[londons_explore]

By the way, the reason big national grids don't allow this scheme is because it is too stable. Ie. if your house, containing some inverters like this, is disconnected from the national grid, it will continue independantly. Your house and the national grid will continue on their own paths, with slightly differing frequencies.

Reconnecting two grids at differing frequencies is hard.

[mschuster91]

> Reconnecting two grids at differing frequencies is hard.

Indeed it is. The last time this happened in 2021 here in Europe, South-Eastern Europe split off, and it took a good hour to get everything balanced again [1].

[1] https://www.entsoe.eu/news/2021/01/26/system-separation-in-t...

[ilyt]

> if your house, containing some inverters like this, is disconnected from the national grid, it will continue independantly. Your house and the national grid will continue on their own paths, with slightly differing frequencies.

That's not the main reason.

The main reason is that if grid is down, workers need to work on it safely and so they need it de-energized, not some random solar installation trying to feed power into it.

Some inverters for that reason have 2 outputs, one for grid, other one for so called EPS (emergency power supply). When grid goes down, usually relay disconnects the two and inverter only feeds power to EPS, and re-connects both outputs only when grid is back up and synced. So you'd shove your important loads onto EPS output and have it grid-independent.

[amluto]

> The main reason is that if grid is down, workers need to work on it safely and so they need it de-energized, not some random solar installation trying to feed power into it.

I’m sure this has motivated someone who makes rules, but I once had occasion to ask some actual line workers replacing equipment serving me, and they laughed. They said that they always assumed the lines were hot at both ends, and if they needed a line to be de-energized for safety, they would deliberately short it out. A pesky little residential inverter was not in the slightest bit concerning to them, anti-islanding or no.

I do believe that closing a switch between the grid and a small residential island could be unpleasant for the island or maybe even for the switch. And closing a large switch connecting an entire neighborhood that somehow formed a functional island and connecting it to the grid without synchronization might genuinely go poorly.

But mostly I think the most important current reason for anti-islanding is that a small residential or light commercial inverter is unlikely to have anywhere near the capacity to power an entire secondary circuit, nor does its owner want it to.

[ilyt]

I doubt it would be required by law if it was just an issue of someone's inverter tripping overcurrent proteciton

> I’m sure this has motivated someone who makes rules, but I once had occasion to ask some actual line workers replacing equipment serving me, and they laughed. They said that they always assumed the lines were hot at both ends, and if they needed a line to be de-energized for safety, they would deliberately short it out. A pesky little residential inverter was not in the slightest bit concerning to them, anti-islanding or no.

Well, that's generally good safety measure.

> But mostly I think the most important current reason for anti-islanding is that a small residential or light commercial inverter is unlikely to have anywhere near the capacity to power an entire secondary circuit, nor does its owner want it to.

Many places do, hell, common problem is grid voltage being too high and inverters turning off because too many neighbours got solar. I'd imagine its pretty easy in peak to have solar exceed local demand even if only part of houses have it. At the very least "to the next transformer".

I wonder whether the move to green energy would change the way grids are build. "Micro-grid" with all houses on street connected to substation with a bunch of batteries that most of the time just stores local peak and sells it back to the residents might be an interesting idea and potentially make grid more resilient overall.

[marssaxman]

> if they needed a line to be de-energized for safety, they would deliberately short it out

An old electrician once showed me his technique for tracing a circuit in a house back to the breaker panel: open the box, expose two wires, cross them with the shaft of a screwdriver, wait for the pop, walk back to the panel and see which breaker tripped. Tada!