[londons_explore]

> And an RS485 or 10BASE-T1S or CAN connection so that they can coordinate their I-V characteristics

Note that simply voltage/frequency alone can communicate everything necessary to evenly distribute load. For example:

Output frequency = 60 + X Hz

Where X varies between -0.1 and 0.1, indicating the percentage of max load the inverter is under(in charge or discharge direction).

Such a scheme will self synchronize any number of inverters, even of varying capacities, into a stable grid, and they will all be the same percentage loaded. The grid can even have a few dumb gas generators on too and remain stable. No fancy comms needed.

[marcosdumay]

That's not simple to create with inverters alone.

Yep, inverters can easily read the frequency change to adapt their output. But a device that reads the network load and translates it into a frequency change is really not trivial. Grids do this because mechanical generators do that translation, but the easiest way to add it to a non-mechanical grid is adding a mechanical device.

Lines exclusively fed by inverters will naturally get undervoltage, instead of frequency shifts.

[londons_explore]

> But a device that reads the network load and translates it into a frequency change is really not trivial.

Nah - it's pretty simple. Have your inverter measure it's own load - for example by measuring the current through the main switching element. Compare that to the max current that switching element can handle. That is your percentage load that you are under. (negative if current goes backwards)

Then use that to decide what frequency you will output, using the formula.

There is one extra element to this... At startup, to avoid large transient currents, you need to start with the 'max switching element current' for the formula at zero, and then increase it smoothly only while the calculated load figure is under 100%. Within a few seconds you will get sync with any existing network. Such a scheme also works for blackstarting an isolated grid, or with many other inverters with the same strategy.

This scheme has a big benefit that there is no need to measure the grid frequency precisely - which is actually a fairly hard thing to do on noisy small grids.

And it turns out thats all thats required to maintain grid sync. As long as supply capacity exceeds demand at all times, this grid will be stable.

[amluto]

Do you have a reference?

If I followed this right, the frequency goes up with load (which seems backwards). So a 50%-loaded grid will be at 60.05 Hz. A newly started inverter will try to output 60 Hz (and fail, since it is unable to single-handedly reduce the frequency). But it will surely end up with nonzero current.

Don’t real spinning synchronous machines work the other way? If they are overloaded, they spin slower, and if they are spinning a bit slower than the grid when connected, the grid will speed them up?

[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!

[ilyt]

It might be pretty complex code wise but not cost wise, you just need to probe current and voltage, which you do anyway

[amluto]

I think this works for discharge, possibly with some extra cleverness needed if the batteries are at different SoCs. But I’m not sure how it could handle being tied to the grid intelligently or how charging could be managed without some additional controls.

Also, can a pure frequency-based scheme handle the case where a whole bunch of inverters are in parallel and a load that’s much larger than any one of them can handle individually starts up?

[londons_explore]

> handle the case where a whole bunch of inverters are in parallel and a load that’s much larger than any one of them can handle individually

Yes, but with caveats... The grid will startup, smoothly (ie. gradually), but during startup every inverter will be at its maximum configured load (with the maximum load config ramped during startup). When the maximum load is hit, the frequency sits at the minimum design frequency (59.9 Hz in this case), and voltage drops instead.

So your large device will see a sine wave at 59.9 Hz that starts at zero volts, and increases in amplitude to 230 volts gradually over many seconds, and as soon as it hits 230 volts, the frequency will almost immediately become 60 Hz.

For large motors, thats a problem. Large motors typically don't like line frequency at a reduced voltage - they can end up not having enough torque to turn, and will just rapidly heat up. They would do better without the gradual startup ramp - but that startup ramp is necessary to ensure the inverters stay in sync.

In reality, this scheme works for AC type motors rated up to about 30% of the capacity of the inverter set, or up to 100% as long as another motor is already running on the same grid.

[londons_explore]

> extra cleverness needed if the batteries are at different SoCs

The scheme as presented keeps the inverters under equal fraction load, but does not guarantee the batteries end up at equal state of charge. The formula can easily be modified to over time achieve this too:

X = 60 + (fraction load of inverter)*0.1 + per_battery_factor

The per_battery_factor would be set based on the batteries state of charge and/or any user instructions from buttons to charge or discharge a specific battery.