Benefits of a single US electrical grid with high-voltage direct current

Because of the "high voltage."

AC versus DC is not all that important by itself. What's important is transmitting electricity at high voltage.

Transmission losses come from the amount of current you pass through a conductor. More current equals more power lost to the resistance of the conductor.

Delivered electric power is equal to voltage multiplied by current. Higher voltage means lower current for the same amount of delivered power, which means lower transmission losses.

The AC versus DC thing comes in because you need lower voltages for practical applications (it's hard to run a light bulb off 100kV) and that means you need to transform between different voltages. Transforming AC between different voltages is pretty easy: an AC transformer is basically just a pair of coils with a metal core, easily built with 19th-century technology. Transforming DC is much harder and requires much more advanced technology.

For the 19th-century battle, this meant that AC was the only one that could be transmitted at extremely high voltages. DC was limited to serving very small areas because it had to be transmitted at the same voltages which would be used at the destinations.

Today, that difference goes away, so the advantages and disadvantages are all about much smaller secondary effects instead.

At the time of Edison, they already had AC transformers that made AC voltage conversions either up or down very easy. All you need is an iron ring, and you wrap some wire around it.

https://en.wikipedia.org/wiki/Transformer#/media/File:Transf...

DC transformers only recently reached comparable performance, thanks to semiconductors, and efficiencies are not as high as a good AC converter without really sophisticated circuits.

Higher voltages and frequencies is how you get less loses as resistance is coupled to current. The lower the current, the less the resistance affects transmission. However you can only increase ac voltage and/or frequency so much before you have another issue, the impedence of the air itself starts to create loses Also, because AC doesn't fully penetrate the wire, you have to run much larger wire sizes to achieve the same affect. At a certain point, you simply can not push more power using AC without resulting to things like superconductors.

DC OTOH, does not have the skin effect issue and so because more desirable in certain cases. In fact, high voltage DC is how they electricity directly to LA all the way from Oregon/Washington state: https://en.wikipedia.org/wiki/Pacific_DC_Intertie

A lot of research and development of semiconductors that can handle the extremely high voltage (250+ kV) required to keep the losses down.

I think ABB (One of the market leaders and early pioneers when they were named ASEA) initially tried to make better underwater transmission systems.

It has to do with the high voltage and the impact of voltage loss.

† I've accumulated a lot of stuff I'm glossing over at the end.

Let's consider a 1 square millimeter cross section wire. Your power lines will be bigger, but it scales up with the area.

Electricity facts refresher:

  ohms measure resistance to electricity movement
  amps measure how many electrons per second you are jamming down a wire
  volts measure how hard you are jamming the electrons down the wire
  watts measure power, this is ultimately what you care about

  volts = amps * ohms
  watts = amps * volts

  this is true for AC and DC

Your copper wire has a resistance of about 17 ohms per kilometer. This wire is allowed to take 3 amps of current when used for power transmission. That makes the voltage drop over a kilometer be 3 * 17, 51 volts.

This voltage drop doesn't depend on the voltage of your power line, just of the current running through it. If you are Mr. Edison with a 110 volt generator, then you have lost almost half of your power just transmitting it one kilometer.

If you are the Pacific DC Intertie sending power from Washington state to Southern California, you are operating at 1000000 volts. You are losing 0.005% of your power per kilometer.

The whole AC/DC thing was because in the early 20th century it was easy to change the voltage of AC power but difficult and expensive to change the voltage of DC power. You could pump AC up to higher voltages for transmission, and bring it back down for domestic use.

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† Notes follow:

The current limits I used for the wire are for common electrical engineering work on devices, I don't know the limits for power transmission lines, but…

I sort of doubt they use copper for transmission lines, too expensive, so the resistances will be higher. Wikipedia to the rescue: The Pacific DC Intertie uses aluminum wire, reinforced with steel, with a 644 mm^2 cross section. (Not clear if that is the aluminum of the whole thing.) They are also pushing 3100 amps which puts us in the ballpark, 1000 times the current in 644 times the cross sectional area. They are operating above the power transmission guideline I picked but below the limit for chassis wiring.

