Comment by PaulDavisThe1st
1 day ago
> There is also the problem of backfeeding, but this is more theoretical
it is not theoretical from the POV of power company employees working on their lines. the power goes out, they go to fix the equipment or wiring, and if the inverters connected to the grid have not shutdown, there could be 10s of kW to even MWs being backfed into the system they are working on.
that's why in the US at least, grid-tied inverters are required to have protection against this. if they do not see power from the grid, they shutdown (and rather quickly).
Hang on. You didn't address the reason they gave in the half sentence your quote skipped. How are those backfeeders going to keep going when there's way way more load than they can possibly supply?
> if they do not see power from the grid, they shutdown
I realize this is a complete noob question but I'm still curious: with enough "backfeeders" could it not in theory be possible that the backeeders see each other's output and keep operating?
Say, a few houses at the end of the line, each with very little electricity consumption (no-one's at home), but with panels and inverters going full tilt, something like that?
Yes, this is a concern with some older inverter designs. They use phase-locked loops which are meant to lock to the grid frequency but if you and your neighbours on the feeder are generating and consuming a roughly equal amount of energy and the feeder loses its connection to the wider grid, the loops can lock to each other and form a stable island. How long this would last in practice is not really known, it would probably be rare for it to happen at all and even rarer to stay stable for more than a few minutes but it has been demonstrated so it could have become an issue. Newer inverter have specific anti-islanding protection to detect situations like this and disconnect if it happens.
(the specific case of very little consumption but panels and inverters going full tilt doesn't work as this would lead to the frequency rapidly increasing, exceeding inverter set points and the local grid collapsing with all the inverter tripping off. The danger scenario is closely matched load and generation, like a real grid)
Thanks, anti-islanding protection must be a fascinating topic. You could look for voltage, frequency, phase anomalies but you also have to balance between being safe and having too many false positives. I'm not sure I'd want to be a line worker knowing there may be many inverters, from various manufacturers and with varying levels of quality, in every direction.
Perhaps a more robust solution would be for the power company to install remote-switchable circuit breakers at various places of the grid. Use some sort of wireless IoT protocol to listen for "on/off" signals and to report back "I'm now on / I'm now off!". Then the power company can disconnect parts of the network by themselves, without trusting that every inverter, generator and battery will do the right thing.
Apparently the anti-islanding features in the inverters prevent this kind of situation. They need several minutes of clean power to start back up, and do other checks.
Again, all linemen are taught to short the wires or they are alive. All grids have regular places where two different lines meet and there is a switch at those points where in the worst case is they can cut the down line close a switch that normally is open and they can back feed a line from a different one and get power to people who are on each side of the break and then they can come back later to fix the break. These switches are miles apart, which means that you cannot be sure while you're working on the line someone else didn't go to that switch and change it.
Also, people have been back peeking the grid from their home generators for decades by now. Solar and inverters are a different variation of that attack, but it is nothing new. Any utility will have procedures because they can't guarantee that's not happening and we learned years ago.
I'm not saying it never happens, only that correct procedure already makes it not an issue
Yep, and they see the power by trying to increase/decrease the frequency of their own output and sensing the resistance from the grid.