> 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 power is essentially a sin wave, and we use 3 different 'phases', which are just shifted sin waves so that we basically always have at least one phase providing power at any time.
Every generator hooked up to the grid must be synchronized to the same 60 hz phases. There are devices called 'phasors' that assist in ensuring generators are lined up with the grid. And now there are networked versions to help coordinate across large regions.
But if two grids are on different phases, then there will be serious problems in connecting them together -- mostly that generators will be fighting against each other and potentially causing damage to things on the grid expecting clean 60hz power. It gets complicated pretty quickly.
Close, Three phase systems actually are a carry over from the days of early induction motors. Three phases can easily be connected to create a rotating magnetic field. We don't just use one phase at a time on the grid. All three are active. Phases A and B and C are all 120 degrees out of phase with each other. (That has nothing to do with the 120volts you know of in your house- it just happens to be another 120 number)
AC has inductive properties. I.e. it can induce electrical currents in wires across a core. (Transformer) DC cannot do this. Before vacuum tubes and other tech AC was one of the few technologies that could be easily stepped up to hundreds of thousands of volts.
Voltage and current have an inverse relationship. So we can step up voltage and reduce the size of the conductor $$$$ other feature is reduced line losses (HVDC actually beats us here but this is another discussion)
Having AC power allows all kinds of cool things! Did you know that your common household 120v feed is simply a single phase being split into two 120v 'legs'. Both 120 degrees out of phase. So if you remember elementary school math the abs() of phase A at its peak of +120 and phase B at it's valley of -120 = 240.
It gets better! We can be cheap on wire with AC systems! I can wire a 'multiwire branch circuit'. With phases a and b that share a neutral. If the current on phase a is 20 amps and the current on phase B is 20 amps what is the return current on my neutral? ( Google Mike Holt Multiwire Branch, Enjoy )
Want more fun. Read about Neutral Earth Voltage and NFPA 70 requirements for agriculture and equipotential planes. :-)
AC is freaking magical. The grid is even more awesome!
> Close, Three phase systems actually are a carry over from the days of early induction motors.
It's not just some historical accident. We continue to use multi-phase power because it enables continuous delivery of power (unlike single-phase, where the delivered power drops to zero at least twice per cycle). And two phases require just as many wires as three phases, with more power delivered for a given mass of copper and insulation, so... we use three.
> If the current on phase a is 20 amps and the current on phase B is 20 amps what is the return current on my neutral?
That depends on the (complex) impedance in each branch. In the pathological case, the current on the neutral could be as high as 40 Amps.
The advantage of three phase over two is that three phase can energize the coils of a three-phase motor so as to create a rotating magnetic field. No extra circuitry is needed.
Two-phase cannot do that when the phases are 180 degrees apart; opposite polarity is as good as a single phase. It would work if the two phases were 90 degrees apart.
A two-phase motor operating on single phase (or 180 degree two phase) needs to generate the 90 degree phase signal with extra circuitry.
Three phases is also smoother power delivery and even loading compared to two.
If both loads were on the same phase that would be a violation of the US NEC (NFPA 70) because use the load would be 40 amps..... And guessing we are taking 12awg, way over allowed current.
In hindsight I did not specify circut ampacity, or wire gauge. Still the question stands, how much current would you read at the panel on the neutral with those two loads on the line :-)
(In my example you have two phases 120 degrees apart. Think US Residential split phase. The actual result is pretty awesome. It's a common test problem for new electricians.)
I won't spoil the result because it's too cool when you get it.
Three-phase systems are far more than a carry-over from early electric motors. Cars, which have no legacy compatibility issues also use three phases. Three-phase power is a very good balance of simplicity of transformation and rectification with even-ness of loading.
You have to remember that alternators and generators are loading mechanical systems, so it's a bad idea to have huge cyclic variations in the load, which stresses the physical system.
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.
> Three phase systems actually are a carry over from the days of early induction motors
Three phase is better than one for transmission: you can pass more power for a given distance with a given budget of copper/aluminium wire. In other words, you have three wires rather than 2, but they can be lighter overall, and require less materials and fewer and less expensive poles.
> Both 120 degrees out of phase. So if you remember elementary school math the abs() of phase A at its peak of +120 and phase B at it's valley of -120 = 240.
