Many thanks, that's very useful feedback.ALAN/ALAN D wrote: ↑Sat Oct 15, 2022 3:37 pm Have used two of those for over a year. They work well.
Good point, thanks, you're right, without some sort of time delay relay this could get into a state where the contactor keeps cycling on and off. I need to sense the combined current, though, as the DNO concern is with the export exceeding the 6kW rating of the big system inverter. It's just about possible that the main system could be generating over 4kW at a time when the second system is generating its 2kW maximum.Stig wrote: ↑Sat Oct 15, 2022 3:38 pmIf you're sensing the combined current and using that to disable the 2nd system won't the relay keep clicking in and out? If you're just sensing from the 1st system and using that to switch off the 2nd system then no problem.Oldgreybeard wrote: ↑Sat Oct 15, 2022 3:02 pm The simplest way to do this seems to be to fit another current transformer to the big PV system AC output, and use that to switch a double pole contactor to isolate the second PV system from the grid. Seems this may be easier than I thought, as I've just found these cheap current sensors: https://www.ebay.co.uk/itm/304631567636 They are adjustable from 2A to 30A, and seem ideal, as I can set it to about 26A and it will trigger a contactor directly.
These are self-powered, from the current flowing through the sensed wire, so can safely be tucked into the consumer unit, with the line conductor from both PV inverters running through the hole.
I think all it needs is a timer relay, so the contactor is switched off when the 6kW threshold is reached, then stays off for, say, 30 minutes. Because of the difference in orientation between the two PV arrays it's pretty unlikely that the 6kW limit will be reached that often, and only then for a short time. Just adding a delay before the second system can turn back on when the output drops should be enough to stop the system "short cycling". Something like this: https://www.ebay.co.uk/itm/134250349231 configured with an "off delay", so when triggered it stays on for the set period, then turns off and allows the second PV system to reconnect.
My gut feeling is that this limiting system may not ever operate, anyway, given that the second system will be in shade by the time the big system peaks in the early afternoon. I only really need this as a way to keep the DNO happy and tick a box in their records to say that the export is always limited to no more than 6kW.