Power grid & generator questions

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Before I begin I have to note that my last thread of this kind got heavily moderated and closed because the moderators did not like the specific way in which I was formulating my questions, so now I want to clarify and reaffirm some of what I have learned through questions without making any statements.
The following are all my questions and should not be taken as facts by anyone


1) So from what I have read is it true that the grid frequency is primarily maintained (influenced) by keeping the rpm's of the synchronous generators attached to the grid constant (within the allowed margins) with the help of "flyball" centrifugal governors (older obsolete technology) or automatic electronics which control (increase or decrease) the torque supplied to the generator rotating axis based on the changes in grid frequency?

2) Since synchronous generator rotor rpm is directly linked to the output (stator) field frequency, does it then mean that in a situation where a large generator has abruptly went offline and the grid frequency has started to decrease (for example from 60hz to 59hz) that electronics would increase the torque supplied to the remaining generators in order to compensate for the decreased frequency?

3) I learned that synchronous generator rotor rpm is "locked" or in step with the stator field frequency (the reason why their called synchronous in the first place), so does that mean that in the mentioned example of a large generator going offline (or other large load coming online) the whole grid frequency for example drops by about 1hz, since all the generators are connected to the grid in parallel and their rotor rpm locked to the stator field, that means that now all the generators rotor rpm drops by some small amount which corresponds to the Hz of frequency lost?

Here is the part I would like to understand deeper, the speed (frequency) governors now "notice" this drop in frequency and apply more torque to each generators prime mover (assuming the prime mover has headroom or additional torque capacity ) what happens now? the generator rotor rpm are locked to grid frequency but grid frequency has dropped so has the rpm, the rotors now get supplied with more torque and by what process the grid frequency now gets back to normal can you please elaborate?
I would also like to post my own quick attempt to answer this. (I think that as the generators prime mover's torque gets increased the rotor B field is now "running" ahead of the stator field (which dropped to 59hz) by some angle, so if this is done on many (all?) generators at the same time it then has the capacity to alter the grid frequency quickly enough to set it back within the specified margins?)


4) It must be that the speed aka frequency governor electronics and even the flyball governors in the old days must have been carefully synchronized ? Because in order to stabilize the grid frequency the response from each generator must have been quick but also the same so that all generators would work in unison correct?


5) Automatic generator speed governor action to stabilize grid frequency is the first and quickest response to grid frequency fluctuation correct? But I read the next (longer) process is when the grid operator coordinates for some plants generators to output more "torque" to stabilize frequency in case of planned generator outages and repairs or other works?



thank you.
 

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  • #2
jim hardy
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so does that mean that in the mentioned example of a large generator going offline (or other large load coming online) the whole grid frequency for example drops by about 1hz,
It drops by a whole lot less than that, more like 0.01 to 0.05 hz . At 59.8 hz we start disconnecting customers to arrest the frequency decline..



Here is the part I would like to understand deeper, the speed (frequency) governors now "notice" this drop in frequency and apply more torque to each generators prime mover (assuming the prime mover has headroom or additional torque capacity ) what happens now? the generator rotor rpm are locked to grid frequency but grid frequency has dropped so has the rpm, the rotors now get supplied with more torque and by what process the grid frequency now gets back to normal can you please elaborate?
You need to think of the grid as the single machine that it is, spread out over a huge area.
It is a rotating machine.
Even though the towers and lines don't move physically, the power flowing through them is moved by angular displacement of the voltage between points of interconnection.
A transmission line is the analogy to a mechanical drive shaft - power through a mechanical shaft is torque X rpm, and torque twists the shaft so there's angular displacement between its ends.

Similarly the grid is just a rotating machine but it's generators that rotate not the wires connecting them..
Like any other rotating machine . if rate of energy outflow(power out) exceeds rate of energy inflow(power in) in it'll slow down . If power in exceeds power out it'll speed up. It really is that simple.

So regions that are short in power generation will run a few degrees behind regions with excess power generation. They remain in step just like people on a conga line.

