Power plant generator improvement

Main Question or Discussion Point

I have looked over the types of generators used in most power plants (Hydro, Nuclear, coal etc) and with different sized and rotor rpm's they all basically share the same principle of operation if I'm not mistaken, which is that the rotor gets supplied via brushes and slip rings with a DC "excitation" current which while flowing through the rotor coils sets up a B field or "pole=pairs" which then travel along the stator coils to induce AC current in them, the frequency of which is directly related to the rpm of the rotor and the number of pole pairs on the rotor.

Now one way to keep the frequency unchanged while operating at different mechanical loads is to alter the DC current through the rotor which I assume is done in most if not all commercial power plant generators correct?

But I assume that it would be more beneficial (especially for some types of power plants like nuclear) to be able to run the reactor-steam turbine-generator set with the power level that is most efficient for a particular desired electrical output of the plant (night vs day peak loads)

So my question is would a generator that has its rpm and output frequency independent from one another be more desirable or in other words could such a generator have any noticeable advances over current generators. If one can vary the rotor rpm but keep the same output frequency of the generator, well the lower rpm or higher rpm would still change the amplitude of the output (voltage) but that could be compensated with varying the strength of the generator's excitation B field.
I am thinking that wind generators would benefit largely from such a device as the rotor rpm and wind loads are varying in short periods of time, so from what I read under certain wind strength the generator becomes too inefficient as the rpm's drop very low.

thank you.

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jim hardy
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Now one way to keep the frequency unchanged while operating at different mechanical loads is to alter the DC current through the rotor which I assume is done in most if not all commercial power plant generators correct?
Not correct. To hold frequency constant we hold turbine RPM constant. We vary field current to adjust voltage.

So my question is would a generator that has its rpm and output frequency independent from one another be more desirable or in other words could such a generator have any noticeable advances over current generators. If one can vary the rotor rpm but keep the same output frequency of the generator, well the lower rpm or higher rpm would still change the amplitude of the output (voltage) but that could be compensated with varying the strength of the generator's excitation B field.
I am thinking that wind generators would benefit largely from such a device as the rotor rpm and wind loads are varying in short periods of time, so from what I read under certain wind strength the generator becomes too inefficient as the rpm's drop very low.

That's how many wind generators work. The term you want to look up is "Double Fed Induction Generator"

http://web.mit.edu/kirtley/binlustuff/literature/wind turbine sys/DFIGinWindTurbine.pdf

Well I assume in nuclear power plants it is far easier to hold turbine rpm constant than it is in say hydro power?
When I visited a hydro plant I got the impression that they indeed control the rpm of the generator by DC field current through the rotor, say for example in springs when water level is higher and there is more flow in the river (more supplied water) the turbines would naturally want to spin faster and they have more torque due to the higher water level, so they increase the DC current which creates a stronger field which then slows down the rotor-turbine assembly so that it indeed spins at fixed rpm for fixed frequency.

But then again yes I see your point, maintaining the same RPM but increasing pole field strength would keep the frequency but increase the induced output voltage of the generator so I guess I have a question how do then they maintain the voltage/B field strength in non controllable mechanical load applications?

Also a question for those who deal or have dealt with steam turbines/nuclear reactors (yes you Jim and hopefully others) would it be beneficial to have a generator that would allow the steam turbine to be run at whatever rpm is more efficient at any given reactor loading condition?
In other words imagine that the generator output would be totally independent from the turbine rpm so that you could run the turbine/reactor at whatever speed is best at that moment, would that solve alot of hassle in terms of synchronization and increase efficiency or would the gains be barely noticeable?

jim hardy
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Well I assume in nuclear power plants it is far easier to hold turbine rpm constant than it is in say hydro power?
No, it is the synchronous generator that holds frequency constant. It is locked to grid frequency and no single machine is big enough to affect the grid (unless you're well along the way to a blackout).

so I guess I have a question how do then they maintain the voltage/B field strength in non controllable mechanical load applications?
A closed loop controller measures generator voltage and adjusts excitation(field current) to hold that voltage at desired value.

would it be beneficial to have a generator that would allow the steam turbine to be run at whatever rpm is more efficient at any given reactor loading condition?
It's not practical. Those things are so huge that any mechanical resonance will be destructive.
Our 1800 RPM turbine was, to the mechanical designers, a 30 mechanical hz machine. No part of it can have a resonance there.
Our longest blades were resonant at 7 hz , and 7 has no integer multiple that's equal to 30.
Our underspeed trip was set at RPM equivalent to 29 mechanical hz to prevent ever reaching 28.

