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My transposed POA values don't match the imported values
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Author:  AndrĂ© Mermoud [ Thu Aug 23, 2012 12:27 pm ]
Post subject:  My transposed POA values don't match the imported values

In PVsyst, the import of POA values uses a retro-transposition (with Hay model) and creates a file with GlobHor and DiffHor hourly values.
During the simulation, the transposition should indeed restitute exactly (within epsilon) the input POA values, provided that the input data are correct and correctly imported, and you use the Hay model in your simulation.

An error in the time defintion may produce very high errors in the morning or the evening, when the sun's height is low (as the POA beam has to be divided by sin(Hsol)).
In case of errors the horizontal beam in the early morning may become several tausends of W/m2... PVsyst limits it to the Clear day model, and this is the main source of your "errors".
(I have event seen POA bad data with significant values before sunrise or after sunset... )

Please carefully check your meteo file using "Tools" / "Meteo Tables and Graphs" / "Check data quality". If there is a time shift you should correct it already at the import time (as the retro-transposition depends on the solar geometry of course).
You can also verify whether your POA and transposition values are identical:
- Either using the Table in Hourly values, and display both POA and Transposed values
- Or with this table, by producing a CSV file and check (in EXCEL) which hourly values are not identical: these will only be morning or evening values!

For correcting the time step you have several possibilities:
- The importing format protocol has a specification "Interval Beginning" and "Interval End", corresponding to the time step definition of your original data.
NB: If your data time step is less than 1 hour, this will act on the interval itself (i.e. produce a time shift of 5 minutes for 5 min steps).
- You can change the timezone of the site used as geographic reference.
- You can eventually shift the column of hours by one hour in your original data file (be careful with 24H00 and 0H00),
- Please make sure that you have defined "Legal Time" and not "Solar Time" as time base.
"Solar time" is the time definition for which the sun passes through south at 12:00 (days don't have 24H anymore). It is never used.

Additional information
You will find more information in the help of the software:
"Meteo Database > Notes on Meteo > Meteonote8_Hourly data quality check" and " ... > Meteonote9_Time shift in meteo files".
Or simply press F1 from the "Check data quality" page.

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