Why is long range (very) high powered electric transmission best done with DC instead of AC?

Question

I was watching a YT video recently where they said something along the lines of “AC current is too inefficient for long distance power transmission, so xyz project needs DC power” and then moved on. I (from my vague memories 20 years ago of school) thought that AC had less loss and inefficiencies. What am I missing/what did I tune out in school?

In: 18

Answer 1

Traditionally, AC power has been used for long distance transmission because you can use transformers to step up the voltage. Since the power losses in a wire are proportional to the current (stepping up voltage steps down current), this minimizes the power loss over a long haul.

However, in recent decades, we’ve invented solid state devices capable of stepping up DC power in the same fashion.

There are some significant benefits with this approach.

First, AC power needs to be frequency coupled. If I run my AC power system at 50 Hz and you run yours at 60 Hz, we can’t directly connect them. However, all DC power runs at the same frequency, so we can tie them together easily.

AC power also has what is termed a ‘skin effect’. The constantly fluctuating power only flows on the outer edges of the conductor. In contrast, DC power flows through the entire conductor. This means you need smaller wires for the same power with DC – and when you’re talking laying down metal conductors for most of a continent, that material savings adds up.

AC power also involves inductive and capacitive losses that aren’t an issue with DC power. When you have a time-varying current, you create magnetic fields. Those magnetic fields interact with the surrounding environment and cause power losses over long distances.

That being said, while we’ve invented ways to step up DC voltage, it’s normally very expensive compared to a traditional transformer. So you’re not going to use it to transmit power a mile from the plant to the local distribution station since you need those expensive installations at both ends. When you’re transmitting power a thousand miles and you still only need two such devices vs. a thousand miles of cable? DC is the way to go.