Wind turbines are a different story. Most i've looked at use huge electronic power converters and doubly fed induction generators to make 60 hz (50 in your neck of the woods?) over a range of RPM.
So your proposal is current practice on wind machines up to 3 or 4 megawatts.

Did i read your questions right ? I'm prone to wander off topic...

old jim

dlgoff
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... would it be beneficial to have a generator that would allow the steam turbine to be run at whatever rpm is more efficient at any given reactor loading condition?
Like Jim says,
... it is the synchronous generator that holds frequency constant. It is locked to grid frequency ...
and that grid has more than one generator to supply our power demands. So to adjust for our varying power demands, each area in the grid has to have Automatic Generation Control (AGC) to vary their generator's governors.

jim hardy
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So to adjust for our varying power demands, each area in the grid has to have Automatic Generation Control (AGC) to vary their generator's governors.
Since the synchronous generators are all connected to one another they rotate in step.
Regions that are exporting power will be slightly ahead in phase of those that are importing power.
University of Tennessee EE department is working on a real time monitoring system that displays that difference in phase angle.
I can't vouch for its accuracy but it is interesting to watch.

Brown regions are exporting power.
Turquoise are holding their own.
Blue are importing power.
Looks like they're having trouble getting readings from Western part of the country.

Watch it in action at http://fnetpublic.utk.edu/anglecontour.html
Over the last few minutes the blue region has got smaller meaning our Northeast is importing less power. Not surprising, it is the middle of the night there so airconditioners are surely shutting off and their water heaters have recovered from evening showers and dish-washing..

old jim

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Ok, I get the fact that they are synchronous machines and that they all run locked to the grid, but then tell me this one thing, no matter how you connect that generator one of it's inherent features is that output Frequency is directly related to RPM, and rotor B field strength at any given rpm is related to the output amplitude aka voltage of the generator, so increasing rpm increases both frequency and output voltage while keeping the same rpm but increasing rotor B field simply increases output voltage keeping the same rpm.

Now keeping this in mind let's say we run a large Hydro plant, water level is rising and flood waters are coming in, sure you would let the excess flow over the dam in spillways but still the torque on the turbine-generator axis is increasing which means it wants to spin faster or pick up rpm which it is not allowed to do because then it would distort the output frequency away from the 50/60 hz, so what do they do? Increase rotor B field (excitation current) to have a stronger B field to drag the rotor rpm back but this increases the output voltage, and I do not see any counter mechanism against this, can you maybe explain in more detail?

I assume you are reading my questions correctly Jim.

@dlgoff well I assume that they don't use mechanical governors any more so they are probably controlled by automated electronics?

So Jim what your saying is that even if someone would have a nice generator that can run its output independently of it's mechanical input it would not be practical at least not for a steam turbine due to a large mass that rotates at sufficiently high speeds and that it can't change it's speed much due to resonance problems?
But I assume the same problem does not exist for hydro turbines due to the very low speed at which they rotate which is probably too low to cause any noticeable vibrations due to the mechanical frequency being very low?

ok one more question for now, still wouldn't it be easier if one could keep the frequency for each generator constant and the generator's rpm would not be related to its frequency so that the only adjustment one would need is to alter the excitation current with respect to output voltage but otherwise let the mechanical load source run at whatever rpm is best at any given case?

so far in this discussion I see that potentially wind turbines would benefit mostly from this. Because the double fed induction generator is still not perfect with respect to its mechanical rpm to output electrical frequency relation as it can only adjust within a given scale of rpm's.

