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Internally, the cells of a PV module are usually connected in series.

This means that the voltages of each cell is added, and the PV module voltage is the sum of all voltages:

V(Module) = V(cell) * NCells in serie.

SubModules

However in case of a deficient or shaded cell, such a big series may lead to a Hot spot effect. In case of high imposed current, the deficient cell may be forced to absorb the total power produced by the cells in series with it, which may be dangerous for the cell.

This is the reason why we have to put a so-called "By pass diode", i.e. a reverse-biased diode deriving the reverse current possibly imposed to the chain. In this situation the damaged cell will only have to absorb the power produced by the concerned section of cells.

In practice, the maximum number of cells for a safe Hot-spot protection is about 20 to 24 cells. Therefore in any PV module, we have a partition in groups of cells, each group protected by a by-pass diode. These groups are named Sub-modules.

The Submodules Layout  parameter in the PV module definition describes the distribution of submodules..

Electrical shadings

Therefore the main objective of the By-pass diode is to protect the PV module.  

However the By-pass diodes have also an effect on the electrical shading losses. The submodules are indeed the basic unit when analyzing the electrical shading losses an an array.  Whatever the number of  shaded cells in a submodule, the resulting I/V curve of the full submodule is quasi-identical. Only the Pmpp voltage is slightly different, but in an array the shaded submodules or strings rarely operate at MPP.

Therefore, when analyzing/constructing the I/V curve of a shaded array, the "Module Layout" tool only deals with sub-modules characteristics (either shaded or unshaded). The individual cells may be completely forgotten.

 

This plot is available in PVsyst  "Tools > Electrical behaviour of PV arrays > Array with shaded cells".

Here you can choose "Number of shaded cells effect > One sub module".