Contributing Factors to Inverter Loss over nominal inv. power

[Tina Dornbusch]

Please help me understand what drives "inverter loss over nominal inv. power" in the loss waterfall. I was under the impression that this was essentially just clipping losses from high DC:AC ratio, but this appears to be incorrect based on project modeling to compare SMA Core1 inverters against SolarEdge inverters. 

For a project in California where I modeled this comparison, I kept all factors the same except the inverter and slight changes in DC:AC ratio due to different inverter sizes. Waterfall losses show: 

• SMA inverters, 1.18 DC:AC ratio: 4.5% inverter loss over nominal inv. power
• SolarEdge inverters with DC optimizers, 1.19 DC:AC Ratio: 0% inverter loss over nominal inv. power

For reference, I ran a similar project in New Jersey which shows: 

• SMA Inverters, 1.42 DC:AC ratio: 2.4% inverter loss over nominal inv. power
• Solar Edge Inverters, 1.37 DC:AC Ratio: 0.8% inverter loss over nominal inv. power 

Is there something in the optimizer function that is causing this substantial difference, and the discrepancy between impacts in NJ and CA?

I will also note that, in trying to understand the difference, I produced an hourly report of the California project with SMA inverters including all inverter losses, which actually left me more confused. I've provided, for reference, the results of summing the hourly values for different inverter losses and dividing by EGrid, which I assume should give us the appropriate loss percentage (but please tell me if I'm thinking of this wrong):

• InvLoss: 7.63%
• IL_Oper: 2.88%
• IL_Pmin: 0.00%
• IL_Vmin: 0.00%
• IL_Vmax: 0.00%
• IL_Imax: 0.00%
• IL_Nigh: 0.02%

Thank you for any help you can provide! 

[Robin Vincent]

Inverter loss over nominal power usually represents the energy lost due to the inverter max AC power. It can also include the energy lost due to the grid maximum power if the option "Account as separate loss" in the power limitation window is not selected.

Having optimizers in your system should have a very limited impact on the inverter clipping losses, as long as they are not limiting the max string power.

With SolarEdge, this will mostly depends on the number of optimizers per string.

In the example below, the max string power cannot be reached. The string operates at constant voltage (850V), but the max optimizer current is 15A so the max output power is limited to 12.75kW.

 

 

In this situation, the clipping that used to be necessary at inverter level will occur at optimizer level.

You can check this by looking at OP_Imax or OP_Pmax values, respectively, the clipping loss due to the max optimizer current and max optimizer power.

NB: There is a known issue in PVsyst 8.0.15 to 8.0.21 that overestimates the current clipping losses. If you are in this situation, please use PVsyst 8.1.0 or later.

 

In the case of the inverter, the percentages are expressed based on EArray, not Egrid. Could that explain the discrepancies you are seeing ?