dlgoff
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@dlgoff well I assume that they don't use mechanical governors any more so they are probably controlled by automated electronics?
Here's a good .pdf file you'll find interesting/helpful: NERC Balancing and Frequency Control.

jim hardy
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a synchronous machine.
is two electromagnets one rotating(field) and one stationary(armature) .
The armature does not rotate but its magnetic field does.
So we have two magnetic fields and if they're not aligned they will try to align,
and that makes torque.
The torque on the rotor multiplied by its RPM gives the power that results,

In that image, if we assign rotation CCW then the rotor field is ahead of the stator field so it feels a retarding torque.
The further ahead the rotor gets the stronger the torque, and the more mechanical power you have to put into the rotor.
That power goes into the armature and comes out as electrical kilowatts.

When the fields are aligned power is zero.
Maximum power is when they are 90 degrees out of alignment.
That is why speed of a synchronous machine doesn't change. Well except for the brief interval when the rotor is advancing or retarding to a new angle.

Think of the rotor dragging the stator's field when generating , and rotor being dragged by the stator's field when motoring.

Increase rotor B field (excitation current) to have a stronger B field to drag the rotor rpm back but this increases the output voltage, and I do not see any counter mechanism against this, can you maybe explain in more detail?
Remember output voltage is set by the grid. There's a concept called "Infinite Bus" which is a voltage source so stout that your machine can't affect its voltage. That's 'the grid' and your machine is connected to it through some very small impedance.
So for this thought experiment just assume terminal voltage is fixed.

Well Now ! If terminal voltage is constant then total flux in the armature must be constant too.
Total armature flux is sum of rotor and stator fluxes
and if you change rotor flux by addig field amps, something must happen to restore total flux back to same value as before.
What might that be?
Recognize that the generator armature has inductance and Inductance is flux (linkages) per amp.
So it is plausible that additional armature current will flow and that additional armature current will restore total flux.

That is indeed what happens. If you raise field current additional amps flow in the armature, and the rotor is dragged backward toward the stator field.
Observe that all we changed was field amps not mechanical power into the rotor.
That ,means those additional armature amps must not carry any kilowatts. And they don't. They are what we call reactive amps and transmit no power.
You'll hear the term "VAR" for Volt Amp Reactive which is your reactive amps X volts.
Reactive amps flow at 90 degrees , and since power = V X I X Cos(θ) reactive amps carry no power.

That's why we say "Excitation adjusts Vars, Steam valves (or penstock if hydro) controls Watts.

It's a beautifully self balancing mechanism that Mother Nature gave us.

Are you studying AC machinery ? Power Angle, Armature Reaction and Synchronous Impedance are the terms that when fully understood will make this intuitive.

old jim

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jim hardy
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My only experience with hydro power is tours of Niagara Falls and Bonneville dams. Both predate me.
In the days they were built there were no electronic multi megawatt power converters.
I will venture a guess that the complexity is the reason nobody attempted a big double fed machine, But it's a guess. @anorlunda ? (@ still refuses to autocomplete)

jim hardy
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My turbine governor was a giant hydraulic analog computer. It measured shaft speed by discharge pressure of a centrifugal pump. Desired speed was set by a valve with adjustable spring force that created a reference pressure . Steam valves were positioned by comparing the two pressures. Giant hydraulic cylinders moved them.

dlgoff
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Now keeping this in mind let's say we run a large Hydro plant, water level is rising and flood waters are coming in, sure you would let the excess flow over the dam in spillways but still the torque on the turbine-generator axis is increasing which means it wants to spin faster or pick up rpm which it is not allowed to do because then it would distort the output frequency away from the 50/60 hz, so what do they do?
Water flowing in to the turbine is controlled by Wicket Gates. This animation, starting at time 1:09, shows how they work.

