SMA rapidshutdown and SMA 6.0 wiring question

[madiaz91911]

Designing my system. 1 row of 21 panels. Going to an SMA Rapid ShutDown. Do I need to add balanced input voltages? Since I have three strings, Can I use three of the input, each with 7 panels. So the output would not be balanced. Input into my SMA 6.0 inverter would be: Imput A would have DC max= 591V @ 9.3A and Input B 296V @ 9.3 A. can I do that?

Thanks

Mike D

[sensij]

If you put 14 panels (two 7 panel strings in parallel) into RSD channel A, that is 18.6 A @ 296 V, if I'm understanding your numbers correctly. RSD channel B would get the other string. I think the inverter will accept that configuration using just mppt A and B, but double check.

[madiaz91911]

The shutdown has 4 inputs. two for each channel (A & B) so, if you connect two strings to say channel A (in ShutDown) internally they get tied in parallel. Hmm, you are right. That would mean 18.6 @ 296... I was typing out loud. So my question is, can the SMA inverter accept that configurartion.... as you stated. can anyone out the confirm that. SMA only speaker to authorized dealers/installers. Thanks for the reply.

[madiaz91911]

Seem like I found the anser. Can anyone confirm. The max amp input for any string on the SMA 6.0 is 10A. So, that means I would need to use two SMA shutdown boxes. Correct? Two strings in one box and one to the other.

[sensij]

Seem like I found the anser. Can anyone confirm. The max amp input for any string on the SMA 6.0 is 10A. So, that means I would need to use two SMA shutdown boxes. Correct? Two strings in one box and one to the other.

Maximum short circuit input is 18 A.

I think if you wire it like this (from page 36 of the installation manual), with the RSD performing the parallel connection, you will be OK, since the combined current is carried by both mppt's A and B.

SMA wiring.JPG

[DaveDE2]

You might want to run just two strings of 10 and 11 panels and two MPPts. The inverter will run more efficiently with higher string voltages. If I understand your numbers correctly, this would lead to DC max of 422V for one string and 464V for the other and nominal operating voltages more in the sweet spot of the inverter for max efficiency (see attached chart). I'm guessing about 375V nominal operating string voltages. If you run string voltages too low, you may be giving up a percent or two in inverter efficiency. I have a system of 51 panels and two Sunnyboy 6.0's and two RSDs but only used two MPPts in each inverter for efficiency reasons.