Unexpected Differences Observed in Results - PVsyst v7.2.11

[kjs55]

PVsyst v7.2.11: The *only* change between the two (bifacial) models is a reduced Grid Power Limitation at the point of interconnection POI (grid injection point). Nothing else changed in the models (carefully confirmed).

Differences in losses before the inverter:

1. Observed in exported numeric loss tree:

- View Factor for rear side (BkVFLss) loss

- Inverter Loss during operation (efficiency) (IL_Oper)

- Inverter Loss over nominal inv. power (IL_Pmax)

2. Observed in PDF output report table named "Balances and main results":

- Effective energy at the output of the array (EArray)

Also, several losses after the inverter (before the grid injection point) changed, which I won't list here unless someone requests it. Thanks.

[Michele Oliosi]

Hi,

I am not sure whether you have already, but you can drop us an email at support@pvsyst.com, ideally including your project and the two differing variants / reports. We will then be able to review what is going on.

[kjs55]

Thanks for your quick reply.

1. Unfortunately, I cannot share the models, b/c the project details are proprietary.
2. I'm curious if the four model outputs I listed are expected to change (in theory) as a result of the one (single) changed model input that I described.
3. If I have time (no guarantee), I'll see if I can reproduce it using the PVsyst Demo bifacial model. In the meantime, please feel free to do the same exercise, especially if the answer to #2 is "No".

[Michele Oliosi]

I see. There is no reason that the BkVFLss changes. What was the magnitude of the change ?

For the losses after and at the inverter yes a change is possible. Indeed, what PVsyst computes is first the power without limitation, and then one readjusts the inverter clipping to obtain the desired (limited) grid injection. Therefore the losses in between may change. Basically, the grid limitation is anyway computed at the level of the inverters.

[kjs55]

Upon reducing Grid AC Power Limitation applied at POI (-9.27%), these changed by the following relative amounts:

BkVFLss: -0.01%

IL_Oper: -0.1%

IL_Pmax: -0.01%

EArray: -1.11%

Plus changes to several losses after the inverter. Would you please comment on EArray? Thanks again.

[Bruno Wittmer]

EArray is the DC power at the inverter input, after the inverter has chosen its operating point on the IV-curve of the PV array.

Therefore it will change if you change the Grid AC power limitation, since the inverter clipping will change.

EArrayMPP is the maximum DC power before the inverter sets its operating point. This value should stay unchanged when you choose a different grid AC power limitation.