Power grids. AC or DC in hindsight? (Split from Nikola Tesla)

[AHHans]

12 hours ago, wumpus said:

The last thing to take away from all the Edison-vs-Westinghouse (instead of Telsa) malarky is that AC is more or less an obsolete installed base that we are stuck with.

Sorry, the complexity and cost of DC-DC voltage conversion - compared to a transformer - makes a grid based on AC the better choice. HVDC power lines are only superior for point to point connections (I don't know of any non point to point HVDC line). With AC you can make a grid, where you can hook up new clients anywhere and have it supply or consume power at need. So even for a clean sheet design I would go with AC for regional and local distribution.

[wumpus]

On 11/21/2020 at 5:09 AM, AHHans said:

Sorry, the complexity and cost of DC-DC voltage conversion - compared to a transformer - makes a grid based on AC the better choice. HVDC power lines are only superior for point to point connections (I don't know of any non point to point HVDC line). With AC you can make a grid, where you can hook up new clients anywhere and have it supply or consume power at need. So even for a clean sheet design I would go with AC for regional and local distribution.

You're forgetting about balancing all the phases across all the generators over the entire grid.  Perhaps Germany doesn't connect large grids, but in the US [lower 48] power is distributed in only 3 grids, and one of them is mostly Texas.  "Hooking up new clients" is trivial.  Hooking up new generators is where the problem lies, and solar and wind power make this  even more of a challenge.  I wouldn't be surprised to see DC connected "AC microgrids" that are small enough that the phase issue is moot, although getting there from here would require significant engineering.

First mile to last mile DC wins on phase issues.  Last mile DC wins on being more compatible with power supplies (especially considering EU directives about power correction).  AC was great from the start of electrification to ~2000, but isn't that great now.

[kerbiloid]

Conjuring railroad fans to tell about the old and new standards of electric network of the electric locomotives, DC vs AC.

(Am not a specialist in it but know that they've changed several decades ago.)

[AHHans]

28 minutes ago, wumpus said:

Perhaps Germany doesn't connect large grids

Well, Germany is part of the synchronous grid of Continental Europe (the largest by connected power in the world), is that big enough?

I'm not saying that an AC grid doesn't need to be managed. And synchronizing a generator to the grid may not be trivial, but is also not a big problem.  In particular a lot less of a problem than designing a DC power grid with multiple voltage levels (you don't want to have the same voltage that is used on the long-range transmission lines in people's homes) and the ability to send power in all directions.

Btw. as you are apparently in the US, you probably have some pole-mounted distribution transformers close to you. How often do they need to be serviced? And replaced? How long does the power supply of a computer typically last?

[YNM]

On 11/21/2020 at 5:09 PM, AHHans said:

Sorry, the complexity and cost of DC-DC voltage conversion - compared to a transformer - makes a grid based on AC the better choice.

Given hindsight 20/20 it is painfully clear for large-scale systems. But in those era grids are unbelievably (at least given modern perspective) small, maybe only covering a city or two, and there were even DC distribution just for certain city blocks.

And it's not like AC adoption necessarily negates any problem - what if, say, all the way back then, you bought generating and transmission equipment from the US, geared for 60 Hz, and also from Europe, geared for 50 Hz, and installed them at the same time ? If so, then, you're welcome in Japan.

Edited November 23, 2020 by YNM

[kerbiloid]

It's not a disadvantage of AC, it's a disadvantage of local preferences.

If a country has one frequency as a standard, they could just buy another frequency manufacturer's products with localized power unit.

Like we here in Russia have no problems with 60 Hz goods because they arre equipped with 50 Hz localized power unit.

Edited November 23, 2020 by kerbiloid

[YNM]

3 hours ago, kerbiloid said:

If a country has one frequency as a standard, they could just buy another frequency manufacturer's products with localized power unit.

Appliances are never largely not a problem, frequency doesn't affect how most AC appliance works as it doesn't change the resistance, induction and capacitance* nor the RMS voltage - actually, not even with voltages, as US's 110/120 V system is actually also 220/240 V system so it's not that much of a problem for someone in the US to install 240V appliances (in fact, they regularly do so) unlike someone in Europe to install 120V appliances - the problem lies in generators and stuff. That's a lot of gearing to change, making sure that the rpm and/or speed changes from multiples (or factors) of 60 to multiples (or factors) of 50 (or the other way around). Might be easier with the rise of renewables but I'm sure the list doesn't stop there, as you can't really gradually change from one frequency to the other without shutting down all of the system.

 

* EDIT : OK, RF equipment and motors are a problem. Though for most RF equipments they are now powered by DC converters ('power bricks') rather than straight off the grid frequency. Motors is the one that remains a problem, though from what I can read here seems like the Japanese themselves figured out how to deal with their peculiar 50/60 Hz situation... though ofc none of this is needed elsewhere so it's not used for products that goes beyond their borders (we use a lot of japanese-company-made equipment and it's all just for 50 Hz).

Though all of this is to show that AC systems aren't necessarily without it's peculiarities, and it does occur in one country on this Earth.

Edited November 23, 2020 by YNM

[mikegarrison]

The key to all this is that high voltages are needed to efficiently transfer power, and AC transformers made high voltages very easy to deal with.

Only recently have people invented solid-state systems that can efficiently and mostly reliably step up or step down DC voltages.

These are still more expensive than transformers, so they are mainly used as bridges between AC systems, to avoid the need to synchronize the different AC grids. They also work much better for underwater power cables.

Like any major piece of infrastructure, once the grid is established it's hard to change it, so I suspect AC grids are going to be around for a really long time to come. It's not like they don't have their own set of advantages, too.

Edited November 23, 2020 by mikegarrison

[YNM]

Yeah, it's hard to change something already well-established most of the time, esp. when it involves a lot of existing physical infrastructure. AC distribution will remain here as a "winner" of the "current wars" (if there were even any such wars to start off with); But it's sometimes important to look back on how things started off like and how it makes some oddities arise.

In the case of Japan, they're now simply trying to put in more conversion capacity between the two grids. With nuclear unlikely to become a preferred generation option and renewables replacing the conventional fuel buffer they had to use, the only way out is to make sure the grid can become effectively larger while remaining on each side of the frequency divide.

 

Maybe the current question will only come back once we've settled on a different planet, and solar panels (and even thermocouples) producing DC power remain the most used solution...

[AHHans]

@wumpus, @kerbiloid, @YNM, @mikegarrison : If you want to continue this discussion, we could move it into a new thread instead of hijacking this one.

[YNM]

I don't think there's much else to discuss on the matter, but it'd be nice to move it to a different thread.

 

EDIT : Thank you moderators !

Edited November 24, 2020 by YNM

[kerbiloid]

I don't see anything to discuss about Tesla, so at your wish.

[Val]

Split off as requested. @AHHans @kerbiloid @YNM

[Shpaget]

DC doesn't suffer from the problem of power factor correction, something that is of significant interest on utility scales, and even as small as industrial users. With more and more switch mode power supplies powering our appliances, I would not be surprised to see power companies penalizing households for poor power factor.

Modern solid state switch mode power supplies can easily deal with voltage shuffling.

Increase in DC power generation (solar panels) is also an important aspect of design.

When we started building the power grid it made sense to use AC simply because transformers were the only reasonable way to convert between voltages and you really need high voltage for long distance distribution. Today, it's really much more even race. I wouldn't be surprised is Mars colony used DC.