Importing DNI

[cvernay]

Dear M. Mermoud,

I try to convert 8760 hourly values of DNI to a .MET file. The input ASCII file only contains DNI + ambient temperature + wind velocity.

It turns out that the DNI used in the processed MET file is slightly different than the initial one, cf. the enclosed jpeg file: some values between 0 and 60 W/m² are set to 0 while other are reduced. Finally, on this example I loose 0.3% of the initial DNI. This value is not very high, but I don't know what would be the result with another example.

Is it a known issue? Is it possible to prevent such a modification?

I am using v5.72.

Best Regards

Christophe Vernay

DNI before Vs. DNI after PVsyst processing

[André Mermoud]

This is typically a time shift between the time definition of your data and the internal time of PVsyst.

Please analyze your meteo file in "Meteo tables and graphs" / "Check Data quality".

You have detailed explanations in the help "Geographical and Meteorological data > Hourly meteorological data > Hourly meteo data quality check" and " >....> Time shift".

[baltide]

Bonjour M. Mermoud,

Cela signifie que PVsyst ne permet pas à ses utilisateurs de rentrer leur données GHI/DNI sans que PVsyst ne fasse un "ajustement" bien que minime sur le DNI, je comprends bien alors que GHI = DHI + DNI * sin (alpha )..dans ce cas la composante temporelle (Time shift) ne devrait être impactant que sur le DNI, hors dans mon cas j'intègre à PVsyst un GHI/DHI et comme cvernay j'obtiens une légère différence entre le DHI_importé dans PVsyst (format horaire sur un an) et le DHI_utilisé_par_PVsyst ?

[André Mermoud]

Once more: If PVsyst applies a correction, it is because the data are not correct, or not correctly imported due to a time shift.

This has implications on the calculations during the simulation, and should be corrected.

The errors usually arise when importing POA measurements, and are due to the fact that if the retro-transposed Global or Beam horizontal values are higher than the clear day model (by a significant amount which is specified in the hidden parameters), PVsyst will limit it. This will arise either in morning or in evening hours, when the sun is low on the horizon (low sin(HSol) varying quickly with the time). With even little time shifts you can easily get Normal beam components of 3000 to 5000 W/m² at the sunrise or sunset. How PVsyst will manage this during the simulation ?

However I don't understand well your assessment that you have differences with Ghi/Dhi, as in this case there are no calculations during the import.

Except if for some hours your Diffuse part is greater than the global part: this doesn't make sense and the simulation will not understand that !!!