils’ caused many structures to collapse by removing key braces from the towers. Because of this, there is now a federal law regarding purposeful damage to transmission lines.
For a more complete telling, pick up Powerline The First Battle of America’s Energy War by Senator Paul Wellstone and Barry M. Casper(University of Minnesota Press)
It seems astro answered my question in the second posting but
wow - this thread has become more interesting than I ever thought it would be.
Well then, I guess I have some more questions.
What about generating AC or DC. Which is easier or cheaper to do?
From what I’ve read so far, it seems DC now has the advantage over AC. Will DC eventually be the preferred method of electrical distribution?
I personally doubt it, especially now that the infrastructure is in place for AC. The minimum distance for DC to be economical is still pretty long, though I think it will get shorter and shorter over time. I think DC is also going to start being used more often to get electricity to places that really need lots of it (like california and the northeast) but I think the extra costs of DC are always going to outweigh its benefits at shorter distances and lower voltages.
engineer_comp_geek
It is funny that you said
“The minimum distance for DC to be economical is still pretty long.”
In Edison’s day (due to different technologies) the statement would have been the maximum distance for DC to be economical would be very short.
One has nothing to do with the other. Telephones have their power supplied by the central office which has battery backups in case of a blackout. The fact that the power distribution for telephones is separate from the main power grid is why it stays up when there is a blackout.
Right, the Telco’s use DC power in the Central Office. The equipment is powered by either DC or AC depending on manufacturer, office location, equipment type etc. The battery backups are often recharged by diesel generators in the case of many large CO’s.