[digerata]

I was initially confused by the statement that direct current has lower transmission losses over long distances.

My recollection was that a big argument for AC over DC going back to Edison v Tesla was that DC required DC generation centers all over the place because of transmission range:

"The primary drawback with the Edison direct current system was that it ran at 110 volts from generation to its final destination giving it a relatively short useful transmission range: to keep the size of the expensive copper conductors down generating plants had to be situated in the middle of population centers and could only supply customers less than a mile from the plant."

Source: https://en.wikipedia.org/wiki/War_of_Currents#Edison.27s_DC_...

Yet, it appears this is no longer the case. https://en.wikipedia.org/wiki/High-voltage_direct_current

Why is that?

[mikeash]

Because of the "high voltage."

AC versus DC is not all that important by itself. What's important is transmitting electricity at high voltage.

Transmission losses come from the amount of current you pass through a conductor. More current equals more power lost to the resistance of the conductor.

Delivered electric power is equal to voltage multiplied by current. Higher voltage means lower current for the same amount of delivered power, which means lower transmission losses.

The AC versus DC thing comes in because you need lower voltages for practical applications (it's hard to run a light bulb off 100kV) and that means you need to transform between different voltages. Transforming AC between different voltages is pretty easy: an AC transformer is basically just a pair of coils with a metal core, easily built with 19th-century technology. Transforming DC is much harder and requires much more advanced technology.

For the 19th-century battle, this meant that AC was the only one that could be transmitted at extremely high voltages. DC was limited to serving very small areas because it had to be transmitted at the same voltages which would be used at the destinations.

Today, that difference goes away, so the advantages and disadvantages are all about much smaller secondary effects instead.

[sliverstorm]

At the time of Edison, they already had AC transformers that made AC voltage conversions either up or down very easy. All you need is an iron ring, and you wrap some wire around it.

https://en.wikipedia.org/wiki/Transformer#/media/File:Transf...

DC transformers only recently reached comparable performance, thanks to semiconductors, and efficiencies are not as high as a good AC converter without really sophisticated circuits.

[deelowe]

Higher voltages and frequencies is how you get less loses as resistance is coupled to current. The lower the current, the less the resistance affects transmission. However you can only increase ac voltage and/or frequency so much before you have another issue, the impedence of the air itself starts to create loses Also, because AC doesn't fully penetrate the wire, you have to run much larger wire sizes to achieve the same affect. At a certain point, you simply can not push more power using AC without resulting to things like superconductors.

DC OTOH, does not have the skin effect issue and so because more desirable in certain cases. In fact, high voltage DC is how they electricity directly to LA all the way from Oregon/Washington state: https://en.wikipedia.org/wiki/Pacific_DC_Intertie

[amalag]

I read further on wikipedia and it mentions that these lines can also be underground. DC cables do not suffer from capacitance issues that AC cables do: "Long underground DC cables have no such issue and can run for thousands of miles." https://en.wikipedia.org/wiki/Electric_power_transmission#Un...

[deelowe]

yeah. HVDC is pretty interesting. Lots of great applications.

[digerata]

The forth paragraph in that link really sums up the difference for my poor brain:

"One advantage of direct current over AC is that DC current penetrates the entire conductor as opposed to AC current which only penetrates to the so-called skin depth. For the same conductor size the effective resistance is greater with AC than DC, so that more power is lost as heat. In general the power losses for HVDC are less than an AC line if the line length is over 500 -600 miles and with advances in conversion technology this distance has been reduced considerably."

[deelowe]

Right. Additionally, frequency plays a role and AC can ionize the air at higher voltages, which increase impedance super-linearly. Basically, high voltage DC is probably going to become a very attractive option as high voltage transistors become commonplace. The reason AC was so attractive previously was b/c it's very easy to convert voltage to current and vice versa with just some coils of wire. With silicon based solutions, that's becoming a much less efficient (and even cost more costly) option.

[kpil]

A lot of research and development of semiconductors that can handle the extremely high voltage (250+ kV) required to keep the losses down.

I think ABB (One of the market leaders and early pioneers when they were named ASEA) initially tried to make better underwater transmission systems.

[jws]

It has to do with the high voltage and the impact of voltage loss.

† I've accumulated a lot of stuff I'm glossing over at the end.

Let's consider a 1 square millimeter cross section wire. Your power lines will be bigger, but it scales up with the area.

Electricity facts refresher:

  ohms measure resistance to electricity movement
  amps measure how many electrons per second you are jamming down a wire
  volts measure how hard you are jamming the electrons down the wire
  watts measure power, this is ultimately what you care about

  volts = amps * ohms
  watts = amps * volts

  this is true for AC and DC

Your copper wire has a resistance of about 17 ohms per kilometer. This wire is allowed to take 3 amps of current when used for power transmission. That makes the voltage drop over a kilometer be 3 * 17, 51 volts.

This voltage drop doesn't depend on the voltage of your power line, just of the current running through it. If you are Mr. Edison with a 110 volt generator, then you have lost almost half of your power just transmitting it one kilometer.

If you are the Pacific DC Intertie sending power from Washington state to Southern California, you are operating at 1000000 volts. You are losing 0.005% of your power per kilometer.

The whole AC/DC thing was because in the early 20th century it was easy to change the voltage of AC power but difficult and expensive to change the voltage of DC power. You could pump AC up to higher voltages for transmission, and bring it back down for domestic use.

␄

† Notes follow:

The current limits I used for the wire are for common electrical engineering work on devices, I don't know the limits for power transmission lines, but…

I sort of doubt they use copper for transmission lines, too expensive, so the resistances will be higher. Wikipedia to the rescue: The Pacific DC Intertie uses aluminum wire, reinforced with steel, with a 644 mm^2 cross section. (Not clear if that is the aluminum of the whole thing.) They are also pushing 3100 amps which puts us in the ballpark, 1000 times the current in 644 times the cross sectional area. They are operating above the power transmission guideline I picked but below the limit for chassis wiring.

Also for transmission lines, the much thicker wires will not be able to dissipate heat as well as the tiny wire I used and there will be a derating of their current capacity from that.

High frequency AC gets weird, it travels on the outside of the wire and the cross sectional area doesn't scale, but 60Hz isn't going to do that much.

If you think "I know, I'll wire my home/datacenter/yacht with 12v DC power!" Then you really care about voltage loss. If you want to move 2000 watts 10 feet with low loss you will be using two copper conductors about the diameter of your thumb. This is why you see 48v used on things like Power Over Ethernet. That is the "low voltage" limit for some regulatory agencies, and they want to get as much power of the tiny conductors as possible.