PV Inverter Datasheet - Detailed PV System Design interaction

Lead in remarks: I wouldn't be asking this question of the Inverter CEC Database had a complete set of parameters for the Enphase S-280 240 Vac microinverter. The database entry is missing values for Mppt_low, Mppt_high, and Vdcmax. Other inverter models also suffer from this shortcoming. So we can get around this issue by using the Inverter Datasheet window...

Enter the data for the Enphase S-280 microinverter into the inverter datasheet window as:
Max ac power: 270; Weighted efficiency: 97; Nominal AC voltage: 240; Maximum DC voltage: 48; Maximum DC current: 10.3; Minimum MPPT DC voltage: 27; Nominal DC voltage: 32; Maximum MPPT voltage: 37; Power consumption during operation: 0; Power consumption at night: 0.06.

Module is chosen as: REC Solar REC280TP BLK

System Design is 1 module per string, 10 modules, 10 inverters

Sizing Messages: Actual DC to AC ratio is 1.04.
The string voltage exceeds the inverter maximum MPPT voltage at reference conditions. Consider using fewer modules per string.

But: string Vmp is 31.8 V - well inside the inverter tracking window. However, string Voc is given as 38.7 V. I can eliminate the "excessive voltage" message by changing the inverter Mppt_high to 39. This seems to indicate that the "excessive voltage" message was caused by comparing the string Voc to the inverter Mppt_high instead of comparing the string Vmp to Mppt_high.

This apparent error occurs in SAM version 2016.3.14, as well as the previous version. I don't mind the "excessive voltage" message if that is the only result of this apparent error, but it is certainly confusing. It does seem out of step with the care that it took to get SAM to the current level of quality and functionality.

Sample file is attached, but will require replacing my local solar resource file with anything else.
BTW, the new utility rate page works quite well to model the PG&E NEM 2.0 tariff that lowers the sell rate by a few cents to exclude certain "non-bypassable" charges in the new NEM 2.0 tariff that has been submitted to the CA CPUC for approval.

Thanks for the great work on SAM,
Bob Mason

Dear Bob,

Your observations are all correct. Keep in mind that the sizing messages on the System Design input page are meant as suggestions rather than as strict design requirements. SAM makes a very conservative, worst-case-scenario assumption when it compares the string Voc to the inverter MPPT window voltages for the sizing message. If you are comfortable with your design, you should ignore the message. SAM will not prevent you from running a simulation if you design a system that falls outside of the conditions it uses to display the sizing messages.

You can use the inverter clipping loss results to see how much clipping occurs in the operation of the system, and make adjustments to your design based on those results rather than on the design ratings. For that reason, you may want to keep the MPPT window voltages at their correct values so that the inverter clipping losses in the results are more meaningful. You can see those in the hourly results, on the Results page either as a graph on the Time Series tab or in a table on the Data tab (the total annual clipping loss appears on the Data tab under Single Values, and in the loss diagram on the Losses tab).

Also, you may be interested in the Inverter Sizing Information macro, which gives more sophisticated feedback on your system design based on both the design ratings and simulation results. To run it, click the Macros button at the bottom left of the SAM window, and then click Inverter Sizing Information.

We will take your feedback into consideration as we explore ways to improve the system sizing messages and other aspects of the photovoltaic model.

Best regards,
Paul.

Thank You for the Data, I used yours as a crib to enter the data for the IQ6+ with Solarworld 340

On this topic... I ran into some behavior I did not expect with respect to mppt voltage. The inverter I was using is the SB3800TL-US-22. There are two entries in the CEC library for this inverter, 2013 and 2014. By selecting the 2013 inverter, the operating voltage of string was limited to 250 V minimum, taking the string off of its maximum power point. Selecting the 2014 inverter allowed the voltage range to extend down to 175 V, keeping the array at its maximium power point and increasing the system output during the hot summer days that forced the Vmp lower.

I saw no error or indication that the Vmp was being clamped to the minimum, except when I drilled into the hourly output in the data table. Perhaps a message similar to the Voc out of range message on the system design screen would be appropriate?

Also, and maybe this is more of a CEC problem, but the actual MPPT operating range for that inverter is 125-500 V, per the datasheet. The *rated* range is 175-480 V, which means the inverter's efficiency outside of that rated range (but inside the operating range) may be less than what the CEC efficiency calculation suggests, but it would still seek the maximum power point. I have no suggestions on what to do with that since the CEC library is what it is, but just wanted to point it out.

Hello,

You are correct that SAM does not report errors based on operating voltages. That is something that we will consider adding to a future version of SAM.

We are investigating several issues with the CEC module and inverter data. There have been some changes in the organizations managing those databases, and we are working on determining how to address those issues.

Best regards,
Paul.