Edit: Here's a resized image showing how Wicket Gates are opened and closed.
Image compliments of https://gerler-engineering.com/terminology/

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thanks @dlgoff for the link I will try to go through it soon.

thanks Jim I don't know how I missed this for all these years while still knowing how generators work but it seems you gave me a good simple explanation,
I wonder how could I have missed this, now I understand what the guy at the Hydro plant was telling me.
So the reason why increased rotor rpm in one of the many generators at a power plant can't increase voltage is because the stator is fed from the grid which then again should have been obvious to me because when one thinks about it after all the stator coils are directly connected to a step up transformer which then feeds into the grid. (330Kv 50hz in my place), I completely forgot that transformers work both ways.

So basically what happens is since the grid sets the frequency and voltage through the transformer into the stator coils, the rotor then can't advance much or retard by much simply because it's mechanical torque is tiny compared to the grid power correct?
so whenever there is enough torque in the mechanical side of the generator , it's rotor will rotate slightly faster than the grid field in the stator causing additional power to be "fed" or induced into the stator coils?

Now I assume that if for whatever reason the rotor rpm drops below the grid frequency related rpm threshold the generator then becomes a motor as the stator field drags the rotor along not the other way around so the generator will actually start to consume a little bit of power from the grid through the transformer right?

Well all in all this seems exactly the same as with an AC induction motor/generator the only difference being that here the rotor can be directly coupled to the stator field instead of slipping always by some small amount due to the difference in rpm necessary for induction to happen in the AC induction motor/generator case.

So I assume that for the synchronous generator any decent amount of rotor DC current will be enough to create a B field which will then be dragged along the stator coils, so what is then the reason for changing the rotor DC current at all? Well one thing that comes to mind is lowering the rotor current in case turbine torque drops so low that the generator starts to work as a motor so that by having a weaker rotor B field the generator can then indeed either still generate small amount of power or at least idle with no power consumption right?
But what then they do in case the turbine has great torque and the rpm picks up, they then simply increase the rotor current to the point where the rotor rpm is only slightly faster than the stator field and leave it there?

jim hardy
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A turbine is a turbine,, eh Don ? Control its power by flow rate through it.

anorlunda
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Sorry for coming to this thread late. Somehow I missed it when first posted. @girts , you have a number of misconceptions.
the rotor gets supplied via brushes and slip rings with a DC "excitation" current which while flowing through the rotor coils sets up a B field or "pole=pairs" which then travel along the stator coils to induce AC current in them
Now one way to keep the frequency unchanged while operating at different mechanical loads is to alter the DC current through the rotor which I assume is done in most if not all commercial power plant generators correct?
It sounds like you think that the power is generated as DC and converted to AC by the generator. False. The generator converts mechanical power (turning the shaft) to AC electrical power. The DC excitation could in principle (but not practically) be replaced by permanent magnets.

So my question is would a generator that has its rpm and output frequency independent from one another be more desirable
No. Steam turbines and hyrdo turbines are optimized to give maximum efficiency at the synchronous RPM. Power generated is varied by changing the rate of steam or water flow, not changing RPM. Wind turbines find it more difficult, so they don't use synchronous generators.

When I visited a hydro plant I got the impression that they indeed control the rpm of the generator by DC field current through the rotor,
No they don't.

no matter how you connect that generator one of it's inherent features is that output Frequency is directly related to RPM, and rotor B field strength at any given rpm is related to the output amplitude aka voltage of the generator, so increasing rpm increases both frequency and output voltage while keeping the same rpm but increasing rotor B field simply increases output voltage keeping the same rpm.
It also sounds like you think more power means higher voltage. That's also false.

We have several PF Insights articles that address these topics. Please read them before posting further
https://www.physicsforums.com/insights/what-happens-when-you-flip-the-light-switch/
https://www.physicsforums.com/insights/ac-power-analysis-part-1-basics/
https://www.physicsforums.com/insights/ac-power-analysis-part-2-network-analysis/

But for the benefit of you and others, let me give the two most important equations about the power grid.

Consider any two points on the power grid. Let point 1 be your favorite power plant, and point 2 be any other plant, or load. But it's best to think of point 2 being the far end of the transmission line several hundred miles away. Here is the formula for the power transmitted from point 1 to point 2. P is power, V is voltage, X is reactance, $\theta$ is angle.

