Solmetric PVA-1000S, User Manual

Page: 138 / 152

7 Interpreting Measured I-V Curves
7-10
5. Steeper slope in horizontal leg. The impact of partial shading on the I-V curve is to
create a notch. In a single PV string, the vertical height or current at which the notch
appears is equal to the reduced short-circuit current of the partially shaded cells. The
horizontal or voltage distance from V
oc
to the notch is related to the number of cell strings
within modules that have been bypassed.
PV Cells Are Damaged
In a cracked cell, a portion of the cell may be electrically isolated. This has the same
effect on the I-V curve as shading of an equivalent area of a normal cell. A notched I-V
curve can result depending on the severity of the PV cell damage.
Cell String Conductor Is Short Circuited
A localized hot spot, as discussed in X. Steeper Slope in Horizontal Leg, can also cause a
bypass diode to turn on, effectively removing a cell string from production.
2. Low Current
Potential causes for the measured I
sc
being higher or lower than predicted are summarized
below, and then discussed in more detail.
Potential causes located in the array include:
•
Uniform soiling
•
Strip shade
•
Dirt dam
•
Module degradation