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Comment by kimpeek | Hacker News Reader

> Direct Current (DC) electric power is an emerging disruptive technological area that has the potential to stimulate economic growth, inspire innovation, increase research and development opportunities, create jobs, and simultaneously advance environmental sustainability.

Was this published in the early 1900s? There is no date and DC is definitely not emerging nor disruptive.

DC won't replace AC for those who rely on remote power production.

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This is, in fact, precisely backward.

DC is great for transmitting power. You crank the voltage, use all of the copper wire (no pesky skin effect), and sync to the grid at the DC-AC conversion point.

The limiting factor to DC was conversion losses. The Pacific DC Intertie needed to use gigantic, toxic mercury vapor tube diodes for the conversion for a very long time.

Now that we use high voltage semiconductors, that's no longer a problem. We easily convert between DC voltages as well as AC with quite remarkable efficiency.

At your typical power distribution frequencies (50 Hz, 60 Hz) the skin effect is negligible.

You are simply wrong.

Skin effect at 60Hz is about 8mm. Power transmission (especially the long distance ones) conductors are normally quite a lot larger than that. Even the wires coming into your house are probably pretty close to that so there will be some effect even if it's not huge.

8 mm (in copper, which is rarely used for powerlines, if at all, it is super expensive and heavy) is huge for a single conductor, and your typical overhead powerline is concentric shells of tens of conductors. Negligible: has no practical effect on the construction. It's in the 4th significant decimal or so for a typical powerline segment, dwarfed by plain resistive losses.

You want those multiple conductor arrangements anyway to reduce the corona discharge.

If you go up to multiple KHz then it will become a problem.

Reactive power losses due to the cable's inductance are probably the largest factor in the efficiency boost of using DC. Also you can use a somewhat higher voltage since you don't have to account for the AC peak, it does mean however that breaking a DC arc is harder since the is no 0 crossing.

Depends greatly on the material used and the cable construction. Typical: 60 strand aluminum (better skin effect properties than copper by the way) around a steel carrier. And yes, those are the largest factors in the boost to DC, but that's mostly because the resistance losses are there regardless so there isn't much else that you could improve on.

If you transmit a lot of power over very long distances then the higher the voltage the lower the current and DC gets rid of the skin losses so there's the case for HVDC transmission lines (which are extremely impressive feats of engineering, as are the substations).

Finally found a good picture of a cross section of a HV AC transmission cable:

https://en.wikipedia.org/wiki/Aluminium-conductor_steel-rein...

Based on that ruler that makes the AL wires about 3 mm each, and the skin depth at 50 Hz would be about 11.5 mm or so, so well within the range where the skin losses are extremely small (they are still there though, and when you're transferring Gigawatts every little bit helps).

For grid transmission over longer hauls it will definitely be the standard, for shorter runs and local distribution we will likely be using AC for a long time to come, possibly forever.

Why won’t it replace it, out of interest?

Same reason why it didn’t win out 100 years ago. It isn’t as efficient.

I don't think it is efficiency, we had no way to step up and step down DC as we can do AC

Yup, transistors especially have given us major breakthroughs in the ability to step up/step down DC.

Also, there have been huge breakthroughs in High-Voltage DC: https://en.wikipedia.org/wiki/High-voltage_direct_current

At certain huge (grid) scales they have found that AC and DC swap "efficiencies" again and we're increasingly starting to see current flows as DC-AC-DC "sandwiches" with DC used by the majority of consumer electronics and DC used for extremely high scale grid transport, and AC still useful in the mid-range transport.

> DC-AC-DC "sandwiches"

Couldn't have said it better, I had a good laugh.

Yes, this is true. To step up/down AC voltages, you only need a transformer, a pair of coiled wire. This is very simple tech.

To step up/down DC. There are ways to do it with solid state electronics. One of the ways I’ve seen is to transform the dc to ac internally, change the voltage, and convert and output DC.

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