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Power Grid: The Game is way more fun than this.
May 17, 2012 7:27 AM   Subscribe

Explain alternating current distribution across "the grid" to me in terms an educated layman can understand. Specifically, I have a question about how to talk about which power plants are providing power to a particular consumer.

Ascii art diagram follows:
POWER PLANT A               100M                   CONSUMER C
+10MW                -----------------------------------     -10.5MW
                                           |x
                                           |
                                           |100KM
                                           |
                                           |x
POWER PLANT B------------------------------------CONSUMER D
+1MW                               100M                                    -0.5MW
Okay, so this is a closed system with a grid of just four entities. The question is, what percentage of the power consumed by D is produced by A?

I know that there's probably a bunch of missing information (like the resistance on the lines) but is there a general tendency? Will it draw more from B because it's much closer, or more from A because it's much bigger? Does it change anything if there are step-up transformers/substations at the points marked X?
posted by 256 to Science & Nature (20 answers total) 5 users marked this as a favorite
 
Because the diagram is not showing up beautifully, a description of the system: There are two power plants, A and B, and two consumers, C and D.

A produces 10MW and is 100 meters from C which consumes 10.5MW.
B produces 1MW and is 100 meters from D which consumes 0.5MW

These two generator/consumer pairs are connected to each other by a 100 kilometer long power line.
posted by 256 at 7:30 AM on May 17, 2012


what percentage of the power consumed by D is produced by A?

None of it.

The 100km link can only transfer power in one direction at a time, from its higher-voltage end to its lower-voltage end. Given that Consumer C is drawing 0.5MW more power than Plant A is producing, the higher-voltage end of the 100km link is on the B-D side, and there is 0.5MW flowing from the B-D end to the A-C end. Plant B is supplying that, along with the 0.5MW consumed by Consumer D.
posted by flabdablet at 7:43 AM on May 17, 2012


Power plant lines do not flow directly into a home. Power plant lines are at VERY high voltage. The power has to be transformed to usable currents.

Lines from a power plant flow to distrubtion points, and from there to transformers on the street, and from there into homes. Power companies have zoned areas. If you live on a certain street, you can not choose your power company. A company has that street - they own the power lines and transformers on that street.

When one power company needs more juice, they can buy extra power from other companies - but that extra power flows into distribution points, and not into homes.

So, your answer is ZERO. A customer only gets power from one distribution point, owned by one power company. (Power companies can buy power from other power companies, but individuals can not.)
posted by Flood at 7:47 AM on May 17, 2012


I don't think that was the substance of 256's question. Replace "Consumer" with "Distribution Point."
posted by scose at 7:54 AM on May 17, 2012 [1 favorite]


OK, maybe Scose is right - maybe your question is about how two power companies flow into the same distribution point, rather than into a consumer household. The answer is still: they don't.

Power does not flow openly into any one distribution point. These distribution points, which serve various functions, like boosting or transforming the current, are owned by individual power companies. On any given day, only power from Power Company A flows to that point.

If power company A needs extra juice, they essentially open a valve. (More like, the have an automated system that removes an insulator, and connects that point to power lines running between power companies). When they open the "valve", they have a meter monitoring the flow of power into that point from power company B.

If the "valve" a distribution point was left open always, then both power companies would be back feeding each others grids. If two power sources are powering the same grid, and you test the voltage on the grid at a certain point, the power from each would be combined, and the two would be essentially indistinguishable.

For example, if you had two portable generators, each producing 6kW of power. And you wanted to feed to appliances, you could wire a system together. So that, coming out of your two power sources is 12kW, and then feed one appliance that uses 9kW and another that uses 3kW. But once the two power sources are wired to feed together, it is no longer distinguishable with particular volt came from with particular power source. It doesnt work that way, since the flow of electricity is real electrons jumping from one atom to the next, and not truly a flow of electrons. The more volts means more electrons jumping, but it does not mean that electron X flowed from power source A to appliance D.
posted by Flood at 8:21 AM on May 17, 2012


My answer, by the way, assumes we're ignoring line losses. In reality, Plant B would need to be producing 1.0+X MW in order to contribute 0.5MW to each of Consumers C and D while allowing for losses of X MW in the 100km line. The value of X depends on the resistance of the line and the voltage it runs at: in general, higher voltages yield lower losses and that's why there probably would indeed be transformers at both X points.
posted by flabdablet at 8:22 AM on May 17, 2012


Probably the best way to think about it is to reframe electrical potential as a reservoir. The suppliers pump water into the pool, and the users pump it out.

It's not really possible to know how much comes from one or the other at any given moment - the usage is decoupled from production.

This glosses over many details (such as reactive power, balance of loads, etc.), and is not a perfect analog*, but it's serviceable enough.