It's not clear what he meant (because this part doesn't really have anything to do with 3-phase), but:
Most houses in the US have 240V service for large loads like A/C, ovens, stoves, dryers, EV chargers, etc. The way you get 240V (in the US) is that you take a single phase, and apply it to a transformer. The transformer's secondary has a grounded center tap, so you get +/- 120V, 180 degrees out of phase (this is called "split phase").
> Having AC power allows all kinds of cool things! Did you know that your common household 120v feed is simply a single phase being split into two 120v 'legs'. Both 120 degrees out of phase. So if you remember elementary school math the abs() of phase A at its peak of +120 and phase B at it's valley of -120 = 240.
The two legs are 180 degrees "out of phase" (that is to say one is simply the negative to the other). The peaks on each are +/- 170 volts, with a total peak across both of 340 volts. But we talk about AC in terms of RMS voltage, which is the DC-equivalent voltage that would perform the same work into a resistor, which is a factor of sqrt(2) for a sine wave, or "120/240 volts".
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?
> 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.
Interesting suggestion, though AFAIK that typically relies on auxiliary onsite power source at some generating plants, as noted in the Wikipedia article intro?
Anything you can find to support grid interconnects as black-start capacity?
Purely speculation based on the small capacity compared to total generated capacity.
From what I understand there can be a number of different techniques for black starting a grid, including pumped storage / hydro generation because they can be started with very modest power. Being able to jumpstart a grid based off of a neighbouring grid seems to make sense.
A large grid is harder to manage and runs the risk of a cascading failure. Especially as HVDC is a lot cheaper and more efficient than it used to be, there's an argument we should actually be breaking our grids down further.
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...
The speed of electricity isn't 1/100th the speed of light. IIRC it's something closer to 70% in copper. The speed electrons move is much lower, however something more like cm or m per second (presumably depends on voltage.) However, this isn't related to the speed of electricity which is analogous to how the speed of water through an (already full) hose isn't related to how soon water starts coming out the end when the tap is turned on.
I've also heard, but I can't explain so you'll have to look it up yourself, that none of this matters when keeping a grid in sync anyway. It just works out.
That's not a very large area, to be fair. That's about the size of half of the United States, in distance/spread. Europe isn't a continent in the traditional sense, it's a region, and not a huge one.
This might counter the grandparent poster if Asia was on the same synchronous grid, but it's not.
I guess there's two ways of looking at it. The distances spanned by the extreme points net or how dense it is. Not that I have the knowledge to relate any of that to real world use cases or consequences.
Pretty much. I mean, that's basically what a VSD is: A 3-phase rectifier back to back with a 3-phase inverter. Then you can have the two sides at different voltages and phases and transfer power back and forth at will.
>"[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 difference between the phases (for their higher-voltage loads).
Now, if you've ever looked at an oven or a dryer (in North America at least), you'll know that they are higher voltage, and look like they are two-phase... But it's really more like one phase that's been mirrored.
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 electrician or EE. I just spent a bunch of time looking into this when I got my 240v table saw and had to hook it up to something.
As proof that I'm not totally bullshitting you: until recently our house had a 120v outlet on a 240v breaker for a baseboard radiator. The electrician was here to fix an unrelated issue, and rejiggered things in the panel to hook it up to a 120v breaker.
As far as I understand it (not very) there is no such thing as two-phase power.
> Don't ask me how grounding works for 240v appliances. I have no idea.
Ground is 'the same' for 240v and 120v on split phase. A usually bare copper wire is run from the ground pin to the service panel, in the main service panel ground and neutral are connected together and to the utility neutral and the grounding rod.
Most 240v outlets are hot-hot-ground, or hot-hot-neutral-ground; some older outlets were hot-hot-neutral and the appliance was supposed to be wired separately to ground (and we can all suppose how often that was done properly, resulting in that outlet type becoming disfavored).
I'm currently rejiggering a 240v wired box for 120v outlets, after finding the wires in the panel and confirming they don't power any other 240v outlets, I'm connecting the white wire to the neutral bus bar, and the black wire to a spare 120v breaker, and then it's normal wiring of the outlet.
> As far as I understand it (not very) there is no such thing as two-phase power.
Because it's obsolete (but still exists in some places like center city PA). Two phase has two windings in the generator 90 degrees apart. Service was delivered via 3, 4, or 5 wires. Three phase beat it for a number of technical and economical reasons.
