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this post was submitted on 25 May 2024
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Yeah, non-USA for this atm, as much fun as it would be to plug such a system into an apartment.
I believe that the US requires that a direct-feed system has to plug into a physical kill switch setup to prevent back-feed of power during an outage.
Still pretty neat, though!
Same for the EU.
Solar inverters also need to follow the grid frequency
They don't follow the grid frequency because the EU or US regulations require it, they follow the grid frequency because physics demands it.
In this house we obey Ohm's law!
Not necessarily. While running parallel to the grid or needs to sync to it of course, but when running in island mode it can do whatever it feels like - if it supports that. My Fronius runs at 52Hz e.g. to keep other generators in the island from starting up.
Interesting - which other generators for example wouldn't you want starting up?
An engineer dabbling in such things explained to me, that it is hard enough to regulate a small island network frequency and voltage-wise from a single point. Reacting to whatever another source (something like another solar inverter out in the garden with a few panels of its own, e.g.) in the same island grid does could easily lead to potentially destructive oscillations in the regulation circuit. Large grids have "mass" - literally, because large generators and electric motors are spinning at whatever speed they are spinning in whatever phase they are in. So small disturbances from regulating too quickly or a little wrong just disappear into that. The same doesn't go for a small island grid, so at Fronius they have decided to put 52Hz on the grid which by standard prevents other sources from syncing. Electric utilities do the same when they have to power small villages from diesel generators temporarily - 52Hz and the house mounted solar generators don't sync.
I think I get that, thanks. So an Island grid is less stable and could cause itself damage if two microinverters say are trying to sync up to each other vs a beefy, stable main grid?
So how does a backup battery system work when islanded? Typically also at 52Hz?
Or can it go into a 60Hz beefy mode?
It would be nice to get all the little island solar inverters working when the grid goes down!
I'm guessing the commenter above is in the EU and operating at 50Hz normally, so running at 60Hz wouldn't be a great idea. A backup battery and such operate in the same way when islanding.
Yep. And in my case, the backup battery is connected to another DC input on the inverter and the inverter pretty much manages everything. As I understand the documentation, there is no other way to use solar AND a battery at the same time as a power source for islanding. Switching over manually with a short disruption in-between is always possible of course, as is charging an AC coupled battery from an islanding solar inverter.
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IIRC some inverters are able to sync up with alternative power sources, but the documentation is extremely limited and seems to be reserved mostly for large-scale systems. I know my Solaredge system has slowly been implementing using both at the same time, but the documentation is pretty unclear as to how this works. I know at the very least it'll allow you to use a 2-wire start to kick a standalone generator on when the batteries are low, but don't know much else about how it's currently set up
Also in the US regular 120v outlets are fed from 1 of 2 transformer legs. If you back fed power through a 120v outlet, roughly half of the circuits in your home would function and the others would be dead.
And disclaimer: no one should do this, but when the transfer switch disconnects from the grid, would it work to jump say a breaker across the L1-L2 hots to share that 120v backfeed over both?
Clearly the 240v appliances won't work in this configuration, but the fridge on one leg and the internet on the other will still work ok, right?
Again, no one try this - it's just a thought exercise.
Yes, it will actually work. I know it's very much not to code, but when we lost power for over 10 days, I did this to keep our furnace running and us from freezing to death since it was -10F out.
I only have a small 120V generator, but hooked both legs to hot and backfed via our EV charger's outlet, since its a 50A circuit. Like you said, nothing 240V worked, but that little 3kW generator did a great job powering basically the whole house with no issues.
That winter was definitely a big driver for me to get a backup battery system so our solar could power the house.
No.
According to the article this system also detects power outages and shuts off when they happen. Just like full-scale solar power systems. But yeah, no physical kill switch.
These systems automatically turn themselves off during an outage.
That's great, but it doesn't matter unless it has the physical cutoff that's required to bring that kind of system up to the current electrical code for such a system.
they have relays (well, most of them. looking at you, Deye), so it should be fine
Physical shutoff via relays is required by the standard. We've just been through a scandal where a manufacturer skimped out on putting them in and had to recall the devices.
I hate technologies that limit cannot use with another manufacturer's battery". Smells monopoly.