That's what that Grideye site that i cited earlier shows. Here it is right now.

upload_2018-8-5_10-28-7.png





right now all that coal in the Dakotas and along the Mississipi, the Nukes in TVA and natural gas in Texas are making more power than the residents there are using. So that region's generators are running a few degrees ahead of those in the Northeast.
Same frequency , just our generators are a fraction of a turn ahead of those in New York. From the looks of that map perhaps as much as 45 electrical degrees.
So--- Power flows from the brownish region where i live toward the blue where some of my kids live.

Too bad the Pacific Northwest part isn't working , it'd be interesting to see what the hydro is up to. Lots of aluminum smelters out there- they're high consumers of electricity.

old jim
 

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  • #3
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Much better than your previous thread @girts, thank you.

1) So from what I have read is it true that the grid frequency is primarily maintained (influenced) by keeping the rpm's of the synchronous generators attached to the grid constant (within the allowed margins) with the help of "flyball" centrifugal governors (older obsolete technology) or automatic electronics which control (increase or decrease) the torque supplied to the generator rotating axis based on the changes in grid frequency?
2) Since synchronous generator rotor rpm is directly linked to the output (stator) field frequency, does it then mean that in a situation where a large generator has abruptly went offline and the grid frequency has started to decrease (for example from 60hz to 59hz) that electronics would increase the torque supplied to the remaining generators in order to compensate for the decreased frequency?
Forget allowed margins, and "major" changes. There is no margin at all, and even the tiniest unbalance causes these changes.

At @jim hardy already said, 1 hz is much to big. Decrease from 60 to 59.97 hertz is considered big.

3) I learned that synchronous generator rotor rpm is "locked" or in step with the stator field frequency (the reason why their called synchronous in the first place), so does that mean that in the mentioned example of a large generator going offline (or other large load coming online) the whole grid frequency for example drops by about 1hz, since all the generators are connected to the grid in parallel and their rotor rpm locked to the stator field, that means that now all the generators rotor rpm drops by some small amount which corresponds to the Hz of frequency lost?
Yes, but it takes some time for the change to spread around. Watch this video, and see the swings in frequency and also see how the frequency changes spread across the continent.



Here is the part I would like to understand deeper, the speed (frequency) governors now "notice" this drop in frequency and apply more torque to each generators prime mover (assuming the prime mover has headroom or additional torque capacity ) what happens now?
Please stop saying torque. It is power that changes. Mechanical power on the turbines, and electric power on the grid.

What happens next is that the power and load come back into balance once again, and frequency stops changing (although not necessarily at 60 hz). I like to think of it the other way around; rate of change of frequency is proportional to the power unbalance on the grid. You started the question with a power plant tripping. That causes an unbalance. In the steady state, when nothing is changing, then there is no unbalance and power generated exactly matches load consumed (plus losses, but neglect losses for now).

4) It must be that the speed aka frequency governor electronics and even the flyball governors in the old days must have been carefully synchronized ? Because in order to stabilize the grid frequency the response from each generator must have been quick but also the same so that all generators would work in unison correct?
Not synchronized, but rather proportional. Each generator changes power (compared to the initial power) proportional to the change in frequency (compared to 60 hz). The frequency change is what triggers the action. If some generator fails to respond for any reason, then the frequency changes some more making the remaining generators work harder. The neat part is that provided the flyball governors have the same gain, then each generator's change is the same percent as all generators. A 10x times bigger power plant, changes MW power 10x times more than a smaller plant.

In real life, millions of people are turning things on and off every second, so the grid sees the average, and in normal circumstances the average changes only slowly.

5) Automatic generator speed governor action to stabilize grid frequency is the first and quickest response to grid frequency fluctuation correct? But I read the next (longer) process is when the grid operator coordinates for some plants generators to output more "torque" to stabilize frequency in case of planned generator outages and repairs or other works?
As I said, once we have a new balance, frequency stops changing, but it does not necessarily stop at 60 hz. That happens in a few seconds. That is where automatic generation control (AGC) comes into play. AGC is done by central computers. It senses the change in frequency, and it orders (via telecommunications) all power plants to change their power enough to restore frequency to 60 hz exactly. That takes maybe 15 minutes to complete. There is an AGC control in each region. Perhaps 20 or so regions in the eastern US+Canada.

