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System Advisor Model (SAM)

SunPower X21-345 MPPT voltage limits warning

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Tom Durston
Topic Author

03 Aug 2018 18:25 #6376 by Tom Durston

SunPower X21-345 MPPT voltage limits warning was created by Tom Durston

When designing PV with the SunPower X21-345-C-AC modules and microinverters with the same model name, the SAM System Design page returns a warning stating "For this combination of module and inverter, it is not possible to size the string to keep string Voc and Vmp within the MPPT voltage limits." The simulation has a significant MPPT power loss.

SunPower does not provide data on the module TCVoc or the microinverter MPPT limits, so I can't check the calculations.

For a workaround, I entered an Inverter Datasheet inverter with most of the data that SAM has for the X21-345-C-AC microinverter, but with the MPPT window widened. Is this solution OK, or do you think the X21-345-C-AC microinverter really does clip power significantly because of MPPT limits?

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pgilman

06 Aug 2018 16:48 #6377 by pgilman

Replied by pgilman on topic SunPower X21-345 MPPT voltage limits warning

Hi Tom,

The warning message on the System Design page is based on the nominal, or nameplate, values from the module and inverter specifications. You can check the simulation results to see if the operating Voc exceeds the MPPT limits: On the Results page, go to the Statistics tab, expand the list under "Hourly Data" and look for the maximum value of "Subarray 1 Open circuit voltage." You can also plot that variable on the Time Series tab or Heat Map tab to see how often the value exceeds the MPPT limit. (On the Heat Map tab, change the color scheme from "Jet" to "Coarse Rainbow" to see the hourly values more distinctly.

You can also see the amount of clipped energy on the Losses tab, or by displaying the "DC inverter MPPT clipping loss (%)" or "Inverter clipping loss DC MPPT voltage limits (kWh/yr)" variables on the Data Tables tab.

In your file, it looks the SAM simulation results show no MPPT clipping for the West roof, and the East roof has MPPT clipping of about 1% of total annual energy.

Best regards,
Paul.

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