PV Start Voltage

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rluongo
Topic Author

01 Aug 2013 19:27 #1733 by rluongo

PV Start Voltage was created by rluongo

Different inverters have different PV Start voltages. The PV Start Voltage of an inverter is the minimum voltage at which the inverter begins pulling power from the array. Per the OEM's technical representative on the matter copied from another PV online forum, "For future reference, yes the start voltage is absolute, on ALL inverters. If the inverter goes offline and does not have the required start DC voltage, it will not start."

The PV start voltage is not a field listed in the Inverter section in SAM, so I assume that SAM is not taking into account whether or not the inverter is actually on, per the OEMs PV start voltage spec.

Am I correct in this assumption? If so, how do you propose that users address this issue?

Thanks!

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Paul Gilman

01 Aug 2013 20:59 #1734 by Paul Gilman

Replied by Paul Gilman on topic PV Start Voltage

Hello,

For now, SAM makes a simplifying assumption that the inverter operates when the DC input power in kWh/h is greater than zero without accounting for the manufacturer's specified minimum voltage. In other words, it does not take into account the inverter DC input voltage (array DC output voltage) to determine whether the inverter operates.

SAM also does not prevent the inverter from operating when the array DC output voltage exceeds the inverter's operating voltage limit, although because that information is available in the inverter database, if that happens, SAM does display a warning message that you can access from the Results page telling you the number of hours that the limit was exceeded.

After running simulations, you can plot in a Time Series graph the hourly DC string voltage ("Subarray 1 DC string voltage (V)" and inverter AC output ("Gross ac output (kWh)" to see how often the inverter operates (Gross ac output > 0) during hours when the input voltage is below the manufacturer's specified start voltage. If it happens infrequently, that simplification is unlikely to have a large impact on the total annual output predicted by the model. If it happens for many hours of the year , you could estimate the reduction in the system's total output and use the "Percent of annual output" input variable on the Performance Adjustment page to account for the reduction. For example, if you think SAM is over-predicting the total annual output by 2% per year, you could use a performance adjustment factor of 98%.

We are investigating options for developing a more detailed PV performance model that takes into account these kinds of effects that we may include in a future version of SAM.

Best regards,
Paul.

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rluongo
Topic Author

01 Aug 2013 22:46 #1735 by rluongo

Replied by rluongo on topic PV Start Voltage

Paul,

Thanks for the response regarding the start-up up voltage, and yes this is in line with expectations, and the error is on order of 2%.

In the mean time I have found another, potentially larger issue. The maximum current of the inverter in question is 30A. This is included in the inverter input page (Idcmax), but I do not think that SAM is checking it. When I export the hourly data for energy, which for an hourly series is equal to power, and voltage, then calculate the amperage, the simulation in fact does go over the maximum amperage. Once I manually correct for the max amperage value of the inverter, the annual energy forecast drops by about 25%.

I believe that this client can rectify the issue by changing the layout of the strings to boost voltage, but my concern is that SAM runs the simulation in an amperage regime above the maximum of the inverter, then reports those energy figures as valid.

Thoughts?

Best,
Rocco

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