But these power unbalances also cause electric clocks to show the wrong time (old style electric clocks are driven by power frequency). So once a week or so, the AGC also changes the frequency of the whole grid a small amount just long enough to bring all those clocks back to the correct time. In today's age with GPS, that seems unnecessary, but before GPS, those electric wall clocks were much more accurate in the long term than any wind-up or battery operated consumer clock.

Jim and I keep asking you to think about angle rather than frequency. Just as
(rate-of-change-of-frequency) is proportional to (power-unbalance),
then
(rate-of-change-of-angle) is proportional to (frequency-unbalance) between any two points.
For the mathematically oriented, that is the second order differential equation that characterizes all power grids.
 
  • #4
jim hardy
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Similarly the grid is just a rotating machine but it's generators (and voltage phasors) that rotate not the wires connecting them..
I've developed a habit of thinking of a sinewave voltage as a rotating arrow, the voltage phasor. A holdover from high school electronics class where we were taught phasor notation.

So to me the concept of angular displacement of voltage between ends of a wire is as natural as twisting a Slinky toy spring and feeling my hands turn relative to one another...

@anorlunda has explained the phenomenon masterfully in his insights article.

girts - If this seems foreign to you i recommend playing with some springs - it helps our subconscious accept the concept.
i realize i'm a bit weird.

old jim
 
  • #5
jim hardy
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Watch this video,
Wow that really dramatizes the dynamics ! Grid is far from a rigid machine, actually it's quite limber.
 
  • #6
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There is an animation that I can't find on Youtube. I would love to make one myself, but I don't have the skills.

Imagine an electric generator driven by an electric motor. If the shafts of the two are bolted together, then the two speeds are identical all the time.

But imagine if the two were connected by a flexible rubber coupler that is able to twist. Then imagine that we paint a dotted line on the rubber coupler before we start. Now, when things are running, the more power we transmit from the motor to the generator, the more the coupler twists. If we had a saddle on the shaft and could ride it like a horse, we would see that dotted line bending. The more power, the more bending. But even with the twist, the RPM of the shafts are the same. That's necessary to keep the amount of twist the same. If the RPMs are not the same, then the rubber twists until it breaks.

If everything switched so that power went in the other direction, then it would twist the other way.

If we think of the angle of voltage as analogous as the twist in that rubber coupler, then the analogy is complete.
Does that help?
 
  • #7
jim hardy
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Does that help?
i think so.

Here's a video with great visuals. You can see the power angle change .

Since the car alternators are multipole, probably twelve or so, you don't see very many mechanical degrees of phase shift. I've done the same thing with four pole 400 kva machines where they can get almost 45 mechanical degrees ahead or behind one another and it's really educational.

old jim
 
  • #8
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ere's a video with great visuals. You can see the power angle change .
Thanks Jim. It did indeed show it clearly. I just wish the guy had gone one more step and showed the power going in the other direction.

@girts, with the two videos in this thread, is it getting clearer?
 
  • #9
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Yes i believe things are getting clearer, all thanks to you guys.

thanks for the responses, they are really educational.

so first of all @Jim, does "electrical degrees" mean the same as mechanical degrees? I assume this is talking about a generator's rotor looking at it from the radial side in terms of the angle of the rotor versus the stator field as a reference frame?


@anorlunda do you want me to talk about power instead of torque because after all the output of a generator (any generator for that matter) is torque multiplied by rpm much like power of an electrical system is volts multiplied by amperes?

I was talking about frequency margins because I read in the NERC paper that there is some minimum threshold under which the speed governors don't even react, even though it is very small.

another question, in the video about how the frequency changes during the storm, I assume it is in real time displayed in seconds? Would it be fair to say that those changes happen at the speed they do over the territory they do because of the speed of electric signals (changes in them) which is something like 2/3c in the grid?


Really nice video @Jim I liked it, so overall let me attempt my own summary of what you both said here as I think I finally got everything.(for anyone else reading this please don't take this as fact rather my attempt at learning!)

