Grid power limitation alters inverter efficiency

[S.Faulkner]

I've noticed that in 6.47, setting a grid power limitation affects the inverter efficiency loss, as well as the Inverter over nom. power (clipping loss). The combined loss seems to be correct (see below), but is it correct that the efficiency changes? I noticed this when I calculated results with and without the grid limitation, so that I could find out how much of the clipping occurs at the inverter and how much at the grid injection point (this affects optimization of the inverter quantities).

Case 1 Without grid limitation:

Inverter Loss during operation (efficiency) -1.68%

Inverter over nom. power (clipping loss) -0.91%

Case 2 With grid limitation:

Inverter Loss during operation (efficiency) -2.12%

Inverter over nom. power (clipping loss) -1.36%

The inverter Euro efficiency is 98.35% so the loss should be around 1.65%, ie the efficiency loss for case 1 (without the grid limitation) looks correct.

I exported the hourly data for each case, and found that if I apply the grid limit in a spreadsheet (apply a cap to E_grid) the additional loss is -0.88%. Combining this with the efficiency and clipping loss for case 1 gives a total loss of -3.44%, and the total for case 2 is -3.46%, ie the total loss appears to be correct.

(I previously added this comment on the end of the Grid Power Limitation post but I thought may be better to raise it as a new topic)

[André Mermoud]

This is quite normal.

If you have a grid limitation, the inverter works more often in the lower part of the powers, where the efficiency is different.

When analyzing your hourly values, you will see that hours without power limitation have the same efficiency.

[S.Faulkner]

Thanks for explaining this, but I don't understand. You seem to be saying that the drop in efficiency is because inverter is more often operating at a lower power, but the difference is very small and the change in efficiency near maximum power doesn't vary much at all. In the case below, the grid limitation is only a few percent below the inverter maximum output, not enough to change the efficiency. Do you mean that changing the IV setpoint to achieve the grid limitation puts the inverter into a lower efficiency operating range?

My feeling is that this isn't useful, and I wonder if others agree. I would like to know the efficiency that is achievable by the inverter without grid limitation, and then the loss that occurs as a result of the grid limitation. But is the assessment done this way so that the effects of the curtailment method are taken into account, such as voltage or current going outside of the inverter's allowable range?

[André Mermoud]

Thank you for your project.

I have indeed found the error in the simulation. This affects the way of distributing the losses (some part of oveload is transferred to the efficiency loss), but doesn't affect the final result.

I have corrected for the version 6.53.