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Comment by mrfusion | Hacker News Reader

I’ve never heard of it increasing exponentially. Also with very high voltage AC we can actually transmit quite far.

With high voltage DC we can transmit farther then high voltage AC (for reasons I don't understand - ask a high voltage engineer), and it avoids all the phase problems with long distance AC.

Capacitance is a major reason; the capacitance of the very large line doesn't need to be charged and discharged at very high voltage 100 or 120 times a second (per half wave of 50 or 60Hz).

Also, an X volts RMS AC line is 2.8X volts peak-to-peak. The peak voltage is limiting with regards to breakdown voltages, and so DC can reach a higher equivalent voltage on a line with the same maximum voltage.

I don’t understand how you change between voltages with dc? Is it as efficient as AC using transformers?

Effectively a giant version of your laptop power supply. The conversion is not quite as efficient as a transformer, and certainly more expensive, but for long distances the whole system can be more efficient than AC.

(The magic component is the "insulated gate bipolar transistor", sometimes using silicon carbide as the semiconducting element. These are capable of multi-kilovolt switching.)

Ask an electrical engineer not me.

The basic idea is you go to high voltage DC at the generation end, run a long ways (across states), turn the DC to AC and then run that through a transformer. It isn't as efficient as a transformer, but it isn't far behind so you more than get it back by the smaller losses in transmission.

I should have elaborated on my exponential claim. Resistive losses are indeed linear, however in the context of a maximum transmission voltage, efficiency is bounded exponentially.

If you consider a voltage source of 1V, a line of 1ohm, with a load of 1ohm (0.25W), the transmission is 50% efficient. With a load of 0.5ohm (0.33W) the line is 33% efficient. with a load of 0ohm it is 0% efficient.

> Also with very high voltage AC we can actually transmit quite far.

We can't transmit efficiently at a quarter the circumference of the globe, which would be required for a continually directly solar powered grid.

It makes sense if you confuse electrical fields with current. Since the exponential increase for transmission is geometrical essentially - which is why we prefer wires for transmission so strongly.

It does. There's also the somewhat complicated issue of reactive power, but there are ways to solve that, such as DC transmission, or capacitor banks.

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