It took me a long time to understand a key reason the electric grid works at all: the rotational inertia of all the big, heavy generators in the system.
As an example, imagine a frictionless generator with no load, spinning at 60 Hz. The rotating mass of the generator stores kinetic energy, and its windings are ready to convert that to electrical energy. Attaching an electrical load slows the rotation unless something is done to replenish that stored kinetic energy.
On a larger scale, loads are switching on and off all the time. Power plant operators need time to react, because adjusting the throttle isn't instantaneous. A natural gas generator might take seconds to throttle up or down; adjusting a nuclear power plant could take a while. In the meantime, energy is drawn from the buffered kinetic energy of every generator on the grid. They are all rotating in sync because every generator is also a motor.
The frequency of the grid is the signal everyone uses to know whether generation matches consumption, averaged over second or minutes or hours.
Indeed, there is special equipment required (the jargon/name escapes me at this moment — it’s been years) to connect a generator to the grid and that also automatically disconnects it if it gets too far from the grid frequency.
IIRC the USA grid SLAs are/were not a simple 60 Hz but 60 times n peaks over some period (10 seconds?) dating back to the era of synchronous electric clocks bc changing load could provide some wobble. I am sure it’s possible to provide tighter conformance these days but I wonder if it matters.
Nuclear can load follow very quickly: cooler water accelerates the reaction by moderating more and cranking up the coolant speed increases the heat delivered. The nucleonics issues are more over a daily shutdown or the like than quick load adjustments.
It would be more accurate to say that, at 10+ second time scales, nuclear can have some convenient load following characteristics. But it takes a while for any load change to feed back through the turbine, then the condenser, then the steam generator, then the primary coolant loop, then the reactor, then ...
And the economics of nuclear - enormous fixed costs, minimal fuel costs - encourages utilities to run them near full power. So they'll have relatively little spare generating capacity, regardless of their load following characteristics.
OTOH, a gas turbine generating plant is essentially a turboprop airplane engine, with the propeller replaced by a large generator. 1950's mechanical control technology can adjust the throttle on a ~1-second time scale as the load on the grid changes.
And the economics of gas turbines - quick & cheap to build, expensive to run - encourages utilities to run them (or not) as demand requires. So they'll usually have lot of spare generating capacity, to meet demand surges.
I believe only reactors used in the navy have this characteristic, the poison transients from adjusting load to quickly (and maybe having to shim for temperature) on a commerical reactor are liable to cause an unplanned shutdown.
The fission events produce a high proportion of fast neutrons. The fission cross-section, ie chance of inducing a fission event when struck by a neutron, depends on the speed of the incoming neutron, and is smaller for the faster neutrons than for slower neutrons.
edit: I should note there's a sweetspot, as the neutrons have to have enough speed to overcome the repulsive force in order to enter the nucleus. Thus the cross-section has a maximum at some energy (speed), and it varies a lot between elements and isotopes.
In a previous life I worked as an intern at a company making SCADA software and the like. The fact that they did this by hand, managing the grid and keeping it in balance, for 60-80 years before any computer existed to help them, and it basically worked, boggles my mind.
They didn't manage it by hand, really. The system manages itself with speed droop flywheel governors. They were purely mechanical systems that made each generator on the grid 'push' a little harder when speed dropped, proportional to the generator capacity. Wince speed drops when load increases, the whole grid manages itself with a few springs and hydraulic valves on each machine.
Occasionally you might need to get a few more units online with fast load, transmission trips, etc but those events are infrequent enough to be well within the ability of humans to manage, especially with old trelatively small power networks.