So as said multiple times synchronous generator rotor rpm is directly related to the stator field frequency as the rotor B field poles are locked in to the stator rotating field, but since the rotor is not a gear physically mechanically attached to the stator it can slip by some angle either in one or the other direction from the stator field much like taking two table magnets one can use force to misalign them from the center position in which they naturally want to stay. I suppose this is the stretching rubber band analogy you wanted me to understand that even though the rpm stay the same the angle between the two parts can change because they are not physically connected rather by B field so in your example the B field is the invisible rubber band that allows for the angle to change , taking @Jim's mechanical axle analogy this would be like the rubber elastic damper fitted into car drive shafts which allows some angle of bending between the two sides of the axle when a sudden large change in torque is applied, surely the analogy falls short in the respect that the drive axle will align back to 0 degrees rather fast while a generator's rotor can run with an angle with respect to the stator field for hours and days if necessary correct?

Another thing I take from the video is that in a standalone(unconnected to grid) condition a synchronous generator's output frequency is directly related to rotor rpm so if the rotor turns slower the output frequency is decreased or if the stator field frequency is dropped again the rotor turns slower (motor case). But if two such generators are connected and one is turned by its prime mover a little faster than the other then instead of each having different rotational rpm both now have the same rpm just that the one with higher power input (originally faster rpm) now acts as a generator while the other which had lower rpm now acts as a motor, even though I understand that this difference in power can only increase up to a certain point where either the generating generator will not be able to add any more power or the motoring generator will not be able to maintain the B field coupling to it's stator and so will fall out of step which I understand for small tabletop devices is not that big of a deal but for large devices with strong B fields and large rotational inertia can have severe mechanical and electrical consequences?

By the way what would those consequences be? Mechanically speaking probably a broken rotor shaft or maybe pole shoes damaged? Electrically what would the consequences be, sudden induced currents in the stator?

Also how often does the angle on a generator change? Because in the video the angle changed everytime he slightly adjusted the frequency (rpm) of one of the motors that spun one of the generators is the nationwide grid also so sensitive?



Now here is a question with regards to this, since the generators attached to the grid form a large network of powerful parallel devices then if the grid frequency drops by a little is it then physically manifesting as very minor change in rpm on all the generators or rather as a change in the angle between rotor and stator field or both? I myself would say that there is some real minor rpm change because otherwise I cannot imagine how else could those old flyball governors could make any corrections as they work on centrifugal force which is directly proportional to rpm.


PS. I hope I'm making progress here, thanks.
 
  • #10
jim hardy
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One step at a time.

so first of all @Jim, does "electrical degrees" mean the same as mechanical degrees? I assume this is talking about a generator's rotor looking at it from the radial side in terms of the angle of the rotor versus the stator field as a reference frame?
No.
A two pole generator makes one electrical cycle every revolution so electrical and mechanical shaft degrees are the same.
A four pole machine makes two electrical cycles every revolution so electrical degrees are twice mechanical shaft degrees.
A six pole machine makes three electrical cycles per revolution so the relation is three electrical degrees per mechanical shaft degree.
And so forth.

If you noticed the three little synchronizing lights in that video - they are brightest when the two machines are 180 electrical degrees apart, and dimmest when at zero electrical degrees - ie in phase.
I tried to count how many times those lights extinguished as the flywheel stripes precessed by one turn. I think i counted seven which says those are fourteen pole alternators. That's a common configuration for automotive, 12 to 18 being typical. https://electrical-engineering-portal.com/lundell-generator-claw-pole-rotor

surely the analogy falls short in the respect that the drive axle will align back to 0 degrees rather fast while a generator's rotor can run with an angle with respect to the stator field for hours and days if necessary correct?
No. Any Mechanical engineer will tell you that any steel bar is in realty a spring just a stiff one. Your car's driveshaft twists albeit imperceptibly whenever it transmits torque. Look up Hooke's Law. https://en.wikipedia.org/wiki/Torsion_spring
The steel shaft of our turbogenerator, an impressive ~1 meter in diameter , twisted 3.2 degrees to transmit ~720 megawatts at 1800 RPM . That'd be how many Newton-Meters of torque ?
 
  • #11
jim hardy
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There often comes a point we have to admit we've got off track somewhere in our basics.

Lavoisier's introduction to his treatise on chemistry made me self examine and go back to basics..
It influenced my thinking in my middle years.

