6 ms·
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
by ooterness 3y ago
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.
- throw0101d 3y agoSee perhaps "Grid frequency volatility in future low inertia scenarios: Challenges and mitigation options" * https://www.sciencedirect.com/science/article/abs/pii/S0306261921002385 https://www.sciencedirect.com/science/article/abs/pii/S03062...
- datadrivenangel 3y agoSynthetic Inertia and Frequency Control Ancillary Services will help resolve this issue.
- sandworm101 3y agoAnd 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.
- Animats 3y agoHere'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. [1] https://www.youtube.com/watch?v=xGQxSJmadm0 https://www.youtube.com/watch?v=xGQxSJmadm0
- peterleiser 3y agoThank 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.
- sandworm101 3y agoThank you. Links like that are why I read HN. That video is totally unique.
- deleted 3y ago[deleted]
- wbl 3y agoNuclear 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.
- pests 3y agoHow does moderating more accelerate the reaction?
- Tabular-Iceberg 3y agoMore neutrons will be able to interact with the fissile nuclei rather than just fly out of the reactor, so you get more fission events.
- magicalhippo 3y agoThe 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.
- bell-cot 3y agoIt 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.
- anonporridge 3y agoThis 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.
- kragen 3y agochemical batteries don't take nearly that long to react!
- ok_dad 3y agoThe control systems do though
- kragen 3y agoi doubt that very much indeed. it isn't practical to run a switching power supply with feedback latencies of multiple milliseconds
- toomuchtodo 3y agoTesla Megapacks detect and respond within ~100ms when operating in grid support mode. Unsure how that compares to what you’d expect. https://www.aurecongroup.com/-/media/files/downloads-library/thought-leadership/aurecon-hornsdale-power-reserve-impact-study-2018.pdf https://www.aurecongroup.com/-/media/files/downloads-library...
- kragen 3y agothey 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'
- ls612 3y agoIn 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.
- idiotsecant 3y agoThey 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.
- gumby 3y agoIndeed, 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.
- th0ma5 3y agoIt matters for forensics of videos to verify when they were created by aligning them with the known power grid frequency changes.
- Scoundreller 3y agoThis is why I suspect anyone with backup generators of being criminals that are up to no good.
- xattt 3y agoDoes AC propagate from larger “pacemaker” sites, or is it in-phase at 60 Hz all throughout an interconnect/grid?
- ooterness 3y agoIgnoring 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.
- maCDzP 3y agoI wonder if there are fly wheels in the grid that are used to store kinetic energy.
- _kb 3y agoThere absolutely are [0]. They're also used in DC's as a buffer as other energy sources spin up. Lawrence Systems did a neat walkthrough of that infra recently [1]. [0]: https://en.wikipedia.org/wiki/Flywheel_storage_power_system#Power_grid_frequency_control https://en.wikipedia.org/wiki/Flywheel_storage_power_system#... [1]: https://youtu.be/gsN_CJJDy_o https://youtu.be/gsN_CJJDy_o