Array MPP Below Inverter MPP Range?

[bradley_borrego]

Dear PVsyst and forum users,

I've searched the forum for an answer to this question and found several discussions that are similar but none that match exactly.

With a fixed tilt array with racks with single modules in landscape with a very high GCR (small row - row spacing) such that there is significant interrow shading for 3 - 4 months of the year and for the entire day for some of those months, how does PVsyst treat the array voltage during those shaded times when the lower 2 rows of cells will be shaded to some extent.

My understanding is that because almost every module and every string is shaded (aside from the first row) the module/string/array voltage would drop by approximately 1/3 of Vmp (I understand that this is dependent on exactly how much shading is occurring and the ratio of direct and diffuse irradiance). Depending on the inverter the array MPP may be outside the MPPT range and the inverter will track to the lower end of the voltage range and hold there regardless of the array voltage, which depending on inverter and array IV curve could have significant impact on the array power output.

Does PVsyst address the voltage of a shaded array in this manner? The hourly output during shaded periods seems to indicate an array voltage very similar to unshaded periods and doesn't seem to be dependent on shading.

Am using V5 of PVsyst.

Thanks in advance.

Regards

Bradley

[André Mermoud]

This is treated in the "Module Layout" electrical shadings part.

After defining the Module Layout, you can perform a pseudo-simulation and observe the behavior of the I/V curves under shadings along the day.

You are right, the MPP voltage will drop, in about the same ratio as the number of submodules shaded. This is the reason why with sheds arrangement, the "beam" production is completely lost as soon as the first bottom cell is shaded (see our FAQ "How to evaluate the effect of by-pass diodes in shaded arrays").

And indeed the MPP voltage often drops below the VmppMin of the inverter.

Up to the version 6.32: the voltage was clipped to the VmppMin,

Since version 6.33: if the Vmpp is under the voltage limit, PVsyst will search for the secondary maximum (which corresponds to the MPP for the diffuse).

In the yearly simulation, both options give very close results.

However the Module layout electrical shadings are not treated in the version 5.

[TumB0]

Depending on the inverter the array MPP may be outside the MPPT range and the inverter will track to the lower end of the voltage range and hold there regardless of the array voltage,??? Edited April 2, 2015 by TumB0

[André Mermoud]

Yes, this is the way of running of the PVsyst simulation, and probably of most inverters.

I don't see what could be the meaning of the Vmppmin specification if this were not the case.

If you choose "Cut" instead of "Limitation" for the behavior at VmppMin/Max, the inverter will stop.