This is also why chemical batteries that can react in milliseconds are such useful buffer tech, even if they can't store more than a few hours of demand.
they have internal control systems that respond a lot faster than that. 100ms is 6 entire cycles of a 60-hertz sine wave, an utter eternity compared to the control mechanisms required to follow that sine wave up and down with pwm/pdm. the 150ms number in that brochure probably has to do with computer networking software, not the battery pack control system itself
but the power conversion control system is relatively slow, taking hundreds of nanoseconds to respond (ten thousand times faster than a few milliseconds). the battery chemistry itself responds enormously faster than that when the current drawn from it changes; the only thing that limits the electrochemical response is the capacitance of the electrical double layer, which in a battery (as opposed to a supercapacitor) is on the order of one nanofarad per ampere. so the tiny changes in the overpotential that turn the electrochemical reactions on or off can take place in well under a single nanosecond
so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height'
> so saying 'chemical batteries that can react in milliseconds' is understating the speed by a factor of ten million. that's like saying 'jet planes that can fly meters per day', 'supercomputers that can do dozens of multiplications per minute', or 'skyscrapers that can reach tens of microns in height'
I checked your analogies for jet planes and skyscrapers, and found them to be well within an order of magnitude of ten million, but you're off on your supercomputer analogy by a factor somewhere in the range of a billion.
I couldn't find a hard definition of what constitutes a supercomputer. The bottom of the Top 500 list is in the single-digit petaflop range and an RTX 4090 has 70 or 80 single-precision teraflops, and presumably you need more than a handful of graphics cards to constitute a supercomputer, so let's say 1 petaflop is our threshold for a supercomputer.
The ratio of a petaflop to 12 flops/minute is 5e15, or 5 million billion. Saying "a factor of ten million is like saying 'supercomputers that can do dozens of multiplications per minute'" is like saying 'the moon orbits meters above the Earth's surface'.
unlike the other examples, the category of 'supercomputer' has shifted over time. initially, i was thinking of a cray-1, which is the machine that popularized the term 'supercomputer' https://books.google.com/ngrams/graph?content=supercomputer&.... it was only about 160 megaflops, which is ten million times 960 calculations per minute. that wouldn't normally be described as 'dozens'; it's about an order of magnitude high. so i should have done my calculations a bit more carefully
you are of course correct that current machines are many orders of magnitude faster than that, and even a single video card is about three orders of magnitude faster
I think this is a bit of a pedantic take. My reading of the GP is to distinguish systems based on chemical batteries from systems based on gravitational batteries rather than the reaction time of the chemical cells specifically.
> i doubt that very much indeed. it isn't practical to run a switching power supply with feedback latencies of multiple milliseconds
That's because the control loop for a grid battery doesn't work the way you probably expect. The grid is effectively an infinite sink, so the voltage and frequency you observe "is what it is". You can't change it[^1], so you can't use voltage feedback from the output of your SMPS to drive the PWM. If you tried to do that with any kind of PID loop, you'd always be pinned at either maximum charge rate or maximum discharge rate[^2].
The correct way to do it is with an inner loop that will target a phase angle between grid voltage and your current. An outer loop (with a longer time constant) will set that target angle by comparing the measured grid frequency to a schedule.
> 100ms is 6 entire cycles of a 60-hertz sine wave
Yes, and?
You're measuring a sine wave, not a square with fast edges. Any voltage noise on the line is going to influence the zero-crossing and thus the measured frequency (which drives your battery controller). If you try have your controller do cycle-by-cycle corrections, you're again going to have your battery always at maximum charge or maximum discharge.
[^1] When Tesla's battery in Australia "saved the grid" when Loy Yang tripped offline, it dumped 150MW into the grid so that the rate of change of grid frequency was reduced. Even with the battery running balls to the wall, the grid was still slowing down.
