# Theory question about power lines

#### Spence

Joined Apr 23, 2010
49
It seems that long distance high voltage DC power lines are possible and indeed are being used over 600km according to wikipedia, which goes on to say that they are economical wrt power losses.

Can anyone explain in fairly basic terms what the theory is and why not A.C. which I always believed to be superior for long transmission lines.

#### russ_hensel

Joined Jan 11, 2009
825
DC has always been better, except for the fact that we generate and use AC and stepping DC up and down may be difficult.

Transmission is better because the line is utilized at 100% of voltage all the time and DC does not radiate for steady currents and voltages.

#### nsaspook

Joined Aug 27, 2009
7,189
DC transmission has been used for a long time with high power links. One of the major problems other than transmission losses is synchronizing the generation frequency of large rotational generators. Very small adjustments in generator frequency are used to balance power from distant stations into the grid with a AC system. With very long AC lines a huge amount of energy is stored in the fields on the transmission lines. This reactive energy makes power shifting very difficult with fast changes in load. With a DC system we decouple the distant frequency feedback-loop allowing for much more stable operation. We "Oregon" supply half of LA power on a DC line.

http://en.wikipedia.org/wiki/Pacific_DC_Intertie
http://web.archive.org/web/20050426161757/www.transmission.bpa.gov/cigresc14/Compendium/Pacific+Pictures.pdf

#### crutschow

Joined Mar 14, 2008
24,750
AC also has reactive losses from reactive currents due to the power line's capacitance and inductance which can be significant in a long line.

#### atferrari

Joined Jan 6, 2004
3,930
Rhetoric question: how is then that we use AC for almost everything?

Thomas Alva was not so wrong then?

#### royl

Joined Feb 6, 2012
4
Hi...just joined this org and I thought that this was an interesting topic....Would I be correct in assuming that utilizing DC for transmission, is only good for a point to point application? in other words, one transmitter and one receiver? Oh, and how do they deal with ohms law then?

#### nsaspook

Joined Aug 27, 2009
7,189
Hi...just joined this org and I thought that this was an interesting topic....Would I be correct in assuming that utilizing DC for transmission, is only good for a point to point application? in other words, one transmitter and one receiver? Oh, and how do they deal with ohms law then?
Cost is a major factor, so most are point to point or dual-line back to back for inter-grid connections.

#### crutschow

Joined Mar 14, 2008
24,750
Rhetoric question: how is then that we use AC for almost everything?

Thomas Alva was not so wrong then?

We use AC for almost everything since AC is easy to change from high voltage to low voltage and back with a transformer. DC is only used when the cost of building an inverter to convert the DC back to AC is justified by the lower loses of the long DC transmission line over an AC line.

And yes, Thomas Alva was wrong, at least for transmission of electrical power over typical inter-city and between-city distances.

#### crutschow

Joined Mar 14, 2008
24,750
Hi...just joined this org and I thought that this was an interesting topic....Would I be correct in assuming that utilizing DC for transmission, is only good for a point to point application? in other words, one transmitter and one receiver? Oh, and how do they deal with ohms law then?
Ohms law is dealt with (or more specifically, it is not violated) as in any other circuit. Why would it be different in a DC transmission line?

#### davebee

Joined Oct 22, 2008
540
That pacific intertie is amazing.

http://en.wikipedia.org/wiki/Pacific_DC_Intertie

The article says that they can use the two wires between Oregon and Los Angeles in a conventional circuit, or they can use Earth ground return as an option, which explains the reason for such an extensive grounding arrangement at each end.

Does that explain why California is warming up? (I'm joking)