Sunny Island 6048-us10 with mnx-240 autoformer question

[wrhartlerode]

I've been using a SI 6048 which has been a/c coupled with an older SB 6000 in an offgird setup with no access to the grid. I used a mnx-240 autoformer and made an e-panel with surge protector. Everything has worked fine on it, but I am upgrading the system a bit. I would like to replace the SB 6000 with a sb 7.7 us 40 that I have or a sb 5.0 us 40, or possibly both? My question is if the mnx 240 autoformer would prevent me from switching to a sunny boy 7.7 us 40. I found a diagram stating it could be max 6kw. I like the idea of having 3 mppt and getting the benefit of oversizing my pv array. I only want to replace the 6000 with the 7.7 us 40 because I have it left over from another project. I guess the other route I could do is buy another SI 6048, and then I could run both the 7.7 and 5.0 and increase my pv array even more. The other option is that I don't need the 240v for my cabin now. I was wondering if I could use the SI 6048 and Sb 7.7 us 40 withought the autoformer and just run120v, but also the benefit of oversize pv. Does anyone have experience with this? Wanting to run it off grid coupled with the sunny boy. I do not have grid power at the place. Just trying to maximize my system using what I have.

 

[Hedges]

First thing to determine is if your PV array is subject to "PID", potential induced degradation. That happens when PV cells are biased below ground (or for some types above), and ions are pushed into the cell, poisoning it.

SB 6000US can be configured either positive or negative ground. (Older SunPower panels required positive ground, most others prefer negative.)

Does the autoformer already have a fan? That would boost power handling somewhat.
With excess power available, you run the risk of transferring too much through the auto transformer and overheating it.
The extra power could be used for 240V loads, or loads on second phase.
Maybe if you programmed a charge limit in SI and limited loads on each phase you'd be OK, but some risk.

Victron makes a transformer with current sensor and relay for protection.

Consider picking up a second SI 6048, often discounted on eBay, and some used.

More MPPT only matters if different length strings (or one heavily shaded)
You can over-panel the 6000US with multiple orientations, so long as all strings same/similar Vmp.

 

[wrhartlerode]

Yeah, I may consider just seeing what the 6000 can handle. The new spot i'm moving the system to has less shading issues and I think it may do better. Never heard of PID, i'll look into it.

 

[Hedges]

Consider a breaker, and a thermostat.
Dryers have thermostats good for maybe 30A, 240v.
I bought some snap-action and some manual reset, for the specific purpose of controlling fan cooling for a transformer, and shutting down if it got too hot.
I pushed a significant amount of power through a small toroid while blowing air over it.
I have some 9kVA toroids I'm going to use for balancing.

You can repurpose a step-down transformer as auto-transformer. Don't use the 120V secondary windings. Use two 240V primary windings, connected in series 240/480V, for 120/240V.