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# BUG: Modelling Dual MPPT

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Hi guys!

Here's my problem.

Program Version: 6.39

Modules Used: Canadian Solar CS6P 260W

Inverters Used: Fronius Symo 24.0-3/480 (US)

So I'm trying to model a split MPPT system where i have 2 orientations (same inclination but different azimuth).

I'm doing 3 strings of 20 panels each on MPPT 1

and 2 strings of 20 panels each on MPPT 2

But when I place these on PVsyst it shows me several distinct errors

1. Maximum PV Power listed is 24.8 (I'm at 26 kW)

2. Nominal AC Power is 27.3 (where did PVsyst get this)?

3. Number of Inverters 0.9

4. Secondary MPPT Nominal AC Power is 20.7

5. Error shown on Secondary Inverter: This array has 5 strings to be distributed onto 3 inverters!

It's driving me nuts!

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You have chosen an inverter with a special feature named "Unbalanced Inputs". This means that you have two different MPPT inputs (in this case, with Imax = 49.5A and 37.5 A respectivaly), named "Main" and "Secondary" inputs.

With such special inverters, PVsyst requires that you define two different sub-arrays, one for each kind of input.

In your case you will define the first sub-array for the main MPPT input, 3 strings and the corresponding orientation.

And a second sub-array for the secondary MPPT input, 2 strings and the corresponding orientation.

NB: You can add sub-arrays either by specifying the number (top left) or by right-clicking on the sub-arrays page titles.

NB: The fractional and strange PNom values are due to the fact that the PNom of each input is shared as function of the Current's shares: 49.5A and 37.5 A lead to PNom = 13.7 kW and 10.3 kW.

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Great! I totally get it now! I was under the assumption that for split MPPT's you can define in a single array multiple combined orientations and just adjust string distribution below. Thanks for this!
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You have chosen an inverter with a special feature named "Unbalanced Inputs". This means that you have two different MPPT inputs (in this case, with Imax = 49.5A and 37.5 A respectivaly), named "Main" and "Secondary" inputs.

With such special inverters, PVsyst requires that you define two different sub-arrays, one for each kind of input.

In your case you will define the first sub-array for the main MPPT input, 3 strings and the corresponding orientation.

And a second sub-array for the secondary MPPT input, 2 strings and the corresponding orientation.

NB: You can add sub-arrays either by specifying the number (top left) or by right-clicking on the sub-arrays page titles.

NB: The fractional and strange PNom values are due to the fact that the PNom of each input is shared as function of the Current's shares: 49.5A and 37.5 A lead to PNom = 13.7 kW and 10.3 kW.

Is there no way to simply bond the 2 MPPT's for roofs that have exactly the same orientation and pitch? I've tried clicking the option to not use the 2nd MPPT but when I save the system detail it says that I have x remaining MPPT's unused. I was under the assumption that by clicking the option to not use the 2nd MPPT, I bonded it with the first (and this is actually supported by the inverter going to it's full Pnom and the sizing factor corresponding to that of a single inverter with a bonded MPPT.

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With such an inverter, this is not advised (or not possible) due to the current limitation.

However with other inverters (those with the secondary input dedicated to one only string, and when it is allowed by the manufacturer), you can indeed use only the main MPPT input. In this case you should inform PVsyst by using the "Adjust" button.

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