5 Interpreting Measured I-V Curves
5-4
A normal I-V curve has a smooth shape with three distinct voltage regions as shown in
Figure 47:
1.
A slightly sloped region above 0 V
2.
A steeply sloped region below Voc
3.
A bend or ‗knee‘ in the curve in the region of the maximum power point
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 have more gradual knees.
The five model points are defined, from left to right, as follows:
SC – First point, at short circuit current Isc
X – Second point, at one-half of the open circuit voltage
MP – Third point, the maximum power point Imp, Vmp
XX – Fourth point, midway between Vmp and Voc
OC – Fifth point, at open circuit voltage Voc
Interpreting I-V Curves
Most array problems show deviations between measured and predicted I-V curves that
fall into one of these categories:
1.
The measured I-V curve shows higher or lower current than predicted
2.
The slope of the I-V curve near Isc does not match the prediction
3.
The slope of the I-V curve near Voc does not match the prediction
4.
The I-V curve has notches or steps
5.
The I-V curve has a higher or lower Voc value than predicted
A single I-V curve may show one or more of these deviations. One or more of these
deviations may indicate a reduction in maximum power produced by the module or string
under test.
NOTE
Deviations from the ideal IV curve may be due to physical problems with the PV array
under test, or may be the result of incorrect model values, settings or measurement
connections. Always select the correct PV module from the on-board PV module list,
