DC vs Gross DC vs Nominal DC

I need some help with determining how SAM treats each of these in a detailed PV model. I'm trying to do some comparative modeling of vertical vs tilted systems and thought "Annual DC" would be what I would use to compare system outputs.

However, the DC answers I get seem to change depending on what inverter is selected (even though I don't care about inverter right now and want to eventually feed these results into a separate inverter model in Pysam). I've worked out that "nominal" seems to be the only one not impacted by inverter selection and also doesn't take into account the flat DC losses in the losses tab. Unfortunately it only seems to be an annual value with no monthly/hourly/half-hourly data. 

Can anyone clarify what is happening in the steps from Nominal DC -> Gross DC -> DC, why there seems to be inverter dependency, and whether "Net DC" is also the same thing as "DC" here? Or possibly how to peek into the actual code for further answers?

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.

 

Selecting different sized inverters changes Annual DC and Annual Gross DC values

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.

I would think that clipping would be a factor in going from DC to AC outputs, but I suppose that could just be overall power clipping, and it's the voltage clipping that is changing the DC values. Am I to understand that Nominal DC is what the module outputs while Gross DC is what goes into the inverter AFTER being clipped down to the appropriate voltage (and applying the flat DC losses)? If that is the case, can you clarify what happens between "Gross DC" and "DC"? 

I was hoping to just compare DC outputs of different systems while ignoring the inverter (planning to feed that data into a separate inverter model later). It sounds like if I truly want independent DC data without the inverter involved I have to use the nominal values, but unfortunately it only looks like I can only get that in an annual value and not an hourly data set.

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.