* it gets even better when you talk about how changes will ripple and reflect around the network, though.
posted by Pogo_Fuzzybutt at 8:29 AM on May 17, 2012


Flood's answer confuses electron flow with energy flow, which is unfortunate. In fact electrons don't flow very far in an AC circuit - they just vibrate back and forth while shoving each other. But in any given wire there is always one end where the shove is coming from, and another end where it's going to; power is never transferred in two directions simultaneously on the same wire.

It's possible to design a system that does transfer power in two different directions simultaneously, by driving each end of the wire at a different frequency and having frequency-dependent loads, but power distribution grids specifically don't do that.

It's not really possible to know how much comes from one or the other at any given moment

Yes it is.
posted by flabdablet at 8:34 AM on May 17, 2012 [2 favorites]


Flood's answer confuses electron flow with energy flow, which is unfortunate. In fact electrons don't flow very far in an AC circuit - they just vibrate back and forth while shoving each other.

flabdablet, I want to clarify your clarification. There is net electron flow in a power grid. Individual electrons don't flow very far, but they "shove" more electrons down the line, and the accumulated effect is more electrons piling up on the positive side of the voltage source, and fewer electrons at the negative side.

It's true that a single electron will never travel very far, but that's somewhat meaningless on the scale of "electricity"; there is net flow.
posted by IAmBroom at 8:57 AM on May 17, 2012


It's not really possible to know how much comes from one or the other at any given moment

Yes it is.


I was sort of hoping to avoid a complicated answer, because I'm not sure where the OP is headed with this question.

I think what the OP is asking is how do payments for electrical service get distributed to producers. In the case of WI, I can choose to get my power from wind sources, and pay a higher fee for it - so how am I using wind electricity vs. nuclear if I am connected to the same grid ? How does the "wind electricity" get to my house and not "nuclear electricity" ? And how do the producers get paid ?

The simple answer is the producers are paid to supply a set amount of potential to the network. And users are charged to draw potential from the network. How the actual prices are set is more complicated; but users know how much they used, and producers know how much they made, so it is possible to allocate costs and funds using that knowledge.

As for which plant the electricity in your house "comes from" at any given moment isn't really knowable. From a layman's perspective, it's not possible to figure out in anything other than a trivial case.

But it isn't particularly important to know "where" the electricty comes from. Just that you paid for that potential to be available to the grid, and the provider made it happen.
posted by Pogo_Fuzzybutt at 9:22 AM on May 17, 2012 [1 favorite]


Individual electrons don't flow very far, but they "shove" more electrons down the line, and the accumulated effect is more electrons piling up on the positive side of the voltage source, and fewer electrons at the negative side.

And when we're talking AC, they just wobble back and forth and never get far from where they started. But the shoving still goes from the source end of the line to the sink end; the line transfers power in an unambiguous direction, from power source to power sink, and that direction has nothing to do with the direction of electron flow.
posted by flabdablet at 9:37 AM on May 17, 2012


The simple answer is the producers are paid to supply a set amount of potential to the network. And users are charged to draw potential from the network.

A little precision in terminology would be good here. What electricity consumers pay producers and distributors for is energy, not potential.
posted by flabdablet at 9:43 AM on May 17, 2012


But it isn't particularly important to know "where" the electricty comes from

...unless you're designing a grid, in which case it's absolutely critical.
posted by flabdablet at 9:47 AM on May 17, 2012 [1 favorite]


Probably the best way to think about it is to reframe electrical potential as a reservoir. The suppliers pump water into the pool, and the users pump it out.

Forget "pool". To an excellent first approximation, there's no energy storage on the grid.

If you must use water analogies think of generators as pumps, motors as turbines, circuits of wire as circuits of pipe, charge as water, voltage as pressure, current as flow rate, and resistance as pipe wall friction.
posted by flabdablet at 10:03 AM on May 17, 2012


Thank you all for the answers, though I'm still a little confused. It seems like the answer is either 0% or 91%?

To clarify, the very real question I am asking with this simplified example is, if my business is directly adjacent to a massive solar power farm and there is also a nuclear power plant about 100km away, is it true and/or meaningful on a sunny day to say that my business is being powered entirely by solar energy?

In the real world case, both power plants are in the same region served by the same electricity distribution company. There are also a handful of other power plants in the same region, but none anywhere near as close as the solar farm.
posted by 256 at 11:57 AM on May 17, 2012


It may be close as the crow flies, but it may not be close as far as how the wires are hooked up.

A truthful answer might be better found by finding out what percentage of the power distributed by the company comes from sources X, Y and Z.
posted by gjc at 3:25 PM on May 17, 2012


if my business is directly adjacent to a massive solar power farm and there is also a nuclear power plant about 100km away, is it true and/or meaningful on a sunny day to say that my business is being powered entirely by solar energy?

