2 strings (different in lengths) connected to one MPPT

[K24]

Hello,

I have a general technical question; is it ok to connect 2 strings with different modules number to one MPPT (18 and 19 for example when using the polystring feature in PV sol).

I've read on some forums that there are losses due the voltage mismatch and the MPPT tracks the string with the lowest performance; but is it really a red line for PV designers or it's just not preferable and I can do it only when necessary?

 

Thanks in advance

Karam

[developer_mh]

Hi Karam,

no, I would not say that connecting strings with different module numbers is a red line in general. Why not try it out in PV*SOL directly? We calculate the losses due to string mismatch with great accuracy, so you can just see how much energy you loose in you PV system and see for yourself if you can accept these losses.

You can also see the resulting IV characteristics of your parallel strings in the results page (under Simulation -> Diagram Editor -> Type of Diaram: Characteristics)

Kind regards,

Martin

[K24]

Hi Martin,

Thanks for the your helpful response!

I used the feature to compare the IV curves as you mentioned. Do you (or any colleague in this forum) have/has an explanasion for the abnormality in the IV curve of MPPT 2 ? (please see the attached screenshots) 

 

Beste Grüße

Karam

[developer_mh]

Hi Kamal,

these kind of bends occur when you have unequal number of modules in your strings. The curve section from 0V to the point where the bend occurs is where the two strings both deliver a current for the given voltage. After the bend, up to the open circuit voltage of the longer string, only the longer string (with 21 modules) can deliver current, the other string is not contributing anymore.

Kind regards,

Martin

[Charita]

Dear Both,

Thank you so much for your information and I’m have if connect strings with different modules number to MPPT (example MPPT 1-2 connection 20 modules per string and MPPT 3-10 connection 18 modules per string) I would like to know if different modules number per string but design separate MPPT will be affect?

[developer_mh]

Hi Charita,

I am not sure if I understand your question correctly. But connecting 20 modules per string to MPPT 1-2 and 18 per string to MPPT 3-10 should be no problem. It all depends on the PV modules and the inverter with their respective electrical data. But if you enter your system design in PV*SOL, you will get precise feedback if your configuration is good or not.

Hope that helps, kind regards,

Martin