# AC vs. DC in power transmission

Many threads that I have read have mentioned in passing that it would be more efficient to use DC to transmit power. Why is that? I was told in school that the reason we use AC was that it was more efficient for long range transmission.

Rob

Transmission efficiency is high when current (and therefore, I[sup]2[/sup]R losses) is low. To keep the curren tlow while delivering the same power means you have to raise the voltage. Until relatively recently, the most efficient and cost-effective means of doing that was to use transformers, which means AC. But now, we have cheaper and more efficient DC-DC converters. DC has the additional benefit of not having stray reactive losses, like AC does–primarily in the form of capacitance between the lines and ground. Thus, overall, DC is better.

It is more efficient to transmit power at higher voltages. We use AC because it is easier to change the high voltages used in long distance transmission to lower voltages that the end user needs.

For the transmission lines there are two reasons why DC might be better than AC.

1. For a given maximum voltage DC can transmit more power. This is because the lines are at the high voltage all the time. With AC the voltage is varying and the average voltage is less than the DC case so the average power is less. A given set of transmission lines has a maximum voltage it can handle to DC would be better there.

2. There is some power losses due to dielectric effects. These are caused by the changing voltage which DC does not have. I don’t know how significant this is in the grand scheme of things.

With advances in high powered switching devices it is getting easier to change high voltage DC to lower voltages so DC transmission lines are starting to be built.

If you want more depth, the Wikipedia article on High Voltage Direct Current talks about the advantages and disadvantages of DC power transmission compred AC.

How is that going to integrate into the AC power grid? Virtually all our electronic devices are designed to run on AC, right?

Like they said.

Moreover, consider a radio antenna used in transmitting. The electrical power you put in one end of that wire is AC, and you never take ANY electrical power out of the other end of the wire. Its transmission efficiency, as a wire, is 0%.

You are correct. The DC needs to be converted to AC at some point. The advances in electronics that make DC to DC voltage converters are also useful in converting DC to AC power.

The pacific DC intertie
http://en.wikipedia.org/wiki/Pacific_DC_Intertie
The sylmar converter station In Los Angeles converts the DC to AC for distribution in LA.

Actually reading the Wikipedia article it looks like the pacific intertie has been DC for 40 years.

Another advantage of DC power distribution is that it does not need to be synchronized like AC power does. IF you have a bunch of generating stations feeding a grid they all have to have the same frequency and phase.

So the real question is, will the ghost of Edison come back to kick Tesla’s ass?

None of this affects the lines going into your house. HVDC transmission is only used to transmit ginormous amounts of power between different parts of the grid. It’s been stepped down several times and converted to AC long before it bounces around inside your plasma screen.

Wikipedia is a great sucker of time. Here is a list of DC transmission lines.
http://en.wikipedia.org/wiki/List_of_HVDC_projects

>So the real question is, will the ghost of Edison come back to kick Tesla’s ass?
Only if he can avoid the ghost of William Kemmler.

There is another thing wire. AC transmission lines are three phase, thus three wires.

DC will require only two wires.

Well for electronic devices, they typically run on DC. It’s just that their power supplies are designed to run on AC, since that’s the distribution system that we have, and convert it to DC that our electronics actually use.

So a few years ago I was talking to some EEs about running low-voltage DC power through a rack of computers to avoid having power supplies in each one. They told me that DC would lose too much power. Is that because of the low-voltage?

Sort of. As I noted upthread, one of the ways to express power loss is the equation P= I[sup]2[/sup]R, which means power lost in a conductor equals the current squared times the resistance of the conductor. You can minimize the power loss, therefore, in one of two ways: either increase the voltage or decrease the resistance. The latter can be done by using heavier and heavier wire, but this gets expensive and impractical, particularly over long distances since you have to square the cross-sectional area to cut the loss in half, per the equation. So, the usual solution is to raise the voltage over the transmission system and step it back down at each substation (in your case, that would be the individual servers.)

There are datacenters designed around DC power distribution (remember that the phone company has run almost all their switching gear on DC for decades.)

Here is an interesting article on the subject.

One of the things our engineers are currently pulling their hair out over is a HV (~380 VDC) distribution system for such datacenters. It’ll take 480 V, 3-phase power and convert it to DC at power levels of around 600 amps, IIRC.

At the moment, however, the prototype is a smoke machine.

Wow, where do you get the five-dimensional wire?

One thing to keep in mind is that AC only reaches its peak voltage twice during the AC cycle. DC on the other hand is always running at peak. AC wiring, insulated standoffs, etc. all has to be designed to handle the peak voltage, which means that for the same wire, you can always transmit more power using DC.

However, there is an increased cost in transformers and switch gear. An AC transformer is just a coil of wire around a hunk of iron. Simple. The lack of a decent DC transformer is what stopped Edison in his tracks. These days we have DC transformers, but they aren’t as simple as a coil of wire. At some point you also have to convert the DC back to AC, which also involves a cost.

Also, disconnecting a high voltage wire isn’t as easy as you might think. The electricity tends to arc across the gap. With AC, the fact that the electricity crosses zero twice per cycle allows the arc to be fairly easily extinguished. DC will draw a much longer arc, requiring different (and more expensive) equipment to handle this.

The extra cost of the switch gear and transformers/inverters gets balanced out with the cheaper cost of wire. Below a certain distance, AC will always be cheaper. At longer distances, DC is cheaper.