- The PR calculation is performed according to IEC 61724, as the AC system yield divided by the plane of array insolation. You can recalculate using the Parameters available at the top segment of Browser of our monitoring platform: (Power exported site/Nominal Power DC)/(Irradiance POA Average/1000).
Power export site measurements are consistent. They originate directly from the main meter and they are similar with some normal losses with the inverter measurements. You can cross-check against the sum of measurements provided by the inverters, as parameter “Power Output Inverters AC”.
Consequently, the problem should be attributed to the irradiance values measured by the pyranometers
- It is generally accepted and a common practice to calculate Performance Ratio when the PoA irradiance is over a threshold - usually set over 100 W/m2. This is because the PR values for low irradiances may be irregular, usually due to the nonlinear behavior of the solar modules and inverters at low irradiance levels. This threshold is a setting in the system, with a default value of 200 W/m2. Setting a very low value for irradiance threshold (e.g. 10W/m2) may cause Performance Ratio values over 100%.
- Shaded and/or dirty pyranometers.
- Pyranometers' inclination and/or azimuth compared to the panels could affect the correct PoA irradiance measurements and consequently the Performance Ratio.
- Another reason for low irradiance values or the reason leading to the high PR values could be the high relative humidity. Humidity could affect the pyranometers’ accuracy. It is recommended that you contact your pyranometers’ vendor to further analyze this behavior.
Modified on: Thu, 30 May, 2019 at 10:27 PM
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