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> because a grid connected system is supposed to shut down when the power fails, the only way out is to stop being grid connected I've just had a tour of a hyd

> because a grid connected system is supposed to shut down when the power fails, the only way out is to stop being grid connected I've just had a tour of a hydrogen fuel cell setup we have here at our OB. I'm no electrician, but from what I could tell, it's plugged into the mains, currently drawing about 3KW, and hydrogen is producing another 12KW (we haven't got much demand for this event - the generator will produce 250kW if we want). It comes in from the mains through an isolating transformer, into the generator and UPS, and then outgoing power to us goes through another isolating transformer (to stop mains hum that we saw last time, in both directions) It seems it's perfectly fine to have this sort of setup connected to the grid in the UK. Obviously it's more expensive to do so than just wiring in your own diesel generator.

jacquesm 5y ago
That's a very nice bit of kit that you have there. Pictures?
LorenPechtel 5y ago
Yup. Fundamentally, the issue is whether there is any means by which locally-generated power can backfeed onto the mains. Typically cogeneration is handled by making the local generator have no ability to create the signal, it can only come from reading the line voltage. The mains voltage dies, there's no more signal to drive it and so even in case of failure it will shut down. Any system which absolutely prevents backfeed is acceptable, your use of the mains charging the UPS and the UPS driving everything is an acceptable way to do it--even if your system goes nuts no circuit exists by which that power can backfeed. It's more typically done by a mechanical isolation switch, you can either be connected to the mains and rely upon the signal from the mains or you can run locally with no physical connection to the mains. The reason for this is repairs. If a lineman opens a breaker the wires after that point must go dead no matter what--if your system backfeeds into the wire you could kill the guy working on the wires.
- quickthrowman 5y ago
  > It's more typically done by a mechanical isolation switch Do you mean (automatic|manual) transfer switch? A transfer switch transfers the load to the generator and isolates the generator from the line side(utility side, or ‘mains’ for the two tiny islands that like silly words like ‘mains’) A ‘mechanical isolation switch’ (safety switch, disconnect switch) is simply a disconnecting means.
  - LorenPechtel 5y ago
    The important part is the isolation. The fact that it also transfers to local power is important to the user, irrelevant to the power company. The power company only cares that there no failure mode that can cause local generation to feed back onto the line. Whether it's a transfer switch, a local inverter that requires line power to operate or some other such system is irrelevant to them.

jacquesm 5y ago

That's a very nice bit of kit that you have there. Pictures?

LorenPechtel 5y ago

Yup. Fundamentally, the issue is whether there is any means by which locally-generated power can backfeed onto the mains. Typically cogeneration is handled by making the local generator have no ability to create the signal, it can only come from reading the line voltage. The mains voltage dies, there's no more signal to drive it and so even in case of failure it will shut down. Any system which absolutely prevents backfeed is acceptable, your use of the mains charging the UPS and the UPS driving everything is an acceptable way to do it--even if your system goes nuts no circuit exists by which that power can backfeed. It's more typically done by a mechanical isolation switch, you can either be connected to the mains and rely upon the signal from the mains or you can run locally with no physical connection to the mains. The reason for this is repairs. If a lineman opens a breaker the wires after that point must go dead no matter what--if your system backfeeds into the wire you could kill the guy working on the wires.

quickthrowman 5y ago
> It's more typically done by a mechanical isolation switch Do you mean (automatic|manual) transfer switch? A transfer switch transfers the load to the generator and isolates the generator from the line side(utility side, or ‘mains’ for the two tiny islands that like silly words like ‘mains’) A ‘mechanical isolation switch’ (safety switch, disconnect switch) is simply a disconnecting means.
- LorenPechtel 5y ago
  The important part is the isolation. The fact that it also transfers to local power is important to the user, irrelevant to the power company. The power company only cares that there no failure mode that can cause local generation to feed back onto the line. Whether it's a transfer switch, a local inverter that requires line power to operate or some other such system is irrelevant to them.