The 240V that you're talking about is what I mentioned as 'mirroring'. There definitely is such a thing as two-phase power, my electric car charger is 208V from two industrial phases, and our reflow oven is 208V, from two similar phases.
Well, it would be the same as single-phase power (to the device) if the devices were not connected to the neutral. The oven actually takes one of the phases (along with the neutral), and uses it to power some accessories.
With the massive caveat that I don't have a first principles understanding of any of this: I don't thing any individual load in your oven takes both legs and the neutral to do work. They're all either 120v or 208v single phase.
Aside: a lot of motors can run on either 240v single phase or 208v single phase (or, presumably, anything in between?). Take a look at Grainger or McMaster-Carr.
At least in Sweden, all houses have a 3-phase, 400V supply. This is very different from the US which just supplies one phase to each house even if the supply itself is 3-phase. This is why you commonly see 200+A main breakers on a house in the US whereas in Sweden, 25A is a pretty common main breaker size.
(It's as if the US grid characteristics were decided by the copper vendors... ;-)
>It's as if the US grid characteristics were decided by the copper vendors...
It was designed without an issue with copper usage. Europe had very little development and quit a bit of rationing while building out the grid post-WW2. The US had no such issue and spent no time working within those constraints.
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 smaller wires, but the disadvantage of increasing the risk if one is electrocuted.
> 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 devices still ship with only two prong plugs (everywhere: Europe, North America, Australia...) even if those prongs are polarized.
> 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.
Find this very scary not having RCD on some plugs even with 120 Volts. Sure 240v with RCD will give you a massive shock but still will very unlikely kill you. No RCD on the other hand can be fatal on 120V:
https://edition.cnn.com/2017/07/18/health/teen-bathtub-elect...
The US is 120V, or split-phase 240 V. Depending where you are, it's allowed to be ±5% at the meter. NEC guidelines say no more than 3% voltage drop from there.
Not just Sweden. In Netherland, 3x25A is also pretty standard. To our surprise, our house turned out to have 1x35A, so when we got a big induction cooker, we had to upgrade to 3x35A.
ConEd standard* service in NYC provides 120/208 three or four wire service from a wye connected transformer bank. With the density of the city it's easier to just run three phase all over and let homes pull alternatingly off the hots. So homes connect A-B, B-C, A-C and so on.
Three wire service means you get two hots and a neutral and is considered a single phase "open wye" service. BUT you can reconstruct the third phase from the two half vectors in each phase using transformers.
Four wire gives you full three phase service so you can run big motors, machinery, lots of lighting and such.
*Though plenty of neighborhoods are fed 120/240 or mixed. I'm on 120/240 yet the next block is 120/208. Sucks because I want my three phase.
I always love this one explanation of why three phase and not some other number of phases, like six or twelve[0]
Starting there is of course an infinite rabbit hole just on youtube to understanding power generation anywhere in the world and how it changed through history
You mention the challenge with connecting out-of-phase grids. This is I believe why the proposed interconnects are high-voltage DC, as mentioned in the article. Power will be loaded in from one AC system, transported, and then offloaded as AC properly synchronized to the recipient grid.
> then there will be serious problems in connecting them together
No there won't. The article mentions DC power converters. They are not cheap, but since HV DC lines are better for long distance power transmission than three phase AC, it's killing two birds with one stone.
"Folks may not realize that power is transmitted in three phases in the US"
My office in the UK has three phase power (I own it.) The phase shift thing is to enable putting more power into a place and nothing to do with "so that we basically always have at least one phase providing power at any time." whatever that means.
My home only has one phase because I don't need to draw that much power here (yet.)
Folks may not realize that power is transmitted in three phases in the US. AC power is essentially a sin wave, and we use 3 different 'phases', which are just shifted sin waves so that we basically always have at least one phase providing power at any time.
Every generator hooked up to the grid must be synchronized to the same 60 hz phases. There are devices called 'phasors' that assist in ensuring generators are lined up with the grid. And now there are networked versions to help coordinate across large regions.
But if two grids are on different phases, then there will be serious problems in connecting them together -- mostly that generators will be fighting against each other and potentially causing damage to things on the grid expecting clean 60hz power. It gets complicated pretty quickly.