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


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


It’s not the AC vs DC itself that’s the reason, but the fact that AC power is very easily converted between different voltages. When it comes to losing power over wires, it’s the current (amps) that actually affect how much power is lost to heat. To mitigate this we can raise the voltage. More volts means less amps for the same amount of power (watts).

Of course, high voltages are dangerous and we don’t want them in our homes. So we need a way to change the voltage fairly easily. *That* is why AC power is used. The way to change voltage with AC is as simple as coiling some wires.

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.

Times changed and tech advanced

AC is easy to change between voltages. Take a hunk of iron, coil two wires around it. Congratulations you now have a transformer! This was feasible in the early 1900s

Converting DC voltages to other DC voltages requires either a DC motor feeding a DC generator, or *semiconductors*

The motors and generators would be insanely large and when deciding on AC or DC they didn’t yet know about semiconductors so they opted for AC so they could convert voltages

Modern times let us use large semiconductor DC-DC converters and inverters at the end to get the AC our devices want

AC has losses on transmission lines due to real wires having capacitance and inductance which while being apparent power, they suck additional real current from the generators which gets impacted by the wire resistance. The skin effect also comes into play on really big power lines, AC current only travels down to a certain depth so really high current connections have multiple (3/4/5) wires in a shape because a single super thick wire wouldn’t carry any current in the middle. DC doesn’t have this problem, you’re only worried about resistance, no other weird effects

It depends on the situation. DC doesn’t experience as much voltage drop over very long distances, but it’s more expensive to transmit.

AC is very cheap and easy to transmit over moderately long distances, but unlike DC, it does experience significant voltage drop along the way. Power companies boost the voltage at various points to compensate for this, so despite the tradeoff, AC still ends up being far more cost effective for power grids.

However, if one needs to transmit power over very long distances (over 1,000km), DC actually overtakes AC in terms of economics and efficiency. In those applications, DC is the winner.