From Edison’s Trunk, Direct Current Gets Another Look

NY Times ^ | November 17, 2011 | MICHAEL KANELLOS

[Erasmus]

AT&T adopted direct current for the phone system because of its inherent stability, which is part of the reason that landline phones often survive storms better than the electric grid...

That is a very strange statement.

DC's "inherent stability?" Why didn't the author simply say that they decided to run the central office phone equipment on DC because they could use batteries that way, thus obtaining the "inherent stability?"

And how did they charge the batteries? Why, with commercial power from the grid, of course. And which type of commercial power was best suited for charging battery banks? What do you know--AC, when transformed and rectified.

[Erasmus]

And even the “best for solar” is a canard. They have to step up the output of solar panels with what? AC technology (inverters). The choice of the type of output from the inverters can be either AC or DC; the fact that the panel output was originally DC is irrelevant.

[Erasmus]

It was really a plot to make it difficult for students to gain admittance to grad school, preying on their susceptance to fads, while impeding their progress, taxing their capacitance to learn, and overcoming their resistance to doing so.

[SunkenCiv]

Thanks neverdem. Photovoltaic arrays 100 miles on a side (four of those) can supply our needs (during the day), but produce DC. Weird coincidence.

[Erasmus]

For years I've wondered if life wouldn't be easier for sound guys like us if the mains were DC. But the only way it could work would be with everything internally powered by those acoustically and electrically whiney DC-DC converters. And of course the reason for their whineyness is that the must generate AC internally.

I guess there's no way to get around AC.

≤}B^(

[central_va]

The real reason for AC is to torture electrical engineering students.

"imagine" that?

Only a EE geek would find that funny...

[Erasmus]

You can still get an HV capacitor, preferably one with a good dielectric such as oil-filled, charge it up to a few hundred volts (or more!), and leave it out on the control room desk.

Another virtue of DC.

≤}B^)

[Erasmus]

And now all that leftover space is getting repopulated by routers and server farms.

≤}B^)

[Pecos]

The bigger problem is that some liberal arts liberal is going to Google “acres of hamsters” and somehow use this forum as justification for promoting the idea as a replacement for Solyndra. ;-) They will get it backwards, and then try to use non-performance as justification for doing it.

[SnuffaBolshevik]

"The real reason for AC is to torture electrical engineering students."

I'm in awe of double Es. My understanding of AC ends at right about this neighborhood:

[Ramcat]

Looks line a Wheatstone bridge used in phone circuits to power ringers and allow both ac and dc in the same circuit. The diodes keep the ac from returning back through the dc circuit?

[thackney]

BTTT

[tophat9000]

Telco CO gear does run on 48DC but not for th
e reasons in this story

[Stepan12]

as the go go gophers say at min. 1.47 -- whooppi doopi

[thackney]

how big would a dc 440 volt cable be

Standard power cable insulations are rated for 600V per the national electric code.

You can get lower voltage levels for thin cables used in instrumentation system or communication or low-voltage systems.

But your standard household wiring is rated for 600V AC or DC.

http://www.southwire.com/products/CuNMB.htm

[Walmartian]

E/IR isn’t just a good idea. It’s the Law!

[Zeppo]

That’s essentially my point.

The author is confused.

[6SJ7]

Can you phase invert a hamster?

[thackney]

At at these distances and power that means you save a tremendous amount of money using AC.

In Edison's day, there was not a realistic way to step-up and step-down the DC voltage as economically as transformers.

In Modern times, however, we do use some DC transmission lines where large amounts of power are transmitted long distances with few (if any) intermediate taps.

Typically these are on the scale of 500VDC. We have transmission lines like this in the US and have done so purely as an economic transmission system and not related to solar or wind power.

HV Direct Current Transmission System
http://www.energy.siemens.com/hq/en/power-transmission/hvdc/

The DC line cost more at the terminals where power is stepped up or stepped down using power electronics. But the Transmission line cost less money and has less line losses. If you go a long enough distance with only power taps at the beginning or end, DC will cost less over a long distance (hundreds of miles).

The towers are easy to identify as they don't have three phases.

For example, the Pacific DC Intertie is a 500kV DC line bringing power from the Northwest hydro-generation to Los Angeles.

This line was first energized back in 1970 and was later increased from 400 to 500 kV. It is now rated for 3,100 MW of transmission.

Pacific HVDC Intertie
http://www.abb.com/industries/ap/db0003db004333/95f257d2f5497e66c125774b0028f167.aspx

[SnuffaBolshevik]

I’ve heard it called a bridge rectifier. I think it converts AC to DC like in your car’s alternator. Hopefully someone will confirm/deny. ;)