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Comment by swatson741 | Hacker News Reader

The article makes this seem like it's a US problem but, the interconnects lay in Canada and Mexico too. I'm guessing that unification is a low priority for them since it seems that the US stands to gain the most from it.

"DC power lines transmit power more efficiently than AC lines do"

This is not at all what is taught at school. Looking into this is interesting. 100 years ago people were debating about whether AC or DC is a better means of transmitting power and, it looks like answers still aren't clear now. I'm interested in knowing what an expert in this thinks.

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What is taught in school is generally a simplification of the truth. However just because that is the case doesn't mean that the "answers still aren't clear now" -- it's just the reality of the limits of the education system (if you tried to explain every topic correctly from first principles you would never get past Newtonian mechanics).

The reason why AC is taught as being "more efficient" is that if you use AC you can easily use a transformer to increase the voltage (decrease the current) of the electricity being transported. That makes the transport more efficient because of I²R power losses. The reason why AC is "easier" to transform is because the classical transformer (two coils of wire and a laminated iron core -- no moving parts and incredibly efficient) requires AC input. Most power grids do use AC because this makes the grid much simpler to produce and

However the part that is missing from high-school education is that AC travelling down power lines at the same voltage as DC is less efficient and more expensive per kilometre. There are many reasons for this -- but the main ones are that the skin effect means AC current only travels on the outer parts of a wire causing higher current density and thus higher losses, the generation of RF interference, and the fact that AC lines have to transport reactive power (AC current reverses direction every half-cycle which essentially means more power loss).

So the most efficient transport mechanism for long distances is to generate AC, transform it up to a high voltage, and then rectify it to DC for the actual transport (and reverse the procedure on the other side). However the rectification procedure requires expensive equipment, which is why HVDC transport is only used for long-distance power transportation.

To transmit power without loss you have to increase the voltage. The easiest way to do this (And in the late 19th century the only way) is with a transformer, and therefore AC.

However, since the 1930s we’ve been able to build high voltage DC lines, and costs have plummet in the last 30-40 years (as we moved away from Hg rectifier tubes to Si high power electronics).

DC has many advantages:

- No capacitive coupling to ground - No reflections due to Z mismatch - You can lay a cable underwater (basically impossible with AC) - you can connect incompatible grids

HVDC is still fairly expensive (obviously compared to a lump of laminated steel) so it’s used for very long lines, or other special needs.

But it’s revolutionized the field.

AC still has many advantages and won’t go anywhere, especially last-mile to medium range:

- arguably safer since an arc fault can self extinguish - switches are much cheaper to build - cheap electric motors - cheap ability to scale V

It looks like most of those points are because you wouldn't necessarily be transmitting power over frequency like you would with AC. But, if we're transmitting over long distances wouldn't we end up transmitting the DC in frequency to avoid attenuation loss?

AC current is more efficiently transformed between voltages.

DC current is more efficiently transmitted (conducted) over long distances at high voltage.

Since the early 20th century, DC transformers have become more efficient, as others have noted.

Given that electric grid transmission occurs at a wide range of voltages (~275kV generation, ~100kV long distance, 50kV local, 110/220V residential/commercial drops), efficient, reliable, cheap transformers were required, giving the initial edge to AC.

https://en.wikipedia.org/wiki/Electrical_grid#Voltage

HVDC is much better than anything else for long distance transmission. But the gain is because of the high voltage part, not the DC one.

The one thing that pushes people into DC is that phase synchronization is a huge problem on high voltage. So, for indirect reasons, DC gets to be the winner right now.

“But the gain is because of the high voltage part, not the DC one.“

What do you mean by that? My understanding is the HVDC lines are still lower V than an AC line (the Soviet even had 500lV lines), and therefore DCs advantages are no coupling to ground, no Z reflections and ability to connect to any grid.

A quick search will lead you somewhere like here: https://en.wikipedia.org/wiki/High-voltage_direct_current.

> Most HVDC links use voltages between 100 kV and 800 kV.

By contrast, high voltage AC lines are around 10kV to (very rarely) 100kV. AC presents some more problems to high voltage than just phase synchronization (so even two point lines are DC), but the gains come mostly from the HV part, and AC is avoided because it doesn't play well with HV, reverting the gains.

Standard (long distance) lines in Europe carry 380kv AC.

Ac transmission lines are regularly 500kv with some famous ones in Quebec being 750kv. I believe there are even higher now.

Lots of good answers in the replies here, but one more thing to keep in mind is that, when the DC vs AC argument was going on in the late 1800s and early 1900s, we were still 40 years away from discovering semiconductors.

Nowadays, a lot of residential loads at least would probably be more efficient with DC coming into the house with the popularization off brushless DC motors and all the technology around the house.

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