$P_{12}=\frac{V_1*V_2}{X_{12}}\sin{\theta_{12}}$
When $\theta$ is small, this can be approximated as:
$P_{12}=\frac{V_1*V_2}{X_{12}}\theta_{12}$

If we think of V and X being constant, then power varies directly with the angle. But the angle is not measured at point 1, it is the angular difference between $V_1$ and $V_2$. You must imagine yourself as a giant, with an oscilloscope or voltmeter probe in each hand, and touching the two probes to two points on the grid hundreds of miles apart.

Next, think what it takes to hold that power constant in time. $\theta_{12}$ must be constant too. To hold $\theta$ constant, the frequencies at both ends must be identical. To change $\theta$ the frequencies must be different for a short time to make the phase angle change. That is why the grid is synchronous.

Now, let's think about the voltages. Let Q be the imaginary power flow (we call it MVAR).

$Q_{12}=\frac{V_1*(V_1-V_2)}{X_{12}}\cos{\theta_{12}}$
When $\theta$ is small, this can be approximated as:
$Q_{12}=\frac{V_1*(V_1-V_2)}{X_{12}}$

As you can see, the predominant term in that is $V_1-V_2$. MVAR flow is roughly proportional to the voltage difference between the two ends. So, increasing $V_1$ has little influence on real power P, but much influence on imaginary power Q. You can simplify all the above into one little sentence,

Power depends on the phase angle of voltage, MVAR depends on the magnitude of voltage.

Memorize that sentence and you know more about the power grid than 99% of all people. (Be careful of one thing; the phase angle is not the angle between current and voltage at the power plant, is is the angle of voltage at the near end of the transmission line compared with the far end of the transmission line.)

Why does Q matter? Q flow is the mechanism for controlling voltages all through the grid, not just at the power plants. It's all explained in the Insights article.

jim hardy
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I don't know how I missed this for all these years while still knowing how generators work but it seems you gave me a good simple explanation,
It's counterintuitive that the rotor is locked in step with the stator field unlike an induction machine.
Surely as a kid you played with magnets placing one atop a table and another underneath it then dragging the upper one around by moving the lower one. That demonstrates to the Doubting Thomas part of our brain that magnets can transmit force . Once that's accepted it's a small step to transmitting torque.

whenever there is enough torque in the mechanical side of the generator , it's rotor will rotate slightly faster than AHEAD OF ! the grid field in the stator causing additional power to be "fed" or induced into the stator coils?
It changes speed only briefly while finding the new angle . Think of field strength as an adjustable spring with a spring constant expressed in megawatts per degree or radian .

Now I assume that if for whatever reason the rotor rpm drops below the grid frequency related rpm threshold the generator then becomes a motor as the stator field drags the rotor along not the other way around so the generator will actually start to consume a little bit of power from the grid through the transformer right?
Exactly. Well, Almost exactly . RPM can be only slightly less and only long enough for the machine to come to its new power angle.
Difference between a motor and a generator is just the direction of power flow..
If you watch the shaft's keyway with a stroboscope synchronized with terminal volts you will see power angle shift with excitation, and you'll see damped oscillations because the machine has inertia and changing field current changes the spring constant coupling that inertia to infinite bus.

You can run that experiment yourself with two car alternators.

Well all in all this seems exactly the same as with an AC induction motor/generator the only difference being that here the rotor can be directly coupled to the stator field instead of slipping always by some small amount due to the difference in rpm necessary for induction to happen in the AC induction motor/generator case.
Yes that's correct.

jim hardy
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Here's a set of three phasor diagrams i made some years ago for a thread with similar questions.

Reactive current either aids or opposes rotor field. It doesn't change torque from the turbine so it doesn't affect power . (Well except during the brief time the machine is moving to its new power angle.)..

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anorlunda
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This thread became very confused. To preserve its value for archival purposes, I had to delete the last 28 posts. Apologies to innocent victims who had their posts deleted; it was not your fault.