"Instead of applying observation to the things we wished to know, we have chosen rather to imagine them. Advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. These errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. The method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning. When matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my Lord Bacon says, to frame the human understanding anew."""
https://web.lemoyne.edu/giunta/lavpref.html
 
  • #12
jim hardy
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Now here is a question with regards to this, since the generators attached to the grid form a large network of powerful parallel devices then if the grid frequency drops by a little is it then physically manifesting as very minor change in rpm on all the generators or rather as a change in the angle between rotor and stator field or both?
Does it not take a tiny and temporary change in speed to fall behind or pull ahead ?
If a region of the grid slows down the nearby generators will pull ahead of it? That will draw more power out of the generators. One of two things will happen.
Either more steam will be admitted to restore grid frequency, or that energy will come out of the turbogenerators' rotating inertia and they will slow down to match grid.
 
  • #13
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I was talking about frequency margins because I read in the NERC paper that there is some minimum threshold under which the speed governors don't even react, even though it is very small.
In modern times, not all generators need an active speed governor at all, but they do need something in emergencies if power changes drastically. They accomplish that by having a governor with a very big dead zone. In the context of your questions, we are talking normal cases, not emergencies.

another question, in the video about how the frequency changes during the storm, I assume it is in real time displayed in seconds? Would it be fair to say that those changes happen at the speed they do over the territory they do because of the speed of electric signals (changes in them) which is something like 2/3c in the grid?
It is in real time. The spread is very much slower than 2/3 c . At 2/3 c, it takes about one millisecond to cross the continent.

Note also in left side of that video how the frequency swings. It doesn't just change from a higher to a lower frequency, it swings all over the place before settling. Power engineers call that math "the swing equations". Tripping a generator is like ringing a big church bell.

the rpm stay the same the angle between the two parts can change because they are not physically connected rather by B field so in your example the B field is the invisible rubber
It seems that you're mixing up two things. The angle between stator and rotor, and the angle between one generator's rotor and another generator's rotor (or the angle of voltage at a load. We are talking about the second kind of angle, and there is no B field coupling remote generators or loads.

Solar panels connect to the grid using inverter electronics. They have no stator or rotor, yet the voltage they generate has a phase angle, and they obey the same regarding angle, frequency, and power. That's part of your confusion; you're making it more difficult than necessary. It is not necessary to even mention stators and rotors at all.

By the way what would those consequences be? Mechanically speaking probably a broken rotor shaft or maybe pole shoes damaged? Electrically what would the consequences be, sudden induced currents in the stator?
It is drastic, but not dramatic. There are many protective relays. On loss of synchronization, one or more of them trip the turbine and open the circuit breaker. Usually, overcurrent is the first one to go. If all or most of the generators trip, then we have a blackout. That's what happened on November 9, 1965.

Also how often does the angle on a generator change?
As often as the power changes, and at the same speed. Remember in the previous thread and in the Insights articles, I showed the equations relating power to angle. Those apply, and they work in real time.

Think all the way back to that first Insights article. When you turn on the light bulb, the angles in the grid must adjust to get more power shipped to your house to supply the bulb. No angle changes, no power.
 
  • #14
jim hardy
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Also how often does the angle on a generator change? It constantly hunts about an equilibrium position. Because in the video the angle changed everytime he slightly adjusted the frequency (rpm) of one of the motors that spun one of the generators is the nationwide grid also so sensitive?
I have watched our generator shaft with a stroboscope just as he did.
It drifts randomly over a fraction of a degree.
That's the interaction of the generator's huge inertia with the local grid's never quite constant load demand.

Any time a steam valve moves it comes to a new angle and drifts about that one. That's the interaction of the mechanical power input from steam bringing that huge inertia to a new angle relative to the grid.
 
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  • #15
jim hardy
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It is drastic, but not dramatic. There are many protective relays. On loss of synchronization, one or more of them trip the turbine and open the circuit breaker. Usually, overcurrent is the first one to go. If all or most of the generators trip, then we have a blackout. That's what happened on November 9, 1965.
We once snapped the shaft at another plant. But @anorlunda is right , normally something acts to prevent that sort of damage.
 
  • #16
dlgoff
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Wow that really dramatizes the dynamics ! Grid is far from a rigid machine, actually it's quite limber.
I was told while working for a power company that typically power "flows" west to east. What's your take on this? Sounds reasonable when not considering large disturbances.
 