Also for transmission lines, the much thicker wires will not be able to dissipate heat as well as the tiny wire I used and there will be a derating of their current capacity from that.

High frequency AC gets weird, it travels on the outside of the wire and the cross sectional area doesn't scale, but 60Hz isn't going to do that much.

If you think "I know, I'll wire my home/datacenter/yacht with 12v DC power!" Then you really care about voltage loss. If you want to move 2000 watts 10 feet with low loss you will be using two copper conductors about the diameter of your thumb. This is why you see 48v used on things like Power Over Ethernet. That is the "low voltage" limit for some regulatory agencies, and they want to get as much power of the tiny conductors as possible.

The current proposed UHV DC systems are not sophisticated switching regulators. They still use 50/60 Hz AC transformers to change the voltage but use electronic commutators to convert this to DC and back to AC (and is reversible). The commutators actually use thyristors (they can only be turned on, and depend on AC momentum to turn off). It means the receiving end of the power must have large AC machines (power generators).

Take a look at: https://en.wikipedia.org/wiki/HVDC_converter It mentions IGBT based commutators which do not require AC machines, but which are less efficient.

I think all the thoughts about the switching harmonics being horrendous are correct.

Something else occurred to me: DC-DC switching is an inherently noisy design. It produces switching spikes which are notoriously difficult to filter.

If you are working on an electronics project which is sensitive to noise, you currently have the option of ditching your cheap, efficient DC-DC bench power supply and instead using a linear power supply, which is far less efficient (instead of producing the desired voltage by converting power, it does so by turning the extra power into heat), but is a very low-noise design.

If we switch to a DC grid, there'd be no point in using a linear bench supply to develop a low-noise project, because those nasty switching spikes you were trying to avoid aren't coming from your supply, they are coming from the grid!

commutation would also be guaranteed (when circuits would be switched) to be highly funky and generating peaks with rebounds... in non linear ways. And with the multiplication of sources (wind/solar) commutation/switching are to be considered. A highly distributed system tends to be already chaotic. Adding chaos to chaos does not seem a good idea.

Not to mention Biot Savart Law that says a long wire generates B ~= I/r

So where the electric current on average nullifies the Magnetic field in AC (other a period), on DC you would have to build with a constant F = qv vect I/r which means a constant magnetic field appearing and inducing resilient constant forces for every other charges in movement. 1/r decrease way less than 1/r² // 1/r3. That is what CE norms on electro pollution wants to avoid. A strong electromagnetical residual signal that can interfere with other electromagnetic devices.

I understand the concerns of persons that never had to learn electricity about the complexity of using Z (impedance) other R (resistance). But Fresnel law makes AC quite easy to grok.

In some countries it is accepted since a long time that the bug in AC is the side effect of loosing the phase (hence introducing peaks up/down phase). That is the reason why companies have to pay not only for the whattage but also for the disruption of phase. Some companies even get discount to regulate the phases (like train companies that uses a lot of electricity in peak fashion at trains start/stop and can accumulate energy selectively to charge huge capacitors and use them to correct a shift of phase).

I sometimes look at the electric cable in my cities. The way they are plugged anarchically and the transformers looking like not maintained, the electric boards with expensive gross protection, the corrosion at the connection points (yes surface effect with rust and water sure it does do weired loss) the fuse used in electricity/electronic devices .... And I feel like I was in Cuba in the 1986 in the middle of embargo.

I don't live in Cuba, this I see in Montréal, SF, miami ...

In supposed developed countries and I wonder if my eyes telling me it is broche à foin are right or my brain listening to all the assertive geeks and citizens telling me the opposite.

You know what, I do not know. I still have a strong feeling technology on the new continent is not as good as it is publicized.

The article doesn't claim that interconnection is impossible, just that it requires extra AC->AC conversion hardware at each interface.