Inverters in database with same max DC voltage as max MPPT DC voltage?

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dcorning
Inverters in database with same max DC voltage as max MPPT DC voltage?

Hey, when modeling a specific system configuration, we came across a very low kWh/kW yield. Everything appeared to be in order, in that the correct models for module and inverter (in this case the Northern Electric: BDM-300X2) were selected, and system design was all in order.

For this inverter the max MPPT voltage is 48V but the maximum input voltage is 60V. It appears in the database to be set to 48V for both max DC input voltage and max MPPT DC voltage, which we believe is causing an error in modeling the kWh produced by the system.

This isn't an issue we've come across with to date, is it possible the database was refreshed with incorrect values for Max DC voltage? It appears to be equal to Max MMPT DC voltage for the majority (but not all) inverters on the list.

Thanks!
Dave

Paul Gilman

Dear Dave,

SAM treats the inverter minimum and maximum MPPT ratings as the minimum and maximum operating voltage. If the array operating voltage is less than the minimum MPPT rating, it sets the array voltage to the minimum rating. If the operating voltage is greater than the MPPT maximum voltage rating, it sets the operating voltage to the maximum limit. It reports the resulting power loss as "inverter voltage clipping loss." The latest inverter database from the California Energy Commission does not report separate MPPT voltage ratings, so we treat the minimum and maximum voltage ratings from the database as the MPPT operating voltage limits.

On the system design page, you should choose the number of modules per string (string length) to avoid or minimize voltage clipping loss. You can use the "System Sizing Macro" to generate a report to determine a good string length for the system.

I'd be happy to review you design if you don't mind either attaching your .sam file to your original post above or emailing it to me.

Best regards,
Paul.

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