  • #17
jim hardy
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From the heartland toward the coast ? Following population gradient ? I never thought about that until seeing those angle maps
 
  • #18
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Ok I think I now see where I misunderstood the angle problem, I was thinking about the angle between rotor and stator if one marked them both at "top dead center" while the rotor is stationary, Ok I see the angle between different generator rotors is what was also shown in the videos, even when they were synchronized electrically, changing one generator prime movers power output influenced the rotor of the other generator by a little, it spun at the same speed but it's angle changed relative to its previous position and then it came back when the power balance was established again. I assume this is what happens in grid and what you were talking about?

In terms of these angles, how does the physical angle of a generator's rotor in a power plant is influenced when a large inductive load comes online which shifts the current lagging the voltage?


@Jim ok I see so only a two pole rotor has its mechanical and electrical degrees being the same, well that makes sense because each single pole approaching and then leaving the center of a stator coil makes 180 degrees in terms of a sine wave or half a cycle so two poles which are of opposite polarity to one another make one full cycle which corresponds to 360 degrees correct?
Continuing this question, as I see nuclear power plant use generators that only have two poles on their rotor which I assume is because of the rather high rpm of the steam turbine, the question then is based also on the video you showed, when the plant needs to be synchronized to the grid aka "switched online" they first spin up the turbine to the speed which approximates? or is the very speed of the grid frequency and then they can switch it online aka connect the stator windings to the grid step up transformer, but they can only do this at the moment when the rotor B field is in phase with the stator grid field? I assume this is easier for a generator that has many pole pairs because then it has multiple 'sweet spots" in a single revolution at which it can be aligned with the grid field of the stator but is much harder for a rotor which has only two poles as it has only one spot for each revolution at which it can be aligned without risking switching it on while out of phase correct? Maybe you can tell me more about how exactly you synchronize a nuke generator to the grid?

PS. yes I paid attention to the blinking lights, I understood their meaning because as the two generators approach the point were their rotors are 180 out of phase they generate the highest PD between them at that point. I watched all 4 parts of those videos, who ever made them has done a good work.



Ok so one more question, in reality then multiple things happen all together, firstly as the grid frequency changes a little due to various factors like large loads etc, the real physical rpm of the generators change by some few rpm which might correspond to the 0.1 or so about change in electrical Hz? Then parallel to this the angle between different generator rotor's may also change as one might have a prime mover that applies more power to the generator when experiencing a frequency decrease while some other generators rotor might have a prime mover already going at full capacity or incapable or adding more power so that generator can add much less to the frequency increase than the first one? Thirdly also the angle between each generator's rotor and stator field changes while all of the before mentioned things happen?

I am starting to think as you advised in terms of load connected to generator without the long powerlines inbetween because after all those powerlines can be simulated by a load of specific inductance, capacitance and resistivity.

@anorlunda does the difference between your example of solar panels and physical rotors and stators is that with an inverter one can control the phase angle better because a semiconductor switch/es are adjustable by electronics versus a large rotor can be adjusted by the help of it's mechanical prime mover and precise timing, or is it not the case? I assume a solar panel inverter's lack of mechanical inertia is simulated by its storage capacitor capacity to supply more power to the grid because sun doesn't change it's strength in the second to minute timeframe, so apart from some stored capacity the energy source itself (sun) is incapable of a sudden increase in power supplied?
 
  • #19
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I was told while working for a power company that typically power "flows" west to east. What's your take on this? Sounds reasonable when not considering large disturbances.
It sounds like nonsense, but in real life it is probably true. As @jim hardy said, from the Heartland to the East Coast. It is also true on the West Coast; power flows east and south to reach southern California. There are several plausible reasons.
  1. Coal & Hydro. Especially in western Pennsylvania and Ohio, power plants were built at the mouths of coal mines. It is easier to transport electricity than coal. Niagara Falls power flows east, Ontario and Quebec hydro power flows southeast. In the West, huge power plants such Navajo, and Palo Verde ship a lot of power westward, and hydro power from the NW flows southward, coal power from Idaho and North Dakota also flows southwest. It's true even in Europe. Look at the map to locate hydro, wind, and coal resources, and compare that with population centers. It is not an east-west bias, but a rural-to-urban bias.
  2. Wind. The wide open spaces and flat landscapes of the heartland are ideal for wind power. The owners of that power would much rather sell it in the East where prices are higher.
  3. NIMBY. On both coasts, the rich privileged people would much rather import electricity than to have a power plant in their back yard. Even green power. Think of the resistance to offshore wind from the residents of Nantucket.
 
