Plug in solar

tfhnota

The DNO process for balcony solar is to be "simplified" so probably an online form of some sort, the interesting bit is if that will be enough to get paid for any export, that has yet to be mentioned. It is a pity that "smart" meters can;t be persuaded to run backwards for small solar exports.

Section62

https://forums.moneysavingexpert.com/discussion/comment/80954684#Comment_80954684

Genuine question - how does the inverter know what it is sensing (presumably an AC voltage?) on the electrical connection to/from the 'mains' is bonafide DNO-supplied electricity, and not coming from a different source?

QrizB

The inverter doesn't know that it's DNO electricity; there's no National Grid watermark on the kilowatt-hours. If it meets the specification for a mains supply, the inverter will accept it.

As an example, a few years back the main feeder to iurocal substation failed. It needed some substantial underground work so the DNO installed a chunky diesel generator at our substation and that fed the hundred-ish homes around me for a month or two. My solar PV worked as it should throughout, feeding our local microgrid.

But - whatever the source is, it needs to have enough load connected to accept the output from the inverter. If there had been a much smaller generator that was only serving my house, there wouldn't have been anywhere to export surplus electricity to and if my inverter had tried to export the supply would have gone out of spec and the inverter would have shut down.

Section62

https://forums.moneysavingexpert.com/discussion/comment/81939345#Comment_81939345

Thanks. That was my understanding, which I think means two things -

1. If the safety features of plug-in solar rely on it not supplying if the voltage/frequency is out of spec, then it wouldn't be safe in a scenario where a (fault) condition doesn't result in the sensed voltage/frequency being out of spec by enough to trigger shutdown. In your 'smaller generator' example it would be possible for the household (or circuit) load to match the locally generated supply closely enough that the voltage/frequency stayed sufficiently within spec to keep the inverter feeding the circuit?

2. Simple plug-in solar wouldn't work in an off-grid situation (like a remote garage or shed) unless there is something else to do load balancing (or similar) to keep the voltage/frequency within spec?

QrizB

Re. 2, yes plugin solar needs a grid to plug in to. It won't work as a standalone off-grid system. Standalone off-grid systems are different (and already available to buy and use).

Re. 1, if the seensed voltage/frequency remains in spec, where's the fault? These inverters can't form a grid (they need to follow an external signal) so what fault scenario are you thinking of where the external supply is simultaneously both present and failed?

Section62

Re. 1, if the seensed voltage/frequency remains in spec, where's the fault? These inverters can't form a grid (they need to follow an external signal) so what fault scenario are you thinking of where the external supply is simultaneously both present and failed?

For example, multiple plug-in solar units on one circuit. The circuit MCB/RCBO trips out due to a fault condition (or is switched off to isolate the circuit) removing grid supply. So long as each unit is still sensing a stable voltage/frequency within the circuit, they won't know the grid supply has gone. So what is it that stops them supplying if nothing else changes?

QrizB

So what is it that stops them supplying if nothing else changes?

None of the grid-tied inverters is capable of independently generating a 230V, 50Hz waveform so they can't follow one another. Without an external reference, they'll lose sync within a cycle (0.02 seconds) and shut down.

born_again

https://forums.moneysavingexpert.com/discussion/comment/81939418#Comment_81939418

Skip to 13:10 & the guy explains what happened when main supply dropped & he used transfer switch to connect their backup ecoflow.

https://youtu.be/MQ103CLK1Ro?si=x3J5YqOb7pPYB01o

QrizB

For some reason I can't click through @born_again 's link. If you have the same problem, try this:

https://www.youtube.com/watch?v=MQ103CLK1Ro

Edit to add:

Note that his Delta 2 isn't a grid-tie inverter, it's one of those off-grid solar energy systems that we briefly mentioned for your question 2 a few posts back. (I think it might be possible to use it for grid-tie if you configure it differently.) So that's not quite the situation that we're discussing here.

Section62

https://forums.moneysavingexpert.com/discussion/comment/81939551#Comment_81939551

Yes, independently. But if two (or more) inverters are providing each other with the required external reference then what stops them?

Hence the question - how do they 'know' they are connected to the grid, vs sensing the output of some other equipment which conforms sufficiently to grid spec?