# Comments on Would we need Alternating Current if superconducting wires existed?

## Parent

# Would we need Alternating Current if superconducting wires existed?

The major advantage of Alternating Current is that it can be transmitted to large distances without significant losses, which is not possible in Direct Current.

Had economical superconducting wires existed, DC could be transmitted to any distance without any loss, and DC is much safer compared to AC.

So, I want to know, do we need AC if long distance transmission is no longer a problem because of superconducting wires?

Would DC be better in that case, or we would still need AC?

## Post

The major advantage of Alternating Current is that it can be transmitted to large distances without significant losses, which is not possible in Direct Current.

**This is incorrect**

Answering the rest of your question is pointless since it is based on a false premise. Instead, I'll elaborate why your statement is wrong.

Advantages of transmitting DC versus AC:

- No skin effect. The entire cross-sectional area of the conductor is used all the time. This means more DC current can be pushed thru the same wires for the same resistive loss compared to AC current.
- Inductance of the transmission line matters much less. The inductance only gets in the way when you try to change the current. In DC systems, this only due to changes in the load. In AC systems it is also due to deliberately changing the direction twice per cycle. This hugely dominates current changes caused by load changes.
- No radiation effects. Long multi-100 km transmission lines are a non-negligible fraction of a wavelength. If care isn't taken, these can radiate power into space. This is usually dealt with by twisting the wires every few km. That balances out, which in theory nulls out, the losses between the three phases.
- No phase synchronization problems. When distances are significant parts of a wavelength, the phase shift caused by the distance matters when you try to combine the power from multiple transmission lines or between different parts of the same grid. If the phases don't match closely, you get large and wasteful loop currents.
- The maximum voltage is the RMS voltage. Each transmission line has some maximum voltage is can handle. With DC, you can set the voltage to this maximum. With AC, the voltage is a sine. The peak of the sine can't exceed the maximum the transmission line can handle, but it's the RMS voltage that matter for power transfer. Since the RMS of a sine is 1/sqrt(2) of its peak, AC systems can only run at about 71% of the DC voltage that the same transmission line could carry.

DC started getting real use a few decades ago for long hauls. One example is the DC line starting at large dams in northern Quebec, extending about 1000 miles to Ayer Massachusetts. There the power is converted to AC and dumped onto a regional transmission line.

Due to advances in converting from DC to AC (the major drawback of DC transmission), we are seeing DC become economical for shorter and shorter runs.

DC is much safer compared to AC

This is also a false assumption.

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