  • #20
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Please stop saying torque. It is power that changes. Mechanical power on the turbines, and electric power on the grid.
Not sure I agree with this.

OK, obviously power is changing, however to me at least its more useful to break it down into specifically what aspect of the system is changing to make that change in power happen, as hashed out ad infinitum, speed is more or less constant, 59Hz, 59.9 Hz, 60Hz, 61Hz, what ever, it can be considered fixed and your math won't be horrendously wrong if you do.

So with constant speed, torque is the thing that has to change with system power (which I assume the OP knows hence the comment about changing torque). This matches up with the power system nicely because under load changes, since more or less constant voltage, its the current that changes. And in electric machines current makes torque and speed makes voltage, so when you say torque is changing on a fixed rpm machine to an e machine guy that's saying your stator current has to change*, but EMF stays ~ constant (*more specifically your torque producing current in the DQ frame).
 
  • #21
jim hardy
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Good for you.
This matches up with the power system nicely because under load changes, since more or less constant voltage, its the current that changes.
We must remain aware that in this discussion, current being AC has both a magnitude AND a phase angle. OP began with comparison to DC so i have trod lightly on the premise he's not a subject matter expert on AC circuits .

(*more specifically your torque producing current in the DQ frame).
Yes.
Before we learn DQ we learn Power = VICos(angle) and spend hours doing polar-rectangular conversions. I was sorta waiting for that subject to broach...
I use those phasor diagrams showing MMF's relative to terminal voltage and they work well for a single machine. But not so well for interconnected machines.

Carry On, Men, for you're better teachers than i .

old jim
 
  • #22
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Also we must not forget the prime mover; the device that is spinning the generator. Most often it is a turbine.

The OP question mentioned governors. The governor senses speed, and acts to open/close the valve admitting steam/water/fuel into the turbine. So the next step is that the turbine makes more mechanical power. That's what I had in mind.

Power=voltage x current
Power=torque x speed
Power=steam flow x enthalpy.

In all those cases there is an extra degree of freedom (such as higher voltage and less current for the same power) that IMO is a distraction from the concepts discussed. Also, IMO is would be best to defer introducing those things until the power & energy concepts are firmly understood. Even emf, flux, and field voltage, are unnecessary for this topic.

And in electric machines current makes torque and speed makes voltage
Not always, no. Sorry, but I'm unsympathetic to that kind of "shop floor" explanation of things. Learn to do the math.
 
  • #23
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Not always, no. Sorry, but I'm unsympathetic to that kind of "shop floor" explanation of things. Learn to do the math.
Might be driving this off topic (again lol), but...

Can you provide an example of a machine based on wires and magnets, where voltage (ie electric field) alone is responsible for torque creation?
 
  • #24
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Can you provide an example of a machine based on wires and magnets, where voltage (ie electric field) alone is responsible for torque creation?
A synchronous generator, which is close the to topic of this thread. You can change voltage by changing field current (not the "electric field" but the current in the field winding of the rotor; also called excitation current). Phase angle does not necessarily change with it, nor does current necessarily change. Power=voltage*current. But more MVAR would flow in that which would change current. However, if you also raise the voltage on the far end of the transmission line, MVAR and current would be restored to the original values.
 
  • #25
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A synchronous generator, which is close the to topic of this thread. You can change voltage by changing field current (not the "electric field" but the current in the field winding of the rotor; also called excitation current). Phase angle does not necessarily change with it, nor does current necessarily change. Power=voltage*current. But more MVAR would flow in that which would change current. However, if you also raise the voltage on the far end of the transmission line, MVAR and current would be restored to the original values.

<Massive edit because I miss read the post!>

Field current changes the flux density, which does change BEMF, however fundamentally Voltage is proportional to rate of change of flux in a loop, so you can change this voltage by changing B, however no matter how much you increase B, you will get zero voltage if the speed is zero.
 
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