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Why Does the U.S. Have Three Electrical Grids?

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Re: Why Does the U.S. Have Three Electrical Grids?

#81

Post by kalleboo »

gumby wrote: post #45
nickff wrote: post #26

Earlier quoted context omitted.

Well, you have to consider that the USA (and Canada) were on the bleeding edge of technology when they adopted electrical grids. They were left with a... legacy, including the type-A plug and 110V single-phase power to residences. In your case, I believe each prong of your two-prong plug gets a different phase, so you have 3 possible pairs of phases to power each circuit from. This has the advantage of requiring smal…

> but the disadvantage of increasing the risk if one is electrocuted. Avoiding this was one of the design parameters of the "Schucho" plug design that is reasonably standard across Europe. You'll note the three prong plug has a longer ground that makes a connection before any hot one does. The US three prong is supposed to work that way too but the ground pin isn't long enough. I continue to be amazed how many device…

> I continue to be amazed how many devices still ship with only two prong plugs (everywhere: Europe, North America, Australia...) even if those prongs are polarized

Because most devices these days have plastic cases (Class II double insulated), so there's nothing metal to touch to get shocked even in the case of a fault. That coupled with how GFCI/RCDs are being widely adopted as well leads to grounding not being as important.

Re: Why Does the U.S. Have Three Electrical Grids?

#82

Post by na85 »

X6S1x6Okd1st wrote: post #69
na85 wrote: post #11

Earlier quoted context omitted.

Fire is a natural phenomenon and I don't see how it can credibly be called an invention. That would be like calling water an invention. The others, though, are certainly inventions.

Do you think that electricity can credibly be called an invention?

No, but I distinguish between Electricity, the physical phenomenon, and Electric Power, the application of electric charge to energize a device. Lightning is the obvious example of electricity that is not electric power.

Re: Why Does the U.S. Have Three Electrical Grids?

#83

Post by hyperdimension »

URSpider94 wrote: post #53
hyperdimension wrote: post #44

Earlier quoted context omitted.

So does my...dishwasher. It actually generates three phase power (maybe stepped sine-wave?) for its main motor. I did a double-take when I saw it (during the course of an unrelated fix,) considering it's powered off a simple (US) 120V 15-amp branch circuit.

Almost all modern motors use inverter circuits to generate multi-phase drive signals. Any time you see a product with a “brushless motor”, that’s what it means.

Interesting. I didn't know that. I always figured that was a method for powering (otherwise) AC motors from DC. I never realized it included multi-phase motors.

Re: Why Does the U.S. Have Three Electrical Grids?

#84

Post by JdeBP »

mauvehaus wrote: post #32
nickff wrote: post #6

Earlier quoted context omitted.

>"[W]e basically always have at least one phase providing power at any time." This isn't really how three-phase power works. In household settings, you're usually getting only one phase, so all your power comes from that phase. Different neighborhoods get different phases, so they're roughly balanced loads. Industrial (and some consumer) users get two (208V) or three phases (480V), and they get power from the differe…

Typical residential service in the US is 240v single phase. The transformer on the pole is putting out 240v, but there's a center tap on the transformer that's bonded to ground. That's your neutral. The other two legs are essentially +120v and -120v relative to ground/neutral. Your dryer hooks up to both legs and sees the full 240v. Don't ask me how grounding works for 240v appliances. I have no idea. I am not an ele…

Enjoy https://youtu.be/jMmUoZh3Hq4 .

Re: Why Does the U.S. Have Three Electrical Grids?

#85

Post by tinix »

jdswain wrote: post #68
aidenn0 wrote: post #5

Earlier quoted context omitted.

The solution is to connect them with HVDC, right? That might not be a great solution for making a single national-grid, but it would allow the 3 grids to mutually borrow from each other, right?

Wouldn't a better solution be to make them all in phase with each other? Then there wouldn't need to be any HVDC interconnect.

Consider the distance, and speed of light, and the speed of electricity (1/100th of that). I don't think it's truly feasible to keep that much physical space in sync. Hmm...

Re: Why Does the U.S. Have Three Electrical Grids?

#86

Post by dredmorbius »

Pfhreak wrote: post #4

> Everything within those separate zones is synched up. So you’ve got your 60 hertz AC wave; 60 times a second the AC power flow is changing direction. And all of the generators, all of the power consumption within each zone is doing that synchronously. But the east is doing it on its own. The West is on a different phase. Same for Texas. Folks may not realize that power is transmitted in three phases in the US. AC p…

Interesting explainer: Why 3 Phase Power? Why not 6 or 12?

https://youtube.com/watch?v=HqZtptHnC2I

(Substation complexity.)

Re: Why Does the U.S. Have Three Electrical Grids?

#88

Post by Gwypaas »

tinix wrote: post #85
jdswain wrote: post #68

Earlier quoted context omitted.

Wouldn't a better solution be to make them all in phase with each other? Then there wouldn't need to be any HVDC interconnect.

Consider the distance, and speed of light, and the speed of electricity (1/100th of that). I don't think it's truly feasible to keep that much physical space in sync. Hmm...

Apparently not a problem to do today.

https://en.m.wikipedia.org/wiki/Synchronous_grid_of_Continen...

Re: Why Does the U.S. Have Three Electrical Grids?

#89

Post by cyphar »

swatson741 wrote: post #55

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…

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.

Re: Why Does the U.S. Have Three Electrical Grids?

#90

Post by dredmorbius »

msisk6 wrote: post #13
aidenn0 wrote: post #5

Earlier quoted context omitted.

The solution is to connect them with HVDC, right? That might not be a great solution for making a single national-grid, but it would allow the 3 grids to mutually borrow from each other, right?

That's exactly right, but the capacity is limited. ERCOT in Texas, for example, has 5 DC ties to the other grids: http://www.ercot.com/content/cdr/html/real_time_system_condi...

> Peter Fairley: So to give you a sense of just what the scale of the transfers is and how small it is, the East and the West interconnects have a total of about 950 gigawatts of power-generating capacity together. And they can share a little over one gigawatt of electricity.

> Steven Cherry So barely one-tenth of one percent.

From TFA.

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