DC vs Gross DC vs Nominal DC

Hi Jeremiah,

Please attach a copy of a .sam file that demonstrates this behavior if you would like me to help troubleshoot.

Best regards,
Paul.

Hi Jeremiah,

Attached is a modified version of your file. I created two cases that are identical exept each uses a different LeadSolar Energy inverter.

The "0.8bifacial DC/AC = 1.9" case uses a 606 Wac inverter with minimum and maximum MPPT voltage ratings of 52-56 Vdc. If the array operating voltage exceeds 56 Vdc, this inverter clips the inverter input voltage to 56 Vdc:

 

The "0.8bifacial DC/AC = 1.6" case uses a 708 Wac inverter with minimum and maximum MPPT voltage ratings of 50-64 Vdc. This inverter clips the inverter input voltage to 64 Vdc.

You can see the effect of inverter clipping in the results by displaying the following variables in the tables on the Data Tables tab or plotting them on the Time Series tab:

• Subarray 1 Operating DC voltage (V)
• Inverter DC input power (kW)
• Inverter clipping loss DC MPPT voltage limits (kW)

The message on the System Design input page under "Electrical Sizing Information" explains that based on the module's nominal voltage ratings, it is not possible to size the strings to avoid clipping. To avoid clipping, you can choose a module with a rated Voc (open circuit voltage) less than the inverter maximum MPPT voltage rating, or you can choose an inverter with a maximum MPPT voltage rating greater than the module's Voc rating.

 

Best regards,
Paul.

Hi Jeremiah,

In the hourly results, Subarray 1 DC power gross (kW) is the output power of Subarray 1 and Inverter DC input power (kW) is the inverter input power. SAM applies the DC loss from the Losses input page to the subarray gross DC power to calculate the inverter input power. For a system with more than one subarray, the inverter input power is the sum of the subarray gross power values adjusted by the DC loss.

Subarray 1 Operating DC voltage (V) is the DC voltage of the array, and Inverter MPPT 1 Nominal DC voltage (V) is the voltage at the inverter input. For systems with one subarray, the Subarray 1 operating voltage is the array operating voltage, and inverter MPPT 1 voltage is the inverter input voltage. For systems with more than one subarray, each subarray's operating voltage is reported separately, and the inverter input voltage depends on whether the inverter has more than one MPPT input, and the option you choose for PV Subarray Voltage Mismatch at the bottom of the System Design page.

If you want to remove the effect of inverter voltage clipping, you can set Vmppt_high on the System Design page to a value higher than the array's maximum operating voltage. To avoid inverter power limiting, you can choose an inverter with a higher power rating.

Best regards,
Paul.