Why Is DC Power Transmission 10X More Efficient Than AC?

I won't argue the "10 times" part, but DC power transmission is more efficient because:

Line inductance and capacitance are unimportant with DC. With AC they contribute to "imaginary" power, where the system has to transmit power (with losses) that does no work, because voltage and current are out of phase.

Line radiation. I don't know how big an effect this is.

DC utilizes a line 100% of the time. An AC insulator has to withstand the peak AC voltage (1.414 times the RMS value) but most of the time the instantaneous voltage is lower and thus the instantaneous power is also lower. It even goes to zero twice per cycle. A DC line will remain at the peak voltage 100% of the time. Which will fill a kiddie pool faster, a garden hose left on or a garden hose repeatedly turned off and on?

Well, as others have ragged on about, the quoted change in efficiency isn't very meaningful. The efficiency of a transmission line depends on many things. And efficiency is received energy divided by transmitted energy.

The situation where DC transmission is better is where the transmission line is long compared to a 1/4 wavelength of the AC transmission frequency. The typical transmission is

60 cps. So 3E8 m/s divided by 60/s, then divided by 4, is 1,250 km. So when you get transmission distances in that range, radiative loss starts to be significant, and it gets tough to match the impedance of the line to the load. Basically what you get is a very large dipole antenna. DC will remove that effect.

It's not a free ride though. DC transmission lines require that you have high voltage DC to transmit. And most generating stations do not produce DC. So you need to get from AC to high voltage DC, and back to AC at the receiving end. This is doable, but it does have losses. It has only been relatively recently that this process has been improved to the point of being useful, say the last 20 years or so. This is because large scale grids have been finding it useful to transmit power over distances of that scale. So people found it worthwhile to start doing the engineering to make the improvements.

Heh heh. A few years ago I was working at the head office of Ontario Power Generation, writing software for their power trading guys. The grid in Ontario is operated by an agency called the IMO. They had a rule that if you offered to sell power, but the grid was congested so that you couldn't get it to the buyer, you got paid compensation.

A utility in Wisconsin noticed they could bid to sell power in New York state, transmitting it through Ontario. And they noticed that the connections were not large enough to carry the amount of power they were offering, if there was even a very small amount of power already on them.

Now the compesation was based on the difference between what they offered to sell it for and what the buyer offered to buy it for. So the guys in Wisconsin were essentially offering it for free to assure they'd win the bid. But the buyers had to pay for it at the market rate in NY. So basically, the guys in Wisconsin were getting paid pretty much the full price. They were getting paid *not* to sell electricity to NY for many hours a day. And in amounts of 100's of MW.

The IMO very quickly had an emergency meeting and changed the rules to disallow this.

Anyway, long distance transmission is sometimes done on DC lines to reduce radiative loss. Socks

Apples and oranges.

Power companies are out to make money.

The economic cost of losses for a DC system can be significantly less than that of an AC system over long distances.

For short distances, the cost of the losses are generally less than the cost of the conversion equipment, so AC is usually used.

There are many reasons why a DC system would be prefered depending on the situation.

Start here:

--
Jim Pennino

Remove .spam.sux to reply.

If we can't get sustainable maybe we can signal ETs for help.

Bret Cahill

Yes, AND 92% efficiency at the same time! Duh!

AND also twice as efficient! eg. 96% efficient. Moron.

Isn't it great when a so-called "scientist" can simply quote mathematical equations as if they were making sense and pretend to win arguments by doing so? [I know it's common practice now] That is "complete bollocks" to use Phil's colorful scientific argument! So just what is 920% "efficiency"? I guess it's a transmission line where nine times as much power comes out as you put in! That would be a WONDERFUL way for Europeans to transmit power from the Sahara to Europe! When mathematics doesn't agree with reality it is MATHEMATICS that is thrown out!

You know, I need to introduce both you and Phil to another troll who speaks latin and loves to correct people who speak "muttish". Her Name is Autyme! [apparently nurse Ratched isn't letting her on the ward computer these days]

Sorry Tom, D+ on this one!

