Comment by cuu508
21 hours ago
> 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.