[^2] Assuming that you haven't set your P and I terms to 0.
it's true, you can drive an h-bridge with a pre-canned pwm waveform whose phase you adjust with a latency measured not just in milliseconds but in seconds, though you'd better have safety mechanisms that respond to a fault on the line faster than that. you don't have to use feedback to generate the control signal! but at least in garden-variety class-d amplifiers reproducing arbitrary waveforms (rather than necessarily a fixed-frequency sine wave), you get better performance if you do use feedback, so it's common practice
(disclaimer, i've never built power electronics, so i could totally be misunderstanding something)
i think you're saying that any kind of a pid loop will alternate between being pinned at maximum or minimum when the plant isn't responding to the control output, unless p = i = 0. i don't think that's correct; i think it's sufficient to not make your p coefficient overwhelmingly large. a sufficiently large i coefficient will also make your control loop tend toward extremes in this situation, but not instantly, and a nonzero i will probably always win in the end when the plant is completely unresponsive. but a moderate p won't have that effect, and d can attenuate it.
also, you can totally use current feedback from the output of your smps to drive the pwm, and that seems to me like it would be a very good idea
> true, you can drive an h-bridge with a pre-canned pwm waveform whose phase you adjust with a latency measured not just in milliseconds but in seconds
I’m not sure what you’re trying to say with this paragraph so I’m not going to respond to it.
> think it's sufficient to not make your p coefficient overwhelmingly large. a sufficiently large i coefficient will also make
What I’m saying is that making the P term small enough not to pin the PWM means that it won’t do anything useful, though I do see I wasn’t originally as clear as I could have been on that point. Either there won’t be enough gain or you will pin the output. There is no meaningful range of values in the middle, especially when the system needs to work outside the laboratory and you can’t tweak the parameters every other day.
If you rely completely on the I term, you’ll have stability problems. So now it requires a lot of D term… and now you’ve gone full circle and we’re back to “the whole thing won’t work without a time constant which makes it totally useless to run a PWM and barely useful for the outer loop.”
> you can totally use current feedback from the output of your smps to drive the pwm, and that seems to me like it would be a very good idea
I did mention that already… along with the fact that you need to set some target for your current feedback using an outer control loop. You can set the target by measuring frequency error averaged across several cycles. Again, you need on the order of 100 ms to make that work.
Whatever way you slice it, the grid backup battery is not going to start exporting power to the grid in less than a cycle, and certainly not in less than a millisecond.
And water. Try to slow down a hydroelectric turbine, increase its load, and you are fighting thousands of tons of moving water flowing through a massive pipe. Any change will take at least the time equivalent to the speed of sound in water through the length of that pipe.
Here's someone doing exactly that, synchronizing a medium-sized hydroelectric generator to the grid.[1] The valve-to-turbine pipe delay isn't a big problem.
This stuff is adjusted on the scale of tens of seconds. Opening the valve makes the whole reservoir slosh for a few minutes, and the operator had to wait that out before trying to achieve sync by hand.
Thank you posting this! Awesome video. As an aside, I went down the rabbit hole on this topic last year when I was considering building my own local grid composed of multiple, heterogeneous generators and solar. Instead, I ended up buying a pair of parallel generators and called it a day.
Ignoring speed-of-light delays, it's completely in-sync across the whole grid. Operators are required to keep things as close as possible to 60 Hz at all times, but it does vary over time.
To be clear, frequency is not the only signal. Operators have voltage and current monitors on most transmission lines, so they can tell where the power is coming from and going to. The whole system is partly automatic and partly held together by people making phone calls.
As an example, imagine a frictionless generator with no load, spinning at 60 Hz. The rotating mass of the generator stores kinetic energy, and its windings are ready to convert that to electrical energy. Attaching an electrical load slows the rotation unless something is done to replenish that stored kinetic energy.
On a larger scale, loads are switching on and off all the time. Power plant operators need time to react, because adjusting the throttle isn't instantaneous. A natural gas generator might take seconds to throttle up or down; adjusting a nuclear power plant could take a while. In the meantime, energy is drawn from the buffered kinetic energy of every generator on the grid. They are all rotating in sync because every generator is also a motor.
The frequency of the grid is the signal everyone uses to know whether generation matches consumption, averaged over second or minutes or hours.