That would be physically true if and only if (a) your business and the solar farm are on the same grid segment (b) the solar farm's power output on that sunny day was more than sufficient to power all of the power consumers connected to that segment.

It would be economically true if your business has a 100%-solar power supply contract with its electricity supplier. In that case, the supplier will be paying all the money it charges your business and others with the same supply contract to supply electrical energy, less its own cut, to solar generators. In order for it to be able to fulfill this contractual requirement, there must be at least as much solar generation capacity on your grid as needed by customers with that form of supply contract.
posted by flabdablet at 7:01 PM on May 17, 2012 [1 favorite]


In your simple case here, everything is nicely balanced and you can probably just use simple ratios to figure out who's power is being used where.

However, it gets much more "fun" in the real world. Generators and loads (i.e. consumers) are connected through a complex set of transmission lines. Using the reservoir analogy from Pogo_Fuzzzybutt, you also have to take into account the diameter of the pipes that lead from the generators to the consumers. If the pipes break, you need to re-route the product through other pipes, and it's quite possible that the most "obvious" source of power can no longer be used at peak capacity because its product can't be moved. When this kind of constraint happens you can have weird price spikes in the wholesale market. I've done work for a generation company in the US, and have seen how the spot price can change by an order of magnitude due to things like this.

Enron was a master of using this feature of the electricity system for nefarious ends - they controlled enough of the infrastructure to force prices to extraordinary levels in the wholesale market in California (and treated customers to rolling blackouts in the process), while PG&E was required by law to sell electricity to consumers at a known rate and therefore couldn't pass on the full cost of the electricity they were providing. They were bankrupted as a result. Hello free market!
posted by clicking the 'Post Comment' button at 7:47 AM on May 18, 2012


In your simple case here, everything is nicely balanced and you can probably just use simple ratios to figure out who's power is being used where.

From an engineering standpoint, everything is always nicely balanced.

At any instant, every device connected to the grid is either generating or consuming electricity; the grid wiring doesn't store energy, just distributes it, and the total power supplied to the grid by all sources, less transmission losses, is equal to the total power being drawn from it by all consumers.

At any instant, every wire that forms part of the grid is either transferring power in one direction or transferring no power at all, either because it's disconnected and therefore not now part of the grid, or because the voltages at its two ends are exactly equal.

If you know that your local power plant's power output right now is S megawatts, and you know that your local grid segment's total consumption is C megawatts, and S is not less than C, then you know that energy must be flowing away from your local grid segment on any long-distance transmission lines connected to it, which means that 100% of the power you are consuming right now is coming from your local plant.

The trouble with this neat picture is that consumers do not in general have access to information about the instantaneous power outputs of their local plants or the instantaneous aggregate power demand for their local grid segments; only the electricity distributors can see those numbers. You can't assume that just because your local plant is rated as capable of generating X megawatts that its operators are currently making it do so. Perhaps it's partially or fully down for maintenance, or it's faulty, or it's a wind farm on a still day, or there is spare capacity available elsewhere and the local distributors can buy electricity in to fulfill their supply obligations for less than it costs to generate with the local plant and they're simply choosing to run it at reduced power or not run it at all.

The complexity of the physical transmission web pales into insignificance next to the complexity of the economic relationships that govern electricity generation and distribution.

If you want to be able to say honestly that your business is 100% solar powered, it doesn't matter at all that you're sited next door to a big fat solar farm. All that matters is the economic agreement you have with your local electricity supplier. If you're paying them for 100% solar power (for which they will probably charge more) then they are obligated to make sure that your energy consumption and that of everybody else who is paying for solar power is all accounted for, and that enough solar plant exists to supply that much energy to the grid (plus extra to allow for transmission losses) and that said plant is run hard enough to do so.

Because your local supplier can only charge you for energy that enters your premises, but they must pay for all the energy they buy from any given generator, you'd think it would be in their interest to try to match local power supply with local power demand so as to minimize the power needing to be transferred via relatively lossy long-distance lines. But as clicking the 'Post Comment' button points out, sometimes the market doesn't work that way. Which is economics, not physics, and cannot therefore be properly understood by people unwilling to let go of their moral code or common sense or both.
posted by flabdablet at 10:00 AM on May 18, 2012


Thank you everyone and especially Flabdalet. I ended up sitting in on a meeting with some power company engineers yesterday and came away with an understanding very similar to what flabdalet lays out in that last answer. And that answer helped wonderfully to clarify the few things that I was still a little confused about.
posted by 256 at 11:44 AM on May 18, 2012


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