Reddit post by an EE explaining it better than I can: https://www.reddit.com/r/AskEngineers/comments/qhear9/commen...
> There are also factors like fault current handling that HN might think is trivial or to be glossed over, but without the ability to eat 10x+ rated load for a brief duration, faults on the grid cannot be addressed and the entire system would collapse into pointlessness.
I don’t understand what you are talking about here. I don’t work in the utility world, I sell and run commercial electrical work, but handling available fault current in my world is as simple as calculating it and providing overcurrent protection with a high enough AIC rating or current limiting fuses. I don’t see why the utility side would be any different.
There are breakers, of course, but they react slowly enough that there will absolutely be a massive overdraw first. Then the breaker will open. Then, some small number of seconds later, it will automatically close.
It will attempt this two to four times before locking out, in case it just needs multiple bursts. It’s called “burning clear”, and it looks just as scary as you’d think… but it does work.
So, solar suppliers need to also survive this.
The lack of rotating mass in a solar site means the rest of the spinning mass of the generators needs to compensate to maintain frequency and voltage, right? So when clouds roll in and the solar field output drops quickly, it’s a challenge for the rest of the system to compensate since any other generator that spins will slow down much more slowly, giving the grid more time to react.
Also, I was not aware that inverters can only handle fault current that is 1.1x the nameplate capacity, that’s a big limitation. I can buy a 20A breaker with 200kaic, which is 10,000x higher than the breaker ampacity, which is extremely helpful for handling fault current.
I'd argue that nothing that uses semiconductors would be suitable for the task. They get you to maybe 2x rated current capacity for a meaningful duration. A spinning turbine can easily handle 10x or more for a much longer duration.
We could put so many redundant transistors in parallel that we have equivalent fault handling, but then we are into some strong economic issues. There's also no room for error with semiconductors. Once you start to disintegrate, it's all over ~instantly. There is no way to control this. A synchronous machine can trade downstream maintenance schedule for more current right now. The failure is much more gradual over time. A human operator can respond quickly enough if the machine is big enough.
The other trivial solution are synchronous condensers. Or just let the generators and maybe even turbines of future emergency reserve thermal plants spin with the grid without consuming any fuel.
Just ensure the proper margins exist in the grid and call in ancillary services as needed.
No need to make it harder than it needs to be.
One way to think about this problem is that our electrical grids are giant machines—in many ways, the largest machines that humanity has every constructed. The enormous machine of the grid is comprised of many smaller connected machines, and many of those have spinning loads with enormous mechanical inertia. Some of those spinning machines are generators (prime movers), and some are loads (like large electric motors at industrial facilities). All of those real, physical machines—in addition to other non-inertia generators and loads—are coupled together through the grid.
In the giant machine of the grid, electricity supply and demand have to be almost perfectly in sync, microsecond to microsecond. If they're not, the frequency of the grid changes. Abrupt changes in frequency translate into not only electrical/electronic problems for devices that assume 60 Hz (or 50, depending on where you are), but into physical problems for the machines connected to the grid. If the grid frequency suddenly drops (due to a sudden drop in generation capacity or sudden drop in load), the spinning masses connected to the grid will suddenly be under enormous mechanical stress that can destroy them.
It's obviously not possible to instantaneously increase or decrease explicit generation in response to spikes or drops in load (or alternatively, instantaneously increase or decrease load in response to spikes or drops in generation). But we don't need to: all of the spinning mass connected to the grid acts as a metaphorical (and literal) flywheel that serves as a buffer to smooth out spikes.
As the generation mix on the grid moves away from things with physical inertia (huge spinning turbines) and toward non-inertial sources (like solar), we need to use other mechanisms to ensure that the grid can smoothly absorb spikes. One way to do that is via spinning reserves (e.g. https://www.sysotechnologies.com/spinning-reserves/). Another way to do it is via sophisticated power electronics that mimic inertia (such as grid-forming inverters, which contrast with the much more common grid-following inverters).
To learn more about this topic, look up ancillary services (e.g. https://en.wikipedia.org/wiki/Ancillary_services). This Shift Key podcast episode is also a great introduction: https://podcasts.apple.com/us/podcast/spains-blackout-and-th...
Not all prime movers are the same with regard to grid dynamics and their impact.
Solar, wind, etc., almost universally rely on some form of inverter. This implies the need for solid state synthetic inertia to provide frequency response service to the grid.
Nuclear, coal, gas, hydropower, geothermal, etc., rely on synchronous machines to talk to the grid. The frequency response capability is built in and physically ideal.
Both can work, but one is more complicated. There are also factors like fault current handling that HN might think is trivial or to be glossed over, but without the ability to eat 10x+ rated load for a brief duration, faults on the grid cannot be addressed and the entire system would collapse into pointlessness. A tree crashing into a power line should result in the power line and tree being fully vaporized if nothing upstream were present to stop the flow of current. A gigantic mass of spinning metal in a turbine hall can eat this up like it's nothing. Semiconductors on a PCB in someone's shed are a different story.