Solmetric PVA-1000S, User Manual

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7 Interpreting Measured I-V Curves
7-4
A normal I-V curve has a smooth shape with three distinct voltage regions as shown in
Figure 58.
1. A slightly sloped region above 0 V (the horizontal leg of the curve)
2. A steeply sloped region below V
oc
(the downward leg of the curve)
3.
A bend or ‘knee’ in the curve between these two regions
In a normal curve, the three regions are smooth and continuous. The shape and location
of the knee depends on cell technology and manufacturer. Crystalline silicon cells have
sharper knees; thin film modules usually have more gradual knees.
The three PV model points are defined, from left to right, as follows:
•
SC – First point, at the predicted short circuit current I
sc
•
MP – Second point, the predicted maximum power point I
mp
, V
mp
•
OC – Third point, at the predicted open circuit voltage V
oc
Interpreting I-V Curves
PV module or string performance problems will cause deviations between the measured
and predicted I-V curves. There are six distinct types of deviation, listed below and in the
following figure. A given PV source under test may show no deviation, one deviation, or
a combination of two or more deviations.
1. 1. Notches or steps
2. 2. Low Current
3. 3. Low voltage
4.