Adding off grid to grid tie system.

[fafrd]

So they say, but I'm not entirely convinced. But it could be true.
It could be isolated from ground, but if not isolated from AC, the AC circuit could provide galvanic connection to ground.

If you isolated both '+' and '-' from SCC, it would be fine regardless of what voltages they might get driven to (so long as not exceeding standoff voltage of disconnect.)
But your SSR, correct, need to see what it does. An isolated PV panel would get shoved around by capacitive coupling to AC.
When connected to SCC, you're depending on microinverter having PV wires isolated from AC side. The transformerless topology I imagined could have a transistor connecting them.

I think a test with Y cables for voltage (likely some DC, possibly AC voltage present) and then biasing with a high resistance and seeing if you can pull to zero volts will answer the question.

Not sure RSD actually isolates. It seems at least some have a buck circuit that drops voltage. For instance, Solar Edge optimizers deliver 1V per panel while in shutdown. SMA now has a way to enable their Secure Power feature with grid down, which suggests they get power from PV while in shutdown.

It’s certainly made more complicated by trying to exploit the multiple MPPTs on a quad-Microinverter versus the line Mppt on my SCC.

If I forget about that and just have a single string, I can just use one meaty SSR to connect the string to the SCC when the grid goes down (and the Microinverter turns off).

Since the SSR will be off when the Microinverter may be moving the + and - voltages around, I just need to make sure any voltage swings can’t cause issues with an OFF SSR.

Or are you ou suggesting that even when there is no grid signal reaching the Microinverter and it is OFF there could be some coupling if AC through it to the PV wires?

Alternatively, I could position the SSR on the SCC output (so it is disconnected from the battery and turned off until the grid goes down). That basically ties the same (single) string to both an inverter and an SCC but only one is powered on at any time.

The 1V drop from the SSR represents closer to 4% of the generated output power (versus ~2.5% of the incoming PV power @ ~40V) but that is at least one ‘solution’ to getting additional power from a small array of ‘backup’ DC-coupled panels when the grid is up and running…