Your terminology seems pointlessly confusing. Presumably you think a 100% efficient system is 20 times as efficient as another 100% system.

The simple answer is that if something is 92% efficient, you can't make it 10 times more efficient.

-- Richard

--
:wq

Never end sentences with prepositions.

Bret Cahill

But it sounds funny to say, "up with which you came."

Paul

Then rewrite the entire sentence:

"Bret doesn't know jack about how to express transmission line efficiency."

Bret Cahill

"Benj"

( snip nauseating verbal diarrhoea)

** Fuck OFF - you asinine, illiterate cretin !!!

.... Phil

"Bret Cahill = TROLL "

** So it is a rote thinker -

as well as a complete f****it.

..... Phil

No argument there.

You obviously have a flawed understanding of the concept of 'efficiency.' It is *precisely* defined as a ratio - a mathematical 'equation' (whether you approve or not):

Note the various definitions here, used for specific cases.

In all cases, the efficiency is the *ratio* of usable energy output to total energy input. Since energy output cannot exceed input, efficiency can never be greater than 1.

To multiplying the efficiency by 2x, you need to do something such as double the output while maintaining a constant input. 96% efficiency is therefore twice as efficient as 48% efficiency.

More apropos to the OP is the following article.

"Transmission efficiency is improved by increasing the voltage using a step-up transformer, which reduces the current in the conductors, while keeping the power transmitted nearly equal to the power input. The reduced current flowing through the conductor reduces the losses in the conductor and since, according to Joule's Law, the losses are proportional to the square of the current, halving the current makes the transmission loss one quarter the original value."

While AC has slight radiative losses, DC has strong resistive losses. In both, higher voltage means lower current is necessary to transmit a given amount of power. AC can be easily (and efficiently) increased in voltages through step-up transformers. High voltage DC is harder and less efficient to produce and to use.

...and they do make sense, especially when you consider that these equations are definitions and not open to debate.

I pretend nothing. Why do you pretend to understand when your argument violates simple arithmetic?

2 x 92% is 184%, and 184% efficiency is impossible as efficiency is defined.

Can you show me *HOW* 2 x 92% = 96%? I don't think so.

By my math, 96% = 1.04347826 * 92%, roughly.

An egregious error, just as wrong as claiming that 2 x 92% = 96%

Tom Davidson Richmond, VA

An infamous feud was carried on between Winston Churchill and Lady Astor.

On one occasion she accused the PM of ending a sentence with a preposition.

Churchill replied, IIRC, "Madam, that is an accusation up with which I shall not put."

Tom Davidson Richmond, VA

"Puppet_Sock"

The situation where DC transmission is better is where the transmission line is long compared to a 1/4 wavelength of the AC transmission frequency. The typical transmission is

60 cps. So 3E8 m/s divided by 60/s, then divided by 4, is 1,250 km. So when you get transmission distances in that range, radiative loss starts to be significant, and it gets tough to match the impedance of the line to the load. Basically what you get is a very large dipole antenna.

Anyway, long distance transmission is sometimes done on DC lines to reduce radiative loss.

** I think this is entirely false.

There is no comparison with a 60 Hz dipole antenna.

AC power transmission is done with 3-phases ( spaced by exactly 120 degrees ) running a few metres apart, all in exact parallel across the country - so the net 60Hz eclectic field at a distance always cancels to ZERO !!.

Ergo, there simply is no "radiation loss" .

The real losses encountered are due to resistive heating, voltage drop and phase angle changes on very long lines.

..... Phil

Why?

jack

Bret Cahill = TROLL

** Cos it is a preposterous thing to do.....

..... Phil

GREAT NEWS PHIL!!!!

Google Groups now can be rigged with a killfile!

Bye Bye, f****it!

---------------- i dont know what you mean by inverters

AFAIK DC cannot be transform to very high volages-

OTHOA AC can be transformed to huge voltages by trasformers yet transformers can work only on AC and hight voltage AC is much more efficient than any low voltage current do i remember correctly ??

ATB Y.Porat

---------------------------

Sez who?

jack