Voltage Outputs

Hello SAM Team,

We are designing a utility solar project and we are running into issues of the inverter not being being able to operate at high enough voltage in multiple instances per year to satisfy dynamic reactive power requirements. This has lead us to dig deeper into what the SAM provided "Inverter MPPT 1 Nominal DC Voltage" and "Operating DC Voltage" mean as well as how they differ in the way SAM calculates them. 
Another related question: When looking at the results, we are noticing that by changing which inverter is being used, the voltage value which corresponds to the lower limit of the MPPT window changes. Shouldn't these DC voltage values be independent of the inverter?

I am happy to provide more context or more information if needed.

Thanks,
Aristide

Hi Aristide,

Assuming you are modeling a system with one subarray, the "Subarray 1 Operating DC voltage" and "Inverter MPPT 1 Nominal DC voltage" should be the same, and represent the inverter input voltage. For systems with more than one inverter, this assumes inverters are connected in parallel and operate at the same voltage.

For systems with more than one subarray, SAM calculates the inverter input voltage based on the option you choose under "PV Subarray Voltage Mismatch" at the bottom of the System Design page. By default, SAM averages the subarray voltages to calculate the inverter input voltage, but you can choose the "Calculate maximum power voltage for array and associated losses due to subarray mismatch" to have SAM iteratively determine a voltage that maximizes the array output (see the Help topic for the System Design page for a description). In this case, by default, the "Inverter MPPT 1 Nominal DC voltage" should be the average of the "Subarray [x] Operating DC voltage" values.

For systems with a single inverter and up to four MPPT inputs, SAM allows each subarray to operate at its own voltage. In that case the "Inverter MPPT [x] Nominal DC voltage" will be the same as the "Subarray [y] Operating DC voltage" as determined by the MPPT input assignments on the System Design page.

For your last question, the MPPT voltage window defined on the Inverter page determines the minimum and maximum value of  "Inverter MPPT [x] Nominal DC voltage." I am not sure I completely understand this question, so more context would be helpful.

Best regards,
Paul.

 

Paul,

Thank you for the response. We are modelling just one subarray and based on your explanation I have a few follow up questions.
1. Is it fair to assume that the input voltage being calculated is then not necessarily maximum power voltage? 
2. Could you provide a brief insight into the process by which that voltage (MPPT or operating DC voltage) is calculated?
3. Are there any conservative assumptions or multipliers that SAM uses to calculate the MMMPT voltage that may result in a decreased MPPT voltage?

Looking forward to your feedback.
Thanks again,
Aristide

Hi Aristide,

1. Yes, for most time steps, the inverter input voltage is the array's maximum power point voltage. Exceptions are when the array MPPT voltage falls outside of the inverter's rated input voltage window defined my the MPPT minimum and maximum voltage parameters, and when the array has more than one subarray (when the inverter input voltage is calculated based on the PV Subarray Mismatch Voltage option as I described above).

2. The array operating voltage calculation depends on the module model. These calculations are described in Gilman, P.; Dobos, A.; DiOrio, N.; Freeman, J.; Janzou, S.; Ryberg, D. (2018) SAM Photovoltaic Model Technical Reference Update. 93 pp.; NREL/TP-6A20-67399. ( PDF 1.8 MB ).

3. The operating voltage depends on the irradiance and weather data from the weather file along with the module input parameters and module model algorithm. I'm not sure how conservative those are, but they are intended to give a reasonable estimate of the operating voltage.

Best regards,
Paul.